CORTICOTROPIN RELEASING FACTOR MEDIATES THE ANTINOCICEPTIVE ACTION OF NITROUS OXIDE IN RATS

Anesthesiology - Volume 99 • Number 3 • September 2003 Copyright © 2003 American Society of Anesthesiologists, Inc. PAIN AND REGIONAL ANESTHESIA

Background:

Exposure to nitrous oxide activates brainstem noradrenergic nuclei and descending inhibitory pathways, which produce the acute antinociceptive action of nitrous oxide. Because corticotropin-releasing factor (CRF) can produce activation of noradrenergic neurons in the locus ceruleus, the authors sought to determine whether it might be responsible for the antinociceptive action of nitrous oxide.

Methods:

Male Sprague-Dawley rats (250–300 g) were exposed for 60 min to room air or 25, 50 or 70% nitrous oxide in oxygen. Brain sections including the hypothalamus were immunostained for both c-Fos (a marker of neuronal activation) and CRF and the percentage of CRF-positive neurons expressing c-Fos was determined. The functional consequences of changes in CRF were investigated by assessing the effect of intracerebroventricular administration of a CRF antagonist (α-helical CRF9–41 , 20 μg/10 μl) on both activation of locus ceruleus noradrenergic neurons and the antinociception (with the tail-flick latency test) produced by nitrous oxide.

Results:

Inhalation of nitrous oxide induced a dose-dependent increase in c-Fos expression in CRF-positive neurons in the paraventricular nucleus of the hypothalamus. Intracerebroventricular administration of CRF antagonist inhibited nitrous oxide-induced c-Fos expression in the locus ceruleus and the antinociceptive effect of nitrous oxide.

Conclusions:

Nitrous oxide activates the CRF system in the brain, which results in stimulation of noradrenergic neurons in the locus ceruleus and its consequent antinociceptive effect.

Introduction

ALTHOUGH the mechanism underlying the analgesic action of nitrous oxide is still unclear, involvement of descending noradrenergic inhibition has been strongly suggested. Nitrous oxide activates noradrenergic neurons in the rat brainstem [1] and increases noradrenaline release in the brain and the spinal cord. [2] [3] Both antagonism of adrenergic receptors [4] and depletion of noradrenaline [3] in the spinal cord inhibit the antinociceptive effect of nitrous oxide. Furthermore, lesioning of brainstem noradrenergic neurons also inhibits nitrous oxide antinociception. [1] These findings suggest that nitrous oxide exerts its antinociceptive effect, at least in part, via the activation of descending noradrenergic inhibitory pathways. However, the mechanisms of activation of noradrenergic neurons by nitrous oxide remain unclear.

Corticotrophin-releasing factor (CRF) is released in response to various types of stressors and is a key mediator of the behavioral, endocrinologic, and physiologic responses to stressors. [5] [6] In addition to its well-described effect on the pituitary gland, CRF acts as a neurotransmitter and activates diverse intracellular signaling pathways. [7] The CRF system is reported to stimulate the locus ceruleus (LC) neurons through direct innervation [8] [9] resulting in the activation of the noradrenergic neuron system in the brain. [10] Furthermore, CRF exerts an antinociceptive action by central and peripheral mechanisms, possibly via activation of descending noradrenergic systems. [6] [11] In this study, we tested the hypothesis that the CRF system in the brain may be involved in the activation by nitrous oxide of the LC neurons and descending noradrenergic-inhibitory pathways. Using immunohistochemical expression of c-Fos protein as a marker of neuronal activation, [12] we sought to determine whether nitrous oxide activated CRF-containing neurons in the paraventricular nucleus (PVN) of the hypothalamus, a prominent site of CRF production. We then examined the effect of intracerebroventricular administration of a CRF antagonist, α-helical CRF9–41 , on nitrous oxide-induced activation of noradrenergic neurons in the LC (as reflected by c-Fos expression). Lastly, to determine whether CRF mediated the antinociceptive action of nitrous oxide, we studied the effect of the CRF antagonist on the prolongation of the tail-flick latency by nitrous oxide.

Materials and Methods

Animals

These experiments were reviewed and approved by Tokyo University Ethical Committee on Animal Research (Tokyo, Japan). Male Sprague-Dawley rats (250–300 g) were housed in a temperature-controlled and humidity-controlled environment and were maintained on a 12-h light/dark cycle. Food and water were available ad libitum. Rats had been habituated to the experimental condition for 2 h for each of 5 consecutive days. All of the behavioral experiments, tissue sampling, and habituation were performed between 8 and 11 am.

Reagents

The CRF antagonist, α-helical CRF9–41 , was purchased from Sigma-Aldrich Japan (Tokyo, Japan) and dissolved in 1% acetic acid with a final concentration of 2 μg/μl. We selected a dose of 20 μg of α-helical CRF9–41 as has been previously reported. [13] [14] Dexmedetomidine was kindly provided by Abbott Japan (Tokyo, Japan).

Immunohistochemistry

Rats were anesthetized by intraperitoneal injection of pentobarbital (100 mg/kg) and transcardially perfused with 100 ml of 0.1 m sodium phosphate-buffered saline, followed by 500 ml of 2% paraformaldehyde and 0.2% picric acid in 0.1 m sodium phosphate buffer cooled to 4°C. After decapitation, the whole brain was removed and immersed in the same fixative for 24 h at 4°C. Tissues were then stored overnight in 30% sucrose solution in 0.1 m phosphate buffer at 4°C for cryoprotection. The brain was sliced into 40-μm-thick sections with a cryotome (CM1800, Leica, Heidelberg, Germany) at −15°C. Every third section including the PVN or the LC was retained and placed in phosphate-buffer solution.

Sections were then double-stained for c-Fos and either CRF or tyrosine hydroxylase (TH). Sections were first incubated for 1 h in blocking solution (5% normal rabbit serum and 0.3% Triton X in phosphate-buffered saline) and then incubated overnight with goat anti-c-Fos antibody (1:5,000, Santa-Cruz Biotechnology, Santa-Cruz, CA) diluted in 1% normal rabbit serum and 0.3% Triton X in phosphate-buffered saline (buffer 1). After vigorous rinsing in buffer 1, sections were incubated for 1 h in biotinylated rabbit anti-goat immunoglobulin (1:200, Chemicon, Temecula, CA) in buffer 1. Sections were vigorously rinsed with 0.3% Triton X in 0.1 m phosphate-buffered saline (buffer 2) and then incubated for 1 h in avidin-biotin-peroxidase complex (Vectra Elite ABC, Vector Laboratories, Burlingame, CA) in buffer 2. Visualization of the reaction product was achieved by incubation for 4 min with diaminobenzidine and nickel-ammonium sulfate in the presence of hydrogen peroxide (diaminobenzidine kit, Vector Laboratories). Sections were rinsed in phosphate buffer and incubated in either goat anti-CRF antibody (1:1,000, Santa Cruz Biotechnology) or anti-TH antibody (1:3,000, Santa Cruz Biotechnology) using the same procedures as for c-Fos immunostaining except that the diaminobenzidine reaction was performed for 2 min in the absence of nickel-ammonium sulfate. As a result, c-Fos positive nuclei were stained black with cytoplasmic CRF or TH expression appearing brown. All incubations were performed at room temperature. After these staining procedures, the sections were rinsed in water and placed on a glass slide. The sections were dehydrated, cleared in 100% xylene, and covered. Care was taken to process samples from different groups simultaneously to minimize the effect of fluctuation in staining.

Tail-flick Latency Test

Tail-flick latencies (TFL) were determined from the mean of three consecutive latencies using a tail-flick apparatus (Muromachi Ikakikai, Tokyo, Japan). The interstimulus interval was approximately 1 min. A high-intensity light was focused on the ventral surface of the middle third of the tail, and the time for the animal to move its tail out of the light beam was automatically recorded. Three different sites of the middle third of the tail were exposed to the light beam to minimize the risk of tissue damage. The same light stimulus intensity was used in all experiments and was preset to give a mean latency of 2.5 s under room air. A cutoff time of 6 s was used to avoid the possibility of tissue damage. If no response had occurred by this time, a value of 6 s was ascribed to the test. Tail-skin temperature was measured immediately before the TFL test with a thermocouple probe mounted on a heat insulating plate (2 × 2 cm). For temperature measurement, the probe was kept in contact with the ventral surface of the tail close to the point heated in the TFL test. The rats were kept on a heating blanket throughout the TFL tests to maintain the tail temperature within 1° of 30°C.

Intracerebroventricular Cannulation.

Rats were anesthetized with intraperitoneal pentobarbital (65 mg/kg) and placed in a stereotaxic apparatus (Muromachi Ikakikai). The skin of the skull was incised in the midline and a hole was drilled (1.5 mm lateral and 1.0 mm posterior to the bregma). A 27-gauge guide cannula (4.0 mm length; Plastics One, Roanoke, VA), inserted with a stylet, was implanted into the left lateral ventricle and fixed to the skull with dental cement and two screws. After the surgical procedure, rats were housed individually for 7 days before experiments were performed. The correct placement of the intracerebroventricular catheter was verified after the experiments by histologic examination of the fixed brain. Only data from animals with correct placement of the cannulae were included.

Intracerebroventricular Administration of CRF Antagonist.

The internal cannula was connected to PE-20 tubing (25 cm) with a three-way stopcock attached on the other end. The tubing and the internal cannula were initially filled with distilled water and then end-loaded either with α-helical CRF9–41 (20 μg/10 μl) or with vehicle using a microsyringe attached to the three-way cock. A small air bubble was placed between the distilled water and the substance to be injected. Rats were anesthetized briefly with halothane and the internal cannula was inserted down through the guide cannula. The three-way cock was opened to the atmosphere and lifted up so that the solution in the PE tubing was delivered into the ventricle by hydrostatic pressure. Either α-helical CRF9–41 or vehicle was injected over a 1-min period and the internal cannula was withdrawn 1 min later. If the solution did not descend by hydrostatic pressure, it was assumed not to be intraventricular and data from the rat were not further analyzed.

Nitrous Oxide and c-Fos Expression in the CRF-positive Neurons of the PVN Rats were individually exposed for 60 min to room air or 25, 50, or 70% nitrous oxide with oxygen in a Plexiglas chamber (25 × 25 × 30 cm, n = 7 per group). An airway gas monitor (Model 254, Datex, Helsinki, Finland) continuously monitored the concentrations of oxygen, nitrous oxide, and carbon dioxide in the chamber, and flow rates were adjusted to maintain the desired concentrations. After 60 min of gas exposure, rats were anesthetized, perfused with fixative, and the brains were removed as described previously. Every third brain section including the PVN (coordinates: 1.5–2.0 mm caudal to the bregma, 0.5–1.0 mm lateral to the midline and 5.5–6.5 mm below the dura) was picked up and double immunostained for c-Fos and CRF expression.

CRF Antagonist and Nitrous Oxide-induced c-Fos in the LC One week after intracerebroventricular cannulation, rats underwent pretreatment with intracerebroventricular administration of vehicle (1% acetic acid) or α-helical CRF9–41 (20 μg/10 μl) during brief halothane anesthesia. Twenty minutes later, rats were exposed to 70% nitrous oxide and 30% oxygen or air for 60 min in a Plexiglas chamber (n = 7 for each of the four groups). The animals were then perfused and fixed as described previously. Every third section of the midbrain was picked up and double immunostained for c-Fos and TH expression.

CRF Antagonist and the Antinociceptive Action of Nitrous Oxide In a different group of rats (n = 7), baseline measurement of TFL (TFLbaseline ) was performed 20 min after intracerebroventricular injection of α-helical CRF9–41 or vehicle (1% acetic acid) after complete recovery from brief halothane anesthesia. The animals were then transferred into a chamber filled with 70% nitrous oxide and 30% oxygen. After 20 min of gas exposure, an examiner blinded to the intracerebroventricular pretreatment performed the TFL measurement (TFLN2O ). On separate occasions with a 1-week interval, individual rats were tested after each pretreatment (α-helical CRF9–41 and the vehicle) performed in a random order. Next, to determine whether CRF system is selectively involved in nitrous oxide analgesia, the effect of α-helical CRF9–41 on the antinociception by dexmedetomidine, a novel α2-adrenergic agonist, was tested in a different group of rats (n = 7). Administration of α2-adrenergic agonist is associated with decrease in the expression of c-Fos in the LC [15] ; hence, the CRF antagonist would not be expected to block the antinociceptive effect. Baseline measurement of TFL (TFLbaseline ) was performed 20 min after intracerebroventricular injection of α-helical CRF9–41 or vehicle. TFL measurement was repeated 20 min after intraperitoneal administration of 50-μg/kg dexmedetomidine (TFLDEX ). On separate occasions with a 1-week interval, individual rats were tested after each pretreatment (α-helical CRF9–41 and the vehicle) performed in a random order.

Statistical Analysis

Three sections containing the largest number of CRF-positive neurons in the PVN were selected in each rat. An examiner who was blinded to the treatment given to the rat scored and averaged the percentage of CRF-positive neurons in the PVN that exhibited c-Fos expression. Data were analyzed using the ANOVA and Bonferroni tests. Of the 10 sections containing the TH-positive LC neurons, the three with the largest number of c-Fos-positive nuclei were counted and aggregated by an investigator blinded to the treatment. Data were analyzed with the ANOVA and Bonferroni tests. TFL data are expressed as percent maximal possible effect (MPE) obtained from the following equation: (TFLN2O or DEX − TFLbaseline )/(6 − TFLbaseline ) × 100, where 6 is the cutoff latency. Percent MPE and TFLbaseline values after each pretreatment protocol were compared using the paired t test. All the data were expressed as mean ± SD and P values less than 0.05 were considered significant.

Results

Nitrous Oxide Induces c-Fos in the CRF-positive Neurons of the PVN In the PVN of the hypothalamus, CRF-positive cytoplasm was stained brown and c-Fos-positive cell nuclei were stained black after immunohistochemical processing (fig. 1). Expression of c-Fos was very limited or absent in rats exposed to room air (fig. 1B). Exposure to 70% nitrous oxide induced a significant increase of c-Fos expression in CRF-positive neurons of the PVN (fig. 1C). In rats exposed to 25, 50, and 70% nitrous oxide, the percentage of CRF-positive neurons in the PVN that expressed c-Fos was 16 ± 3, 32 ± 4, and 86 ± 17%, respectively (fig. 2). In rats exposed to 70% nitrous oxide, c-Fos-positive neurons were also identified in other regions of the brain including the thalamic and cortical nuclei, LC, parabrachial nucleus, periaqueductal gray, and cuneiform nucleus. c-Fos expression was not induced by nitrous oxide in CRF-positive neurons of the stria terminalis and central nucleus of the amygdala, two other prominent regions of CRF production.

Discussion

In this study, we have demonstrated that nitrous oxide activates CRF-positive neurons in the PVN as evidenced by c-Fos expression. Intracerebroventricular administration of a CRF antagonist, α-helical CRF9–41 , inhibited c-Fos expression, and hence the activation of the LC neurons by nitrous oxide exposure. The CRF antagonist also inhibited the antinociceptive effect of nitrous oxide as evaluated with the TFL test. Because the CRF antagonist had no effect on the antinociceptive action of dexmedetomidine (an α2 agonist that inhibits the LC neurons), [15] the action of CRF is selective for the antinociceptive action of nitrous oxide. These results suggest that nitrous oxide activates the CRF-containing neurons in the brain, which are causally related to its antinociceptive effect through the activation of noradrenergic systems.

Corticotropin-releasing factor is a key mediator of various stress responses. The exogenous administration of CRF produces behavioral activation and enhances behavioral responses to stressors. [5] Studies using CRF antagonists have provided evidence to suggest that brain CRF systems play an important role in mediating behavioral, endocrinologic, and physiologic responses to stressors. [5] [16] Evidence indicates that CRF acts as a neurotransmitter; CRF receptors couple to multiple G-proteins to activate diverse intracellular signaling pathways. [7] The CRF system reportedly enhances neuronal activity in the LC, the largest collection of noradrenergic neurons in the brain, [8] [17] and increases norepinephrine release. [18] [19] Furthermore, anatomical evidence indicates that the LC neurons are directly innervated by CRF neurons, [9] and intracerebroventricular or local administration of CRF activates the noradrenergic LC neurons. [20] In addition, a feed-forward interaction between CRF and noradrenergic systems has been postulated, [5] because noradrenergic innervation in the PVN further stimulates release of CRF. [21] Because the CRF system is activated under stressful conditions, a variety of stressors can activate the LC neurons through the CRF system. [22] [23] Indeed, the CRF concentration in the LC is modulated by stress. [17] [24] Central CRF neuronal systems are involved in mediating the stress-induced changes in the dynamics of brain noradrenergic systems in rats. [25] Furthermore, intracerebroventricular or direct administration of CRF antagonists prevented the stress-induced activation of the LC neurons [26] It has also been reported that CRF produces antinociception [27] ; for example, intracerebroventricular injection of CRF produces antinociception in the hot-plate test, [11] and although the precise mechanism is still unclear, several studies suggest that descending inhibitory mechanisms may be involved. [6] [28] The antinociceptive effect of intracerebroventricular administration of CRF is antagonized by 6-hydroxydopamine and prazosin, thereby suggesting the involvement of the noradrenergic system in CRF-induced antinociception. [11] Considering that CRF activates the LC, the CRF system may mediate endogenous antinociception by triggering descending inhibitory pathways originating from the LC. This may also apply to stress-induced analgesia. Cold-water swim analgesia was inhibited by lesioning of the PVN, [29] a prominent site of CRF production. Furthermore, several studies indicate that acute stress-induced analgesia is mediated via a noradrenergic system in animal models indicative of the interaction between the CRF and noradrenergic pathways in antinociception. [30] [31]

The antinociceptive action of nitrous oxide has also been strongly linked to descending noradrenergic inhibitory pathways. [4] We previously demonstrated that nitrous oxide activated noradrenergic neurons in the LC and in other brainstem noradrenergic nuclei, and lesioning of these nuclei, which gives rise to the descending inhibitory pathways, inhibited the antinociceptive effect of nitrous oxide. [1] Also, we and others have shown that nitrous oxide increases norepinephrine release in the brain and the spinal cord. [2] [3] Both antagonism of the adrenergic receptors [4] and depletion of norepinephrine [3] in the spinal cord inhibited the antinociceptive effect of nitrous oxide.

Although the mechanisms of LC activation by nitrous oxide have been unclear, the involvement of the CRF system in the brain is strongly suggested by the fact that exposure to nitrous oxide activated CRF containing neurons of the rat PVN and that intracerebroventricular administration of CRF antagonist inhibited the activation of the LC neurons by nitrous oxide. In addition, the antinociceptive action of nitrous oxide is also mediated, at least in part, by the CRF system in the brain because intracerebroventricular administration of CRF antagonist resulted in inhibition of nitrous oxide-induced antinociception but did not inhibit antinociception affected by dexmedetomidine. Taken together, we speculate that nitrous oxide activates the CRF system in the brain resulting in the stimulation of the LC and descending noradrenergic inhibitory pathways. Although this study focused on the activation of the LC, other noradrenergic nuclei in the brainstem, namely A5 and A7, could also be involved in the antinociceptive action of nitrous oxide. In fact, these nuclei originate descending noradrenergic inhibitory pathways that are activated by inhalation of nitrous oxide. [1]

Corticotropin-releasing factor induces transcription of proopiomelanocortin, which is a precursor of corticotropin, melanocyte-stimulating hormone, and β-endorphin. Furthermore, about 20% of CRF neurons in the PVN contain enkephalin. [32] Thus, activation of CRF system by nitrous oxide could lead to the antinociceptive action through opioidergic mechanisms. In fact, several studies have shown that opiate system could be involved in the antinociceptive effect of nitrous oxide. [4] Although the mechanism underlying the activation of the CRF system by nitrous oxide is still unclear, it may result from the recently reported N-methyl-d-aspartate (NMDA)-antagonistic effect of nitrous oxide. [33] [34] Nitrous oxide undergoes one electron reduction, and therefore is producing free-radical products. [35] The ionotropic NMDA receptor contains a redox-sensitive site, which might be a target for nitrous oxide. [36] The noncompetitive NMDA receptor antagonist MK801 activates CRF neurons in the rat PVN, [37] and some NMDA antagonists have been shown to exert antinociceptive effects at least partly through a supraspinal mechanism, [38] [39] as has been shown in nitrous oxide antinociception. However, we should also note that the majority of studies using pure NMDA antagonists have failed to show antinociception to acute noxious stimuli. Although animals were exposed to nitrous oxide as a sole anesthetic in our study, this gas is usually used in combination with other agents such as isoflurane or propofol in the clinical arena. The descending inhibitory mechanism of nitrous oxide antinociception can be inhibited by the concomitant use of general anesthetics because of the suppressive effect of these agents. In fact, our unpublished data showed that nitrous oxide-induced c-Fos expression in CRF neurons was dose-dependently inhibited by the concomitant use of isoflurane or propofol. [40] [41] Evidence indicates that volatile anesthetics antagonize nitrous oxide antinociception in the rat [42] ; therefore, nitrous oxide is a less effective analgesic in healthy volunteers coadministered sevoflurane. [43] However, our results do not refute the additive minimum alveolar concentration effects of gas anesthetics, because definition of minimum alveolar concentration is based on anesthetic, not analgesic, effects. Furthermore, our study does not exclude the possibility that mechanisms of nitrous oxide antinociception other than descending noradrenergic inhibition may exist. Direct evidence of CRF release by nitrous oxide exposure is lacking in our study because the activation of CRF-containing neurons does not necessarily indicate that CRF secretion is enhanced. In fact, CRF-containing neurons are reported to contain other neuropeptides such as neurotensin and enkephalin. [32] However, the finding that the CRF antagonist inhibited both the activation of brainstem nuclei and the antinociceptive action of nitrous oxide indicates the direct involvement of CRF in these actions of nitrous oxide. Although there are several technical difficulties, direct demonstration of nitrous oxide-mediated CRF release in the brain would corroborate our findings.

Conclusion

In summary, we have used c-Fos expression as a marker of neuronal activation and demonstrated that nitrous oxide activates CRF-containing neurons in the PVN of the rat hypothalamus. Intracerebroventricular administration of CRF antagonist inhibited the activation of the LC by nitrous oxide. In addition, the CRF antagonist inhibited the antinociceptive action of nitrous oxide as measured by the tail-flick assay. These data indicate that the CRF system plays a pivotal role in the mechanism of nitrous oxide-induced acute antinociception.

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41. Obara M , Sawamura S , Hagihara R , Hanaoka K : Propofol inhibits nitrous oxide-induced activation of corticotropin releasing factor-containing neurons in rat paraventricular nucleus of hypothalamus (abstract) . Anesthesiology 2001 ; 95 : A714

42. Goto T , Marota JJ , Crosby G : Volatile anaesthetics antagonize nitrous oxide and morphine-induced analgesia in the rat . Br J Anaesth 1996 ; 76 : 702–6 Abstract 43. Janiszewski DJ , Galinkin JL , Klock PA , Coalson DW , Pardo H , Zacny JP : The effects of subanesthetic concentrations of sevoflurane and nitrous oxide, alone and in combination, on analgesia, mood, and psychomotor performance in healthy volunteers . Anesth Analg 1999 ; 88 : 1149–54 Abstract

Shigehito Sawamura, M.D., Ph.D.,*

Mizuki Obara, M.D.,†

Kenji Takeda, M.D.,†

Mervyn Maze, M.B., Ch.B.,‡

Kazuo Hanaoka, M.D., Ph.D.§

*Assistant Professor,

†Postdoctoral Clinical Fellow, §Professor and Chairman, Department of Anesthesiology, Tokyo University Hospital.

‡Professor and Chairman, Department of Anaesthetics and Intensive Care, Imperial College, London, United Kingdom.

Received from the Department of Anesthesiology, Tokyo University Hospital, Tokyo, Japan.

Submitted for publication March 7, 2003.

Accepted for publication May 28, 2003.

Support was provided solely from institutional and/or departmental sources.

Address reprint requests to Dr. Sawamura: Department of Anesthesiology, Tokyo University Hospital, 7-3-1, Hongo, Bunkyo, Tokyo, 113-8655, Japan. Address electronic mail to: sawamura-tky@umin.ac.jp. Individual article reprints may be purchased through the Journal Web site, www.anesthesiology.org.

PACIENTE DROGADICTO, CUIDADOS E INTERAÇÕES MÉDICAS

Interações farmacológicas realcionadas com o alcool

O álcool etílico (etanol) é um agente sedativo e hipnótico consumido como droga social, sendo de uso legal (assim como o tabaco). O alcoolismo é um distúrbio complexo, sendo um problema importante do ponto de vista clínico e de saúde pública. As interações entre o etanol e outras drogas podem apresentar efeitos clínicos importantes que resultam de alterações na farmacocinética ou na farmacodinâmica da segunda droga.

As mais freqüentes interações farmacocinéticas entre droga e álcool ocorrem como resultado da proliferação induzida pelo álcool no retículo endoplasmático liso das células hepáticas. Assim, a ingestão prolongada de álcool, sem lesão para o fígado, pode aumentar a biotransformação metabólica de outras drogas. Já o uso agudo de álcool é capaz de inibir o metabolismo de outras drogas, devido à alteração do metabolismo ou à alteração do fluxo sangüíneo hepático. Este efeito agudo do álcool pode contribuir para o perigo comum de misturá-lo com outras drogas. As fenotiazinas, os antidepressivos tricíclicos e as drogas sedativas são as mais importantes capazes de interagirem com o álcool através deste mecanismo.

As interações farmacodinâmicas do álcool também têm grande importância clínica. A interação aditiva com outros agentes sedativos e hipnóticos é mais significativa. O álcool também potencializa os efeitos farmacológicos de muitas drogas não sedativas, incluindo os vasodilatadores e os fármacos hipoglicêmicos orais. O álcool também aumenta a ação antiplaquetária da aspirina.

Diversas interações relacionadas com o álcool são encontradas embaixo:

Fármaco - Efeito - Mecanismo Provável

Benzodiazepínicos -Depressão aumentada do SNC - Potencialização dos efeitos depressores

Barbitúricos - Depressão aumentada do SNC - Soma de efeitos - Inibição do metabolismo.

Opióides- Depressão aumentada do SNC - Soma dos efeitos

Antiespasmódicos -Depressão aumentada do SNC - Soma dos efeitos

Anti-histamínicos - Depressão aumentada do SNC - Soma dos efeitos

Metronidazol - Efeito antiabuso

Dissulfiram - Inibição enzimática

Cloranfenicol - Efeito antiabuso

Sulfaniluréias - Alteração do efeito hipoglicemiante - Indução enzimática

Isoniazida - Diminuição do efeito antimicrobiano - Indução enzimática

Paracetamol - Aumento da hepatotoxicidade - Aumento da síntese de metabólitos reativos

Anestésicos gerais - Diminuição do efeito - Tolerância farmacodinâmica cruzada

Interações farmacológicas realcionadas com o fumo

O número de substâncias liberadas pelo tabaco quando utilizado na forma cigarro (monóxido de carbono, cianureto de hidrógeno, aldeídos, benzopirenos, nicotina, pesticidas, alquitrienos e nitrosaminas) proporciona um grande potencial para interações medicamentosas, tanto farmacodinâmicas, quanto farmacocinéticas. Vários fármacos têm seus efeitos diminuídos à custa de indução do metabolismo hepático (como a teofilina, a imipramina, a amitriptilina, a desipramina, a nortriptilina, a cafeína e a heparina). Os benzopirenos, por exemplo, estimulam a atividade do citocromo P448 (similar ao citocromo P450) e, portanto, aumenta a biotransformação de medicamentos que utilizam essa via oxidativa.

Se um paciente em tratamento com teofilina pára de fumar, como em uma internação hospitalar, é necessário reduzir sua dosagem em aproximadamente 50%. É útil dosar os níveis plasmáticos de teofilina nesses casos.

O cigarro talvez ainda possa diminuir a absorção de insulina a partir dos sítios subcutâneos devido a vasoconstrição. Também pode ocorrer um aumento do risco de acidente vascular encefálico e de cardiopatia isquêmica em mulheres que estejam ingerindo contraceptivos orais (principalmente se esta for maior de 35 anos).

Além disso, várias outras interações entre o tabaco e diversos medicamentos podem ocorrer, tornando o hábito de fumar, extremamente difundido em todas as sociedades, ainda mais prejudicial a saúde da população.

Diversas interações relacionadas com o tabaco são encontradas abaixo:

Fármaco - Efeito -Mecanismo Provável

Anticoncepcionais - Aumento dos efeitos adversos dos anticoncepcionais orais - Alteração do metabolismo hormonal

Antidepressivos tricíclicos - Diminuição da concentração plasmática - Indução enzimática

Antipirina - Diminuição do efeito - Indução enzimática

Benzodiazepínicos -Diminuição do efeito - Indução enzimática

Cafeína - Aumento do clearance - Indução enzimática

Fenacetina - Diminuição da concentração plasmática - Indução enzimática

Furosemida - Diminuição da resposta diurética - Aumento do clearance renal

Heparina - Aumento da eliminação - Não estabelecido

Insulina - Diminuição do efeito - Antagonismo por liberação de substâncias endógenas

Propoxifeno - Diminuição ou inibição do efeito - Não estabelecido

Propanolol - Diminuição do efeito - Indução enzimática

Teofilina - Diminuição da concentração sérica - Indução enzimática

Vacinas anti-gripe - Menor proteção - Indução enzimática

Vitaminas C, B, B12 - Aumento das necessidades diárias - Deficiência na absorção?

Warfarina - Diminuição da concentração sérica - Indução enzimática

Interações farmacológicas realcionadas com a maconha

A maconha (Cannabis sativa), ou Marijuana, é a droga ilegal mais comumente usada nos Estados Unidos, sendo que, no final da década de 70, 11% dos alunos secundários do último ano admitia seu uso diário. Os efeitos farmacológicos característicos da maconha fumada são produzidos, em sua maioria, pelo agente denominado Delta-9-tetraidrocanabinol (Delta-9-THC), sendo que estes variam com a dose utilizada.

Vários efeitos medicinais da maconha já foram descritos, incluindo efeitos antinauseantes (podendo ser utilizados para combater os efeitos causados pelo tratamento por quimioterapia), efeitos relaxantes musculares, anticonvulsivantes e redução da pressão intra-ocular (tratamento do glaucoma).

Os efeitos nocivos mais comumente conhecidos são as alterações do humor, da percepção e da motivação, comprometimento das funções cognitivas, da aprendizagem e da memória.

Interações medicamentosas da maconha com outras drogas são pouco conhecidas: pode interagir com anestésicos gerais (como o halotano e o ciclopropano), potencializando os efeitos destas drogas, criando assim uma condição potencialmente perigosa.

Também já foi descrito a interação com o alcalóide fisostigmina (um inibidor das colinesterases), podendo levar a uma séria depressão do SNC, condição que foi associada a suicídios. O mecanismo desta interação pode ser devido a um efeito anticolinesterásico da maconha.

Outra interação já descrita foi com bloqueadores ganglionares, causando hipotensão.

Interações farmacológicas entre anestésicos locais e cocaína

A cocaína é um agente simpatomimético que estimula a liberação de noradrenalina e inibe sua recaptação nas terminações nervosas adrenérgicas. Isto implica dizer que os usuários de cocaína são pacientes de risco para toda e qualquer complicação cardiovascular. Em doses suficientes, induz hipertensão arterial e taquicardia, aumentando o débito cardíaco e as necessidades de oxigênio. Esta atividade do SNA simpático pode diminuir a perfusão das artérias coronárias e acarretar uma isquemia significante, arritmia ventricular, angina pectoris e infarto do miocárdio, efeitos estes que têm sido exaustivamente relatados.

Enquanto o efeito da cocaína permanecer ativo é também alto o risco de efeitos adversos, caso os vasoconstritores do grupo das aminas simpatomiméticas forem inadvertidamente injetados no sistema vascular sangüíneo.

Em virtude do risco potencial que isto representa para o cirurgião-dentista, é proposto o seguinte protocolo no atendimento de pacientes usuários de cocaína:

1. Através da anamnese, procurar identificar o usuário de cocaína. Como na grande maioria dos casos o paciente não relata ou não assume sua condição de usuário da droga, deve-se incluir a seguinte pergunta no roteiro de anamnese: Você faz uso de cocaína? Obviamente o profissional deverá esclarecer o teor da pergunta, descrevendo o grande risco de interação da droga com certos tipos de vasoconstritores contidos nas soluções anestésicas.

Obs.: Caso o cirurgião-dentista suspeite das informações dadas pelo paciente, alguns sinais físicos característicos como euforia, agitação, tremores, dilatação das pupilas e alteração no ritmo cardíaco (avaliada através do pulso carotídeo), assim como lesões de pele na região ventral do antebraço ou da mucosa nasal, podem auxiliar na identificação deste grupo de pacientes);

2. Documentar no prontuário clínico que o paciente foi esclarecido sobre os riscos da interação;

3. Evitar o uso de soluções anestésicas locais contendo vasoconstritores do tipo da adrenalina, noradrenalina, fenilefrina ou levonordefrina, ou utilizar fios de retração gengival impregnados com adrenalina, ao menos que o paciente declare que não usou a droga nas últimas 24 horas (informação esta que também deve ser documentada no prontuário clínico). Se o cirurgião-dentista suspeitar do contrário, deve adiar a consulta;

4. Na anestesia local destes pacientes, a escolha da solução anestésica deve recair na prilocaína 3% associada à felipressina (Biopressin®, Citanest® ou Citocaína®),nos procedimentos de média duração (60 a 90 minutos),ou na mepivacaína 3% sem vasoconstritor, quando a intervenção exige uma anestesia pulpar de no máximo 20 minutos para intervenções na maxila, e de até 40minutos para intervenções na mandíbula.

5. Nas urgências odontológicas, quando não se pode adiar a intervenção, avalie inicialmente o risco/benefício do atendimento ser realizado em ambulatório ou ambiente hospitalar. Considere também a sedação prévia com ansiolíticos do grupo dos benzodiazepínicos.

Conclusão

A dependência de drogas é um problema social complexo. É considerado um distúrbio, e por isso deve receber um enfoque mais amplo, envolvendo cuidados especiais no tratamento médico-odontológico.As drogas têm efeitos diretos sobre as estruturas da boca (mucosa, dente, língua), sendo a cárie e a doença periodontal doenças de elevada prevalência em viciados em droga, além da ocorrência de leucoplasias e carcinomas.

Quanto ao aspecto social, o uso de drogas causa alterações comportamentais nos indivíduos, com alteração do humor e perda da auto-estima, levando a um descuido com a saúde geral e bucal. Além disso, a dependência química causa efeitos deletérios progressivos sobre a saúde sistêmica.

Deve-se sempre lembrar que qualquer substância que venha a ser ingerida pelo indivíduo sem a intenção de promover um efeito terapêutico (como no caso dos alimentos, do tabaco, etc.) poderá levar a ocorrência de interações medicamentosas com qualquer medicamento que aquela pessoa estiver utilizando, causando alterações na eficácia terapêutica e na toxicidade destas substâncias. Assim, quando o cirurgião-dentista vai prescrever um determinado fármaco, ou quando o farmacêutico vai dispensá-lo, eles devem sempre considerar os hábitos individuais de cada indivíduo, aconselhando-os (no caso do cirurgião-dentista) a alterar seus hábitos para evitar possíveis complicações durante o tratamento devido a interações adversas deste tipo (por exemplo, os inibidores da monoaminooxidase causam hipertensão e a tiramina - uma substância presente no queijo, vinho chocolate, abacaxi, presunto, etc. - também causa hipertensão).

Diante disso, o indivíduo usuário ativo de drogas, lícitas ou não, deve ser tratado integralmente, associando-se aos cuidados médicos e psiquiátricos, uma abordagem educativa para valorização da saúde bucal e dos cuidados necessários para manter a integridade das estruturas buco-dentais.

Fonte: Alexandre Carlos Evangelista da Silva , Helder Fernandes de Oliveira e Tiago de Oliveira Campos .