Chlordiazepoxide microinjected into the region of the dorsal raphe nucleus eliminates the interference with escape responding produced by inescapable shock whether administered before inescapable shock or escape testing.

Systemic administration of benzodiazepines before exposure to inescapable shock (IS) blocks the enhanced fear conditioning and escape learning deficits that follow exposure to IS, whereas administration before the subsequent behavioral testing eliminates the enhanced fear but not the interference with escape (N?=?44 male rats). The failure of benzodiazepines to reduce the IS-produced escape learning deficit when given before testing is inconsistent with a recent proposal that interference with escape is mediated by an IS-induced sensitization of dorsal raphe nucleus (DRN) activity. The present experiments demonstrate that chlordiazepoxide will block both the enhancement of fear and interference with escape responding when given before either IS or testing if microinjected in the region of the DRN. This suggests that systemic benzodiazepines fail to block escape deficits when given before testing because action at a site distant from the DRN counters the effect of the drug at the DRN. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dissociation of interference with the speed and accuracy of escape produced by inescapable shock.

Benzodiazepines and naltrexone administered before inescapable shock block behavioral consequences of the inescapable shock such as poor shuttle box escape, reduced activity in reaction to shock, reduced social interaction, and so on. Anxiogenic β-carboline derivatives such as FR-7142 can produce these effects by themselves. In the present study, neither diazepam nor naltrexone had any effect on the interference with Y-maze choice escape accuracy produced by inescapable shock even though they both eliminated the reduction in Y-maze escape response speed produced by inescapable shock. Analogously, FG-1742 did not lead to a reduction in Y-maze choice escape response accuracy even though it did show escape responding. These data imply that inescapable shock interferes with escape choice learning and escape response speed by different mechanisms, the former not involving fear-anxiety processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of fear in mediating shuttle escape learning deficit produced by inescapable shock.

The relation between the shuttlebox escape deficit produced by prior inescapable shock (IS) and fear during shuttlebox testing as assessed by freezing was investigated in rats. IS rats learned to escape poorly and were more fearful than either escapably shocked subjects or controls, both before and after receiving shock in the shuttlebox. However, fear and poor escape performance did not covary with the manipulation of variables designed to modulate the amount of fear and the occurrence of the escape deficit. A 72-hr interval between IS and testing eliminated the escape deficit but did not reduce preshock freezing. Diazepam before testing reduced both preshock and postshock fear in the shuttlebox but had no effect on the escape deficit. Naltrexone had no effect on fear but eliminated the escape deficit. This independence of outcome suggests that the shuttlebox escape deficit is not caused by high levels of fear in IS subjects. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of the amygdala and dorsal raphe nucleus in mediating the behavioral consequences of inescapable shock

It has been argued that exposure to inescapable shock produces later behavioral changes such as poor shuttle box escape learning because it leads to the conditioning of intense fear, which later transfers to the shuttle box test situation and interferes with escape. Both fear, as assessed by freezing, and escape were measured in Sprague-Dawley rats 24 hr after exposure to inescapable shock. Lesions of the basolateral region and central nucleus of the amygdala eliminated the fear that transfers to the shuttle box after inescapable shock, as well as the fear conditioned in the shuttle box by the shuttle box shocks. However, the amygdala lesions did not reduce the escape learning deficit produced by inescapable shock. In contrast, dorsal raphe nucleus lesions did not reduce the fear that transfers to the shuttle box after inescapable shock, but eliminated the enhanced fear conditioning in the shuttle box as well as the escape deficit. The implications of these results for the role of fear and anxiety in mediating inescapable shock effects are discussed.     

Failure to learn to escape by rats previously exposed to inescapable shock is partly produced by associative interference.

2 experiments demonstrated that the effects of prior exposure to inescapable shock on the subsequent acquisition of an escape response in rats is determined by the nature of the contingency that exists between responding and shock termination during the escape learning task, and not by the amount of effort required to make the response or the amount of shock that the S is forced to receive during each trial. Exp I, using 48 male Simonsen rats, showed that inescapably shocked Ss did not learn to escape shock in a shuttle box if 2 crossings of the shuttle box were required (fixed ratio, FR, -2) to terminate shock, but did learn this FR-2 response if a brief interruption of shock occurs after the 1st crossing of the FR-2. Exp II with 72 Ss showed that inescapably shocked Ss learned a single-crossing escape response as rapidly as did controls, but were severely retarded if a brief delay in shock termination was arranged to follow the response. Results are discussed in terms of the learned helplessness hypothesis, which assumes that prior exposure to inescapable shock results in associative interference. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Catecholamine depletion in mice upon reexposure to stress: Mediation of the escape deficits produced by inescapable shock.

Reports 9 experiments with 372 male Swiss-Webster mice in which, immediately following exposure to 60 inescapable shocks, Ss had significantly reduced hypothalamic norepinephrine (NE). Within 24 hrs NE levels returned to control values. Reexposure to as few as 10 shocks 24 hrs after initial stress exposure resulted in significant decline of NE. At this interval after shock, escape performance was severely disrupted, with a large proportion of Ss exhibiting numerous failures to escape shock. Increasing brain dopamine (DA) and NE by levodopa treatment prior to shock prevented the escape deficits. Conversely, pairing 5 inescapable shocks with NE depletion by FLA-63, or both DA and NE depletion by alpha-methylparatyrosine, disrupted escape performance 24 hrs later. Residual drug effects, state dependence, or sustained amine turnover could not account for the behavioral changes. Data are discussed in terms of catecholamine mediation of escape performance through variations in response maintenance abilities. It is suggested that long-term effects of inescapable shock may be due to sensitization effects or conditioned amine depletion. (28 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dissociation of long-term analgesia and the shuttle box escape deficit caused by inescapable shock.

Previous studies indicate that inescapably shocked rats perform poorly on a 2-way shuttlebox escape task 24 hrs after shock. Because inescapably shocked rats become analgesic upon reexposure to a small amount of shock 24 hrs after inescapable shock (IS), they are likely to be analgesic during the shuttlebox escape task. Ss receiving an equivalent amount of escapable shock display neither the escape deficit nor the analgesia. Both the analgesia and the escape deficit respond in a similar fashion to the manipulation of a variety of other variables. These findings have led to the suggestion that the analgesia (long-term analgesia) may cause the IS-produced escape deficit. However, the present 2 experiments with 72 male albino rats demonstrated that 2 pituitary manipulations that completely eliminate the analgesia have no effect on the escape deficit. Both hypophysectomy and dexamethasone administration blocked the analgesic consequences of IS but did not reduce the magnitude of the escape deficit. Therefore, the long-term analgesia produced by IS does not cause the deficit in shuttlebox escape performance displayed by inescapably shocked rats. Results indicate that the pituitary is not essential in the production of this escape deficit. (47 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Interaction of Pavlovian conditioning with a zero operant contingency: Chronic exposure to signaled inescapable shock maintains learned helplessness effects.

Four studies, with 372 male Holtzman rats, examined the effect of Pavlovian contingencies and a zero operant contingency (i.e., uncontrollability) on subsequent shock-escape acquisition in the shuttle box using triads consisting of escapable-shock (ES), yoked inescapable-shock (IS), and no-shock (NS) rats. After exposure to 50 signals and shocks per session for 9 sessions, interference with shuttlebox escape acquisition for IS Ss was a monotonically increasing function of the percentage of signal–shock pairings during training (Exp I), with 50% pairings producing little or no impairment. Without regard to signaling, ES Ss performed as well as NS Ss. Exp II demonstrated that training and test conditions led to substantial and equal impairment in IS Ss preexposed for 1 session to 100 or 50% signal–shock pairings or to unsignaled shocks. In Exp III, chronic exposure to 100% signaled ISs resulted in impairment only if the signal (light) was present during the shuttlebox test. The continuous presence of the signal during the test contrasted with its discrete (5-sec) presentation during training and suggested that an antagonistic physiological reaction rather than a specific competing motor response had been conditioned. Exp IV provided evidence for possible conditioned opioid mediation. Findings suggest that chronic exposure to uncontrollable shocks maintains the impairment produced by acute exposure only if the shocks are adequately signaled. (63 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Forebrain afferents to the rat dorsal raphe nucleus demonstrated by retrograde and anterograde tracing methods

The dorsal raphe nucleus through its extensive efferents has been implicated in a great variety of physiological and behavioural functions. However, little is know about its afferents. Therefore, to identify the systems likely to influence the activity of serotonergic neurons of the dorsal raphe nucleus, we re-examined the forebrain afferents to the dorsal raphe nucleus using cholera toxin b subunit and Phaseolus vulgaris-leucoagglutinin as retrograde or anterograde tracers. With small cholera toxin b subunit injection sites, we further determined the specific afferents to the ventral and dorsal parts of the central dorsal raphe nucleus, the rostral dorsal raphe nucleus and the lateral wings. In agreement with previous studies, we observed a large number of retrogradely-labelled cells in the lateral habenula following injections in all subdivisions of the dorsal raphe nucleus. In addition, depending on the subdivision of the dorsal raphe nucleus injected, we observed a small to large number of retrogradely-labelled cells in the orbital, cingulate, infralimbic, dorsal peduncular, and insular cortice, a moderate or substantial number in the ventral pallidum and a small to substantial number in the claustrum. In addition, we observed a substantial to large number of cells in the medial and lateral preoptic areas and the medial preoptic nucleus after cholera toxin b subunit injections in the dorsal raphe nucleus excepting for those located in the ventral part of the central dorsal raphe nucleus, after which we found a moderate number of retrogradely-labelled cells. Following cholera toxin b subunit injections in the dorsal part of the central dorsal raphe nucleus, a large number of retrogradely-labelled cells was seen in the lateral, ventral and medial parts of the bed nucleus of the stria terminalis whereas only a small to moderate number was visualized after injections in the other dorsal raphe nucleus subdivisions. In addition, respectively, a substantial and a moderate number of retrogradely-labelled cells was distributed in the zona incerta and the subincertal nucleus following all tracer injections in the dorsal raphe nucleus. A large number of retrogradely-labelled cells was also visualized in the lateral, dorsal and posterior hypothalamic areas and the perifornical nucleus after cholera toxin b subunit injections in the dorsal part of the central raphe nucleus and to a lesser extent following injections in the other subdivisions. We further observed a substantial to large number of retrogradely-labelled cells in the tuber cinereum and the medial tuberal nucleus following cholera toxin b subunit injections in the dorsal part of the central dorsal raphe nucleus or the lateral wings and a small to moderate number after injections in the two other dorsal raphe nucleus subdivisions. A moderate or substantial number of labelled cells was also seen in the ventromedial hypothalamic area and the arcuate nucleus following cholera toxin injections in the dorsal part of the central dorsal raphe nucleus and the lateral wings and an occasional or small number with injection sites located in the other subdivisions. Finally, we observed, respectively, a moderate and a substantial number of retrogradely-labelled cells in the central nucleus of the amygdala following tracer injections in the ventral or dorsal parts of the central dorsal raphe nucleus and a small number after injections in the other subnuclei. In agreement with these retrograde data, we visualized anterogradely-labelled fibres heterogeneously distributed in the dorsal raphe nucleus following Phaseolus vulgaris-leucoagglutinin injections in the lateral orbital or infralimbic cortice, the lateral preoptic area, the perifornical nucleus, the lateral or posterior hypothalamic areas, the zona incerta, the subincertal nucleus or the medial tuberal nucleus. (ABSTRACT TRUNCATED)     

The inhibitory effect of 8-OH-DPAT on the firing activity of dorsal raphe serotoninergic neurons in rats is attenuated by lesion of the frontal cortex

The dose-response inhibitory effect of 8-OH-DPAT on the firing rate of dorsal raphe serotoninergic neurons was shifted 10-fold to the right after acute fronto-cortical deafferentation. This finding suggests that the inhibitory effect of 8-OH-DPAT on the dorsal raphe firing rate might be mediated indirectly by the frontal cortex.     

Ventromedial septal lesions in rats reduce the effects of inescapable shock on escape performance and analgesia.

In 2 experiments with 104 male Sprague-Dawley rats, lesions of the ventromedial septum (VMS) reduced or eliminated several effects of exposure to inescapable shock, but lesions of the dorsolateral septum did not. Exp I demonstrated that VMS lesions reduced the loss in body weight produced by inescapable shock and eliminated the subsequent (24 hrs later) interference with escape performance (learned helplessness). Exp II demonstrated that VMS lesions reduced the analgesia that occurs immediately following inescapable shock and the analgesia reinstated by exposure to escapable shock 24 hrs later. Findings indicate that VMS lesions reduce several responses to inescapable shock and suggest the possibility that all of these effects may reflect a unitary deficit. It is hypothesized that VMS lesions reduce these effects of exposure to inescapable shock either by reducing the ability of the rats to learn that their responses and shocks were uncorrelated or by reducing the emotional impact of this lack of correlation. (52 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Beta-endorphinergic innervation of mu and delta receptor containing neurons in the dorsal raphe nucleus

A pre-embedding double immunostaining technique was used to determine the role of beta-endorphin in synapse, particularly in neurons with a postsynaptic membrane containing micro-1 or delta-1 opioid receptors. A small number of beta-endorphin immunoreactive axon terminals in the dorsal raphe nucleus was found to make direct synapses on micro-1 or delta-1 opioid receptor-immunoreactive dendrites, some of which showed immunostaining of their postsynaptic membranes, although with low frequencies. These results suggest that beta-endorphin can play a direct role through the micro-1 or delta-1 opioid receptors at synapses, but the main route would be through other opioid receptor at the synapse or even not through the synapse.     

Risperidone inhibits 5-hydroxytryptaminergic neuronal activity in the dorsal raphe nucleus by local release of 5-hydroxytryptamine

1. The effects of risperidone on brain 5-hydroxytryptamine (5-HT) neuronal functions were investigated and compared with other antipsychotic drugs and selective receptor antagonists by use of single cell recording and microdialysis in the dorsal raphe nucleus (DRN). 2. Administration of risperidone (25-400 micrograms kg-1, i.v.) dose-dependently decreased 5-HT cell firing in the DRN, similar to the antipsychotic drug clozapine (0.25-4.0 mg kg-1, i.v.), the putative antipsychotic drug amperozide (0.5-8.0 mg kg-1, i.v.) and the selective alpha 1-adrenoceptor antagonist prazosin (50-400 micrograms kg-1, i.v.). 3. The selective alpha 2-adrenoceptor antagonist idazoxan (10-80 micrograms kg-1, i.v.), in contrast, increased the firing rate of 5-HT neurones in the DRN, whereas the D2 and 5-HT2A receptor antagonists raclopride (25-200 micrograms kg-1, i.v.) and MDL 100,907 (50-400 micrograms kg-1, i.v.), respectively, were without effect. Thus, the alpha 1-adrenoceptor antagonistic action of the antipsychotic drugs might, at least partly, cause the decrease in DRN 5-HT cell firing. 4. Pretreatment with the selective 5-HT1A receptor antagonist WAY 100,635 (5.0 micrograms kg-1, i.v.), a drug previously shown to antagonize effectively the inhibition of 5-HT cells induced by risperidone, failed to prevent the prazosin-induced decrease in 5-HT cell firing. This finding argues against the notion that alpha 1-adrenoceptor antagonism is the sole mechanism underlying the inhibitory effect of risperidone on the DRN cells. 5. The inhibitory effect of risperidone on 5-HT cell firing in the DRN was significantly attenuated in rats pretreated with the 5-HT depletor PCPA (p-chlorophenylalanine; 300 mg kg-1, i.p., day-1 for 3 consecutive days) in comparison with drug naive animals. 6. Administration of risperidone (2.0 mg kg-1, s.c.) significantly enhanced 5-HT output in the DRN. 7. Consequently, the reduction in 5-HT cell firing by risperidone appears to be related to increased availability of 5-HT in the somatodendritic region of the neurones leading to an enhanced 5-HT1A autoreceptor activation and, in turn, to inhibition of firing, and is probably only to a minor extent caused by its alpha 1-adrenoceptor antagonistic action.     

Extracellular signal-regulated kinase 1/2 involvement in the enhancement of contextual fear conditioning by nicotine.

Contextual fear conditioning is enhanced by nicotine, but the cellular mechanisms underlying this effect are unknown. Extracellular signal regulated kinase 1/2 (ERK 1/2) has been shown to play an integral role in the formation of contextual fear memories. As such, it is possible that ERK 1/2 is involved in the enhancement of contextual fear conditioning by nicotine. To determine whether ERK 1/2 plays a role in this enhancement, a dose of SL327 (a selective, systemic ERK 1/2 inhibitor) that is subthreshold for inhibiting contextual fear conditioning was coadministered with nicotine prior to training, testing, or both training and testing of contextual fear conditioning in C57BL/6 mice. When administered prior to training, this subthreshold dose of SL327 attenuated the enhancement of contextual fear conditioning by nicotine to levels similar to those of vehicle-treated animals. When administered prior to testing, the subthreshold dose of SL327 did not significantly alter conditioning. These results suggest that activation of ERK 1/2 by nicotine during acquisition leads to an enhancement of contextual fear conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of response restriction during exposure to inescapable shock upon subsequent one-way and shuttle-avoidance performance in rats.

Investigated the effects of signaled inescapable shock on subsequent avoidance performance in 3 experiments with male Holtzman rats (N = 188). Exp I indicated that prior shock exposure (PSE) facilitated 1-way and shuttle avoidance. When Ss were preshocked in a harness so that free mobility was not possible, the facilitative effects of PSE on shuttle, but not 1-way avoidance performance, were largely reduced. Exp II indicated that activity during CS periods following PSE was greater among unrestrained than restrained Ss. Exp III showed that immobilization via injection of succinylcholine chloride did not affect the facilitative effects of PSE relative to that of Ss preshocked in a harness. Results are interpreted in terms of response repertoire changes produced by PSE in conjunction with the response requirement of the avoidance task. (French summary) (17 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Expression of galanin and nitric oxide synthase in subpopulations of serotonin neurons of the rat dorsal raphe nucleus

The dorsal raphe nucleus (DR) of the rat was studied with triple labeling immunofluorescence histochemistry to evaluate the quantitative relationships between neurons expressing 5-hydroxytryptamine (5-HT), the neuropeptide galanin (GAL) and the nitric oxide (NO) synthesizing enzyme NO synthase (NOS). In addition retrograde tracing studies were performed. It could be established that a high percentage (between 40 and 60%) of the 5-HT neuron profiles in the ventromedial and dorsomedial subgroups of the DR contained both GAL and NOS after colchicine treatment. This triple coexistence was lower in the dorso-lateral subgroup and much lower in the lateral subgroup (down to 5%). All GAL neuron profiles contained 5-HT, and they constituted up to 80% of all labeled profiles in the rostral ventromedial and caudal dorsomedial subgroups, with the lowest percentage in the lateral subgroup (45%). The percentage of 5-HT-alone neuron profiles in these four subgroups varied between 15-40%. The proportion of 5-HT/NOS neurons was low (a few percent) at all levels, and this was the case also for NOS-alone neuron profiles except in the lateral subgroup, where 10-20% were of this type. It could be established that some 5-HT/GAL/NOS neurons project to the striatum. These retrogradely labeled cells were mainly found in the mid-line subgroups. In the striatum a moderately dense 5-HT fiber network and numerous NOS-positive cell bodies and fibers could be observed. However, only a few, weakly fluorescent GAL fibers were found and in a small number of cases it could be shown or was likely that 5-HT and GAL coexisted. No evidence for coexistence between 5-HT and NOS was obtained. The present findings strongly suggest that a large proportion of the DR 5-HT neurons can synthesize and release two additional messenger molecules, GAL and NO. Furthermore, even if the 5-HT/GAL/NOS neurons project to the striatum, the amounts of GAL and NOS transported to the terminal ramifications in this area are very low. This is in agreement with a very low GAL synthesis in the DR under normal circumstances, which is also indicated by the fact that colchicine treatment is needed to visualize GAL-like immunoreactivity in DR cell bodies. It is possible that NO, and perhaps GAL, in 5-HT neurons exert their main actions at the somatic and dendritic level in the DR.     

Destruction of serotonergic neurons in the median raphe nucleus blocks circadian rhythm phase shifts to triazolam but not to novel wheel access

Systematic treatment of hamsters with triazolam (TRZ) or novel wheel (NW) access will yield PRCs similar to those for neuropeptide Y. Both TRZ and NW access require an intact intergeniculate leaflet (IGL) to modulate circadian rhythm phase. It is commonly suggested that both stimulus types influence rhythm phase response via a mechanism associated with drug-induced or wheel access-associated locomotion. Furthermore, there have been suggestions that one or both of these stimulus conditions require an intact serotonergic system for modulation of rhythm phase. The present study investigated these issues by making serotonin neuron-specific neurotoxic lesions of the median or dorsal raphe nuclei and evaluating phase response of the hamster circadian locomotor rhythm to TRZ treatment or NW access. The expected effect of TRZ injected at CT 6 h on the average phase advance was virtually eliminated by destruction of serotonin neurons in the median, but not the dorsal, raphe nucleus. No control or lesioned animal engaged in substantial wheel running in response to TRZ. By contrast, all median raphe-lesioned hamsters that engaged in substantial amounts of running when given access to a NW had phase shifts comparable to control or dorsal raphe-lesioned animals. The results demonstrate that serotonergic neurons in the median raphe nucleus contribute to the regulation of rhythm phase response to TRZ and that it is unlikely that these neurons are necessary for phase response to NW access. The data further suggest the presence of separate pathways mediating phase response to the two stimulus conditions. These pathways converge on the IGL, a nucleus afferent to the circadian clock, that is necessary for the expression of phase response to each stimulus type.     

Double dissociation between the involvement of the bed nucleus of the stria terminalis and the central nucleus of the amygdala in startle increases produced by conditioned versus unconditioned fear

The amplitude of the acoustic startle response is reliably enhanced when elicited in the presence of bright light (light-enhanced startle) or in the presence of cues previously paired with shock (fear-potentiated startle). Light-enhanced startle appears to reflect an unconditioned response to an anxiogenic stimulus, whereas fear-potentiated startle reflects a conditioned response to a fear-eliciting stimulus. We examine the involvement of the basolateral nucleus of the amygdala, the central nucleus of the amygdala, and the bed nucleus of the stria terminalis in both phenomena. Immediately before light-enhanced or fear-potentiated startle testing, rats received intracranial infusions of the AMPA receptor antagonist 2, 3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)-quinoxaline (3 microg) or PBS. Infusions into the central nucleus of the amygdala blocked fear-potentiated but not light-enhanced startle, and infusions into the bed nucleus of the stria terminalis blocked light-enhanced but not fear-potentiated startle. Infusions into the basolateral amygdala disrupted both phenomena. These findings indicate that the neuroanatomical substrates of fear-potentiated and light-enhanced startle, and perhaps more generally of conditioned and unconditioned fear, may be anatomically dissociated.     

Inescapable shock-induced potentiation of morphine analgesia in rats: involvement of opioid, GABAergic, and serotonergic mechanisms in the dorsal raphe nucleus

Exposure of rats to inescapable shock (IS) potentiated the analgesic response to a low dose (1 mg/kg) of morphine 24 hr later. This effect was blocked by naltrexone (10 micrograms), diazepam (5 micrograms), or 8-hydroxy-2-(di-n-propylamine)-tetralin (8-OH-DPAT; 1 microgram) microinjected into the dorsal raphe nucleus (DRN) 15 min before IS. When microinjected into the DRN at the time of tail-flick testing, 8-OH-DPAT also effectively prevented this effect. Further, intra-DRN administration of a beta-carboline mimicked the effects of IS, because rats treated with methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (1 microgram) and simply restrained displayed potentiated morphine analgesia 24 hr later. These data suggest that this phenomenon shares mechanisms in common with other effects of IS at the level of the DRN.     

Role of the nucleus raphe magnus in antinociception produced by ABT-594: immediate early gene responses possibly linked to neuronal nicotinic acetylcholine receptors on serotonergic neurons

Recently, a novel cholinergic channel modulator, (R)-5-(2-azetidinylmethoxy)-2-chloropyridine (ABT-594), was shown to produce potent analgesia in a variety of rodent pain models when administered either systemically or centrally into the nucleus raphe magnus (NRM). The purpose of the present study was to investigate the possible supraspinal contribution of ABT-594 by assessing its ability to induce expression of the immediate early gene c-fos, a biochemical marker of neuronal activation, in the NRM of rats. Putative serotonergic neurons in the NRM, a medullary nucleus proposed to be involved in descending antinociceptive pathways, were identified immunohistochemically using a monoclonal antibody (mAb) against tryptophan hydroxylase. ABT-594 (0.03-0.3 micromol/kg, i.p.) produced a dose-dependent induction of Fos protein that was blocked by the central nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (5 micromol/kg, i.p.) but not by the peripheral nAChR antagonist hexamethonium (15 micromol/kg, i.p.). Immunohistological studies using mAb 299 revealed the expression of alpha4-containing nAChRs in the NRM. The alpha4 immunostaining was dramatically reduced by pretreating (30 d) animals with the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), which was previously shown to substantially attenuate the antinociceptive actions of ABT-594. In a double immunohistochemical labeling experiment, coexpression of the serotonin marker tryptophan hxdroxylase and the alpha4 nAChR subunit in NRM neurons was observed. These results suggest that the analgesic mechanism of ABT-594 may in part involve the activation of the NRM, a site where alpha4-containing nAChRs are expressed by serotonergic neurons.