Role of the basolateral nucleus of the amygdala in endocannabinoid-mediated stress-induced analgesia

Recent work in our laboratories has demonstrated that an opioid-independent form of stress-induced analgesia (SIA) is mediated by endogenous ligands for cannabinoid receptors-anandamide and 2-arachidonoylglycerol (2-AG) [A.G. Hohmann, R.L. Suplita, N.M. Bolton, M.H. Neely, D. Fegley, R. Mangieri, J.F. Krey, J.M. Walker, P.V. Holmes, J.D. Crystal, A. Duranti, A. Tontini, M. Mor, G. Tarzia, D. Piomelli, An endocannabinoid mechanism for stress-induced analgesia, Nature 435 (2005) 1108-1112]. The present study was conducted to examine the contribution of cannabinoid CB1 receptors in the basolateral nucleus of the amygdala (BLA) and central nucleus of the amygdala (CeA) to nonopioid SIA. SIA was induced by continuous footshock (3 min 0.9 mA) and quantified behaviorally using the tail-flick test. Microinjection of the CB1 antagonist/inverse agonist rimonabant (SR141716A) into the BLA, a limbic forebrain region with high densities of CB1 receptors, suppressed SIA relative to control conditions. By contrast, the same dose administered into the CeA, where CB1 immunoreactivity is largely absent, or outside the amygdala did not alter SIA. To examine the contribution of endocannabinoids in the BLA to SIA, we used selective pharmacological inhibitors of the anandamide-degrading enzyme fatty-acid amide hydrolase (FAAH) and the 2-arachidonoylglycerol-degrading enzyme monoacylglycerol lipase (MGL). The FAAH inhibitor URB597 and MGL inhibitor URB602, at doses that enhanced SIA following microinjection in the midbrain periaqueductal gray, did not alter SIA relative to control conditions. Our findings suggest that CB1 receptors in the BLA but not the CeA contribute to SIA, but pharmacological inhibition of endocannabinoid degradation at these sites does not affect the expression of stress antinociception.     

Long-term potentiation (LTP) in the central amygdala (CeA) is enhanced after prolonged withdrawal from chronic cocaine and requires CRF1 receptors

The amygdala is part of the brain reward circuitry that plays a role in cocaine-seeking and abstinence in animals and cocaine craving and relapse in humans. Cocaine-seeking is elicited by cocaine-associated cues, and the basolateral amygdala (BLA) and CeA are essential in forming and communicating drug-related associations that are thought to be critical in long-lasting relapse risk associated with drug addiction. Here we simulated a cue stimulus with high-frequency stimulation (HFS) of the BLA-CeA pathway to examine mechanisms that may contribute to drug-related associations. We found enhanced long-term potentiation (LTP) after 14-day but not 1-day withdrawal from 7-day cocaine treatment mediated through N-methyl-d-aspartate (NMDA) receptors (NRs), L-type voltage-gated calcium channels (L-VGCCs), and corticotropin-releasing factor (CRF)(1) receptors; this was accompanied by increased phosphorylated NR1 and CRF(1) protein not associated with changes in NMDA/AMPA ratios in amygdalae from cocaine-treated animals. We suggest that these signaling mechanisms may provide therapeutic targets for the treatment of cocaine cravings.     

Corticotropin-releasing factor type II (CRF-sub-2) receptors in the bed nucleus of the stria terminalis modulate conditioned defeat in Syrian hamsters (Mesocricetus auratus)

In Syrian hamsters (Mesocricetus auratus), social defeat produces a subsequent increase in submissive and defensive behavior and a loss of normal territorial aggression, which the authors have called conditioned defeat. In this study, the authors investigated the effect of blocking corticotropin-releasing factor (CRF) Type I and Type II receptors on conditioned defeat. Intracerebroventricular infusion of the CRF-sub-2 receptor antagonist antisauvagine-30 prior to testing significantly reduced conditioned defeat compared with vehicle controls, whereas the CRF-sub-1 receptor antagonist CP-154,526 had no effect. Also, infusion of antisauvagine-30 into the bed nucleus of the stria terminalis (BNST) 15 min, but not immediately, prior to testing reduced conditioned defeat in a dose-dependent manner. The authors' results provide evidence that CRF-sub-2 receptors in the BNST, but not CRF-sub-1 receptors, are an important component in the neural circuitry regulating conditioned defeat.     

Activation of basolateral amygdala corticotropin-releasing factor 1 receptors modulates the consolidation of contextual fear

The basolateral amygdala complex (BLA) and central amygdala nucleus (CeA) are involved in fear and anxiety. In addition, the BLA contains a high density of corticotropin-releasing factor 1 (CRF₁) receptors in comparison to the CeA. However, the role of BLA CRF₁ receptors in contextual fear conditioning is poorly understood. In the present study, we first demonstrated in rats that oral administration of DMP696, the selective CRF₁ receptor antagonist, had no significant effects on the acquisition of contextual fear but produced a subsequent impairment in contextual freezing suggesting a role of CRF₁ receptors in the fear memory consolidation process. In addition, oral administration of DMP696 significantly reduced phosphorylation of cyclic AMP response element-binding protein (pCREB) in the lateral and basolateral amygdala nuclei, but not in the CeA, during the post-fear conditioning period. We then demonstrated that bilateral microinjections of DMP696 into the BLA produced no significant effects on the acquisition of conditioned fear but reduced contextual freezing in a subsequent drug-free conditioned fear test. Importantly, bilateral microinjections of DMP696 into the BLA at 5 min or 3 h, but not 9 h, after exposure to contextual fear conditioning was also effective in reducing contextual freezing in the conditioned fear test. Finally, microinfusions of either DMP696 into the CeA or a specific corticotropin-releasing factor 2 receptor antagonist in the BLA were shown to have no major effects on disrupting either contextual fear conditioning or performance of contextual freezing in the drug-free conditioned fear test. Collectively, results implicate a role of BLA CRF₁ receptors in activating the fear memory consolidation process, which may involve BLA pCREB-induced synaptic plasticity.     

Differential expression of glycine receptor subunits in the rat basolateral and central amygdala

The amygdalar complex is a limbic structure that plays a key role in emotional processing and fear conditioning. Although inhibitory transmission in the amygdala is predominately GABA-ergic, neurons of the amygdala are also known to express glycine receptors. The subtype and function of these glycine receptors within the synaptic circuits of the amygdala are unknown. In this study, we have investigated the relative expression of the four major glycine receptor subunits (α1–3 and β) in the rat basolateral (BLA) and central amygdala (CeA), using real-time PCR and protein biochemistry. We demonstrate that α1, α2, α3, and β subunits are all expressed in the BLA and CeA with α2 being the predominant α-subunit in both nuclei. Electrophysiological recordings from BLA and CeA neurons in acute brain slices indicated that differences in relative expression of these subunits were correlated with the pharmacological properties of native glycine receptors expressed on these neurons. We conclude that glycine receptors assembled in BLA neurons are largely α1β-containing heteromultimers whereas receptors assembled in neurons of the central amygdala are primarily α2β-, α3β- or α1β-containing heteromultimers, with a minor component of α2 or α3 homomeric receptors also expressed.     

Involvement of central amygdalar and bed nucleus of the stria terminalis corticotropin-releasing factor in behavioral responses to social defeat

The authors investigated whether corticotropin-releasing factor (CRF) within the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST) is a critical component of the neural circuitry mediating conditioned defeat. In this model, hamsters that have experienced social defeat subsequently display only submissive-defensive agonistic behavior instead of territorial aggression. Conditioned defeat was significantly reduced following infusion of the CRF receptor antagonist D-Phe CRF((12-41)) into the BNST but not into the CeA. In another experiment, hamsters given unilateral lesions of the CeA and infusions of D-Phe CRF((12-41)) into the contralateral BNST displayed significantly less submissive behavior than did controls. These data suggest that CRF acts within a neural circuit that includes the amygdala and the BNST to modulate agonistic behavior following social defeat.     

Role of dopamine receptor mechanisms in the amygdaloid modulation of fear and anxiety: Structural and functional analysis

Dopamine plays an important role in fear and anxiety modulating a cortical brake that the medial prefrontal cortex exerts on the anxiogenic output of the amygdala and have an important influence on the trafficking of impulses between the basolateral (BLA) and central nuclei (CeA) of amygdala. Dopamine afferents from the ventral tegmental area innervate preferentially the rostrolateral main and paracapsular intercalated islands as well as the lateral central nucleus of amygdala activating non-overlapping populations of D1- and D2-dopamine receptors located in these structures. Behaviorally, the intra-amygdaloid infusion of D1 agonists and antagonists elicits anxiogenic and anxiolytic effects respectively on conditioned and non-conditioned models of fear/anxiety suggesting an anxiogenic role for D1 receptors in amygdala. The analysis of the effects of D2 agonists and antagonists suggest that depending of the nature of the threat the animal experiences in anxiety models either anxiogenic or anxiolytic effects are elicited. It is suggested that D1- and D2-dopamine receptors in the amygdala may have a differential role in the modulation of anxiety. The possibility is discussed that D1 receptors participate in danger recognition facilitating conditioned–unconditioned associations by the retrieval of the affective properties of the unconditioned stimuli, and in the control of impulse trafficking from cortical and BLA regions to BLA and CeA nuclei respectively whereas D2 receptors have a role in setting up adaptive responses to cope with aversive environmental stimuli.     

Role of the extended amygdala in short-duration versus sustained fear: a tribute to Dr. Lennart Heimer

The concept of the "extended amygdala", developed and explored by Lennart Heimer, Jose de Olmos, George Alheid, and their collaborators, has had an enormous impact on the field of neuroscience and on our own work. Measuring fear-potentiated startle test using conditioned stimuli that vary in length we suggest that the central nucleus of the amygdala (CeA) and the lateral division of the bed nucleus of the stria terminalis (BNST(L)) are involved in short-term versus long-term fear responses we call phasic versus sustained fear, respectively. Outputs from the basolateral amygdala (BLA) activate the medial division of the CeA (CeA(M)) to very rapidly elicit phasic fear responses via CeA(M) projections to the hypothalamus and brainstem. The BLA also projects to the BNST(L), which together with other BNST(L) inputs from the lateral CeA (CeA(L)) initiate a slower developing, but sustained fear response, akin to anxiety. We hypothesize this occurs because the CeA(L) releases the peptide corticotropin releasing hormone (CRF) into the BNST(L) which facilitates the release of glutamate from BLA terminals. This activates the BNST(L) which projects to hypothalamic and brainstem areas similar to those innervated by the CeA(M) that mediate the specific signs of fear and anxiety. The generality of this idea is illustrated by selective studies looking at context conditioning, social defeat, drug withdrawal and stress induced reinstatement.     

Basolateral amygdala NMDA receptors are selectively involved in the acquisition of taste-potentiated odor aversion in the rat

In the taste-potentiated odor aversion (TPOA) paradigm, animals acquire a strong aversion to an odor that is followed by delayed intoxication only if a gustatory stimulus is presented with the odor during conditioning. Although previous work has shown that N-methyl-D-aspartate (NMDA) receptors in the basolateral nucleus of the amygdala (BLA) play a role in the acquisition of TPOA, the present study aimed at describing the process in which NMDA receptors in the BLA are involved during acquisition of TPOA. Male Long-Evans rats received intra-BLA infusions of the competitive NMDA receptor antagonist D,L-2-2-amino-5-phosphonovalerate (D-APV; 0.05 and 0.50 microg) immediately before or after the odor-taste conditioned stimulus (CS) presentation, or immediately before the test. Results showed that D-APV impaired acquisition of TPOA when infused before, but not after, the CS presentation, but did not affect retrieval. These results suggest that NMDA receptors of the BLA are involved in the formation of potentiation--by taste--of the olfactory memory trace, but not in the maintenance of this process.     

Pro- and anti-nociceptive effects of corticotropin-releasing factor (CRF) in central amygdala neurons are mediated through different receptors

Corticotropin-releasing factor (CRF) is not only a stress hormone but also acts as a neuromodulator outside the hypothalamic-pituitary-adrenocortical axis, playing an important role in anxiety, depression, and pain modulation. The underlying mechanisms remain to be determined. A major site of extra-hypothalamic expression of CRF and its receptors is the amygdala, a key player in affect-related disorders such as anxiety. The latero-capsular division of the central nucleus of the amygdala (CeLC) is also important for pain modulation and pain affect. This study analyzed the effects of CRF on nociceptive processing in CeLC neurons and the contribution of CRF1 and CRF2 receptors and protein kinases A and C. Extracellular single-unit recordings were made from CeLC neurons in anesthetized adult rats. All neurons responded more strongly to noxious than innocuous mechanical stimulation of the knee. Evoked responses and background activity were measured before and during administration of CRF into the CeLC by microdialysis. CRF was administered alone or together with receptor antagonists or protein kinase inhibitors. CRF (0.01-1 microM; concentrations in microdialysis probe; 15 min) facilitated the evoked responses more strongly than background activity; a higher concentration (10 microM) had inhibitory effects. Facilitation by CRF (0.1 microM) was reversed by a selective CRF1 receptor antagonist (NBI27914, 10 microM) but not a CRF2 receptor antagonist (astressin-2B, 100 microM) and by a protein kinase A (PKA) inhibitor (KT5720, 100 microM) but not a protein kinase C inhibitor (GF109203X, 100 microM). Inhibitory effects of CRF (10 microM) were reversed by astressin-2B. These data suggest that CRF has dual effects on amygdala neurons: CRF1 receptor-mediated PKA-dependent facilitation and CRF2 receptor-mediated inhibition.     

Stimulation of basolateral amygdaloid serotonin 5-HT2C receptors promotes the induction of long-term potentiation in the dentate gyrus of anesthetized rats

We have previously found that the induction of hippocampal long-term potentiation (LTP) is modulated by neuron activities in the basolateral amygdala (BLA). However, little is known about what neurotransmitter system in the BLA contributes to modulation of hippocampal LTP. In the present study, we investigated possible involvement of BLA serotonergic system in the induction of LTP at the perforant path (PP)-dentate gyrus (DG) granule cell synapses of anesthetized rats. The induction of PP-DG LTP was significantly inhibited by intra-BLA injection of the 5-HT₂ receptor antagonist cinanserin (25–50 nmol), but not by intra-BLA injection of the 5-HT_1,7 receptor antagonist methiothepin (50 nmol), the 5-HT₃ receptor antagonist ondansetron (50 nmol) or the 5-HT₄ receptor antagonist RS23597-190 (100 nmol). In addition, intra-BLA injection of the 5-HT_2C receptor agonist MK212 (50 nmol) facilitated the induction of PP-DG LTP. These results suggest that the induction of PP-DG LTP is promoted by activation of 5-HT_2C receptors in the BLA.     

Two intra-amygdaloid pathways to the central amygdala exhibit different mechanisms of long-term potentiation

Synaptic plasticity in the amygdala is thought to underlie aversive or rewarding learning and emotional memories. In this study, different mechanisms were found to underlie synaptic plasticity in lateral (LA) and basolateral (BLA) amygdala pathways to the primary output nucleus of the amygdala, the central amygdala (CeA). Specifically, 1) long-term potentiation (LTP) at the BLA-CeA synapses was independent of inhibition and mediated through N-methyl-d-aspartate receptors (NMDARs) and L-type voltage-gated calcium channels (VGCCs), and 2) LTP in the LA-CeA pathway was gated by inhibition and mediated through VGCCs but not NMDARs.     

Hyperphagia induced by GABAA receptor-mediated inhibition of the nucleus accumbens shell: dependence on intact neural output from the central amygdaloid region

To investigate the role of corticolimbic input in modulating feeding-related nucleus accumbens (Acb) circuitry, researchers temporarily deactivated sites within the basolateral amygdaloid complex (BLA) or central amygdaloid region (CeA) via GABA(A) agonist (muscimol) infusions and measured feeding responses following muscimol infusions into the Acb shell. Hyperphagia elicited by intra-Acb shell muscimol was not altered by coinfusions of intra-BLA muscimol. In contrast, muscimol infusions into the CeA dose-dependently reduced feeding elicited either by intra-Acb shell GABA(A) receptor stimulation or by food deprivation and produced a syndrome of forepaw treading. Intra-CeA tetrodotoxin infusions also blocked intra-Acb shell muscimol-induced hyperphagia. Hence, feeding elicited by intra-Acb shell GABA(A) receptor stimulation requires intact neural output from the CeA but not the BLA.     

Differential effects of CRF1 and CRF2 receptor antagonists on pain-related sensitization of neurons in the central nucleus of the amygdala

As a hormone in the hypothalamic-pituitary-adrenocortical (HPA) axis corticotropin-releasing factor (CRF) mediates stress responses. CRF can also act as a neuromodulator of synaptic transmission outside the HPA axis. A major site of extrahypothalamic expression of CRF and its G-protein-coupled receptors is the amygdala, a key player in affect-related disorders such as anxiety. The laterocapsular division of the central nucleus of the amygdala (CeLC) is important for the modulation of pain affect. This study determined the effects of CRF1 and CRF2 receptor antagonists in CeLC neurons in an arthritis pain model. Extracellular single-unit recordings were made from CeLC neurons in anesthetized adult rats. All neurons responded more strongly to noxious than to innocuous mechanical stimulation (compression) of peripheral tissues, including the knee. Evoked responses and background activity were measured before and during the development of a kaolin/carrageenan-induced knee joint arthritis. Drugs were administered into the CeLC by microdialysis before and/or after arthritis induction. All CeLC neurons showed increased responses to mechanical stimuli ("sensitization") 5-6 h postinduction of arthritis. A selective CRF1 receptor antagonist (NBI27914; 1-100 microM, concentration in microdialysis probe; 15 min) inhibited evoked responses and background activity in arthritis (n = 9) but had no effect under normal conditions before arthritis (n = 9). In contrast, a selective CRF2 receptor antagonist (Astressin-2B; 1-100 microM, 15 min) had no effect in arthritis (n = 7) but increased the neurons' responses under normal conditions (n = 8). These data suggest that CRF1 receptors in the amygdala contribute to pain-related sensitization, whereas the normally inhibitory function of CRF2 receptors is lost in the arthritis pain model.     

Synaptic properties and postsynaptic opioid effects in rat central amygdala neurons

An important output of amygdaloid nuclei, the central nucleus of the amygdala (CeA) not only mediates negative emotional behaviors, but also participates in the stimulus-reward learning and expression of motivational aspects of many drugs of abuse, and links environmentally stressful conditions such as fear to endogenous pain-inhibiting mechanisms. The endogenous opioid system in the CeA is crucial for both reward behaviors and environmental stress-induced analgesia. In this study using whole-cell voltage-clamp recordings, we investigated synaptic inputs and the postsynaptic effects of opioid agonists in CeA neurons. We found that synaptic inputs evoked within the CeA were mediated by both glutamate and GABA, but those evoked from the basolateral amygdala were primarily glutamatergic. Based on membrane properties, three types of cells were characterized. Type A neurons had no spike accommodation while type B neurons displayed characteristic accommodating response. Type A neurons were further classified as either A1 or A2, based on differences in resting membrane potential and the amplitude of after-hyperpolarizing potential. micro-Opioid receptor agonists hyperpolarized a subpopulation of CeA neurons, of which the vast majority was type A1. This micro agonist-induced hyperpolarization was mediated by the opening of inwardly rectifying potassium channels. In contrast, the kappa-opioid receptor agonist hyperpolarized only type B neurons. These results illustrate three types of CeA neurons with distinctive membrane properties and differential responses to opioid agonists. They may represent functionally distinct CeA cell groups for the integration and execution of CeA outputs in the aforementioned CeA functions.     

Noradrenergic agonist administration into the central nucleus of the amygdala increases the tail-flick latency in lightly anesthetized rats

The amygdala is a medial forebrain structure with an established role in nociceptive modulation, including the expression of stress-induced hypoalgesia (SIH). Projections from the locus coeruleus increase levels of noradrenaline in the amygdala during acute stress. alpha(2)-Noradrenergic receptor agonists have significant clinical utility as analgesic agents. We therefore hypothesized that alpha(2)-noradrenergic activation of the amygdala may result in behaviorally measurable hypoalgesia. Lightly anesthetized rats underwent microinjection of the alpha(2)-noradrenergic agonist clonidine into the amygdala and intermittent measurement of thermal nociception using the tail-flick latency (TFL). Bilateral microinjection of clonidine into the central nucleus of the amygdala (CeA) resulted in a significant, dose-dependent increase in TFL. This effect was blocked by systemic pre-treatment with the alpha(2)-antagonist yohimbine or by local pre-injection of the alpha(2)-antagonist idazoxan but not by local pre-injection of the alpha(1)-antagonist WB-4101. When injected alone, no antagonist resulted in a significant change in TFL compared with baseline. Clonidine injection into the amygdala but outside the CeA, including the basolateral nucleus of the amygdala, did not significantly alter TFL. These results demonstrate that anatomically and pharmacologically specific activation of alpha(2)-receptors in the CeA in lightly anesthetized rats results in behaviorally measurable antinociception.     

Possible routes of visceral information in the rat brain in formation of conditioned taste aversion.

When ingestion of a taste stimulus is paired with internal malaise, the animal remembers the taste and rejects its ingestion thereafter. This learning is referred to as conditioned taste aversion (CTA). To establish CTA in adult male Wistar rats, 0.1% saccharin and an i.p. injection of 0.15 M LiCl were used as the conditioned and unconditioned stimuli, respectively. Neuroanatomical study using the tracer method was performed to examine the ascending routes from the lateral part of the parabrachial nucleus (PBlat) which receives general visceral information and suggested the three possible routes to the amygdala: (1) direct route to the central nucleus of the amygdala (CeA); (2) diencephalic route to the basolateral nucleus of the amygdala (BLA) involving the zona incerta (ZI) and the midline and intralaminar thalamic complex (MITC); and (3) cortical route to the BLA involving insular cortex (IC). Rats with excitotoxic lesions of each of the CeA, ZI, MITC or IC had only a small or negligible effect on the acquisition of CTA. However, single lesions of the BLA and combined lesions of the ZI and IC, but not CeA and IC, almost completely abolished the acquisition of CTA. These results together with previous findings suggest that visceral (or unconditioned stimulus) information in the PBlat is sent to the BLA which is essential for the acquisition of CTA via the functionally important two parallel routes, the diencephalic and cortical routes, with either being able to create the aversion.     

Somatosensory regulation of serotonin release in the central nucleus of the amygdala is mediated via corticotropin releasing factor and gamma-aminobutyric acid in the dorsal raphe nucleus

Noxious cutaneous stimulation increases, whereas innocuous cutaneous stimulation decreases serotonin (5-HT) release in the central nucleus of the amygdala (CeA) in anesthetized rats. In the present study, we investigated the contribution of corticotropin releasing factor (CRF) receptors and gamma-aminobutyric acid (GABA) receptors in the dorsal raphe nucleus (DRN) to those responses. Release of 5-HT in the CeA was monitored by microdialysis before and after 10-min stimulation by pinching or stroking. Increased 5-HT release in the CeA in response to pinching was abolished by CRF₂ receptor antagonism in the DRN. Decreased 5-HT release in the CeA in response to stroking was abolished by either CRF₁ receptor antagonism or GABA_A receptor antagonism in the DRN. These results suggest that opposite responses of 5-HT release in the CeA to noxious versus innocuous stimulation of the skin are due to separate contributions of CRF₂, CRF₁ and GABA_A receptors in the DRN.     

Effects of stress on AMPA receptor distribution and function in the basolateral amygdala

Stress is a growing public health concern and can lead to significant disabilities. The neural response to stressors is thought to be dependent on the extended amygdala. The basolateral amygdala (BLA) is responsible for associations of sensory stimuli with emotional valence and is thought to be involved in stress-induced responses. Previous behavioral and electrophysiological experiments demonstrate that, in response to stress, changes occur in glutamatergic neurotransmission within the BLA and, in particular in transmission at AMPA receptors. Given the established role of AMPA receptors in memory and synaptic plasticity, we tested the hypothesis that stress produces alterations in the distribution of these receptors in a way that might account for stress-induced alterations in amygdala circuitry function. We examined the subcellular localization of GluR1 subunits of the AMPA receptor and the electrophysiological characteristics of BLA principal neurons in an animal model of unpredictable stress. Compared to controls, we demonstrated an increase in the ratio of labeled spines to labeled dendritic shafts in the BLA of rats 6 and 14 days post-stress, but not 1 day post-stress. Furthermore, the frequency of mini-EPSCs was increased in stressed animals without a change in general membrane properties, mini-EPSC amplitude, or in paired pulse modulation of glutamate release. Taken together, these data suggest that the shift of GluR1-containing AMPA receptors from dendritic stores into spines may be in part responsible for the persistent behavioral alterations observed following severe stressors.     

Cocaine pre-exposure enhances CRF-induced expression of c-fos mRNA in the central nucleus of the amygdala: an effect that parallels the effects of cocaine pre-exposure on CRF-induced locomotor activity

There is evidence that cocaine pre-exposure produces changes in the responsivity of central corticotropin-releasing factor (CRF) systems and that these systems mediate some of the drug-related behavioural effects of acute stressors. The present experiment was conducted to assess the effects of repeated cocaine exposure on CRF-induced neuronal activation within two regions of the extended amygdala, the central nucleus of the amygdala (CeA) and lateral bed nucleus of the stria terminalis (BNST). In addition, CRF-induced neuronal activation was compared with CRF-induced locomotor activity. Rats were injected for 7 days with cocaine (days 1 and 7 in test chambers; days 2-6 in homecages) or saline. After 10 drug-free days, locomotor responsiveness to intracerebroventricular (i.c.v.) injections of CRF and Vehicle was assessed over 2-h test periods. Twenty-four to 48 h following testing for locomotor activity, animals were injected with either CRF or Vehicle, 30 min before being sacrificed. Subsequently, the brains were processed by in situ hybridization for c-fos mRNA, a widely used marker of neuronal activation, in the CeA and BNST. In CeA, i.c.v. CRF enhanced the expression of c-fos mRNA in cocaine, but not saline, pre-exposed animals; in the same animals, i.c.v. CRF resulted in enhanced locomotor activity in cocaine, but not saline, pre-exposed animals. The results demonstrate that repeated exposure to cocaine changes the neuronal response to CRF in the CeA; furthermore, they suggest that these changes in the CeA could potentially be of functional significance in the effects of repeated cocaine exposure on CRF-induced locomotor activity.