Opioid actions on neurons of rat lateral amygdala in vitro

Pain-related cortico-limbic plasticity and opioid signaling

Neuroplasticity in cortico-limbic circuits has been implicated in pain persistence and pain modulation in clinical and preclinical studies. The amygdala has emerged as a key player in the emotional-affective dimension of pain and pain modulation. Reciprocal interactions with medial prefrontal cortical regions undergo changes in pain conditions. Other limbic and paralimbic regions have been implicated in pain modulation as well. The cortico-limbic system is rich in opioids and opioid receptors. Preclinical evidence for their pain modulatory effects in different regions of this highly interactive system, potentially opposing functions of different opioid receptors, and knowledge gaps will be described here. There is little information about cell type- and circuit-specific functions of opioid receptor subtypes related to pain processing and pain-related plasticity in the cortico-limbic system. The important role of anterior cingulate cortex (ACC) and amygdala in MOR-dependent analgesia is most well-established, and MOR actions in the mesolimbic system appear to be similar but remain to be determined in mPFC regions other than ACC. Evidence also suggests that KOR signaling generally serves opposing functions whereas DOR signaling in the ACC has similar, if not synergistic effects, to MOR. A unifying picture of pain-related neuronal mechanisms of opioid signaling in different elements of the cortico-limbic circuitry has yet to emerge. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Shared Mechanisms of GABAergic and Opioidergic Transmission Regulate Corticolimbic Reward Systems and Cognitive Aspects of Motivational Behaviors

The functional interplay between the corticolimbic GABAergic and opioidergic systems plays a crucial role in regulating the reward system and cognitive aspects of motivational behaviors leading to the development of addictive behaviors and disorders. This review provides a summary of the shared mechanisms of GABAergic and opioidergic transmission, which modulate the activity of dopaminergic neurons located in the ventral tegmental area (VTA), the central hub of the reward mechanisms. This review comprehensively covers the neuroanatomical and neurobiological aspects of corticolimbic inhibitory neurons that express opioid receptors, which act as modulators of corticolimbic GABAergic transmission. The presence of opioid and GABA receptors on the same neurons allows for the modulation of the activity of dopaminergic neurons in the ventral tegmental area, which plays a key role in the reward mechanisms of the brain. This colocalization of receptors and their immunochemical markers can provide a comprehensive understanding for clinicians and researchers, revealing the neuronal circuits that contribute to the reward system. Moreover, this review highlights the importance of GABAergic transmission-induced neuroplasticity under the modulation of opioid receptors. It discusses their interactive role in reinforcement learning, network oscillation, aversive behaviors, and local feedback or feedforward inhibitions in reward mechanisms. Understanding the shared mechanisms of these systems may lead to the development of new therapeutic approaches for addiction, reward-related disorders, and drug-induced cognitive impairment.

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Amygdala, neuropeptides, and chronic pain-related affective behaviors

Neuropeptides play important modulatory roles throughout the nervous system, functioning as direct effectors or as interacting partners with other neuropeptide and neurotransmitter systems. Limbic brain areas involved in learning, memory and emotions are particularly rich in neuropeptides. This review will focus on the amygdala, a limbic region that plays a key role in emotional-affective behaviors and pain modulation. The amygdala is comprised of different nuclei; the basolateral (BLA) and central (CeA) nuclei and in between, the intercalated cells (ITC), have been linked to pain-related functions. A wide range of neuropeptides are found in the amygdala, particularly in the CeA, but this review will discuss those neuropeptides that have been explored for their role in pain modulation. Calcitonin gene-related peptide (CGRP) is a key peptide in the afferent nociceptive pathway from the parabrachial area and mediates excitatory drive of CeA neurons. CeA neurons containing corticotropin releasing factor (CRF) and/or somatostatin (SOM) are a source of long-range projections and serve major output functions, but CRF also acts locally to excite neurons in the CeA and BLA. Neuropeptide S (NPS) is associated with inhibitory ITC neurons that gate amygdala output. Oxytocin and vasopressin exert opposite (inhibitory and excitatory, respectively) effects on amygdala output. The opioid system of mu, delta and kappa receptors (MOR, DOR, KOR) and their peptide ligands (β-endorphin, enkephalin, dynorphin) have complex and partially opposing effects on amygdala function. Neuropeptides therefore serve as valuable targets to regulate amygdala function in pain conditions. This article is part of the special issue on Neuropeptides.

Opioids differentially modulate two synapses important for pain processing in the amygdala

BACKGROUND AND PURPOSE

Pain is a subjective experience involving sensory discriminative and emotionally aversive components. Consistent with its role in pain processing and emotions, the amygdala modulates the aversive component of pain. The laterocapsular region of the central nucleus of the amygdala (CeLC) receives nociceptive information from the parabrachial nucleus (PB) and polymodal, including nociceptive, inputs from the basolateral nucleus of the amygdala (BLA). Opioids are strong analgesics and reduce both the sensory discriminative and the affective component of pain. However, it is unknown whether opioids regulate activity at the two nociceptive inputs to the amygdala.

EXPERIMENTAL APPROACH

Using whole-cell electrophysiology, optogenetics, and immunohistochemistry, we investigated whether opioids inhibit the rat PB-CeLC and BLA-CeLC synapses.

KEY RESULTS

Opioids inhibited glutamate release at the PB-CeLC and BLA-CeLC synapses. Opioid inhibition is via the μ-receptor at the PB-CeLC synapse, while at the BLA-CeLC synapse, inhibition is via μ-receptors in all neurons and via δ-receptors and κ-receptors in a subset of neurons.

CONCLUSIONS AND IMPLICATIONS

Agonists of μ-receptors inhibited two of the synaptic inputs carrying nociceptive information into the laterocapsular amygdala. Therefore, μ-receptor agonists, such as morphine, will inhibit glutamate release from PB and BLA in the CeLC, and this could serve as a mechanism through which opioids reduce the affective component of pain and pain-induced associative learning. The lower than expected regulation of BLA synaptic outputs by δ-receptors does not support the proposal that opioid receptor subtypes segregate into subnuclei of brain regions.

Electrophysiological Actions of N/OFQ

Whilst the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) has similar intracellular coupling mechanisms to opioid receptors, it has distinct modulatory effects on physiological functions such as pain. These actions range from agonistic to antagonistic interactions with classical opioids within the spinal cord and brain, respectively. Understanding the electrophysiological actions of N/OFQ has been crucial in ascertaining the mechanisms by which these agonistic and antagonistic interactions occur. These similarities and differences between N/OFQ and opioids are due to the relative location of NOP versus opioid receptors on specific neuronal elements within these CNS regions. These mechanisms result in varied cellular actions including postsynaptic modulation of ion channels and presynaptic regulation of neurotransmitter release.

Light and electron microscopic analysis of enkephalin-like immunoreactivity in the basolateral amygdala, including evidence for convergence of enkephalin-containing axon terminals and norepinephrine transporter-containing axon terminals onto common targets

Modulatory interactions of opioids and norepinephrine (NE) in the anterior subdivision of the basolateral nucleus of the amygdala (BLa) are critical for the consolidation of memories of emotionally arousing experiences. Although there have been several studies of the noradrenergic system in the amygdalar basolateral nuclear complex (BLC), little is known about the chemical neuroanatomy of opioid systems in this region. To address this knowledge gap the present study first examined the distribution of met-enkephalin-like immunoreactivity (ENK-ir) in the BLC at the light microscopic level, and then utilized dual-labeling immunocytochemistry combined with electron microscopy to investigate the extent of convergence of NE and ENK terminals onto common structures in the BLa. Antibodies to ENK and the norepinephrine transporter (NET) were used in these studies. Light microscopic examination revealed that a subpopulation of small nonpyramidal neurons expressed ENK-ir in all nuclei of the BLC. In addition, the somata of some pyramidal cells exhibited light to moderate ENK-ir. ENK+ axon terminals were also observed. Ultrastructural analysis confined to the BLa revealed that most ENK+ axon terminals formed asymmetrical synapses that mainly contacted spines and shafts of thin dendrites. ENK+ terminals forming symmetrical synapses mainly contacted dendritic shafts. Approximately 20% of NET+ terminals contacted a structure that was also contacted by an ENK+ terminal and 6% of NET+ terminals contacted an ENK+ terminal. These findings suggest that ENK and NE terminals in the BLa may interact by targeting common dendrites and by direct interactions between the two types of terminals.

Mu opioid receptor localization in the basolateral amygdala: An ultrastructural analysis

Receptor binding studies have shown that the density of mu opioid receptors (MORs) in the basolateral amygdala is among the highest in the brain. Activation of these receptors in the basolateral amygdala is critical for stress-induced analgesia, memory consolidation of aversive events, and stress adaptation. Despite the importance of MORs in these stress-related functions, little is known about the neural circuits that are modulated by amygdalar MORs. In the present investigation light and electron microscopy combined with immunohistochemistry was used to study the expression of MORs in the anterior basolateral nucleus (BLa). At the light microscopic level, light to moderate MOR-immunoreactivity (MOR-ir) was observed in a small number of cell bodies of nonpyramidal interneurons and in a small number of processes and puncta in the neuropil. At the electron microscopic level most MOR-ir was observed in dendritic shafts, dendritic spines, and axon terminals. MOR-ir was also observed in the Golgi apparatus of the cell bodies of pyramidal neurons (PNs) and interneurons. Some of the MOR-positive (MOR+) dendrites were spiny, suggesting that they belonged to PNs, while others received multiple asymmetrical synapses typical of interneurons. The great majority of MOR+ axon terminals (80%) that formed synapses made asymmetrical (excitatory) synapses; their main targets were spines, including some that were MOR+. The main targets of symmetrical (inhibitory and/or neuromodulatory) synapses were dendritic shafts, many of which were MOR+, but some of these terminals formed synapses with somata or spines. All of our observations were consistent with the few electrophysiological studies which have been performed on MOR activation in the basolateral amygdala. Collectively, these findings suggest that MORs may be important for filtering out weak excitatory inputs to PNs, allowing only strong inputs or synchronous inputs to influence pyramidal neuronal firing.

Mu-opioid receptor knockout mice are more sensitive to chlordiazepoxide-induced anxiolytic behavior

We have previously demonstrated benzodiazepine binding in the cortex and hippocampus of mu-opioid receptor knockout (KO) mice. It is known that benzodiazepine receptors are involved in regulating anxiety-like behaviors. Thus, the present study was designed to examine whether there are changes in anxiety-like behavior in mice lacking mu-opioid receptors. To produce anxiolytic activity (less anxiety), the prototype benzodiazepine receptor agonist chlordiazepoxide (CDP, 5 mg/kg) was intraperitoneally administered in wild type (WT) and mu-opioid receptor KO mice. We found that compared to WT mice, mu-opioid receptor KO mice showed enhanced anxiolytic activity to CDP, including increased number of entries into open arm, increased percentage of the time spent in open arms, and decreased percentage of the time spent in enclosed arms in the elevated plus-maze test. We also assessed protein expression of the gamma-aminobutyric acid (GABA) synthetic enzyme (glutamic acid decarboxylase; GAD). Western blotting data indicated that neither the lack of mu-opioid receptors nor CDP treatment altered cortical or hippocampal GAD₆₅ or GAD₆₇ protein expression. These data indicate that compared with WT, mu-opioid receptor KO mice experienced less anxiety and exhibited enhanced anxiolytic activity to CDP treatment, and these effects were not dependent on GAD₆₅ or GAD₆₇ protein expression. Our previous and present data suggest that the anxiolytic activity displayed in mu-opioid receptor KO mice is associated with upregulation of the benzodiazepine receptor system.

The role of the endogenous opioid system in polycystic ovary syndrome

OBJECTIVE

To review the complex role of the opioid system in reproduction and carbohydrate metabolism, abnormalities in the opioid system in women with polycystic ovary syndrome (PCOS), and the role of opioid antagonists in the management of PCOS-related infertility.

DESIGN

Pertinent articles were identified through a computer PubMed search. References of selected articles were hand searched for additional citations.

CONCLUSION(S)

Endogenous opioids are generally considered inhibitory central neurotransmitters. Peripherally, opioids are involved in the regulation of pancreatic islet function, hepatic insulin clearance, and glucose metabolism, potentially contributing to the pathogenesis of hyperinsulinemia and insulin resistance in PCOS. The presence of sex steroids is required for normal function of the opioid system in both GnRH secretion and carbohydrate metabolism. In women with PCOS, growing evidence suggests dysregulation of the opioid system both centrally and peripherally, with complex interactions. The opioid system effects on carbohydrate metabolism appear to be modulated by obesity. Finally, naltrexone has been demonstrated to successfully augment traditional ovulation induction regimens, but has limited support as a single ovulation induction agent for PCOS.

Opioids: from physical pain to the pain of social isolation

The opioid systems play an important role in mediating both physical pain and negative affects (eg, the pain of social isolation). From an evolutionary perspective, it is not surprising that the neurocircuitry and neurochemistry of physical pain would overlap with that involved in complex social emotions. Exposure to trauma as well as a range of gene variants in the opioid system may be associated with alterations in opioid systems function, with changes in reward processing, and with vulnerability to substance abuse. A role for interventions with opioid agents in depression and anxiety disorders has been suggested.

Postsynaptic mechanisms underlying responsiveness of amygdaloid neurons to cholecystokinin are mediated by a transient receptor potential-like current

Projection neurons of mouse basolateral amygdala responded to CCK with an inward current at a holding potential of -70 mV. This response was mediated by CCK2 receptors as indicated by agonist and antagonist effectiveness, and conveyed via G-proteins of the G(q/11) family as it was abolished in gene knockout mice. Maximal current amplitude was insensitive to extracellular potassium, cesium, and calcium ions, respectively, whereas amplitude and reversal potential critically depended upon extracellular sodium concentration. The current reversed near -20 mV consistent with activation of a mixed cationic channel reminiscent of transient receptor potential (TRP) channels. Extracellular application of the non-selective TRP channel blockers 2-APB, flufenamic acid, Gd3+, and ruthenium red, respectively, inhibited CCK induced inward currents. Single cell PCR confirmed the expression of TRPC1,4,5 and coexpression of TRPC1 with TRPC4 or TRPC5 in some cells. CCK responses were associated with depolarization leading to an increase in cell excitability.

The role of delta opioid receptors in the anxiolytic actions of benzodiazepines

The anxiolytic effects of benzodiazepines appear to involve opioid processes in the amygdala. In previous experiments, overexpression of enkephalin in the amygdala enhanced the anxiolytic actions of the benzodiazepine agonist diazepam in the elevated plus maze. The effects of systemically administered diazepam are also blocked by injections of naltrexone into the central nucleus of the amygdala. The current studies investigated the role of delta opioid receptors in the anxiety-related effects of diazepam. Three days following bilateral stereotaxic injections of viral vectors containing cDNA encoding proenkephalin or beta-galactosidase (control vector), the delta opioid receptor antagonist naltrindole (10 mg/kg, s.c.) attenuated the enhanced anxiolytic effects of 1-2 mg/kg diazepam in rats overexpressing preproenkephalin in the amygdala. Despite this effect, naltrindole failed to attenuate the anxiolytic action of higher diazepam doses (3 mg/kg) in animals with normal amygdalar enkephalin expression. Similarly, the mu opioid receptor antagonist, beta-funaltrexamine (20 mg/kg, s.c.), had no effect on the anxiolytic effect of diazepam alone. These data support a role for delta opioid receptors in the opioid-enhanced anxiolytic effects of diazepam.

Atypical anxiolytic-like response to naloxone in benzodiazepine-resistant 129S2/SvHsd mice: role of opioid receptor subtypes

RATIONALE

Mice of many 129 substrains respond to environmental novelty with behavioural suppression and high levels of anxiety-like behaviour. Although resistant to conventional anxiolytics, this behavioural phenotype may involve stress-induced release of endogenous opioids.

OBJECTIVES

To assess the effects of opioid receptor blockade on behavioural reactions to novelty stress in a chlordiazepoxide-resistant 129 substrain.

MATERIALS AND METHODS

Experiment 1 contrasted the effects of the broad-spectrum opioid receptor antagonist naloxone (1.0-10.0 mg/kg) in C57BL/6JOlaHsd and 129S2/SvHsd mice exposed to the elevated plus-maze. Experiments 2-4 examined the responses of 129S2/SvHsd mice to the mu-selective opioid receptor antagonist beta-funaltrexamine (2.5-10.0 mg/kg), the delta-selective antagonist naltrindole (2.5-10.0 mg/kg) and the kappa-selective antagonist nor-binaltorphimine (2.5-5.0 mg/kg).

RESULTS

129 mice displayed higher levels of anxiety-like behaviour and lower levels of general exploration relative to their C57 counterparts. Although naloxone failed to alter the behaviour of C57 mice, both doses of this antagonist produced behaviourally selective reductions in open-arm avoidance in 129 mice. Surprisingly, none of the more selective opioid receptor antagonists replicated this effect of naloxone: beta-funaltrexamine was devoid of behavioural activity, naltrindole suppressed rearing (all doses) and increased immobility (10 mg/kg), while nor-binaltorphimine (5 mg/kg) nonspecifically increased percent open arm entries.

CONCLUSIONS

Recent evidence suggests differential involvement of opioid receptor subtypes in the anxiolytic efficacy of diverse compounds including conventional benzodiazepines. The insensitivity of 129 mice to the anxiolytic action of chlordiazepoxide, coupled with their atypical anxiolytic response to naloxone (but not more selective opioid receptor antagonists), suggests an abnormality in anxiety-related neurocircuitry involving opioid-GABA interactions.

Generalization of amygdala LTP and conditioned fear in the absence of presynaptic inhibition

Pavlovian fear conditioning, a simple form of associative learning, is thought to involve the induction of associative, NMDA receptor-dependent long-term potentiation (LTP) in the lateral amygdala. Using a combined genetic and electrophysiological approach, we show here that lack of a specific GABA(B) receptor subtype, GABA(B(1a,2)), unmasks a nonassociative, NMDA receptor-independent form of presynaptic LTP at cortico-amygdala afferents. Moreover, the level of presynaptic GABA(B(1a,2)) receptor activation, and hence the balance between associative and nonassociative forms of LTP, can be dynamically modulated by local inhibitory activity. At the behavioral level, genetic loss of GABA(B(1a)) results in a generalization of conditioned fear to nonconditioned stimuli. Our findings indicate that presynaptic inhibition through GABA(B(1a,2)) receptors serves as an activity-dependent constraint on the induction of homosynaptic plasticity, which may be important to prevent the generalization of conditioned fear.

Microinjection of naltrexone into the central, but not the basolateral, amygdala blocks the anxiolytic effects of diazepam in the plus maze

The amygdala is involved in behavioral and physiological responses to fear, and the anxiolytic properties of several drugs are localized to this region. Activation of endogenous opioid systems is known to occur in response to stress and a growing body of literature suggests that opioid systems regulate the properties of anxiolytic drugs. These experiments sought to elucidate the role of opioid receptors in the central (CeA) and basolateral (BLA) nuclei of the amygdala in regulating the anxiolytic properties of ethanol and diazepam. Male rats fitted with cannula received bilateral microinjections of the nonselective opioid receptor antagonist naltrexone (NAL) immediately followed by systemic delivery of either ethanol (1 g/kg) or diazepam (2 mg/kg) in the elevated plus maze. Both diazepam and ethanol decreased anxiety-like behavior. Delivery of NAL into the CeA blocked the anxiolytic properties of diazepam. Delivery of NAL into the BLA slightly increased open arm avoidance, but had no effect on the anxiolytic properties of diazepam. Microinjection of NAL into either nucleus failed to block the effects of ethanol. These results were specific to the anxiolytic properties of diazepam, since baseline behaviors were unaffected by microinjection of NAL. Microinjection of lidocaine produced results distinct from NAL and failed to block the anxiolytic actions of diazepam. These studies indicate distinct roles for opioid receptor systems in the CeA and BLA in regulating the anxiolytic properties of diazepam in the elevated plus maze. Further, opioid receptor systems in the CeA and BLA do not regulate the anxiolytic properties of ethanol in this test.

Co-Consumption of Benzodiazepines in Heroin Users, Methadone-Substituted and Codeine-Substituted Patients

Concomitant consumption of benzodiazepines (BZDs) and opioids is a major problem in patients with opioid dependence. It may have substantial impact on morbidity, mortality and clinical course. The current retrospective study aims to determine whether there are differences in the additional use of BZDs among addicts regularly taking methadone or codeine medications in treatment and untreated addicts injecting heroin. The records of 1,685 patients admitted for detoxification were analyzed using bivariate analysis and multiple logistic regression analysis. Demographic and drug related variables were considered, both as possible confounders and predictors of concomitant BZD use. Daily intake of BZDs was reported in 44.4% of the patients. Patients treated with methadone or codeine medications report daily intake of BZDs significantly more often than the heroin-dependent patients (p < 0.01). Using multiple regression analyses, the results were confirmed as independent from the assessed possible confounders. Further we found that daily use of alcohol or barbiturates, early onset of opioid use (p < 0.01), unemployment, having a substance dependent family member with, and a history of imprisonment (for all p < 0.05) were associated with concomitant daily consumption of BZDs in opioid dependent subjects. These finding underline the need to further explore the causes, interactions and consequences of concomitant BZD and opiate use.

Interface of physical and emotional stress regulation through the endogenous opioid system and mu-opioid receptors

Unraveling the pathways and neurobiological mechanisms that underlie the regulation of physical and emotional stress responses in humans is of critical importance to understand vulnerability and resiliency factors to the development of a number of complex physical and psychopathological states. Dysregulation of central stress response circuits have been implicated in the establishment of conditions as diverse as persistent pain, mood and personality disorders and substance abuse and dependence. The present review examines the contribution of the endogenous opioid system and mu-opioid receptors to the modulation and adaptation of the organism to challenges, such as sustained pain and negative emotional states, which threaten its internal homeostasis. Data accumulated in animal models, and more recently in humans, point to this neurotransmitter system as a critical modulator of the transition from acute (warning signals) to sustained (stressor) environmental adversity. The existence of pathways and regulatory mechanisms common to the regulation of both physical and emotional states transcend classical categorical disease classifications, and point to the need to utilize dimensional, "symptom"-related approximations to their study. Possible future areas of study at the interface of "mind" (cognitive-emotional) and "body" (physical) functions are delineated in this context.

Mechanisms of somatostatin-evoked responses in neurons of the rat lateral amygdala

The effects of somatostatin in the rat lateral amygdala (LA) in vitro were investigated through whole cell recording techniques. Somatostatin induced an inwardly rectifying K+ current in approximately 98% of LA projection neurons. Half-maximal effects were obtained by 189 nM somatostatin. The effects of somatostatin were insensitive to tetrodotoxin, reduced by Ba2+, occluded or abolished by the presence of nonhydrolysable GTP or GDP analogues, respectively, and blocked or mimicked by a somatostatin receptor type 2 antagonist (BIM-23627) or somatostatin receptor type 2 agonist (L-779,976), respectively, while somatostatin receptor type 1, 3 and 4 agonists were ineffective (L-797,591, L-796,778, L-803,087). Responses to somatostatin were associated with membrane hyperpolarization and decrease in input resistance, resulting in a dampening of cell excitability. It is suggested that these cellular mechanisms contribute to the role of somatostatin in decreasing anxiety behaviour as well as to anticonvulsant and antiepileptogenic actions of somatostatin or somatostatin agonists in the amygdala.

Opioids inhibit lateral amygdala pyramidal neurons by enhancing a dendritic potassium current

Pyramidal neurons in the lateral amygdala discharge trains of action potentials that show marked spike frequency adaptation, which is primarily mediated by activation of a slow calcium-activated potassium current. We show here that these neurons also express an alpha-dendrotoxin- and tityustoxin-Kalpha-sensitive voltage-dependent potassium current that plays a key role in the control of spike discharge frequency. This current is selectively targeted to the primary apical dendrite of these neurons. Activation of micro-opioid receptors by application of morphine or d-Ala(2)-N-Me-Phe(4)-Glycol(5)-enkephalin (DAMGO) potentiates spike frequency adaptation by enhancing the alpha-dendrotoxin-sensitive potassium current. The effects of micro-opioid agonists on spike frequency adaptation were blocked by inhibiting G-proteins with N-ethylmaleimide (NEM) and by blocking phospholipase A(2). Application of arachidonic acid mimicked the actions of DAMGO or morphine. These results show that micro-opioid receptor activation enhances spike frequency adaptation in lateral amygdala neurons by modulating a voltage-dependent potassium channel containing Kv1.2 subunits, through activation of the phospholipase A(2)-arachidonic acid-lipoxygenases cascade.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Naltrexone potentiates both amnestic and anxiolytic effects of chlordiazepoxide in mice

Studies have suggested that opioid antagonists potentiate the anxiolytic effect but not the amnestic action of chlordiazepoxide (CDZ). We investigated the effects of naltrexone (NAL) on the anxiolytic and amnestic effects of CDZ in mice tested in the plus-maze discriminative avoidance task (DAT). Mice are conditioned to choose between two enclosed arms (one of which aversive) while avoiding the two open arms of the apparatus. This task measures memory (time spent in the aversive vs. time in the non-aversive enclosed arms) and anxiety (time spent in the open arms). Mice treated with saline (SAL) or 5 mg/kg NAL, and SAL or 2.5 mg/kg CDZ were submitted to DAT training. The test was performed 24 h later, without aversive stimuli. In the training, NAL + CDZ group showed higher percent time spent in the open arms than all the other groups. In the test, NAL + CDZ (but not SAL + CDZ) group showed higher percent time spent in the aversive enclosed arm than SAL + SAL and NAL + SAL groups. The data suggest that NAL potentiates the small decreases in anxiety and retention induced by a subeffective dose of CDZ.

Opioid receptor blockade in rat nucleus tractus solitarius alters amygdala dynorphin gene expression

It has been suggested that an opioidergic feeding pathway exists between the nucleus of the solitary tract (NTS) and the central nucleus of the amygdala. We studied the following three groups of rats: 1) artificial cerebrospinal fluid (CSF) infused in the NTS, 2) naltrexone (100 microg/day) infused for 13 days in the NTS, and 3) artificial CSF infused in the NTS of rats pair fed to the naltrexone-infused group. Naltrexone administration resulted in a decrease in body weight and food intake. Also, naltrexone infusion increased dynorphin, but not enkephalin, gene expression in the amygdala, independent of the naltrexone-induced reduction in food intake. Gene expression of neuropeptide Y in the arcuate nucleus and neuropeptide Y peptide levels in the paraventricular nucleus did not change because of naltrexone infusion. However, naltrexone induced an increase in serum leptin compared with pair-fed controls. Thus chronic administration of naltrexone in the NTS increased dynorphin gene expression in the amygdala, further supporting an opioidergic feeding pathway between these two brain sites.

Presynaptic modulation of synaptic transmission by opioid receptor in rat subthalamic nucleus in vitro

Presynaptic modulation of synaptic transmission in rat subthalamic nucleus (STN) neurons was investigated using whole-cell patch-clamp recordings in brain slices. Evoked GABAergic inhibitory postsynaptic currents (IPSCs) were reversibly reduced by methionine enkephalin (ME) with an IC(50) value of 1.1 +/- 0.3 microM. The action of ME was mimicked by the mu-selective agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO), and was partially blocked by the mu-selective antagonists naloxonazine and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP). Evoked GABA(A) IPSCs were also inhibited by the delta-selective agonist [D-Pen(2,5)]-enkephalin (DPDPE), but not by the kappa-selective agonist (+)-(5 alpha,7 alpha,8 beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U-69593) and the orphan receptor agonist orphanin FQ/nociceptin (OFQ). DPDPE-induced inhibition was completely blocked by the delta-selective antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864). ME, DAMGO and DPDPE increased the paired-pulse ratio of IPSCs. Evoked excitatory postsynaptic currents (EPSCs) were reversibly reduced by ME with an IC(50) value of 0.35 +/- 0.14 microM. Inhibition by ME was associated with an increase in the paired-pulse ratio of EPSCs. The action of ME was mimicked by DAMGO, and blocked by naloxonazine. DPDPE had little effect on evoked EPSCs. Neither U-69593 nor OFQ had any effect. ME significantly decreased the frequency of spontaneous miniature EPSCs (mEPSCs) without change in their amplitude. The action of ME was mimicked by DAMGO. DPDPE had no effect. The presynaptic voltage-dependent potassium conductance blocker 4-aminopyridine (4-AP, 100 microM) abolished the inhibitory effects of ME on evoked IPSCs and EPSCs. In contrast, 4-AP only partially blocked the actions of baclofen. These results suggest that opioids inhibit inhibitory synaptic transmission in the STN through the activation of presynaptic mu- and delta- receptors. In contrast, inhibition of excitatory synaptic inputs to the STN occurs through the activation of only mu-receptors. Both inhibitions may be mediated by blockade of voltage-dependent potassium conductance.

mu-Opioid receptors and limbic responses to aversive emotional stimuli

Functional neuroimaging studies implicate limbic and paralimbic activity in emotional responses, but few studies have sought to understand neurochemical mechanisms which modulate these responses. We have used positron emission tomography to measure mu-opioid receptor binding, and cerebral blood flow in the same subjects, and demonstrated that the baseline binding potential and the regional cerebral blood flow in the left inferior temporal pole are functionally related. Higher baseline mu-opioid receptor binding potential was associated with lower regional cerebral blood flow in this region during presentation of emotionally salient stimuli. This is consistent with an inhibitory/anxiolytic role of the endogenous opioid system in limbic regions of the temporal lobe and basal forebrain.

Control of glutamate and GABA release by nociceptin/orphanin FQ in the rat lateral amygdala

The actions of the heptadecapeptide termed nociceptin or orphanin FQ (N/OFQ) and the recently discovered putative precursor product nocistatin were examined on synaptic transmission in putative projection cells of the rat lateral amygdala using the whole-cell patch-clamp technique. N/OFQ decreased evoked non-NMDA receptor-mediated excitatory postsynaptic current (EPSC) amplitudes in a concentration-dependent manner, with a half-maximal inhibitory effect elicited by 21.8 +/- 7.5 nM and a Hill coefficient of 0.8 +/- 0.2 (n = 22). Responses were maximally suppressed to 70.3 +/- 1.7 % of the control value. The effect of N/OFQ was prevented by 1 microM [Phe1[psi](CH2-NH)Gly2]NC(1-13)NH2 (Phe[psi]N/OFQ), a substance known as an antagonist/partial agonist of the ORL receptor. GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) elicited through intra-amygdaloid stimulation were reduced to 48.0 +/- 6.8 % by 1 microM N/OFQ (n = 5). Nocistatin had no measurable effect on evoked synaptic currents or membrane properties of recorded neurons. N/OFQ reduced the frequency of spontaneous miniature EPSCs and IPSCs to 74.0 +/- 2.6 % and 84.4 +/- 1.1 %, respectively, without affecting the amplitudes. The present findings indicate that N/OFQ, but not nocistatin, inhibits the release of glutamate and GABA in the lateral amygdala, presumably by acting on presynaptic release sites. These mechanisms may add to the role of N/OFQ in reducing stress vulnerability as recently proposed on the basis of behavioural and genetic approaches.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Alfentanil-induced activation: a promising tool in the presurgical evaluation of temporal lobe epilepsy patients

Pharmacologic activation of epileptic foci has been used experimentally with the hope that it may accelerate the presurgical evaluation of patients with medically intractable epilepsy. In this article, we will review the existing literature on these activating tests giving emphasis on the opioid analogs, and particularly alfentanil. Alfentanil is an opioid analog with rapid anesthetic effect, which has been known to trigger epileptiform discharges in epilepsy patients. 58 temporal lobe epilepsy (TLE) patients were studied with alfentanil activation during electrocorticography, at the Epilepsy Surgery Unit (ING, Brazil). An increase of the interictal epileptiform discharges was observed originating from hippocampal and parahippocampal regions (96.5%). To a lesser extent, alfentanil activated the basal and lateral temporal regions. Electrographic seizures were observed in 38%. In addition, we performed continuous video-EEG (VT/EEG) monitoring, with scalp and bilateral foramen ovale electrodes, in 12 TLE patients. The results of spontaneously observed seizures were compared with the electrographic changes following alfentanil activation (50-75 microg/kg, i.v.). In seven cases, alfentanil triggered focal electrographic seizures, ipsilaterally to the side generating the spontaneous seizures and in two patients it produced bilateral sequential activation of the temporal lobes. Ictal SPECTs during the alfentanil test showed hyperperfusion at the lateral temporal regions, ipsilaterally to the activated area or bilaterally. In summary, our study confirms the activating effect of alfentanil, and provides a strong evidence for its selective activating effect on the temporal lobes of TLE patients. The ictal SPECT during alfentanil activation did not offer any additional advantage for the localization of the ictal onset.

Opioid modulation of ventral pallidal inputs

While the ventral pallidum (VP) is known to be important in relaying information between the nucleus accumbens and target structures, it has become clear that substantial information processing occurs within the VP. We evaluated the possibility that opioid modulation of other transmitters contained in VP afferents is involved in this process. Initially, we demonstrated that opioids hyperpolarized VP neurons in vitro and suppressed spontaneous firing in vivo. The ability of opioids to modulate other transmitters was determined using microiontophoretically applied ligands and extracellular recordings of VP neurons from chloral hydrate-anesthetized rats. With neurons that responded to iontophoresed opioid agonists, the ejection current was reduced to a level that was below that necessary to alter spontaneous firing. This "subthreshold" current was used to determine the ability of mu opioid receptor (microR) agonists to alter VP responses to endogenous (released by electrical activation of afferents) and exogenous (iontophoretically applied) transmitters. microR agonists decreased the variability and enhanced the acuity (e.g., "signal-to-noise" relationship) of VP responses to activation of glutamatergic inputs from the prefrontal cortex and amygdala. By contrast, microR agonists attenuated both the slow excitatory responses to substance P and GABA-induced inhibitions that resulted from activating the nucleus accumbens. Subthreshold opioids also attenuated inhibitory responses to stimulating midbrain dopaminergic cells. These results suggest that a consequence of opioid transmission in the VP is to negate the influence of some afferents (e.g., midbrain dopamine and accumbal GABA and substance P) while selectively potentiating the efficacy of others (e.g., cortical and amygdaloid glutamate). Interpreted in the context of opiate abuse, microR opioids in the VP may serve to diminish the influence of reinforcement (ventral tegmental area and nucleus accumbens) in the transduction of cognition (prefrontal cortex) and affect (amygdala) into behavior. This may contribute to drug craving that occurs even in the absence of reward.

Presynaptic dopaminergic function in the nucleus accumbens following chronic opiate treatment and precipitated withdrawal

Naloxone treatment at three days following implantation of pellets containing morphine base increased uptake of tritiated dopamine by the nucleus accumbens but did not alter efflux of tritiated dopamine by the nucleus accumbens or tritiated norepinephrine by the hippocampus. At six days following placement of pellets containing morphine base, withdrawal score was increased after treatment with either saline or naloxone, indicating that animals were undergoing spontaneous opiate withdrawal. Fractional efflux of tritiated dopamine was decreased at this time point following intermittent stimulation with 317 and 1000 microM 4-aminopyridine, for striatal slices obtained from animals pretreated with either saline or naloxone. For the nucleus accumbens at six days after placement of morphine pellets, similar decreases in the efflux of tritiated dopamine were only observed in slices obtained from naloxone treated animals. Fractional dopamine efflux was also diminished after in vitro exposure to rising concentrations of 4-aminopyridine, amphetamine, or cocaine for tissue obtained from the nucleus accumbens, but not for slices from the striatum at six days following morphine pellet implantation. In conclusion, deficits in dopamine efflux by the nucleus accumbens occur at a time when animals are undergoing spontaneous opiate withdrawal at six days following morphine pellet implantation, but do not occur at an earlier time point when withdrawal is precipitated by naloxone treatment. These deficits are apparent for brain slices obtained from the striatum or nucleus accumbens after exposure to rising concentrations of different in vitro treatments, with tissue obtained from the nucleus accumbens being more sensitive than the striatum to dopamine efflux produced by a wider range of treatments.

Postsynaptic mechanisms underlying responsiveness of amygdaloid neurons to nociceptin/orphanin FQ

Effects of nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid-like orphan receptor (ORL), were investigated in the rat lateral (AL) and central (ACe) amygdala in vitro. Approximately 98% of presumed projection neurons in the AL responded to N/OFQ with an increase in inwardly rectifying potassium conductance, resulting in an impairment in cell excitability. Half-maximal effects were obtained at 30.6 nM; the Hill coefficient was 0.63. In the ACe, 31% of the cells displayed responses similar to that in the AL, 44% were nonresponsive, and 25% responded with a small potassium current with a linear current-voltage relationship. Responses to N/OFQ were reduced by 100 microM Ba2+, were insensitive to 10 microM naloxone, and were blocked by a selective ORL antagonist, [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 (IC50 = 760 nM). Involvement of G-proteins was indicated by irreversible effects and blockade of action of N/OFQ during intracellular presence of GTP-gamma-S (100 microM) and GDP-beta-S (2 mM), respectively, and prevention of responses after incubation in pertussis toxin (500 ng/ml). These mechanisms may contribute to the role of N/OFQ in the reduction of fear responsiveness and stress that have recently been suggested on the basis of histochemical and behavioral studies.

Role of opioidergic and monoaminergic neurotransmission in the GnRH release mechanism of EBP-primed OVX rats

We examined the effect of intracerebroventricular (i.c.v.) administration of mu-opioid agonist, morphine, and its antagonist naloxone followed by morphine on the activities of monoamine-metabolizing enzymes, namely tyrosine hydroxylase (TH) and monoamine oxidase (MAO) along with adenosinetriphosphatase (Na+, K+ -ATPase), the enzyme responsible for the maintenance of ionic gradients across the membrane, in seven discrete regions of brain from estrogen- and progesterone-primed ovariectomized rats. TH activity decreased after morphine treatment in some areas such as the median eminence-arcuate region (ME-ARC), the amygdala, and the thalamus, showing statistically significant change. MAO activity increased in all the areas studied, but more appreciable change was observed in medial preoptic area (mPOA), the ME-ARC region, and the cortex. Pronounced increase in Na+, K+ -ATPase enzyme activity was observed after the drug treatment. Naloxone given prior to morphine injection resulted in recovery of the enzyme activities in most of the areas studied. Our study may provide insights into the precise opioidergic modulation of gonadotropin releasing hormone (GnRH) release mechanisms through the involvement of monoaminergic system, elucidating the basis of various neuronal dysfunctions and their management in opioid addicts.

Association between the amygdala and nucleus of the solitary tract in mu-opioid induced feeding in the rat

The central nucleus of the amygdala (CNA) and the nucleus of the solitary tract (NTS) are important in the regulation of ingestive behavior. We evaluated whether opioid-opioid signaling between the CNA and rostral NTS (rNTS) affect feeding behavior. To test this, rats were doubly cannulated with one cannula placed in the rNTS and one cannula in the CNA, allowing for co-administration of an opioid agonist into one site and an opioid antagonist into the other. Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO) (2 nmol) injected into the CNA (CNA DAMGO) increased feeding more than two-fold compared to the vehicle-injected rats. This increase in food intake was blocked when doses of 26.5 and 79 nmol of naltrexone (NTX) were injected into the rNTs. In the reverse situation, rNTS DAMGO increased food intake above control levels, and CNA NTX blocked DAMGO-induced feeding when administrated in doses of 26.5 and 79 nmol. This suggests that a bi-directional opioid-opioid signaling pathway exists between the CNA and the rNTS which influences feeding via mu-opioid receptors.

Naloxone potentiates the effects of subeffective doses of anxiolytic agents in mice

The ability of the opioid receptor antagonist naloxone to potentiate the effects of subeffective doses of chlordiazepoxide, diazepam and buspirone was evaluated in Swiss mice tested in an elevated plus maze. Diazepam (0.5 mg/kg), chlordiazepoxide (2.5 mg/kg) and buspirone (2 mg/kg) were ineffective per se but, when combined with naloxone (10 mg/kg), they increased the proportion of open arm entries as did higher doses of the anxiolytic agents (diazepam 1.5 mg/kg, chlordiazepoxide 5 mg/kg and buspirone 4 mg/kg). Naloxone alone (10 mg/kg) had no intrinsic effect. These data suggest that naloxone is able to potentiate the effects of anti-anxiety agents.

Ultrastructural localization of mu-opioid receptors in the superficial layers of the rat cervical spinal cord: extrasynaptic localization and proximity to Leu5-enkephalin

Many of the analgesic effects of opiate drugs and of endogenous opioid ligands, such as Leu5-enkephalin (LE) are thought to be mediated in part by mu-opioid receptors (MOR) in the dorsal horn of the spinal cord. To establish the cellular sites for the spinally mediated analgesic effects of MOR activation and the potential anatomical substrates for interactions with LE, we examined the ultrastructural localization of MOR and LE immunoreactivities in the adult rat cervical spinal cord (C3-C5). Anti-MOR sera recognizing the carboxyl terminal domain of MOR was localized using immunoperoxidase and immunogold-silver methods. mu-opioid receptor-like immunoreactivity (MOR-LI) was observed mainly in the superficial layers of the dorsal horn. Electron microscopy of this region revealed that small unmyelinated axons and axon terminals constituted 48% (91/189) and 15% (28/189), respectively, while dendrites comprised 36% (68/189) of the total population of neuronal profiles containing the MOR. MOR-LI was localized mainly along extrasynaptic portions of the plasma membrane in both axons and dendrites. In sections dually labeled for MOR and LE, 21% (14/68) of the dendrites containing MOR-LI closely apposed or received synaptic contact from axon terminals exhibiting LE reaction product. The results provide the first ultrastructural evidence that within the dorsal horn of the spinal cord, LE, as well as exogenous opiates may alter both axonal release of neurotransmitters and postsynaptic responsiveness of target neurons to afferent input through activation of extrasynaptic MOR.

Synaptic transmission and plasticity in the amygdala. An emerging physiology of fear conditioning circuits

Numerous studies in both rats and humans indicate the importance of the amygdala in the acquisition and expression of learned fear. The identification of the amygdala as an essential neural substrate for fear conditioning has permitted neurophysiological examinations of synaptic processes in the amygdala that may mediate fear conditioning. One candidate cellular mechanism for fear conditioning is long-term potentiation (LTP), an enduring increase in synaptic transmission induced by high-frequency stimulation of excitatory afferents. At present, the mechanisms underlying the induction and expression of amygdaloid LTP are only beginning to be understood, and probably involve both the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) subclasses of glutamate receptors. This article will examine recent studies of synaptic transmission and plasticity in the amygdala in an effort to understand the relationships of these processes to aversive learning and memory.

Systemic morphine and local opioid effects on neuronal activity in the medial prefrontal cortex

Behavioral studies support the importance of the medial prefrontal cortex in the circuitry of drug-reinforced behavior, yet the neurophysiological correlates of this phenomenon remain unclear. The present study evaluates opioid neuropharmacology in the medial prefrontal cortex of the anesthetized rat. The effects of both systemic and local application of mu agonists on individual neurons in the medial prefrontal cortex were examined. Systemic morphine was found to inhibit (63%), excite (4%) or have no effect on (33%) spontaneous firing. The inhibitory response was reversed by systemic naloxone in 77% of the cases. Electrophoretic application of a mu-selective agonist, [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin, had mixed effects on cell activity. While most cells exhibited no change in firing rate (53%), 38% showed inhibition of spontaneous activity. The [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin-evoked inhibitory responses were antagonized by electrophoresis of naloxone (86%). These results indicate that the medial prefrontal cortex might directly mediate some portion of the overall response to opiates in reinforcement or self-administration paradigms. The naloxone-reversible inhibition of firing seen following both systemic and local application of predominantly mu-selective agonists argues for a direct involvement of medial prefrontal cortical neurons in opiate-induced effects. However, the smaller percentage of cells inhibited by local versus systemic application of mu agonists also supports an influence of other brain circuitry in this response.

Met5-enkephalin is localized within axon terminals in the subfornical organ: vascular contacts and interactions with neurons containing gamma-aminobutyric acid

Met5-enkephalin inhibits sodium and water excretion and antagonizes the central actions of angiotensin II in subfornical organ of rat brain. We examined the ultrastructural basis for enkephalin modulation in this circumventricular region. Additionally, we examined the possibility that there might be cellular sites for functional interactions involving Met5-enkephalin and gamma-aminobutyric acid (GABA), a known inhibitory transmitter throughout the central nervous system. Met5-enkephalin and GABA were identified in single coronal sections through the subfornical organ using immunoperoxidase and silver-enhanced immunogold labeling methods, respectively. Enkephalin-like immunoreactivity was most prominently localized within axon terminals. These were distributed primarily in the central, highly vascular, regions of the subfornical organ. Enkephalin-labeled terminals were apposed to the basement membranes of fenestrated capillaries and also formed symmetric, inhibitory type synapses with neurons. In terminals associated with either blood vessels or neurons, the enkephalin immunoreactivity was enriched in large (80-150 nm) dense core vesicles. The immunoreactive vesicles were usually located within portions of the axon in close proximity to astrocytic processes. In contrast, smaller vesicles in the same terminals were more often aggregated near the basement membrane of the capillaries and the active zone of the synapse. The targets of enkephalin-immunoreactive terminals were either unlabeled or GABA-labeled dendrites of local neurons. Enkephalin was also co-localized with GABA in perikarya and in axon terminals. Terminals containing only GABA were far more abundant than those containing enkephalin or enkephalin and GABA. GABA-immunoreactive terminals formed symmetric synapses on unlabeled dendrites some of which also received convergent input from terminals containing enkephalin. Additionally, the enkephalin-immunoreactive terminals were closely apposed to GABA-labeled and unlabeled terminals. These results suggest sites for nonsynaptic release of Met5-enkephalin from dense core vesicles in contact with astrocytes near blood vessels and synaptic complexes in the rat subfornical organ. Moreover, the observed dual localization and pre- and postsynaptic associations between neurons containing Met5-enkephalin and GABA indicate that inhibitory effects of opioids in the subfornical organ may be mediated or potentiated by GABA.