Altered gating of opiate receptor-modulated K+ channels on amygdala neurons of morphine-dependent rats

Duration of peripheral nerve blocks in opioid-tolerant individuals: A study protocol

BACKGROUND

Peripheral nerve blocks effectively alleviate postoperative pain. Animal studies and human research suggest that opioid tolerance may reduce the effectiveness of local analgesics. The reduced effectiveness has been observed in opioid-tolerant humans and animals undergoing spinal and infiltration anaesthesia with both lidocaine and bupivacaine. However, the impact on peripheral nerve blocks in humans has not been evaluated. This study aims to assess the onset time and duration of a radial nerve block in opioid-tolerant individuals compared to opioid-naive individuals. We hypothesise that peripheral nerve blocks may be less effective in producing sensory and motor blockades in opioid-tolerant individuals compared to their opioid-naive counterparts.

METHODS

Twenty opioid-tolerant individuals will be matched by sex and age with opioid-naïve counterparts. Participants will receive an ultrasound-guided radial nerve block. The primary outcome is the difference in the duration of sensory nerve blockade between the two groups. The secondary outcomes include the onset time of sensory blockade, onset time of motor blockade, and difference in duration of motor nerve blockade.

CONCLUSION

This study will compare the effectiveness of a peripheral nerve block between opioid-tolerant and opioid-naïve individuals. Any found differences could support a specific postoperative protocol for opioid-tolerant individuals regarding the use of peripheral nerve blocks.

A novel Oprm1-Cre mouse maintains endogenous expression, function and enables detailed molecular characterization of μ-opioid receptor cells

Key targets of both the therapeutic and abused properties of opioids are μ-opioid receptors (MORs). Despite years of research investigating the biochemistry and signal transduction pathways associated with MOR activation, we do not fully understand the cellular mechanisms underlying opioid addiction. Given that addictive opioids such as morphine, oxycodone, heroin, and fentanyl all activate MORs, and current therapies such as naloxone and buprenorphine block this activation, the availability of tools to mechanistically investigate opioid-mediated cellular and behavioral phenotypes are necessary. Therefore, we derived, validated, and applied a novel MOR-specific Cre mouse line, inserting a T2A cleavable peptide sequence and the Cre coding sequence into the MOR 3'UTR. Importantly, this line shows specificity and fidelity of MOR expression throughout the brain and with respect to function, there were no differences in behavioral responses to morphine when compared to wild type mice, nor are there any alterations in Oprm1 gene expression or receptor density. To assess Cre recombinase activity, MOR-Cre mice were crossed with the floxed GFP-reporters, RosaLSLSun1-sfGFP or RosaLSL-GFP-L10a. The latter allowed for cell type specific RNA sequencing via TRAP (Translating Ribosome Affinity Purification) of striatal MOR+ neurons following opioid withdrawal. The breadth of utility of this new tool will greatly facilitate the study of opioid biology under varying conditions.

Morphine Efficacy, Tolerance, and Hypersensitivity Are Altered After Modulation of SUR1 Subtype KATP Channel Activity in Mice

ATP-sensitive potassium (K_ATP) channels are found in the nervous system and are downstream targets of opioid receptors. K_ATP channel activity can effect morphine efficacy and may beneficial for relieving chronic pain in the peripheral and central nervous system. Unfortunately, the K_ATP channels exists as a heterooctomers, and the exact subtypes responsible for the contribution to chronic pain and opioid signaling in either dorsal root ganglia (DRG) or the spinal cord are yet unknown. Chronic opioid exposure (15 mg/kg morphine, s.c., twice daily) over 5 days produces significant downregulation of Kir6.2 and SUR1 in the spinal cord and DRG of mice. In vitro studies also conclude potassium flux after K_ATP channel agonist stimulation is decreased in neuroblastoma cells treated with morphine for several days. Mice lacking the K_ATP channel SUR1 subunit have reduced opioid efficacy in mechanical paw withdrawal behavioral responses compared to wild-type and heterozygous littermates (5 and 15 mg/kg, s.c., morphine). Using either short hairpin RNA (shRNA) or SUR1 cre-lox strategies, downregulation of SUR1 subtype K_ATP channels in the spinal cord and DRG of mice potentiated the development of morphine tolerance and withdrawal. Opioid tolerance was attenuated with intraplantar injection of SUR1 agonists, such as diazoxide and NN-414 (100 μM, 10 μL) compared to vehicle treated animals. These studies are an important first step in determining the role of K_ATP channel subunits in antinociception, opioid signaling, and the development of opioid tolerance, and shed light on the potential translational ability of K_ATP channel targeting pharmaceuticals and their possible future clinical utilization. These data suggest that increasing neuronal K_ATP channel activity in the peripheral nervous system may be a viable option to alleviate opioid tolerance and withdrawal.

Estimating Mental Health Conditions of Patients with Opioid Use Disorder

Objectives

Noninvasive estimation of cortical activity aberrance may be a challenge but gives valuable clues of mental health in patients. The goal of the present study was to characterize specificity of electroencephalogram (EEG) electrodes used to assess spectral powers associated with mental health conditions of patients with opioid use disorder.

Methods

This retrospective study included 16 patients who had been diagnosed with opioid use disorder in comparison with 16 sex- and age-matched healthy controls. EEG electrodes were placed in the frontal (FP1, FP2, F3, F4, F7, F8, and Fz), central (C3, C4, and Cz), temporal (T3, T4, T5, and T6), parietal (P3, P4, and Pz), and occipital scalp (O1 and O2). Spectral powers of δ, θ, α, β, and γ oscillations were determined, and their distribution was topographically mapped with those electrodes on the scalp.

Results

Compared to healthy controls, the spectral powers at low frequencies (<8 Hz; δ and θ) were increased in most electrodes across the scalp, while powers at the high frequencies (>12 Hz; β and γ) were selectively increased only at electrodes located in the frontal and central scalp. Among 19 electrodes, F3, F4, Fz, and Cz were highly specific in detecting increases in δ, θ, β, and γ powers of patients with opioid use disorders.

Conclusion

Results of the present study demonstrate that spectral powers are topographically distributed across the scalp, which can be quantitatively characterized. Electrodes located at F3, F4, Fz, and Cz could be specifically utilized to assess mental health in patients with opioid use disorders. Mechanisms responsible for neuroplasticity involving cortical pyramidal neurons and μ-opioid receptor regulations are discussed within the context of changes in EEG microstates.

GABAB receptors within the central nucleus of amygdala may involve in the morphine-induced incentive tolerance in female rats

Objective(s):

Central nucleus of amygdala (CeA) is the most important region for morphine-induced reward, and GABAergic system plays an important role on morphine reinforcement. The influence of CeA administration of GABA_B receptor agonist and antagonist on the expression and acquisition of morphine-induced incentive tolerance using conditioned place preference (CPP) paradigm was investigated in the present study. Our purpose was to evaluate the role of CeA GABA_B receptors in morphine tolerance.

Materials and Methods:

Seven days after surgery and cannulation, the experiments were begun. Subcutaneous (SC) injections of morphine induced CPP. Administration of one daily dose of morphine (12.5 mg/kg) for 3 days in order to develop tolerance to the drug reduced the conditioning induced by morphine (7.5 mg/kg, SC). GABA_B receptor agonist, baclofen (1.5, 6 and 12 µg/rat) or GABA_B receptor antagonist, CGP35348 (1.5, 6 and 12 µg/rat) were injected into the CeA 5 min before the experiments in the test day (expression of tolerance) or 5 min before each injection of morphine (12.5 mg/kg) (acquisition of tolerance).

Results:

It was shown that injections of baclofen (1.5 and 12 µg/rat) reduced acquisition, whereas the dose of 6 µg/rat of the drug exacerbated the acquisition of morphine tolerance. Baclofen at all doses significantly increased the expression of tolerance to morphine. Administration of CGP35348 (1.5, 6 and 12 µg/rat) reduced the acquisition and expression of morphine tolerance.

Conclusion:

These results confirmed the importance of GABA_B receptors with in the CeA in morphine tolerance in female rats.

Opioid-induced Loss of Local Anesthetic Potency in the Rat Sciatic Nerve

BACKGROUND

Previous evidence suggests that opioid-tolerant patients are less responsive to local anesthetics (LAs) for postoperative pain management.

METHODS

To determine whether this apparent loss of LA potency is due to an intrinsic change in the peripheral nerve, the effect of systemic morphine was assessed on the potency of lidocaine-induced block of the compound action potential in isolated rat sciatic nerves. Analgesic efficacy was assessed with the heat withdrawal assay.

RESULTS

While acute administration of 10 mg/kg morphine had no detectable influence on lidocaine potency, seven daily subcutaneous injections of morphine produced a three-fold decrease in potency (EC50 for block A and C waves for naive rats were [mean ± SD] 186 ± 32 μM [n = 6] and 201 ± 31 μM [n = 6], respectively, vs. 608 ± 53 μM [n = 6] and 613 ± 42 μM [n = 6], respectively [P < 0.001], in nerves from rats that had received seven daily injections of morphine [10 mg/kg]). This loss in potency was both dose-dependent and injection number dependent, such that the magnitude of the loss of lidocaine potency was significantly (n = 6; P < 0.01) correlated (r = 0.93) with the development of morphine tolerance. Interestingly, despite the complete recovery of analgesic efficacy within days after cessation of morphine administration, the morphine-induced decrease in lidocaine potency was fully manifest even 35 days after the last morphine injection. Coadministration of naloxone (1 mg/kg, intraperitoneally), but not of naloxone methiodide (1 mg/kg, subcutaneously), with each of seven daily injections of morphine blocked the decrease in lidocaine potency.

CONCLUSIONS

These preclinical data suggest that the morphine-induced decrease in LA potency is due, at least in part, to the intrinsic changes in the peripheral nerve. Identification of the underlying mechanisms may suggest strategies for more effective postoperative pain management in the growing population of opioid-tolerant patients.

Early single Aspirin-triggered Lipoxin blocked morphine anti-nociception tolerance through inhibiting NALP1 inflammasome: Involvement of PI3k/Akt signaling pathway

Clinical usage of opioids in pain relief is dampened by analgesic tolerance after chronic exposure, which is related to opioid-associated neuroinflammation. In the current study, which is based on a chronic morphine tolerance rat model and sustained morphine treatment on primary neuron culture, it was observed that Akt phosphorylation, cleaved-Caspase-1-dependent NALP1 inflammasome activation and IL-1β maturation in spinal cord neurons were significantly enhanced by morphine. Moreover, treatment with LY294002, a specific inhibitor of PI3k/Akt signaling, significantly reduced Caspase-1 cleavage, NALP1 inflammasome activation and attenuated morphine tolerance. Tail-flick tests demonstrated that pharmacological inhibition on Caspase-1 activation or antagonizing IL-1β dramatically blocked the development of morphine tolerance. The administration of an exogenous analogue of lipoxin, Aspirin-triggered Lipoxin (ATL), caused a decline in Caspase-1 cleavage, inflammasome activation and mature IL-1β production and thus attenuated the development of morphine tolerance by inhibiting upstream Akt phosphorylation. Additionally, treatment with DAMGO, a selective μ-opioid receptor peptide, significantly induced Akt phosphorylation, Caspase-1 cleavage and anti-nociception tolerance, all of which were attenuated by ATL treatment. Taken together, the present study revealed the involvement of spinal NALP1 inflammasome activation in the development of morphine tolerance and the role of the μ-receptor/PI3k-Akt signaling/NALP1 inflammasome cascade in this process. By inhibiting this signaling cascade, ATL blocked the development of morphine tolerance.

μ-Opioid inhibition of Ca2+ currents and secretion in isolated terminals of the neurohypophysis occurs via ryanodine-sensitive Ca2+ stores

μ-Opioid agonists have no effect on calcium currents (I(Ca)) in neurohypophysial terminals when recorded using the classic whole-cell patch-clamp configuration. However, μ-opioid receptor (MOR)-mediated inhibition of I(Ca) is reliably demonstrated using the perforated-patch configuration. This suggests that the MOR-signaling pathway is sensitive to intraterminal dialysis and is therefore mediated by a readily diffusible second messenger. Using the perforated patch-clamp technique and ratio-calcium-imaging methods, we describe a diffusible second messenger pathway stimulated by the MOR that inhibits voltage-gated calcium channels in isolated terminals from the rat neurohypophysis (NH). Our results show a rise in basal intracellular calcium ([Ca(2+)]i) in response to application of [D-Ala(2)-N-Me-Phe(4),Gly5-ol]-Enkephalin (DAMGO), a MOR agonist, that is blocked by D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a MOR antagonist. Buffering DAMGO-induced changes in [Ca(2+)]i with BAPTA-AM completely blocked the inhibition of both I(Ca) and high-K(+)-induced rises in [Ca(2+)]i due to MOR activation, but had no effect on κ-opioid receptor (KOR)-mediated inhibition. Given the presence of ryanodine-sensitive stores in isolated terminals, we tested 8-bromo-cyclic adenosine diphosphate ribose (8Br-cADPr), a competitive inhibitor of cyclic ADP-ribose (cADPr) signaling that partially relieves DAMGO inhibition of I(Ca) and completely relieves MOR-mediated inhibition of high-K(+)-induced and DAMGO-induced rises in [Ca(2+)]i. Furthermore, antagonist concentrations of ryanodine completely blocked MOR-induced increases in [Ca(2+)]i and inhibition of I(Ca) and high-K(+)-induced rises in [Ca(2+)]i while not affecting KOR-mediated inhibition. Antagonist concentrations of ryanodine also blocked MOR-mediated inhibition of electrically-evoked increases in capacitance. These results strongly suggest that a key diffusible second messenger mediating the MOR-signaling pathway in NH terminals is [Ca(2+)]i released by cADPr from ryanodine-sensitive stores.

Tramadol-induced blockade of delayed rectifier potassium current in NG108-15 neuronal cells

Tramadol is a centrally acting analgesic drug used mainly in the moderate to severe pain control. In this study, the effects of this agent on ion currents of NG108-15 neuronal cells were investigated. This cell line expresses Kv3.1a mRNAs and exhibits the activity of delayed rectifier K(+) (K(DR)) channels. Tramadol suppressed the amplitude of delayed rectifier K(+) current (I(K(DR))) in a concentration-dependent manner with an IC(50) values of 25 microM. Tramadol (30 microM) also shifted the steady-state inactivation of I(K(DR)) to a more negative membrane potential by approximately -15 mV. The role of the K(DR) channel, particularly as a member of the Kv3 superfamily, is to stabilize the resting potential and to reduce the width of action potentials in the time-coding neurons. Tramadol-induced block of I(K(DR)) observed in this study could be partly responsible for its anti-depressant action.

Voltage-dependent kappa-opioid modulation of action potential waveform-elicited calcium currents in neurohypophysial terminals

Release of neurotransmitter is activated by the influx of calcium. Inhibition of Ca(2+) channels results in less calcium influx into the terminals and presumably a reduction in transmitter release. In the neurohypophysis (NH), Ca(2+) channel kinetics, and the associated Ca(2+) influx, is primarily controlled by membrane voltage and can be modulated, in a voltage-dependent manner, by G-protein subunits interacting with voltage-gated calcium channels (VGCCs). In this series of experiments we test whether the kappa- and micro-opioid inhibition of Ca(2+) currents in NH terminals is voltage-dependent. Voltage-dependent relief of G-protein inhibition of VGCC can be achieved with either a depolarizing square pre-pulse or by action potential waveforms. Both protocols were tested in the presence and absence of opioid agonists targeting the kappa- and micro-receptors in neurohypophysial terminals. The kappa-opioid VGCC inhibition is relieved by such pre-pulses, suggesting that this receptor is involved in a voltage-dependent membrane delimited pathway. In contrast, micro-opioid inhibition of VGCC is not relieved by such pre-pulses, indicating a voltage-independent diffusible second-messenger signaling pathway. Furthermore, relief of kappa-opioid inhibition during a physiologic action potential (AP) burst stimulation indicates the possibility of activity-dependent modulation in vivo. Differences in the facilitation of Ca(2+) channels due to specific G-protein modulation during a burst of APs may contribute to the fine-tuning of Ca(2+)-dependent neuropeptide release in other CNS terminals, as well.

Presynaptic delta opioid receptors regulate ethanol actions in central amygdala

Endogenous opioid systems are implicated in the reinforcing effects of ethanol consumption. For example, delta opioid receptor (DOR) knockout (KO) mice show greater ethanol consumption than wild-type (WT) mice (Roberts et al., 2001). To explore the neurobiological correlates underlying these behaviors, we examined effects of acute ethanol application in brain slices from DOR KO mice using whole-cell patch recording techniques. We examined the central nucleus of amygdala (CeA) because the CeA is implicated in alcohol reinforcement (Koob et al., 1998). We found that the acute ethanol effects on GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) were greater in DOR KO mice than in WT mice. Ethanol increased the frequency of miniature IPSCs (mIPSCs) significantly more in DOR KO mice than in WT mice. In CeA of WT mice, application of ICI 174864 [[allyl]2-Tyr-alpha-amino-isobutyric acid (Aib)-Aib-Phe-Leu-OH], a DOR inverse agonist, augmented ethanol actions on mIPSC frequency comparable with ethanol effects seen in DOR KO mice. Superfusion of the selective DOR agonist D-Pen(2),D-Pen(5)-enkephalin decreased the mean frequency of mIPSCs; this effect was reversed by the DOR antagonist naltrindole. These findings suggest that endogenous opioids may reduce ethanol actions on IPSCs of CeA neurons in WT mice through DOR-mediated inhibition of GABA release and that the increased ethanol effect on IPSCs in CeA of DOR KO mice could be, at least in part, due to absence of DOR-mediated inhibition of GABA release. This result supports the hypothesis that endogenous opioid peptides modulate the ethanol-induced augmentation of GABA(A) receptor-dependent circuitry in CeA (Roberto et al., 2003).

Ion channels and intracellular signaling proteins as potential targets for novel therapeutics for addictive and depressive disorders

Modern neuroscience is placing increased emphasis on understanding how the activity of ion channels and intracellular molecules in the central nervous system affect behavior. An improved understanding of the brain and the biological bases of conditions such as addictive and depressive disorders is important because it should ultimately enable the design of innovative treatments for these conditions. The development of rational therapies that are based on knowledge of what is different about the addicted or depressed brain would be an important advance. Here, we describe how multidisciplinary studies that combine numerous approaches (behavioral analysis, physiology, molecular biology, and genetic engineering) have begun to provide important advances that have helped to establish causal relationships between the pathophysiology of these conditions and behavior. This type of work has identified classes of molecules on the outside of cells (receptors and ion channels) that receive signals from other cells and initiate cellular events that have short-term effects on the neurons. It has also identified other classes of molecules that are inside of cells (signal transduction molecules) that can have immediate effects on cell function (e.g., ion channel phosphorylation), as well longer term effects (alterations in protein expression) that affect the ways in which neurons function within circuits. Innovative treatments that block, negate, or even reverse the extracellular or intracellular neuroadaptations resulting from exposure to drugs of abuse or stress might be more effective than current therapies because they directly target the molecular processes that cause maladaptive behaviors.

Effects of morphine on gene expression in the rat amygdala

Influence of morphine self-administration on gene expression in the rat amygdala was studied using rat genome DNA arrays U34A from Affymetrix. Animals were trained to self-administer morphine, each having two 'yoked' control animals, receiving passive injections of either morphine or saline. After 40 sessions of self-administration, amygdalae were removed, total RNA was isolated and used to prepare probes for Genechip arrays. The treatment was found to significantly change abundance of 29 transcripts. Analysis by means of reverse transcription real-time PCR showed significant changes in abundance of five transcripts: gamma protein kinase C (PKC), upstream binding factor 2 (UBF2), lysozyme, noggin and heat shock protein 70 (hsp70). After 30 days of forced abstinence from morphine self-administration, abundance of hsp70 and lysozyme returned to basal levels. Changes in abundance of UBF2 persisted, and abundance of three additional genes, namely nuclear factor I/A, gamma1 subunit of GABAA receptor and the neuronal calcium sensor 1, changed. Additionally, acute as well as chronic intraperitoneal morphine administration changed the abundance of PKC gamma, gamma1 subunit of GABAA and hsp70 genes.

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.

Molecular mechanisms of excitatory signaling upon chronic opioid agonist treatment

Opioid receptor agonists mediate their analgesic effects by interacting with Gi/o protein-coupled opioid receptors. Acute treatment with opioid agonists is thought to mediate analgesia by hyperpolarization of presynatic neurons, leading to the inhibition of excitatory (pain) neurotransmitters release. After chronic treatment however, the opioid receptors gradually become less responsive to agonists, and increased drug doses become necessary to maintain the therapeutic effect (tolerance). Analgesic tolerance is the result of two, partially overlapping processes: a gradual loss of inhibitory opioid function is accompanied by an increase in excitatory signaling. Recent data indicate that chronic opioid agonist treatment simultaneously desensitizes the inhibitory-, and augments the stimulatory effects of the opioids. In the present paper we review the molecular mechanisms that may have a role in the augmentation of the excitatory signaling upon chronic opioid agonist treatment. We also briefly review our recent experimental data on the molecular mechanism of chronic opioid agonist-mediated functional sensitization of forskolin-stimulated cAMP formation, in a recombinant Chinese hamster ovary cell line stably expressing the human delta-opioid receptor (hDOR/CHO). To interpret the experimental data, we propose that chronic hDOR activaton leads to activation of multiple redundant signaling pathways that converge to activate the protein kinase, Raf-1. Raf-1 in turn phosphorylates and sensitizes the native adenylyl cyclase VI isoenzyme in hDOR/CHO cells, causing a rebound increase in forskolin-stimulated cAMP formation upon agonist withdrawal.

Modulation of voltage-dependent potassium currents by opiates in facial motoneurons of neonatal rats

We examined the modulation of rat facial motoneurons (FMNs) by opiates in a slice preparation (7-15 days old) using whole-cell patch clamp techniques. Although application of methionine enkephalin (ME) did not change the peak value of the transient outward current (A-current, IA), it reduced the persistent voltage-dependent K(+) currents (IKs) in a dose-dependent manner. The reduction was antagonized by naloxone (40 microM). IKs were reduced only by mu-selective agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO, 2-121.6 microM). This reduction was antagonized by naloxone (40 microM) or the mu-selective antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2) (CTOP, 1 microM). Agonists for other opiate receptors (delta- and kappa-opiate receptor) showed no effect on IKs. In accord with the effects on IKs, DAMGO (100 microM) prolonged the duration of the action potential evoked in Ca(2+)-free external solution containing 4-aminopiridine (1mM). These results suggest that the activation of mu-opiate receptors contributes to signal transduction in FMNs primarily by modulating action potential duration.

Opioid receptor modulation of a metabolically sensitive ion channel in rat amygdala neurons

We have used single-channel patch-clamp recordings to study opiate receptor effects on freshly dissociated neurons from the rat amygdalohippocampal area (also called the posterior nucleus of the amygdala), an output nucleus of the amygdala implicated in appetitive behaviors. Dissociated cells included a distinct subpopulation that was 30-40 micrometer in diameter, multipolar or pyramidal in shape, and immunoreactive for neuron-specific enolase, mu opioid receptors, and galanin. In whole-cell perforated-patch recordings, these cells responded to low concentrations of mu opioid agonists with a hyperpolarization. In cell-attached single channel recordings, these cells expressed a large variety of K(+)-permeable ion channels, including 20-100 pS inward rectifiers and 150-200 pS apparent Ca(2+)-activated K(+) channels, none of which appeared sensitive to the presence of opioid drugs. In contrast, a 130 pS inwardly rectifying channel was selectively activated by mu opioid receptors in this same subpopulation of cells and was active only in the presence of opioid agonists, and inhibited in the presence of antagonists. Channels identical to the 130 pS channel in conductance and voltage sensitivity were activated in the absence of opioids, when the cells were treated with glucose-free medium or with the metabolic inhibitor rotenone. The sulfonylurea drug tolbutamide inhibited 130 pS channel openings elicited by opioids. Thus, a subpopulation of amygdala projection neurons expresses a metabolically sensitive ion channel that is selectively modulated by opiate receptors. This mechanism may allow opioid neurotransmitters to regulate ingestive behaviors, and thus, opiate drugs to influence reward pathways.

Opioid receptor modulation of a metabolically sensitive ion channel in rat amygdala neurons

We have used single-channel patch-clamp recordings to study opiate receptor effects on freshly dissociated neurons from the rat amygdalohippocampal area (also called the posterior nucleus of the amygdala), an output nucleus of the amygdala implicated in appetitive behaviors. Dissociated cells included a distinct subpopulation that was 30-40 micrometer in diameter, multipolar or pyramidal in shape, and immunoreactive for neuron-specific enolase, mu opioid receptors, and galanin. In whole-cell perforated-patch recordings, these cells responded to low concentrations of mu opioid agonists with a hyperpolarization. In cell-attached single channel recordings, these cells expressed a large variety of K(+)-permeable ion channels, including 20-100 pS inward rectifiers and 150-200 pS apparent Ca(2+)-activated K(+) channels, none of which appeared sensitive to the presence of opioid drugs. In contrast, a 130 pS inwardly rectifying channel was selectively activated by mu opioid receptors in this same subpopulation of cells and was active only in the presence of opioid agonists, and inhibited in the presence of antagonists. Channels identical to the 130 pS channel in conductance and voltage sensitivity were activated in the absence of opioids, when the cells were treated with glucose-free medium or with the metabolic inhibitor rotenone. The sulfonylurea drug tolbutamide inhibited 130 pS channel openings elicited by opioids. Thus, a subpopulation of amygdala projection neurons expresses a metabolically sensitive ion channel that is selectively modulated by opiate receptors. This mechanism may allow opioid neurotransmitters to regulate ingestive behaviors, and thus, opiate drugs to influence reward pathways.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Morphine and clonidine activate different K+ channels on rat amygdala neurons

In cell-attached patch-clamp recordings from freshly dissociated neurons of the rat amygdalohippocampal area, clonidine principally activated a 95-pS (picosiemens) inwardly rectifying K+-permeable channel, whereas morphine acting at mu-opioid receptors activated a different 130-pS channel. Clonidine's effects were largely antagonized by the alpha(2)-adrenoceptor antagonist idazoxan, but were poorly mimicked by agmatine. These results partially contradict the prevailing hypothesis that alpha(2) and opioid receptors act through the same ion channel transduction pathways.