Stimulation of endogenous opioid release displaces mu receptor binding in rat hippocampus

Kappa Opioid Receptors Mediate Heterosynaptic Suppression of Hippocampal Inputs in the Rat Ventral Striatum

Kappa opioid receptors (KORs) are highly enriched within the ventral striatum (VS) and are thought to modulate striatal neurotransmission. This includes presynaptic inhibition of local glutamatergic release from excitatory inputs to the VS. However, it is not known which inputs drive this modulation and what impact they have on the local circuit dynamics within the VS. Individual medium spiny neurons (MSNs) within the VS serve as a site of convergence for glutamatergic inputs arising from the PFC and limbic regions, such as the hippocampus (HP). Recent data suggest that competition can arise between these inputs with robust cortical activation leading to a reduction in ongoing HP-evoked MSN responses. Here, we investigated the contribution of KOR signaling in PFC-driven heterosynaptic suppression of HP inputs onto MSNs using whole-cell patch-clamp recordings in slices from adult rats. Optogenetically evoked HP EPSPs were greatly attenuated after a short latency (50 ms) following burst-like PFC electrical stimulation, and the magnitude of this suppression was partially reversed following blockade of GABA_ARs (GABA Type A receptors), but not GABA_BRs (GABA Type B receptors). A similar reduction in suppression was observed in the presence of the KOR antagonist, norBNI. Combined blockade of local GABA_ARs and KORs resulted in complete blockade of PFC-induced heterosynaptic suppression of less salient HP inputs. These findings highlight a mechanism by which strong, transient PFC activity can take precedence over other excitatory inputs to the VS.SIGNIFICANCE STATEMENT Emerging evidence suggests that kappa opioid receptor (KOR) activation can selectively modulate striatal glutamatergic inputs onto medium spiny neurons (MSNs). In this study, we found that robust cortical stimulation leads to a reduction in ongoing hippocampal-evoked MSNs responses through the combined recruitment of local inhibitory mechanisms and activation of presynaptic KORs in the ventral striatum (VS). These processes are likely to facilitate the efficient transfer of cortical information through the VS during critical decision making by dampening competing information from less salient excitatory inputs. These data provide a novel mechanism through which VS information processing could influence decision making, a function thought to occur primarily in the PFC.

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.

Endogenous opioids mediate left dorsolateral prefrontal cortex rTMS-induced analgesia

The concurrent rise of undertreated pain and opiate abuse poses a unique challenge to physicians and researchers alike. A focal, noninvasive form of brain stimulation called repetitive transcranial magnetic stimulation (rTMS) has been shown to produce acute and chronic analgesic effects when applied to dorsolateral prefrontal cortex (DLPFC), but the anatomical and pharmacological mechanisms by which prefrontal rTMS induces analgesia remain unclear. Data suggest that DLPFC mediates top-down analgesia via gain modulation of the supraspinal opioidergic circuit. This potential pathway might explain how prefrontal rTMS reduces pain. The purpose of this sham-controlled, double-blind, crossover study was to determine whether left DLPFC rTMS-induced analgesia was sensitive to μ-opioid blockade. Twenty-four healthy volunteers were randomized to receive real or sham TMS after either intravenous saline or naloxone pretreatment. Acute hot and cold pain via quantitative sensory testing and hot allodynia via block testing on capsaicin-treated skin were assessed at baseline and at 0, 20, and 40 minutes after TMS treatment. When compared to sham, real rTMS reduced hot pain and hot allodynia. Naloxone pretreatment significantly reduced the analgesic effects of real rTMS. These results demonstrate that left DLPFC rTMS-induced analgesia requires opioid activity and suggest that rTMS drives endogenous opioidergic pain relief in the human brain. Further studies with chronic dosing regimens of drugs that block or augment the actions of opiates are needed to determine whether TMS can augment opiates in chronic or postoperative pain management.

Endogenous opioid peptides contribute to associative LTP in the hippocampal CA3 region

The medial and lateral perforant path projections to the hippocampal CA3 region display distinct mechanisms of long-term potentiation (LTP) induction, N-methyl-d-aspartate (NMDA) and opioid receptor dependent, respectively. However, medial and lateral perforant path projections to the CA3 region display associative LTP with coactivation, suggesting that while they differ in receptors involved in LTP induction they may share common downstream mechanisms of LTP induction. Here we address this interaction of LTP induction mechanisms by evaluating the contribution of opioid receptors to the induction of associative LTP among the medial and lateral perforant path projections to the CA3 region in vivo. Local application of the opioid receptor antagonists naloxone or Cys2-Tyr3-Orn5-Pen7-amide (CTOP) normally block induction of lateral perforant path-CA3 LTP. However, these opioid receptor antagonists failed to block associative LTP in lateral perforant path-CA3 synapses when it was induced by strong coactivation of the medial perforant pathway which displays NMDAR-dependent LTP. Thus strong activation of non-opioidergic afferents can substitute for the opioid receptor activation required for lateral perforant path LTP induction. Conversely, medial perforant path-CA3 associative LTP was blocked by opioid receptor antagonists when induced by strong coactivation of the opioidergic lateral perforant path. These data indicate endogenous opioid peptides contribute to associative LTP at coactive synapses when induced by strong coactivation of an opioidergic afferent system. These data further suggest that associative LTP induction is regulated by the receptor mechanisms of the strongly stimulated pathway. Thus, while medial and lateral perforant path synapses differ in their mechanisms of LTP induction, associative LTP at these synapses share common downstream mechanisms of induction.

Mu opioid receptor activation normalizes temporo-ammonic pathway driven inhibition in hippocampal CA1

The hippocampus of the mammalian brain is important for the formation of long-term memories. Hippocampal-dependent learning can be affected by a number of neurotransmitters including the activation of μ-opioid receptors (MOR). It has been shown that MOR activation can alter synaptic plasticity and network oscillations in the hippocampus, both of which are thought to be important for the encoding of information and formation of memories. One hippocampal oscillation that has been correlated with learning and memory formation is the 4-10 Hz theta rhythm. During theta rhythms, inputs to hippocampal CA1 from CA3 (Schaffer collaterals, SC) and the entorhinal cortex (perforant path) can integrate at different times within an individual theta cycle. Consequently, when excitatory inputs in the stratum lacunosum-moleculare (the temporo-ammonic pathway (TA), which includes the perforant path) are stimulated approximately one theta period before SC inputs, the TA can indirectly inhibit SC inputs. This inhibition is due to the activation of postsynaptic GABA(B) receptors on CA1 pyramidal neurons. Importantly, MOR activation has been shown to suppress GABA(B) inhibitory postsynaptic potentials in CA1 pyramidal neurons. Therefore, we examined how MOR activation affects the integration between TA inputs and SC inputs in hippocampal CA1. To do this we used voltage-sensitive dye imaging and whole cell patch clamping from acute hippocampal slices taken from young adult rats. Here we show that MOR activation has no effect on the integration between TA and SC inputs when activation of the TA precedes SC by less than one half of a theta cycle (<75 ms). However, MOR activation completely blocked the inhibitory action of TA on SC inputs when TA stimulation occurred approximately one theta cycle before SC activation (>150 ms). This MOR suppression of TA driven inhibition occurred in both the SC input layer of hippocampal CA1 (stratum radiatum) and the output layer of CA1 pyramidal neurons (stratum pyramidale). Thus MOR activation can have profound effects on the temporal integration between two primary excitatory pathways to hippocampal CA1 and subsequently the resultant output from CA1 pyramidal neurons. These data provide important information for understanding how acute or chronic MOR activation may affect the integration of activity within hippocampal CA1 during theta rhythm.

Age- and hormone-regulation of opioid peptides and synaptic proteins in the rat dorsal hippocampal formation

Circulating estrogen levels and hippocampal-dependent cognitive functions decline with aging. Moreover, the responses of hippocampal synaptic structure to estrogens differ between aged and young rats. We recently reported that estrogens increase levels of post-synaptic proteins, including PSD-95, and opioid peptides leu-enkephalin and dynorphin in the hippocampus of young animals. However, the influence of ovarian hormones on synaptic protein and opioid peptide levels in the aging hippocampus is understudied. Here, young (3- to 5-month-old), middle-aged (9- to 12-month-old), and aged (about 22-month-old) female rats were ovariectomized and then, 4 weeks later, subcutaneously implanted with a silastic capsule containing vehicle or 17β-estradiol. After 48 h, rats were subcutaneously injected with progesterone or vehicle and sacrificed 1 day later. Coronal sections through the dorsal hippocampus were processed for quantitative peroxidase immunohistochemistry of leu-enkephalin, dynorphin, synaptophysin, and PSD-95. With age, females showed opposing changes in leu-enkephalin and dynorphin levels in the mossy fiber pathway, particularly within the hilus, and regionally specific changes in synaptic protein levels. 17β-estradiol, with or without progesterone, altered leu-enkephalin levels in the dentate gyrus and synaptophysin levels in the CA1 of young but not middle-aged or aged females. Additionally, 17β-estradiol decreased synaptophysin levels in the CA3 of middle-aged females. Our results support and extend previous findings indicating 17β-estradiol modulation of hippocampal opioid peptides and synaptic proteins while demonstrating regional and age-specific effects. Moreover, they lend credence to the "window of opportunity" hypothesis during which hormone replacement can modulate hippocampal structure and circuitry to improve cognitive outcomes.

Hippocampal dynorphin immunoreactivity increases in response to gonadal steroids and is positioned for direct modulation by ovarian steroid receptors

The hippocampal formation (HF) is involved in modulating learning related to drug abuse. While HF-dependent learning is regulated by both endogenous opioids and estrogen, the interaction between these two systems is not well understood. The mossy fiber (MF) pathway formed by dentate gyrus (DG) granule cell axons is involved in some aspects of learning and contains abundant amounts of the endogenous opioid peptide dynorphin (DYN). To examine the influence of ovarian steroids on DYN expression, we used quantitative light microscopic immunocytochemistry to measure DYN levels in normal cycling rats as well as in two established models of hormone-treated ovariectomized (OVX) rats. Rats in estrus had increased levels of DYN-immunoreactivity (ir) in the DG and certain CA3 lamina compared with rats in proestrus or diestrus. OVX rats exposed to estradiol for 24 h showed increased DYN-ir in the DG and CA3, while those with 72 h estradiol exposure showed increases only in the DG. Six hours of estradiol exposure produced no change in DYN-ir. OVX rats chronically implanted with medroxyprogesterone also showed increased DYN-ir in the DG and CA3. Next, dual-labeling electron microscopy (EM) was used to evaluate the subcellular relationships of estrogen receptor (ER) alpha-, ERbeta and progestin receptor (PR) with DYN-labeled MFs. ERbeta-ir was in some DYN-labeled MF terminals and smaller terminals, and had a subcellular association with the plasmalemma and small synaptic vesicles. In contrast, ERalpha-ir was not in DYN-labeled terminals, although some DYN-labeled small terminals synapsed on ERalpha-labeled dendritic spines. PR labeling was mostly in CA3 axons, some of which were continuous with DYN-labeled terminals. These studies indicate that ovarian hormones can modulate DYN in the MF pathway in a time-dependent manner, and suggest that hormonal effects on the DYN-containing MF pathway may be directly mediated by ERbeta and/or PR activation.

Upregulation of opioid receptor binding following spontaneous epileptic seizures

Animal and limited human data suggest an important anticonvulsant role for opioid peptides and their receptors. We aimed to provide direct human in vivo evidence for changes in opioid receptor availability following spontaneous seizures. We scanned nine patients within hours of spontaneous temporal lobe seizures and compared their postictal binding of the non-subtype selective opioid receptor PET radioligand [11C]diprenorphine (DPN), quantified as a volume-of-distribution (VD), with interictal binding and with binding changes in 14 healthy controls, controlling for a range of behavioural variables associated with opioid action. A regionally specific increase of opioid receptor availability was evident in the temporal pole and fusiform gyrus ipsilateral to the seizure focus following seizures (Z 5.01, P < 0.001, 16 432 mm3). Within this region, there was a negative correlation between VD and log10 time since last seizure (r = -0.53, P < 0.03), compatible with an early increase and gradual return to baseline. [11C]DPN VD did not undergo systematic changes between time points in controls. This study provides direct human in vivo evidence for changes in opioid receptor availability over a time course of hours following spontaneous seizures, emphasizing an important role of the opioid system in seizure control.

Plastic processes in the dentate gyrus: a computational perspective

The dentate gyrus has the capacity for numerous types of synaptic plasticity that use diverse mechanisms and are thought essential for the storage of information in the hippocampus. Here we review the various forms of synaptic plasticity that involve afferents and efferents of the dentate gyrus, and, from a computational perspective, relate how these plastic processes might contribute to sparse, orthogonal encoding, and the selective recall of information within the hippocampus.

Opioid systems in the dentate gyrus

Opiate drugs alter cognitive performance and influence hippocampal excitability, including long-term potentiation (LTP) and seizure activity. The dentate gyrus (DG) contains two major opioid peptides, enkephalins and dynorphins, which have opposing effects on excitability. Enkephalins preferentially bind to delta- and mu-opioid receptors (DORs and MORs) while dynorphins preferentially bind to kappa-opioid receptors (KORs). Opioid receptors can also be activated by exogenous opiate drugs such as the MOR agonist morphine. Enkephalins are contained in the mossy fiber pathway, in the lateral perforant path (PP) and in scattered GABAergic interneurons. MORs and DORs are predominantly in distinct subpopulations of GABAergic interneurons known to inhibit granule cells, and are present at low levels within granule cells. MOR and DOR agonists increase excitability and facilitate LTP in the molecular layer. Anatomical and physiological evidence is consistent with somatodendritic and axon terminal targeting of both MORs and DORs. Dynorphins are in the granule cells, most abundantly in mossy fibers but also in dendrites. KORs have been localized to granule cell mossy fibers, supramammillary afferents to granule cells, and PP terminals. KOR agonists, including endogenous dynorphins, diminish the induction of LTP. Recent evidence indicates that opiates and opioids also modulate other processes in the hippocampal formation, including adult neurogenesis, the actions of gonadal hormones, and development of neonatal transmitter systems.

Role of hippocampal CA3 mu-opioid receptors in spatial learning and memory

The dorsal CA3 region of the hippocampus is unique in its connectivity, sensitivity to neurotoxic lesions, and its ability to encode and retrieve episodic memories. Computational models of the CA3 region predict that blocking mossy-fiber and/or perforant path activity to CA3 would cause impairments in learning and recall of spatial memory, respectively. Because the CA3 region contains micro-opioid receptors and receives inputs from the mossy-fiber and lateral perforant pathways, both of which contain and release opioid peptides, we tested the hypothesis that inactivating micro-opioid receptors in the CA3 region would cause spatial learning and memory impairments and retrieval deficits. In this study, male Sprague Dawley rats were trained in a Morris water maze after a single bilateral intrahippocampal injection of either saline or the selective and irreversible micro-opioid receptor antagonist beta-funaltrexamine (beta-FNA) into area CA3. We found that micro-opioid receptor binding decreased 24 hr after beta-FNA injection and returned to control levels 11 d after injection. Injections of beta-FNA into the CA3 region, but not into the ventricles, caused a significant impairment in the acquisition of spatial learning without causing sensory or motor deficits. New learning was not affected once micro-opioid receptor levels replenished (>11 d after injection). In pretrained animals, beta-FNA significantly impaired spatial memory retrieval and new (reversal) learning. These data are consistent with theoretical models of CA3 function and suggest that CA3 micro-opioid receptors play an important role in the acquisition and retrieval of spatial memory.

Mu opioid receptors are in discrete hippocampal interneuron subpopulations

In the rat hippocampal formation, application of mu opioid receptor (MOR) agonists disinhibits principal cells, promoting excitation-dependent processes such as epileptogenesis and long-term potentiation. However, the precise location of MORs in particular inhibitory circuits, has not been determined, and the roles of MORs in endogenous functioning are unclear. To address these issues, the distribution of MOR-like immunoreactivity (-li) was examined in several populations of inhibitory hippocampal neurons in the CA1 region using light and electron microscopy. We found that MOR-li was present in many parvalbumin-containing basket cells, but absent from cholecystokinin-labeled basket cells. MOR-li was also commonly in interneurons containing somatostatin-li or neuropeptide Y-li that resembled the "oriens-lacunosum-moleculare" (O-LM) interneurons innervating pyramidal cell distal dendrites. Finally, MOR-li was in some vasoactive intestinal peptide- or calretinin-containing profiles resembling interneurons that primarily innervate other interneurons. These findings indicate that MOR-containing neurons form a neurochemically and functionally heterogeneous subset of hippocampal GABAergic neurons. MORs are most frequently on interneurons that are specialized to inhibit pyramidal cells, and are on a limited number of interneurons that target other interneurons. Moreover, the distribution of MORs to different neuronal types in several laminae, some relatively far from endogenous opioids, suggests normal functional roles that are different from the actions seen with exogenous agonists such as morphine.

Neurons with mu opioid receptors interact indirectly with enkephalin-containing neurons in the rat dentate gyrus

In the dentate gyrus, mu opioid receptors (MORs) and their enkephalin agonists have overlapping distributions and influence excitability and plasticity. Released endogenous enkephalins can activate at least some of these MORs; however, whether these interactions involve synaptically associated profiles or more distant associations and whether some subcellular compartments (e.g., terminals or dendrites) are more likely to be targeted than others are not known. To elucidate the relationships between potential sites of enkephalin release and MORs, MOR1 and leucine-enkephalin (LE) immunoreactivities were localized in the hilus by electron microscopy, using immunoperoxidase and immunogold markers. Of the 573 MOR-immunoreactive (ir) profiles analyzed, most were axons and terminals (51 and 30%, respectively), and fewer were dendrites (12%), glia (3%), or unclassifiable (4%). Most MOR-ir profiles resembled interneuron processes, while most LE-ir terminals resembled mossy fibers. One third of MOR-ir profiles were within 3 microm and approximately half were within 4 microm of the nearest LE-ir profile. In contrast, few (3%) MOR-ir profiles contacted LE-ir profiles; only 16% of these contacts included observable synapses, and very few profiles (0.5%) colocalized MOR and LE immunoreactivity. MOR-ir axons, terminals, and dendrites were not distributed differently relative to LE-ir profiles. These results suggest that activation of hilar MORs by LE usually involves short-range volume transmission and that dendritic MORs are as likely as axonal and terminal MORs to be activated by released LE. However, the greater abundance of MOR-ir axons and terminals compared to dendrites indicates that presynaptic profiles are a more prominent target for enkephalins and exogenous MOR agonists such as morphine.

Long-term potentiation in direct perforant path projections to the hippocampal CA3 region in vivo

The perforant path constitutes the primary projection system relaying information from the neocortex to the hippocampal formation. Long-term synaptic potentiation (LTP) in the perforant path projections to the dentate gyrus is well characterized. However, surprisingly few studies have addressed the mechanisms underlying LTP induction in the direct perforant path projections to the hippocampus. Here we investigate the role of N-methyl-D-aspartate (NMDA) and opioid receptors in the induction of LTP in monosynaptic medial and lateral perforant path projections to the CA3 region in adult pentobarbital sodium-anesthetized rats. Similar to LTP observed at the medial perforant path-dentate gyrus synapse, medial perforant path-CA3 synapses display LTP that is blocked by both local and systemic administration of the competitive NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid [(+/-)-CPP]. By contrast, LTP induced at the lateral perforant path-CA3 synapses is not blocked by either local or systemic administration of this NMDA receptor antagonist. The induction of LTP at lateral perforant path-CA3 synapses, which is blocked by the opioid receptor antagonist naloxone, is also blocked by the selective mu opioid receptor antagonist Cys(2)-Tyr(3)-Orn(5)-Pen(7)-amide (CTOP), but not the selective delta opioid receptor antagonist naltrindole (NTI). CTOP was without effect on the induction of medial perforant path-CA3 LTP. The selective sensitivity of lateral perforant path-CA3 LTP to mu-opioid receptor antagonists corresponds with the distribution of mu-opioid receptors within the stratum lacunosum-moleculare of area CA3 where perforant path projections to CA3 terminate. These data indicate that both lateral and medial perforant path projections to the CA3 region display LTP, and that LTP induction in medial and lateral perforant path-CA3 synapses are differentially sensitive to NMDA receptor and mu-opioid receptor antagonists. This suggests a role for opioid, but not NMDA receptors in the induction of LTP at lateral perforant path projections to the hippocampal formation.

Ultrastructural evidence for presynaptic mu opioid receptor modulation of synaptic plasticity in NMDA-receptor-containing dendrites in the dentate gyrus

Physiological studies have demonstrated that long-term potentiation (LTP) induction in N-methyl-D-aspartate (NMDA) receptor containing dentate granule cells following lateral perforant path stimulation is opioid dependent, involving mu-opioid receptors (MORs) on gamma-aminobutyric acid (GABA)-ergic neurons. To determine the cellular relationships of MORs to postsynaptic NMDA receptor-containing dendrites, immunoreactivity (-I) against MOR and the NMDA receptor subunit 1 (NMDAR1) was examined in the outer molecular layer of the dentate gyrus using electron microscopy. MOR-I was predominantly in axons and axon terminals. NMDAR1-I was almost exclusively in spiny dendrites, but was also in a few terminals. Using immunogold particles to localize precisely NMDAR1, one-third of the NMDAR1-I was detected on the dendritic plasmalemma; in dendritic spines plasmalemmal immunogold particles were near synaptic densities. Many MOR-labeled axons and terminals contacted NMDAR1-labeled dendrites. MOR-labeled terminals formed symmetric (inhibitory-type) synapses on NMDAR1-labeled dendritic shafts or nonsynaptically contacted NMDAR1-labeled shafts and spines. MOR-labeled axons often abutted NMDAR1-containing dendritic spines which received asymmetric (excitatory-type) synapses from unlabeled terminals. Occasionally, MOR-labeled terminals and dendrites were apposed to NMDAR1-containing terminals. These results provide anatomical evidence that endogenous enkephalins or exogenous opioid agonists could inhibit GABAergic terminals that modulate granule cell dendrites, thus boosting depolarizing events in granule cells and facilitating the activation of NMDA receptors located on their dendrites.

Hippocampal mossy fiber activity evokes Ca2+ release in CA3 pyramidal neurons via a metabotropic glutamate receptor pathway

Mossy fiber activity can evoke Ca2+ release from internal stores in CA3 neurons, but the physiological conditions under which this occurs and the mechanisms underlying the release are not understood. Using rat hippocampal slices we report here that short trains of mossy fiber stimulation activate group I metabotropic glutamate receptors (mGluRs) on CA3 pyramidal neurons and elicit waves of Ca2+ release from inositol 1,4,5-trisphosphate (IP3) sensitive internal stores that propagate from stratum lucidum to the soma and in some cases distally out the dendrites. Activation of mGluR1,5 receptors by an agonist trans-azetidine-2,4-dicarboxylic acid (tADA) applied to stratum lucidum was also sufficient to induce waves of Ca2+ release. This release was blocked by internal heparin, but not by dantrolene, suggesting the involvement of IP3 rather than ryanodine receptors in not only the initial release but also in the maintenance of the propagating waves. Release could be facilitated by Ca2+ influx through voltage-gated Ca2+ channels, which is consistent with the known Ca2+ sensitivity of IP3 receptors.These results provide insight into the mechanisms and conditions of Ca2+ release in CA3 neurons and demonstrate the powerful influence mossy fiber input can have on these neurons.

Effects of mineralocorticoid and glucocorticoid receptors on long-term potentiation in the CA3 hippocampal field

We have previously shown that the two types of adrenal steroid receptors, mineralocorticoid MR. and glucocorticoid GR. produce opposite effects on long-term potentiation LTP. in the dentate gyrus in vivo. and CA1 hippocampal field in vitro. More specifically, MR activation enhanced and prolonged LTP, whereas GR activation suppressed LTP in these areas and also produced a long-term depression LTD. of the synaptic response. In the present experiment we investigated acute effects of MR and GR activation on LTP induction in the mossy fiber and commissural associational input to the CA3 hippocampal field, since the mechanisms underlying LTP induction in these two pathways differ, the former being N-methyl-D-aspartate receptor NMDAR. independent while the latter being NMDAR-dependent. Rats were either adrenalectomized ADX or adrenally intact. ADX animals were acutely injected with either the specific MR agonist, aldosterone, the specific GR agonist RU 28362 or vehicle. One hour following the injection, the animals were prepared for electrophysiological recording stimulation. Field potential recordings were performed in the radiatum or laconosum moleculare layers of the CA3 field, with stimulation of either the mossy fibers or the commissural associational input from the contralateral hemisphere. We also replicated our previous findings by recording in the dentate gyrus with stimulation of the medial perforant pathway, in the same animals. As observed in our previous study in the dentate gyrus, we found an enhancement and a suppression of LTP with MR and GR activation, respectively. Similarly, for the commissural associational input to CA3, MR activation enhanced LTP, while GR activation reduced it. In contrast, for the mossy fiber input to CA3, neither MR nor GR activation significantly affected LTP induction. These results indicate that adrenal steroids may modulate LTP induction in the hippocampus via an interaction with glutamatergic NMDAR.

GABAB-Receptor-mediated currents in interneurons of the dentate-hilus border

GABA(B)-receptor-mediated inhibition was investigated in anatomically identified inhibitory interneurons located at the border between the dentate gyrus granule cell layer and hilus. Biocytin staining was used to visualize the morphology of recorded cells. A molecular layer stimulus evoked a pharmacologically isolated slow inhibitory postsynaptic current (IPSC), recorded with whole cell patch-clamp techniques, in 55 of 63 interneurons. Application of the GABA(B) receptor antagonists, CGP 35348 (400 microM) or CGP 55845 (1 microM) to a subset of 25 interneurons suppressed the slow IPSC by an amount ranging from 10 to 100%. In 56% of these cells, the slow IPSC was entirely GABA(B)-receptor-mediated. However, in the remaining interneurons, a component of the slow IPSC was resistant to GABA(B) antagonists. Subtraction of this antagonist resistant current from the slow IPSC isolated the GABA(B) component (IPSC(B)). This IPSC(B) had a similar onset and peak latency to that recorded from granule cells but a significantly shorter duration. The GABA(B) agonist, baclofen (10 microM), produced a CGP 55845-sensitive outward current in 19 of 27 interneurons. In the eight cells that lacked a baclofen current, strong or repetitive ML stimulation also failed to evoke an IPSC(B), indicating that these cells lacked functional GABA(B) receptor-activated potassium currents. In cells that expressed a baclofen current, the amplitude of this current was approximately 50% smaller in interneurons with axons that projected into the granule cell dendritic layer (22.2 +/- 5.3 pA; mean +/- SE) than in interneurons with axons that projected into or near the granule cell body layer (46.1 +/- 10.0 pA). Similarly, the IPSC(B) amplitude was smaller in interneurons projecting to dendritic (9.4 +/- 2.7 pA) than perisomatic regions (34.3 +/- 5.1 pA). These findings suggest that GABA(B) inhibition more strongly regulates interneurons with axons that project into perisomatic than dendritic regions. To determine the functional role of GABA(B) inhibition, we examined the effect of IPSP(B) on action potential firing and synaptic excitation of these interneurons. IPSP(B) and IPSP(A) both suppressed depolarization-induced neuronal firing. However, unlike IPSP(A), suppression of firing by IPSP(B) could be easily overcome with strong depolarization. IPSP(B) markedly suppressed N-methyl-D-aspartate but not AMPA EPSPs, suggesting that GABA(B) inhibition may play a role in regulating slow synaptic excitation of these interneurons. Heterogeneous expression of GABA(B) currents in hilar border interneurons therefore may provide a mechanism for the differential regulation of excitation of these cells and thereby exert an important role in shaping neuronal activity in the dentate gyrus.

Evidence for tonic activation of NK-1 receptors during the second phase of the formalin test in the Rat

Behavioral, electrophysiological, and autoradiographic experiments were done to study the second nociceptive phase in the formalin test. In initial experiments, this second phase was attenuated by 1-10 mg of the NK-1 receptor antagonist CP-99,994, given subcutaneously 10, 30, or 60 min before formalin (n = 8-10) and by 20 microgram given intrathecally 20 min after formalin (n = 13); the inactive isomer CP-100,263 was ineffective. In electrophysiological experiments on single dorsal horn neurons in vivo, the excitatory responses to subcutaneous formalin injection (50 microliter, 2.5%) were attenuated by subsequent intravenously administration of the NK-1 receptor antagonist CP-96,345 (0.5 mg/kg; n = 8), given 35-40 min after formalin, but not by the inactive enantiomer CP-96,344 (0.5 mg/kg; n = 9). Finally, autoradiographic binding of exogenous [(125)I]BH-substance P in the lumbar cord was reduced at 5 and 25 min after formalin (50 microliter, 1 or 5%), with an intermediate level of reduction at 12 min. These data are interpreted as evidence that the second phase of nociceptive scores in the formalin test is attributable at least partially to tonic activation of NK-1 receptors at the spinal level, whether because of a temporally limited release of substance P, for example only during the first phase, but a slow removal or breakdown of substance P, or, more likely, because of tonic release from primary afferents throughout the second phase. Irrespective of the mechanism, it can be concluded that at least some of the persistent nociceptive effects associated with peripheral inflammation, or at least those provoked by subcutaneous injection of formalin, are mediated via continuous activation of NK-1 receptors at the level of the spinal dorsal horn; this may relate directly to mechanisms underlying prolonged nociceptive pains in humans.

Morphometry of a peptidergic transmitter system: dynorphin B-like immunoreactivity in the rat hippocampal mossy fiber pathway before and after seizures

While the morphometry of classical transmitter systems has been extensively studied, relatively little quantitative information is available on the subcellular distribution of peptidergic dense core vesicles (DCVs) within axonal arbors and terminals, and how distribution patterns change in response to neural activity. This study used correlated quantitative light and electron microscopic immunohistochemistry to examine dynorphin B-like immunoreactivity (dyn B-LI) in the rat hippocampal mossy fiber pathway before and after seizures. Forty-eight hours after seizures induced by two pentylenetetrazol injections, light microscopic dyn B-LI was decreased dorsally and increased ventrally. Ultrastructural examination indicated that, in the hilus of the dentate gyrus, these alterations resulted from changes that were almost entirely restricted to the profiles of the large mossy-like terminals formed by mossy fiber collaterals (which primarily contact spines), compared to the profiles of the smaller, less-convoluted terminals found on the same collaterals (which primarily contact aspiny dendritic shafts). Dorsally, mossy terminal profile labeled DCV (/DCV) density dropped substantially, while ventrally, both mossy terminal profile perimeter and /DCV density increased. In all terminal profile examined, /DCVs also were closely associated with the plasma membrane. Following seizures, there was a reorientation of /DCVs along the inner surface of mossy terminal profile membranes, in relation to the types of profiles adjacent to the membrane: in both the dorsal and ventral hilus, significantly fewer /DCVs were observed at sites apposed to dendrites, and significantly more were observed at sites apposed to spines. Thus, after seizures, changes specific to: (1) the dorsoventral level of the hippocampal formation, (2) the type of terminal, and (3) the type of profile in apposition to the portion of the terminal membrane examined were all observed. An explanation of these complex, interdependent alterations will probably require evoking multiple interrelated mechanisms, including selective prodynorphin synthesis, transport, and release.

Mu opioids enhance mossy fiber synaptic transmission indirectly by reducing GABAB receptor activation

The cellular mechanisms underlying mu opioid facilitation of mossy fiber (MF) long-term potentiation (LTP) and synaptic transmission were investigated in the rat hippocampal slice. Naloxone (10 microM) significantly inhibited the induction of mossy fiber LTP, an effect attributed by Derrick and Martinez [B.E. Derrick, J.L.J. Martinez, Opioid receptor activation is one factor underlying the frequency dependence of mossy fiber LTP induction, J. Neurosci. 14 (1994) 4359-4367] to antagonism of endogenous opioid peptide action. We found that the inhibitory effects of naloxone were not blocked by bicuculline, suggesting that endogenous opioids did not enhance mossy fiber LTP by depressing GABAA inhibition. [d-Ala2, NMePhe4, Glyol5] enkephalin, DAMGO (300 nM), a mu opioid agonist, mimicked the action of endogenous opioids, enhancing both mossy fiber LTP induction and paired-pulse facilitation. DAMGO potentiation of the paired-pulse facilitation of mossy fiber response was also insensitive to bicuculline but was blocked by the mu selective antagonist CTOP. Further analysis of the cellular mechanism showed that the depletion of internal Ca2+ stores by thapsigargin (1 microM), or inhibition of protein kinases by application of staurosporine (1 microM) did not block the DAMGO facilitation of mossy fiber-CA3 synaptic transmission. However, application of phaclofen (100 microM GABAB receptor antagonist or SCH 50911, a more potent GABAB antagonist significantly inhibited the DAMGO effect (49+/-15%; 51+/-19% inhibition, P<0.05). The data indicate that the DAMGO effect on the mossy fiber pathway is partially mediated by a reduction in GABA activation of GABAB receptors. These findings further suggest that endogenous opioid peptides activate mu opioid receptors to facilitate mossy fiber LTP and synaptic transmission in rat hippocampus partially by GABAB receptor-mediated disinhibitory mechanism.

Enkephalin-containing interneurons are specialized to innervate other interneurons in the hippocampal CA1 region of the rat and guinea-pig

Enkephalins are known to have a profound effect on hippocampal inhibition, but the possible endogenous source of these neuropeptides, and their relationship to inhibitory interneurons is still to be identified. In the present study we analysed the morphological characteristics of met-enkephalin-immunoreactive cells in the CA1 region of the rat and guinea-pig hippocampus, their coexistence with other neuronal markers and their target selectivity at the light and electron microscopic levels. Several interneurons in all subfields of the hippocampus were found to be immunoreactive for met-enkephalin. In the guinea-pig, fibres arising from immunoreactive interneurons were seen to form a plexus in the stratum oriens/alveus border zone, and basket-like arrays of boutons on both enkephalin-immunoreactive and immunonegative cell bodies in all strata. Immunoreactive boutons always established symmetric synaptic contacts on somata and dendritic shafts. Enkephalin-immunoreactive cells co-localized GABA, vasoactive intestinal polypeptide and calretinin. Postembedding immunogold staining for GABA showed that all the analysed enkephalin-immunoreactive boutons contacted GABAergic postsynaptic structures. In double-immunostained sections, enkephalin-positive axons were seen to innervate calbindin D28k-, somatostatin-, calretinin- and vasoactive intestinal polypeptideimmunoreactive cells with multiple contacts. Based on these characteristics, enkephalin-containing cells in the hippocampus are classified as interneurons specialized to innervate other interneurons, and represent a subset of vasoactive intestinal polypeptide- and calretinin-containing cells. The striking match of ligand and receptor distribution in the case of enkephalin-mediated interneuronal communication suggests that this neuropeptide may play an important role in the synchronization and timing of inhibition involved in rhythmic network activities of the hippocampus.

Endogenous activation of mu and delta-1 opioid receptors is required for long-term potentiation induction in the lateral perforant path: dependence on GABAergic inhibition

Opioid peptides costored with glutamate have emerged as powerful regulators of long-term potentiation (LTP) induction in several hippocampal pathways. The objectives of the present study were twofold: (1) to identify which opioid receptor types (mu, delta, or kappa) regulate LTP induction at lateral perforant path-granule cell synapses and (2) to test the hypothesis that endogenous opioids regulate LTP induction via modulation of GABAergic inhibition. LTP of lateral perforant path-evoked field EPSPs was induced selectively by high-frequency stimulation applied to the outer third of the molecular layer of the dentate gyrus of rat hippocampal slices. No changes in medial perforant path responses occurred. LTP was blocked when high-frequency stimulation was applied in the presence of the mu receptor antagonist CTAP, the selective delta-1 receptor antagonist BNTX, or the delta-1 and delta-2 receptor antagonist naltrindole. By contrast, the kappa-1 opioid receptor antagonist NBNI had no effect on LTP induction. The role of GABAergic inhibition was investigated by comparing the effect of naloxone on LTP induction in slices maintained in standard buffer and picrotoxin-containing buffer. Naloxone blocked LTP in standard buffer, whereas normal LTP was induced in picrotoxin-treated, disinhibited slices. Finally, NMDA receptor blockade completely inhibited LTP in both standard and disinhibited slices. The results show that mu and delta-1 opioid receptors regulate LTP induction and that this mechanism critically depends on GABAergic inhibition. A key issue then becomes how endogenous opioids fine-tune the activity of intact inhibitory networks in the dentate gyrus, effectively gating synaptic plasticity in specific dendritic strata.

Changes in hippocampal circuitry after pilocarpine-induced seizures as revealed by opioid receptor distribution and activation

The pilocarpine model of temporal lobe epilepsy was used to study the time-dependent changes in dentate gyrus circuitry after seizures. Seizures caused a decrease in mu- and delta-opioid receptor immunoreactive (MOR-IR and DOR-IR, respectively) neurons in the hilus and MOR-IR neurons in the granule cell layer. Additionally, diffuse DOR-IR, MOR-IR, and GABA immunoreactivities (GABA-IR) were increased in the inner molecular layer. Using the in vitro hippocampal slice preparation to study the physiological consequences of the anatomical changes, we found that the disinhibitory effects of the mu-opioid receptor agonist [D-Ala2, MePhe4,Gly-(ol)5]-enkephalin (DAMGO) and the GABAA receptor antagonist bicuculline were greatly depressed 5-13 d after pilocarpine injection but returned to control levels within 6 weeks. The amplitudes of monosynaptic evoked IPSCs and the effects of DAMGO on this parameter were also slightly decreased 5-13 d after pilocarpine injection but significantly increased at 6 weeks. DAMGO significantly decreased the mean amplitude of spontaneous IPSCs (sIPSCs) at 6 weeks after pilocarpine injection but not in controls. The delta-opioid receptor agonist [D-Pen2,5]-enkephalin (DPDPE) principally inhibited excitatory transmission in saline-treated animals without affecting either sIPSCs or evoked IPSCs. The DPDPE-induced inhibition of excitatory transmission became more pronounced at 6 weeks after pilocarpine injection. These results illustrate the anatomical reorganization and functional changes in dentate gyrus circuitry evident in an animal model of temporal lobe epilepsy and provide evidence of compensatory changes after trauma to the hippocampal formation.

Cellular and subcellular localization of delta opioid receptor immunoreactivity in the rat dentate gyrus

To study a potential locus of action of opioids in the rat dentate gyrus, we examined the localization of the delta opioid receptor (DOR) by immunocytochemistry. Two antisera raised to unique, non-overlapping peptide sequences located within the extracellular N-terminal sequence of DOR were tested. By light microscopy, numerous neurons in the central hilar region were intensely labeled for DOR, while the granule cell layer contained light DOR immunoreactivity. To further characterize hilar neuron cell types which contained DOR, sections through the dentate gyrus were double labeled using immunofluorescence with antisera to DOR and either gamma-aminobutyric acid (GABA), neuropeptide Y (NPY), or somatostatin-28 antisera. Most DOR-labeled perikarya also contained GABA and NPY, while a subpopulation contained somatostatin. Electron microscopic examination of sections labeled for DOR revealed that the immunoreactivity was common in profiles which exhibited the morphological characteristics of granule cells, as well as those of non-granule cells. DOR immunoreactivity was located at postsynaptic sites within neuronal perikarya (2%), dendrites (27%), and dendritic spines (22%); as well as in presynaptic axon terminals (25%) and glia (23%) (n = 279). In dendrites and dendritic spines, DOR immunoreactivity was most often associated with the plasmalemmal surface near asymmetric synapses. In axon terminals, DOR immunoreactivity primarily surrounded small, clear vesicles, and was less consistently found on the plasmalemmal surface. The distribution of DOR-labeled profiles overlapped with, but was not restricted to regions known to contain enkephalin. These data suggest that opiates acting at the DOR can modulate both hilar neurons and granule cells both pre- and postsynaptically.

Actions of endogenous opioids on NMDA receptor-independent long-term potentiation in area CA3 of the hippocampus

The opioid peptides represent a major class of neurotransmitter in the vertebrate nervous system and are prevalent in the hippocampus. There is considerable interest in the physiological function of the opioids contained in the mossy fiber pathway. The release of opioids from mossy fibers shows a strong frequency dependence. Long-term potentiation (LTP) at this synapse, an NMDA receptor-independent form of LTP, also depends on high-frequency synaptic activity, and this has led to speculation that endogenous opioids may be a critical factor in LTP induction. Previous reports using extracellular recordings have provided evidence for and against a role for opioids in mossy fiber LTP. Using single-cell recording techniques, we have tested the hypothesis that endogenous opioids are required for mossy fiber LTP induction. We recorded from a defined population of synapses that had EPSCs with fast rise times, short latencies, and monophasic decays, consistent with a proximally terminating synapse. The opioid antagonist naloxone prevented mossy fiber LTP in the rat, but had no effect on the commissural/associational system, a nonopioid-containing pathway. The action of naloxone was not mediated through disinhibition because GABAA receptors were pharmacologically blocked in these experiments. We also tested the hypothesis that variations in postsynaptic receptor subtype distribution between species might explain previous controversies regarding the role of endogenous opioids. In contrast to the rat, LTP of the mossy fiber field potential in guinea pig was not blocked by naloxone. Our data suggest that opioids may be the presynaptically released, frequency-dependent, associative factor for mossy fiber LTP induction.

Associative, bidirectional modifications at the hippocampal mossy fibre-CA3 synapse

Long-term potentiation (LTP) and long-term depression (LTD) are activity-dependent changes in synaptic strength that may serve as the cellular mechanisms of information storage in the vertebrate brain. The mossy fibre-CA3 synapse displays NMDA (M-methyl-D-aspartate) receptor-independent forms of LTP and LTD that were thought to be non-associative. Here we report that the mossy fibre-CA3 synapse displays each of the known types of LTD in vivo, including associative, heterosynaptic and homosynaptic LTD. These types of LTD are induced when only two of the three conditions necessary for mossy fibre LTP induction are provided. Because some of these conditions can be provided by convergent CA3 afferents, each type of LTD can be induced in an associative manner, which suggests that LTD is involved in associative information storage. Similar to the induction of NMDA receptor-dependent LTD and LTP at other cortical synapses, mossy fibre LTD occurs when synaptic conditions are insufficient to induce LTP, and both LTP and LTD induction are influenced by previous synaptic activity, consistent with the view that common principles govern activity-dependent plasticity at cortical synapses.

Endogenous opioid regulation of hippocampal function

Endogenous opioid peptides modulate neural transmission in the hippocampus. Procnkephalin-derived peptides have been demonstrated to act at mu and delta opioid receptors to inhibit GABA release from inhibitory interneurons, resulting in increased excitability of hippocampal pyramidal cells and dentate gyrus granule cells. Prodynorphin-derived peptides primarily act at presynaptic kappa opioid receptors to inhibit excitatory amino acid release from perforant path and mossy fiber terminals. Opioid receptors reduce membrane excitability by modulating ion conductances, and in this way they may decrease voltage-dependent calcium influx and transmitter release. Synaptic plasticity in the hippocampus also is modulated by endogenous opioids. Enkephalins facilitate long-term potentiation, whereas dynorphins inhibit the induction of this type of neuroplasticity. Further, opioids may play important roles in hippocampal epilepsy. Recurrent seizures induce changes in the expression of opioid peptides and receptors. Also, enkephalins have proconvulsant effects in the epileptic hippocampus, whereas dynorphins may function as endogenous anticonvulsants.

Depression of LTP in rat dentate gyrus by naloxone is reversed by GABAA blockade

Long-term potentiation (LTP) of the lateral perforant path (LPP) to dentate granule cell (DGC) synapse is suppressed by the opioid antagonist, naloxone, and thus appears to be dependent upon the release of endogenous opioids from the LPP. It has been suggested that endogenous opioids enhance LTP by depressing GABAA inhibition. As one test of this hypothesis, we determined whether blockade of GABAA inhibition would alleviate the naloxone block of LTP in the LPP. Consistent with the hypothesis that endogenous opioids enable LTP by disinhibition of the DGCs, naloxone no longer blocked LTP in the presence of the GABAA antagonist, bicuculline methiodide. Furthermore, although blockade of mu receptors suppressed LTP of the slope of the population excitatory potential (pEPSP), blockade of both mu and delta opioid receptors was needed to suppress LTP of both the pEPSP and the orthodromic population spike (OPS).

A role for hippocampal opioids in long-term functional plasticity

In the granule cells of the hippocampus, glutamate coexists with opioid peptides derived from the proenkephalin and prodynorphin genes. The functional significance of this coexistence has been unclear but recent evidence suggests that the dynorphins and enkephalins play a crucial role in regulating the efficiency of neurotransmission at granule-cell synapses. Together with evidence that the level of opioid activity in this pathway can change dramatically according to the physiological or pathological state of the tissue, this information focuses attention on granule-cell opioids as primary mediators of hippocampal plasticity.

Ultrastructural heterogeneity of enkephalin-containing terminals in the rat hippocampal formation

Opioid peptides, including leu-enkephalin (LE), are important neuromodulators in the hippocampal formation where they may play a role in learning and memory as well as epileptogenesis. We examined the cellular substrates that underlie the function of LE in each lamina of the rat hippocampal formation by immunocytochemistry at the electron microscopic level in single section analysis. LE-like immunoreactivity (LE-LI) was primarily associated with large dense-core vesicles (80-100 nm), usually found in axons and axon terminals, but was also observed in perikarya and occasionally in dendrites. The morphology and synaptic associations of LE-LI-containing terminals were strikingly distinct in each region of the hippocampal formation. In the molecular layer of the dentate gyrus, terminals with LE-LI were typically small (0.6 microns) and formed primarily asymmetric (excitatory type) synapses on single dendritic spines, which is consistent with the presence of LE in the lateral perforant path. In the hilus of the dentate gyrus, two types of LE-containing terminals were present: (1) small round terminals that were heterogeneous in size (0.4-1 microns) and in type of contact formed and (2) larger (3-5 microns) terminals exhibiting the characteristic morphology of mossy fiber boutons that formed asymmetric synapses on spines. This variation in morphology and the type of contact suggests LE may have a heterogeneous influence on diverse hilar interneurons. In the CA3 region of the hippocampus, LE-LI was localized to large mossy fiber boutons (3-7 microns) that formed multiple asymmetric synapses on complex spiny dendritic processes and often formed puncta adherentia with the shafts of large CA3 pyramidal cell dendrites, indicating that this peptide may be directly released onto pyramidal cells. At the border of stratum radiatum and lacunosum moleculare in the CA1 region of the hippocampus, LE-labeled terminals averaged 0.8 microns in diameter and often formed symmetric (inhibitory type) synapses on dendritic shafts, which is consistent with a role in disinhibition. In conclusion, these heterogeneous cellular interactions indicate that LE has diverse functional roles and mechanisms of action within each lamina of the hippocampal formation and may directly and indirectly modulate hippocampal cell activity.

Immunocytochemical localization of delta opioid receptors in mouse brain

An affinity-purified anti-peptide antibody generated against the carboxy-terminal region of the delta opioid receptor was used to localize delta opioid receptors in mouse brain. delta Opioid receptor immunoreactivity was found in axons and nerve terminals in regions of the olfactory bulb, hippocampal formation, cerebral and cerebellar cortex, midbrain and hindbrain. The immunocytochemical distribution correlated well, though not completely with autoradiographic distribution of delta opioid receptors in mouse brain using either [3H][2-D-penicillamine, 5-D-penicillamine]-enkephalin (DPDPE) or [3H]naltrindole. Confocal microscopy of double-labeled tissue provided direct evidence that delta opioid receptors are principally expressed on GABAergic terminals in the hippocampus. These anatomical findings complement extensive physiological studies to provide a more detailed description of endogenous opioid circuitry.

Frequency-dependent associative long-term potentiation at the hippocampal mossy fiber-CA3 synapse

The mossy fiber-CA3 synapse displays an N-methyl-D-aspartate-receptor-independent mu-opioid-receptor-dependent form of long-term potentiation (LTP) that is thought not to display cooperativity or associativity with coactive afferents. However, because mossy fiber LTP requires repetitive synaptic activity for its induction, we reevaluated cooperativity and associativity at this synapse by using trains of mossy fiber stimulation. Moderate-, but not low-, intensity trains induced mossy fiber LTP, indicating cooperativity. Low-intensity mossy fiber trains that were normally ineffective in inducing LTP could induce mossy fiber LTP when delivered in conjunction with trains delivered to commissural-CA3 afferents. Associative mossy fiber LTP also could be induced with single mossy fiber pulses when delivered with commissural trains in the presence of a mu-opioid-receptor agonist. Our findings suggest a frequency-dependent variation of Hebbian associative LTP induction that is regulated by the release of endogenous opioid peptides.

Endogenous opioid regulation of norepinephrine release in guinea pig hippocampus

Release of endogenous norepinephrine was detected in guinea pig hippocampal slices using a radioligand displacement assay. Focal electrical stimulation released endogenous norepinephrine and caused a calcium-dependent reduction in specific [3H]propranolol binding at beta-adrenergic receptors in the brain slice. The mu-opioid agonist PL017 decreased norepinephrine release, and the inhibition by PL017 could be blocked by the opioid antagonist naloxone. Endogenous opioid peptides concomitantly released by tissue stimulation also decreased norepinephrine release in a naloxone-sensitive manner. These results support the hypothesis that endogenous opioids can regulate excitability in the hippocampus by presynaptic modulation of norepinephrine release.

8-Bromo-cAMP blocks opioid activation of a voltage-gated potassium current in isolated hippocampal neurons

We have previously shown that mu-selective opioid agonists activate both an inward rectifying and a voltage-gated potassium conductance in acutely dissociated non-pyramidal neurons from rat hippocampus. We now report that the opioid-activated voltage-gated potassium conductance was blocked by the membrane permeable cAMP analogue 8-bromo-cAMP. In contrast, 8-bromo-cGMP failed to inhibit opioid activation of the voltage-gated potassium current. These results suggest that the opioid-activated potassium channel is regulated by cAMP-dependent phosphorylation.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.

Focal stimulation of the mossy fibers releases endogenous dynorphins that bind kappa 1-opioid receptors in guinea pig hippocampus

Physiological release of endogenous opioids in guinea pig hippocampal slices was detected in an in vitro competition binding assay using [3H]U69,593, a kappa 1-selective radioligand. Veratridine-induced opioid release caused a decrease in [3H]U69,593 binding that was blocked by either tetrodotoxin addition or the removal of calcium from the incubation buffer. Focal electrical stimulation of opioid peptide-containing afferent pathways resulted in a decrease in [3H]U69,593 binding, whereas stimulation of a major afferent lacking endogenous opioid immunoreactivity had no effect. The addition of 6-cyano-7-nitroquinoxaline-2,3-dione blocked the reduction in [3H]U69,593 binding caused by perforant path stimulation, but not the reduction caused by mossy fiber stimulation, suggesting that the primary source of endogenous kappa ligands was likely to be the dentate granule cells. Antisera against dynorphin A(1-8) or dynorphin B peptides inhibited the effects of mossy fiber stimulation in the [3H]U69,593 displacement assay. Antisera against other prodynorphin- and proenkephalin-derived opioid peptides had no effect. As shown by receptor autoradiography, the distribution of kappa 1 binding sites was limited to the molecular layer of the dentate gyrus and the presubiculum region of temporal hippocampal slices. These results indicate that prodynorphin-derived opioids released under physiological conditions from the mossy fibers act at kappa 1 receptors in the guinea pig dentate gyrus.

Opioids activate both an inward rectifier and a novel voltage-gated potassium conductance in the hippocampal formation

Opioid receptors were found to activate two different types of membrane potassium conductance in acutely dissociated neurons from the CA1/subiculum regions of the adult rat hippocampal formation. Opioid-responsive neurons were distinguished based on their morphology and electrophysiological responses. In one population of neurons having a multipolar, nonpyramidal cell shape, mu-selective opioid agonists increased an inward rectifying potassium current. Opioid activation of the inward rectifying conductance resulted in small outward potassium currents at resting membrane potentials and increased inward currents at hyperpolarized potentials. In a second population of nonpyramidal neurons, mu opioid agonists increased a novel voltage-gated potassium current. This current was blocked by internal CsCl2, unaffected by external BaCl2 or CdCl2, irreversibly activated by intracellular GTP-gamma-S, and inactivated by sustained depolarization. In contrast to the inward rectifying conductance, the voltage-gated conductance was not activated at resting membrane potentials or hyperpolarized potentials. The opioid-activated, voltage-gated conductance represents a new class of G protein-regulated potassium current in the brain.