Delta opioid receptor activation is required to induce LTP of synaptic transmission in the lateral perforant path in vivo

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

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

Mature granule cells of the dentate gyrus--Passive bystanders or principal performers in hippocampal function?

The dentate gyrus is the main entrance of highly processed information to the hippocampus which derives from associative cortices and it is one of the few privileged areas in the brain where adult neurogenesis occurs. This creates the unique situation that neurons of diverse maturation stages are part of one neuronal network at any given point in life. While recently adult-born cells have a low induction threshold for long-term potentiation several studies suggest that following maturation granule cells are poorly excitable and they exhibit reduced Hebbian synaptic plasticity to an extent that it was even suggested that they functionally retire. Here, we review the functional properties of mature granule cells and discuss how plasticity of intrinsic excitability and alterations in excitation-inhibition balance might impact on their role in hippocampal information processing.

Delta Opioid Receptors: Learning and Motivation

Delta opioid receptor (DOR) displays a unique, highly conserved, structure and an original pattern of distribution in the central nervous system, pointing to a distinct and specific functional role among opioid peptide receptors. Over the last 15 years, in vivo pharmacology and genetic models have allowed significant advances in the understanding of this role. In this review, we will focus on the involvement of DOR in modulating different types of hippocampal- and striatal-dependent learning processes as well as motor function, motivation, and reward. Remarkably, DOR seems to play a key role in balancing hippocampal and striatal functions, with major implications for the control of cognitive performance and motor function under healthy and pathological conditions.

Pronounced differences in signal processing and synaptic plasticity between piriform-hippocampal network stages: a prominent role for adenosine

KEY POINTS

Extended trains of theta rhythm afferent activity lead to a biphasic response facilitation in field CA1 but not in the lateral perforant path input to the dentate gyrus. Processes that reverse long-term potentiation in field CA1 are not operative in the lateral perforant path: multiple lines of evidence indicate that this reflects differences in adenosine signalling. Adenosine A1 receptors modulate baseline synaptic transmission in the lateral olfactory tract but not the associational afferents of the piriform cortex. Levels of ecto-5'-nucleotidase (CD73), an enzyme that converts extracellular ATP into adenosine, are markedly different between regions and correlate with adenosine signalling and the efficacy of theta pulse stimulation in reversing long-term potentiation. Variations in transmitter mobilization, CD73 levels, and afferent divergence result in multivariate differences in signal processing through nodes in the cortico-hippocampal network.

ABSTRACT

The present study evaluated learning-related synaptic operations across the serial stages of the olfactory cortex-hippocampus network. Theta frequency stimulation produced very different time-varying responses in the Schaffer-commissural projections than in the lateral perforant path (LPP), an effect associated with distinctions in transmitter mobilization. Long-term potentiation (LTP) had a higher threshold in LPP field potential studies but not in voltage clamped neurons; coupled with input/output relationships, these results suggest that LTP threshold differences reflect the degree of input divergence. Theta pulse stimulation erased LTP in CA1 but not in the dentate gyrus (DG), although adenosine eliminated potentiation in both areas, suggesting that theta increases extracellular adenosine to a greater degree in CA1. Moreover, adenosine A1 receptor antagonism had larger effects on theta responses in CA1 than in the DG, and concentrations of ecto-5'-nucleotidase (CD73) were much higher in CA1. Input/output curves for two connections in the piriform cortex were similar to those for the LPP, whereas adenosine modulation again correlated with levels of CD73. In sum, multiple relays in a network extending from the piriform cortex through the hippocampus can be differentiated along three dimensions (input divergence, transmitter mobilization, adenosine modulation) that potently influence throughput and plasticity. A model that incorporates the regional differences, supplemented with data for three additional links, suggests that network output goes through three transitions during the processing of theta input. It is proposed that individuated relays allow the circuit to deal with different types of behavioural problems.

Deletion of the NMDA receptor GluN2A subunit significantly decreases dendritic growth in maturing dentate granule neurons

It is known that NMDA receptors can modulate adult hippocampal neurogenesis, but the contribution of specific regulatory GluN2 subunits has been difficult to determine. Here we demonstrate that mice lacking GluN2A (formerly NR2A) do not show altered cell proliferation or neuronal differentiation, but present significant changes in neuronal morphology in dentate granule cells. Specifically, GluN2A deletion significantly decreased total dendritic length and dendritic complexity in DG neurons located in the inner granular zone. Furthermore, the absence of GluN2A also resulted in a localized increase in spine density in the middle molecular layer, a region innervated by the medial perforant path. Interestingly, alterations in dendritic morphology and spine density were never seen in dentate granule cells located in the outer granular zone, a region that has been hypothesized to contain older, more mature, neurons. These results indicate that although the GluN2A subunit is not critical for the cell proliferation and differentiation stages of the neurogenic process, it does appear to play a role in establishing synaptic and dendritic morphology in maturing dentate granule cells localized in the inner granular zone.

The role of δ-opioid receptors in learning and memory underlying the development of addiction

Opioids are important endogenous ligands that exist in both invertebrates and vertebrates and signal by activation of opioid receptors to produce analgesia and reward or pleasure. The μ-opioid receptor is the best known of the opioid receptors and mediates the acute analgesic effects of opiates, while the δ-opioid receptor (DOR) has been less well studied and has been linked to effects that follow from chronic use of opiates such as stress, inflammation and anxiety. Recently, DORs have been shown to play an essential role in emotions and increasing evidence points to a role in learning actions and outcomes. The process of learning and memory in addiction has been proposed to involve strengthening of specific brain circuits when a drug is paired with a context or environment. The DOR is highly expressed in the hippocampus, amygdala, striatum and other basal ganglia structures known to participate in learning and memory. In this review, we will focus on the role of the DOR and its potential role in learning and memory underlying the development of addiction.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Comparison of interval timing behaviour in mice following dorsal or ventral hippocampal lesions with mice having δ-opioid receptor gene deletion

Mice with cytotoxic lesions of the dorsal hippocampus (DH) underestimated 15 s and 45 s target durations in a bi-peak procedure as evidenced by proportional leftward shifts of the peak functions that emerged during training as a result of decreases in both 'start' and 'stop' times. In contrast, mice with lesions of the ventral hippocampus (VH) displayed rightward shifts that were immediately present and were largely limited to increases in the 'stop' time for the 45 s target duration. Moreover, the effects of the DH lesions were congruent with the scalar property of interval timing in that the 15 s and 45 s functions superimposed when plotted on a relative timescale, whereas the effects of the VH lesions violated the scalar property. Mice with DH lesions also showed enhanced reversal learning in comparison to control and VH lesioned mice. These results are compared with the timing distortions observed in mice lacking δ-opioid receptors (Oprd1(-/-)) which were similar to mice with DH lesions. Taken together, these results suggest a balance between hippocampal-striatal interactions for interval timing and demonstrate possible functional dissociations along the septotemporal axis of the hippocampus in terms of motivation, timed response thresholds and encoding in temporal memory.

Impaired hippocampus-dependent and facilitated striatum-dependent behaviors in mice lacking the δ opioid receptor

Pharmacological data suggest that delta opioid receptors modulate learning and memory processes. In the present study, we investigated whether inactivation of the delta opioid receptor modifies hippocampus (HPC)- and striatum-dependent behaviors. We first assessed HPC-dependent learning in mice lacking the receptor (Oprd1(-/-) mice) or wild-type (WT) mice treated with the delta opioid antagonist naltrindole using novel object recognition, and a dual-solution cross-maze task. Second, we subjected mutant animals to memory tests addressing striatum-dependent learning using a single-solution response cross-maze task and a motor skill-learning task. Genetic and pharmacological inactivation of delta opioid receptors reduced performance in HPC-dependent object place recognition. Place learning was also altered in Oprd1(-/-) animals, whereas striatum-dependent response and procedural learning were facilitated. Third, we investigated the expression levels for a large set of genes involved in neurotransmission in both HPC and striatum of Oprd1(-/-) mice. Gene expression was modified for several key genes that may contribute to alter hippocampal and striatal functions, and bias striatal output towards striatonigral activity. To test this hypothesis, we finally examined locomotor effects of dopamine receptor agonists. We found that Oprd1(-/-) and naltrindole-treated WT mice were more sensitive to the stimulant locomotor effect of SKF-81297 (D1/D5), supporting the hypothesis of facilitated striatonigral output. These data suggest, for the first time, that delta receptor activity tonically inhibits striatal function, and demonstrate that delta opioid receptors modulate learning and memory performance by regulating the HPC/striatum balance.

Delta opioid receptors colocalize with corticotropin releasing factor in hippocampal interneurons

The hippocampal formation (HF) is an important site at which stress circuits and endogenous opioid systems intersect, likely playing a critical role in the interaction between stress and drug addiction. Prior study findings suggest that the stress-related neuropeptide corticotropin releasing factor (CRF) and the delta opioid receptor (DOR) may localize to similar neuronal populations within HF lamina. Here, hippocampal sections of male and cycling female adult Sprague-Dawley rats were processed for immunolabeling using antisera directed against the DOR and CRF peptide, as well as interneuron subtype markers somatostatin or parvalbumin, and analyzed by fluorescence and electron microscopy. Both DOR- and CRF-labeling was observed in interneurons in the CA1, CA3, and dentate hilus. Males and normal cycling females displayed a similar number of CRF immunoreactive neurons co-labeled with DOR and a similar average number of CRF-labeled neurons in the dentate hilus and stratum oriens of CA1 and CA3. In addition, 70% of DOR/CRF dual-labeled neurons in the hilar region co-labeled with somatostatin, suggesting a role for these interneurons in regulating perforant path input to dentate granule cells. Ultrastructural analysis of CRF-labeled axon terminals within the hilar region revealed that proestrus females have a similar number of CRF-labeled axon terminals that contain DORs compared to males but an increased number of CRF-labeled axon terminals without DORs. Taken together, these findings suggest that while DORs are anatomically positioned to modulate CRF immunoreactive interneuron activity and CRF peptide release, their ability to exert such regulatory activity may be compromised in females when estrogen levels are high.

Prenatal morphine exposure attenuates the maintenance of late LTP in lateral perforant path projections to the dentate gyrus and the CA3 region in vivo

Previously we reported that prenatal exposure to morphine twice daily during gestation decreases proenkephalin levels in adult progeny within the brain, including the dentate gyrus, and alters mu and delta opioid receptors in the hippocampal CA3 region. The lateral aspect of the perforant path contains and releases enkephalin-derived opioid peptides, and induction of long-term potentiation (LTP) in lateral perforant path projections to both the dentate gyrus and the hippocampal CA3 region is blocked by antagonists of opioid receptors. Thus LTP induction at these synapses involves opioid receptor activation mediated by the release of proenkephalin-derived opioid peptides with lateral perforant path activation. Here we show in adult behaving animals, neither LTP induction nor the early phase of LTP (E-LTP) maintenance is altered by prenatal morphine exposure in the lateral perforant path projections to the dentate gyrus and the CA3 region. However, maintenance and longevity of late LTP (L-LTP), as reflected in the magnitude of LTP over days, was attenuated in animals prenatally exposed to morphine. In contrast, in medial perforant path projections to the dentate gyrus and CA3 region, both LTP induction and the maintenance of E- and L-LTP were unaffected by prenatal morphine treatment. Thus a brief prenatal exposure to the opiate morphine produces sustained, and possibly permanent, alterations in L-LTP in the opioidergic lateral perforant path projection. This suggests that prenatal morphine exposure disrupts LTP via disruption of opioid mechanisms involved in LTP maintenance or via disruption of opioid receptor activation during LTP induction, which can subsequently alter LTP maintenance.

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.

Running has differential effects on NPY, opiates, and cell proliferation in an animal model of depression and controls

Physical activity has documented beneficial effect in treatment of depression. Recently, we found an antidepressant-like effect of running in an animal model of depression, the Flinders Sensitive Line (FSL) and demonstrated that it was associated with increased hippocampal cell proliferation. In this study, we analyzed levels of mRNAs encoding the neuropeptide Y (NPY) and the opioid peptides dynorphin and enkephalin in hippocampus and correlated these to cell proliferation in the FSL and in the 'nondepressed' Flinders Resistant Line (FRL) strain, with/without access to running wheels. Running increased NPY mRNA in dentate gyrus and the CA4 region in FSL, but not in FRL rats. NPY mRNA increase was correlated to increased cell proliferation in the subgranular zone of dentate gyrus. Baseline dynorphin and enkephalin mRNA levels in the dentate gyrus were lower in the FSL compared to the FRL strain. Running had no effect on dynorphin and enkephalin mRNAs in the FSL strain but it decreased dynorphin mRNA, and there was a trend to increased enkephalin mRNA in the FRL rats. Thus, it would appear that the CNS effects of running are different in 'depressed' and control animals; modification of NPY, a peptide associated with depression and anxiety, in depressed animals, vs effects on opioids, associated with the reward systems, in healthy controls. Our data support the hypothesis that NPY neurotransmission in hippocampus is malfunctioning in depression and that antidepressive treatment, in this case wheel running, will normalize it. In addition, we also show that the increased NPY after running is correlated to increased cell proliferation, which is associated with an antidepressive-like effect.

NMDA receptor antagonists block heterosynaptic long-term depression (LTD) but not long-term potentiation (LTP) in the CA3 region following lateral perforant path stimulation

High-frequency stimulation of lateral perforant path is accompanied by a heterosynaptic long-term depression (LTD) of medial perforant path synaptic responses in both the dentate gyrus and the CA3 region of the hippocampus. We reported previously that LTP induction at lateral perforant path-CA3 synapses is unaffected by NMDA antagonists. However, it is not known if heterosynaptic LTD that is observed in the CA3 region following lateral perforant path stimulation also is independent from NMDA receptors. We address this question in anesthetized adult rats using systemic administration of the competitive NMDA receptor antagonist CPP. Induction of lateral perforant path-CA3 LTP produced a sustained heterosynaptic depression of medial perforant path-CA3 responses. Systemic administration of CPP (10 mg/kg) was ineffective in blocking the induction of LTP at lateral perforant path-CA3 responses. However, heterosynaptic LTD of medial perforant path-CA3 responses was blocked completely by CPP. These data indicate that NMDA receptors are not required for the induction of lateral perforant path-CA3 LTP, but are involved in the induction of heterosynaptic LTD that accompanies lateral perforant path activity. The requirement for NMDA receptors for heterosynaptic LTD suggests one functional role of NMDA receptors at termination fields of the lateral perforant path.

Fimbrial control of bidirectional synaptic plasticity of medial perforant path-dentate transmission

Lesions of the fimbria-fornix (FF) tract cause profound impairments of cognitive ability in animals. Our previous study showed that spatial performance correlates with long-term potentiation (LTP) of the dentate gyrus (DG), but not of the CA1 region, in rats with bilateral FF lesions, suggesting that FF lesions selectively inhibited LTP in the DG. The cortical input to the DG is anatomically and physiologically divided into two types of afferents, i.e., the medial perforant path (MPP) and the lateral perforant path (LPP), which show distinct synaptic properties. To elucidate the difference in the FF modulation of these two inputs, field responses were recorded from MPP- or LPP-DG synapses in anesthetized rats. MPP-DG synapses of rats with FF lesions displayed neither LTP in response to theta-burst stimulation nor long-term depression (LTD) in response to low-frequency burst stimulation. In contrast to the MPP, LPP-DG synapses showed normal LTP in rats with FF lesions. The low-frequency burst stimulation could not induce LTD at LPP-DG synapses in either intact or FF-lesioned rats. These results suggest that the FF pathway selectively supports the mechanisms of bidirectional synaptic plasticity at MPP-DG synapses. This study provides new insights into external control of information processing in the hippocampus.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

Somatostatin depresses long-term potentiation and Ca2+ signaling in mouse dentate gyrus

The selective loss of somatostatin (SST)-containing interneurons from the hilus of the dentate gyrus is a hallmark of epileptic hippocampus. The functional consequence of this loss, including its contribution to postseizure hyperexcitability, remains unclear. We address this issue by characterizing the actions of SST in mouse dentate gyrus using electrophysiological techniques. Although the majority of dentate SST receptors are located in the outer molecular layer adjacent to lateral perforant path (LPP) synapses, we found no consistent action of SST on standard synaptic responses generated at these synapses. However, when SST was present during application of high-frequency trains that normally generate long-term potentiation (LTP), the induction of LTP was impaired. SST did not alter the maintenance of LTP when applied after its induction. To examine the mechanism by which SST inhibits LTP, we recorded from dentate granule cells and examined the actions of this neuropeptide on synaptic transmission and postsynaptic currents. Unlike findings in the CA1 hippocampus, we observed no postsynaptic actions on K(+) currents. Instead, SST inhibited Ca(2+)/Ba(2+) spikes evoked by depolarization. This inhibition was dependent on N-type Ca(2+)currents. Blocking these currents also blocked LTP, suggesting a mechanism through which SST may inhibit LTP. Our results indicate that SST reduction of dendritic Ca(2+) through N-type Ca(2+) channels may contribute to modulation of synaptic plasticity at LPP synapses. Therefore the loss of SST function postseizure could result in abnormal synaptic potentiation that contributes to epileptogenesis.

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.

Simultaneous activation and opioid modulation of long-term potentiation in the dentate gyrus and the hippocampal CA3 region after stimulation of the perforant pathway in freely moving rats

Recent investigations indicate monosynaptic activation by the perforant pathway (pp) of the dentate gyrus and the CA3 region. While short-term potentiation and long-term potentiation (LTP) and its opioid modulation are frequently described for the dentate gyrus, data for the CA3 region are rare. Therefore, evoked potentials and opioid modulation of LTP were directly compared in both target regions of the pp. Male Wistar rats were chronically implanted with a bipolar stimulation electrode in the pp (angular bundle) and two recording electrodes in the dorsal dentate gyrus and the CA3 region. Stimulation of the pp in the freely behaving animals induced short-latency evoked potentials in both target structures which were compared with respect to waveform, latency, amplitude and signs of short- and long-term neuronal plasticity. The short-latency potential in the CA3 region seemed to be a monosynaptic potential which displayed LTP sensitive to the N-methyl-D-aspartate receptor antagonist, MK 801, and depotentiating stimulation. After application of specific opioid antagonists at the mu-, delta- and kappa-opioid receptor subtypes, naloxone, funaltrexamine, naltrindole and binaltorphimine, different effects on induction and maintenance of LTP of the population spike were found both within the dentate gyrus and between the dentate gyrus and the CA3 region. The results show marked diminution of LTP in the dentate gyrus only for naloxone and naltrindole and only small, if any, effects of naloxone on LTP in the CA3 region. Thus, neuronal plasticity in the direct perforant pathway input to the CA3 region seems not to be under such substantial opioidergic control. LTP would be inducible in that region even when LTP in the input formation, the dentate gyrus, and transsynaptic LTP via the mossy fibres are blocked.

Preferential cytoplasmic localization of delta-opioid receptors in rat striatal patches: comparison with plasmalemmal mu-opioid receptors

The activation of delta-opioid receptors (DORs) in the caudate-putamen nucleus (CPN) produces regionally distinct changes in motor functions, many of which are also influenced by opioids active at micro-opioid receptors (MORs). These actions most likely occur in MOR-enriched patch compartments in the CPN. To determine the functional sites for DOR activation and potential interactions involving MOR in these regions, immunoperoxidase and immunogold-silver labeling methods were applied reversibly for the ultrastructural localization of DOR and MOR in single rat brain sections containing patches of the CPN. DOR immunoreactivity was commonly seen within the cytoplasm of spiny and aspiny neurons, many of which also expressed MOR. In dendrites and spines, DOR labeling was preferentially localized to membranes of the smooth endoplasmic reticulum and spine apparatus, whereas MOR showed a prominent plasmalemmal distribution. DOR- and/or MOR-labeled spines received asymmetric, excitatory synapses, some of which showed notable perforations, suggesting the involvement of these receptors in activity-dependent synaptic plasticity. DORs were more frequently detected than were MORs within axon terminals that formed either asymmetric synapses with spine heads or symmetric synapses with spine necks. Our results suggest that in striatal patches, DORs, often in cooperation with MORs, play a direct modulatory role in controlling the postsynaptic excitability of spines, whereas presynaptic neurotransmitter release onto spines is mainly influenced by DOR activation. In comparison with MOR, the prevalent association of DOR with cytoplasmic organelles that are involved in intracellular trafficking of cell surface proteins suggests major differences in availability of these receptors to extracellular opioids.

Effects of adult neurogenesis on synaptic plasticity in the rat dentate gyrus

Ongoing neurogenesis in the adult hippocampal dentate gyrus (DG) generates a substantial population of young neurons. This phenomenon is present in all species examined thus far, including humans. Although the regulation of adult neurogenesis by various physiologically relevant factors such as learning and stress has been documented, the functional contributions of the newly born neurons to hippocampal functions are not known. We investigated possible contributions of the newly born granule neurons to synaptic plasticity in the hippocampal DG. In the standard hippocampal slice preparation perfused with artificial cerebrospinal fluid (ACSF), a small (10%) long-term potentiation (LTP) of the evoked field potentials is seen after tetanic stimulation of the afferent medial perforant pathway (MPP). The induction of this ACSF-LTP is resistant to a N-methyl-D-aspartate (NMDA) receptor blocker, D,L-2-amino-5-phosphonovaleric acid (APV), but is completely prevented by ifenprodil, a blocker of NR2B subtype of NMDA receptors. In contrast, slices perfused with picrotoxin (PICRO), a GABA-receptor blocker, revealed a larger (40--50%), APV-sensitive but ifenprodil-insensitive LTP. The ACSF-LTP required lower frequency of stimulation and fewer stimuli for its induction than the PICRO-LTP. All these characteristics of ACSF-LTP are in agreement with the properties of the putative individual new granule neurons examined previously with the use of the whole cell recording technique in a similar preparation. A causal relationship between neurogenesis and ACSF-LTP was confirmed in experiments using low dose of gamma radiation applied to the brain 3 wk prior to the electrophysiological experiments. In these experiments, the new cell proliferation was drastically reduced and ACSF-LTP was selectively blocked. We conclude that the young, adult-generated granule neurons play a significant role in synaptic plasticity in the DG. Since DG is the major source of the afferent inputs into the hippocampus, the production and the plasticity of new neurons may have an important role in the hippocampal functions such as learning and memory.

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.

Dynorphin selectively augments the M-current in hippocampal CA1 neurons by an opiate receptor mechanism

Most electrophysiological studies of opioids on hippocampal principal neurons have found indirect actions, usually through interneurons. However, our laboratory recently found reciprocal alteration of the voltage-dependent K(+) current, known as the M-current (I(M)), by kappa and delta opioid agonists in CA3 pyramidal neurons. Recent ultrastructural studies have revealed postsynaptic delta opiate receptors on dendrites and cell bodies of CA1 and CA3 hippocampal pyramidal neurons (HPNs). Reasoning that previous electrophysiological studies may have overlooked voltage-dependent postsynaptic effects of the opioids in CA1, we reevaluated their role in CA1 HPNs using the rat hippocampal slice preparation for intracellular current- and voltage-clamp recording. None of the delta and mu; receptor-selective opioids tested, including [D-Pen(2,5)]-enkephalin (DPDPE), [D-Ala(2)]-deltorphin II (deltorphin), [D-Ala(2), NMe-Phe(4), Gly-ol]-enkephalin (DAMGO), and [D-Ala(2), D-Leu(5)] enkephalin (DADLE), altered membrane properties such as I(M) or Ca(2+)-dependent spikes in CA1 HPNs. The nonopioid, Des-Tyr-dynorphin (D-T-dyn), also had no effect. By contrast, dynorphin A (1-17) markedly increased I(M) at low concentrations and caused an outward current at depolarized membrane potentials. The opioid antagonist naloxone and the kappa receptor antagonist nor-binaltorphimine (nBNI) blocked the I(M) effect. However, the kappa-selective agonists U69,593 and U50,488h did not significantly alter I(M) amplitudes when averaged over all cells tested, although occasional cells showed an I(M) increase with U50,488h. Our results suggest that dynorphin A postsynaptically modulates the excitability of CA1 HPNs through opiate receptors linked to voltage-dependent K(+) channels. These findings also provide pharmacological evidence for a functional kappa opiate receptor subtype in rat CA1 HPNs but leave unanswered questions on the role of delta receptors in CA1 HPNs.

Lesions of the medial or lateral perforant path have different effects on hippocampal contributions to place learning and on fear conditioning to context

The axons of the neurons in the medial and lateral components of the entorhinal cortex (MEC and LEC) form the medial and lateral perforant paths (MPP and LPP) which represent the major source of cortical input to the hippocampus. Anatomical, physiological, and pharmacological studies have shown that MPP and LPP are distinct. Unfortunately, assessment of the functional significance of damage to either of these pathways has not used tasks known to be sensitive to hippocampal function in the rodent. In this study, we performed dissociated lesions of MPP and LPP using a combined physiological and anatomical method. Rats with lesions of either the MPP or the LPP were tested on place learning in the water task and on a discriminative fear conditioning to context task. The results indicated that the MPP, but not LPP, lesions resulted in impaired place learning. The context discrimination data revealed an amygdala-like, reduced fear effect of MPP lesions and an enhanced discriminative fear conditioning to context effect of LPP lesions. Consistent with a two-stage model of spatial learning proposed by Buzsaki (Buzsaki G. Two-stage model of memory trace formation: a role for 'noisy' brain states. Neuroscience 1989;31(3):551-570), the impairment in the water task can be interpreted as reflecting the higher efficiency of the MPP synapses in activating hippocampal neurons. The context discrimination results can be explained by either a dissociation of sensory information that reaches the MEC and LEC, or alternatively, by a dissociation between the limbic nature of the MEC and the sensory nature of the LEC.

Hippocampal theta activity after systemic administration of a non-peptide delta-opioid agonist in freely-moving rats: relationship to D1 dopamine receptors

Hippocampal theta activity was acquired and processed off-line from digitized EEG recordings after subcutaneous (s.c.) administration of the non-opioid delta agonist BW 373U86 (0.5-2.5 mg/kg) in freely-moving rats. Relative theta power spectral analysis, implemented by a signal processing software, showed that BW 373U86 induced a dose-dependent increase in the slow component of theta band (Type 2 theta), while movement-related fast theta band (Type 1 theta) failed to show significant changes. Moreover, the increase in relative Type 2 theta power showed a maximal change at 1 mg/kg of BW 373U86, while higher doses, although effective in increasing relative Type 2 theta, induced locomotion and irregularly increased Type 1 hippocampal theta activity. The administration of 10.0 mg/kg of the delta antagonist Naltrindole (NLI) 30 min before BW 373U86, abolished hippocampal Type 2 theta increase. The rise of relative Type 2 theta power induced by BW 373U86 (1-2.5 mg/kg) was greatly attenuated by 0.1 mg/kg of the selective dopamine (DA) D1 antagonist SCH 23390. Administration of 0.1 mg/kg of SCH 23390 alone did not modify hippocampal Type 2 theta. These results indicate that delta receptors modulate the expression of hippocampal Type 2 theta and dopamine, through D1 receptors, exerts a permissive role on this influence.

Involvement of cAMP-dependent protein kinase in mu-opioid modulation of NMDA-mediated synaptic currents

We have previously reported dual effects of mu-opioids on N-methyl-D-aspartate (NMDA)-receptor-mediated synaptic events in the hippocampal dentate gyrus: an indirect facilitating effect via suppression of GABAergic interneurons (disinhibition) and a direct inhibitory effect in the presence of gamma-aminobutyric acid-A (GABA(A)) antagonists. The cellular mechanism underlying the inhibitory effect of mu-opioids remains to be determined. In the present study we examine the role of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) in mu-opioid-induced inhibition of NMDA currents in rat hippocampal slices. NMDA-receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) were evoked by stimulating the lateral perforant path and were recorded from dentate granule cells with the use of whole cell voltage-clamp techniques in the presence of the GABA(A) antagonist and a non-NMDA type of glutamate receptor antagonist. Two selective mu-agonists, [N-MePhe3, D-Pro4]-morphiceptin and [D-Ala2, N-MePhe4, Gly-ol5]-enkephalin, induced dose-dependent inhibition of NMDA EPSCs in a concentration range of 0.3-10 microM. This inhibitory effect could be completely reversed by the opioid antagonists naloxone or prevented by a selective mu-antagonist cyprodime, but was not affected by removal of Mg2+ from the external perfusion medium. Intracellular application of pertussis toxin (PTX) into the granule cell via whole cell recording pipettes completely prevented mu-opioid-induced reduction in NMDA currents, suggesting that a postsynaptic mechanism involving PTX-sensitive G proteins might be responsible for the inhibitory action of mu-opioids. Further studies were conducted to identify the intracellular messengers that coupled with G proteins and transduced the effect of mu-opioids in granule cells. The adenylate cyclase activator forskolin was found to enhance NMDA-receptor-mediated synaptic responses and to reverse the inhibitory effect of mu-opioids. Sp-cAMPS, a specific PKA activator, also enhanced NMDA EPSCs, whereas the PKA inhibitor Rp-cAMPS reduced NMDA EPSCs and occluded further inhibition of the current by mu-opioids. These findings strongly suggest that NMDA receptor function is subject to the modulation by PKA, and that mu-opioids can inhibit NMDA currents through suppression of the cAMP cascade in the postsynaptic neuron. Combined with our previous findings, the present results also indicate that mu-opioids can modulate NMDA-receptor-mediated synaptic activity in a complex manner. The net effect of mu-opioids in the dentate gyrus may depend on the interplay between its disinhibitory action, which facilitates NMDA-receptor-mediated responses, and its inhibitory action on the cAMP cascade.

Effect of GABA(B) receptors on synaptic interactions in dentate gyrus granule neurons of the rat

Dendritic arborization permits convergence of synaptic inputs and their integration in single neurons. The granule neuron in the dentate gyrus represents a relatively simple example where anatomically and functionally distinct medial and lateral perforant pathways terminate on different regions of the dendritic tree. High-frequency stimulation of either pathway alone results in the induction of long-term potentiation. However, whether the potentiated synapses in different parts of the dendrites interact is not known. In this study we have compared long-term potentiation and synaptic interactions in the lateral and medial perforant pathways in the "disinhibited" hippocampal slice preparation in the presence of the GABA(A) receptor blocker bicuculline. The data show that the magnitude of long-term potentiation induced by tetanic stimulation was similar in both pathways, but differences between the two pathways were revealed after two or more tetanizations. A significantly smaller capacity for further long-term potentiation in the lateral, as compared to the medial, perforant pathway was found and can be attributed to stronger postsynaptic GABA(B) inhibition in distal dendrites of granule neurons. Blockade of GABA(B) inhibition with CGP36742 (100 microM) unmasked additional long-term potentiation in the lateral pathway. Presynaptically, GABA(B) receptors produced a short-lasting heterosynaptic depression in the medial pathway, which was reduced by CGP36742. Coincident activation of the two pathways boosted long-term potentiation only in the medial pathway. We propose that the interactions between the two pathways are orchestrated to maximize associative long-term potentiation in the medial pathway; this may be important for types of learning attributed to the hippocampus.

LTP in the lateral perforant path is beta-adrenergic receptor-dependent

Norepinephrine induces an activity-independent long-lasting depression of synaptic transmission in the lateral perforant path input to dentate granule cells, whereas high frequency stimulation induces activity-dependent long-term potentiation (LTP). We investigated the role of endogenous activation of beta-adrenergic receptors in LTP of the lateral and medial perforant paths under conditions affording selective stimulation of these pathways in the rat hippo-campal slice. Propranolol (1 microM), a beta-receptor antagonist, blocked LTP induction of both lateral and medial perforant path-evoked field excitatory postsynaptic potentials. The results indicate a broad requirement for norepinephrine in different types of synaptic plasticity, including activity-independent depression and activity-dependent LTP in the lateral perforant path.

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.

Orphanin FQ inhibits synaptic transmission and long-term potentiation in rat hippocampus

It is known that opioid peptides acting on opioid receptors can modulate hippocampal synaptic functions. Although a novel member of the opioid receptor family, ORL1 receptors, that displays high-sequence homology with classical opioid receptors is abundant in the hippocampus, little is known regarding its role in synaptic function. The present study was designed to investigate whether activation of the ORL1 receptor by its natural ligand, orphanin FQ, could modulate synaptic transmission and synaptic plasticity in the hippocampus. The actions of orphanin FQ in the CA1 and dentate gyrus were examined by field potential recordings in response to stimulation of Schaffer collaterals and perforant path, respectively. Our results showed that orphanin FQ, but not the inactive analog des-Phe1-orphanin FQ, reduced both the slope of the excitatory postsynaptic potentials and population spike amplitude. The inhibitory effect of orphanin FQ is dose dependent and probably involves a presynaptic mechanism, as suggested by the significantly increased paired-pulse facilitation evoked in the presence of orphanin FQ. In addition, orphanin FQ was found to inhibit the induction of long-term potentiation at the Schaffer collateral-CA1 synapse. These results demonstrate that orphanin FQ can function as an inhibitory modulator regulating synaptic transmission and synaptic plasticity in the hippocampus, suggesting that activation of ORL1 receptors may play an important role in synaptic plasticity involved in learning and memory.

Ultrastructural relationships between leu-enkephalin- and GABA-containing neurons differ within the hippocampal formation

Electrophysiological studies have suggested that the excitatory actions of opioids in the hippocampal formation are mediated by inhibition of interneurons containing GABA; however, an anatomical basis for this interaction has never been established. Thus, we sought to determine the relationship between leu-enkephalin (LE)-containing axon terminals and GABAergic neurons using dual labeling immunohistochemistry and electron microscopy. In the CA1 region of the hippocampus, LE-labeled terminals (n = 99) were in direct contact with GABA-labeled perikarya and dendrites (18%), and directly apposed to GABA-labeled axon terminals (14%). In the molecular layer of the dentate gyrus, LE-containing terminals (n = 125) occasionally apposed GABA-containing terminals (8%). In the hilus of the dentate gyrus, LE-containing terminals (n = 165) often contacted GABA-containing perikarya and dendrites (39%), but rarely apposed GABA-containing terminals (3%). In the CA3 region of the hippocampus, only a few LE-labeled mossy fiber boutons (n = 102) contacted the shafts of GABA-labeled dendrites (4%). The results demonstrate that leu-enkephalin-containing terminals have a different anatomical relationship with GABA-containing profiles in each subregion of the hippocampal formation. In the CA1 region of the hippocampus, the data support the numerous electrophysiological studies indicating that LE functions in modulating inhibitory GABAergic neurons by both pre- and postsynaptic mechanisms. In the outer molecular layer of the dentate gyrus the localization suggests some presynaptic regulation of GABAergic terminals. In the hilus of the dentate gyrus, the study also supports the contention that LE may have an important role in regulating inhibition of GABA-containing neurons. In comparison, in the CA3 region of the hippocampus, LE may have a more limited role in regulating GABAergic inhibition by direct association.

Long-term potentiation and learning

Long-term potentiation (LTP), a relatively long-lived increase in synaptic strength, remains the mot popular model for the cellular process that may underlie information storage within neural systems. The strongest arguments for a role of LTP in memory are theoretical and involve Hebb's Postulate, Marr's theory of hippocampal function, and neural network theory. Considering LTP research as a whole, few studies have addressed the essential question: Is LTP a process involved in learning and memory? The present manuscript reviews research that attempts to link LTP with learning and memory, focusing on studies utilizing electrophysiological, pharmacological, and molecular biological methodologies. Most evidence firmly supports a role for LTP in learning memory. However, an unequivocal experimental demonstration of a contribution of LTP to memory is hampered by our lack of knowledge of the biological basis of memory and of the ways in which memories are represented in ensembles of neurons, the existence of a variety of cellular forms of LTP, and the likely resistance of distributed memory stores to degradation by treatments that incompletely disrupt LTP.

Synaptic recruitment during long-term potentiation at synapses of the medial perforant pathway in the dentate gyrus of the rat brain

Long-term potentiation (LTP) in synapses of the medial perforant pathway of the rat dentate gyrus has been studied using the whole-cell voltage clamp technique and a standard hippocampal slice preparation. The rate of LTP induction by 2-4 brief trains of stimuli at 100 Hz, paired with postsynaptic depolarization to -20 mV, in individual granule neurons was only 42% but the average magnitude was large. In a representative series of nine experiments the average potentiation was 339% (s.d. 255%). The variable magnitude of LTP appeared to be related to the relative size of the NMDA receptor dependent current in individual neurons. LTP was further characterized by the selective enhancement of the AMPA (but not the NMDA) component in the excitatory synaptic responses. This selective enhancement of the AMPA component and a graphical variance analysis suggest that the large magnitude of LTP in dentate gyrus can be best explained by recruitment of previously silent synapses by a combination of pre- and post-synaptic mechanisms.

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.

Ultrastructural localization of delta-opioid receptor and Met5-enkephalin immunoreactivity in rat insular cortex

The insular cortex has been implicated in the reinforcing properties of opiates as well as in the integration of responses to sensory-motor stimulation. Moreover, the delta-opioid receptor (DOR) and the endogenous opioid ligand, Met5-enkephalin (ENK) are known to be prominently distributed in insular limbic cortex. To examine the anatomical sites for opioid activation of DOR in rat insular cortex, we used immunoperoxidase for detection of an antiserum raised against a peptide sequence unique to the DOR alone, and in combination with immunogold-silver labeling for ENK. Light microscopy showed intense DOR-like immunoreactivity (DOR-LI) in pyramidal cells and interneurons in deep laminae, and in varicose processes in both superficial and deep layers of the insular cortex. Ultrastructural analysis of layers V and VI in insular cortex showed that the most prominent immunoperoxidase labeling for DOR was in dendrites. This labeling was associated with asymmetric excitatory-type junctions postsynaptic to unlabeled terminals. Dendritic DOR-LI was also distributed along selective portions of non-synaptic plasma membranes and subsurface organelles. In dually labeled sections, dendrites containing DOR-LI sometimes received synaptic input from ENK-labeled terminals or more infrequently colocalized with ENK. Other axon terminals were exclusively immunolabeled for DOR or more rarely contained both DOR and ENK immunoreactivity. Within labeled axon terminals, distinct segments of the plasma membrane and membranes of immediately adjacent synaptic vesicles showed the largest accumulation of the peroxidase reaction product for DOR. These results indicate that in rat insular cortex DOR is primarily heteroreceptive, but also serves an autoreceptive function on certain ENK-containing neurons. Our results also provide the first ultrastructural evidence that in rat insular cortex endogenous opioids interact through the DOR (1) to modulate the postsynaptic responses to other excitatory afferents and (2) to presynaptically regulate the release of other neurotransmitters. The modulatory actions on both ENK-containing and non-ENK-containing neurons may contribute significantly to the reinforcing properties of exogenous opiates acting on the DOR in limbic cortex.

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).

Long-term potentiation and depression in the intermediate gray matter of rat spinal cord in vitro

Field potentials recorded from a group of cells in the intermediomedial gray matter of spinal cord slices from neonatal rats showed long-term potentiation and long-term depression in response to brief bursts of high frequency conditioning stimulation. Both N-methyl-D-aspartic acid-dependent and N-methyl-D-aspartic acid-independent long-term potentiation was seen. The long-term depression was partially reversed, but not prevented from occurring, by the opiate antagonist naloxone. The frequency of occurrence of long-term potentiation and depression was affected by the inhibitory blockers bicuculline and strychnine in a fashion consistent with the hypothesis that a slight depolarization favoured long-term potentiation and a larger depolarization favoured long-term depression.

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.

Acute tolerance to the excitatory effects of opioids in the rat hippocampus

Prolonged iontophoretic administrations of delta- and mu-selective opioid receptor agonists were conducted in the hippocampus of rats, in order to study the possible development of acute tolerance to the excitatory effects of the opioids. Acute tolerance (AT) to the excitatory effects of the delta-selective opioid receptor agonist Tyr-D-Ser-Gly-Phe-Leu-Thr (DSLET) was observed when the drug was applied locally for 3-5 min in the CA1 hippocampal pyramidal neurons. The acute tolerance was expressed as a decrease in the commissurally evoked spike responsiveness during peptide's administration and led to a long-lasting potentiation of the population spike (PS) upon its withdrawal. In all cases, where AT and spike potentiation were evident, the population excitatory postsynaptic potential (pEPSP) remained unaltered. Pharmacological studies of AT and long-lasting spike potentiation showed the following: (1) the nonselective opioid receptor antagonist, naloxone, while effective in blocking the excitatory effects of DSLET when applied prior and during the application of the latter, failed to exhibit any effect on the long-lasting potentiating effect of the opioid; and (2) during the spike potentiation phase, administration of DSLET exhibited a depressant effect towards baseline values. This depressant effect of the opioid was evident 2-3 min from the beginning of the application and was completely antagonized by naloxone. The above results show that the development of acute tolerance to the excitatory effects of the DSLET led to long-lasting spike potentiation, which manifests a withdrawal phenomenon.