Quantitative autoradiographic analysis of mu and delta opioid binding sites in the rat hippocampal formation

Acute Delta 9-tetrahydrocannabinol administration differentially alters the hippocampal opioid system in adult female and male rats

Our prior studies demonstrated that the rat hippocampal opioid system can undergo sex-specific adaptations to external stimuli that can influence opioid-associated learning processes. This opioid system extensively overlaps with the cannabinoid system. Moreover, acute administration of Δ⁹ Tetrahydrocannabinoid (THC), the primary psychoactive constituent of cannabis, can alter cognitive behaviors that involve the hippocampus. Here, we use light and electron microscopic immunocytochemical methods to examine the effects of acute THC (5 mg/kg, i.p., 1 h) on mossy fiber Leu-Enkephalin (LEnk) levels and the distribution and phosphorylation levels of delta and mu opioid receptors (DORs and MORs, respectively) in CA3 pyramidal cells and parvalbumin dentate hilar interneurons of adult female and male Sprague-Dawley rats. In females with elevated estrogen states (proestrus/estrus stage), acute THC altered the opioid system so that it resembled that seen in vehicle-injected females with low estrogen states (diestrus) and males: (1) mossy fiber LEnk levels in CA2/3a decreased; (2) phosphorylated-DOR levels in CA2/3a pyramidal cells increased; and (3) phosphorylated-MOR levels increased in most CA3b laminae. In males, acute THC resulted in the internalization of MORs in parvalbumin-containing interneuron dendrites which would decrease disinhibition of granule cells. In both sexes, acute THC redistributed DORs to the near plasma membrane of CA3 pyramidal cell dendrites, however, the dendritic region varied with sex. Additionally, acute THC also resulted in a sex-specific redistribution of DORs within CA3 pyramidal cell dendrites which could differentially promote synaptic plasticity and/or opioid-associated learning processes in both females and males.

Sex and chronic stress alter delta opioid receptor distribution within rat hippocampal CA1 pyramidal cells following behavioral challenges

Following oxycodone (Oxy) conditioned place preference (CPP), delta opioid receptors (DORs) differentially redistribute in hippocampal CA3 pyramidal cells in female and male rats in a manner that would promote plasticity and opioid-associative learning processes. However, following chronic immobilization stress (CIS), males do not acquire Oxy-CPP and the trafficking of DORs in CA3 pyramidal neurons is attenuated. Here, we examined the subcellular distribution of DORs in CA1 pyramidal cells using electron microscopy in these same cohorts.

CPP

Saline (Sal)-females compared to Sal-males have more cytoplasmic and total DORs in dendrites and more DOR-labeled spines. Following Oxy-CPP, DORs redistribute from near-plasmalemma pools in dendrites to spines in males.

CIS

Control females compared to control males have more near-plasmalemmal dendritic DORs. Following CIS, dendritic DORs are elevated in the cytoplasm in females and near-plasmalemma in males.

CIS plus CPP

CIS Sal-females compared to CIS Sal-males have more DORs on the plasmalemma of dendrites and in spines. After Oxy, the distribution of DORs does not change in either females or males.

Conclusion

Following Oxy-CPP, DORs within CA1 pyramidal cells remain positioned in naïve female rats to enhance sensitivity to DOR agonists and traffic to dendritic spines in naïve males where they can promote plasticity processes. Following CIS plus behavioral enrichment, DORs are redistributed within CA1 pyramidal cells in females in a manner that could enhance sensitivity to DOR agonists. Conversely, CIS plus behavioral enrichment does not alter DORs in CA1 pyramidal cells in males, which may contribute to their diminished capacity to acquire Oxy-CPP.

Oxycodone injections not paired with conditioned place preference have little effect on the hippocampal opioid system in female and male rats

Oxycodone (Oxy) conditioned place preference (CPP) in Sprague Dawley rats results in sex-specific alterations in hippocampal opioid circuits in a manner that facilitates opioid-associative learning processes, particularly in females. Here, we examined if Oxy (3 mg/kg, I.P.) or saline (Sal) injections not paired with behavioral testing similarly affect the hippocampal opioid system. Sal-injected females compared to Sal-injected males had: (1) higher densities of cytoplasmic delta opioid receptors (DOR) in GABAergic hilar dendrites suggesting higher baseline reserve DOR pools and (2) elevated phosphorylated DOR levels, but lower phosphorylated mu opioid receptor (MOR) levels in CA3a suggesting that the baseline pools of activated opioid receptors vary in females and males. In contrast to CPP studies, Oxy-injections in the absence of behavioral tests resulted in few changes in the hippocampal opioid system in either females or males. Specifically, Oxy-injected males compared to Sal-injected males had fewer DORs near the plasma membrane of CA3 pyramidal cell dendrites and in CA3 dendritic spines contacted by mossy fibers, and lower pMOR levels in CA3a. Oxy-injected females compared to Sal-injected females had higher total DORs in GABAergic dendrites and lower total MORs in parvalbumin-containing dendrites. Thus, unlike Oxy CPP, Oxy-injections redistributed opioid receptors in hippocampal neurons in a manner that would either decrease (males) or not alter (females) excitability and plasticity processes. These results indicate that the majority of changes within hippocampal opioid circuits that would promote opioid-associative learning processes in both females and males do not occur with Oxy administration alone, and instead must be paired with CPP.

Chronic immobilization stress primes the hippocampal opioid system for oxycodone-associated learning in female but not male rats

In adult female, but not male, Sprague Dawley rats, chronic immobilization stress (CIS) increases mossy fiber (MF) Leu-Enkephalin levels and redistributes delta- and mu-opioid receptors (DORs and MORs) in hippocampal CA3 pyramidal cells and GABAergic interneurons to promote excitation and learning processes following subsequent opioid exposure. Here, we demonstrate that CIS females, but not males, acquire conditioned place preference (CPP) to oxycodone and that CIS "primes" the hippocampal opioid system in females for oxycodone-associated learning. In CA3b, oxycodone-injected (Oxy) CIS females relative to saline-injected (Sal) CIS females exhibited an increase in the cytoplasmic and total densities of DORs in pyramidal cell dendrites so that they were similar to Sal- and Oxy-CIS males. Consistent with our earlier studies, Sal- and Oxy-CIS females but not CIS males had elevated DOR densities in MF-CA3 dendritic spines, which we have previously shown are important for opioid-mediated long-term potentiation. In the dentate gyrus, Oxy-CIS females had more DOR-labeled interneurons than Sal-CIS females. Moreover, Sal- and Oxy-CIS females compared to both groups of CIS males had elevated levels of DORs and MORs in GABAergic interneuron dendrites, suggesting capacity for greater synthesis or storage of these receptors in circuits important for opioid-mediated disinhibition. However, more plasmalemmal MORs were on large parvalbumin-containing dendrites of Oxy-CIS males compared to Sal-CIS males, suggesting a limited ability for increased granule cell disinhibition. These results suggest that low levels of DORs in MF-CA3 synapses and hilar GABAergic interneurons may contribute to the attenuation of oxycodone CPP in males exposed to CIS.

Sex Differences in the Rat Hippocampal Opioid System After Oxycodone Conditioned Place Preference

Although opioid addiction has risen dramatically, the role of gender in addiction has been difficult to elucidate. We previously found sex-dependent differences in the hippocampal opioid system of Sprague-Dawley rats that may promote associative learning relevant to drug abuse. The present studies show that although female and male rats acquired conditioned place preference (CPP) to the mu-opioid receptor (MOR) agonist oxycodone (3 mg/kg, I.P.), hippocampal opioid circuits were differentially altered. In CA3, Leu-Enkephalin-containing mossy fibers had elevated levels in oxycodone CPP (Oxy) males comparable to those in females and sprouted in Oxy-females, suggesting different mechanisms for enhancing opioid sensitivity. Electron microscopy revealed that in Oxy-males delta opioid receptors (DORs) redistributed to mossy fiber-CA3 synapses in a manner resembling females that we previously showed is important for opioid-mediated long-term potentiation. Moreover, in Oxy-females DORs redistributed to CA3 pyramidal cell spines, suggesting the potential for enhanced plasticity processes. In Saline-injected (Sal) females, dentate hilar parvalbumin-containing basket interneuron dendrites had fewer MORs, however plasmalemmal and total MORs increased in Oxy-females. In dentate hilar GABAergic dendrites that contain neuropeptide Y, Sal-females compared to Sal-males had higher plasmalemmal DORs, and near-plasmalemmal DORs increased in Oxy-females. This redistribution of MORs and DORs within hilar interneurons in Oxy-females would potentially enhance disinhibition of granule cells via two different circuits. Together, these results indicate that oxycodone CPP induces sex-dependent redistributions of opioid receptors in hippocampal circuits in a manner facilitating opioid-associative learning processes and may help explain the increased susceptibility of females to opioid addiction acquisition and relapse.

Sex differences in subcellular distribution of delta opioid receptors in the rat hippocampus in response to acute and chronic stress

Drug addiction requires associative learning processes that critically involve hippocampal circuits, including the opioid system. We recently found that acute and chronic stress, important regulators of addictive processes, affect hippocampal opioid levels and mu opioid receptor trafficking in a sexually dimorphic manner. Here, we examined whether acute and chronic stress similarly alters the levels and trafficking of hippocampal delta opioid receptors (DORs). Immediately after acute immobilization stress (AIS) or one-day after chronic immobilization stress (CIS), the brains of adult female and male rats were perfusion-fixed with aldehydes. The CA3b region and the dentate hilus of the dorsal hippocampus were quantitatively analyzed by light microscopy using DOR immunoperoxidase or dual label electron microscopy for DOR using silver intensified immunogold particles (SIG) and GABA using immunoperoxidase. At baseline, females compared to males had more DORs near the plasmalemma of pyramidal cell dendrites and about 3 times more DOR-labeled CA3 dendritic spines contacted by mossy fibers. In AIS females, near-plasmalemmal DOR-SIGs decreased in GABAergic hilar dendrites. However, in AIS males, near-plasmalemmal DOR-SIGs increased in CA3 pyramidal cell and hilar GABAergic dendrites and the percentage of CA3 dendritic spines contacted by mossy fibers increased to about half that seen in unstressed females. Conversely, after CIS, near-plasmalemmal DOR-SIGs increased in hilar GABA-labeled dendrites of females whereas in males plasmalemmal DOR-SIGs decreased in CA3 pyramidal cell dendrites and near-plasmalemmal DOR-SIGs decreased hilar GABA-labeled dendrites. As CIS in females, but not males, redistributed DOR-SIGs near the plasmalemmal of hilar GABAergic dendrites, a subsequent experiment examined the acute affect of oxycodone on the redistribution of DOR-SIGs in a separate cohort of CIS females. Plasmalemmal DOR-SIGs were significantly elevated on hilar interneuron dendrites one-hour after oxycodone (3 mg/kg, I.P.) administration compared to saline administration in CIS females. These data indicate that DORs redistribute within CA3 pyramidal cells and dentate hilar GABAergic interneurons in a sexually dimorphic manner that would promote activation and drug related learning in males after AIS and in females after CIS.

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Females have more near-plasmalemmal DORs in pyramidal CA3 dendrites than males.

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Acute stress in males relocates DORs in CA3 & GABA dendrites to promote activation.

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Chronic stress in females relocates DORs in GABA dendrites in females to promote activation.

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Chronic stress in males relocates DORs in GABA dendrites opposite of females.

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DOR-stress relocation may contribute to sexually dimorphic effects on drug related learning.

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.

Recent advances on the δ opioid receptor: from trafficking to function

Within the opioid family of receptors, δ (DOPrs) and μ opioid receptors (MOPrs) are typical GPCRs that activate canonical second-messenger signalling cascades to influence diverse cellular functions in neuronal and non-neuronal cell types. These receptors activate well-known pathways to influence ion channel function and pathways such as the map kinase cascade, AC and PI3K. In addition new information regarding opioid receptor-interacting proteins, downstream signalling pathways and resultant functional effects has recently come to light. In this review, we will examine these novel findings focusing on the DOPr and, in doing so, will contrast and compare DOPrs with MOPrs in terms of differences and similarities in function, signalling pathways, distribution and interactions. We will also discuss and clarify issues that have recently surfaced regarding the expression and function of DOPrs in different cell types and analgesia.

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.

Delta-opioid receptors mediate unique plasticity onto parvalbumin-expressing interneurons in area CA2 of the hippocampus

Inhibition is critical for controlling information transfer in the brain. However, the understanding of the plasticity and particular function of different interneuron subtypes is just emerging. Using acute hippocampal slices prepared from adult mice, we report that in area CA2 of the hippocampus, a powerful inhibitory transmission is acting as a gate to prevent CA3 inputs from driving CA2 neurons. Furthermore, this inhibition is highly plastic, and undergoes a long-term depression following high-frequency 10 Hz or theta-burst induction protocols. We describe a novel form of long-term depression at parvalbumin-expressing (PV+) interneuron synapses that is dependent on delta-opioid receptor (DOR) activation. Additionally, PV+ interneuron transmission is persistently depressed by DOR activation in area CA2 but only transiently depressed in area CA1. These results provide evidence for a differential temporal modulation of PV+ synapses between two adjacent cortical circuits, and highlight a new function of PV+ cells in controlling information transfer.

The novel micro-opioid receptor antagonist, [N-allyl-Dmt(1)]endomorphin-2, attenuates the enhancement of GABAergic neurotransmission by ethanol

AIMS

We investigated the effects of [N-allyl-Dmt(1)]endomorphin-2 (TL-319), a novel and highly potent micro-opioid receptor antagonist, on ethanol (EtOH)-induced enhancement of GABA(A) receptor-mediated synaptic activity in the hippocampus.

METHODS

Evoked and spontaneous inhibitory postsynaptic currents (eIPSCs and sIPSCs) were isolated from CA1 pyramidal cells from brain slices of male rats using whole-cell patch-clamp techniques.

RESULTS

TL-319 had no effect on the baseline amplitude of eIPSCs or the frequency of sIPSCs. However, it induced a dose-dependent suppression of an ethanol-induced increase of sIPSC frequency with full reversal at concentrations of 500 nM and higher. The non-specific competitive opioid receptor antagonist naltrexone also suppressed EtOH-induced increases in sIPSC frequency but only at a concentration of 60 microM.

CONCLUSION

These data indicate that blockade of micro-opioid receptors by low concentrations of [N-allyl-Dmt(1)]endomorphin-2 can reverse ethanol-induced increases in GABAergic neurotransmission and possibly alter its anxiolytic or sedative effects. This suggests the possibility that high potency opioid antagonists may emerge as possible candidate compounds for the treatment of ethanol addiction.

Hypoxia modulates cholinergic but not opioid activation of G proteins in rat hippocampus

Intermittent hypoxia, such as that associated with obstructive sleep apnea, can cause neuronal death and neurobehavioral dysfunction. The cellular and molecular mechanisms through which hypoxia alter hippocampal function are incompletely understood. This study used in vitro [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTP gamma S) autoradiography to test the hypothesis that carbachol and DAMGO activate hippocampal G proteins. In addition, this study tested the hypothesis that in vivo exposure to different oxygen (O(2)) concentrations causes a differential activation of G proteins in the CA1, CA3, and dentate gyrus (DG) regions of the hippocampus. G protein activation was quantified as nCi/g tissue in CA1, CA3, and DG from rats housed for 14 days under one of three different oxygen conditions: normoxic (21% O(2)) room air, or hypoxia (10% O(2)) that was intermittent or sustained. Across all regions of the hippocampus, activation of G proteins by the cholinergic agonist carbachol and the mu opioid agonist [D-Ala(2), N-Met-Phe(4), Gly(5)] enkephalin (DAMGO) was ordered by the degree of hypoxia such that sustained hypoxia > intermittent hypoxia > room air. Carbachol increased G protein activation during sustained hypoxia (38%), intermittent hypoxia (29%), and room air (27%). DAMGO also activated G proteins during sustained hypoxia (52%), intermittent hypoxia (48%), and room air (43%). Region-specific comparisons of G protein activation revealed that the DG showed significantly less activation by carbachol following intermittent hypoxia and sustained hypoxia than the CA1. Considered together, the results suggest the potential for hypoxia to alter hippocampal function by blunting the cholinergic activation of G proteins within the DG.

Layer selective presynaptic modulation of excitatory inputs to hippocampal cornu Ammon 1 by mu-opioid receptor activation

Chronic and acute activation of mu-opioid receptors (MOR) in hippocampal cornu Ammon 1 (CA1) disrupts rhythmic activity, alters activity-dependent synaptic plasticity and impairs spatial memory formation. In CA1, MORs act by hyperpolarizing inhibitory interneurons and suppressing inhibitory synaptic transmission. MOR modulation of inhibitory synaptic function translates into an increase in excitatory activity in all layers of CA1. However, the exact anatomical sites for MOR actions are not completely known. Therefore, we used voltage-sensitive dye imaging, whole cell patch clamping, photolysis of alpha-carboxy-2-nitrobenzyl ester, trifluoroacetic acid salt (CNB) -caged GABA, and micro-sectioned slices of rat hippocampus to investigate the effect of MOR activation in CA1. First, we investigated the effect of MOR activation using a MOR agonist [d-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO) on the direct activation of GABA receptors by photolysis of CNB-caged GABA in all layers of CA1. MOR activation did not affect hyperpolarizations due to direct GABA receptor activation in any layer of CA1, but MOR activation did suppress GABAergic inhibitory postsynaptic potentials suggesting that MOR activation acts by presynaptically inhibiting interneuron function. We next examined whether MOR activation was equivalently effective in all anatomical layers of CA1. To do this, cuts were made between anatomical layers of CA1 and isolated layers were stimulated electrically (five pulses at 20 Hz) to produce excitatory postsynaptic potentials (EPSPs). Under these conditions, MOR activation significantly increased EPSP areas in stratum radiatum (SR), stratum pyramidale (SP) and stratum oriens (SO) relative to stratum lacunosum-moleculare (SLM). When compared with the effect of GABA(A) and GABA(B) receptor antagonists on EPSP areas, the effect of DAMGO was proportionately larger in SR, SP and SO than in SLM. We conclude that MOR activation is more effective at directly modulating activity in SR, SP and SO, and the smaller effect in SLM is likely due to a smaller MOR inhibition of GABA release in SLM.

Effects of mu-opioid receptor modulation on GABAB receptor synaptic function in hippocampal CA1

Activation of mu-opioid receptors (MORs) alters information coding, synaptic plasticity, and spatial memory in hippocampal CA1. In CA1, MORs act by inhibiting GABA release onto both GABA(A) and GABA(B) receptors exclusively. MOR activation can facilitate excitatory inputs in CA1 dendritic layers by inhibiting synaptic activation of GABA(A) receptors. In this study, we use voltage-sensitive dye imaging to show that MOR activation by the MOR agonist DAMGO suppressed GABA(B) inhibitory postsynaptic potentials in all layers of CA1. When stimulating excitatory input in stratum oriens (SO), stratum radiatum (SR), or stratum lacunosum-moleculare (SLM) with five pulses at 20 Hz in the presence of bicuculline (50 microM), DAMGO (1 muM) was most effective at increasing the amplitude of the last excitatory event. This effect was reversed by the MOR antagonist CTOP (1 muM) and occluded by the GABA(B) receptor agonist CGP 55845 (10 microM). DAMGO was less effective at increasing the amplitude of later excitatory events compared with the effect of CGP 55845. DAMGO was relatively ineffective at increasing the amplitude of excitatory inputs in SLM but had significantly greater effects on excitatory events as they propagated to stratum pyramidale (SP). When stimulating in SR, DAMGO was least effective at increasing excitatory amplitudes in SLM and most effective in SP and SO. Finally, DAMGO was equally effective at increasing excitatory activity amplitudes in all layers of CA1 after stimulating in SO. Therefore MOR suppresses GABA(B) synaptic hyperpolarizations in all layers of CA1 and most effectively facilitates excitatory activity in CA1 output layers.

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.

Field-specific changes in hippocampal opioid mRNA, peptides, and receptors due to prenatal morphine exposure in adult male rats

Alterations in the opioid system in the hippocampal formation and some of the possible functional consequences were investigated in adult male rats that were prenatally exposed to either saline or morphine (10 mg/kg twice daily on gestational days 11-18). In situ hybridization and Northern blots were used to measure proenkephalin and prodynorphin mRNA, and radioimmunoassays quantified proenkephalin- and prodynorphin-derived peptide levels in the dentate gyrus, CA3, and CA1 subfields of the hippocampal formation. Prenatal morphine exposure in male rats decreases proenkephalin and increases prodynorphin mRNA selectively in the granule cell layer of the dentate gyrus. Similarly, met-enkephalin peptide levels are decreased and dynorphin B peptide levels are increased in the dentate gyrus but not CA3 or CA1 of prenatally morphine-exposed males. In addition, there are decreases in dynorphin-derived peptides in the CA3 subfield. Receptor autoradiography revealed increases in the density of micro but not delta receptor labeling in discrete strata of specific hippocampal subfields in morphine-exposed males. Because alterations in the hippocampal opioid system suggest possible alterations in the excitability of the hippocampal formation, changes in opioid regulation of seizures were examined. Morphine exposure, however, does not alter the latency to onset or number of episodes of wet dog shakes or clonic seizures induced by infusion of 10 nmol [D-Ala2, MePhe4, Gly-ol5]enkephalin into the ventral hippocampal formation. Interestingly, a naloxone (5 mg/kg) injection 30 min before bicuculline administration reverses the increased latency to onset of clonic and tonic-clonic seizures in morphine-exposed males. Thus, the present study suggests that exposure of rats to morphine during early development alters the hippocampal opioid system, suggesting possible consequences for hippocampal-mediated functions.

Mu-opioid receptors facilitate the propagation of excitatory activity in rat hippocampal area CA1 by disinhibition of all anatomical layers

Hippocampal mu-opioid receptors (MORs) have been implicated in memory formation associated with opiate drug abuse. MORs modulate hippocampal synaptic plasticity acutely, when chronically activated, and during drug withdrawal. At the network level, MORs increase excitability in area CA1 by disinhibiting pyramidal cells. The precise inhibitory interneuron subtypes affected by MOR activation are unknown; however, not all subtypes are inhibited, and specific interneuron subtypes have been shown to preferentially express MORs. Here we investigate, using voltage-sensitive dye imaging in brain slices, the effect of MOR activation on the patterns of inhibition and on the propagation of excitatory activity in rat hippocampal CA1. MOR activation augments excitatory activity evoked by stimulating inputs in stratum oriens [i.e., Schaffer collateral and commissural pathway (SCC) and antidromic], stratum radiatum (i.e., SCC), and stratum lacunosum-moleculare (SLM; i.e., perforant path and thalamus). The augmented excitatory activity is further facilitated as it propagates through the CA1 network. This was observed as a proportionately larger increase in amplitudes of excitatory activity at sites distal from where the activity was evoked. This facilitation was observed for excitatory activity propagating from all three stimulation sites. The augmentation and facilitation were prevented by GABAA receptor antagonists (bicuculline, 30 microM), but not by GABAB receptor antagonists (CGP 55845, 10 microM). Furthermore, MOR activation inhibited IPSPs in all layers of area CA1. These findings suggest that MOR-induced suppression of GABA release onto GABAA receptors augments all inputs to CA1 pyramidal cells and facilitates the propagation of excitatory activity through the network of area CA1.

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.

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.

Slide-binding characterization and autoradiographic localization of delta opioid receptors in rat and mouse brains with the tetrapeptide antagonist [3H]TIPP

Slide-binding and autoradiographic studies were performed on cryostat sections from brains of adult Sprague-Dawley rats and BALB C mice to describe the binding characteristics of the tetrapeptide [3H]TIPP, an antagonist with high specificity and affinity for the delta opioid receptors. Steady-state binding of [3H]TIPP to cryostat sections of brain paste was reached in 120-180 min of incubation. Specific [3H]TIPP binding resulted in maximal numbers of binding sites (Bmax) of 15.59 and 23.91 fmol/mg protein, and dissociation constants (Kd) of 0.46 and 0.85 nM for rat and mouse brain paste sections, respectively. TIPP displayed the highest affinity for delta opioid receptors in inhibiting specific [3H]TIPP binding, with IC50 values of 0.82 nM and 0.14 nM in rat and mouse brain sections, respectively. While DPDPE was also effective in displacing the specific binding of [3H]TIPP (IC50 = 3.18 +/- 0.53 nM and 0.63 +/- 0.42 nM in rat and mouse brain paste sections, respectively), other subclass-selective or nonopioid ligands were much less effective, or ineffective. Autoradiographic localization of [3H]TIPP binding revealed the characteristic distribution of delta opioid receptors in both species. In consequence of its antagonistic nature, and of its unnatural amino acid residue, which makes this ligand more resistant to biodegradation, [3H]TIPP is a superior ligand for evaluation of the binding characteristics and autoradiogaphic distribution of the delta opioid receptors.

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.

Immunohistochemical localization of mu-opioid receptors in the central nervous system of the rat

Of the three major types of opioid receptors ( mu, delta, kappa) in the nervous system, mu-opioid receptor shows the highest affinity for morphine that exerts powerful effects on nociceptive, autonomic, and psychological functions. So far, at least two isoforms of mu-opioid receptors have been cloned from rat brain. The present study attempted to examine immunohistochemically the distribution of mu-opioid receptors in the rat central nervous system with two kinds of antibodies to recently cloned mu-opioid receptors (MOR1 and MOR1B). One antibody recognized a specific site for MOR1, and the other bound to a common site for MOR1 and MOR1B. Intense MOR1-like immunoreactivity (LI) was seen in the 'patch' areas and subcallosal streak in the striatum, medial habenular nucleus, medial terminal nucleus of the accessory optic tract, interpeduncular nucleus, median raphe nucleus, parabrachial nuclei, locus coeruleus, ambiguous nucleus, nucleus of the solitary tract, and laminae I and II of the medullary and spinal dorsal horns. Many other regions, including the cerebral cortex, amygdala, thalamus, and hypothalamus, also contained many neuronal elements with MOR1-LI. The distribution pattern of the immunoreactivity revealed with the antibody to the common site for MOR1 and MOR1B (MOR1/1B-LI) was almost the same as that of MOR1-LI. Both MOR1-LI and MOR1/1B-LI were primarily located in neuronal cell bodies and dendrites. However, the immunoreactivities were observed in the accessory optic tract, fasciculus retroflexus, solitary tract, and primary afferent fibers in the superficial layers of the medullary and spinal dorsal horns. The presynaptic location of MOR1-LI and MOR1/1B-LI was confirmed by lesion experiments: Enucleation, placing a lesion in the medial habenular nucleus, removal of the nodose ganglion, or dorsal rhizotomy resulted in a clear reduction of the immunoreactivities, respectively, in the nuclei of the accessory optic tract, some subnuclei of the interpeduncular nucleus, nucleus of the solitary tract, or laminae I and II of the spinal dorsal horn. The results indicate that the mu-opioid receptors are widely distributed in the brain and spinal cord, mainly postsynaptically and occasionally presynaptically. Opioids, including morphine, may inhibit the excitation of neurons via the postsynaptic mu-opioid receptors, and also suppress the release of neurotransmitters and/or neuromodulators from axon terminals through the presynaptic mu-opioid receptors.

Alterations in opiate receptor binding in the hippocampus of aged Long-Evans rats

Quantitative in vitro autoradiography was used to examine [3H]D-Ala2, MePhe4, Gly-015 enkephalin (DAGO) (mu-agonist) and [3H]diprenorphine (general opiate antagonist) binding sites in the hippocampal formation of young (6-8 months) and aged (25-28 months) Long-Evans rats. Age-related changes in these binding sites were restricted to specific regions but were not generally dependent on the ligand used. No reliable age-related changes in opiate binding were observed in the CA1 field or subicular region. In contrast, a decrease in the density of binding was found in both dorsal and ventral hippocampus within the CA3 field of aged brains. An age-related decrease in opiate binding within the dentate gyrus differed in its topography at dorsal and ventral levels of the hippocampus. A uniform decrease of opiate receptor binding was found throughout the dorsal dentate gyrus, while a more localized decrease of these sites occurred in hilar and granular layers of the ventral dentate gyrus. These results indicate that a decrease of opiate binding in the hippocampal formation is largely localized to the CA3 region and dentate gyrus of aged rats. These findings are discussed with reference to age-related changes in hippocampal pathways containing opioid peptides. The implications for hippocampal opioid function in learning and age-related cognitive decline are also considered.

Search for the structures initiating seizures triggered by intraventricular injection of the mu opioid agonist dermorphin in rats

Free-moving rats received intraventricular (i.c.v.) or intravenous (i.v.) injections of the mu opioid agonist dermorphin (DRM). The EEG activity of the cortex and of several structures near the injected lateral ventricle was recorded. The intravenous injections of DRM did not induce epileptiform activity. The intracerebroventricular injections of DRM triggered several types of electrical seizures and interictal spikes. With the aim of determining which structure gave rise to the epileptiform discharges, we compared the time relationships of epileptiform phenomena occurring in different structures. Epileptiform discharges, at once generalized, appeared first in the CA3 area of the ventral hippocampus, with involvement of the CA1 area of ventral hippocampus, the entorhinal cortex and the amygdala following immediately. We conclude that, after intracerebroventricular injection of a mu opiate agonist, epileptiform activity originates in the CA3 area of the ventral hippocampus.

The apparent affinity of morphine-3-glucuronide at mu1-opioid receptors results from morphine contamination: demonstration using HPLC and radioligand binding

Equilibrium binding studies in sheep thalamic homogenates indicated that morphine-3-glucuronide (M3G) had an apparent affinity for mu1-opioid binding sites (IC50 = 178 +/- 40 nM, Ki = 116 +/- 25 nM, mean +/- s.e.m., n = 4) similar to that reported by Pasternak and co-workers (1). However, when the chemical purity of M3G was investigated using high-performance-liquid-chromatography (HPLC) with electrochemical detection, it was found to be contaminated with 0.5% (molar basis) of morphine. Reduction of the morphine contamination of M3G to 0.08% resulted in a 7.2-fold decrease in apparent binding affinity (IC50 = 1279 +/- 287 nM, Ki = 766 +/- 30 nM, mean +/- s.e.m., n = 4), indicating that the small percentage of morphine present in the M3G raw material drug is the likely explanation for M3G's apparent binding to mu1-opioid receptors.

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.

Immunohistochemical localization of mu-opioid receptors in rat brain using antibodies generated against a peptide sequence present in a purified mu-opioid binding protein

Light-microscope visualization in rat brain of a pattern of distribution of immunoreactivity, which included immunolabeled perikarya and beaded processes, was achieved using an immunoaffinity purified polyclonal antibody, Ab165, which recognizes the amino acid sequence, IRNLRQDRSKYY, found in the mu-opioid binding protein purified in our laboratory. Immunohistochemical staining with Ab165 was carried out by the avidin-biotin procedure. Antibody, preabsorbed with antigen, served as control. Extensive immunoreactivity was seen in the hippocampal formation, the amygdaloid complex, the striatal complex, cortical regions, select areas of the thalamus and hypothalamus and in laminae I and II of the dorsal horn in spinal cord. The distribution of immunoreactivity in the rat brain of antibody 165, which recognizes a purified mu-opioid binding protein, is concordant with the distribution of mu-opioid binding sites as determined by other laboratories in autoradiographic, electrophysiological and immunocytochemical studies. These findings have enabled us to distinguish areas possessing large fields of mu-opioid receptor containing cell bodies from areas possessing dense networks of immunolabeled neuronal processes or mixtures of both.

Expression of mu opioid receptor mRNA in rat brain: an in situ hybridization study at the single cell level

The mu (mu) opioid receptors, which mediate the effects of morphine, are widely distributed in brain. We have examined the distribution of mRNA encoding a mu opioid receptor in rat brain with in situ hybridization histochemistry at the single-cell level to obtain information about the cell types synthesizing this receptor. Only neurons, not glia, were labeled in discrete brain regions. High levels of labeling were detected in the thalamus, striosomes of the caudate-putamen, globus pallidus, and brain regions involved in nociception, arousal, respiratory control, and, possibly, addiction. The general distribution of the receptor mRNA paralleled that of mu opioid binding sites with some notable exceptions. These include the cerebral cortex, which contains binding sites, but very few labeled neurons. No labeling was observed in the cerebellum, a region devoid of mu binding sites. Three main findings emerged from these experiments: 1) the mRNA was present in regions mediating both the therapeutic (analgesia) and the unwanted (respiratory depression, addiction) effects of morphine, 2) the mRNA was very densely expressed by neurons known to receive dense enkephalin-containing inputs, and 3) the dissociation between the presence of binding sites and absence of mRNA in some brain regions supports a presynaptic localization of mu opioid receptors in these areas. Alternatively, other subtypes of mu opioid receptors may be encoded by a different mRNA. These results provide new insights into the receptor types and neuronal circuits involved in the effects of endogenous opioids and morphine.

mu-Opioid receptors modulate noradrenaline release from the rat hippocampus as measured by brain microdialysis

The modulation of noradrenaline (NA) release via presynaptic opioid receptors in the hippocampus of freely moving rats was studied by the use of brain microdialysis. Extracellular levels of NA were estimated by assaying its concentrations in the perfusion fluid using high-performance liquid chromatography (HPLC) with electrochemical detection (ECD). Spontaneous NA levels were reduced by tetrodotoxin (1 microM) co-perfusion and were increased by peripheral administration of desipramine (5 and 10 mg/kg, i.p.). Addition of potassium (K+, 60 and 120 mM) to the perfusion fluid evoked a concentration-dependent release of NA. K+ (120 mM)-evoked NA release was markedly reduced by removal of calcium (Ca2+) from the perfusion fluid. These results indicate that both the spontaneous and the K(+)-evoked NA release measured by the use of brain microdialysis coupled with HPLC-ECD can be used as indices of neuronal release from the noradrenergic nerve terminals. A mu-opioid receptor agonist, morphine (0.01-10 microM), when co-perfused with K+ (120 mM), produced a reduction of K(+)-evoked NA release in a concentration-dependent manner. Neither co-perfusion with a high concentration of [D-Pen2, D-Pen5]-enkephalin (DPDPE) (10 microM), an agonist selective for delta-opioid receptors, nor with U-69593 (10 microM), an agonist selective for kappa-opioid receptors, modified the K+ (120 mM)-evoked release of NA. Morphine-induced (1 microM) inhibition of NA release was blocked by a mu-opioid receptor antagonist, naltrexone (3 and 9 mg/kg, i.p). Naltrexone by itself did not alter the spontaneous NA levels or the K(+)-evoked NA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of mu-opioid receptor-mediated presynaptic inhibition in the rat hippocampus in vitro

1. The electrophysiological action of the mu-opioid receptor-preferring agonist D-Ala2, MePhe4, Met(O)5-ol-enkephalin (FK 33-824) on synaptic transmission has been studied in area CA3 of organotypic rat hippocampal slice cultures. 2. FK 33-824 (1 microM) had no effect on the amplitude of pharmacologically isolated N-methyl-D-aspartate (NMDA) or non-NMDA receptor-mediated EPSPs. 3. FK 33-824 (10 nM to 10 microM) reduced the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) that were elicited in pyramidal cells with local stimulation after pharmacological blockade of excitatory amino acid receptors. This effect was reversible, dose-dependent, and sensitive to naloxone and the mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP). FK 33-824 at 1 microM caused a mean reduction in the amplitude of the monosynaptic IPSP of 70%. 4. Neither delta- nor kappa-receptor-preferring agonists had any effect on excitatory or inhibitory synaptic potentials. 5. The disinhibitory action of FK 33-824 was blocked by incubating the cultures with pertussis toxin (500 ng/ml for 48 h) or by stimulation of protein kinase C with phorbol 12,13-dibutyrate (PDBu, 0.5 microM). 6. The depression of monosynaptic IPSPs by FK 33-824 was unaffected by extracellular application of the K+ channel blockers Ba2+ or Cs+ (1 mM each). 7. FK 33-824 produced a decrease in the frequency of miniature, action potential-independent, spontaneous inhibitory synaptic currents (mIPSCs) recorded with whole-cell voltage-clamp techniques, but did not change their mean amplitude. Application of the Ca2+ channel blocker Cd2+ (100 microM) or of nominally Ca(2+)-free solutions did not alter either the frequency and amplitude of mIPSCs or the reduction of mIPSC frequency induced by FK 33-824. 8. The effect of FK 33-824 on spontaneous mIPSCs was prevented by naloxone, and by incubation of cultures with pertussis toxin. 9. These results indicate that mu-opioid receptors decrease GABA release presynaptically by a G protein-mediated inhibition of the vesicular GABA release process, and not by changes in axon terminal K+ or Ca2+ conductances that are sensitive to extracellular Ba2+, Cs+ or Cd2+.

Opioid receptor regulation of 5-hydroxytryptamine release from the rat hippocampus measured by in vivo microdialysis

The modulation of serotonin (5-HT) release by opioid receptors in the hippocampus of the awake, unrestrained rat was evaluated by use of in vivo microdialysis. The hippocampus was perfused with Ringer's solution (2 microliters/min), and extracellular levels of 5-HT and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were estimated by assaying their concentration in the dialysate by HPLC-ECD. Addition of potassium (K+, 60 and 120 mM) to the perfusate evoked a concentration-dependent release of 5-HT, but did not alter extracellular 5-HIAA levels. Co-perfusion of morphine (0.1 to 10 microM) with K+ (120 mM) produced a concentration-dependent reduction of 5-HT release. Naltrexone (0.03 to 3 mg/kg, i.p.), a relatively selective mu-opioid receptor antagonist, blocked in a dose-dependent manner the morphine (10 microM)-induced inhibition of 5-HT release. Naltrexone alone did not alter significantly either extracellular 5-HT levels or the release of 5-HT evoked by K+. Neither co-perfusion with [D-Pen2, D-Pen5]-enkephalin (DPDPE, 1 to 10 microM), an agonist selective for delta-opioid receptors, nor with U-69593 (10 microM), an agonist selective for kappa-opioid receptors, modified the K+ (120 mM)-evoked release of 5-HT. These findings indicate that mu-opioid receptors modulate the physiological release of 5-HT from serotonergic neurons in the rat hippocampus.

Effect of deltorphin on behavior and hippocampal electrical activity in rabbits

To clarify the role of delta-opioid receptors on modulation of hippocampal electrical activity and behavior, deltorphin (DT), a naturally occurring heptapeptide that selectively binds to delta-opioid receptors, was intravenously (IV) administered to rabbits. For this purpose, at 8-day intervals, the effects of IV administration of normal saline and IV infusion of synthetic DT (1 mg/kg b.wt. for 2 min) on the spontaneous behavior in neutral environment, both in absence of any external stimulus and after the introduction of a stuffed predator, were examined in seven adult male rabbits, on separate and successive occasions. During each session of experimental procedure, hippocampal EEG was also recorded by telemetry. Behavioral activity showed an increase in alert and reactive immobility after the peptide injection in comparison with that observed during control period (saline administration). Under DT treatment the frequency of hippocampal electrical activity decreased, and a reduction in rhythmicity of electrical pattern was also observed in presence of stressful stimulus. These findings show that DT may affect neural and behavioral elements related to the control of attentional and emotional processes, suggesting a modulating role of delta-opioid receptors.

Dissociation of μ and δ opioid receptor-mediated reductions in evoked and spontaneous synaptic inhibition in the rat hippocampus in vitro

Modulation of gamma-aminobutyric acid (GABA)-mediated inhibition, and glutamate-mediated excitation by highly selective mu and delta opioid agonists was studied using intracellular recordings of CA1 pyramidal neuron synaptic responses in superfused hippocampal slices. Equimolar concentrations of the selective mu agonist, [Tyr-(D-Ala)-Gly-(N-Me-Phe)-Gly-ol]-enkephalin (DAGO), or the delta selective agonist, [D-Pen2,D-Pen5]-enkephalin (DPDPE), reversibly increased the amplitudes of excitatory post-synaptic potentials (EPSPs), evoked by Schaffer collateral/commissural stimulation, without altering the input resistance or resting membrane potential of these CA1 pyramidal neurons. The increased EPSP amplitudes resulting from superfusion with the enkephalin analogs were qualitatively similar to those caused by the GABAA receptor antagonist, bicuculline methiodide (BMI). Specific stimulation/recording protocols and micro-lesions of the slices were used to evoke relatively pure forms of recurrent and feed-forward GABA-mediated inhibitory post-synaptic potentials (IPSPs). The mu opioid agonist DAGO reduced both recurrent and feed-forward IPSPs, while the delta agonist DPDPE had no effect upon these responses. To test the hypothesis that the enhancement of pyramidal neuron EPSPs by delta (and mu) opioids was due to the reduction of an inhibitory potential that was coincident with the EPSP, DPDPE or the mu agonist, DAGO, were applied while recording monosynaptic IPSPs following the elimination of EPSPs by the glutamate receptor antagonists, D,L-2-amino-5-phosphonovalerate (APV) and 6,7-dinitroquinoxaline-2,3-dione (DNQX). The mu agonist, DAGO, reversibly reduced these pharmacologically isolated IPSPs, while the delta agonist, DPDPE, had no effect upon these responses. Despite the fact that the delta agonist, DPDPE, had no effect on recurrent, feed-forward or monosynaptic evoked IPSPs, this enkephalin did reversibly reduce the frequency of spontaneously occurring IPSPs, measured using whole-cell recordings with pipettes containing 65 mM KCl. The mu agonist, DAGO, and the GABAA antagonist, BMI, similarly reduced spontaneous IPSP rates. We conclude from these data that mu and delta opioid receptor activation increases EPSPs via the reduction of a form of GABAergic inhibition that is difficult to characterize, and which may be distinct from conventional feed-forward and recurrent inhibition. Furthermore, delta opioids seem to reduce this form of GABAergic inhibition selectively, while mu opioids reduced this inhibition, and conventional feed-forward and recurrent IPSPs as well.

Opioid inhibition of GABA release from presynaptic terminals of rat hippocampal interneurons

Opiates and the opioid peptide enkephalin can cause indirect excitation of principal cortical neurons by reducing inhibitory synaptic transmission mediated by GABAergic interneurons. The mechanism by which opioids mediate these effects on interneurons is unknown, but enkephalin hyperpolarizes the somatic membrane potential of a variety of neurons in the brain, including hippocampal interneurons. We now report a new, more direct mechanism for the opioid-mediated reduction in synaptic inhibition. The enkephalin analog D-Ala2-Met5-enkephalinamide (DALA) decreases the frequency of miniature, action potential-independent, spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) without causing a change in their amplitude. Thus, we conclude that DALA inhibits the action potential-independent release of GABA through a direct action on interneuronal synaptic terminals. In contrast, DALA reduces the amplitude of action potential-evoked, GABA-mediated IPSCs, as well as decreases their frequency. This suggests that the opioid-mediated inhibition of non-action potential-dependent GABA release reveals a mechanism that contributes to reducing action potential-evoked GABA release, thereby decreasing synaptic inhibition.

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

The role of opioid receptors in long-term potentiation (LTP) of the medial (MPP) and lateral (LPP) divisions of the perforant path-granule cell projection was investigated in urethane anesthetized rats. A stimulating electrode was positioned in the dorsomedial or ventrolateral aspect of the angular bundle for selective activation of the MPP and LPP, respectively. A push-pull cannula served to focally perfuse artificial cerebrospinal fluid (ACSF) across the perforant path terminal zone, while perforant path evoked potentials were monitored in the dentate hilus. Robust LTP of the excitatory postsynaptic potential (EPSP) initial slope and population spike height was induced by high frequency stimulation (400 Hz, 8 bursts of 8 pulses) applied to the medial or lateral perforant path in rats perfused with standard medium. In the lateral perforant path, a putative proenkephalin system, LTP of the EPSP and population spike was blocked when ACSF containing 100 microM of the opioid receptor antagonist naloxone was present during the tetanus, while perfusion with 0.1 microM naloxone prevented EPSP potentiation but only reduced the magnitude of the population spike increase. Naloxone had no effect on LTP induction in the MPP. Importantly, 0.1 microM ICI 174,864, a selective antagonist of delta opioid receptors, blocked LTP of synaptic transmission in the LPP while leaving the population spike increase intact. The results indicate that LTP of synaptic transmission in the LPP requires activation of delta opioid receptors, while 'non-delta' opioid receptors may be involved in augmenting granule cell output. This opioid receptor-dependent LTP illustrates peptidergic regulation of synaptic plasticity in the hippocampus.

Naloxone blocks antiepileptogenic properties of an in vitro electroconvulsive shock model

Electroconvulsive shock-induced seizures elevate seizure thresholds in humans and interfere with kindling in animals; opioids may mediate the anticonvulsant mechanism. In a potential model of acute electroconvulsive shock in hippocampal slices, a high-intensity tetanus via the mossy fibers substantially delayed subsequent in vitro kindling through the Schaffer collaterals. Naloxone blocked this effect, implicating the opioid system in the antiepileptogenic properties of this model.

Differential effects of mu- and delta-receptor selective opioid agonists on feedforward and feedback GABAergic inhibition in hippocampal brain slices

Previous studies have suggested that opioid receptor activation in the hippocampus increases pyramidal neuron excitability by reducing GABAergic inhibition. This hypothesis has received support with regard to mu-receptor agonists but has not been adequately tested with selective delta-receptor agonists. In the present investigation we compared the effects of the selective mu-opioid receptor agonist [Tyr-(D-Ala)-Gly-(N-Me-Phe)-Gly-ol]-enkephalin (DAGO) and the delta-receptor agonist [D-Pen2,D-Pen5]-enkephalin (DPDPE) to those of bicuculline methiodide (BMI) on extracellularly recorded feedforward (FFW) and recurrent (feedback; FB) inhibition. It was discovered that the control population spike response, evoked by Schaffer collateral/commissural axon stimulation, increased in response to DAGO, DPDPE, and BMI, while the secondary or test response increased only in the presence of DAGO and BMI. The resulting hypothesis that delta-opioid receptor activation facilitates synaptically evoked responses independently of a reduction of inhibition was investigated by examining the effect of DPDPE on the field EPSP response recorded in stratum radiatum of CA1, or postsynaptically on a burst response activated through antidromic stimulation of pyramidal neurons in low calcium medium. delta-Opioid receptor activation had no effect on either the field EPSP response or the burst response, suggesting that neither synaptic transmission nor postsynaptic excitability were augmented. Finally, the possibility that DPDPE acts to enhance pyramidal cell excitability independently of GABAergic transmission was further investigated by examining responses to both mu- and delta-opioid agonists following treatment with BMI (30 microM). Responses to DPDPE and DAGO were completely blocked by this treatment, supporting the involvement of a GABAergic circuit in the actions of these enkephalins. These results suggest that the delta-opioid receptor agonist DPDPE may mediate a reduction in GABAergic inhibition which is not detectable using paired stimulation techniques designed to examine FFW and FB inhibition in the hippocampal slice.

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.

Evidence for mu opiate receptors on inhibitory terminals in area CA1 of rat hippocampus

The mechanism of disinhibition produced by opioid peptides was studied using intracellular recording in area CA1 of rat hippocampal slices. The mu-selective opioid peptide [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAGO) reversibly depressed directly-activated, monosynaptic inhibitory postsynaptic potentials (IPSPs) evoked in the presence of the excitatory amino acid receptor antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D,L-2-amino-5-phosphonovalerate (APV) in a naloxone-sensitive manner. Depression of monosynaptic inhibitory postsynaptic potentials (IPSPs) by DAGO was not prevented by 1-2 mM Ba2+. DAGO reversibly depressed monosynaptic IPSPs when applied locally close to the recording site, but was ineffective when applied close to the stimulating site in stratum radiatum. These results suggest that DAGO disinhibits pyramidal neurons in area CA1 of the rat hippocampus by activating mu opiate receptors located on the terminals of inhibitory neurons, and by a Ba(2+)-insensitive mechanism.

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.

Differential effects of colchicine lesions of dentate granule cells on wet dog shakes and seizures elicited by direct hippocampal stimulation

Direct electrical stimulation of either the dorsal or ventral hippocampal formation elicits wet dog shakes and overt seizures. Destruction of dentate granule cells in the dorsal hippocampal formation does not significantly reduce the number of wet dog shakes elicited by ventral hippocampal stimulation. However, destruction of dentate granule cells in the ventral hippocampus virtually eliminates wet dog shaking elicited by dorsal hippocampal stimulation. Destruction of either dorsal or ventral dentate granule cells lowers the threshold for eliciting forelimb clonus with rearing. These results suggest that dentate granule cells in the ventral hippocampus are essential for wet dog shakes elicited by intrahippocampal stimulation. However, dentate granule cells throughout the hippocampal formation appear to play an important inhibitory role in the spread of seizure activity within the hippocampus.

Granule cells in the ventral, but not dorsal, dentate gyrus are essential for kainic acid-induced wet dog shakes

Intrahippocampal injections of colchicine selectively destroy dentate granule cells. Wet dog shaking elicited by systemic administration of kainic acid is eliminated by bilateral destruction of ventral dentate granule cells but unaffected by bilateral destruction of dorsal dentate granule cells. This implies that ventral dentate granule cells are essential for the generation of kainic acid-induced wet dog shakes.

Colchicine lesions of ventral, but not dorsal, dentate granule cells attenuate wet dog shakes elicited by perforant path stimulation

Intrahippocampal injections of colchicine selectively destroy dentate granule cells. Wet dog shaking elicited by perforant path stimulation is unaffected by bilateral destruction of dorsal dentate granule cells but virtually eliminated by bilateral destruction of ventral dentate granule cells. This implies that ventral dentate granule cells are essential for the generation of perforant path stimulation-induced wet dog shakes.

Chronic morphine exposure blocks opioid effects on both the early and late inhibitory postsynaptic potentials in hippocampal CA1 pyramidal cells

The mu-opioid agonist, [N-MePhe3,D-Pro4]morphiceptin (PL017), significantly decreased the conductance changes measured during both the early and late inhibitory postsynaptic potentials (IPSP) in CA1 pyramidal cells. Although the conductance change during the early IPSP was much larger than that during the late IPSP, the relative decrease in conductance caused by 1 microM PL017 was similar for both. Chronic morphine treatment of rats prior to hippocampal slice preparation resulted in a loss of PL017 (1 microM) effects on both the early and late IPSPs. These results suggest that opioids have an equal ability to alter both early and late IPSPs in the CA1, that these effects are equally sensitive to chronic morphine, and that these measurements are a sensitive means of determining opioid tolerance in the hippocampus.