Electrophysiological effects of selective opioid agonists on spontaneous and evoked neuronal activity in the dentate gyrus of the hippocampus in vivo

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.

Morphine-induced potentiation in the dentate gyrus of the hippocampus involves norepinephrine

We examined the role of norepinephrine in the morphine-induced potentiation in the dentate gyrus of the hippocampus in chronically implanted freely moving rats. The population spikes of the field potentials in the dentate gyrus following perforant path stimulation were recorded before and after morphine injection. We found that a single dose of morphine sulfate (5 mg/kg, i.v.) resulted in a long-lasting augmentation in the amplitudes of population spikes. When pretreated with propranolol (5 mg/kg, i.v.), a beta-adrenoceptor antagonist, the morphine-induced potentiation was significantly attenuated. These results suggested that an increase in norepinephrine release in the hippocampus contributed to the morphine-induced potentiation.

Morphine augments excitatory synaptic transmission in the dentate gyrus through GABAergic disinhibition

The present study investigated the effect of morphine on synaptic transmission and long-term potentiation (LTP) in the dentate gyrus using rat hippocampal slice preparations. Field excitatory postsynaptic potential (fEPSP) and population spike (PS), evoked by stimulation of the perforant path, were recorded from the dentate molecular layer and the stratum granulosum, respectively. Following application of 10 microM morphine, PS amplitude increased gradually in 10 min and was eventually potentiated by approximately 50%. The phenomenon showed a concentration-dependent manner and was completely canceled by naloxone, a mu opioid receptor antagonist. Furthermore, morphine-induced PS augmentation was not detected in disinhibited hippocampal slices, which suggests that the inhibitory input to the dentate granule cells was required for the facilitatory effect of morphine. Neither fEPSP nor tetanus-induced LTP of PS was altered by morphine application. The data support the hypothesis that mu opioid receptor activity modulates inhibitory recurrent circuits in the dentate gyrus and thereby, indirectly plays a regulatory role for hippocampal excitatory neurotransmission.

Selective opposite modulation of dentate granule cells excitability by mu and kappa opioids in rat hippocampal slices

The effects of opioids having affinity for kappa (U50,488H and U54,494A) and mu receptors (DAMGO and methadone) were tested on the excitability of the dentate and CA1 neurons in rat hippocampal slices. Slice perfusion with 25 microM U50,488H or with 12 microM U54,494A produced within 60 min a significant (P < 0.05) decrease in the amplitude of the primary dentate population spike (PS). A similar decrease occurred on the CA1 PS amplitude only at concentrations higher than 100 microM of U50,488H or 50 microM of U54,494A. Slice perfusion with 0.5 microM DAMGO, or 100 microM methadone produced an increase in the amplitude of the primary dentate and CA1 PS and the appearance of secondary PSs. Slice perfusion with 12 microM U50,488H or with 25 microM of methadone significantly (P < 0.05) decreased or increased, respectively, the rate of appearance of the dentate posttetanic potentiation (PTP) and long-term potentiation (LTP) after a 100 Hz tetanic stimulation of the perforant path. The same concentration of U50,488H or methadone did not affect the rate of appearance of the CA1 PTP and LTP after a 100 Hz tetanic stimulation of the Schaffer collaterals. The data, providing evidence for a selective opposite modulation by mu and kappa opioids on the basal and stimulated dentate neuronal excitability, indicate the dentate area as a target within the hippocampus for an opposite influence between mu and kappa opioids on neuronal excitability.

Site-selective acute desensitization following local administration of opioid in the hippocampus

Electrophoretic administration of the mu selective opioid agonist [D-Ala2, NMe-Phe4, Gly-ol]-Enkephalin (DAMGO) in the dentate gyrus of the hippocampus acutely produces a marked increase in the responsiveness of dentate granule cells to perforant path stimulation. This can be measured by an increase in the primary population spike (PS) amplitude and by disinhibition in the paired-pulse (PP) paradigm. Concomitantly, the spontaneous single unit activity of interneurons is usually inhibited. We have observed that after prolonged (usually 10-20 min) local (electrophoretic) administration of DAMGO, a second, late effect is noted, suggesting acute desensitization. There is a loss of the disinhibition seen in the PP paradigm while the primary PS shows only some increased variability in response to stimulation. Furthermore, in a time course parallel to the loss of disinhibition, single cell activity initially inhibited by DAMGO appears to lose its responsiveness. Pretreatment with kappa or delta opioid agonists, or with GABA agonists and antagonists, does not affect the development of this desensitization suggesting selective involvement of the mu receptor. We further propose a regional specificity within the hippocampus since we are unable to detect evidence of desensitization to opioid in CA1 using the same techniques.

Electrophysiological effects of Mu-selective opioids on hilar neurons in the hippocampus in vivo

Although mu-selective opioids have been shown to produce dramatic effects on neurons within the CA1 and dentate regions of the rat hippocampus, little is known regarding their effects on neurons within the hilus, a region of potential importance in several disease states. We studied the neurophysiologic responses of hilar neurons recorded extracellularly to electrophoretic [D-Ala2, NMe-Phe4, Gly-ol]-enkephalin (DAMGO) and systemic morphine (MS) in anesthetized rats. We found that hilar cells could be readily divided into two categories, based on their pattern of spontaneous activity and response to perforant path stimulation. Cells that discharged in a bursting-type pattern formed a homogeneous group electrophysiologically. The response of these cells to opioids was dependent on route of administration, with the spontaneous activity of all cells tested increasing following electrophoretically administered DAMGO, and remaining unchanged in response to systemic MS. Cells that discharged in a non-bursting pattern showed some electrophysiologic variation, as well as some differential response to opioids. However, the spontaneous activity in the majority of non-bursting cells increased following electrophoretic administration of DAMGO. In these cells, MS produced similar, although usually less dramatic, effects. Comparison with intracellular data suggests that the bursting cells in our study correlate most closely with hilar "mossy cells," while the non-bursting action potentials were recorded from other cells, primarily putative interneurons. We conclude that mu-selective opioids produce excitation of mossy cells, probably through an indirect mechanism, with the primary site of action occurring on cells in the granule cell layer. This regional excitation may help to mediate the effects of locally administered mu-selective opioids within the dentate gyrus.

Endogenous opiates: 1994

This article is the 17th installment of our annual review of research concerning the opiate system. It includes papers published during 1994 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics covered this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.