Mu- and delta-opioid receptor binding peaks and kappa-homogeneity in the molecular layers of rat hippocampal formation

Ovarian steroids alter mu opioid receptor trafficking in hippocampal parvalbumin GABAergic interneurons

The endogenous hippocampal opioid systems are implicated in learning associated with drug use. Recently, we showed that ovarian hormones regulate enkephalin levels in the mossy fiber pathway. This pathway overlaps with parvalbumin (PARV)-basket interneurons that contain the enkephalin-activated mu opioid receptors (MORs) and are important for controlling the "temporal timing" of granule cells. Here, we evaluated the influence of ovarian steroids on the trafficking of MORs in PARV interneurons. Two groups of female rats were analyzed: cycling rats in proestrus (relatively high estrogens) or diestrus; and ovariectomized rats euthanized 6, 24 or 72 h after estradiol benzoate (10 microg, s.c.) administration. Dorsal hippocampal sections were dually immunolabeled for MOR and PARV and examined by light and electron microscopy. As in males, in females MOR-immunoreactivity (-ir) was in numerous PARV-labeled perikarya, dendrites and terminals in the dentate hilar region. Variation in ovarian steroid levels altered the subcellular distribution of MORs in PARV-labeled dendrites but not terminals. In normal cycling rats, MOR-gold particles on the plasma membrane of small PARV-labeled dendrites (area <1 microm2) had higher density in proestrus rats than in diestrus rats. Likewise, in ovariectomized rats MORs showed higher density on the plasma membrane of small PARV-labeled dendrites 72 h after estradiol exposure. The number of PARV-labeled cells was not affected by estrous cycle phase or estrogen levels. These results demonstrate that estrogen levels positively regulate the availability of MORs on GABAergic interneurons in the dentate gyrus, suggesting cooperative interaction between opioids and estrogens in modulating principal cell excitability.

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.

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.

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.

Multiple heteroreceptors on limbic thalamic axons: M2 acetylcholine, serotonin1B, beta 2-adrenoceptors, mu-opioid, and neurotensin

Ligand binding to many transmitter receptors is much higher in layer Ia of rat posterior cingulate cortex than it is in other layers, and this is where most axons from the anterior thalamus terminate. The present study explores the possibility that a number of receptors may be expressed on axons from limbic thalamic nuclei that terminate in layer Ia. Unilateral thalamic lesions were placed in rats and, 2 weeks later, five ligand binding protocols, coverslip autoradiography, and single grain counting techniques were used to quantify binding in control and ablated hemispheres. Binding to the following receptor subtypes was analyzed: M2 acetylcholine, 3H-oxotremorine-M, or 3H-AF-DX 116 with 50 nM pirenzepine; serotonin1B, 125I-(-)-cyanopindolol with 30 microM isoproterenol; beta 2-adrenoceptors, 125I-(-)-cyanopindolol with 1 microM serotonin and 10 microM atenolol; mu-opioid, 3H-T[r-D-Ala-Gly-MePhe-Gly-ol; neurotensin, 3H-neurotensin. Thalamic lesions reduced binding in two laminar patterns. In one pattern, there was a major reduction in binding in most superficial layers with that in layer Ia ranging from 50 to 70% for binding to M2 muscarinic and serotonin1B receptors. Binding to beta 2-adrenoceptors was also reduced in most superficial layers but to a lesser extent. In the second pattern, reductions were limited to layer I with losses in layer Ia of 20-30% for mu-opioid and neurotensin receptors. In no instance was layer Ia binding completely abolished (i.e., postlesion peaks remained). Since the transmitters for each of the five receptors analyzed in this study are not synthesized by anterior or laterodorsal thalamic neurons, these receptors are heteroreceptors. The greatest postlesion reduction in M2 binding was for AF-DX 116 and so most M2 heteroreceptors are of the "cardiac" subtype. Finally, the diverse population of heteroreceptors on limbic thalamic axons provides for presynaptic modulation by a wide range of transmitter systems and suggests that thalamocortical transmission may not be a simple, unmodulated event.

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.

Microinjection of dynorphin into the hippocampus impairs spatial learning in rats

The effect of hippocampal dynorphin administration on learning and memory was examined in spatial and nonspatial tasks. Bilateral infusion of dynorphin A(1-8) (DYN; 10 or 20 micrograms in one microliters) into the dorsal hippocampus resulted in a dose-related impairment of spatial working memory in a radial maze win-stay task. Subsequent experiments found that acquisition of a reference memory task in the water maze was impaired by DYN injections (20 micrograms/microliters) in the dorsal hippocampus, but not in the ventral hippocampus, and that this impairment could be blocked by naloxone. In a nonspatial task, posttraining DYN injections in the dorsal hippocampus had no effect on retention of step-through passive avoidance. These results suggest that dynorphin specifically interferes with spatial learning and memory, and that this effect is mediated by opioid receptors in the dorsal hippocampus.

Kappa opioid receptors stimulate phosphoinositide turnover in rat brain

The effects of various subtype-selective opioid agonists and antagonists on the phosphoinositide (PI) turnover response were investigated in the rat brain. The kappa-agonists U-50,488H and ketocyclazocine produced a concentration-dependent increase in the accumulation of IP's in hippocampal slices. The other kappa-agonists Dynorphin-A (1-13) amide, and its protected analog D[Ala]2-dynorphin-A (1-13) amide also produced a significant increase in the formation of [3H]-IP's, whereas the mu-selective agonists [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephalin and morphine and the delta-selective agonist [D-Pen2,5]-enkephalin were ineffective. The increase in IP's formation elicited by U-50,488H was partially antagonized by naloxone and more completely antagonized by the kappa-selective antagonists nor-binaltorphimine and MR 2266. The formation of IP's induced by U-50,488H varies with the regions of the brain used, being highest in hippocampus and amygdala, and lowest in striatum and pons-medulla. The results indicate that brain kappa- but neither mu- nor delta-receptors are coupled to the PI turnover response.

Laminar distributions of muscarinic acetylcholine, serotonin, GABA and opioid receptors in human posterior cingulate cortex

Experimental animal studies have demonstrated a number of receptor localizations on specific cortical afferents and neurons. The present study of human posterior cingulate cortex evaluates the laminar distributions of particular receptors and their likely association with components of the neuropil. Coverslip autoradiographic and single grain counting techniques were used followed by heterogeneity analysis in which the layer of peak binding and an index of heterogeneity were determined for each ligand. The index was calculated by determining specific binding by layer as a percentage of binding in all layers. The differences from an absolutely homogeneous distribution, i.e. 11.1% for each of nine layers, were subtracted and the absolute laminar differences summed to form the index. High indices of over 15 reflected heterogeneous binding patterns in neocortex. The binding of ligands for muscarinic acetylcholine, serotonin, opioid, GABA and beta adrenoceptors was evaluated. Pirenzepine binding peaked in layer II of area 23a but was extremely homogeneous with an index of heterogeneity of 8.9. In contrast, oxotremorine-M binding had a peak in layer IIIc and an index of 16.4, while AF-DX 116 binding peaked in layer IIIa-b and had an index of 30.6. Of the ligands for serotonin uptake and receptor binding paroxetine binding was evenly distributed in layers I-III and had a low index of heterogeneity of 9.8. Ketanserin binding was also homogeneous and, since it had an index of 8.9, this pattern was virtually the same as that for paroxetine. In contrast, serotonin and 8-hydroxy-2-(di-n-propylamino)tetralin binding peaked in layer II and had very high indices of 20.8 and 50.3, respectively, suggesting only a limited association with that of the paroxetine distribution. Finally, there were three layers which contained peaks in binding for ligands for opioid, GABA and beta adrenoceptors. Firstly, layer Ia had peak dynorphin-A binding, the latter of which had an index of 22.6. Secondly, Tyr-D-Ala-Gly-MePhe-Gly-ol and 2-D-penicillamine-5-D-penicillamine-enkephalin binding peaked in layer II and had indices of 8.6 and 17.4, respectively. Thirdly, muscimol and (-)-cyanopindolol binding peaked in layer IIIa-b and had indices of 29.6 and 11.1, respectively. When viewed in the context of experimental animal studies, it is likely that heterogeneities in oxotremorine-M and paroxetine binding are associated with the termination of the thalamic and raphe nuclei, respectively. While serotonin 2 receptors are co-distributed with serotonin uptake sites, serotonin 1A receptors have a significant mismatch with these sites.(ABSTRACT TRUNCATED AT 400 WORDS)