Excitatory action of opioid peptides and opiates on cultured hippocampal pyramidal cells

Opiate alkaloids antagonize postsynaptic glycine and GABA responses: correlation with convulsant action

Opiate alkaloid and opioid peptide actions on spontaneous neuronal activity and postsynaptic amino acid responsiveness were assessed using intracellular recording techniques applied to murine spinal cord neurons in primary dissociated cell culture. Application of opiates was by superfusion and amino acids by iontophoresis. Glycine and GABA but not glutamate responses were antagonized by the opiate alkaloids. Since opiate effects on glycine and GABA responses were not naloxone-reversible, only weakly stereospecific, and not produced by the opioid peptide [D-Ala2]-Met-enkephalinamide, it is unlikely that these effects were mediated by opiate receptors. Opiate depression of glycine inhibition was correlated with the induction of paroxysmal depolarizations in cultured spinal cord neurons, suggesting that antagonism of inhibitory amino acid transmission may underlie the convulsant actions of high concentrations of the opiate alkaloids.

Postsynaptic inhibitory effects of Met- and Leu-enkephalin on endocrine and adjacent neurones in the preoptic-septal region of the guinea pig

Iontophoretic application of enkephalins induced inhibitory effects on unit activity of endocrine and adjacent neurones in the preoptic-septal region. Antagonism or lack of antagonism of these effects by naloxone indicated an action of enkephalins through different opiate receptors. Inhibitory effects of enkephalins were obtained during iontophoretic application of Mg2+, showing that these opioid peptides acted postsynaptically. Because enkephalin acts on endocrine neurones, these opioid peptides might be involved in the control of gonadotrophic hormone release by acting on cell bodies of LH-RH neurones.

Organotypic monolayer cultures of nervous tissue

Cultivation of nervous tissue by means of the roller-tube technique yields thin organotypic cultures. Explants or slices prepared from 1- to 20-day-old rats are embedded in a plasma clot on flying coverslips and cultivated for weeks in roller-tubes. Due to the flattening of the tissue, individual nerve cells are often arranged in monolayer thickness and can, therefore, be viewed with phase-contrast microscopy. This technique is utilized to culture and co-culture nervous tissue derived from various brain regions. The degree of organotypic organization depends on the age of the animals used for culturing. Stable intracellular recordings arae obtained from nerve cells which are impaled under visual control. In view of the accessibility of individual living cells, this approach seems to be particularly well-suited for physiological and pharmacological studies on morphologically identified nerve cells.

Intracellular correlates of morphine excitation in the hippocampal slice preparation

The influences of morphine and opioid peptides on hippocampal CA1 pyramidal cells were investigated using intracellular recordings from the in vitro slice preparation. Morphine applied to somal and basal dendritic areas of CA1 cells via a pressure ejection system confirmed a number of excitatory actions of opiates and opioid peptides in this brain region. These included an increase in the amplitude and duration of orthodromically (radiatum) elicited EPSPs and a decrease in amplitude of the following IPSP. The increase in EPSP amplitude was accompanied by a reduction in stimulus intensity necessary for eliciting the action potential. Morphine delivered to the slice in this manner induced synaptically elicited and spontaneous multiple spike burst discharges. In slices maintained in 10(-4) M pentobarbital, morphine reversed the presumably GABA mediated long-duration depolarization following orthodromic stimulation. Finally, depending on the specific site of application (apical or basal dendritic region) of the opiate, morphine produced two different effects on the resting membrane potential and input resistance of CA1 pyramidal cells. These findings are discussed as to whether opiates act directly upon excitatory influences in the hippocampus, or via blockade of GABA mediated inhibitory mechanisms.

Absence of seizure activity following focal cerebral injection of enkephalins in a primate

Baboons (Papio papio) with photosensitive have been chronically prepared with guide cannulae and deep electrodes to study the effects of focal injections of opioids. In the hippocampus, amygdala and thalamus (centre median) 50--100 micrograms morphine, 20--100 micrograms Met-enkephalin or 2--10 micrograms FK 33,824 do not induce local or general electrographic or motor signs of epilepsy. The acute epileptogenic effect of morphine and enkephalins observed in rats is not a general phenomenon whereas the anticonvulsant action of opioids acting on mu-receptors is seen in rodents and primates.

Morphine and opioid peptides reduce inhibitory synaptic potentials in hippocampal pyramidal cells in vitro without alteration of membrane potential

We used intracellular recording in the hippocampal slice in vitro to characterize further the mechanisms behind the unusual excitatory action of opiates and opioid peptides on hippocampal pyramidal cells in vivo. No significant effect on resting membrane potential, input resistance, or action potential size in cortical area 1 (CA1) pyramidal cells was observed with morphine sulfate, beta-endorphin, [Met5]enkephalin, or [D-Ala2, D-Leu5]enkephalin at 1-50 microM. However, in all cells studied, these agents markedly reduced the size of inhibitory postsynaptic potentials generated by stimulation of the stratum radiatum or alveus. Excitatory postsynaptic potentials were also diminished in many of these cells. The effects of the opioids were antagonized by naloxone. These results are consistent with excitation of pyramidal neurons by a disinhibitory mechanism.

Development of acute tolerance during exposure of hippocampal explants to an opioid peptide

Bath application of FK 33-824, a stable enkephalin analogue, altered synaptic potentials and induced sustained depolarization shifts in hippocampal pyramidal cells. In a proportion of cells, these excitatory effects diminished and then disappeared within 6-8 min during FK 33-824 perfusion. This desensitization was temperature-sensitive. In contrast, no desensitization to similar excitatory effects of bicuculline was seen after acute application. This GABA-antagonist also excited cells which first had been rendered tolerant to the action of FK 33-824, but cross-tolerance occurred between FK 33-824 and (D-Ala)2-(D-Leu)5-enkephalin.

Involvement of mu-receptors in the opioid-induced generation of bursting discharges in hippocampal pyramidal cells

Cultured hippocampal pyramidal cells responded to field stimulation with a short latency excitation followed by a long-lasting inhibition. This sequence was transformed into a bursting response by bath application of 10(-8) M FK 33-824, 10(-6) M (D-Ala)2(D-Leu)5-enkephalin and 10(-5) M bremazocine. Bremazocine and ethylketocyclazocine stereospecifically blocked the effects of FK 33-824. The results indicate that the excitatory responses were predominantly mediated by mu-receptors.

Morphological differentiation of nerve cells in thin organotypic cultures derived from rat hippocampus and cerebellum

Explants of cerebellum and hippocampus were cultured by means of the roller-tube technique. Large nerve cells such as Purkinje cells and pyramidal cells were injected with the fluorescent dye lucifer yellow. The results demonstrate that cultured neurons grow dendritic arborizations in a pattern that is reminiscent of their in situ counterparts. This finding supports the view that under the described culturing conditions nerve cells show a high degree of differentiation despite an altered biochemical and topographical environment.

Effects of opioid peptides on synaptic potentials in explants of rat hippocampus

Bath application of 10(-8) M FK 33-824 (an enkephalin analogue) and 10(-7) M beta-endorphin reversibly induced stimulus-evoked bursting activity with depolarization shifts in cultured hippocampal pyramidal cells. Statistical analysis of the data revealed that in the majority of cells, inhibitory postsynaptic potentials were markedly decreased and excitatory postsynaptic potentials increased prior to the development of bursting activity, although some inhibition persisted in cells exposed to opioid peptides. In a minority of cells, no alteration in synaptic potentials were observed to precede the stimulus-evoked bursts induced by the opioid peptides.

Localization and opiate receptor binding of enkephalin, CCK and ACTH/beta-endorphin in the rat central nervous system

We have examined the distributions of ACTH, CCK and enkephalin immunoreactive nerves in the rat central nervous system and compared the pattern obtained with that of opiate receptors. In addition, a radioreceptor assay has been employed for studying the possible functional interactions between these peptides and opiate receptors. Our results suggest that: (a) The distribution of enkephalin, CCK and ACTH/beta-endorphin immunoreactive nerve terminals is sufficiently similar to suggest functional interaction between these neuropeptides. (b) The CCK immunoreactive nerves display a distribution similar to that of enkephalin, but the main endogenous CCK forms do not bind to opiate receptors. However, opioid peptides and CCK may interact in many brain regions via binding to different, but functionally interacting receptors. (c) The ACTH immunoreactive nerves, which seem also to contain beta-endorphin, shows a partially overlapping distribution with the enkephalinergic systems. Further, ACTH and its fragments bind to opiate receptors. This suggests that ACTH could be an endogenous opioid ligand.

Behavioural effects of (-)naloxone in mice from four inbred strains

To study the role of endogenous opioid peptides in the regulation of behavioural responses to novelty, male mice from the inbred strains SRH, SRL, C57BL/6, and DBA/2 were injected IP with either saline alone, or the opiate antagonist naloxone, dissolved in saline, in dosages of 4 or 8 mg/kg. After 10 min, the animals were placed individually for 20 min in a novel environment and some 12 behavioural components were recorded. Naloxone reduced grooming and incipient rearing in all four strains and it reduced sniffing, leaning against the wall, and locomotor activity in some of them. Object-sniffing, object-learning, defecation, freezing, and Straub tail elevation remained unaffected. The results for grooming and locomotor activity are largely in agreement with reports from others. Rather unexpectedly, the drug enhanced rearing responses in all strains. Although in several cases, a genotype-treatment interaction became apparent, the observed strain differences usually persisted and the correlations found between the behavioural components did not alter much. The naloxone-induced reductions in sniffing, leaning, locomotion, and grooming suggest endogenous opioid involvement in the control of behavioural activation in a novel situation. The increases in rearing possibly result from an additional agonist action of naloxone.

Endogenous opiates: 1980

Vast amounts of research have been done that have attempted to delineate the pharmacological and physiological effects of the endogenous opiate peptides. A great deal of knowledge has also been accumulated in a limited time span concerning the types and locations of the opiate receptors and peptides, as well as their functions. In 1980, reports were made concerning the effects of these peptides on analgesia, on tolerance and dependence, on activity, on learning and memory, on schizophrenia and other types of emotional disturbances, and on physiological responses such as eating and drinking, cardiovascular responses, and sexual function. Additional understanding was also gained concerning their interactions with neurotransmitters, other neuropeptides, and hormones. These and other studies published only in 1980 are reviewed in this paper, which is the third of an annual series.

Comparative immunocytochemical localization of putative opioid ligands in the central nervous system

We report a detailed comparative immunocytochemical mapping of enkephalin, CCK and ACTH/beta-endorphin immunoreactive nerves in the central nervous system of rat and guinea pig. Enkephalin immunoreactivity was detected in many groups of nerve cell bodies, fibers and terminals in the limbic system, basal ganglia, hypothalamus, thalamus, brain stem and spinal cord. beta-endorphin and ACTH immunoreactivity was limited to a single group of nerve cell bodies in and around the arcuate nucleus and in fibers and terminals in the midline areas of the hypothalamus, thalamus and mesencephalic periaqueductal gray with lateral extensions to the amygdaloid area. Cholecystokinin immunoreactive nerve fibers and terminals displayed a distribution similar to that of enkephalin in many regions; but striking differences were also found. An immunocytochemical doublestaining technique, which allowed simultaneous detection of two different peptides in the same tissue section, showed that enkephalin-, CCK- and ACTH/beta-endorphin-immunoreactive nerves although closely intermingled in many brain areas, occurred separately. The distributions of nerve terminals containing these neuropeptides showed striking overlaps and also paralleled the distribution of opiate receptors. This may suggest that enkephalin, CCK, ACTH and beta-endorphin may interact with each other and with opiate receptors.

Inhibition of opiate receptor binding by zinc ions: possible physiological importance in the hippocampus

In the rat and guinea pig hippocampus, the staining pattern for enkephalin (by immunocytochemistry) and for zinc (by the Timm method) is congruent and confined to the mossy fiber system. The stereospecific binding of 3H-enkephalinamide (2-D-Ala-5-L-methionine) to opiate receptors can be totally blocked by zinc ions, both in the hippocampus and in the cerebral cortex, the basal ganglia and the rest of the forebrain. Scatchard analysis of binding isotherms indicates that this inhibitory effect involves a decrease in receptor affinity, whereas the number of binding sites is unaffected. Thiol reductants can reactivate Zn2+-inhibited opiate receptors with a potency correlating to their redox potential (Eo). Thus, our data suggest that zinc ions represent modulators of opiate receptor binding in the hippocampus and that they work through a redox reaction with essential SH-groups of opiate receptors.

Excitatory action of opioid peptides and opiates on cultured hippocampal pyramidal cells

Bath application of low concentrations of opioid peptides and higher concentrations of opiates increased the amplitude and duration of excitatory postsynaptic potentials of pyramidal cells and induced long-lasting depolarization shifts. These actions were reversible and blocked by the opiate antagonist naloxone. Synaptic isolation of the cells by exposure of the cultures to 8 mM Mg2+ not only abolished all spiking and synaptic activity, but also obliterated the peptide effects on pyramidal cells, although these cells were still excited by bath-applied glutamate. The opioid peptides had no detectable effect on resting membrane potential and on the input resistance of the penetrated cells. Experiments in which pyramidal cells were synaptically activated by field stimulation provided direct evidence for a disinhibitory action of the peptides.