Opioid peptides excite pyramidal neurons and evoke epileptiform activity in hippocampal transplants in oculo

In vitro electrophysiological analysis of mature rat hippocampal transplants in oculo

We have investigated the maturation of isolated rat hippocampus grafted into the anterior chamber of the eye. Electrophysiological responses from transplants were compared to those recorded from the in vitro hippocampal slice preparation. Intracellular recording demonstrated that the passive membrane characteristics of intraocular hippocampal neurons were similar to those of the CA1 pyramidal cells in the in vitro slice preparation. However, the slow after-hyperpolarization which normally follows depolarization-induced action potentials was reduced or completely absent in the intraocular transplants, and the excitatory postsynaptic potential (EPSP) evoked by local stimulation was prolonged. The duration of the EPSP was reduced by perfusion with D-aminophosphonovaleric acid (2.5-50 microM), an N-methyl-D-aspartate receptor antagonist. Normal levels of glutamate decarboxylase (a marker for gamma-aminobutyric acidergic neurons) were found in the transplants, and responses to adenosine, bicuculline, and norepinephrine were similar in the in oculo transplants and in vitro slices. The data suggest that although many properties of hippocampal neurons are intrinsically determined, other aspects of the physiology of mature hippocampus either fail to develop, or develop abnormally in the absence of external inputs in oculo.

Alterations in excitatory and GABAergic inhibitory connections in hippocampal transplants

Solid pieces of embryonic hippocampal tissue were implanted in a cavity formed by aspiration of the fimbria-fornix and the overlying cingulate cortex in adult rats. Six to 8 months after the transplantation, chronic recording electrodes were implanted into the graft and the host hippocampi for the recording of electroencephalogram and unit activity in the freely moving animal. Irregularly occurring sharp waves or electroencephalogram spikes and concurrent synchronous discharge of large groups of neurons dominated the electrical activity of the grafts, in contrast to the situation in normal animals. Light microscopy and GABA immunocytochemistry in the grafts revealed that the three major cell types of the hippocampal formation, i.e. pyramidal neurons, dentate granule cells and GABA-immunoreactive interneurons were present in the hippocampal grafts. At the ultrastructural level, however, significant alterations in connectivity were observed. The most striking finding was the absence or sparse occurrence of synapses on the axon initial segments of pyramidal neurons. The axon initial segments are normally densely covered by GABAergic synapses derived from a specialized type of interneuron, the chandelier or axo-axonic cell. On the other hand, numerous GABA-immunoreactive terminals were found in synaptic contact with somata of pyramidal neurons, suggesting that other types of GABAergic interneurons and their efferent connections may have developed in a normal manner. The cell bodies of pyramidal neurons received, in addition, several asymmetric synapses from GABA-negative terminals. These presumably excitatory synapses are not present on the somata of pyramidal cells in the normally developing hippocampus. We hypothesize that the somatic excitatory synapses originate, at least in part, from the axon collaterals of the neighbouring pyramidal cells in the graft. We suggest that the hyperexcitability of the neuronal circuitry within the graft is due to reduced inhibition (lack of axo-axonic synapses) coupled with increased collateral excitation of the pyramidal neurons.

The effects of procaine HCl on population cellular and evoked response activity within the limbic system of the cat. Evidence for differential excitatory action of procaine in a variety of limbic circuits

1. The effects of intravenous injections of procaine HCl on population cellular activity in limbic tissue and overlying cortex, and on transmission of evoked activity between limbic structures was investigated in awake cats. Clear dose-related increases in cellular activity were seen in amygdala and ventral hippocampus. Changes in cellular activity in the nucleus accumbens and temporal neocortex were also dose-related, but in a complex time-dependent manner. Changes in ventromedial hypothalamus only appeared at the second highest dose of procaine. 2. Procaine facilitated transmission of evoked excitatory activity from the amygdala to the ventromedial hypothalamus, but only after a considerable delay from the time of injection. On the other hand, procaine had no effect on activity evoked in the ventral hippocampus, nucleus accumbens or temporal cortex by amygdala stimulation. 3. It was concluded that intravenous procaine functions as an excitant of limbic system cells, and that procaine alters synaptic transmission in some, but not all, output pathways from the amygdala. The neuroexcitant effects of procaine appear to be idiosyncratic, however, varying over dose with limbic and cortical area examined.

Basic science and clinical aspects of procaine HCl as a limbic system excitant

The literature in animals and humans which indicate that systemic procaine HCl activates limbic tissue is reviewed. Studies in cats which suggest that procaine excites limbic cells by reducing neural inhibition are then described. Evidence that power spectral analysis of high frequency EEG bands (omega or 31-55 cps) in the temporal cortical EEG reflects degree of limbic (amygdala) excitation in animals and humans is reviewed. Studies in cats are described which show that procaine selectively increases omega band activity in the amygdala and temporal cortex in a dose related fashion which parallels dose related increases in amygdaloid neural activity. Preliminary results of combining intravenous procaine and omega band analysis of scalp EEG in humans to predict therapeutic response to carbamazepine in borderline personality and affective disorder patients are then described. The effects of procaine on omega are compared to the effects of direct electrical stimulation of human limbic system in complex partial seizure patients undergoing assessment for temporal lobectomy. The results tentatively support the hypothesis that some psychiatric patients have hyperexcitable limbic systems, and those that do, show a positive behavioural response to carbamazepine.

Survival and growth of neurons with enkephalin-like immunoreactivity in fetal brain areas grafted to the anterior chamber of the eye

Areas of fetal rat brain and spinal cord known to contain enkephalin-like immunoreactive cell bodies and/or terminal fields were transplanted to the anterior chamber of the eye of adult rats. Enkephalin-like immunoreactive neurons survive and produce an enkephalin-like immunoreactive fiber network within grafts of spinal cord, ventral medulla oblongata, ventrolateral pons, tectum, locus coeruleus, substantia nigra and the areas containing columna fornicis and globus pallidus. Although single intraocular grafts of neocortex do not apparently contain enkephalin-like immunoreactive fibers, such grafts contain a variable amount of sparsely distributed enkephalin-like fibers when sequentially grafted in oculo with either locus coeruleus or spinal cord. Combinations of locus coeruleus and globus pallidus contained a rich enkephalin fiber network in the locus coeruleus part and a sparse innervation of the globus pallidus part. We conclude that enkephalin-like immunoreactive neurons in small areas of fetal rat brain can be successfully transplanted to the anterior chamber of the eye. They are able to survive and develop to maturity in complete isolation from the rest of the brain. In general, the enkephalin-like immunoreactive fiber density in the various single grafts approximated that of their brain counterparts in situ. Fiber formation can be reinitiated in mature enkephalin-like immunoreactive neurons by addition of new brain target areas. Thus, the technique permits establishment of isolated, defined enkephalin systems and pathways accessible to functional analysis.

Comparison of neuronal activity in septal and hippocampal grafts developing in the anterior eye chamber of the rat

Septal and hippocampal tissue of the rat embryos (17-18 days of gestation) was transplanted into the anterior eye chamber of adult rats. Four to six months later the grafts with the piece of iris were transferred into an incubating chamber, and extracellular registration of neuronal activity was performed. Septal grafts (SG) were characterised by the presence of a greater number (70%) of spontaneously-active units, than the hippocampal grafts (HG, 41%). The level of activity in SG units was higher (mean 2.2 spikes X s-1, maximal 24 spikes X s-1), than in HG (mean 1.0 spikes X s-1, maximal 3.0 spikes X s-1). Among spontaneously active HG units, the cells with 'complex spikes' constituted the majority; in SG, besides the units with irregular single spike activity, the neurons with regular pacemaker-like and rhythmic burst (0.5-5 per s) activity were present. Stimulation of interface between iris and a graft evoked a single-spike or short burst responses in HG units. In SG, responses were tonic with gradual increase of discharge frequency or with repetitive bursts; in some cases suppression of the activity was observed. Responses of HG units followed repetitive stimulation up to 30-50 Hz and were characterized by prominent frequency potentiation. This was not observed in the SG units. These data show that embryonal nervous tissue differentiating in the anterior eye chamber develops some organotypic features of spontaneous and evoked activity similar to the activity of corresponding structures in normal conditions and in slices of adult brain.

Pro- and anticonvulsant actions of morphine and the endogenous opioids: involvement and interactions of multiple opiate and non-opiate systems

The proconvulsant actions of high doses of systemic morphine are probably mediated by 3 different systems. One of them produces non-convulsant electrographic seizures and can be activated separately from the others both by intracerebroventricular injections as well as microinjections into discrete subcortical areas. The enkephalins and beta-endorphin, when administered to the same loci, produce similar effects. Pharmacological evidence suggests that specific opiate receptors of the delta-subtype mediate the epileptiform effects produced by this system. The second system mediating proconvulsant effects of systemic morphine is not mediated by stereo-specific opiate receptors. It produces behavioral convulsions, and the GABA-ergic system has been implicated in its action. A third proconvulsant action of systemic morphine can be activated separately from the other two systems by administering this compound with other convulsive agents or manipulations. Specific mu-type opiate receptors are implicated in this effect. In addition to potent proconvulsant effects, systemic morphine also has anticonvulsant properties which are mediated by specific opiate mu-receptors. The conditions under which morphine acts as a proconvulsant rather than an anticonvulsant agent are, as yet, not understood.

Opiate and non-opiate aspects of morphine induced seizures

The intraperitoneal administration of morphine hydrochloride at doses of 300 mg/kg produced analgesia, catalepsy, and electrographic spiking in rats that developed into electrographic seizure patterns after approximately 2.5 h. Whereas naltrexone (12 mg/kg) reversed analgesia and catalepsy, and diminished electrographic spiking, it precipitated electrographic seizure activity similar to that observed following intraperitoneal morphine alone. These seizures were accompanied by behavioral convulsions. No tolerance to these seizures developed with repeated paired administration of morphine and naltrexone or in morphine tolerant rats, but rather potentiation was observed. The epileptogenic effects were found to be potentiated in amygdaloid kindled rats, as well. It was concluded that morphine at these doses activates two different epileptogenic mechanisms, one mediated by opiate receptors, the other not. The possibility of the simultaneous activation of a morphine sensitive anticonvulsant mechanism is discussed.

Opiates, endorphins and the developing organism: a comprehensive bibliography

A comprehensive bibliography of the literature concerned with opiates, endorphins, and the developing organism is presented. A total of 1378 clinical and laboratory references, with citations beginning in 1875, are recorded. A series of indexed accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics. The clinical section is subdivided into: age of subject examined; maternal aspects; effects on the fetus; pharmacology, physiology, and the withdrawal syndrome; and "other" effects on the offspring. The laboratory section is subdivided into: type of opiate/endorphin studied; species utilized; and major subject areas explored.

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.

An iontophoretic survey of opioid peptide actions in the rat limbic system: in search of opiate epileptogenic mechanisms

Iontophoretic and micropressure drug application and lesion techniques were used to investigate the cellular source of rat limbic system epileptiform responses to opioid peptides [19]. Iontophoretically applied morphine, methionine enkephalin or beta-endorphin inhibited the spontaneous or glutamate-activated firing of the great majority of single neurons in medial and lateral septum, amygdala and cingulate cortex. These inhibitions in firing were antagonized by iontophoresis of naloxone. In contrast to inhibitory effects in other limbic areas, morphine and the opioid peptides predominantly excited CA1 and CA3 pyramidal neurons in a naloxone-sensitive manner, as previously reported [36]. On rare occasions, iontophoretically applied beta-endorphin evoked repetitive waveforms similar to interictal population EPSPs or spikes. Micropressure application of opiates and peptides also excited hippocampal neurons indicating such responses were not current-induced artefacts. The possible role of the excitatory cholinergic septal hippocampal pathway in the facilitatory response of hippocampal units to the opiates was tested with iontophoretically applied atropine and scopolamine, or lesions of septal nuclei. None of these manipulations reduced the opioid-induced excitations; rather, septal lesions enhanced excitatory and epileptiform responses to the opiates. These results support the hypothesis that opiate-evoked epileptiform activity in the limbic system arises from enhanced pyramidal cell activity in the hippocampal formation, probably by a non-cholinergic mechanism.

Electrophysiological interactions of enkephalins with neuronal circuitry in the rat hippocampus. II. Effects on interneuron excitability

The effects of active and inactive enkephalin derivatives and naloxone on putative interneurons were studied in the in vitro hippocampal slice. Inhibitory interneurons were recorded from extracellularly, and identified electrophysiologically on the basis of their characteristic action potential shape and pattern of evoked firing in response to single and multiple electrical stimuli. Active enkephalin derivatives elicited a dose-dependent depression in excitability whereas inactive derivatives had no effect. Naloxone reliably and reproducibly antagonized the depressant action of active enkephalins. These data confirm the hypothesis outlined in the preceding communication, that the direct effect of enkephalins in the hippocampus is a depression of firing of inhibitory neurons, and support the hypothesis that enkephalin-induced excitations of pyramidal cells are brought about by disinhibition.

Opioid peptides may excite hippocampal pyramidal neurons by inhibiting adjacent inhibitory interneurons

The atypical excitation by opiates and opioid peptides of hippocampal pyramidal cells can be antagonized by iontophoresis of naloxone, the gamma-aminobutyric acid antagonists bicuculline, or magnesium ion. The recurrent inhibition of these cells evoked by transcallosal stimulation of the contralateral hippocampus is blocked by enkephalin but only shortened by acetylcholine. The results suggest that the opioids excite pyramidal neurons indirectly by inhibition of neighboring inhibitory interneurons (probably containing gamma-aminobutyric acid). This mechanism may be pertinent to the electrographic signs of addictive drugs.