Electrophysiological effects of enkephalin analogs in rat cortex

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Opioid effects on activation of neurons in the medial prefrontal cortex

1. The effects of opioids have been characterized in portions of the neural circuitry proposed to underly the development and maintenance of addiction. One possible mechanism is modulation of function of endogenous transmitters. 2. Cells in the prefrontal cortex, a brain area involved in cognitive function and processes relevant to addiction, are described that exhibit morphine-associated attenuation of activation response to glutamate but not acetylcholine. 3. The predominantly excitatory response of prefrontal cortical cells to local application of glutamate and acetylcholine were differentially modified by systemic and local application of opioids. 4. Local mu opioid effects mimic those of systemic morphine to a more limited degree. 5. Morphine attenuates the response of prefrontal cortical cells to activation of excitatory afferents from the mediodorsal thalamus, and to a lesser degree, from the basolateral amygdala and the hippocampus. 6. Morphine modulation of prefrontal excitatory activation is naloxone-reversible.

Opioid actions on neurons of rat lateral amygdala in vitro

Intracellular recordings were made from neurons in the lateral nucleus of the amygdala, in a slice of rat brain that was superfused in vitro. [Met5]enkephalin (3-30 microM) and the mu receptor selective agonist DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol; 0.3-3 microM) hyperpolarized about 50% of cells; this was blocked by naloxone and by the mu receptor antagonist CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2). The pA2s for naloxone and CTOP were 8.3 and 7.7, respectively. DPDPE (Tyr-D-Pen-Gly-Phe-D-Pen: delta receptor selective) and U50488 (trans-(+-)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide methane sulfonate; kappa receptor selective) had no effect. Synaptic potentials mediated by gamma-aminobutyric acid (GABA) acting at GABAA receptors were elicited by focal stimulation of the slice in a combination of 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (10 microM) and 4-aminophosphonovaleric acid (30 microM). They were inhibited by up to 60% by DAMGO and by DPDPE. The action of DAMGO was blocked by CTOP but not by the delta-selective antagonist ICI174864 (N,N-bisallyl-Tyr-Aib-Aib-Phe-Leu-OH, Aib = aminoisobutyrate). The action of DPDPE was blocked by ICI174864 but not by CTOP. Depolarizations elicited by addition of GABA to the superfusing solution were not affected by opioids. It is concluded that activation of mu opioid receptors hyperpolarizes about 50% of lateral amygdala neurons. Activation of either mu or delta receptors also inhibits presynaptically the release of GABA.

Interaction of putative neurotransmitters in rostral ventrolateral medullary cardiovascular neurons

As recent immunohistochemical evidence has shown the coexistence of putative neurotransmitters in the rostral ventrolateral medulla (RVLM), we have investigated the possibility that there may be an interaction of putative transmitters on the firing frequency of cardiovascular neurons in the RVLM. Extracellular activity was recorded from 37 spontaneously firing units in the right RVLM of urethane anaesthetized and artificially ventilated rats. Nine of these units were classified as cardiovascular neurons because: (i) they were silenced by baroreceptor activation (1-3 micrograms phenylephrine i.v.); and (ii) they showed rhythmicity of their spontaneous activity in synchrony with the cardiac cycle. Microiontophoresis of combinations of near threshold amounts of L-glutamate (GLU; 10 nA), acetylcholine (Ach; 30 nA) and substance-P (SP; 60 nA) showed a synergistic interaction of these substances with one another in eliciting changes in firing frequency of cardiovascular neurons. These results show that GLU and Ach, GLU and SP and Ach and SP interact synergistically to influence the firing frequency of cardiovascular neurons in the RVLM and suggest that these substances play a physiological role in the neural control of the circulation.

Met- and leu-enkephalins inhibit rat cortical neurons intracellularly recorded in vivo while morphine excites them: evidence for naloxone-sensitive and naloxone-insensitive effects

The action of enkephalin-analogues (D-ala2-D-leu5-enkephalin and D-ala2-D-met-enkephalin) and morphine, iontophoretically applied, was tested on rat cortical neurons intracellularly recorded "in vivo". Inhibition of cellular excitability of 60% of the tested cells followed the iontophoretic administration of opioid peptides. 50% of the inhibited cells were also hyperpolarized. The amplitude of membrane hyperpolarization was related to the value of the membrane potential. In 13 out of the 30 inhibited cells the change in membrane input resistance was measured; the input resistance was decreased by 30%. In 8 cells, hyperpolarized by the opioid peptides, the depolarizing postsynaptic potentials, evoked by cortical stimulation, were also reduced in amplitude. Naloxone, iontophoretically applied, reversed and/or prevented the peptide responses. On the same neurons, morphine induced a bursting pattern of spiking activity and increased the membrane input resistance: this action was naloxone-insensitive. The reported results suggest that opioid peptides and morphine activate, respectively, naloxone-sensitive and naloxone-insensitive mechanisms on the same cortical neurons, leading to different and, in some respect, opposite effects on the neuronal activity.

Electrophysiological effects of monoamine-derived aldehydes on single neurons in neocortex and cerebellum in rats

The electrophysiological effects of aldehydes derived from several monoamines were studied on single neurons in the cerebellum and neocortex of rats. The aldehydes derived from dopamine (3,4-dihydroxyphenylacetaldehyde) and serotonin (5-hydroxy-3-acetaldehyde) were prepared as stable disulfite complexes, from which free aldehydes were extracted. Serotonin and 5-hydroxy-3-acetaldehyde caused pronounced depression of firing rates both of cerebellar Purkinje neurons and neurons in prefrontal cortex. When locally applied from multibarrel micropipettes by pressure ejection, 5-hydroxy-3-acetaldehyde was twice as potent in the neocortex as in the cerebellum, and was equipotent with serotonin in both brain areas. The aldehyde of tryptamine also caused depressions of neuronal activity in cerebellum, but only at 5-fold higher doses than were effective for 5-hydroxy-3-acetaldehyde. 3,4-Dihydroxyphenylacetaldehyde was without effect in prefrontal cortex, but had mixed responses in the cerebellum. The results show that monoamine-derived aldehydes are physiologically active. It is possible that changes in the steady state level of these aldehydes caused by drugs such as ethanol and barbiturates might influence the electrophysiological properties of neurons in the central nervous system.

Action of morphine on rat cortical neurons intracellularly recorded in vivo: evidence for an excitatory postsynaptic effect which is naloxone insensitive

The action of morphine, applied either iontophoretically (40-200 nA balanced current) or systemically (5-10 mg/kg, intraperitoneally) to rat cortical neurons, was investigated in vivo, using intracellular electrodes. Morphine increased the apparent input resistance and increased the number of both spontaneous and evoked action potentials. Several cells, which normally generated single spikes, generated bursting potentials; neurons with bursting activity increased their activity. Naloxone, iontophoretically or systemically applied, did not reverse or prevent the morphine-induced excitation. The iontophoretic administration of cadmium suggested that the effects of morphine were due, at least in part, to a postsynaptic site of action. It is suggested that the increase of cellular excitability induced by morphine could contribute to its production of seizures in cortex.

D-ala2-Metenkephalinamide blocks the synaptically elicited cortical spreading depression in rats

Spreading depression (SD) was elicited in rats anesthetized with pentobarbital by a train of 8 electrical pulses (0.1 ms, 10 Hz) applied to parietal cortex. Local application of 50 micrograms of D-ala2-metenkephalinamide (DAME) on the stimulated area evoked one or two SD waves followed by an increase of SD threshold from 40 V to 90 V. This effect could be partly prevented by naloxone (1 mg/kg i.p.) and reversed by local application of 4-aminopyridine (10(-3) M, 2 microliters), which reduced SD threshold to 5 and 20 V in normal and DAME-treated cortex, respectively. It is argued that DAME exerts an inhibitory effect on cortical neurons and that the initial SD facilitation is due to initial blockade of inhibitory neurons in the superficial cortical layers.

Effects of neuropeptides on rat cortical neurons: laminar distribution and interaction with the effect of acetylcholine

The effects of the microiontophoretic application of five different peptides (cholecystokinin octapeptide sulfated form, cholecystokinin octapeptide non-sulfated form, vasoactive intestinal polypeptide, angiotensin-II and substance P) on cortical neurons were studied in rats anaesthetized with urethane. Vertical electrode penetrations were made in the first somatic sensory cortex and the laminar position of the neurons determined by the reconstruction of the tracks based on extracellular dye deposits. The first type of effect observed was an excitation of some cortical neurons. These neurons were mostly found in infragranular layers, specially in layer Vb. Pyramidal tract neurons were more often excited by peptides than the cortical population taken as a whole. Substance P excited the largest percentage of neurons, followed by vasoactive intestinal polypeptide and cholecystokinin octapeptide sulfated form, whereas angiotensin II and cholecystokinin octapeptide non-sulfated form were the least potent in terms of frequency of neurons excited as well as of amplitude of the responses. The vast majority of the neurons excited by a peptide could also be excited by acetylcholine. A second and independent effect of peptides was observed: the neuronal excitation induced by acetylcholine could be depressed by the simultaneous application of peptide. This depressing effect was also the most frequently observed with substance P, followed by cholecystokinin and vasoactive intestinal polypeptide.

A comparison of the effects of morphine, enkephalin, kyotorphin and D-phenylalanine on rat central neurones

1 Morphine, Met-enkephalin, kyotorphin and D-phenylalanine have been applied by microiontophoresis to neurones in the globus pallidus and cerebral cortex of rats anaesthetized with urethane. 2 In the pallidum, most cells were inhibited by all the agonists, with a high correspondence between cells inhibited by Met-enkephalin and D-phenylalanine and by Met-enkephalin and kyotorphin. Whereas responses to Met-enkephalin were readily antagonized by naloxone, responses to kyotorphin and D-phenylalanine were not. 3 In the cerebral cortex a high proportion of cells was excited by all four agonists and antagonism by naloxone was less consistent than in pallidum. 4 It is concluded that the naloxone-reversible analgesic effects of kyotorphin and D-phenylalanine may be mediated indirectly, rather through an activation of opiate receptors.

Trophic effects of brain areas on the developing cerebral cortex: II. Electrophysiology of intraocular grafts

Electrophysiological correlates of locus coeruleus-induced growth stimulation in rat cortical grafts homologously transplanted to the anterior chamber of the eye were studied. Neurons in growth-stimulated grafts manifested a slow sustained spontaneous discharge similar to that found in rat cortex in situ. Local administration of glutamate markedly augmented this discharge. In contrast, neurons from nonstimulated grafts fired in high frequency bursts separated by long pauses, and this discharge was comparatively insensitive to glutamate. Poststimulus inhibition after local stimulation of the transplant surface was readily observed in the growth-stimulated grafts, but absent in all non-stimulated grafts tested. Moreover, superfusion of picrotoxin, which antagonizes GABA-mediated inhibitory pathways, reversibly converted the growth-stimulated graft discharge pattern into one characteristic of non-stimulated grafts. Taken together with the data in the preceding paper, the results demonstrate the importance of extrinsic inputs for functional development of neuronal circuits within neocortex.

Enkephalin inhibition of inhibitory input to CA1 and CA3 pyramidal neurons in the hippocampus

Enkephalin-induced excitation in the hippocampus has been attributed to the attenuation of inhibitory input as well as to augmentation of excitatory input to pyramidal neurons. We have further examined these possible mechanisms of enkephalin action, as well as the possibility that enkephalins may be affecting intrinsic membrane properties, by recording intracellularly from CA1 and CA3 pyramidal cells in the guinea pig hippocampal brain slice preparation. It was observed that the inhibitory synaptic potential was significantly decreased in the presence of leucine enkephalin and D-alanine, D-leucine-enkephalin (DADL), whereas the excitatory synaptic potential, revealed by block of the inhibitory postsynaptic potential (IPSP) by bicuculline, was unaltered. In addition, the response of pyramidal cells to pressure-applied GABA was unaffected by enkephalin, as were the voltage-dependent membrane conductances. The increase in excitability which was observed in both field potential and intracellular recordings to drop application of DADL must, then, be due to a purely presynaptic block of inhibitory interneurons in both the CA1 and CA3 areas of the hippocampus.

Stereoselectivity of opiate antagonists in rat hippocampus and neocortex: responses to (+) and (-) isomers of naloxone

The relative potencies of the (+) and (-) isomers of naloxone in antagonizing electrophysiological responses to D-alanine2-methionine enkephalinamide were compared in rat frontal cortex and hippocampus. In the in vitro hippocampus, the (-) isomer was found to be at least a 100 times more potent than the (+) isomer in antagonizing opiate-induced changes in field potentials. Similar stereoselectivity was observed in vivo in both frontal cortex and hippocampus in terms of the antagonism of enkephalin-induced changes in spontaneous cell firing. The direct effects of (+) and (-)-naloxone were examined as well. In hippocampus both in vivo and in vitro, no differential effect was observed, whereas in the neocortex (-)-naloxone was considerably more potent than the (+) isomer in eliciting depressions of spontaneous activity. These direct effects of naloxone in the cortex do not appear to be due to an antagonism of the effects of endogenously released opioids. These results demonstrate that the stereoselectivity of naloxone isomers in antagonizing electrophysiological responses to opiates in the cortex and hippocampus parallels that previously observed in other brain regions and in other tissues. In addition, they suggest that naloxone may have interactions with other unknown opiate (or possibly non-opiate) receptors which are of physiological significance.

Electrophysiological support in favor of multiple opiate receptors in the caudate and the central gray of the rat

1. The present study compares the direct actions of morphine on two brain sites known to be rich in opiate receptors, namely, the caudate nucleus and the central gray. Recordings and morphine injections were made through a multibarrel glass micropipette using microiontophoresis. 2. Four different patterns of neuronal response to increasing currents of morphine were recorded in both brain regions. 3. Differences in the response to morphine between the two sites were detected in morphine-dependent rats. While the caudate neurons exhibited super-sensitivity to morphine, the neurons in the central gray displayed tolerance, and in some instances, dependence was evident when naloxone was administered. 4. The distribution of spontaneously active neurons within these two brain areas was found to be different in morphine-naive and morphine-dependent rats. 5. The electrophysiological findings of this study support the hypothesis of multiple opiate receptors.

Opiate receptor gradients in monkey cerebral cortex: correspondence with sensory processing hierarchies

In order to obtain information on the possible functions of endogenous opiates in the primate cerebral cortex, we assessed the distribution of mu-like opiate receptors (which selectively bind 3H-labeled naloxone) and delta-like opiate receptors (which selectively bind 3H-labeled D-Ala2, D-Leu5-enkephalin) throughout the cerebral cortex of the rhesus monkey. Stereospecific [3H]naloxone binding sites increased in a gradient along hierarchically organized cortical systems that sequentially process modality-specific sensory information of a progressively more complex nature. Specific [3H]enkephalin binding sites, in contrast, were relatively evenly distributed throughout the cerebral cortex. These results, in combination with electrophysiological studies of monkeys and humans, suggest that mu-like opiate receptors may play a role in the affective filtering of sensory stimuli at the cortical level, that is, in emotion-induced selective attention.

Reactions to ischemic pain: interactions between individual, situational and naloxone effects

Fifty-two paid volunteers participated in two separate factorial investigations of the effects of naloxone on time tolerance of and affective reactions to ischemia, as a function of the interaction between expectations of involvement in the experimental situation and experimental variables involving stress or suggestions of analgesia. Naloxone-induced reduction in tolerance to ischemia interacted significantly with the level of involvement expectancies. The suggestion of analgesia provided no significant naloxone-saline discrimination, but there was a significant interaction between variable memory task conditions and drug effects on the time ischemia was tolerated. These findings suggest that naloxone-opiate receptor interactions may depend on individual differences in attitudes to the situation, but may be potentiated by select environmental stimuli. Analyses of the effects of treatment on affective reactions to ischemia failed to show consistent results.

Influence of unilateral olfactory bulbectomy on opiate and other binding sites in the contralateral bulb

The neurochemical consequences of unilateral olfactory bulbectomy (UBX) in mice were determined in the remaining olfactory bulb at various times after surgery. The most significant finding was a progressive decline in opiate ligand (dihydromorphine) and naloxone) binding that appeared within 11 days after surgery and persisted throughout the study. Statistically significant declines in spiroperidol (-67%), clonidine (-48%) and muscimol (-16%) binding were also observed 90 days after surgery. At 180 days postsurgery we observed a 20% increase in diazepam binding. No effect of UBX on dihydroalprenolol, quinucludinylbenzilate or serotonin ligand binding was observed. Bulbectomy resulted in a moderate decrease (-28%) in DOPA decarboxylase activity 14 days after surgery, which returned to normal by 30 days. Glutamic acid decarboxylase activity decreased by 37% 7 days after UBX, returned to normal by 14 days after surgery and then increased by 25% 90 days after UBX. Unilateral bulbectomy had no effect on cholineacetyltransferase activity in the remaining bulb. Thus, following a unilateral procedure, one bulb cannot necessarily serve as a valid control for the other. Possible explanations for the neurochemical changes observed are discussed.

Electrophysiological interactions of enkephalins with neuronal circuitry in the rat hippocampus. I. Effects on pyramidal cell activity

Effects of enkephalins on hippocampal pyramidal cell activity were studied in situ and in the in vitro hippocampal slice. Active enkephalin derivatives produced a dose-dependent naloxone-reversible excitation in both preparations whereas inactive enkephalin derivatives had no effect. Several different types of experiments, carried out in the slice, strongly suggest that this excitation is due to blockade of inhibitory pathways. First, when the pyramidal cell population spike is increased during enkephalin administration, no change is seen in the simultaneously recorded EPSP. Second, the magnitude of the enkephalin effect is highly correlated with the amount of inhibition, as judged by paired-pulse stimulation, initially present in the slice. Third, if inhibitory pathways are depressed by a brief period of hypoxia, enkephalin has little effect. Finally, enkephalin responses are mimicked by picrotoxin, which selectively antagonizes inhibitory input to the pyramidal neuron. Since enkephalins do not block the effects of GABA, the putative inhibitory transmitter, these data suggest that opioid peptides depress the inhibitory interneurons and disinhibit the pyramidal cells.

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

The effect of opiate peptide administration on the electrical activity of intraocular hippocampal transplants was studied. Similar to observations in situ, the administration of beta-endorphin or methionine enkephalin produces a concentration-dependent increase in the firing rate of identified pyramidal neurons within hippocampal formation transplants. In addition, these peptides elicit a profound increase in 'EEG' amplitude, which ultimately develops into epileptiform activity. The ability of naloxone to either reverse or prevent the peptide-induced changes in both single unit and EEG activity supports the hypothesis that the excitatory response of the hippocampus to opioid peptides is mediated via an opiate receptor. The results of this study also suggest that the excitatory response to the opiate peptides in hippocampus is the result of alterations in intrinsic neuronal circuitry and is not dependent upon extra-hippocampal afferents.