Actions of opiates upon single unit activity in the cortex of naive and tolerant rats

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Opioid abuse and brain gene expression

Opiate addiction is a central nervous system disorder of unknown mechanism. Neuronal basis of positive reinforcement, which is essential to the action of opioids, relies on activation of dopaminergic neurons resulting in an increased dopamine release in the mesolimbic brain structures. Certain aspects of opioid dependence and withdrawal syndrome are also related to the activity of noradrenergic and serotonergic systems, as well as to both excitatory and inhibitory amino acid and peptidergic systems. The latter pathways have been recently proven to be involved both in the development of dependence and in counteracting the states related to relapse. An important role in neurochemical mechanisms of opioid reward, dependence and vulnerability to addiction has been ascribed to endogenous opioid peptides, particularly those acting via the mu- and kappa-opioid receptors. Opiate abuse leads to adaptive reactions in the nervous system which occur at the cellular and molecular levels. Recent research indicates that intracellular mechanisms of signal transmission-from the receptor, through G proteins, cyclic AMP, MAP kinases to transcription factors--also play an important role in opioid tolerance and dependence. The latter link in this chain of reactions may modify synthesis of target genes and in this manner, it may be responsible for opiate-induced long-lasting neural plasticity.

Single neurone studies of opioid tolerance and dependence at the ventrobasal thalamic level in an experimental model of clinical pain, the arthritic rat

The aim of this electrophysiological study was to investigate the effects of an acute injection of morphine (1 mg/kg i.v.) or the opioid antagonist naloxone (0.6-2 mg/kg i.v.) on thalamic ventrobasal (VB) neuronal activities recorded in arthritic rats rendered tolerant/dependent by pretreatment with relatively low doses of morphine. Recordings were performed in animals immobilized by i.v. injections of gallamine triethiodide (Flaxedil) and artificially ventilated under a moderate gaseous anesthesia (mixture of one-third O2, two-thirds N2O, 0.5-0.6% halothane). This level of anesthesia, as checked by the electrocorticogram, was stable and appeared sufficiently deep, since no sign of suffering or stress could be detected. The efficacy of morphine on VB neuronal responses induced by mild stimulation of the joints was greatly reduced in morphine-pretreated arthritic rats, compared to naive animals (mean neuronal inhibition of 35 vs 85%, respectively). This indicates that the tolerance phenomena observed in behavioral studies are reflected at the VB level, on neurons involved in pain processes. In addition, naloxone (0.6, 1 and 2 mg/kg i.v.) induced a dramatic increase in the evoked (52, 88 and 93%) and spontaneous (64, 211 and 292%) VB neuronal activities recorded in morphine-pretreated arthritic rats, while these activities were not significantly altered in naive arthritic rats. The time-courses of the modifications induced by naloxone in morphine-pretreated arthritic animals were similar to those of the naloxone-precipitated morphine withdrawal observed in freely moving rats. These findings may represent the neuronal correlate at the VB level of the withdrawal response and/or the hyperalgesia induced in tolerant arthritic rats by high doses of naloxone.

A general role for adaptations in G-proteins and the cyclic AMP system in mediating the chronic actions of morphine and cocaine on neuronal function

Previous studies have shown that chronic morphine increases levels of the G-protein subunits Gia and Goa, adenylate cyclase, cyclic AMP-dependent protein kinase, and certain phosphoproteins in the rat locus coeruleus, but not in several other brain regions studied, and that chronic morphine decreases levels of Gia and increases levels of adenylate cyclase in dorsal root ganglion/spinal cord (DRG-SC) co-cultures. These findings led us to survey the effects of chronic morphine on the G-protein/cyclic AMP system in a large number of brain regions to determine how widespread such regulation might be. We found that while most regions showed no regulation in response to chronic morphine, nucleus accumbens (NAc) and amygdala did show increases in adenylate cyclase and cyclic AMP-dependent protein kinase activity, and thalamus showed an increase in cyclic AMP-dependent protein kinase activity only. An increase in cyclic AMP-dependent protein kinase activity was also observed in DRG-SC co-cultures. Morphine regulation of G-proteins was variable, with decreased levels of Gia seen in the NAc, increased levels of Gia and Goa in amygdala, and no change in thalamus or the other brain regions studied. Interestingly, chronic treatment of rats with cocaine, but not with several non-abused drugs, produced similar changes compared to morphine in G-proteins, adenylate cyclase, and cyclic AMP-dependent protein kinase in the NAc, but not in the other brain regions studied. These results indicate that regulation of the G-protein/cyclic AMP system represents a mechanism by which a number of opiate-sensitive neurons adapt to chronic morphine and thereby develop aspects of opiate tolerance and/or dependence. The findings that chronic morphine and cocaine produce similar adaptations in the NAc, a brain region important for the reinforcing actions of many types of abused substances, suggest further that common mechanisms may underlie psychological aspects of drug addiction mediated by this brain region.

Localization and ontogeny of cells expressing preprodynorphin mRNA in the rat cerebral cortex

The regional distribution of preprodynorphin (PPD) mRNA-containing cells (PPD cells) in the rat cerebral cortex was investigated by in situ hybridization histochemistry using a synthetic oligonucleotide probe. In the isocortex, PPD cells were small or medium-sized and were mainly located in layer V. While they were less numerous in the allocortex than in the isocortex. Only a few labeled cells were seen in the piriform and entorhinal cortices. In the hippocampal formation, labeled cells were observed in the granular layer of the dentate gyrus. An ontogenetic study revealed that PPD mRNA-containing cells appeared on postnatal day 7 in the isocortex and the allocortex and on day 14 in the dentate gyrus. Thereafter, they increased in number and signal intensity to reach a plateau on postnatal day 14 in both the isocortex and the allocortex and on day 35 in the dentate gyrus. The time-course of development of PPD mRNA-containing neurons in the cerebral cortex suggested that PPD-derived peptide has a neuromodulator and/or neurotransmitter role in these regions of the brain.

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.

Regulation of G proteins by chronic morphine in the rat locus coeruleus

A possible role for G proteins in contributing to the chronic actions of opiates was investigated in the rat locus coeruleus (LC). The LC is a relatively homogeneous brain region that appears to play an important role in mediating acute and chronic opiate action in animals, as well as in humans. It was found that chronic, but not acute, treatment of rats with morphine, under conditions known to induce states of opiate tolerance and dependence, produced an increase in the level of pertussis toxin-mediated ADP-ribosylation of G proteins in the LC. The morphine-induced increase in ADP-ribosylation occurred in both Gi and Go, and was observed over a 30-fold range of NAD concentrations used. Concomitant treatment of rats with the opiate receptor antagonist naltrexone blocked the ability of morphine to produce this effect. In contrast, chronic morphine had no effect on pertussis toxin-mediated ADP-ribosylation of Gi and Go in the other brain regions studied, including the neostriatum, frontal cortex, and dorsal raphe. Chronic morphine also had no effect on cholera toxin-mediated ADP-ribosylation of Gs in the LC and these other brain regions. Preliminary immunoblot analysis revealed that increased ADP-ribosylation levels of the alpha subunit of Go in the LC were associated with equivalent increases in the immunoreactivity of this protein in this brain region. It is possible that the observed regulation of G-proteins by morphine in the LC represents part of the changes that underlie opiate addiction in these neurons.

Cellular and molecular mechanisms of drug dependence

The molecular and cellular actions of three classes of abused drugs--opiates, psychostimulants, and ethanol--are reviewed in the context of behavioral studies of drug dependence. The immediate effects of drugs are compared to those observed after long-term exposure. A neurobiological basis for drug dependence is proposed from the linkage between the cellular and behavioral effects of these drugs.

The lateral vestibular nucleus is a site for the depressant action of benzodiazepines on the crossed extensor reflex

The effects of diazepam microinjected into the subcerebellar nuclei, the lateral vestibular nucleus (LVN), the red nucleus and the substantia nigra were investigated using the crossed extensor reflex (CER) model in chloralose-anesthetized rats. Microinjections of diazepam at doses of 1 and 25 ng/animal into the left LVN, ipsilateral to the testing muscle, produced a dose-related CER depression, while injections of diazepam at a dose of up to 100 ng/animal into the right LVN and into other regions tested produced little effect. The CER depression produced by diazepam could be clearly reversed by a subsequent intravenous administration of Ro 15-1788, a benzodiazepine antagonist. Electrical lesions of the left LVN decreased the amplitude of CER in otherwise untreated rats. Intravenously administered diazepam at a dose of 0.01 or 0.05 mg/kg decreased spontaneous firing of LVN neurons. Iontophoretically applied GABA or flurazepam, a water-soluble benzodiazepine, decreased the spontaneous firing of LVN neurons. Depressant actions on neuronal activity in the LVN produced by both diazepam and flurazepam were clearly antagonized by intravenously administered Ro 15-1788, while it failed to reverse the depression produced by GABA. These findings indicate that the LVN is a major site of the CER depressant action of benzodiazepines and a possible mechanism of action is discussed.

Are astroglial cells involved in morphine tolerance?

Morphine gives rise to a cascade of events in the nervous system affecting, among others, neurotransmitter metabolism. Tolerance develops for various effects shortly after administration of the drug. Also, physical dependence develops and can be demonstrated by precipitation of withdrawal reactions. Biochemical events in nervous tissue have been extensively studied during morphine treatment. This overview will focus upon brain protein metabolism since macromolecular events might be of importance for development of long-term effects, such as tolerance and physical dependence. Both dose- and time-dependent changes in brain protein synthesis and the syntheses of specific proteins have been demonstrated after morphine treatment, although methodological considerations are important. Different experimental models (animal and tissue culture models) are presented. It might be interesting to note that astroglial protein synthesis and the secretion of proteins to the extracellular medium are both changed after morphine treatment, these having been evaluated in astroglial enriched primary cultures and in brain tissue slices. The possibility is suggested that proteins released from astroglial cells participate in the communication with other cells, including via synaptic regions, and that such communication might of significance in modifying the synaptic membranes during morphine intoxication.

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.

Effects of naloxone and repeated stimulus presentation on cortical somatosensory evoked potential (SEP) amplitude in the rat

The effects of naloxone (1 mg/kg) and of repeated stimulus presentation were investigated on a cortical somatosensory evoked potential elicited by innocuous stimulation of the spinal trigeminal tract in awake, minimally restrained rats. The amplitude of an early cortical component (9-ms latency) habituated after administration of saline but did not habituate after naloxone. Naloxone also enhanced the response decrement of late somatosensory evoked potential components (60 to 120 ms) produced by repeated stimulation. The amplitude of midlatency components (14 to 50 ms) did not change after administration of saline and repeated stimulus presentation. However, the amplitude of these components increased after administration of naloxone and repeated stimulus presentation.

Amino acid incorporation during morphine intoxication. II: Electrophoretic separation of extracellular proteins from cerebral hemisphere slices and astroglia-enriched primary cultures

Radioactively labeled proteins were identified in a broad molecular weight range in the incubation medium of rat cerebral hemisphere slice preparations or primary cultures after incubation with radioactive valine. Morphine chloride caused an increase in labeled media proteins with MW approximately 40,000 and 65,000 in both preparations, whereas a fraction with MW, approximately 80,000 decreased in amount. In the culture preparation a MW approximately 15,000 fraction also decreased after morphine treatment. The effects on the MW 40,000 and 65,000 fractions by morphine could not be blocked by naloxone hydrochloride, an opiate antagonist. Labeled media proteins from primary cultures obtained from various brain regions showed many similarities. After 10(-5) or 10(-6) M morphine, protein fractions with MW approximately 15,000, 25,000, 40,000, 65,000-70,000, 80,000, and 100,000 changed with increases or decreases in the various morphine concentrations. The 3H-labeling of the MW approximately 40,000 fraction increased in all cultures, where it decreased. In conclusion, proteins synthesized within cells in brain slices or in astroglia-enriched cultures are released or secreted into the incubation media. Some protein fractions are affected in different directions by morphine. The effects were not correlated with the presence of classical opiate receptors on the cells. This was so even though more changes in media proteins occurred in astroglial cultures from opiate receptor-rich brain regions than in similar cultures from opiate receptor-poor regions. The possible significance of proteins synthesized in astroglial cells and released extracellularly in opiate action is discussed.

Actions of D-Ala2-D-Leu5-enkephalin and dynorphin A (1-17) on neocortical neurons in vitro

Intracellular recordings were made from neocortical neurons in vitro. Application of D-Ala2-D-Leu5-enkephalin (DADL) by different methods produced a decrease in EPSP amplitude and in the amplitude of L-glutamate-induced depolarizations without changes in membrane potential or membrane input resistance. The DADL effects were blocked by naloxone and persisted when synaptic transmission was depressed, suggesting DADL acts on postsynaptically located opiate receptors. With dynorphin A (1-17), depolarizations, hyperpolarizations, decreases and increases in EPSP were observed, but never an anti-glutamate effect.

Alpha and gamma interferons' effects on cortical and hippocampal neurons: microiontophoretic application and single cell recording

Responses of 96 extracellular spontaneous active cortical and hippocampal neurons to microiontophoretically applied four types of alpha interferons (alpha-IFNs), one type of gamma interferon (gamma-IFN) as well as three fractions of gamma-IFNs were examined. All four types of alpha-IFN ejections increased the discharge of the majority of neuron tested. Significant differences of the number of cells excited and the intensity of the excitation among the 4 types of alpha-IFN were observed. The most significant effects were induced by Cantell's human leukocyte alpha-IFN followed by the Hoffman-LaRoche recombinant alpha-IFN which exhibited a dose dependent effect (i.e., each higher dose of IFN affected more neurons and intensified the excitation) on the hippocampal and cortical cells respectively. Neither the IFN carrier (albumin), nor the gamma-IFNs and its fractions, as well as current ejection, altered the extracellular spontaneous active of these 96 cortical and hippocampal neurons respectively. These observations show that the immunomodulator alpha-IFNs, but not gamma-IFNs, exerts excitatory effects on neuronal activity recorded from these two brain structures and support the view that the brain is capable of communicating with the immune system.

Excitatory effects of iontophoretically applied substance P on neurons in the nucleus tractus solitarius of the cat: lack of interaction with opiates and opioids

The effects of iontophoretically applied substance P (SP), (D-Pro2, D-Trp7,9)-SP and (D-Pro4, D-Trp7,9,10)-SP were studied on neurons identified by their histological location in the nucleus tractus solitarius (NTS), their response to vagal or carotid sinus nerve stimulation and eventually their functional correlation with the central respiratory drive. Potent and consistent excitatory effects of SP were found supporting its role as a putative excitatory transmitter in the NTS. The effects of SP and L-glutamate (Glu) were differentiated by the relative insensitivity of SP-induced excitations to levorphanol and Met-enkephalin.

Simultaneous measurement of cholecystokinin- and vasoactive intestinal polypeptide-like immunoreactivity from cat frontal cortex in vitro: effect of morphine and D-Ala2-D-Leu5-enkephalin

The two peptides vasoactive intestinal polypeptide (VIP) and cholecystokinin (CCK) have been demonstrated to be discretely distributed in the cerebral cortex. This distribution closely parallels the distribution of mu- and delta-opiate receptors in the frontal cortex. The basal efflux and potassium-stimulated release of VIP- and CCK-immunoreactivity was studied in the presence and absence of morphine and D-Ala2-D-Leu5-enkephalin (DADL), agents with relative affinity for the mu and delta receptors, respectively. The basal efflux of VIP- and CCK-immunoreactivity was not affected by these opiates; however, the potassium-stimulated release of VIP-immunoreactivity was profoundly inhibited in a dose-dependent manner by both morphine (ED50 = 1 X 10(-9) M) and DADL (ED50 = 3.02 X 10(-9) M). The inhibition produced by either morphine or DADL was shown to be reversed by naloxone.

Differential effects of interferon on ventromedial hypothalamus and dorsal hippocampus

Three incremental doses of recombinant alpha-interferon (IF) were applied iontophoretically to hippocampal and hypothalamic cells. IF produced a dose-dependent long-lasting excitation in hippocampal neurons, whereas, in the hypothalamus alpha-IF elicited biphasic responses. The highest IF currents induced changes in the amplitude of the action potentials on both structures. Our observations suggest that IF could have a selective effect on the specific areas studied.

Novel effects of interferon on the brain: microiontophoretic application and single cell recording in the rat

Interferon (IF), one of the most controversial drugs in cancer therapy, induces a variety of CNS side effects. Therefore, IF was tested on single neuronal activity and compared with other drugs. Recombinant leukocyte A IF, morphine sulfate and L-glutamate were applied microiontophoretically to 18 cortical and 29 thalamic neurons. The majority of the cortical cells were excitated by IF while most of the thalamic cells did not respond to IF; however, morphine and glutamate elicited on these neurons the expected effects. IF produced a long-lasting increase in firing discharges and exhibited dose-response characteristics.

Effects of iontophoretically applied (+)- and (-)-naloxone on rat hypothalamic and septal neurons

The purpose of this study was to investigate the possible independent actions of both (+)-and (-)-naloxone on individual neurons in the preoptic/anterior hypothalamus (POAH) and septal area (SA) of the rat brain. Morphine and (-)-naloxone were applied to 31 neurons in the SA (n = 11) and the POAH (n = 20). Morphine depressed the spontaneous activity in 19 of 31 neurons. (-)-Naloxone at currents less than 10 nA did not influence these neurons. However, (-)-naloxone applied in excess of 10 nA reduced spontaneous activity in 28 of 29 neurons. This effect of (-)-naloxone was stereospecific; (+)-naloxone did not alter the spontaneous rate in 12 of 14 cells when alternately applied with (-)-naloxone at the same current intensity. Application of (+)- and (-)-naloxone at supramaximal currents produced a diminution of spike amplitude and an increase in the duration of the action potential. The results of this study indicate that naloxone reduces spontaneous activity via two mechanisms. One involves a direct stereospecific action, and a second produces a non-specific reduction in spike amplitude and a prolongation of spike duration.

Caudate neuronal response to microiontophoretically injected morphine in naive and morphine-dependent rats

1. The response of caudate nucleus neurons to morphine was found to be dose-dependent and could be divided into two classes: neurons which responded monophasically either by increase or decrease in their firing rate, and neurons whose response can be described as biphasic, exhibiting increase followed by decrease in their firing rate or vice versa, with the increase in morphine concentration. These responses were found in both naive and morphine-dependent rats. 2. Naloxone antagonized the effects of morphine in 74 out of 102 neurons tested. 3. Caudate neurons of morphine-dependent rats showed super-sensitivity to morphine compared to naive rats. 4. Differences were found in the distribution of the spontaneously active neurons between naive and morphine-dependent rats, indicating the existence of two different opiate receptor populations within the caudate nucleus.

Neurons in the frontal cortex of the rat carry multiple opiate receptors

Acute desensitization to the inhibitory action of iontophoretically applied opiate alkaloids and opioid peptides was used to investigate the possibility of multiple opiate receptors located on single neurons in the frontal cortex of rats. Such short term exposure resulted in adaptive processes which were similar to tolerance to and dependence on opiate agonists occurring after chronic treatment. Neurons desensitized to methionine-enkephalin (ME) or D-Ala2,D-Leu5-enkephalin (DADL) became subsensitive to morphine, whereas cells desensitized to morphine remain sensitive to the inhibitory action of the opioid peptides. This lack of cross-desensitization may suggest the existence of multiple opiate receptors on the same cell.

Inhibitory actions produced by local electrical stimulation in the caudal spinal trigeminal nucleus in rat

Inhibitory actions induced by local electrical stimulation (LES) on the tooth pulpal afferent activities were investigated in the caudal part of the rat spinal trigeminal nucleus. For the LES, ipsilateral Yin-Hsiang (intrasegmental point) or Ho-Ku (extrasegmental point) was used as a cathodal point which was stimulated electrically by a single pulse of 0.1 msec in duration or by 0.1 msec-pulse train at 45 Hz for 15 min. We found at least three types of inhibitions in the caudal trigeminal nucleus: Type I--this inhibition is the most forceful, caused by naloxone-reversible endogenous opiate system with a slow onset and prolonged aftereffect. This inhibition is presumably postsynaptic action. Type II--this is evoked by postsynaptical acting inhibition and begins within milliseconds after the stimulus is applied. Type III--this inhibition is elicited by presynaptic action and also begins within milliseconds after the onset of the stimulation. Type I and II inhibitions are evoked by stimulating either intra- or extrasegmental LES points, however, Type III is produced by stimulation of intrasegmental and rarely provoked by extrasegmental point stimulation. Naloxone failed to reverse Type II and III inhibitions. During LES, Type I to III inhibitions co-work for producing the suppressive effect and after the cessation of its stimulation, only Type I inhibition produces the so-called aftereffect of the LES.

Periaqueductal gray neurons response to microiontophoretically injected morphine in naive and morphine-dependent rats

The attempt of this study was to investigate the direct effects of increasing doses of morphine on the neuronal activity of the periaqueductal gray in morphine-naive and morphine-dependent rats. The microiontophoresis technique was used for this purpose. The four different responses induced by morphine exhibited dose-related patterns. Naloxone antagonized these responses in about 40% of the cases. Differences were found in the sensitivity of the neurons of morphine between naive and morphine-dependent rats. The phenomena of acute tolerance, chronic tolerance and dependence have been found. The results of this study indicate the presence of different neural populations in the periaqueductal gray in relation to their response to morphine, supporting the notion that subpopulations of opiate receptors exist within this brain area.

Postsynaptic inhibitory actions of catecholamines and opioid peptides in the bed nucleus of the stria terminalis

Effects of catecholamines, enkephalins and related compounds on electrical activity of the bed nucleus of stria terminalis (BST) were studied in vitro on thin BST sections prepared from guinea pig brains. Norepinephrine (NE) and epinephrine (E) suppressed field potentials elicited by a single shock to the stria terminalis (ST). The effects of NE and E were mimicked by phenylephrine and blocked by phenoxybenzamine. Isoproterenol and dichloroisoproterenol were without effect. NE and E suppressed the spontaneous firing of BST neurons and discharges elicited by ST stimulation. Dopamine was a less potent depressant. [D-Ala2]-Met-enkephalinamide (EKA) suppressed the field potentials and spike discharges elicited by ST stimulation. Spikes occurring spontaneously or during administration of glutamate were also suppressed by EKA. The action of EKA was blocked by naloxone. Late inhibition induced by stimulation of the lateral division of the ST was blocked by naloxone in about a third of the neurons examined. These results indicate that norepinephrine suppresses the activity of BST neurons by activating postsynaptic alpha-receptors. It is also suggested that opioid peptides mediate inhibitory control of the amygdala over the BST.

Development of acute opioid tolerance and dependence in rat striatal neurones

Striatal neurones in the rat were frequently observed to develop tachyphylaxis to the specific, naloxone-antagonisable depressant effects of methionine- and leucine-enkephalin on spontaneous and L-glutamate-evoked activity. This loss of responsiveness to the enkephalins occurred within a few minutes of the repeated or prolonged application of these peptides and is suggested to reflect a form of acute tolerance. As with chronic opiate tolerance, the acute tolerance of single striatal neurones to the enkephalins appeared to be associated with dependence on these opioids, as evinced by the withdrawal-like hyperactivity that occurred upon terminating the application of the peptides or upon the microelectrophoresis of naloxone.

Calcium reverses the inhibitory action of morphine on neuroeffector transmission in the mouse vas deferens

In the mouse vas deferens, the amplitude of excitatory junction potentials (e.j.p.s.) recorded intracellularly from smooth muscle cells was found to be proportional to stimulus intensity. Normorphine (0.4-2-10 microM) reduced the amplitude of these postsynaptic transients and shifted the stimulus-response curve to the right; i.e. in its presence, higher stimulus intensities were required to elicit an e.j.p. of a similar size to one generated in its absence. Naloxone (0.4 microM) reversed the inhibitory effect of normophine (2 microM). Membrane potentials were unaffected by the concentrations of normorphine employed in solutions of varying ionic compositions. Manoeuvres designed to increase intracellular free calcium, that is increasing the extracellular Ca+ ion concentration (from 2.5 to 5 or 10 mM), removing Mg2+ ions from a medium containing 5 mM Ca2+ or applying 4-aminopyridine (100 microM), enhanced the e.j.p. amplitude and reversed the inhibitory effect of normorphine. Lowering the concentration of Ca2+ ions (from 2.5 to 1 mM) or increasing the concentration of Mg2+ ions (from 1.2 to 4.8 mM) in the bathing solution reduced the amplitude of e.j.p.s. Short trains of impulses (3Hz) facilitated the amplitude of successive e.j.p.s., probably by elevating the intracellular Ca2+ ion concentration. The inhibitory effect of normorphine upon these transients was inversely proportional to the length of the train. It is concluded that the reversal of the effect of normorphine by calcium does not occur at the level of the opiate receptor, and that the opiate depresses the stimulated release of the excitatory transmitter by a reduction in the supply of Ca2+ ions to the stimulus-release coupling mechanism in the sympathetic nerve terminals.

A demonstration of naloxone-precipitated opiate withdrawal on single neurones in the morphine-tolerant/dependent rat brain

1 A comparison has been made between the effects of microelectrophoretically applied naloxone on single neurones in the frontal cerebral cortex and the striatum of naive and of morphine-tolerant/dependent rats, anaesthetized with a mixture of alpha-chloralose and urethane. 2 Specificity of the results obtained was evaluated by contrasting the effects of alternate applications of the (+)- and (-)-isomers of naloxone to the same neurones. 3 In naive rats naloxone had predominantly no effect, only a few cells revealing non-specific depressant responses to the drug. 4 In morphine-tolerant/dependent rats a higher proportion of neurones responded to naloxone; either with stereospecific excitatory responses, in which the activity evoked by L-glutamate or acetylcholine was increased, or with a non-specific inhibition, similar to that observed in naive animals. 5 It is suggested that these excitatory responses to microelectrophoretically applied (-)-naloxone represent opiate withdrawal responses at the single neurone level and that they reflect a latent hyperexcitability of the postsynaptic membrane in the morphine-tolerant/dependent state.

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.

Cellular site of opiate dependence

Experiments on whole animals, guinea pig isolated ileum, individual neurones in situ and cultured neuroblastoma--glioma hybrid cells indicate that opiate dependence and associated tolerance develop within neurones bearing specific opiate receptors (opiate-sensitive neurones). The essential change appears to be an hypertrophy of the cyclic AMP system, in response to inhibition by opiate of a neuronal adenylate cyclase.

Morphine-naloxone interactions: a role for nonspecific morphine excitatory effects in withdrawal

The opiate antagonist naloxone precipitates withdrawal when given either 15 minutes after or 1 minute before a single injection of morphine in drug-naïve mice. We propose that withdrawal signs arise from a synergistic mixture of excitatory influences that are direct (agonistic action on nonspecific opiate receptors) and indirect (sensory and affective disorders, stress, hormonal and neurotransmitter dysfunction, and so forth). The predominant effects during precipitated withdrawal are assumed to be direct, whereas during abstinence in tolerant animals they are indirect.

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.

Unit activity of limbic system neurons: effects of morphine, diazepam and neuroleptic agents

The effects of morphine, diazepam and three neuroleptic agents (chloropromazine, perphenazine and haloperiodol) on neuronal firing rats were studied in the limbic system of immobilized cats. Parietal craniotomy was carried out under 1.5--4.0% halothane. Extracellular potentials from single cells in the cingulate gyrus, septum and lateral hypothalamic areas were recorded using glass-coated, platinum-iridium microelectrodes. In general, intravenous adminstration of morphine sulfate augmented the spontaneous firing rates of most of the neurons studied. In contrast, diazepam produced a marked attenuation of both spontaneous and morphine augmented firing rates, whereas the neuroleptic agents had no significant or consistent effects on the morphine augmented firing rates of neurons in these limbic areas. These data indicate that the limbic system may play an important role in the behavioral excitement in cats induced by morphine administration and also the depressant effect of the tranquilizer diazepam. In contrast, the inability of the neuroleptic agents to antagonize the morphine augmented neuronal firing rates suggest these agents may act outside the limbic areas studied here.

Specific versus non-specific actions of opioids on hippocampal neurones in the rat brain

An investigation has been made into the pharmacological specificity of the actions of microelectrophoretically applied opioids on neurones in the rat hippocampus, a structure containing a low concentration of specific receptors for these substances. The majority of hippocampal neurones remained unaffected by morphine or enkephalin. Some neurones, however, displayed either inhibitory or excitatory responses to the opioids. Of the inhibitory effects, a few appeared to be specific, in that they could be antagonized by naloxone, but most of the other inhibitory responses were found to be potentiated by this drug. Similarly, naloxone not only failed to antagonize, but frequently potentiated the excitatory responses to the opioids. Further evidence for the predominantly non-specific nature of the responses of hippocampal neurones to opioids was provided by experiments with the stereoisomers levorphanol and dextrorphan. Neurones could be found which were either inhibited or excited by both enantiomers. Stereospecific responses, when observed, were inhibitory. Although non-specific, the excitatory effects of enkephalin and morphine on hippocampal neurones were greatly reduced in morphine tolerant/dependent rats. Indeed, in the hippocampus of these animals, the opioids had predominantly inhibitory effects which were potentiated, not antagonized, by naloxone. It is concluded that the low concentration of opiate receptors in the rat hippocampus renders neurones within this structure sensitive to a variety of nonspecific opioid actions.