Noradrenergic terminal excitability: effects of opioids

Role of glucocorticoids on noradrenergic and dopaminergic neurotransmission within the basolateral amygdala and dentate gyrus during morphine withdrawal place aversion

Aversive memories related to drug withdrawal can generate a motivational state leading to compulsive drug taking. However, the mechanisms underlying the generation of these withdrawal memories remain unclear. Limbic structures, such as the basolateral amygdala (BLA) and the dentate gyrus (DG) of the hippocampus, play a crucial role in the negative affective component of morphine withdrawal. Given the prominent role of glucocorticoids (GCs), noradrenaline (NA), and dopamine (DA) in memory-related processes, in the present study, we employed the conditioned place aversion (CPA) paradigm to uncover the role of GCs on NA and DA neurotransmission within the BLA and NA neurotransmission within the DG during opiate-withdrawal conditioning (memory formation consolidation), and after reexposure to the conditioned environment (memory retrieval). We observed that adrenalectomy impaired naloxone-induced CPA. Memory retrieval was associated with an increase in dihydroxyphenylacetic acid (DOPAC) levels in the BLA in morphine-addicted animals in a GC-independent manner. Importantly, NA turnover was related with the expression of withdrawal physical signs during the conditioning phase and with locomotor activity during the test phase. On the other hand, reduced DA concentration in the BLA was correlated with the CPA score. Our results indicate that while noradrenergic system is more associated with the somatic consequences of withdrawal, dopaminergic neurotransmission modulates the affective state. Nevertheless, it seems necessary that both systems work together with GCs to enable aversive-memory formation and recall.

Significance and Remembrance: The Role of Neuromodulatory Systems

It is now well known that the retention of newly-acquired information can be modulated by drugs or hormones administered shortly following training. It is generally thought that such treatments influence retention by modifying processes underlying the storage of information. The fact that susceptibility to posttraining memory modulation is seen in many species, including bees, fish, birds, and mammals, argues that some common time-dependent memory storage processes have been conserved in evolution. Recent research findings have provided strong support for the view that such susceptibility to posttraining influences provides opportunity for modulation of memory storage by endogenous neurohormonal systems. In rats and mice, posttraining administration of hormones such as epinephrine that are normally released by training experiences enhances subsequent retention. Comparable effects are found with posttraining administration of opiate receptor antagonists such as naloxone. Findings of recent experiments indicate that these treatments affect memory by influencing the release of norepinephrine within the amygdaloid complex. The endogenous regulation of memory storage appears to involve interaction of neurohormones and transmitters in activating brain systems involved in memory storage.

Prazosin + Naltrexone Decreases Alcohol Drinking More Effectively Than Does Either Drug Alone in P Rats with a Protracted History of Extensive Voluntary Alcohol Drinking, Dependence, and Multiple Withdrawals

BACKGROUND

Prazosin (PRZ; an α1 -adrenergic receptor antagonist) and naltrexone (NTX; a nonspecific opioid receptor antagonist) each decrease alcohol drinking when administered to rats selectively bred for high voluntary alcohol drinking (alcohol-preferring or "P"), and the combination of PRZ + NTX decreases alcohol drinking more effectively than does either drug alone. As drug responsiveness can depend on history of alcohol drinking and dependence, we investigated whether various schedules of PRZ and NTX administration, alone or in combination, are effective in decreasing alcohol drinking in male P rats with a history of protracted voluntary alcohol drinking, dependence, and repeated withdrawals closely resembling human alcoholism.

METHODS

Male P rats became alcohol-dependent during 1 year of ad libitum 24 h/d access to food, water, and 20% alcohol with repetitive temporary alcohol withdrawals. Four sequential studies then addressed effects of oral PRZ (2 mg/kg) and NTX (10 mg/kg), alone or together, on alcohol drinking during: (i) daily alcohol access with daily drug treatment, (ii) intermittent alcohol access with daily drug treatment, (iii) intermittent alcohol access with occasional drug treatment, and (iv) postdeprivation reinstatement of alcohol access.

RESULTS

The combination of PRZ + NTX consistently suppressed alcohol drinking during daily or intermittent alcohol access conditions and when drug treatment was either daily or occasional. PRZ + NTX was consistently more effective than either drug alone. The reduction in alcohol drinking was not due to sedation, motor effects, or malaise.

CONCLUSIONS

Both daily and "as-needed" treatment with PRZ + NTX are highly effective in suppressing daily, intermittent, and postdeprivation alcohol drinking in male P rats with a protracted history of alcohol dependence and repeated withdrawals. This drug combination may be especially effective for treating individuals with long histories of heavy alcohol abuse, dependence, and repeated relapse, as commonly encountered in clinical practice.

Memory modulation

Our memories are not all created equally strong: Some experiences are well remembered while others are remembered poorly, if at all. Research on memory modulation investigates the neurobiological processes and systems that contribute to such differences in the strength of our memories. Extensive evidence from both animal and human research indicates that emotionally significant experiences activate hormonal and brain systems that regulate the consolidation of newly acquired memories. These effects are integrated through noradrenergic activation of the basolateral amygdala that regulates memory consolidation via interactions with many other brain regions involved in consolidating memories of recent experiences. Modulatory systems not only influence neurobiological processes underlying the consolidation of new information, but also affect other mnemonic processes, including memory extinction, memory recall, and working memory. In contrast to their enhancing effects on consolidation, adrenal stress hormones impair memory retrieval and working memory. Such effects, as with memory consolidation, require noradrenergic activation of the basolateral amygdala and interactions with other brain regions.

Endogenous morphine and codeine. Possible role as endogenous anticonvulsants

Exogenously administered morphine can have both convulsive or anticonvulsive effects, depending on the dose and species. The levels of the endogenous opiate alkaloids morphine and codeine were significantly elevated in specific rat brain regions by the convulsive drug, pentylenetetrazole, as well as by the anticonvulsant drugs, carbamazepine and phenytoin. Morphine and codeine levels in peripheral tissues (heart, lung, spleen and adrenal) were unaffected by these drugs. Maximal increases in morphine levels were seen in the hypothalamus and striatum (2-10-fold), while lesser increases occurred in the midbrain and brain stem (2-4-fold). Codeine levels were also markedly increased in hypothalamus (5-10 fold), In contrast to morphine, codeine levels were also increased in the hippocampus (2-10-fold), but were unchanged in the striatum. These studies suggest that the endogenous alkaloids morphine and codeine are involved in the modulation of convulsions and that morphine and/or codeine may act as an endogenous anticonvulsant.

Effects of footshock-, psychological- and forced swimming-stress on the learning and memory processes: involvement of opioidergic pathways

Modulation of learning and memory acquisition, retention and retrieval in the one trial passive avoidance learning task in mice by three inescapable stresses, i.e., footshock (FS), psychological (PSY) and forced swimming (SW) were investigated. Pre-, post-training and pre-test FS-stress (2 mA, 0.2 Hz, 1 sec for 30 min) and pre-training PSY-stress (communication box, 5 min) resulted in enhanced test latencies. On the contrary, SW-stress (20 degrees C, 5 min) immediately or 1 hr after training impaired retention latencies that tended to recover after 2 hr post-training SW-stress, suggesting that at least 2 hr are required to consolidate newly acquired information. In contrast, pre-stress naloxone (Nx), which did not affect FS- and PSY-stress induced facilitatory effects, returned to control levels the impaired retention latencies induced by SW-stress. Taken collectively, these results imply the involvement of an opioid-dependent mechanism in the modulation of memory by SW-stress and non-opioid in the case of FS- and PSY-stress. Furthermore, they suggest that different mechanisms are involved in stress-induced memory modifications and the production of stress-induced analgesia (SIA) since in the latter, FS and PSY but not SW stress produce Nx-sensitive antinociception.

Interaction of GABAergic and beta-noradrenergic drugs in the regulation of memory storage

These experiments examined the interaction of drugs affecting noradrenergic and GABAergic systems, administered post-training, in influencing retention of an inhibitory avoidance response. Male CD1 mice (23-28 g) were trained in an inhibitory avoidance task, given immediate post-training ip injections of saline or GABAergic and adrenergic drugs administered either alone or concurrently. Retention was tested 48 h later. In agreement with extensive previous evidence, the GABAergic antagonist bicuculline (0.3, 1.0, or 3.0 mg/kg) produced dose-dependent (inverted-U) enhancement of retention and the GABAergic agonist muscimol (1.0 mg/kg) impaired retention. The retention-enhancing effects of bicuculline were blocked by concurrent administration of the beta-nor-adrenoceptor antagonist propranolol (2.0 mg/kg). Also in agreement with previous evidence, the beta-adrenoceptor agonist clenbuterol (0.030, 0.100, or 0.300 mg/kg, ip) produced dose-dependent (inverted-U) enhancement of retention. Clenbuterol also blocked the retention-impairing effects of muscimol (1.0 mg/kg). In addition, propranolol (2.0 mg/kg) potentiated the retention impairing effects of muscimol (1.0 or 3.0 mg/kg, ip). These findings support the view that GABAergic systems modulate memory through an interaction with beta-noradrenergic mechanisms.

Neuromodulatory systems and memory storage: role of the amygdala

This article reviews findings of research examining the interaction of peripheral adrenergic systems with cholinergic, opioid peptidergic and GABAergic systems in modulating memory storage. It is well established that retention is enhanced by posttraining systemic or intra-amygdala injections of adrenergic agonists, opiate antagonists and GABAergic antagonists. These influences appear to be mediated by activation of NE receptors within the amygdala, as intra-amygdala injections of beta-adrenergic antagonists block the memory-modulating effects of hormones and drugs affecting these systems. Furthermore, these influences also appear to involve, at a subsequent step, activation of a cholinergic system: atropine blocks the memory-enhancing effects of adrenergic agonists and opiate and GABAergic antagonists and oxotremorine attenuate the memory-impairing effects of opiate agonists and GABAergic agonists. These findings suggest that the amygdala integrates the memory-modulating effects of neuromodulatory systems activated by learning experiences.

Evidence that cholecystokinin-enhanced retention is mediated by changes in opioid activity in the amygdala

Mice, partially trained to avoid footshock in a T-maze, showed enhanced retention relative to vehicle-injected mice when treated peripherally with arecoline, D-amphetamine, cholecystokinin octapeptide (CCK-8), epinephrine or naloxone. Both intra-amygdaloid and intraventricular injections of beta-endorphin resulted in amnesia. D-amphetamine and arecoline blocked the amnestic effect of beta-endorphin administered into the amygdala but it required higher doses for CCK-8, epinephrine and naloxone to block the amnestic effect of beta-endorphin. The effects of CCK-8, epinephrine and naloxone showed a differential ability to block amnesia induced by beta-endorphin intraventricularly with epinephrine and naloxone preventing amnesia but CCK-8 not improving retention. This data suggests that the memory enhancement produced by peripherally administered CCK-8 involves the amygdala and that both CCK-8 and epinephrine interact with opioid amnestic mechanisms within the amygdala to alter memory processing.

Potentiated hypnotic action with a combination of fentanyl, a calcium channel blocker and an alpha 2-agonist in rats

An investigation was made of the hypnotic-anaesthetic effects in rats of subcutaneous coadministration of fentanyl (25-100 micrograms.kg-1), clonidine (100-300 micrograms.kg-1) and verapamil (1-5 mg.kg-1). Hypnotic-anaesthetic efficacy was assessed via loss of the righting reflex. In the doses used, none of the three drugs alone was associated with appreciable hypnotic-anaesthetic effects. Coadministration of fentanyl and clonidine resulted in a dose-related enhancement of the anaesthetic potency, without change in the duration of hypnotic action. Verapamil coadministration failed to increase the anaesthetic efficacy of binary combinations of fentanyl and clonidine, but a marked prolongation of the duration of hypnotic action was observed (P less than 0.001). These results suggest the existence of unreported interactions between these three drugs in the production of hypnotic-anaesthetic action in rats.

Mechanisms of opioid actions on neurons of the locus coeruleus

The locus coeruleus (LC) has provided a useful model for pioneering studies of the mechanisms underlying the acute and chronic actions of opioid drugs. Acutely, opioids inhibit the electrical activity of single neurons in the rat and guinea pig LC. Inhibition is due to a membrane hyperpolarisation. In these cells, opioids act on mu-receptors to increase the opening of inwardly rectifying potassium channels, thus leading to hyperpolarisation. The mu-receptors are coupled to potassium channels via G-proteins which are sensitive to inactivation by pertussis toxin. This coupling process is quite direct, in that it does not involve freely diffusible intracellular second messengers. Agonists specific for other receptors, such as alpha 2- and somatostatin-receptors, are capable of opening the same population of potassium channels on LC neurons. Following chronic treatment of animals with morphine, a specific deficit develops in the ability of mu-receptors to open potassium channels, producing reduced sensitivity of LC neurons to inhibition by opioids.

Axonal sprouting of noradrenergic locus coeruleus neurons following repeated stress and antidepressant treatment

Plastic changes in axon terminals of NA LC neurons following repeated stress and antidepressant treatments were examined using electrophysiological or morphological methods. For stress treatment, rats restrained in a small cage were immersed up to the neck in warm water for 10 min daily. Electrophysiological experiments were performed under urethane anesthesia on the day following the termination of stress treatment. To quantify the density of cortical axon terminals arising in the LC, the percentage of LC neurons activated antidromically from the cerebral cortex was assessed. The percentage of LC neurons showing antidromic response to cortical stimulation was increased in the animals stressed for two weeks but not for one week. Since threshold currents for antidromic activation were not changed by the stress treatment, the observed changes were interpreted as morphological (axonal sprouting) rather than physiological consequences in NA axon terminals of LC neurons. To test the ability of antidepressants to induce the regeneration of central NA axons, local injections of 6-OHDA were made bilaterally into the symmetrical sites of the FC. Two weeks after the 6-OHDA injections, the same cortical site of one hemisphere was infused with the antidepressant MPL, DMI, or MIA, and the corresponding site of the other hemisphere with SAL. The density of glyoxylic acid-induced catecholamine fibers was greater in the cortical hemisphere infused with the antidepressants than that infused with SAL. These findings indicate that repeated mild stress and antidepressant treatments induce sprouting of NA LC axons in the cerebral cortex. Axonal sprouting of LC neurons can explain both the delayed onset of the clinical response to antidepressants and subsensitivity of beta-adrenoceptors following repeated stress and antidepressant treatments, and may be a common mechanism for the clinical efficacy of antidepressant drugs and electroconvulsive shock. Furthermore, the findings suggest the possibility that axonal retraction or degeneration of central NA neurons may be involved, at least in part, in the pathology of clinical depression.

Involvement of the amygdaloid complex in neuromodulatory influences on memory storage

Neuromodulatory systems activated by training experiences appear to play a role in influencing memory storage processes. The research summarized in this paper examined the effects, on memory, of posttraining administration of treatments affecting adrenergic, opioid peptidergic and GABAergic systems. When administered after training, drugs affecting these systems all produce dose- and time-dependent effects on memory storage. The drug effects on memory are blocked by lesions of the amygdaloid complex as well as lesions of the stria terminalis, a major amygdala pathway. The effects of drugs affecting these neuromodulatory systems are also blocked by injections of beta-adrenergic antagonists administered to the amygdaloid complex. Thus, the findings suggest that the neuromodulatory systems affect memory storage through influences involving the activation of beta-adrenergic receptors within the amygdala. These findings are consistent with the view that the amygdala is involved in regulating the storage of memory in other brain regions.

Activation of mu- and delta-opioid receptors present on the same nerve terminals depresses transmitter release in the mouse hypogastric ganglion

1. The inhibitory actions of mu- and delta-opioid receptor agonists on the strong, single fibre synaptic input to neurones contained in the mouse hypogastric ganglion have been examined. 2. The opioid agonists [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAMGO, 10 nM-10 microM), morphine (10-30 [D-Ser2,Leu5,Thr6]enkephalin (DSLET, 3 nM-1 microM), [D-Pen2,D-Pen5]enkephalin (DPDPE, 10 nM-10 microM), all depressed the single fibre, all-or-nothing, nicotinic, excitatory synaptic potential (e.p.s.p.) recorded in mouse hypogastric ganglion neurones. U50488H (0.3-1 microM) was without effect. 3. The effect of DSLET, but not that of DAMGO, was reversed by the delta-opioid receptor-selective antagonist, ICI 174864 (0.3 microM). Naloxone (0.3 microM) antagonized the effect of both DSLET and DAMGO. 4. The site of action of the mu- and delta-receptor agonists was on the presynaptic terminals, since at the concentrations which depressed the e.p.s.p. these drugs did not affect the resting membrane potential or input resistance of the postganglionic neurone body, nor did they depress the postganglionic, nicotinic response to exogenously applied acetylcholine. 5. Quantal analysis further confirmed the presynaptic site of action; mu- and delta-opioid receptor agonists decreased the mean number of quanta released per stimulus but did not reduce the mean amplitude of the quantal unit. 6. It was concluded that mu- and delta-opioid receptors were located on the same presynaptic nerve terminals since, in the same neurones, mu- and delta-opioid receptor agonists depressed the same single fibre inputs. 7. The potassium channel blockers barium and quinine, at concentrations known to block opioidactivated somatic potassium conductances, reduced slightly but did not abolish the mu- and delta-opioid receptor-mediated inhibition of the e.p.s.p.

Electrophysiological evidence for terminal sprouting of locus coeruleus neurons following repeated mild stress

To see if repeated mild stress causes plastic changes in central noradrenergic terminal axons, the density of terminal axons arising in locus coeruleus (LC) neurons of rats was quantified by antidromic stimulation technique. After the termination of stress treatments (immersion in warm water for 10 min daily) for 1 or 2 weeks, electrophysiological experiments were performed under urethane anesthesia. The frequency of LC neurons activated antidromically from the cerebral cortex increased in rats stressed for 2 weeks but not for 1 week. Since the increased frequency of antidromic responses was not due to a change in terminal excitability, the change observed here is considered to be morphological (terminal sprouting) rather than a physiological consequence. The results suggest that LC neurons dynamically alter their terminal morphology in response to environmental stimuli.

Effects of pertussis toxin on opioid regulation of catecholamine release from rat and guinea pig brain slices

Opioid agonists selective for mu-, delta-, and kappa-receptors are all capable of regulating the stimulated release of noradrenaline from three terminal fields (cortex, hippocampus, and cerebellum) of the noradrenergic projections from locus coeruleus in the guinea pig brain. Intracerebroventricular injections of pertussis toxin abolished the ability of a mu-selective agonist and of a delta-selective agonist to inhibit stimulated noradrenaline release, but left unaffected the concentration-related inhibition of NE release by a kappa agonist. Thus, mu- and delta-receptors have been shown to be coupled to their effector system in these noradrenergic neurons via guanyl nucleotide binding proteins (G proteins) which are sensitive to pertussis toxin, while kappa-receptors in the same neurons appear to be coupled through a different mechanism which is significantly less sensitive to pertussis toxin. In contrast to opioid receptor regulation of noradrenaline release in guinea pig hippocampus, mu-, but not delta- or kappa-agonists are capable of regulation of stimulated noradrenaline release from rat hippocampus and cortex, and kappa-, but not mu- or delta-agonists are capable of inhibiting the stimulated release of dopamine from rat striatum and cortex. Pertussis toxin injections significantly attenuated mu-agonist inhibition of noradrenaline release, but had no effect on the ability of a kappa-selective agonist to regulated dopamine release, confirming the insensitivity of the kappa-receptor-effector coupling system to pertussis toxin.

The ionic mechanisms underlying opioid actions

Recent experiments using intracellular recording techniques in vitro have revealed that common ionic mechanisms may explain the actions of opioid drugs. Evidence is now available from studies on guinea pig gut myenteric and submucous plexi, from preparations of spinal cord and dorsal root ganglia, from brain slices including the locus coeruleus and from neuroblastoma/glioma hybrid cells. The concensus is that mu opioid receptors activate an outward potassium conductance, possibly by way of adenylate cyclase. Activation of the receptor increases the membrane permeability to potassium ions and thus produces a membrane hyperpolarisation and conductance increase, plus an indirect inhibition of calcium entry during the action potential. Kappa opioids appear to inhibit directly the entry of calcium through voltage-dependent calcium channels, although to date there is no conclusive evidence that this mechanism of action can be extended to neurones of the central nervous system. The mechanism of action of delta opioids has only recently been investigated and initial evidence suggests they increase a potassium conductance similar to that increased by mu opioids. However, work in neuroblastoma x glioma hybrid cells has suggested that in these cells at least, receptor activation depress a component of voltage-dependent calcium current. The link between the receptor and the calcium channel involves a G-protein, Go.

Memory-enhancing effects of posttraining naloxone: involvement of beta-noradrenergic influences in the amygdaloid complex

Rats (220-250 g) were bilaterally implanted with cannulae in the amygdala, trained on an inhibitory avoidance response and two weeks later, on a Y-maze discrimination response. Immediately following the training on each task, they were injected intraperitoneally (i.p.) or intra-amygdally. Retention was tested one week after training for each task. Retention of the Y-maze task was assessed by discrimination reversal training. Naloxone administered i.p. (3.0 mg/kg) significantly facilitated retention of both tasks in unoperated control rats as well as in rats implanted bilaterally with amygdala cannulae. The memory-enhancing effect of naloxone i.p. was blocked by propranolol (0.3 or 1.0 microgram) injected in the amygdala, but not when this beta-noradrenergic antagonist was injected (0.3 micrograms) into either the caudate or the cortex dorsal to the amygdala. Further, intra-amygdala injections of the beta 1-adrenoceptor blocker atenolol (0.3 or 1.0 microgram) and the beta 2-adrenoceptor blocker zinterol (0.3 or 1.0 microgram), in doses which were ineffective when administered alone, blocked naloxone-induced (3.0 mg/kg, i.p.) memory facilitation. In contrast, posttraining intra-amygdala administration (1.0 micrograms) of the alpha-antagonists prazosin (alpha 1) or yohimbine (alpha 2) did not attenuate the memory-enhancing effects of systemically administered naloxone. These findings support the view that naloxone-induced enhancement of memory is mediated by the activation of beta- but not alpha-noradrenergic receptors located within the amygdaloid complex.

Electrophysiological correlates of presynaptic opiate receptor activation: reduction in norepinephrine-mediated inhibition from the locus coeruleus

Inhibitory responses of rat cerebellar Purkinje cells to locus coeruleus (LC) stimulation and iontophoresis of norepinephrine (NE) were examined before and after administration of morphine to determine whether the inhibitory modulation of NE release by opiates results in a functional impairment in noradrenergic synaptic action. Administration of morphine systemically (0.2-1.2 mg/kg, i.v.) or by iontophoresis reduced inhibitions in Purkinje firing elicited by LC stimulation without affecting depressions in activity induced by postsynaptic applications of NE. This antagonistic effect of morphine on LC-induced inhibition was reversed or prevented by naloxone and mimicked by administration of levorphanol but not dextrorphan. Morphine increased the excitatory response of Purkinje cells to monosynaptic input from the parallel fibers, whereas it blocked gamma-aminobutyric acid-induced inhibitions in firing via a non-opiate receptor-mediated mechanism. These results demonstrate that morphine interferes with synaptic inhibition derived from the LC and suggest that this may occur via activation of presynaptic opiate receptors residing on noradrenergic nerve terminals.

Fetal addiction to methadone: postnatal abstinence syndrome and development of visual evoked potentials

Female rats were given different doses of methadone for the whole duration of pregnancy. The effects of methadone on birthweight and abstinence syndrome were studied from birth to maturation. Visual Evoked Potentials (VEP) from onset to 90 days of age were studied with averaging and frequency analysis techniques. The intensity of the withdrawal syndrome was dependent on the doses of the drug administered to the mothers, but not on the postnatal therapy (phenobarbital or methadone) administered to offspring. VEP latencies and amplitudes were not affected by fetal addiction to methadone. Thus, any long lasting alteration of spontaneous and evoked electrical activities in the visual system of fetal addicted young rats can be excluded. Only transient slight alterations of visual evoked responses are observed during the abstinence syndrome.

Membrane potential measured during potassium-evoked release of noradrenaline from rat brain neurons: effects of normorphine

A single slice of rat pons that contained the locus ceruleus (LC) or two slices of cerebellum were loaded with [3H]noradrenaline; superfusion with high (35 or 60 mM) potassium solutions evoked a release of 3H. In the presence of normorphine, the release of 3H evoked by 35 mM potassium and 60 mM potassium was reduced. In some of those experiments in which the release of 3H from the LC slice was measured, an intracellular microelectrode was used to measure membrane potential. This showed that solutions of increased potassium concentration depolarized the neurons to a potential at which inward calcium currents flowed (calcium action potentials occurred). Normorphine hyperpolarized the neurons; during this hyperpolarization the depolarization caused by 35 mM potassium did not reach the threshold for significant calcium entry. The results suggest that the inhibition by normorphine of transmitter release evoked by solutions of raised potassium concentration could result in part from the membrane hyperpolarization caused by the normorphine.

Receptors on individual neurones

Membrane potential or ionic conductance of neurones of the mammalian central or peripheral nervous system maintained in vitro can be measured over periods of several hours. Drugs or transmitters which change potential or conductance can be applied repeatedly under equilibrium conditions, and pharmacological null methods used to characterize the receptors with which they interact. The method offers an advantage over ligand binding studies on nervous tissue because both agonist and antagonist affinities can be estimated on individual functioning cells. The results to date suggest the hypothesis that a given receptor subtype is always associated with the same change in ion conductance, and the corollary that distinct ion conductances affected by the same transmitter result from interactions with different receptor subtypes.

Neonatal monosodium glutamate administration alters noradrenergic measures in the brainstem of the mouse

Mice treated neonatally with MSG (4 mg/g) were compared to saline-injected controls on a number of neurochemical parameters of brainstem noradrenergic activity. MSG treatment resulted in an attenuation of brainstem norepinephrine (NE) decline after alpha-methyl-p-tyrosine administration. Neonatal MSG administration did not result in alterations in the steady state levels of brainstem NE or MOPEG. The synthesis of NE was slightly increased in the pons-medulla of MSG-treated mice as indexed by pargyline-induced NE accumulation. NE release, however, appeared diminished as reflected by a significant (p less than 0.05) decrease in the ratio of normetanephrine to NE found in the pons-medulla of MSG-treated mice given pargyline. The results suggest that MSG-induced damage to the arcuate nucleus produces selective alterations in brainstem NE systems. These alterations may reflect the toxic action of MSG on the opiomelanocortin neurons of the arcuate nucleus or other descending systems that are damaged by MSG. The loss of the descending opiomelanocortin input to the brainstem could result in these types of neurochemical consequences since the pharmacologic action of opiate drugs results in a selective enhancement of brainstem NE turnover in rodents.

The effect of morphine on monoamine release and content in guinea-pig brain slices

The effect of morphine on the efflux of (3H) monoamines as well as the endogenous monoamine contents in electrically stimulated brain slices was investigated. Only at a concentration at high as 30 microM did the drug reduce the tritium efflux and counteracted the monoamine depletion caused by prolonged electrical stimulation. This effect was antagonized by Naloxone 10 microM. Besides the good agreement between the two methods used to evaluate drug effects the discrepancy between morphine concentrations active on the neurosecretory process and those effective in the whole animal is stressed. The opioids may act in vivo either by modulating the firing rate of the monaminergic neurons or by affecting other related neuronal pools.

Opiate activation of potassium conductance inhibits calcium action potentials in rat locus coeruleus neurones

Opiates act on mu-receptors to increase the potassium conductance of rat locus coeruleus neurones. Opiates also depress the rate of rise and peak amplitude of calcium action potentials in these cells. The action of opiates on calcium action potentials was prevented by two procedures which blocked the opiate-induced potassium current, intracellular caesium and extracellular barium. This indicates that the opiate reduction in calcium entry is secondary to an increased potassium current.

Vasoactive intestinal peptide alters membrane potential and cyclic nucleotide levels in retinal horizontal cells

Vasoactive intestinal peptide stimulated the synthesis of adenosine 3',5'-monophosphate in fractions of isolated carp horizontal cells. When applied extracellularly to isolated and cultured horizontal cells, the peptide also induced a slow depolarization (30 to 40 millivolts) accompanied by a decrease in membrane resistance. However, analogs of adenosine 3',5'-monophosphate applied extracellularly or intracellularly, and forscolin applied extracellularly, had no effect on the membrane potential of cultured horizontal cells, indicating that the induced depolarization was not related to the accumulation of adenosine 3',5'-monophosphate in these cells.