Search

Alcoholism and alcohol abuse create costly social and economic problems in many nations. Recent studies indicate that alcohol exposure during adolescence may convey unique risks for subsequent neurocognitive deficits and problem drinking. Although GABA A receptor function is one of the principle neurochemical targets of ethanol action in the adult brain, little is known about the effects of alcohol on this system during adolescence. Adolescent (30-day-old) and adult (90-day-old) male rats were intermittently exposed to ethanol for 1 month. At various times after the end of the exposure period, synaptoneurosomes were prepared from their cerebral cortices. GABA A receptor-mediated 36 Cl − influx was measured in the absence and presence of the neurosteroid 3α,21-dihydroxy-5α-pregnan-20-one (THDOC). In tissue from ethanol-exposed animals, sensitization to the potentiating effects of the neurosteroid was apparent 5 and 12 days after ethanol withdrawal. This sensitization was more apparent at the low concentrations of THDOC in animals pretreated with ethanol as adolescents. Sensitization to the potentiating effects of a neurosteroid is an enduring phenomenon, persistent long after the acute phase of ethanol withdrawal, and may be indicative of long-term changes in GABA A receptor function. Enhanced neurosteroid sensitization in animals pretreated as adolescents is consistent with the notion that adolescence is a period of unique sensitivity to the effects of ethanol. This uniqueness may now be extended to the chronic effects of ethanol.

The role of magnesium ions in the inhibitory effect of ethanol on NMDA receptor-mediated population synaptic potentials (pEPSPs) in area CA1 of the hippocampus of the adult rat, was studied. The excitatory amino acid (non-NMDA) receptor antagonist, DNQX and the GABAA channel antagonist, picrotoxin, were used to pharmacologically isolate NMDA-mediated pEPSPs. In the presence of a physiological concentration of magnesium (1.0 mM), ethanol (25–100 mM) inhibited NMDA-mediated pEPSPs, with an apparent EC50 of approximately 50 mM. The ability of ethanol to inhibit NMDA-mediated pEPSPs was reduced when the slices were incubated in the absence of magnesium. Concentrations of ethanol, in the range of 50–200 mM (apparent EC50 100 mM), were required to inhibit NMDA-mediated pEPSPs, in the absence of added magnesium. Combination studies of these two antagonists indicated that the sensitivity of NMDA-mediated pEPSPs to one antagonist was not altered by the presence of the other. This finding suggests that the affinity of each antagonist binding site is not affected by the presence of the other antagonist. In the case of ethanol, its low maximum antagonist efficacy may require larger concentrations of ethanol to inhibit NMDA-mediated pEPSPs, in the absence of other non-competitive antagonists such as magnesium.

Infusion of benzodiazepine (BDZ) receptor ligands into the medial septum (MS) produces a bidirectional modulation of spatial memory retention. The present experiments sought to determine the effects of BDZ ligands upon synaptic responses and long-term potentiation (LTP) in the dentate gyrus following electrical stimulation of the angular bundle. Intraseptal infusion of the BDZ agonist, chlordiazepoxide, decreased the amplitude of the evoked population spike and increased paired-pulse facilitation at a 150-ms interstimulus interval (ISI) in a dose-dependent manner. Intraseptal infusion of the BDZ antagonist, flumazenil (10 nmol), enhanced the amplitude of the dentate population spike and also increased paired-pulse facilitation at the 150-ms ISI. There was no effect of either BDZ receptor ligand upon the slope of the rising phase of the evoked population excitatory postsynaptic potential (pEPSP). Intraseptal flumazenil also significantly enhanced the magnitude of dentate LTP induced by high-frequency stimulation of the angular bundle. Intraseptal chlordiazepoxide failed to alter LTP induction. These results indicate that intraseptal infusion of an amnestic dose of the BDZ ligand, chlordiazepoxide, decreases the excitatory responsiveness of the dentate gyrus to its synaptic input from entorhinal cortex. In contrast, the promnestic BDZ ligand, flumazenil, enhances dentate granule cell responsivity, and facilitates synaptic plasticity in the dentate gyrus network. Taken together these data suggest that the memory impairing and memory enhancing action of these compounds may be a function of their ability to alter hippocampal physiology during a critical phase of memory. The potential role of septodentate cholinergic and GABAergic projections in the present observation is discussed.

We assessed the effects of systemically injected baclofen, a GABA B agonist, on single and paired-pulse responses in the dentate gyrus of urethane-anesthetized rats, in vivo. Baclofen (10 mg/kg) significantly increased the duration of single excitatory responses. This increase was blocked by the GABA B receptor antagonist, CGP 35348, indicating that baclofen was acting through GABA B receptors. To determine the mechanism underlying this increase in response duration, the NMDA antagonist, d -2-amino-5-phosphonopentanoic acid ( d -APV), was administered intracerebroventricularly (i.c.v.) after baclofen. d -APV by itself had no effect on the duration of the population excitatory post-synaptic potential (EPSP). However, when infused after baclofen, d -APV blocked the baclofen induced increase in EPSP duration. This indicates the prolonged EPSP duration caused by baclofen resulted from an enhancement of an NMDA receptor mediated component of the response. We then examined the effect of baclofen on population responses to paired stimuli. Baclofen attenuated paired-pulse inhibition of population spike amplitudes at a 25 ms interstimulus interval. CGP-35348 reduced the effect of baclofen on paired-pulse inhibition, indicating that baclofen suppressed paired-pulse inhibition by acting on GABA B receptors. In contrast to its disinhibitory effect at the 25 ms interval, baclofen had an inhibitory effect on responses evoked at a 150 ms interstimulus interval. Under control conditions, we observed that when stimuli were delivered 150 ms apart, both the EPSP duration and population spike amplitude evoked by the second stimulus were enhanced. Baclofen suppressed this enhancement. We conclude that GABA B receptor activation by baclofen reduces short interval inhibition and this reduction of inhibition leads to an enhanced NMDA-receptor mediated response. We also conclude that by reducing inhibition, baclofen occludes paired-pulse disinhibition found at longer interstimulus intervals. In contrast, CGP-35348 directly blocks paired-pulse disinhibition. Thus, GABA B receptor activation can regulate synaptic inhibition, in turn influencing NMDA receptor activation and synaptic transmission. Thus, pharmacologic manipulation of GABA B receptors may provide a powerful approach for regulating synaptic transmission in the hippocampus.

All-or-none electrographic seizures (EGSs) were studied in hippocampal slices from young (21- to 38-day-old) rats in medium containing low (0 mM) or physiological (0.9 mM) levels of magnesium, with and without the GABAB agonist baclofen. Extracellular recording and stimulation were performed in stratum pyramidale and stratum radiatum of CA3, respectively. EGS activity was induced by exposure to low-Mg medium or by delivering repetitive stimulus trains in physiological Mg medium. After EGS activity had stabilized, the EGSs were tested for all-or-none behavior by varying the number of pulses in a train. An EGS was considered all-or-none if subthreshold stimulation produced no afterdischarge bursts, and if the EGS duration was largely independent of the number of suprathreshold stimulus pulses. According to this measure, EGSs in Mg-free + baclofen medium were all-or-none. EGSs evoked in physiological Mg medium were also all-or-none, although the threshold was higher, and the EGS duration lower, than in Mg-free medium. This all-or-none characteristic was observed whether the EGSs were induced by prior exposure to Mg-free medium or by repetitive stimulation, and in the presence and absence of baclofen. The all-or-none characteristic suggests that while the triggering mechanism for EGSs is strongly dependent on stimulus intensity, regenerative mechanisms — independent of stimulus intensity — are responsible for the maintenance of EGSs. EGSs are also terminated by mechanisms not dependent on stimulus intensity.

Baclofen is an analog of the inhibitory neurotransmitter, GABA, which is used clinically to control spasticity. Recent evidence has accumulated showing this compound to have profound inhibitory effects upon hippocampal neural activity at both the cellular and circuit levels, and to attenuate epileptiform bursting in the hippocampal slice. However, it does not appear as an anticonvulsant on most traditional drug screens. Baclofen can produce inhibition by increasing potassium conductance, and therefore may fail to appear efficacious in typical anticonvulsant screens due to techniques that cause rapid and massive increases in interstitial potassium. We tested the hypothesis that baclofen is less effective at attenuating epileptiform bursting in the hippocampal slice under conditions of elevated extracellular potassium. Male Sprague-Dawley rats were decapitated and hippocampal slices were prepared. Epileptiform bursting was induced by bathing the slices in an artificial cerebrospinal fluid solution which contained either 7.0 mM K+ or 30 microM bicuculline methiodide, or by stimulus train-induced bursting. In each of these media, baclofen was applied in a random presentation of concentration format. Baclofen attenuated epileptiform bursting in both bicuculline and elevated K+, although considerably higher concentrations were necessary to attenuate bursting in high K+ than in bicuculline or after stimulus train-induced bursting. These results further support the antiepileptic actions of baclofen and provide evidence that this drug may be of value for attenuating epileptiform activity when there is not a tonic elevation of interstitial brain potassium.

Adult male hooded rats were tested pretreatment for spontaneous alternation and open-field behaviors. Animals were then intubated with either 5 or 7 mg/kg trimethyltin chloride (TMT Cl) or the isotonic saline vehicle. Post-treatment, animals were again tested for spontaneous alternation and open-field behaviors during three separate 5-day periods: days 14-18, days 56-60 and days 106-110. Additionally, both body weight and water intake were assessed. The larger dose of TMT Cl resulted in significant, but temporary reduction in body weight, elevated water intake for approximately 3 weeks, and a persistent increase in open-field activity. The smaller dose of TMT Cl resulted in elevated water intake for approximately 3 weeks and a temporary increase in open-field activity. Since the tendency to enter choice arms decreased in all groups across testing sessions, the effects of TMT Cl on spontaneous alternation could not be accurately assessed.

An alcohol self-selection test was first given to adult male rats of either the Sprague-Dawley or Long-Evans strain in which the concentrations available with water were increased from 3% to 30% over an eight-day period. Subsequently, the animals were anesthetized and, using stereotaxic procedures, a 1.2 or 2.4 nmole dose of kainic acid was infused bilaterally into the cerebral ventricle (ICV) over a 30-sec interval and in a total volume of 10 microliters. When the same alcohol self-selection test was repeated two weeks post-operatively, alcohol intake was significantly suppressed in terms of both g/kg intake per day as well as proportion of alcohol to water selected. Alcohol intake of the control rats infused with the CSF carrier vehicle was unchanged. When a much longer interval of 7-10 min was used to infuse the 2.4 nmole dose of kainic acid ICV, the intake of alcohol of this group also was not significantly changed. Post-mortem histological analysis of forebrain tissue of the kainic acid infused rats confirmed cytological damage to the hippocampus, particularly in cell fields CA3 and CA4, which has been reported previously. Since the hippocampus has been implicated recently in the mechanisms underlying alcohol drinking, our results suggest that a pathological lesion of this limbic-forebrain structure could influence the degree to which alcohol is self-administered in a free-choice situation.

Baclofen (Lioresal, Ciba-Geigy) is an analog of the inhibitory neurotransmitter GABA and is used clinically to control spasticity. Recent studies have demonstrated that this compound produces a marked inhibition of synaptically evoked responses in area CA3 of the hippocampal slice, suggesting that this drug could influence behavior mediated by the limbic system. In the present study, male rats of the Fischer-344 strain were trained on a one-trial passive avoidance task and tested for retention 1 week later. After the training trial, separate groups of rats received either 5 or 10 mg/kg/4 ml IP of baclofen or the distilled H2O vehicle immediately, 10 min, or 60 min after training. One week later, the rats that received baclofen immediately after training reentered the test chamber with a significantly higher frequency than controls, although no differences in vacillatory responses were observed between groups. Similar effects were observed following posttrial administration of chlordiazepoxide. In a separate experiment rats were tested for locomotor activity after receiving the same doses of baclofen. Although baclofen decreased activity during a 30-min period after dosing, rats exposed to baclofen showed no significant change in activity relative to controls 1 week later. These data are consistent with the interpretation that baclofen may interfere with memory consolidation or retention.

Organic leads are present in the environment from a variety of sources and produce metabolites such as triethyl lead (TEL) which have relatively long half-lives in the brain. However, little is known at present about the possible neurological consequences of exposure to these compounds. The susceptibility of an animal to experimentally induced seizures has been used as a reliable index of neural function to assess neurotoxic insult, and the present study assessed the effect of TEL upon this index. Adult male rats of the Fischer-344 strain were divided into groups and treated with 7.88 mg/kg of TEL or the control vehicle at 1, 7, 14 or 28 days prior to a single challenge with 35 mg/kg of pentylenetetrazole (PTZ). The rats treated with TEL were more susceptible to PTZ-induced seizures than were the controls. This effect was statistically significant on days 7, 14 and 28 after treatment, although some elevation of seizure scores was observed among TEL-treated animals on day 1. These results demonstrate a functional change in the central nervous system after exposure to TEL, which persists for at least 4 weeks after exposure.

Sixteen male Fischer-344N rats were trained on a eight-arm radial maze task for food reinforcement. The effects of various doses of baclofen (1.25 or 2.50 mg/kg) and scopolamine (0.188, 0.375, and 0.750 mg/kg) were determined alone and in combination. Relative to vehicle controls, baclofen alone did not affect performance in the radial arm maze (number correct in the first eight responses, total errors) or the time required to complete the maze. Scopolamine alone decreased the number of correct responses in the first eight arm choices, while increasing both the number of errors and the time necessary to complete the maze. When the two drugs were co-administered, baclofen had no effect on the number of errors or time required to complete the maze in the presence of scopolamine; however, in combination with the high dose of scopolamine, it significantly increased the number of errors made during the first eight choices. Baclofen thus can exacerbate some radial arm maze dificits produced by an anticholinergic drug. In a subsequent experiment to test the interaction between scopolamine and baclofen using a nonlearned behavior, baclofen (1.25 and 2.5 mg/kg) did not affect motor activity, whereas all doses of scopolamine (0.188-0.75 mg/kg) increased activity. The higher dose of baclofen attenuated scopolamine-induced hypermotility by 50%, but the lower dose of baclofen was not effective. These data demonstrate pharmacological interactions between baclofen, a drug used clinically for spaticity, and a drug having anticholinergic properties.

We recently have described a new model of ictal-like electrographic activity in the hippocampal slice. When magnesium is eliminated from the medium bathing the hippocampal slice, spontaneously occurring electrical events which closely resemble electrographic seizures can be recorded extracellularly from area CA3. In order to begin to understand the mechanisms of initiation and termination of these seizures, the present study investigated the role of GABAa-mediated inhibition in these processes. Prior to the onset of a seizure recorded from stratum pyramidale of CA3, a twin-pulse stimulus to stratum radiatum of CA3 evoked twin EPSPs. Following the seizure, the same stimulation triggered a strong epileptiform burst. This is consistent with the known reduction of GABAa inhibition after seizures in vivo. Addition of bicuculline, picrotoxin, or penicillin, which reduce the efficacy of GABAa -mediated inhibition in this system, caused triggered epileptiform bursting to occur prior to the seizure as well. They also lowered the threshold for the production of seizures, facilitating their spontaneous occurrence or elicitation by fewer stimulus pulses. This suggests that the GAB Aa inhibition present in magnesium-free (0-Mg) plus baclofen medium tends to suppress the onset of seizures and raises the seizure threshold. However, these drugs did not prolong the ictal events. This suggests that in this model, GABAa-mediated inhibition is not responsible for termination of the seizure-like activity. Although there is some potentiation of the tonic firing phase of the seizures under these conditions, there is no gross change in the morphology of the events when inhibition is suppressed. In this model, therefore, GABAa-mediated inhibition plays a limited role in determining the structure and duration of the ictal events, but may contribute to the seizure threshold.

Recently, we have reported that the exposure of hippocampal slices in vitro to artificial cerebrospinal fluid (ACSF) containing no added magnesium results in ictal-like (ictaform) activity in area CA3 of the hippocampal formation. Other reports describe such activity in slices of entorhinal cortex (EC) under similar conditions. Because of the close interrelationship between the entorhinal area and the hippocampal formation, we have begun to study, in vitro, brain slices which contain both the entorhinal cortex and the hippocampal formation. In these slices, we have found that, in the magnesium-free (0-Mg 2+ ) model, there is good electrical communication between area CA3 and the EC. Simultaneous recordings of the activity in the EC and CA3 showed that, when the circuitry linking the two areas was intact, the EC tended to initiate the ictaform activity and lead CA3. However, late in the event, CA3 could lead EC. Furthermore, interictal-like spontaneous bursting in CA3 led to a disorganized pattern of ictaform activity in EC. Finally, when the EC was separated from the hippocampal formation, both areas were capable of ictaform activity which was temporally unrelated. This model provides the opportunity to explore the relationship between two epileptogenic areas in vitro, and to compare and contrast the morphology of the ictaform activity present in both structures. As such, it may prove valuable in both pharmacological and physiological studies of seizure disorders.

Prenatal exposure to ethanol has been associated with birth defects ranging in severity from physical dysmorphias and profound mental retardation to more subtle compromises of cognitive and behavioral function. Recent evidence has shown the hippocampus to be damaged both morphologically and neurochemically after such exposure in experimental animals. The functional implications of these changes have just recently begun to be addressed. We now report that long-term potentiation and potassium-induced excitability are decreased in hippocampal slices from adult animals exposed to ethanol in-utero. These deficits reflect a decrease in the plasticity of the hippocampal formation. This alteration may be one factor contributing to the memory and learning deficits associated with in-utero exposure to ethanol.

Recent studies have demonstrated that (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801), a use-dependent blocker of N- methyl- d -aspartate (NMDA)-activated membrane channels, attenuates the development of long-term potentiation in vitro and kindling in vivo. Both of these phenomena are manifestations of physiological plasticity related to behavioural states and the results of these studies add to the gathering evidence for the involvement of the NMDA receptor/channel system in such processes. In the present experiment, slices of hippocampus, prepared from rats, were electrically stimulated to produce either long-term potentiation of the CA1 population spike or stimulus train-induced epileptiform bursting in area CA3. At 10 μM, MK-801 attenuated the development of long-term potentiation, but had no attenuating effect upon the previously-potentiated population spike. Similarly, 10 μM MK-801 attenuated the development of epileptiform activity in area CA3, but had little or no effect on the previously-established bursting in area CA3. These data support the suggestion that MK-801 exerts an antiepileptogenic, but not an anticonvulsant effect, at concentrations which also inhibit long-term potentiation.

Previous studies have shown that electrographic seizures (EGSs) can occur spontaneously for several hours in the hippocampal slice in Mg-free medium containing the GABAb agonist baclofen. We report here that similar all-or-none EGSs could be triggered in physiological Mg medium (with or without baclofen) if the EGSs were induced by several stimulus trains, and sometimes transiently exposed to Mg-free medium.

A control alcohol (ethanol) preference test was first given to adult male Sprague-Dawley rats which were provided water together with alcohol increased in concentrations from 3 to 30% over 8 days. Then each rat was anesthetized and a two-stage, bilateral lesion was made stereotaxically by microinjection of kainic acid in a dose of 0.5 micrograms into the hippocampus, cingulate gyrus or periaqueductal gray. Specifically selected sites were based on earlier experiments in which the application of a tetrahydroisoquinoline alkaloid was found to induce an increase in alcohol drinking. Two weeks later, the 8-day test of self-selection for alcohol was repeated. Although neurological signs of kainic acid lesions were apparent, no significant differences in alcohol drinking in terms of proportion or g/kg intake occurred after either the first or second stage of the lesion. These results show that local destruction of an alkaloid-sensitive site does not appear to be responsible for induction of aberrant drinking of alcohol.

Male rats of the Long-Evans strain were divided into two equal groups of nine each and given either 7.0 mg/kg trimethyltin (TMT) or 0.9% saline by intragastric gavage. The pattern of self-selection of alcohol in concentrations of 3%--30% was examined in both groups at 21 and 150 days following the gavage. The TMT-treated rat consistently drank less alcohol than did the controls at every concentration of alcohol. This difference in alcohol intake was equally significant when the rats were tested in a food-contingent, schedule-induced polydipsia situation. Further, although the TMT-lesioned animal consumed fewer calories per day in the form of alcohol, their overall daily caloric intakes were slightly higher than those of the controls. These results are interpreted as a consequence of damage to structures of the forebrain and as part of a syndrome of behavioral and neurological pathology.

The purpose of this investigation was to determine the functional relationship between putative satiety peptides and endogenous norepinephrine (NE) activity in the hypothalamus. Permanent guide cannulae for push-pull perfusion were implanted stereotaxically in Sprague-Dawley rats so as to rest above the medial or lateral hypothalamus (LH). Post-operatively, the animals were either satiated with food and water, both available ad lib, or fasted for 18-22 hr prior to an experiment. To perfuse a site in the LH, paraventricular (PVN) or ventromedial nucleus (VMN), a concentric 29-23 ga push-pull cannula system was lowered to a pre-determined site, in most cases after catecholamine stores had been pre-labeled with [3H]-NE. During control tests, an artificial CSF was perfused at a rate of 20-25 microliter/min for 5-8 min with a 5 min interval between each sample. The addition of cholecystokinin (CCK) in a concentration of 2.0-6.0 ng/microliter to the CSF perfused in PVN or VMN of the satiated rat enhanced the efflux of NE; however, in the fasted animal CCK often suppressed the catecholamine's release. Perfused in the LH, CCK exerted opposite effects, typically augmenting NE output when the rat was fasted but not affecting the amine's activity during the sated condition. Proglumide (1.2 micrograms/microliter) attenuated CCK's effect in releasing NE when the antagonist was perfused in the PVN of the satiated rat. Similar experiments in which neurotensin (NT) was perfused in the LH, PVN and VMN revealed virtually the same inverse effects on NE release in the fasted and satiated rat, which again were anatomically specific. Finally, insulin and 2-deoxy-D-glucose (2-DG) exerted similar state-dependent effects on the release of NE within LH and PVN. Overall, the results suggest that CCK or other neuroactive peptide could serve as a "neuromodulator" of the pre-synaptic release of NE within classical hypothalamic structures which are thought to underlie both hunger and satiety. The state-dependent nature of the peptides' activity on the noradrenergic feeding mechanism implies that these substances constitute a pivotal portion of the profile of factors which impinge functionally upon the hypothalamic neurons responsible for the feeding response and its cessation.

In male rats of the Long-Evans strain, either 7.0 mg/kg of trimethyltin (TMT) or 0.9% NaCl was administered by intragastric gavage. After a period of recovery from the typical signs of trimethyltin toxicity, each rat was tested at 72-hr intervals for its locomotor activity in an open field apparatus, the floor of which was divided into square grids. The baseline activity of each of the trimethyltin-treated rats was significantly greater than the saline-treated controls. (d-Amphetamine, injected intraperitoneally in a dose of 0.5 or 2.0 mg/kg, augmented the hyperactivity of the trimethyltin-treated animals. However, a 4.0 mg/kg dose of d-amphetamine markedly attenuated the hyperactivity of trimethyltin-treated rats while elevating that of the controls. Since trimethyltin produced an autism-like behavioral disorder involving hyperactivity, perseveration, aggressiveness and impairment in problem-solving and memory function, the placating effect of amphetamine supports the proposition that the pathology due to trimethyltin may represent an experimental analogue to the hyperkinetic syndrome in children.

The effect of magnesium-free medium on the electrical activity in CA3 of the rat hippocampal slice was examined. Magnesium removal resulted in the development of spontaneous and triggered interictal-like bursting, followed by spontaneous ictal-like events and finally periodic clustered bursts. The ictal-like events consisted of a tonic firing phase and a phase of clustered burst discharges resembling the tonic and clonic phases of seizures. The return to normal medium resulted in spontaneous and triggered interictal-like bursts.