Behavioral effects following subacute inhalation exposure to m-xylene or trimethylbenzene in the rat: a comparative study.

Trimethylbenzene (TMB), like xylene (dimethylbenzene), is a significant constituent of some industrial solvent mixtures. In earlier studies, we found that in the rat a subacute low-level inhalation exposure to some of the TMB isomers may result in behavioral alterations detectable weeks after the exposure [Neurotoxicol Teratol 19;1997:327; Int J Occup Med Environ Health 11;1998:319]. The purpose of the present study was to compare m-xylene (XYL) and each of the TMB isomers: 1,2,3-TMB (hemimellitene - HM), 1,2,4-TMB (pseudocumene - PS), and 1,3,5-TMB (mesitylene - MES) with respect to the ability for inducing behavioral effects in the rat. The rats (10-11 animals per group) were exposed repeatedly for 4 weeks (6 h per day, 5 days per week) to XYL (XYL group), HM (HM group), PS (PS group) or MES (MES group) at 100 ppm, or sham exposed (C group) in 1.3 cu/m dynamic inhalation chambers. Starting 2 weeks after exposure the following forms of rat's behavior were assessed: radial maze performance, spontaneous activity in an open field, learning and retention of passive and active (two-way) avoidance response, and heat-induced paw licking before and after a 2 min footshock (a test for assessment of the stress response). None of the solvent-exposed groups differed considerably from the control one with respect to the radial maze performance. Compared to control rats, the rats of the XYL, PS and MES groups, but not those of HM group, showed a significantly higher spontaneous locomotor activity in the open field, an impaired passive avoidance learning and significantly longer paw-lick latencies 24 h after footshock. Acquisition, but not retention, of the two-way active avoidance response was significantly impaired in all solvent-exposed groups. The XYL group did not differ significantly from PS, MES or HM group in any of the behavioral parameters. The above results show that a short-term exposure to any of the TMB isomers or m-xylene at concentration as low as 100 ppm may induce persistent behavioral alterations in the rat.

Long-term effects of acute exposure to chlorphenvinphos on behavioural responsiveness to amphetamine and scopolamine in rats.

We investigated the effect of acute exposure to chlorphenvinphos (CVP) (2-chloro-1 (2,4-dichlorophenyl)-vinyl-diethyl-phosphate), an organophosphate anticholinesterase, on the amphetamine- and scopolamine-induced open-field locomotion in Wistar rats. CVP was administered at a single i.p. dose of 1.0 mg/kg (1/10 of LD50). In part of the rats cholinesterase (ChE) was determined. Three hours after CVP injection, the ChE activity decreased by about 27%. It returned to the preinjection level within 14 days after the exposure. In the behavioural part of the experiment, the animals were challenged with 1.0 mg/kg amphetamine or 0.75 mg/kg scopolamine three weeks after CVP exposure, i.e. after a period of time sufficient for cholinesterase recovery. It has been found that in the CVP-exposed rats, the behavioural responses to amphetamine or scopolamine challenge (the increase in locomotor activity) was significantly reduced compared to the controls. This suggests that acute exposure to CVP produced an increase in cholinergic activity which persisted long after ChE activity had returned to normal.

Spontaneous spike-wave discharges in rat neocortex and their relation to behaviour.

A certain proportion of laboratory rats of various strains show spontaneous nonconvulsive ECoG seizures in the form of bursts of spike-and-wave discharges (SWD). Since in the majority of behavioural experiments the EEG is not controlled, the experimenter is usually unaware of this fact. The purpose of the present work was to find out whether the SWD trait is related to the rats behavioural performance in selected test situations. The experiment was performed on two groups of male Wistar rats, outbreds, aged six (group 6M, n = 17) and 24 months (group 24M, n = 14). First, in both groups the following forms of behaviour were assessed: (1) seeking water reward in an 8-arm radial maze, (2) exploration of a new object, (3) inhibition of a locomotor response (passive avoidance), and (4) paw-lick response to a thermal stimulus (54.5 degrees C) applied to the feet before and after intermittent footshock. The rats were then implanted with intrabrain electrodes and the level of SWD activity was assessed. Rats of the 24M group, compared with those of the 6M one, showed a significantly shorter exploratory response to a new object and diminished responsiveness to heat. The groups did not differ, however, in passive avoidance and radial maze performance. The analysis of 3-h ECoG sections revealed SWD bursts in 73% and nearly 93% of rats from groups 6M and 24M, respectively. The groups did not differ in the number of bursts or in the total duration of SWD activity. A correlation analysis of pooled data from both groups revealed that the exploration time of a new object was significantly (negatively) correlated with the number of SWD episodes. The total duration of SWD activity, and the number of perseveration errors in the radial maze, was significantly (positively) correlated with the total duration of SWD activity. The results suggest that SWD rats are behaviourally impaired in some test situations.

Hyposensitivity to amphetamine following exposure to chlorphenvinphos--protection by amphetamine preexposure.

We investigated the effect of an acute exposure to chlorphenvinphos (CVP), an organophosphate anticholinesterase, on amphetamine-induced open-field locomotion in rats. CVP was administered in a single i.p. dose of 1.0 mg/kg (1/10 of the LD50). All animals were challenged with 1.0 mg/kg amphetamine (AMPH) three weeks after the CVP exposure, i.e. after a time sufficient for acetylcholinesterase recovery. Some rats were also given AMPH three weeks before the CVP exposure. In rats challenged with AMPH only once after the CVP exposure, AMPH-induced open-field locomotion was significantly reduced. Such an effect was not observed in rats given AMPH three weeks before the CVP exposure. The results suggest that a single CVP exposure may result in persistent dopaminergic hyposensitivity, and that an amphetamine pretreatment may protect the rat against this effect.

Behavioural research and nonconvulsive spontaneous electrocortical seizures in laboratory rats. A note of concern.

A proportion of subjects of rat strains and lines commonly used for behavioural research in neurotoxicology, neuropharmacology and related fields show spontaneous generalized nonconvulsive EEG seizures. The seizures have the form of bursts of spike and wave discharges (SWD) reminiscent of human petit-mal epilepsy. Behavioural manifestations of SWD seizures are poor and, therefore, their occurrence is not taken into account either during selection or during grouping of rats for the experiment. This paper comprises a short discussion of literature data which allow one to suspect that the rats with SWD seizures differ from normal nonepileptic ones with respect to behavioural performance in some test situations and behavioural effects of exposure to drugs or other chemicals. Thus, considering the fact that the proportion of subjects with SWD seizures may vary considerably in rat samples, it seems likely that the broad use of rat populations heterogenous with respect to this EEG pathology may be responsible, at least in part, for inconsistent results of similar experiments performed at different laboratories. Therefore, attempts should be made in order to reduce the use of such rat population in behavioural research, particularly those intended to supply data which may consist the basis of hygiene standards of exposure to xenobiotics.

Long-term behavioural effects of a repeated exposure to chlorphenvinphos in rats.

The aim of the present study was to investigate persistent neurobehavioural effects of repeated low-level exposure to chlorphenvinphos ((2-chloro-1-(2,4-dichlorophenyl) vinyl diethyl phosphate-CVP) in rats. The rats received 10 i.p. injections of CVP at daily doses of 0.5 mg/kg or 1.0 mg/kg (one injection/day, five days/week) which corresponded to 1/20 and 1/10 of LD50, respectively, for this species. In a part of the rats, cholinesterase (ChE) activity in blood (plasma and erythrocytes), and in the selected brain regions was determined at arbitrarily chosen time after the last exposure. The determinations showed that the level of ChE inhibition was dose-related, but the compartments studied differed in the magnitude of this effect. The differences in the level of ChE inhibition between the compartments were particularly evident in rats which had received CVP at the 1.0 mg/kg dose; in these animals 3 h after exposure the ChE activity in erythrocytes, plasma and the brain corresponded to 78%, 48% and 67-70%, respectively, of the control value. Enzyme activity returned to the control level after 14 days in plasma and after 35 days in erythrocytes. In rats receiving CVP at daily doses of 0.5 mg/kg, ChE activity in plasma was decreased by 40.8% and that in erythrocytes by 21.4% 3 h after the last exposure. The activity of ChE in plasma returned to the control level within four days and that in erythrocytes within 14 days. In these rats, in all the brain regions studied except brainstem, ChE activity was not reduced significantly. In rats selected for behavioural tests, the following behavioural aspects were investigated: response to novelty in an open field, acquisition and extinction of a one-way active avoidance response, and the magnitude and persistence of the footshock-induced analgesia (hot-plate test). Testing in the open field was performed before the exposure and then 1, 3 and 6 weeks after the last exposure. The remaining tests were performed after the exposure. The interval between testing and the last CVP injection was sufficient for recovery of ChE activity. It has been found that in rats of both exposure groups the response to novelty in the open field, i.e. the increase in locomotor and exploratory activity in the presence of a new object, was reduced, albeit nonsignificantly, compared to the unexposed animals. In rats which received CVP at the dose of 1.0 mg/kg, acquisition of the one-way active avoidance response was facilitated. No differences between groups were found during extinction of this response. In the hot-plate test, in rats exposed repeatedly to 1.0 mg/kg CVP, the footshock-induced increase in the latency of the paw-lick response to heat (54.5 degrees C) was stronger and more persistent than in the unexposed animals. The above results show that some neurobehavioural effects of exposure to organophosphorous (OP) compounds may be detected after a time sufficient for recovery of ChE activity.

Effects of acute exposure to aromatic hydrocarbons C 9 on locomotor activity in rats. Trimethylbenzene isomers.

This study was performed to find out whether in acute exposure to trimethylbenzene (TMB) isomers the dose effect relationship is linear or biphasic. In experiments performed on rats, the effect of four solvents was studied: three TMB isomers: 1,3,5-TMB (mesitylene), 1,2,3-TMB (hemimellitene), and 1,2,4-TMB (pseudocumene), and toluene, known for its biphasic activity, was used as a reference compound. The solvents were dissolved in olive oil and administered to rats orally at the doses of 0.008, 0.016, and 0.032 mol/kg. Spontaneous locomotor activity was assessed with the open-field test. Solvent concentrations in peripheral blood were determined parallelly by gas chromatography on separate groups of animals. Statistics employed a two-way analysis of variance (ANOVA) and Tukey's test. The results showed that oral administration of toluene at a dose of 0.008 mol/kg induced biphasic changes in the animal locomotor activity. It was found that TMB at applied doses increased slightly the animal locomotor activity, but the magnitude of changes did not indicate their stimulating effect. Contrary to toluene, no time-effect relationship was observed after administration of trimethylbenzene isomers. The mean blood concentrations of solvents were dose-related. The highest concentrations were observed after toluene administration.

Organic solvents and time-dependent sensitization.

The nervous system is main target of the toxic action of most of organic solvents. There is little doubt that occupational solvent exposure may result in persisting neurobehavioural disturbances--the organic solvent syndrome. Recently, the solvents are quoted among possible causes of the abnormal condition referred to as multiple chemical sensitivity (MCS), and which is characterized by a psychosomatic over-reactivity to a variety of chemicals present in food or ambient air. According to some authors, MCS is a manifestation of the time-dependent sensitization (TDS), a phenomenon of progressive increase in responsiveness to chemical agents following acute or intermittent exposure, and related to some functional aberrations within limbic structures. TDS is commonly induced by psychostimulant drugs. The purpose of the present paper was to show, based on the literature data, that under circumstances of acute and repeated exposure, some solvents (mainly toluene) exert effect on behaviour and on the functional state of some neurotransmitter systems similar to that exerted by drugs known to induce TDS. Of special importance is the fact that in case of solvents the behavioural and biochemical changes suggestive of sensitization appear after exposure at levels close to those admissible in the occupational exposure, and that the concentration-effect relationship is nonlinear (an inverted U curve). To date, however, only a few of the existing data may be regarded as a direct evidence of the solvent-induced TDS. It is mainly due to the fact that the experimental protocol of a TDS study does not match the experimental routine of neurotoxicity assessment. Some data suggest that some solvents are possibly unable to induce TDS. The necessity to assess the commonly used solvents for their ability to induce TDS has been emphasized.

[Behavioral effects of exposure as indicators of neurotoxicity]. / Zachowanie jako wskaznik skutków neurotoksycznych.

All the guidelines and methodological recommendations issued by international organizations, as well as proposals published by independent groups of scientists, emphasize the importance of behavioural testing in the process of neurotoxicity assessment. It stems from the conviction that behaviour, being the 'end product' of the concerted action of many neural subsystems, is the most sensitive indicator of the presence of adverse effects within the nervous system. The purpose of the paper presented was to highlight the structural and functional properties of the nervous system, owing to which the exposure-induced disturbance of the functional balance may initiate processes resulting in a significant change in the primary behavioural response. The final result may be the restoration of the functional balance (compensation, tolerance) or to the contrary, an increased susceptibility to a disturbing factor (sensitization). In both instances the magnitude of the behavioural effect is being changed from one exposure to another which make it rather an unreliable indicator of neurotoxicity. Therefore, conclusions concerning neurotoxicity of a chemical, based on the behavioural end-points, should be drawn with great caution, especially while establishing NOAELs and LOAELs.

Behavioural changes following a four-week inhalation exposure to hemimellitene (1,2,3-trimethylbenzene) in rats.

Trimethylbenzene isomers (TMBs): 1,2,4-TMB (pseudocumene--PS), 1,2,3-TMB (hemimellitene--HM) and 1,3,5-TMB (mesitylene--MES) are important constituents of solvent mixtures. In the US, the adopted TLV-TWA value for TMBs is 125 mg/m3 or 25 ppm (ACGIH 1996). Recent experiments at our laboratory have revealed an impaired learning of passive and active avoidance responses and a longer persistence of an effect of footshock (increase in latency of the paw-lick response to heat) in rats tested several weeks after a four-week inhalation exposure (6h/day, five days/week) to PS at a concentration of 100 or 250 ppm (15). The concentration-effect relationship appeared to be nonlinear; the effect of 100 ppm HM was more pronounced than that of 250 ppm. In the present experiment we investigated the effects of a repeated four-week (6h/day, 5 days/week) inhalation exposure to HM at concentrations of 0, 25, 100 or 250 ppm on radial-maze performance, open-field activity, passive and active avoidance learning, and on the shock-induced changes in latency of the paw-lick response to heat (hot-plate test). The tests were performed between days 14 and 61 after the last exposure. No significant effects on radial-maze performance and open-field activity were noted in any of the dose groups. In the remaining tests effects of exposure were noted but, similarly as in the case of PS exposure, the concentration-effect relationship was not linear. In rats exposed to HM at 25 or 100 ppm, but not 250 ppm, learning of the passive avoidance, i.e. refraining from performance of a punished response (stepping off an elevated platform) was significantly impaired. Moreover, in rats exposed to 100, but not 250 ppm of HM, acquisition of the two-way active avoidance in the shuttle-box was slower and the footshock-induced increase in latency of the paw-lick response to heat persisted longer than in the unexposed animals. The results suggest that a low-level inhalation exposure to HM, just like low-level exposure to PS, may lead to long-lasting disturbances in the CNS functions. The nonlinear concentration-effect relationship observed in the case of both TMB-s requires clarification in further studies.

[Multiple chemical sensitivity: a new type of toxicity?]. / Nadwrazliwosc na czynniki chemiczne. Nowy rodzaj skutków toksycznych?

Multiplechemical sensitivity (MCS) is a chronic condition manifested by the appearance of variable symptoms, involving many systems and organs, after exposure to extremely low levels of chemicals, mainly pesticides and solvents. The paper discusses briefly the main hypotheses concerning causes and mechanisms of MCS development. It was emphasized that during neurotoxicity assessment is necessary to pay more attention to these aspects of toxic effects of chemicals likely to generate MSc.

Behavioral changes following 4-week inhalation exposure to pseudocumene (1,2,4-trimethylbenzene) in the rat.

Pseudocumene (1,2,4-trimethylbenzene, TMB) is a component of several solvent mixtures. During recent studies on rats we investigated the effect of a 4-week (6 h/day, 5 days/week) inhalation exposure to TMB at concentrations of 0, 25, 100, or 250 ppm on radial maze performance, open field activity, passive avoidance, active two-way avoidance, and shock-induced changes in the pain sensitivity reflecting the magnitude of the shock-induced fear response (hot plate test). The tests were performed between days 14 and 54 after the last exposure. The radial maze performance was not disturbed in any dose group. During testing in the open field grooming was significantly increased in rats exposed to 100 ppm TMB. In rats exposed to 100 and 250 ppm TNB, a foot shock applied after stepping off an elevated platform (a safe area) resulted in a significantly smaller increase in the step-down latency (i.e., passive avoidance, on days 3 and 7 after the foot shock) than in sham-exposed animals. Learning of a two-way active avoidance was slightly retarded in rats exposed to 250 ppm of TMB. Results of the hot plate test revealed no differences between groups in the paw sensitivity to heat (54.5 degrees C) before a 2-min intermittent food shock, but in rats exposed to 100 and 250 ppm of TMB the foot shock-induced fear response persisted apparently longer. These results suggest that inhalation exposure to TMB may lead to long-lasting disturbances in CNS functions.

Retardation of the age-related increase in spontaneous cortical spike-wave discharges (SWD) in rats after a 28-day inhalation (SWD) in rats after a 28-day inhalation exposure to an industrial solvent, pseudocumene (1,2,4-trimethylbenzene).

It has been hypothesized that exposure to neurotoxins may hasten the process of brain ageing. Volatile hydrocarbons are in common use as solvents and their neurotoxic properties are acknowledged. In the rat, the age-related neurodegenerative changes in the brain develop together with an increased occurrence of bursts of spontaneous spike-wave discharges (SWD) in the neocortex. Therefore, the number and/or duration of SWD bursts may serve as an index allowing to distinguish between young and old brains (7). Measuring the SWD activity after exposure may thus reveal the effect of the studied neurotoxicant on brain ageing. Pseudocumene (1,2,4-trimethylbenzene, TMB), is a component of industrial solvent mixtures. The present study investigated the effect of a 4-week (6h/day, 5 days/week) inhalation exposure to TMB at concentrations of 0, 25, 100 or 250 ppm on the occurrence of SWD bursts. EEG recordings were performed before and one day, 30 days and 4 months after exposure. In rats exposed to TMB at 0 and 25 ppm, the level of the SWD activity increased progressively after exposure. In rats exposed to TMB at 100 or 250 ppm, the SWD activity did not increase during the post-exposure period or even dropped below the pre-exposure level. This result indicates that the persistent changes in the rat central nervous system (CNS), following the exposure to TMB, differ in some respects from those which develop in the course of normal ageing.

[Possible consequences of AChE inhibition in organophosphate poisoning. A new approach to an old problem]. / Mozliwe nastepstwa inhibicji AChE w zatruciu zwiazkami fosforoorganicznymi. Problem odkrywany od nowa.

Organophosphorus pesticides (OP) inhibit irreversibly acetylcholinesterase (AChE). The known function of AChE is hydrolysis of the neurotransmitter, acetylcholine (ACh). Therefore, the consequences of OP intoxication are usually perceived as results of transient excess of ACh in cholinergic synapses of the central and peripheral nervous system as well as in cholinergically innervated organs and tissues. Since a long time it has been known, however, that in the nervous system the localization pattern of AChE does not overlap the localization of cholinergic neurons, and several recent findings show that apart from being the ACh hydrolyzing enzyme, AChE is also a neuromodulator participating in the phenomenon of neuronal plasticity i.e. induction of longterm changes in synaptic efficacy. It is likely then, that out of the known effects of OP poisoning regarded as results of cholinergic hyperactivity some are due to the loss of the nonenzymatic, neuromodulatory role of AChE. They may include the longterm alterations of cognitive function which may follow longterm occupational OP exposure or OP poisoning.

[Neurotoxicity of penicillin and other beta-lactam antibiotics--importance in clinical practice]. / Neurotoksycznosc penicyliny i innych antybiotyków beta-laktamowych--znaczenie w praktyce klinicznej.

Epileptogenic properties of penicillin, applied either directly to the brain or systemically, have been known for many years and may be also of clinical importance. In the present work different aspects of this action, including clinical ones, as well as risk factors for the occurrence of epileptic seizures, and mechanism of penicillin action in the central nervous system have been shortly described. Moreover, results of experimental studies confirming epileptogenic properties of many antibiotics, mainly b-lactams, have been mentioned. Data from experimental studies, revealing the possibility of long-term adverse effects of repeated injections of convulsants, have been emphasized, and the absence of relevant clinical studies has been pointed out. Widespread usage of beta-lactam antibiotics should encourage medical personnel to pay more attention to all these problems in everyday clinical practice.

Persisting behavioural and electroencephalographic effects of exposure to chlorphenvinphos, an organophosphorous pesticide, in laboratory animals.

Organophosphorous compounds (OPs) constitute a large proportion of insecticides used all over the world. Their insecticidal properties and acute toxicity in nontarget species derive from the inhibition of acetylcholinesterase (AChE) which disturbs the cholinergically mediated neurotransmission. OPs do not accumulate in living organisms and the acute signs and symptoms disappear as the AChE activity returns to normal level. Therefore, they are regarded as relatively safe. However, as some literature data suggest, after either acute or prolonged exposure to OPs subtle neurobehavioral impairments may persist long after normalization of AChE activity. The possibility that OPs exposure may induce such long-term effects is nowadays a problem of great concern for the regulatory agencies. Here we present a review of studies from our laboratory aimed at detecting neurobehavioural effects of exposure to chlorphenvinphos, an OP pesticide ((2-chloro-1-(2,4-dichlorophenyl) vinyl diethyl phosphate)--CVP)) in laboratory animals. In Poland, CVP is manufactured (250 tons/year) and used for crop protection. In a series of experiments we have demonstrated that: a) in rabbits, two i.p. exposures to CVP at the same sublethal dose at three-month interval resulted in a similar inhibition of blood AChE activity but the effect of the second exposure on body temperature and hippocampal EEG was smaller and less consistent than that of the first one. This would indicate that some permanent changes within the CNS may occur even after a single exposure to CVP; b) in rats, under conditions of repeated i.p. exposure to CVP (one injection/day for ten days) at a symptomatic (3.0 mg/kg) dose inhibiting blood and brain AChE activity by about 80%, the tolerance to CVP, assessed from the spontaneous locomotor behaviour, developed within four to five days. However, single exposure to CVP at a symptomatic (3.0 mg/kg) or subsymptomatic (1.0 mg/kg, less than 50% AChE inhibition) dose, or repeated exposure (one injection/day, for ten days) at subsymptomatic doses (1.0 mg/kg or 0.5 mg/kg) resulted in subtle changes in complex behaviours detectable after AChE activity in blood and in the brain had returned to the normal level. The changes--neophobia in the open field, an increased and more persistent emotional response to a stressful stimulus, and increased EEG arousal response to an external pain signalling stimulus--suggest an increased reactivity of the system or systems responsible for the induction of fear; c) direct intrahypothalamic injections of CVP, unlike those of oxotremorine, a direct stimulant of cholinergic muscarinic receptors, did not induce overt changes in the animal (rabbit) behaviour and EEG. This would indicate that the changes in the CNS functions after CVP exposure may be the consequence of increased cholinergic activity due to AChE inhibition rather than to a direct stimulation of cholinergic muscarinic receptors by CVP. The above findings provide experimental evidence that health effects of exposure to CVP, may persist after recovery of AChE activity in blood and in the brain.

The influence of repeated systemic penicillin injections at subconvulsive doses on spontaneous spike-wave discharges in the rat.

Changes in nonconvulsive spontaneous epileptic activity-Spike-Wave Discharges (SWD)-induced by repeated intraperitoneal (i.p.) administration of crystalline penicillin (PC) at subconvulsive doses were evaluated in imp-DAK rats. Three groups received ten daily i.p. injections of PC at doses of 750,000, 500,000 and 250,000 IU/kg b.w. For comparison, another classic convulsant, pentylenetrazol (PTZ) was applied in the same way to another group. PTZ was also given to all rats before and after injection series for better evaluation of changes in CNS excitability. Repeated PC injections resulted in a progressive increase in the basal level of the spontaneous SWD activity and in an increase in the SWD response to PC, which was statistically significant in the case of the dose 750,000 IU/kg. Moreover, in all rats given PC the response to PTZ (increase in SWD activity) was reduced. The results obtained in this and previous experiment suggest that in the course of repeated systemic Pc administration adaptive changes in the rat CNS develop which prevent the convulsive effects of Pc but promote the occurrence of the spontaneous nonconvulsive SWD activity.

Interaction of chlorphenvinphos with cholinergic receptors in the rabbit hypothalamus.

The purpose of this study was to find out whether chlorphenvinphos (CVP), an organophosphorous pesticide, interacts with the muscarinic cholinergic receptors in CNS. To attain this goal, the effects of intrahypothalamic injections of oxotremorine (Ox), a muscarinic agonist, and physostigmine (Phys), a carbamate anticholinesterase, were compared with those produced by intrahypothalamic injections of CVP in the rabbit. It was found that the infusion of Ox (20 micrograms) as well as Phys (200 micrograms) into the anterior hypothalamus leads to an increase in the 4-7 Hz theta rhythm in the hippocampus and to the appearance of behavioral symptoms suggestive of a threat response. In the case of Ox, the effects could be prevented by injections of 20 micrograms scopolamine, a muscarinic antagonist. Pretreatment of the hypothalamus with 100 micrograms hemicholinium (HC-3) did not prevent the effects of Phys injected 2 h later. (HC-3 prevents the resynthesis of acetylcholine by blocking choline reuptake. This leads to a gradual depletion of ACh stores and to an inhibition of the cholinergic transmission). It suggests that Phys activates directly postsynaptic muscarinic receptors. Intrahypothalamic injections of CVP in doses of up to 1360 micrograms produced no overt changes in behavior nor in the hippocampal EEG of the rabbit and did not prevent the effect of subsequent injections of Ox. This suggests that CVP is neither an agonist nor antagonist of the muscarinic receptors in the rabbit hypothalamus.

Effects of repeated systemic penicillin injections on nonconvulsive and convulsive epileptic seizures in the rat.

Changes in the spontaneous and induced epileptic activity in the course of repetitive systemic i.p. administration of crystalline penicillin (Pc) were examined in imp-DAK rats. In all the rats used, nonconvulsive seizures characterized by bursts of spike and wave discharges (SWD) in the neocortex occurred spontaneously. A single i.p. injection of Pc at doses of 1,000,000, 1,500,000 or 2,000,000 IU/kg resulted in a transient increase in SWD activity. For the latter two doses, Pc injections also induced convulsive activity, i.e. single spikes and trains of spikes accompanied by myoclonies. When 1,500,000 IU/kg of Pc was administered repeatedly (six injections, one every 48 h), the amount of the convulsive activity induced by successive injections decreased but the increase in the number of SWD bursts became more pronounced. This result gives rise to some questions about the development of tolerance to the epileptogenic Pc effects in the course of repeated administration of this antibiotic.

Development of the age-related spontaneous spike-wave discharges in rat neocortex and exposure to a model neurotoxin.

In laboratory rats an epileptic-like spontaneous neocortical activity in the form of bursts of spike and wave discharges (SWD) develops gradually with age. High incidence of the SWD episodes is accompanied by other indices characteristic of advanced age: memory disturbances and atrophic changes within basal forebrain structures (Buzsaki et al. 1988b, Gage et al. 1988). Accordingly, it has been proposed that the number and duration of the SWD episodes be regarded as a diagnostic marker to distinguish between young and old brains (Buzsaki et al. 1988a). It is suspected that exposure to neurotoxins may accelerate the progress of age-related neurodegeneration by predisposing neurons to premature death and thus hasten the appearance of the age-related functional deficits (Weiss 1990). Analysing the development of SWD activity in exposed rats may be helpful for an assessment of the potency of the neurotoxin under study to exert such an effect. In the present work the influence of a three-month exposure to a model neurotoxin, ethanol (ETOH), on the development of the SWD activity in imp-DAK rats was investigated. It has been found that in rats given 10% ETOH solution as the only drink for three months, the incidence of the SWD episodes increased markedly. The increase was most clearly seen after ETOH withdrawal and on the 90th day after exposure no tendency to decline could be observed. The obtained data indicate that exposure to exogenous substances may exert a distinguishable long-lasting influence on the development of the SWD activity.