Joy Rm

A pharmacological study in the kindling model of epilepsy

The anticonvulsant properties of carbamazepine were evaluated in the kindled amygdaloid seizure model in rats. Carbamazepine significantly raised the threshold for seizures, reduced the duration of elicited afterdischarges and attenuated the severity of seizures in previously-kindled rats, at doses that did not cause sedation or ataxia. A similar reduction in the duration of elicited afterdischarges and severity of seizures was seen after suprathreshold stimulation (400 mu A) with doses of carbamazepine that were without obvious sedative or ataxic effects. After acute intraperitoneal injections (solvent = 2% Tween-80 and 70% propylene glycol), the maximum anticonvulsant effectiveness against suprathreshold stimulation was seen at 30 min. When administered daily (13 days) during acquisition or development of kindling, carbamazepine (25 and 50 mg/kg, i.p.) had variable effects on kindling. Neither dose consistently reduced the duration of elicited afterdischarges during the acquisition phase. Both groups tended to reduce the developing seizure, with the smaller dose of carbamazepine (25 mg/kg) resulting in a more consistent and significant reduction in severity of seizures. No significant differences in number of daily stimulations needed to reach fully kindled seizures were found. Previous studies have reported variable results with carbamazepine and the kindled amygdaloid seizure in rats. The present study provides a comprehensive evaluation of carbamazepine in this model of epilepsy and discusses the results with regard to the finding reported previously.

Convulsive properties of chlorinated hydrocarbon insecticides in the cat central nervous system

Abstract The convulsive properties of subgroups of chlorinated hydrocarbon insecticides were compared in locally anesthetized, paralyzed cats. Dieldrin, endrin, and lindane proved to be rapidly acting, potent convulsants. They produced indistinguishable changes in EEG and evoked-response studies. Hypersynchrony, rhythmic bursts of spikes and waves, and isolated spikes characterized the preictal state. Seizures were of a generalized tonic-clonic type. Responses in sensory and motor cortex to sensory nerve stimulation were enhanced three- to fivefold. Aldrin and heptachlor produced similar effects but only after a delay of 20–30 min and at two to three times higher doses. DDT effects were more variable. Although it produced convulsive behavior, EEG effects differed, being characterized by sustained hypersynchrony. Responses to sensory nerve stimulation were usually, but not always, enhanced. Kelthane produced no measurable effects on EEG or evoked responses at all doses below those causing death by cardiovascular collapse. These data demonstrate that the epoxide compounds (dieldrin and aldrin) and lindane are directly toxic to the mammalian CNS and do not require metabolism to an active metabolite. Second, for the cyclodienes the epoxides are more potent and more rapid in development of effects than are the nonepoxide analogs.

NMDA receptors have a dominant role in population spike-paired pulse facilitation in the dentate gyrus of urethane-anesthetized rats.

Paired-pulse facilitation was studied at the perforant path-granule cell synapses in the dentate gyrus of urethane-anesthetized rats. Extracellular field potentials comprising excitatory postsynaptic potentials (EPSPs) and population spikes (PSs) were used to compare facilitation of both responses at interpulse intervals between 10 and 1000 ms. In this model system EPSPs, produced at stimulus intensities well below the PS threshold, exhibited paired-pulse facilitation (PPF) at intervals less than 40 ms. Between 40 and 100 ms both responses were of equal size and from 200 to 1000 ms the second response exhibited paired-pulse depression (PPD). With higher stimulus intensities, PSs exhibited early (10-30 ms) and late (400-1000 ms) PPD and intermediate interval (40-200 ms) PPF. The enhanced PS amplitude during facilitation was accompanied by a modest decrease in EPSP slope and increase in PS latencies to onset and to peak. If PS amplitude was increased to the same extent by simply increasing stimulus intensity, an increase in EPSP slope and decrease in PS latencies to onset and peak were observed. Current source-density analysis revealed that the current sink generated by synaptic activation of granule cell dendrites was indistinguishable between control and facilitated responses up to the onset of the PS. However, the facilitated response exhibited a marked increase in the duration and amplitude of current flowing into the dendrites during the late phase of the EPSP. PPF of the PS was attenuated or blocked by the administration of the known NMDA receptor-ion channel blockers, MK-801, dextromethorphan and ketamine. The depressant effects of these pharmacological agents on facilitation shared the same time course as facilitation itself, peaking at 60 ms and lasting approximately 200 ms. These data suggest that facilitation in this situation is due to postsynaptic rather than presynaptic modulations, and is based upon an increase in the NMDA-mediated component of the evoked response. No increase in transmitter release, per se, could be demonstrated.

Aminophylline and kindled seizures.

The effects of aminophylline on amygdaloid and cortically kindled rats was studied. Rats implanted with chronic amygdaloid electrodes received either saline or 150 mg/kg, i.p., aminophylline 20 min prior to their first stimulation. On the first stimulation, aminophylline-treated rats had dramatically longer afterdischarge durations and more severe seizure ranks. When fully kindled, the animals were retested with saline or aminophylline. Again, the aminophylline-treated animals had longer afterdischarge durations than the saline-treated rats. In a second experiment, fully amygdaloid kindled rats were pretreated with various doses of aminophylline and stimulated 20 min later. With suprathreshold stimulation, a dose-dependent increase was noted in the afterdischarge duration. During seizure threshold determinations, aminophylline pretreatment markedly prolonged afterdischarge durations without significantly changing seizure severity or threshold. When animals were treated with the adenosine agonist, 2-chloroadenosine, prior to kindled amygdaloid stimulation, the elicited afterdischarge was shortened. The effect was antagonized where treatment with both 2-chloroadenosine and aminophylline occurred prior to amygdaloid stimulation. Rats with neocortical electrodes were also exposed to various doses of aminophylline while in a stable, partially developed kindled stage and again when fully kindled. At both stages, afterdischarge duration was increased by aminophylline in a dose-dependent manner. The partially developed, cortically kindled animals were more responsive to aminophylline than were those fully kindled and they tended to have greater increases in afterdischarge duration and seizure rank. These data demonstrate that aminophylline acts to prolong afterdischarges elicited at various stages of kindling from both amygdaloid and cortical sites.

Cumulative after-discharge as the principal factor in the acquisition of kindled seizures

Abstract The role of after-discharge in the development of behavioral and electrographic seizures (kindling) was evaluated. Rats were stimulated (amygdala) once daily for 10 days 30 min after an intraperitoneal injection of diazepam, phenobarbital, propranolol or vehicle. Following a 10 day period to allow for drug elimination, the daily stimulations (without drug) resumed until all animals responded with fully kindled seizures. Only the lowest dosage groups of diazepam (2 mg/kg) and phenobarbital (15 mg/kg) showed a savings in the number of stimulations to kindle in the non-drug state. This suggests that little or no kindling development had occurred in the drug state with the high doses (4 or 8 mg/kg diazepam, 30 mg/kg phenobarbital). Propranolol treated rats kindled with the same number of stimulations as their control group. As an alternative measure of kindling, the number of seconds of cortical after-discharge accumulated in the drug state, the non-drug state, and the grand total required to kindle were averaged and compared between the drug and control groups. The average grand total number of seconds of after-discharge accumulated by any of the drug or control 188 and 352 sec. None of the grand totals accumulated by any of the drug or control groups differed significantly. The lowest dosage groups treated with diazepam and phenobarbital accumulated significantly less afterdischarge in both the drug and non-drug state than the total for the control. These data suggest that a specific quantity of after-discharge must be elicited to kindle (200–300 sec) and that any after-discharge elicited in the presence of phenobarbital or diazepam will contribute to that quantity.

Caffeine modification of kindled amygdaloid seizures

Rats were kindled during exposure to caffeine (50 mg/kg) or saline given IP twenty minutes before daily electrical stimulation of the amygdala until 3 kindled amygdaloid seizures (KAS) occurred. They were then stimulated for 3 days without drug pretreatment followed by 5 additional days with drug pretreatment. There were no significant differences between the two groups in the number of daily stimulations or in the total seconds of cumulative afterdischarge (AD) needed to reach the first KAS. During kindling, the daily average AD tended to be longer in the caffeine treated group. This difference became significant (greater than 200% saline) when the KAS was reached. When KAS animals were stimulated without caffeine pretreatment, the average AD returned to control lengths. When put back on caffeine pretreatment, the average AD was again increased. Caffeine (6-50 mg/kg, IP) was also evaluated in previously kindled rats using suprathreshold (400 mu AMP) and threshold (20 microA increments) seizures. Caffeine had no consistent effect on threshold values. However, 12-50 mg/kg of caffeine increased seizure severity and AD durations after threshold stimulation. With suprathreshold stimulation, the length of the AD was significantly increased only after the highest dose of caffeine. It would appear that caffeine lengthens induced afterdischarges both during the acquisition phase of kindling and in the fully kindled subject. Caffeine does not appear to lower seizure thresholds or increase the rate of acquisition of the KAS in the doses tested in this model.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased inhibition in dentate gyrus granule cells following exposure to GABA-uptake blockers

Rats anesthetized with urethane had stimulating and recording electrodes placed in the perforant pathway and in the dentate gyrus. They were then exposed to increasing doses of either the vehicle control dimethylsulfoxide (DMSO) or one of two gamma-aminobutyric acid (GABA)-uptake blockers (SKF-100330A or SKF-89976A). Analysis of evoked field potentials from dentate granule cells indicated that the only effect of the GABA uptake blockers was to increase the threshold for evoking the field population spikes (PS). No other measure of excitatory postsynaptic potentials (EPSPs) or PSs were significantly affected. The lack of effect on evoked EPSP by these drugs suggests no direct effect on transmitter release at this synapse, while the increase in PS threshold suggests a slight decrease in granule cell excitability. The effects of the two GABA-uptake blockers on synaptically mediated facilitation and inhibition was tested by using paired-pulse paradigms. Both GABA-uptake blockers increased early GABA-mediated inhibition to a greater extent than they reduced synaptically mediated facilitation. Neither GABA uptake blocker appeared to effect the late inhibition seen at paired-pulse intervals of 400-1000 ms which is presumably associated with calcium-activated increases in potassium conductance. These effects on granule cell responses occurred at doses found previously not to be associated with side effects and yet to be anticonvulsant in unanesthetized rats. These data confirm in vivo that SKF-100330A and SKF-89976A increase GABA-mediated inhibition. The effect on granule cell excitability and late inhibition are minimal. Although facilitation was reduced by exposure to these drugs, the mechanism of this reduction (direct or prolongation of early inhibition) cannot be determined.

Dose-dependent proconvulsant and anticonvulsant actions of the alpha2 adrenergic agonist, xylazine, on kindled seizures in the rat

The effects of the alpha 2 adrenergic agonist, xylazine, was evaluated on kindling acquisition and on kindled seizure expression in rats. Dose-dependent proconvulsant and anticonvulsant properties were found. The proconvulsant effects were observed at low (0.3 mg/kg) doses. In previously kindled rats these consisted of a decrease in afterdischarge threshold and an increase in the length and severity of the accompanying seizure. This dose also facilitated the rate of kindling in naive subjects. The anticonvulsant effects were observed at higher dose levels (3-20 mg/kg) which also produced sedation and ataxia. If these effects upon kindling are related to the adrenergic actions of xylazine, then it is proposed that the proconvulsant effects are associated with alpha 2 receptor activation and the anticonvulsant effects with alpha 1 receptor activation.

Characteristics of the prolonged inhibition produced by a range of pyrethroids in the rat hippocampus

Eight different synthetic pyrethroids were examined to determine their effects on the excitability of hippocampal granule cells in urethane-anesthetized rats. A paired stimulus approach was used. All eight prolonged the depression of granule cell excitability that follows stimulation of their major synaptic input, the perforant path. The magnitude of this effect depended upon the class to which the pyrethroid belonged. Type I pyrethroids (those primarily producing tremor) prolonged the depression of granule cell excitability for shorter periods than did type II pyrethroids (those primarily producing salivation and choreoathetosis) or pyrethroids producing a mixed type of intoxication. No overlap was found between groups. To determine whether the difference observed between type I and type II pyrethroids was the result of an infelicitous selection of doses, cismethrin (type I) was tested over a dose range of 1.5-24 times the conscious rat iv LD50. Even at the highest dose, the prolongation remained well below that produced by type II pyrethroids. The effect of deltamethrin was shown to be consistent with the production or potentiation of a surmountable inhibitory response. This action of deltamethrin was antagonizable by mephenesin and lidocaine, but not by picrotoxin or halothane. The type of effect, its time course, and the antagonism data suggest that type II pyrethroids enhance inhibition in the dentate gyrus. This action does not appear to be mediated by GABAA receptors.

Type I and type II pyrethroids increase inhibition in the hippocampal dentate gyrus of the rat

Urethane-anesthetized rats were prepared for stimulation of the perforant path and for recording from the granule cell region of the hippocampal dentate gyrus. Subjects were administered varying doses of allethrin (a prototype type I pyrethroid) or deltamethrin (a prototype type II pyrethroid), and the excitability of the perforant path and granule cells was tested. Both pyrethroids produced a dose-dependent decrease in the responsiveness of granule cells, following stimulation of the perforant path, that lasted up to 100 msec. Analysis suggested that the pyrethroid-induced effects were associated with an increase in interneuronally mediated inhibition. Neither the perforant path axon or terminal nor the granule cell was affected by doses which appeared to affect interneurons. Basal excitability of the granule cells was also decreased by deltamethrin. This effect may have been secondary to an increase in tonic inhibition evoked by the same mechanisms responsible for the increase in phasic inhibition.