Subconvulsive Doses Of Picrotoxin Research Articles

Effects of acute and chronic treatment on the pro- and anti-convulsant actions of CL 218, 872, PK 8165 and PK 9084, putative ligands for the benzodiazepine receptor.

CL 218,872 is a triazolopyridazine that acts at the benzodiazepine binding site. At low doses (0.5-7.5 mg kg-1) it is proconvulsant when combined with subconvulsant doses of picrotoxin but not when combined with pentetrazol (leptazol, pentylenetetrazol). At high doses (20-60 mg kg-1) CL 218,872 counteracted seizures caused by pentetrazol but not those caused by picrotoxin. There was tolerance to the proconvulsant effects after five days of treatment and to the anticonvulsant effects after 15-20 days. Two phenylquinolines, PK 8165 and PK 9084, that also act at the GABA-benzodiazepine receptor complex have proconvulsant actions in combination with picrotoxin. Significant tolerance to these effects had not developed even after 20 days of treatment. It is concluded that three different sites on the GABA-benzodiazepine complex mediate the pro- and anti-convulsant actions of CL 218,872 and the proconvulsant actions of PK 8165 and PK 9084.

GSM radiation triggers seizures and increases cerebral c-Fos positivity in rats pretreated with subconvulsive doses of picrotoxin

This study investigated the effects of mobile-phone-type radiation on the cerebral activity of seizure-prone animals. When rats transformed into an experimental model of seizure-proneness by acute subconvulsive doses of picrotoxin were exposed to 2 h GSM-modulated 900 MHz radiation at an intensity similar to that emitted by mobile phones, they suffered seizures and the levels of the neuronal activity marker c-Fos in neocortex, paleocortex, hippocampus and thalamus increased markedly. Non-irradiated picrotoxin-treated rats did not suffer seizures, and their cerebral c-Fos counts were significantly lower. Radiation caused no such differences in rats that had not been pretreated with picrotoxin. We conclude that GSM-type radiation can induce seizures in rats following their facilitation by subconvulsive doses of picrotoxin, and that research should be pursued into the possibility that this kind of radiation may similarly affect brain function in human subjects with epileptic disorders.

Memory impairment after incomplete pharmacological kindling

In rats trained to run for food reinforcement, kindling was reproduced by daily injections of subconvulsant doses of picrotoxin until the stage of recurrent clonic seizures. Subsequent behavioral test revealed no difference between kindled and control rats in retention and retrieval of memory traces. However these groups differed significantly in the dynamics and pattern of extinction: kindled animals needed much more time to stop running under conditions of non-reinforcement. A relearning test showed that kindling did not affect learning ability. Weakened inhibition after incomplete pharmacological kindling is indicative of a congnitive dysfunction characteristic of early stages of epilepsy.

Effects of desipramine and alprazolam in the forced swim test in rats after long-lasting termination of chronic exposure to picrotoxin and pentylenetetrazol

Rats were treated for 5 weeks with three subconvulsant doses of picrotoxin (PTX) and pentylenetetrazol (PTZ) per week to induce a persistent reduction of the GABA A receptor function which results in chemical kindling. Fifteen days after termination of this treatment schedule, the effect of desipramine (DMI) and alpraxolam (ALP) on immobility time in the forced swim test (FST) was evaluated. Chronic PTX and PTZ did not alter the immobility time. Acute PTX and PTZ reduced the immobility of rats chronically treated with vehicle but not of those exposed chronically to PTX and PTZ. Chronic PTX did not influence the anti-immobility effect of DMI, but blocked that of ALP. Chronic PTZ markedly potentiated the anti-immobility effect of DMI but blocked that of ALP. Concomitant administration of chlordiazepoxide prevented the effects of chronic PTX and PTZ. These findings suggest that a long-lasting reduction in GABA A receptor function, unlike acute reduction, does not play an important role in the mobility of rats in the FST and in the anti-immobility effect of DMI while it blocks that of ALP.

The influence of activation of the superior colliculi on convulsive activity during picrotoxin kindling.

It has been demonstrated in experiments in rats in which kindling was induced by repeated subconvulsive doses of picrotoxin (1.0 mg/kg) that the bilateral microinjection of picrotoxin, bicuculline, and penicillin into the deep layers of the superior colliculi of the corpora quadrigemina leads to the appearance of a generator of pathologically intensified excitation. An increase in the latent period of the initial convulsive manifestations, the suppression of clonic-tonic convulsive attacks, as well as the prevention of generalized epileptic activity induced by the systemic utilization of picrotoxin were observed under the conditions of the appearance of a generator of pathologically intensified excitation. An inference was drawn regarding the role of the superior colliculi in the mechanisms of the suppression of convulsive activity and regarding their inclusion in the antiepileptic system.

Motor and emotional disturbances developing in rats receiving daily subthreshold injections of picrotoxin

MOTOR AND EMOTIONAL DISTURBANCES DEVELOPING IN RATS RECEIVING DAILY SUBTHRESHOLD INJECTIONS OF PICROTOXIN Go N. Kryzhanovskii,* A. A. Shandra, UDC 616.8-009.1+616.89-008.48]- R. Fo Makul'kin, L. S. Godlevskii~ 092.9-02:615.214.31 and A~ M. Mazarati KEY WORDS: picrotoxin kindling; seizure syndrome; akinetic syndrome; defensive be- havior syndrome~ pathological system~ pathological determinant Chronic administration of metrazol in a subconvulsive dose causes the development of pre- disposition to seizures, manifested as an increase in the intensity of responses to metrazol, ranging from individual twitches to generalized clonicotonic convulsions [3, 8, i0]. The phe- nomenon also is characterized by lowering of the seizure threshold relative to metrazol for a long period of time after its administration has ceased, i.e., it has the basic features of kindling and is defined as pharmacologic kindling [3, 8]. Besides the seizure changes, dur- ing kindling due to repeated injections of lidocaine in animals, stable behavioral changes arise [12]. Hence the importance of studying whether pharmacologic kindling can develop as a re- sult of administration of other epileptogens, differing in the mechanism of their convulsant action from metrazol and lidocaine, and also of studying changes arising under these circum- stances in behavior. ~ The work was devoted to a study of the development of epileptic activ- ity (EpA) in rats during chronic administration of subconvulsive doses of picrotoxin, whose epileptogenic effect is linked with its action on the GABA-ionophore complex [13], and also to a study of behavioral disorders arising under these conditions. EXPERIMENTAL METHOD Experiments were carried out on male Wistar rats weighing 170-250 g. Each group consisted of no fewer than i0 animals. Picrotoxin, in doses of 5, 2.5, 1.5, or 1.25 mg/kg was injected intraperitoneally in a volume of 0.5 ml of 0.9% NaCI solution daily under identical conditions (the same time of day, in a room with the same intensity of illumination and with the same intensity of illumination and with the same noise effect). Animals of the control group re- ceived injections of physiological saline under identical conditions. The animals were ob- served for 90 min after receiving the injection of picrotoxin. The intensity of the seizures was expressed in points [3]. The latent period of the first seizure responses and of general- ized convulsions also was determined. Defensive responses were evaluated in accordance with the character of the animals' behavior when attempts were made to handle them, and these also were expressed in points [12]. To investigate the electrical activity of the brain, rats were anesthetized with hexobarbital (i00 mg/kg) and monopolar constantan electrodes were inserted into the hippocampus, caudate nucleus, sensomotor cortex, and cortex of the cerebellar vermis~ stereotaxically taking coordinates from the atlas [ii]; the reference electrode was fixed in the nasal bones. The results were subjected to statistical analysis [7]. EXPERIMENTAL RESULTS Injection of picrotoxin (5 mg/kg) caused generalized convulsions or spasms of the fore- limbs (the intensity of the convulsions was 3.6 • 0.2 points). After injection of picrotoxin in a dose of 2.5 mg/kg clonic convulsions were observed in eight of the 12 animals (1.8 • 0.7 points), whereas in a dose of 1.5 mg/kg spasms were observed in four of the 12 rats (0.3 • 0.2 points). After injection of picrotoxin in a dose of 1.25 mg/kg no seizure responses were observed. This dose was chosen for chronic administration. *Academician of the Academy of Medical Sciences of the USSR. Laboratory of General Pathology of the Nervous System, Institute of General Pathology and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Department of Pathologi- cal Physiology, N. I. Pirogov Odessa Medical Institute. Translated from Byulleten' Eksperimental' no• Biologii i Meditsiny, Vol. 108, No. 7, pp. 18-21, July, 1989. Original article submitted Octo- ber I, 1988. 0007-4888/89/0007-0923512.50 9 1989 Plenum Publishing Corporation 923

Modulatory roles of GABAergic mechanisms in post-rotatory nystagmus in the rabbit

1. 1. To determine how the GABAergic mechanism operates in the generation of post-rotatory nystagmus, an experiment was performed with GABAergic drugs in rabbits. 2. 2. Subconvulsive doses of picrotoxin (0.3–0.6 mg/kg, i.v.) and bicuculline (0.1 mg/kg, i.v.) decreased the number of post-rotatory nystagmus beats, whereas strychnine sulphate, at a subconvulsive dose (0.1 mg/kg i.v.), increased it. 3. 3. Diazepam (1 mg/kg, i.v.) remarkably increased the number of post-rotatory nystagmus beats. Pretreatment with picrotoxin (0.45 mg/kg, i.v.), bicuculline (0.1 mg/kg, i.v.) or semicarbazide-HCl (180 mg/kg, i.v.) antagonized the effects of diazepam (1 mg/kg, i.v.). 4. 4. GABAergic mechanisms may play a modulatory role in the production of nystagmus rhythm. Strychnine-sensitive neurons involved in the vestibular mechanism may behave in a different manner from picrotoxin-sensitive neurons.

Proconflict effect of benzodiazepine receptor inverse agonists and other inhibitors of GABA function

GABA seems to be a neurotransmitter with great impact on conflict behaviour in rats. We studied the effects of different types of GABA function inhibitors on conflict behaviour in rats. Among these inhibitors, the benzodiazepine (BZ) receptor inverse agonists are a new type of compound downregulating GABA-mediated functions allosterically. The most effective proconflict inducing compounds were pentylenetetrazol and the three BZ inverse agonists β-CCM, β-CCE and ZK 90886. The BZ receptor inverse agonists, FG 7142, DMCM and CGS 8216, the GABA antagonist bicuculline and the GABA synthesis inhibitor isoniazid were moderately active. Only a weak effect was seen with just subconvulsive doses of picrotoxin, a chloride channel inhibitor. These results show that the mode of GABA function inhibition determines the degree to which proconflict action is elicited and that proconflict effects and proconvulsant or convulsant effects may be separated. Evidence is presented that proconflict action in rats is predictive of an anxiogenic action in man.

Interactions of two phenylquinolines with picrotoxin and benzodiazepines in vivo and in vitro

PK 8165 and PK 9084 are phenylquinolines with high affinity for the benzodiazepine binding site. Both phenylquinolines were proconsulvant in combination with subconvulsant doses of picrotoxin and pentylenetetrazole in mice. Both chlordiazepoxide (10 mg/kg) and the imidazodiazepine, RO 15-1788 (10 mg/kg) prevented these tonic-clonic convulsions. In vitro studies of rat cuneate nucleus indicated that the proconvulsant actions of PK 8165 and PK 9084 could be explained by their direct, albeit slightly different, interactions with the GABA-receptor complex. PK 8165 (100 μM) alone had no effect on responses to the GABA analogue muscimol, but enhanced the potency of picrotoxin as an antagonist of muscimol and reduced the potency of flurazepam as a potentiator of muscimol. PK 9084 (100 μM) alone caused a small antagonism of muscimol, but did not affect the potency of picrotoxin; flurazepam reversed the effect of PK 9084.

On the convulsant action of Ro 5-4864 and the existence of a micromolar benzodiazepine binding site in rat brain.

The benzodiazepine Ro 5-4864 (60 mg/kg) produced convulsions in mice that could be antagonised either by diazepam (2-4 mg/kg) or by Ro 15-1788 (10-20 mg/kg). In mice and rats subconvulsant doses of Ro 5-4864 were proconvulsant when combined with subconvulsant doses of picrotoxin or pentylenetetrazole. Ro 15-1788 antagonised the tonic convulsions triggered by the drug combinations when it was given at the same time as Ro 5-4864; this antagonism was not observed when the drugs were injected at different times. In contrast to a previous report, we could find no evidence that Ro 5-4864 antagonised seizures induced by electroshock. Using two different ligand-binding techniques, no evidence was seen for the existence in rat brain of the previously reported "micromolar" benzodiazepine receptor, a suggested site of action of Ro 5-4864.

Muscimol-induced myoclonic jerks in mice

Muscimol produced myoclonic jerks in mice. The manifestation of this response required a minimum dose of 2 mg/kg (i.p.). These jerks, involving the hind quarter of the animals, commenced after a latent period of 6–10 min, were more or less rhythmic, peaked between 27 and 45 min and slowly tapered off thereafter. The maximum responses seen after 2 and 3 mg/kg (i.p.) doses of muscimol were 48 and 76 jerks per min, respectively. It is not known whether the response to muscimol is spinal in origin or whether a metabolite is involved. This effect of muscimol was not blocked by subconvulsive doses of picrotoxin or bicuculline. Drug interaction studies showed that treatments which resulted in decreased activity of the central norepinephrine system, or which enhanced the activity of the central serotoninergic system, effectively blocked the response to muscimol. The present study has not only brought to light a new aspect of the central action of muscimol, but it also seems that this response may prove to be valuable as an animal model for the evaluation of antimyoclonic drugs. One interesting aspect of this response is that it is strain-specific.

Effect of GABA agonist and antagonists on cardiovascular and sympathetic responses in SHR and WKY rats.

Injection of muscimol (4 mg/kg, i.a.) to unanesthetized SHR rats reduced the arterial blood pressure and heart rate more than in WKY rats. These cardiovascular responses were accompanied by a moderate increase in plasma norepinephrine level and a marked increase in plasma epinephrine level, in the SHR rats. In the WKY rats, only a moderate increase in plasma epinephrine level was seen. Injection of subconvulsive doses of picrotoxin (2 mg/kg, i.a.) or bicuculline (1 mg/kg, i.a.) to SHR rats, elicited a higher and a more prolonged increase of the arterial blood pressure than in WKY rats. Plasma norepinephrine and epinephrine levels increased to the same extent in both the SHR and WKY rats following picrotoxin injection. These results suggest that the SHR rats differ from the WKY rats in the reactivity of GABAergic responses in cardiovascular centers which also modulate the peripheral sympathetic tone.

Morphine-induced catalepsy in mice. Modification by drugs acting on neurotransmitter systems

Morphine-induced catalepsy was demonstrated in mice under appropriate conditions. Dose-response curves were obtained within specific limits of dose. Central muscarinic blockade with atropine sulfate but not peripheral blockade with atropine methyl bromide enhanced the cataleptic response. Centrally acting physostigmine and peripherally acting neostigmine also enhanced the catalepsy. The results with neostigmine suggest a peripheral site of action with these drugs. Amantadine and d,1-dopa enhanced the catalepsy. These appeared similar to the atropine in that they caused increased rigidity and hyperreflexia. Aminooxyacetic acid enhanced while subconvulsive doses of picrotoxin and bicuculline had little effect. Five-hydroxytryptophan had no effect.