Injections of picrotoxin and bicuculline into the amygdaloid complex of the rat: An electroencephalographic, behavioural and morphological analysis

Abstract

Bicuculline methiodide (0.5–3 nmol) and picrotoxin (0.5–4 nmol) were injected uni- or bilaterally into the rat amygdala and the resulting behavioural, electroencephalographic and morphological alterations were studied. In rats treated unilaterally with lowest doses of either bicuculline or picrotoxin (0.5 and 1 nmol) increase in the locomotor activity, occasional myoclonus of the hindlimbs and wet dog shakes were observed. At doses of 2–3 nmol, both γ-aminobutyrate antagonists produced a sequence of repetitively occurring behavioural alterations including limbic gustatory automatisms, tremor and myoclonus of the forelimbs, head nodding and rearing, that developed over 15–30 min and built up progressively into the recurrent motor limbic seizures lasting for 1–6 h. In animals injected bilaterally with either bicuculline (0.5–3 nmol) or picrotoxin (0.5–3 nmol) motor limbic seizures rapidly developed into the status epilepticus lasting for several hours. Bicuculline and picrotoxin produced both ictal and interictal epileptiform activity in the electroencephalogram. A spectrum of electroencephalographic changes consisted of high voltage fast activity, slow and fast voltage spiking, paraoxysmal bursts and periods of postictal depression. The earliest electrographic alterations appeared in the amygdala and then rapidly spread to cortical areas. Electrographic seizures started 1–10 min after unilateral injections of large doses of bicuculline and pictrotoxin (2–4 nmol). Ictal periods lasted for 1–2 min, recurred every 5–10 min and were followed by periods of depression of the electrographic activity. Bilateral injections of large doses of both γ-aminobutyrate antagonists (2–3 nmol) resulted in the status epilepticus. Morphological examination of frontal forebrain sections with light microscopy revealed a widespread damage to the amygdala, olfactory cortex, substantia nigra, thalamus, hippocampus and neocortex. Pretreatment of animals with diazepam prevented the build-up of convulsive activity and brain damage produced by bicuculline or picrotoxin. Muscimol retarded the appearance and shortened the duration of convulsive activity, but did not alter the sequence and intensity of seizures. The results indicate that γ-aminobutyrate antagonists, bicuculline and picrotoxin when directly applied to the amygdala can elicit in rats motor limbic seizures, epileptic changes in the electroencephalogram indicative of repetitive limbic seizures, and status epilepticus accompanied by seizure-related brain damage. It is proposed that intraamygdaloid injections of either bicuculline or picrotoxin in rats provide a particularly useful model for studying mechanisms of temporal lobe epilepsy.