Diurnal variations of IR-Met-enkephalin in the brain of pentylenetetrazol-kindled rats

Pentylenetetrazol (PTZ) kindling was induced in male Wistar rats by daily i.p. injections of 40 mg/kg of the convulsant agent. Immunoreactive (IR)-Met-enkephalin was quantified in the amygdala, hippocampus and hypothalamus 17 days after the last stimulus, in groups of 6–7 rats, every 4 h, beginning at 08.00 h. IR-Met-enkephalin level displayed diurnal variations in brain regions of control animals. In the amygdala and the hippocampus the peptide peaked at 24.00 h and in the hypothalamus at 20.00 h; the troughs were at 08.00, 16.00 and 08.00 h, respectively. Diurnal variations were abolished in the amygdala and hypothalamus of kindled rats. In the amygdala the effect was characterized by an IR-Met-enkephalin increase at 04.00, 08.00 and 12.00 h; in the hypothalamus the peptide was enhanced at 08.00 and 12.00 h; in the hippocampus IR-Met-enkephalin increased at 12.00 h and showed a displacement of the peak during the dark phase. The results suggest that PTZ kindling in rats produces a long-lasting alteration on diurnal variations of IR-Met-enkephalin levels in limbic structures.