Opioid peptide systems following a subconvulsant dose of pentylenetetrazolin rats

The development of epilepsy and a progressive increase in susceptibility to seizures may involve changes in inhibitory and excitatory systems from the beginning of the process. The present study was focused to analyze the opi oid peptide changes induced by a chemical sub-convulsant stimulation. Exper iments were carried out to determine opioid peptide release, mu receptor bi nding and proenkephalin expression in rat brain, as well as nociceptive res ponses, following the administration of a sub-convulsant dose of pentylenet etrazol (PTZ) (30 mg/kg, i.p.). Membrane binding experiments revealed reduc ed number of mu binding sites (B-max) in cortex and amygdala, but not in st riatum and hippocampus, an effect that was evident only 24 h, but not 28 da ys, after PTZ treatment. In situ hybridization experiments suggested a sign ificant enhancement of proenkephalin mRNA expression in specific brain regi ons 24 h after PTZ treatment. Microdialysis combined with a universal opioi d peptide radioimmunoassay revealed extracellular opioid peptide levels to be elevated in the amygdala (137%) 90 min after PTZ administration. Evaluat ion of nociceptive responses using the Randall-Selitto test showed an analg esic effect short term (30-90 min) after PTZ injection. Collectively, these data provide evidence for a significant activation of opioid peptide syste ms as a consequence of the administration of a sub-convulsant dose of PTZ. These neurochemical changes may play an important role in the progression o f epileptogenesis. (C) 1999 Elsevier Science B.V. All rights reserved.