Homolateral cerebrocortical changes in neuropeptide and receptor expression after minimal cortical infarction

A cortical infarct of 2 mm diameter was obtained in the parietal cortex after a craniotomy, disruption of the dura mater and topical application of 3 M KCl. It has been shown previously that the presence of a small cortical infarct induces an increase in immediate early gene messenger RNA expression followed by an increase in neuropeptide and glutamic acid decarboxylase messenger RNA expression. Glutamate, acting at N-methyl-D-aspartate receptors, is held responsible for these changes, since they are blocked by pretreatment with dizocilpine. In the present study, we have analysed the consequences of the dramatic changes in messenger RNA expression on the level of immediate early gene products c-fos and zif 268, and on that of neuropeptides by using immunohistochemistry. After just 1 h, an increase in c-fos- and zif 268-like immunoreactivity is observed in the entire cortical hemisphere homolateral to the infarct, and is no longer detected after 6 h. An increase in cholecystokinin octapeptide-, substance P-, neuropeptide Y- and somatostatin-like immunoreactivity is observed in the entire cortical hemisphere homolateral to the infarct after three days, and is no longer detected after 30 days. To investigate if these dramatic increases in neuropeptide immunoreactivities may have functional consequences, we studied the level of cholecystokinin receptors by autoradiographic binding using [125I]cholecystokinin-8S and in situ hybridization for the detection of cholecystokinin-b receptor messenger RNA. A decrease in cholecystokinin binding sites and cholecystokinin-b receptor messenger RNA is observed in the entire cortical hemisphere homolateral to the infarct after three days, and is no longer detected after nine days. This study shows that a topical stimulation has diffuse effects, reaching regions far from the site of the lesion, and some of them are still strongly present after nine days. The increase in neuropeptide messenger RNAs is followed by an increase in the protein products of these genes, which may modify the neurotransmission. As a corollary to this, a decrease in cholecystokinin binding sites occurs. This may have further consequences on signal transduction pathways. This decrease in cholecystokinin binding sites is associated with a decrease in the cholecystokinin-b receptor messenger RNA, and this is the first example of a decrease in messenger RNA levels in this experimental model.