ELECTROPHYSIOLOGICAL EVIDENCE AGAINST A NEUROTRANSMITTER ROLE OF CORTICOTROPIN-RELEASING HORMONE (CRH) IN PRIMARY SOMATOSENSORY CORTEX

The possible neurotransmitter role of corticotropin-releasing hormone (CRH) was studied in the primary somatosensory cortex of the rat. Elec trical activity of single neurones was recorded in layers II-VI of cor tex, and in the region of the locus coeruleus. Iontophoresis and press ure ejection were employed to locally apply CRH, and changes in sponta neous, synaptically driven and iontophoretically driven firing were ex amined, In the cortex, of 62 neurones recorded most (51) were complete ly unaffected by high and prolonged current/pressure ejections of CRH. Depression of firing was occasionally seen (8 of 62), while a very fe w (3) were weakly excited. Of 25 cells studied with vibrissal stimulat ion to evoke excitatory synaptic responses, responses in two cells wer e depressed and in two they were enhanced. Activity that was evoked by iontophoretic ejection of excitatory amino acids, such as glutamate, was depressed in 6 of 40 cells (none were enhanced). Such effects as w ere seen were weak and often difficult to reproduce. The effect of CRH on depressions produced by GABA was also tested in four experiments. No effects on the amplitude or duration of the depressions were observ ed. In contrast recordings made in the midbrain, in the region of the locus coeruleus, resulted in over half the neurones (11 of 20) showing clear reproducible excitatory responses to CRH applications. Solution s used in the experiments were analysed using chromatography, radioimm unoassay and bioassay, and no significant degradation of the peptide w as found compared with the synthetic standard (CRH (1-41)). The data p rovide evidence against CRH acting as a neurotransmitter or modulator in primary cortex, suggesting that the CRH which is localized in certa in types of cortical cells is involved in other processes. (C) 1998 El sevier Science B.V. All rights reserved.