CENTRAL EFFECTS OF 5-HT ON RESPIRATORY AND HYPOGLOSSAL ACTIVITIES IN THE ADULT CAT

The activities of the diaphragmatic, internal intercostal and hypoglos sal-innervated muscles were studied in adult decerebrate cats in respo nse to 5-HT and related agents (8-OH-DPAT and DOI). The drugs were pla ced on the floor of the IVth ventricle. The mean respiratory frequency (Fi) increased (124-193% of the control value) within 3 min of the 5- HT application, and decreased thereafter (30-90%). The mean Ti and Te changed similarly, but opposite to Fi. With some delay, the hypoglossa l-innervated muscles were tonically activated or exhibited increased a ctivities. Methysergide pretreatment completely blocked the effect of 5-HT on all the respiratory parameters and the hypoglossal-innervated muscles activities. The responses to and OH-DPAT and DOI indicate that 5-HT modulates the respiratory frequency via activation of both 5-RT( 1A) and 5-HT2 receptors. Nevertheless, the effect of 5-HT on both the expiratory and hypoglossal-innervated muscles seems to depend on 5-HT2 , receptors activation only.