Abstract
Intrathecal (i.th.) administration of substance P (SP, 6.5nmol) at the Th 8–10 level in conscious rats increased blood pressure (carotid artery), heart rate and plasma catecholamine concentrations. The responses were antagonized by the intravenous (i.v.) but not i.th. pretreatment with the 5-HT2-receptor antagonists ketanserin and ritanserin and intrathecally administered serotonin (5-HT, 10 Μg). The pressor response and the increase in plasma noradrenaline concentrations were also antagonized by i.v. or i.th. pretreatment with the 5-HT1A-agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT). In contrast the pressor response to SP was facilitated by the 5-HT1A-antagonist 1-pindolol (i.v. or i.th). Pretreatment with SP (i.th) reduced the hypotensive response to i.v. 8-OH-DPAT. These results demonstrate functional interactions between SP and serotonergic mechanisms in the central system, but the precise location and nature were not elucidated.

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