Spinally-mediated behavioural responses evoked by intrathecal high-dose morphine: possible involvement of substance P in the mouse spinal cord

Intrathecal (i.t.) administration of morphine in the spinal subarachnoid space of mice produced a severe hindlimb scratching followed by biting and licking. The onset of the scratching behaviour was observed 60–70 s after i.t. injection of morphine (60 and 90 nmol), and had a duration of 3–4 min. The morphine-induced behaviour was increased additively by i.t. co-administration of substance P (SP). This characteristic behavioural response was inhibited dose-dependently by i.t. co-administration of the tachykinin NK-1 receptor antagonists, sendide and CP-96,345. Significant antagonistic effects of SP (1–7), a putative antagonist for NK-1 receptors and [ d-Phe 7, d-His 9jSP (6–11), a selective antagonist for SP receptors, were observed against the morphine-induced behaviour. Pretreatment with i.t. SP antiserum and i.t. capsaicin resulted in reduction of the response to morphine. I.t. administration of somatostatin (SOM) antiserum, cysteamine, a relatively selective depletor of SOM and cyclo-SOM, a SOM receptor antagonist, produced no inhibitory effect on the morphine-induced behaviour. These results demonstrate that a spinal system of neurones containing SP may be involved in elicitation of the behavioural episode following i.t. injection of morphine in mice.