Spinal 5‐HT‐receptors and tonic modulation of transmission through a withdrawal reflex pathway in the decerebrated rabbit

1 In decerebrated, non‐spinalized rabbits, intrathecal administration of either of the selective 5‐HT 1A ‐receptor antagonists (S)WAY‐100135 or WAY‐100635 resulted in dose‐dependent enhancement of the reflex responses of gastrocnemius motoneurones evoked by electrical stimulation of all myelinated afferents of the sural nerve. The approximate ED 50 for WAY‐100635 was 0.9 nmol and that for (S)WAY‐100135 13 nmol. Intrathecal doses of the antagonists which caused maximal facilitation of reflexes in non‐spinalized rabbits had no effect in spinalized preparations. 2 In non‐spinalized animals, intravenous administration of (S)WAY‐100135 was significantly less effective in enhancing reflexes than when it was given by the intrathecal route. 3 When given intrathecally, the selective 5‐HT 2A/2c ‐receptor antagonist, ICI 170,809, produced a bell‐shaped dose‐effect curve, augmenting reflexes at low doses (≥ 44 nmol), but reducing them at higher doses (982 nmol). Idazoxan, the selective a 2 ‐adrenoceptor antagonist, was less effective in enhancing reflex responses when given intrathecally after ICI 170,809 compared to when it was given alone. Intravenous ICI 170,809 resulted only in enhancement of reflexes and the facilitatory effects of subsequent intrathecal administration of idazoxan were not compromised. 4 The selective 5‐HT 3 ‐receptor blocker ondansetron faciliated gastrocnemius medialis reflex responses in a dose‐related manner when given by either intrathecal or intravenous routes. This drug was slightly more potent when given i.v. and it did not alter the efficacy of subsequent intrathecal administration of idazoxan. 5 None of the antagonists had any consistent effects on arterial blood pressure or heart rate. 6 These data are consistent with the idea that, in the decerebrated rabbit, 5‐HT released from descending axons has multiple roles in controlling transmission through the sural‐gastrocnemius medialis reflex pathway. Thus, it appears 5‐HT tonically inhibits transmission between sural nerve afferents and gastrocnemius motoneurones by an action at spinal 5‐HT 1A ‐receptors. Spinal 5‐HT 2A/2C ‐receptors may mediate a weak inhibition of transmission in the spinal cord, but more convincing evidence was obtained for their involvement in descending facilitatory tone. Further, some of the facilitatory consequences of spinal α 2 ‐adrenoceptor blockade may be mediated through 5‐HT 2 type receptors. Spinal 5‐HT 3 receptors do not appear to have a major role in tonic modulation of the sural‐gastrocnemius medialis reflex.