A novel 5-HT receptor or a combination of 5-HT receptor subtypes may mediate depression of a spinal monosynaptic reflex in vitro

The monosynaptic reflex (MSR), recorded in vitro from the neonatal rat spinal cord, was depressed by 5-hydroxytryptamine (5-HT) and 5-HT receptor agonists. The results, together with our previous findings, indicate an apparent rank order of potency: 5-carboxamidotryptamine (5-CT) > sumatriptan > methysergide > 5-HT > 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) > 5-methoxytryptamine (5-MeOT) = 1-[3-(trifluoromethyl)phenyl]-piperazine (TFMPP) > α-methyl-5-hydroxytryptamine (α-Me 5-HT) > (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). The equipotent molar ratios were 5-CT 0.12, sumatriptan 0.4, methysergide 0.72, 5-HT 1.0, 8-OH-DPAT 13.3, 5-MeOT 26.7, TFMPP 31, α-Me 5-HT 402 and DOI > 3333. Time for peak depression from start of agonist application was 3–4 min for 5-HT, 5 min for sumatriptan and 5-MeOT, 5–7 min for α-Me 5-HT and 12 min for 8-OH-DPAT. The half-time for recovery from peak depression was 1.5 ± 0.3 min for 5-HT, 2.8 ± 0.3 min for 5-MeOT, 5.3 ± 1.5 min for sumatriptan, 13 ± 2.9 min for 8-OH-DPAT and > 30 min for a-Me 5-HT. 8-OH-DPAT induced depression of the reflex (IC 50 0.85, 0.7–1.0 μM, geometric mean and 95% confidence limits) was blocked by spiperone (1 μM, apparent pA 2 6.3) suggesting mediation via 5-HT 1A receptors. Sumatriptan induced depression of the reflex (IC 50 12.1, 7.9–18.4 nM) was blocked by methiothepin (1 μM, apparent pA 2 7.4) consistent with mediation via 5-HT 1B receptors. 5-MeOT depression (IC 50 0.8, 0.4–1.5 μM) was blocked by ketanserin (1 μM, apparent pA 2 7.0) consistent with mediation by 5-HT 2C receptors. α-Me 5-HT was not a potent agonist and the depression (IC 50 12.07, 9.4–15.5 μM) was not blocked by ketanserin (1 μM). DOI at concentrations up to 100 μM caused only a 12% depression of the MSR. It was concluded that the MSR can be depressed by activation of 5-HT 1A, 5-HT 1B or sub-synaptic 5-HT 2 (possibly 5-HT 2C) receptors. 5-HT 1A and 5-HT 1B receptors may be extra-synaptic. Blockade of 5-HT receptor subtypes with a mixture of ketanserin (1 μM), methiothepin (1 μM), spiperone (1 μM) and ondansetron (1 μM) did not displace the 5-HT concentration-depression curve. Since a major contribution by 5-HT 3 and 5-HT 4 receptors has been ruled out, 5-HT may act via a novel 5-HT receptor to depress the MSR.