1 Monosynaptic (MSR) and polysynaptic (PSR) segmental reflex responses were recorded from a ventral root of the neonate rat hemisected spinal cord. Amplitudes of the two components were monitored with a peak height detector. 2 5-Hydroxytryptamine (5-HT) depressed the MSR and PSR in a concentration-dependent manner. The IC50 for MSR depression was 9.5 + 3.2pM and for PSR depression was 9.0 + 4.8pm.3 Blockade of neuronal uptake of 5-HT by citalopram (0.1 M) greatly increased sensitivity to 5-HT. In the presence of citalopram, the IC50 for MSR depression was 30 + 18 nM and for PSR depression was 89 + 23 nM.4 5-HT did not depress the MSR or the PSR by releasing glycine since strychnine (1 pM) did not prevent these actions of 5-HT. Exogenous 5-HT may act at a mixture of 5-HT receptor subtypes to depress the MSR.

No abstract

1 The monosynaptic reflex (MSR), recorded in vitro from the neonatal rat spinal cord, was depressed by 5-hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), methysergide and R( + )-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), and also by the selective 5-HTlD agonists, sumatriptan and N-methyl-3-(1-methyl-1-piperidinyl)-1H-indole-5-ethane sulphonamide (GR 85548).2 Ketanserin (1 gM) and methiothepin (1 gM) reduced the duration of depressions elicited by 5-CT, but not those produced by 5-HT, sumatriptan, GR 85548, methysergide or 8-OH-DPAT.3 The IC50 for MSR depression by 5-CT was 3.6, 2.1 -6.2nM (n=4), by sumatriptan was 15.2, 12.9-18.0nM (n=32), by GR 85548 was 18.4, 11.7-29.1 nM (n =12), by methysergide was 29.8, 10.2-87.1nM (n=4) and by 8-OH-DPAT was 0.21, 0.11-0.43 MM (n=3) (geometric means and 95% confidence limits). 4 Ketanserin (0.1 or 1 UM) antagonized competitively responses to sumatriptan (apparent pA2 7.8 ± 0.1, n = 5), GR 85548 (apparent pA2 7.6, unpaired data, n = 5), methysergide (apparent pA2 7.9 ± 0.12, n=4) and 8-OH-DPAT (apparent pA2 8.3 ± 0.1, n= 3). Concentration-response curves to 5-CT showed a smaller, parallel shift to the right (apparent pA2 6.8 ± 0.1, n = 4), but responses to 5-HT were unaffected by ketanserin (1 gM) (n = 4). Neither with these concentrations nor with concentrations in the range 1-3 nm was there any unequivocal blockade of responses to sumatriptan. 9 It is concluded that sumatriptan, GR 85548, methysergide and 8-OH-DPAT depress the MSR in the neonate rat spinal cord via ketanserin-sensitive receptors, which have some similarities to 5-HTLIT1 receptors but which are not blocked by GR 127935. 5-HT released by tryptaminergic pathways may act via the same receptors to depress the MSR. 5-HT applied to the cord probably acts via a different, possibly novel 5-HT receptor to depress the MSR.