The 5-HT1-like receptors mediating inhibition of sympathetic vasopressor outflow in the pithed rat: operational correlation with the 5-HT1A, 5-HT1B and 5-HT1D subtypes

1. It has been suggested that the inhibition of sympathetically-induced vasopressor responses produced by 5-hydroxytryptamine (5-HT) in pithed rats is mediated by 5-HT₁-like receptors. The present study has re-analysed this suggestion with regard to the classification schemes recently proposed by the NC-IUPHAR subcommittee on 5-HT receptors.
2. Intravenous (i.v.) continuous infusions of 5-HT and the 5-HT₁ receptor agonists, 8-OH-DPAT (5-HT_1A), indorenate (5-HT_1A), CP 93,129 (5-HT_1B) and sumatriptan (5-HT_1B/1D), resulted in a dose-dependent inhibition of sympathetically-induced vasopressor responses.
3. The sympatho-inhibitory responses induced by 5-HT, 8-OH-DPAT, indorenate, CP 93,129 or sumatriptan were analysed before and after i.v. treatment with blocking doses of the putative 5-HT receptor antagonists, WAY 100635 (5-HT_1A), cyanopindolol (5-HT_1A/1B) or GR 127935 (5-HT_1B/1D). Thus, after WAY 100635, the responses to 5-HT and indorenate, but not to 8-OH-DPAT, CP 93,129 and sumatriptan, were blocked. After cyanopindolol, the responses to 5-HT, indorenate and CP 93,129 were abolished, whilst those to 8-OH-DPAT and sumatriptan (except at the lowest frequency of stimulation) remained unaltered. In contrast, after GR 127935, the responses to 5-HT, CP 93,129 and sumatriptan, but not to 8-OH-DPAT and indorenate, were abolished.
4. In additional experiments, the inhibition induced by 5-HT was not modified after 5-HT₇ receptor blocking doses of mesulergine.
5. The above results suggest that the 5-HT₁-like receptors, which inhibit the sympathetic vasopressor outflow in pithed rats, display the pharmacological profile of the 5-HT_1A, 5-HT_1B and 5-HT_1D, but not that of 5-HT₇, receptors.