In vivo modulation of vagal‐identified dorsal medullary neurones by activation of different 5‐Hydroxytryptamine 2 receptors in rats

In in vivo experiments, DOI (a 5‐HT 2 receptor agonist), MK‐212 (a 5‐HT 2C receptor agonist), and BW‐723C86 (a 5‐HT 2B receptor agonist) were applied by ionophoresis to neurones in the rat nucleus tractus solitarius (NTS) receiving vagal afferent input. The majority of the putative ‘monosynaptically’ vagal activated cells were inhibited by both MK‐212 (4/6) and DOI (2/4), but unaffected by BW‐723C86 (12/14). In contrast, ‘polysynaptically’ activated NTS cells were excited by both BW‐723C86 (13/19) and DOI (9/10). Inactive ‘intermediate’ cells were inhibited by BW‐723C86 (9/12), MK‐212 (5/6) and DOI (3/4), whilst active cells of this group were excited by BW‐723C86 (7/13) and DOI (5/5). The selective 5‐HT 2B receptor antagonist LY‐202715 significantly reduced the excitatory actions of BW‐723C86 on ‘intermediate’ and ‘polysynaptic’ cells (13/13), but not the inhibitory effects observed on inactive Group 2 cells ( n =5) whereas the selective 5‐HT 2C receptor antagonist RS‐102221 reversed the inhibitory effects of MK‐212 and DOI on ‘monosynaptic and ‘intermediate’ neurones. Cardio‐pulmonary afferent stimulation inhibited two of four putative ‘monosynaptically’ activated calls and all four inactive intermediate cells. These were also inhibited by DOI and MK‐212. In contrast, cardio‐pulmonary afferents excited all five active intermediate cells and all six putative ‘polysynaptically’ activated NTS cells, while all were also previously excited by BW‐723C86 and/or DOI. In conclusion, these data demonstrate that neurones in the NTS are affected differently by 5‐HT 2 receptor ligands, in regard of their vagal postsynaptic location, the type of cardio‐pulmonary afferent they receive and the different 5‐HT 2 receptors activated.British Journal of Pharmacology (2000) 131 , 1445–1453; doi: 10.1038/sj.bjp.0703722