The receptor subtype(s1 mediating the enhancement of facial motoneuron excitability by serotonin (5-HT) was evaluated by means of single-cell recording in vivo (in the anesthetized rat) and in vitro in brain slices. In vivo, microiontophoretic application of the broad-spectrum 5-HT, agonist 5-carboxamidotryptamine (5-CT), the 5-HT,/ 5-HTlc agonist l-[2,5-dimethoxy-4-methylphenyll-2-aminopropane (DOM), but not the selective 5-HTlA agonist 8-OH-2[di-n-propylaminoltetralin (8-OH-DPAT), produced a 5-HT-like enhancement of facial motoneuron excitability. Intravenous administration of the 5-HT2/5-HTlc antagonists ritanserin and LY 53857 in vivo blocked the facilitatory effects of 5-HT and DOM, but not norepinephrine (NE). Similarly, in brain slices, bath application of ritanserin blocked the effects of 5-HT, DOM, and 5-CT, but not NE on facial motoneurons. Intracellular recordings showed that DOM induced a slow depolarization and an increase in evoked spikes, but these effects were of lesser magnitude and longer duration than those produced by 5-HT. Taken together, these results indicate a role for 5-HT2 and/or 5-HTIc but not 5-HTIA receptors in serotonergic enhancement of facial motoneuron excitability since 5-HTs effect was 1) at least partially mimicked by the selective 5-HT2/5-HTlc agonist DOM, 2) mimicked by the broad-spectrum 5-HT, agonist 5-CT but not the selective 5-HTlA agonist 8-OH-DPAT, and 3) blocked by the 5-HT2/ 5-HTIc antagonists ritanserin and LY 53857.
This study examined the response of facial motoneurons, recorded in vivo in the anesthetized rat and in vitro in the brain slice preparation, to ligands selective than one 5-H f receptor subtype.