Autonomic efferents affect intake of imbalanced amino acid diets by rats

An anorectic response occurs following ingestion of imbalanced amino acid (IMB) diets. There are three phases o this response: 1, recognition of the IMB diet; 2, conditioned development of an aversion to the IMB diet; and 3, adaptation. Blockade of peripheral serotonin-3 (5-HT3) receptors or vagotomy attenuates Phase 2 of the anorectic response. We investigated whether sympathetic efferents interact with the ventral gastric branch (VGB), by cutting it (X), or with the 5-HT3 receptor in these responses. First, VGBX and sham-operated (SHAM) groups were injected with vehicle or phenoxybenzamine (α-blocker), or nadolol (β-blocker) before introducing the IMB diet. At 3 h suppression of the IMB diet ingestion was unchanged, showing no sympathetic efferent effect on Phase 1. Intake of the IMB diet increased 12-24 h later only in the SHAM + phenoxybenzamine group, so the VGB was necessary for α-blockade to enhance IMB diet intake during Phase 2 or possibly Phase 3. On days 2-5, intakes by the SHAM + phenoxybenzamine, VGBX + phenoxybenzamine and VGBX + nadolol groups were elevated. Therefore, α-blockade enhanced adaptation alone, but VGBX was necessary for β-receptor blockade to augment Phase 3 adaptation. Both sympathetic efferents and the VGB are involved in Phases 2-3. Second, rats received vehicle or nadolol or scopolamine (nonselective muscarinic blocker) or pirenzepine (muscarinic M-1 receptor blocker), w+/− tropisetron (5-HT3 blocker). Pirenzepine attenuated the tropisetron effect between 6-9 h, but then pirenzepine and nadolol enhanced the tropisetron effect between 9-12 h. Scopolamine attenuated the tropisetron effect between 9-12 h. While neither experiment showed effects during the recognition phase, the autonomic and serotonergic systems interact in the learned and adaptive responses to IMB diets.