Oesophageal premotor mechanisms in the rat

The functional organization and neurotransmitter mechanisms of oesophageal premotor neurons in the subnucleus centralis of the nucleus tractus solitarii (NTSc) were investigated in vivo in the anaesthetized rat and in vitro in a rat brainstem slice preparation. -- Results from neurophysiological and pharmacological investigations indicate that the striated muscle of the oesophagus shows segmentally organized reflex responses to local distension, i.e. the proximal portion produces monophasic peristalsis, whereas the distal portion generates rhythmic peristaltic motility. The afferent limbs of these loops synapse in the NTSc. Organotopically organized projections from the NTSc to the compact formation of the nucleus ambiguus (AMBc) represent the chief internuncial pathway connecting oesophageal reflex interneurons (or premotor neurons) with motoneurons of the efferent limb. -- Neurons in the NTSc receive both excitatory amino acid (EAA)ergic and cholinergic inputs. However, vagal afferents from the oesophagus employ an EAA rather than acetylcholine to convey excitatory input to ipsilateral NTSc premotor neurons. Cholinergic input to the NTSc arises from propriobulbar sources and serves to facilitate oesophageal stage coupling, to promote aboral propagation of peristalsis and to generate slow esophagomotor rhythms. -- Results from in vitro experiments reveal that activation of muscarinic cholinoceplors (mAChRs) in the NTSc region evokes rhythmic synaptic activity in single oesophageal motoneurons in the AMBc, demonstrating that the interneuronal network in the NTSc can generate rhythmic esophagomotor activity in the absence of phasic sensory input. -- When stimulated via N-methyl-D-aspartate (NMDA) receptors and/or mAChRs in the presence of tetrodotoxin (TTX), single neurons of the NTSc region generate two distinct rhythmic oscillations that mirror two types of in vivo peristaltic rhythm. These observations support the idea that the oesophageal premotor neurons possess conditional oscillator properties. -- In summary, the present investigations provide strong evidence that the premotor neuronal circuits in the NTSc function as esophagomotor pattern generators. Subject to regulation by EAAergic and cholinergic inputs, these premotor circuits generate segmentally and/or coordinatively organized esophagomotor patterns