Sensory afferent neurotransmission in caudal nucleus tractus solitarius--common denominators

The nucleus of the solitary tract (NTS) receives a wide range of sensory inputs including gustatory, gastrointestinal and cardiorespiratory which are loosely segregated viscerotopically to subnuclei. Our laboratory has focused on a dorsomedial area of caudal NTS (mNTS) which is critical for cardiovascular reflexes. Using a brainslice, we study primarily mNTS neurons mono-synaptically activated by solitary tract stimulation. mNTS neurons show varying degrees of delayed excitation, spike frequency adaptation and after hyperpolarizations. Sensory afferent transmission is mediated by glutamate acting at post-synaptic non-NMDA receptors. Glutamate release depends on at least four different presynaptic calcium channels with N-type predominating. This profile of presynaptic calcium channels in NTS is also present at the peripheral soma, but absent from the baroreceptor sensory endings. Many peptides are associated with these sensory neurons and several modulate glutamatergic transmission in mNTS. Angiotensin II facilitates excitatory responses to sensory afferent activation by a presynaptic mechanism. Caudal NTS appears to have a framework of synaptic and cellular mechanisms in common with other NTS areas and peptides may play a critical role modulating this framework.