Neurotransmitter and neuromodulator activity in the gustatory zone of the nucleus tractus solitarius

The rostral nucleus of the solitary tract (rNST) is the first central relay in the gustatory pathway. While previous investigations have provided a wealth of information on the pattern of central terminations of gustatory afferent fibers, the morphology of synaptic connections of rNST neurons and responses of second order neurons to taste stimuli applied to the tongue, little is known regarding the neurophysiological characteristics of synaptic transmission in rNST. We have used an in vitro brain slice preparation of the rNST to study the intrinsic biophysical properties, neuropharmacology and synaptic responses of rNST neurons. These experiments have revealed that rNST neurons respond to the excitatory amino acid neurotransmitter glutamate, as well as the inhibitory amino acid neurotransmitter gamma amino butyric acid (GABA). By use of glutamate receptor agonists and antagonists we have shown that rNST neurons have AMPA/kainate and NMDA ionotropic glutamate receptors, as well as matabotropic glutamate receptors. In addition, rNST neurons respond to both GABA(A) and GABA(B) receptor agonists. The nature of the transmission at the synapse between primary afferent fibers and second order neurons in rNST has been examined by electrical stimulation of the solitary tract to elicit post-synaptic potentials (PSP). Three types of monosynaptic PSP result from stimulation of the solitary tract: excitatory post-synaptic potentials, inhibitory postsynaptic potentials, and a complex mixture of excitatory and inhibitory potentials. These new discoveries provide details about synaptic transmission in rNST and thereby clarify the underlying mechanism by which gustatory information is processed.