NEUROTRANSMITTER AND NEUROMODULATOR ACTIVITY IN THE GUSTATORY ZONE OFTHE NUCLEUS-TRACTUS-SOLITARIUS

The rostral nucleus of the solitary tract (rNST) is the first central relay in the gustatory pathway. While previous investigations have pro vided a wealth of information on the pattern of central terminations o f gustatory afferent fibers, the morphology of synaptic connections of rNST neurons and responses of second order neurons to taste stimuli a pplied 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 bioph ysical properties, neuropharmacology and synaptic responses of rNST ne urons. These experiments have revealed that rNST neurons respond to th e excitatory amino acid neurotransmitter glutamate, as well as the inh ibitory amino acid neurotransmitter gamma amino butyric acid (GABA). B y use of glutamate receptor agonists and antagonists we have shown tha t rNST neurons have AMPA/kainate and NMDA ionotropic glutamate recepto rs, as well as matabotropic glutamate receptors. In addition, rNST neu rons respond to both GABA(A) and GABA(B) receptor agonists. The nature of the transmission at the synapse between primary afferent fibers an d second order neurons in rNST has been examined by electrical stimula tion of the solitary tract to elicit post-synaptic potentials (PSP). T hree types of monosynaptic PSP result from stimulation of the solitary tract: excitatory post-synaptic potentials, inhibitory postsynaptic p otentials, and a complex mixture of excitatory and inhibitory potentia ls. These new discoveries provide details about synaptic transmission in rNST and thereby clarify the underlying mechanism by which gustator y information is processed.