The broad, long term objectives of this project are to discover how neurons in the brain encode information about taste stimuli. To further our understanding of taste processing, the present project is focused on temporal coding in the nucleus of the solitary tract (NTS) the first central relay in the rat taste system. In addition to receiving information about taste directly from the tongue, the NTS also receives input from a number of higher order brain structures. In the present proposal, we will investigate the descending influence of two of these structures, the gustatory cortex (GC) and the lateral hypothalamus (LH) on temporal coding in the NTS and on learned behavior. HYPOTHESES are a) centrifugal input from the GC to the NTS modulates the temporal characteristics of taste- evoked spike trains and that these changes will be expressed as changes in taste acuity and discrimination., and b) LH-NTS input regulates behavioral responses to taste, i.e. licking, and thus controls the events around which the temporal code is referenced. SPECIFIC AIMS: 1) Using selective optogenetic (OG) stimulation of the GC input to the NTS, test the hypothesis that this input will change the temporal characteristics of taste responses (including temporal pattern and firing rate) increasing the quantity of information conveyed about taste quality (Experiment 1.1) and enhancing discrimination of similar tastants (Experiment 1.2). We will use Go-no-Go (GnG) and conditioned taste aversion (CTA) paradigms (in separate groups of rats) as tests of taste discrimination. For all experiments, it is hypothesized that inhibition of GC-NTS input will have opposite effects. 2) Test the hypothesis that LH-NTS input modifies lick- related and anti-lick neural activity in rostral NTS neurons according to palatability (Experiment 2.1) and that that selective OG enhancement of LH-NTS terminals will modify the performance in GnG and CTA paradigms through a specific modification in the ability to control initiation and inhibition of lick bouts (Experiment 2.2). Both excitatory and inhibitory viruses will be used.