Role of PBN Subnuclei in Oromotor Behaviors

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Taste information travels from the mouth to the brain within nerves that terminate in a part of the medulla oblongata called the nucleus of the solitary tract (NST). The NST is interconnected with another part of the brainstem called the parabrachial nucleus (PBN). Both the PBN and NST act as processing and distribution centers for taste information. They send some taste information to higher brain centers so that the taste quality can be perceived. The PBN and NST also send outputs to motor centers in the brainstem that produce motor responses to the taste input. Within the PBN, there are two distinct regions that respond to taste input. One of these regions (CM/VL) responds best to salt on the anterior tongue while the other (EM/EL) responds best to bitter taste on the posterior tongue. Due to this difference in responses to taste input, it is hypothesized that the two PBN areas control different motor responses. Specifically, it is hypothesized that CM/VL controls ingestive behaviors (like mouth movements and tongue protrusions) while EM/EL controls aversive behaviors (like gapes). Undergraduate Biology and Psychology students and their faculty mentors, will address these hypotheses by stimulating discrete areas of the PBN while observing motor responses. This procedure will allow the construction of a functional map of the PBN. Specifically, CM/VL and EM/EL will be stimulated in separate groups of conscious rats using implanted electrode-cannula systems. First, these areas will be electrically stimulated while oral motor behaviors are videotaped. Following electrical stimulation, injection of glutamate (an excitatory neurotransmitter) through the cannula into the same location within the PBN will occur. The combination of electrical and chemical stimulation will allow accurate assessment of the specific neural elements responsible for the behavioral effects. The motor responses will be correlated with the histological localization of the stimulation sites. As mentioned, based on previous findings, it is hypothesized that stimulation of CM/VL will generate ingestive behaviors, while stimulation of EM/EL will evoke aversive responses.This study employs a straightforward strategy to define the effects of taste-responsive PBN regions on oral motor behaviors. The potential impact of the project on the understanding of central processing of taste input and the function of the PBN is substantial. Specifically, elucidation of a functional topography (map) within the PBN would significantly improve the understanding of the central mechanisms producing oral motor responses to taste input.