Project II is designed to identify pathways and targets in forebrain through which visceral signals received in the nucleus tractus solitarius (NTS) act to influence electrocortical activity, mood, and behavior. Specifically, the hypothesis will be tested that visceral stimuli research the cerebral cortex, particularly prefrontal regions, by projections from NTS to the midline- intralaminar thalamic complex (MITC). Study I is designed to address whether visceral afferents reach the MITC, disynaptically, via the NTS. Evidence will be sought using axonal tracers to, first define the visceral thalamus and to determine whether visceral-thalamic circuit neurons (l) are organized viscerotopially in NTS, and (ii) are characterized by neurotransmitter specificity, and (iii) receive synaptic input from primary visceral afferents, as established by electron microscopy (EM). Study II uses a comparable strategy to determine whether the NTS projects to the MITC, indirectly, by way of neurons in the rostral ventrolateral medulla (n.RVL). Evidence will be sought to determine whether projections from n.RVL to thalamus are catecholaminergic and arise from subsets of C1 neurons. Study III investigates whether neurons in NTS and n.RVL relay by way of MITC to viscerolimbic areas of prefrontal and cingulate cortex, hippocampus, or striatum. Experiments will evaluate (l) if targets within MITC relate topographically to afferent sources, particularly the NTS and n.RVL; and (ii) by EM, whether (a) the NTS terminates on thalamic neurons innervating the medial prefrontal cortex and (b) adrenergic neurons synapse in the visceral thalamus. In Study IV, a structural basis will be sought for parallel processing of visceral afferents by investigating whether visceral-thalamic circuit neurons branch collaterals to other major areas of relevance in emotional and autonomic processing. Dual- retrograde transport methods are used to determine whether the visceral thalamus is innervated by axon collaterals of neurons terminating in cerebral cortex, the amygdala and nucleus accumbens, or other sites in thalamus. Another question to be addressed is whether thalamic afferents issues collaterals to nuclei subserving autonomic/endocrine reflex regulation, the n.RVL, thoracic intermediolateral cell column or the paraventricular hypothalamic nucleus.
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