Neurotransmission Of Visceral Information In The Parabrachial Nucleus Of The Rat

The parabrachial nucleus (PB) has been shown to be a major relay site for visceral afferents ascending through the ventrobasal thalamus and terminating in the insular cortex. The purpose of these investigations was to determine the neurotransmitter mechanisms in the PB involved in relaying visceral sensory information to the thalamus in the rat. Using electrophysiological techniques in the cloral hydrate anesthetised male Wistar rat, our results show that the visceral pathway from the PB to the thalamus appears to use two neuronal systems, one mediating the relay of specific visceral information via glutamatergic N-methyl D-aspartate (NMDA) receptors in the PB, and another set of receptors is able to significantly reduce the spontaneous activity of visceral thalamic neurons. In addition, we have provided evidence for the actions of the peptide neurotensin (NT) in enhancing the visceral signal and the actions of the peptides substance P (SP), calcitonin gene-related peptide (CGRP) and cholecystokinin (CCK) in reducing the visceral signal reaching the thalamus. Somatostatin (SOM) was shown to selectively affect the spontaneous activity of visceral thalamic neurons. The peptides NT, SP, CGRP and CCK were shown to modulate the visceral signal to the thalamus by working at a site prior to the adrenergic or glutamatergic receptor in the PB. SOM was shown to work on the same neurons with the adrenergic receptors. The final set of investigations demonstrated the modality specific changes in the immunohistochemical labeling of these peptides into the PB following visceral afferent activation. The results demonstrated that an increase in baroreceptor activity increased the staining of NT, but decreases the staining of CGRP and SP in the PB. The peptides CCK and SOM only changed during gastrointestinal activation, and an increase in this modality decreased the release of these two peptides into the PB. These investigations have provided evidence for peptide and neurotransmitter mechanisms in the PB, a major relay nucleus for ascending visceral information to the insular cortex