The parabrachial nucleus in the golden hamster: Anatomical organization, location and properties of taste-evoked activity, and connections

The ascending central gustatory system has two brainstem synapses in rodents and lagomorphs: the nucleus of the solitary tract (NST) in the medulla and the parabrachial nucleus (PBN) in the pons. However, some ascending taste information bypasses the PBN and projects directly to the thalamus in primates. Gustation in the golden Syrian hamster (Mesocricetus auratus) has been well studied behaviorally and physiologically, but little is known about the organization of its central nervous system. In the present series of studies the hamster PBN was examined cytoarchitecturally, physiologically, and hodologically. Comparison of several mammalian central gustatory systems will allow a better understanding of functional anatomy. As in rat, multiple anatomically distinct subdivisions are evident within the PBN in Nissl-stained material. Multi-unit neural activity evoked by taste stimuli applied to the anterior tongue is localized to two of the subdivisions, the central medial (CM) and ventral lateral (VL). This mapped taste-evoked activity within the PBN in relation to its anatomical organization for the first time. Previous studies have implicated the medial PBN in subserving a gustatory function and the lateral PBN in subserving a visceral function. The VL subdivision may be a substrate for taste and visceral interactions. Response properties of PBN neurons are site specific within CM, with sucrose-sensitive sites, which respond best to ternary mixtures, located more rostrally than NaCl-sensitive sites. Also, NaCl-sensitive sites are more common in the PBN than NST. Both PBN CM and VL efferents project to the gustatory thalamus bilaterally. Thus, PBN efferents are the key to taste's bilateral thalamo-cortical sensory representation. Efferents from VL also project to the ipsilateral central nucleus of the amygdala, an area implicated in subserving emotional and mnemonic behaviors. Thus, the functional organization of the hamster central gustatory system suggests the significance of two separate taste-responsive areas within the PBN and confirms differences in laterality of two distinct mammalian central taste-related pathways.