Postnatal development of the parabrachial gustatory zone in rat: dendritic morphology and mitochondrial enzyme activity.

Previous studies have shown that behavioral and neurophysiological responses to tastes develop during rat's postnatal life. The present experiments evaluated morphological and metabolic development of neurons in the gustatory zone of the caudal parabrachial nucleus (PBNc) of rat. Histological reconstruction studies were conducted to establish coordinate systems for PBNc gustatory zones in developing rats. Reliability of coordinate systems were evaluated in separate experiments following infusions of horseradish peroxidase in the thalamic taste area. Morphological and Golgi impregnation studies were performed to characterize neuronal and dendritic architecture in PBNc gustatory zones defined by coordinates. Conventional histochemical studies were performed for the mitochondrial respiratory enzymes cytochrome C oxidase (CO; EC 1.9.3.1) succinate dehydrogenase (SDH; EC 1.3.99.1), and NADH-dehydrogenase (NADH-DH; EC 1.6.99.3). Results show that two somatic morphologies can be statistically characterized in PBNc gustatory zones: Multipolar somatic types and fusiform somatic types. Multipolar and fusiform neurons of neonatal and adult rats project axons to the thalamic taste area, and dendrites of these neurons grow extensively between approximately 16 days after birth to approximately 35 days after birth. Activity of CO, SDH, and NADH-DH increases in the PBNc gustatory zones during the period of dendritic growth, and continues to increase slightly to approximately 45 days. These results provide the first demonstration of postnatal morphological and metabolic developmental in a central gustatory relay. Postnatal development of gustatory system therefore appears similar to that reported for other sensory systems, to the extent that morphological and metabolic development accompanies the ontogeny of taste responses.