Evolution of gustatory reflex systems in the brainstems of fishes

The great number of species of teleosts permits highly specialized forms to evolve to occupy particular niches.This diversity allows for extreme variations in brain structure according to particular sensory or motor adaptations.In the case of the taste system,goldfish(Carassius auratus L.,1758)and some carps have evolved a specialized in-traoral food-sorting apparatus along with corresponding specializations of gustatory centers in the brainstem.A comparison of circuitry within the complex vagal lobe of goldfish,and of the simpler gustatory lobes in catfish(Ictalurus punctatus Rafinesque,1818)shows numerous similarities in organization and neurotransmitters.Double labeling studies using horseradish peroxidase and biotinylated dextran amine in catfish shows a direct projection from the vagal lobe to the motoneurons of nucleus ambiguus which innervate oropharyngeal musculature.Therefore,a three neuron reflex arc connects gustatory input to motor output.In the vagal lobe of goldfish,a similar three neuron arc can be identified:from primary gustatory afferent,to vagal lobe interneuron,thence to dendrites of the vagal motoneurons that innervate the pharyngeal muscles.Therefore,despite large differences in the gross appearance of the vagal gustatory systems in the brains of catfish and goldfish,the essential connectivity and circuitry is similar.This suggests that evolutionary change in the central nervous system largely proceeds by rearrangement and elaboration of existing systems,rather than by addition of new structures or circuits.

c-Fos expression in rat brainstem following intake of sucrose or saccharin

To examine whether the activation of brainstem neurons during intake of a sweet tastant is due to orosensory signals or post-ingestive factors,we compared the distribution of c-Fos-like immunoreactivity(c-FLI)in the nucleus of the solitary tract(NST)and parabrachial nucleus(PBN)of brainstem following ingestion of 0.25 M sucrose or 0.005 M saccharin solutions.Immunopositive neurons were localized mainly in the middle zone of the PBN and four rostral-caudal subregions of the NST.Intake of sucrose increased the number of FLI neurons in almost every subnucleus of the PBN(F_((2,13))=7.610,P=0.023),in addition to the caudal NST at the level of the area postrema(F_((2,13))=10.777,P=0.003)and the NST intermediate zone(F_((2,13))=7.193,P=0.014).No significant increase in the number of c-Fos positive neurons was detected in response to saccharin ingestion,although there was a trend towards a modest increase in a few select NST and PBN nuclei.These results suggest that the PBN and NST may be involved in sweet taste perception and modulation of sweet tastant intake,but the significantly enhanced intensity of Fos expression induced by sucrose indicates that PBN/NST neuronal activity is driven by the integrated effects of sweet taste sensation and post-ingestive signals.