The role of the parabrachial nucleus in taste processing and feeding.

The parabrachial nucleus (PBN) was identified as a taste relay in rodents in 1971. Early recordings suggested that the PBN transmitted a faithful representation of taste activity from the nucleus of the solitary tract (NTS). However, its role assumed greater significance as its subnuclei were shown to deal with different aspects of taste, visceral sensations, hedonics, and conditioned aversions. The discovery of parallel projections from PBN to the thalamus and to ventral forebrain, and evidence that the former carried sensory information while the latter signaled hedonics, conferred on PBN a central role in guiding feeding. Thus, it was surprising to discover that the PBN is not a taste relay in primates. So arose a distinction between rodents, in which parallel processing of taste and hedonic information is the rule, and primates, where serial processing through the cortex precedes a hedonic assessment. Where does the integration of taste and hedonics occur, and how does this affect feeding? Neurons in both NTS and PBN of rodents are modified by changing physiological conditions. That altered activity parallels and perhaps directs the rodent's feeding behavior. Information from primate NTS implies no such modification. These interactions are reserved for orbitofrontal cortex and ventral forebrain. The implication is that in rodents, hindbrain alterations not only control the reflexes associated with taste, but also direct food selection through the PBN-ventral forebrain projections. In primates, the apparatus is in place for an independent cognitive analysis unaltered by physiological state, upon which a hedonic assessment is subsequently overlaid.