Newborn rats' first suckling experience: taste differentiation and suckling plasticity.

The present study examined responsiveness of newborn rats to a surrogate nipple providing fluids with basic tastes (sweet, sour, bitter and salty) and assessed the effects of this first gustatory experience on subsequent responding to the nipple itself (empty nipple) or the nipple providing water. Responsiveness (attachment to and ingestion from a surrogate nipple providing saccharin, saline, quinine or ammonium chloride) was compared with that toward a nipple providing water. Compared to water, saline and quinine significantly reduced attachment to and ingestion from the nipple, while saccharin and milk significantly increased attachment behavior. Ammonium chloride increased attachment but not ingestive behavior. Suckling experience with saline, quinine and ammonium chloride attenuated both attachment and ingestive behaviors when subjects were tested 1 h later with an empty surrogate nipple or a nipple providing water. Experience with saccharin and milk (but not water) increased both measures. The data suggest that in newborn rats, as early as a few hours after birth, mechanisms of gustatory detection have control over suckling behavior. Initial experience with the tastants available from the nipple in the first suckling episode may alter further responsiveness to the nipple itself, mediated perhaps by mechanisms of appetitive and aversive conditioning.