EFFECTS OF OROCHEMICAL STIMULATION ON POSTNATAL-DEVELOPMENT OF GUSTATORY RECIPIENT ZONES WITHIN THE NUCLEUS OF THE SOLITARY TRACT

Previous receptor damage studies and artificial rearing (AR) studies i n rat have demonstrated that orochemical stimulation between the postn atal ages of P2 and 814 is necessary for development of primary gustat ory axons and terminal endings in the rostral nucleus of the solitary tract (NST). Objectives of the present experiment were to evaluate the qualitative nature of orochemical stimulation and amount of orochemic al stimulation that is necessary to produce normal axonal and terminal development in gustatory recipient zones of the rostral and intermedi ate NST. To this end, ultrapure water, 30, 150, and 500 mM NaCl, 80 an d 340 mM lactose, whole rat milk, and rat milk that had been subject t o extensive dialysis (12-14 kD MWCO) was delivered to independent grou ps of rat pups during AR procedures. Unstimulated AR animals and match ed mother-reared (MR) animals were used as controls. AR animals receiv ed experimental orochemical stimulation between the ages of P4 and P10 , and were thereafter returned to lactating dams until the time of wea ning; MR animals received experimental orochemical stimulation during the course of normal nursing. Following maturation, anterograde fluore scent dual-labeling experiments were conducted to map the course and d istribution of primary gustatory axons within the NST. Results show th at experimental stimulation with water during AR procedures is not suf ficient to produce normal development of primary gustatory axons and t erminal endings in the gustatory NST. Stimulation with 30, 150, and 50 0 mM NaCl produced normal axonal development in the NST, as did 80 and 340 mM lactose, whole rat milk, and dialyzed rat milk. These results indicate that orochemical stimulation with qualitatively different tas tants during critical periods of postnatal development is sufficient t o produce normal development of primary gustatory axons within the ros tral NST. In a second experiment, rat pups received either 1 day or 3 days of stimulation with. 150 mM NaCl to determine the time-course of stimulation necessary to produce normal development of terminal fields in the rostral NST. Results indicated that 1 day of stimulation with NaCl was not sufficient to produce normal development of rostral NST a xons, whereas 3 days stimulation with NaCl was effective in producing normal axonal development. Therefore, like other sensory systems, the gustatory system is susceptible to anatomical modification by taste st imulation during critical or sensitive periods of postnatal developmen t.