Age-related decrease of the chorda tympani nerve terminal field in the nucleus of the solitary tract is prevented by dietary sodium restriction during development.

Institution of a low-NaCl diet beginning at embryonic day 3 and continued throughout pre- and postnatal development has widespread effects on the neuroanatomical organization of the first gustatory relay in the nucleus of the solitary tract. To determine when these effects are expressed postnatally, the terminal field of the chorda tympani nerve was compared between sodium-restricted and sodium-replete rats at postnatal days 15-17, postnatal days 25-27, postnatal days 35-37, and adults. Total terminal fields were significantly larger in postnatal days 35-37 and adult sodium-restricted rats compared with aged-matched controls. The group-related differences appear related more to a remodeling of the terminal field in the dorsal zone of the terminal field in controls. Specifically, the terminal field volume in the dorsal zone in controls decreased dramatically from postnatal days 25-27 to postnatal days 35-37 and then again from postnatal days 35-37 to adulthood. In contrast, the fields did not change during development in sodium-restricted rats. These findings suggest that remodeling of the chorda tympani field occurs in controls at about the developmental period of taste response maturation. The lack of remodeling in sodium-restricted rats may be explained by a corresponding lack of functional response development to sodium salts. These results also illustrate the specificity and extent of how early dietary manipulations shape the developing brainstem.

Behavioral taste responses of developmentally NaCl-restricted rats to various concentrations of NaCl.

The behavioral taste responses of developmentally NaCl-restricted rats were examined with a brief-exposure taste test. Neurophysiological and morphological alterations have been reported in rats whose dietary sodium is restricted during pre- and postnatal development, yet there exists little data discerning their behavioral response to tastants. Control and developmentally NaCl-restricted rats were maintained on a low-NaCl diet and trained to lick from individually presented sipper tubes. Each subject received 4 days of testing on various NaCl concentrations. Results indicate that developmentally NaCl-restricted rats have dramatically increased lick rates to NaCl solutions. These responses are likely due to some combination of factors including (a) numbers and type of active chorda tympani fibers, (b) compensatory responses to NaCl-solutions from other nerves of the oral cavity, and (c) increased sensitivity of central taste systems to NaCl.

Mammalian bombesin-like peptides extend the intermeal interval in freely feeding rats.

We evaluated the effects of systemic delivery of amphibian bombesin and its mammalian homologues on the length of the postprandial intermeal interval. Adult male rats, feeding ad libitum, were injected intraperitoneally (i.p.) 5 min after the end of the first nocturnal meal with 0 (vehicle), 2.5, 5, or 10 nM/kg of tetradecapeptide bombesin (BN), gastrin-releasing peptide(1-27) (GRP1-27), the C-terminal decapeptide of GRP(18-27) (GRP18-27), the C-terminal decapeptide of neuromedin B (NMB23-32), or combinations of equimolar doses of GRP1-27 and NMB23-32. BN produced a potent, dose-related extension (maximum of 177%) of the first postprandial intermeal interval; GRP1-27 produced a lesser but significant prolongation (maximum of 47%); the combination of GRP1-27 and NMB23-32 produced an intermediate prolongation (maximum of 70%); GRP18-27 alone and NMB23-32 alone failed to produce any significant change. Peptide effects were limited to the first postprandial intermeal interval. The results demonstrate that systemic, postprandial injection of BN, GRP1-27, or the combination of GRP1-27 and NMB23-32 extends the duration of the postprandial intermeal interval. The results suggest that the endogenous peptides, released in the gastrointestinal tract by ingested food, have a potent satiety action, selectively lengthening the intermeal interval.

Changes in taste threshold over the life span of the Sprague-Dawley rat.

Taste thresholds for sucrose, NaCl, QHCl and citric acid were examined over the lifetime of seven rats. Significant yet subtle decreases in taste sensitivity were observed in the oldest subjects only.