NaCl preference increases during pregnancy and lactation: assessment using brief access tests.

Pregnancy and lactation are characterized by increases in NaCl intake, as determined by long-term consumption tests, which cannot examine the relative contribution of taste and postingestive factors to this phenomenon. Consequently, in this study, changes in NaCl preference during pregnancy and lactation were studied in nulliparous Long-Evans rats using a brief access test (lickometer). In Experiment 1, rats were maintained on a Na(+)-adequate diet (0.03% Na(+)), habituated to lickometer testing, and subsequently assessed during pregnancy and lactation with three 30-s exposures to each of seven taste solutions: 0.075 M sucrose (base), 0.089 M NaCl in base, 0.158 M NaCl in base, 0.281 M NaCl in base, 0.5 M NaCl in base, 0.158 M NaCl and 0.281 M NaCl. Results indicated higher lick rates to the 0.5 M NaCl in base, 0.158 M NaCl and 0.281 M NaCl solutions during late pregnancy and late lactation (Day 13 and beyond). In Experiment 2, a comparison of two diets differing in sodium content (0.03% vs. 0.3% Na(+)) determined that these changes in NaCl preference during pregnancy and lactation were unrelated to dietary sodium. Thus, the apparent increase in NaCl preference during pregnancy and lactation, independent of dietary sodium, suggests that this change in preference is not in response to physiological sodium need.

NaCl detection thresholds: comparison of Fischer 344 and Wistar rats.

Adult Fischer 344 (F344) rats fail to display any preference for NaCl solutions at concentrations typically preferred by other rat strains. To determine whether this behavior is due to a strain difference in NaCl detection threshold, a conditioned taste aversion (CTA) was first established to a suprathreshold concentration of NaCl (0.1 M). Then, a series of dilute NaCl solutions, ranging from 0.0 to 0.011 M NaCl, were presented to F344 (n = 16) and Wistar (n = 16) rats. The lowest concentration at which there was a reliable difference in the preference scores of conditioned and control rats was defined as the detection threshold. Results indicate that the detection threshold for NaCl lies between 0.001 and 0.002 M NaCl for both F344 and Wistar rats. The addition of the sodium channel blocker amiloride to the NaCl solutions raised the detection threshold 10-fold to 0.03-0.04 M NaCl for both strains of rats. These results suggest that the NaCl detection thresholds of F344 and Wistar rats are similar and that these strains do not differ in the degree to which amiloride raises this threshold.

Hormone replacement modifies cholecystokinin-induced changes in sucrose palatability in ovariectomized rats.

The taste reactivity test was used to examine the effect of CCK-octapeptide (CCK-8) on the palatability of a sucrose solution in ovariectomized rats either receiving hormonal replacement (estradiol and progesterone; OVX + HRT), or treated with vehicle only (OVX + VEH). Statistical analyses revealed that the OVX + HRT rats treated with CCK-8 exhibited a robust decrease in ingestive responses, and an increase in aversive responses and passive drips to the intraoral sucrose infusions, relative to treatment with the NaCl vehicle. In contrast, a weak effect of CCK-8 on ingestive responses, no significant effect on the frequency of aversive responses, and a reduced effect on passive drips was observed in the OVX + VEH rats. These results show that CCK-8 modifies sucrose palatability, and that this effect is modulated by gonadal hormone levels.

Taste reactivity responses in rats: influence of sex and the estrous cycle.

Gonadal hormones (e.g., estradiol) may regulate feeding by producing a shift in the taste or palatability of food items. This study examined the impact of endogenous gonadal hormones on palatability by investigating sex differences in taste responsivity, as well as the effect of the estrous cycle on taste responsivity, in a rodent model. In the taste reactivity test, male and female Long-Evans rats received a brief (1 min) intraoral infusion of one of three tastants: sucrose (0.3 M), quinine (0.0003 M), and a sucrose-quinine mixture (0.3 M sucrose and 0.0003 M quinine). Statistical analyses indicated that female rats tested during diestrus or proestrus produced significantly more ingestive responses than did male rats and fewer aversive responses than did both male rats and female rats tested during estrus or metestrus (P < 0.05). These results indicate a sex difference in taste responsivity in the rat that is modulated by the reproductive status of female rats. This finding implies a role of gonadal hormones in the regulation of taste responsivity in the rat.

Morphine-induced modification of quinine palatability: effects of multiple morphine-quinine trials.

Morphine pretreatment attenuates aversive taste reactions elicited by quinine solution when assessed by the taste reactivity test. To determine whether this effect changes across trials, rats were administered morphine (2 mg/kg, subcutaneously) 30 min before a 5-min intraoral infusion of quinine solution (0.05%) on each of eight trials. Neither tolerance nor sensitization developed to morphine-induced attenuation of quinine aversiveness; morphine suppressed quinine-elicited aversive reactions on each trial. In addition, when tested in the absence of morphine, rats displayed a reduced aversion to quinine, suggesting that quinine became conditionally less aversive following previous pairings with morphine.