The development of aversive responses to quinine in hyperthyroid rats

Thyroid hormones and learning-associated neuroplasticity

Thyroid hormones (THs) are crucial for brain development and maturation in all vertebrates. Especially during pre- and perinatal development, disruption of TH signaling leads to a multitude of neurological deficits. Many animal models provided insight in the role of THs in brain development, but specific data on how they affect the brain's ability to learn and adapt depending on environmental stimuli are rather limited. In this review, we focus on a number of learning processes like spatial learning, fear conditioning, vocal learning and imprinting behavior and on how abnormal TH signaling during development shapes subsequent performance. It is clear from multiple studies that TH deprivation leads to defects in learning on all fronts, and interestingly, changes in local expression of the TH activator deiodinase type 2 seem to have an important role. Taking into account that THs are regulated in a very space-specific manner, there is thus increasing pressure to investigate more local TH regulators as potential factors involved in neuroplasticity. As these learning processes are also important for proper adult human functioning, further elucidating the role of THs in developmental neuroplasticity in various animal models is an important field for advancing both fundamental and applied knowledge on human brain function.

Abnormal taste preference for saccharin in hypothyroid rats

Taste preferences for saccharin in concentrations ranging from 0.16 mM to 50 mM were determined in rats made hypothyroid with radioactive iodine and in their littermate controls. Hypothyroid rats demonstrated taste preferences for saccharin which were similar to those of controls only at very low (0.016 mM) or very high (49.0 mM) saccharin concentrations. At these concentrations of tastant, the preferences for tastant and water were similar to one another. At a concentration of 5.1 mM, preferences were also very similar in both groups but were very high. At intermediate saccharin concentrations of 1.1 and 3.0 mM, hypothyroid animals showed significantly lower percent preferences for the sweet tastant than did controls, mean +/- SEM (62.48 +/- 5.97 vs. 82.92 +/- 4.60, p = 0.0002) for the 1.1 mM concentration and (74.98 +/- 5.12 vs. 89.40 +/- 2.54, p = 0.0029) for the 3.0 mM concentration. These changes in taste preference for saccharin in hypothyroid rats were similar in direction and magnitude to those previously published by this laboratory using sucrose as the tastant. Thus, hypothyroid rats demonstrate abnormalities in taste preference for both the nonnutritive sweetener, sodium saccharin, as well as for the nutritive sweetener, sucrose.

A new method for the artificial raising of infant rats: the palate cannula

Chronic removal of infant rats from their mother prior to the onset of weaning is complicated by the fact that young rats do not easily suckle from an artificial nipple. Thus, a method of artificial raising is advantageous for developmental investigations of nutrition or ingestive behaviors during the suckling period. The intragastric cannula has become a popular method for this purpose. However, for many studies, it would be advantageous if the diet could be administered to the mouth and actually swallowed by the young rat. We developed a new cannulation procedure which accomplishes these goals. Infant rats were removed from their mother on postnatal day 13 and fitted with a cannula that opened into the oral cavity through the hard palate. Liquid diet was administered by an infusion pump through the cannula for the subsequent 5 days. Growth was assessed by daily measures of body and organ weight. The results indicate that from postnatal day 13 on, the palate cannula can allow the continuation of normal growth patterns and eliminates certain complicating factors associated with other forms of artificial raising.

Intake and behavioral responsiveness to taste stimuli in infant rats from 1 to 15 days of age

Infant rats from 1 to 15 days of age received oral infusions of various taste solutions, and their intake of and behavioral responses to the infusions were observed. From 1 day of age on, pups responded to strong acid and quinine solutions with certain characteristic aversive responses and suppressed intake (Experiment 1). Although even very young (1- and 3-day-old) pups suppressed their intake of strong quinine and acid solutions, and rejected them by showing certain aversive responses (gaping and forelimb flailing), other components of the adultlike aversion response sequence (chin scraping and paw treading) did not appear until approximately 12 days of age. This suggests that the failure of pups to engage in chin scraping and paw treading prior to 12 days is not due to an inability to detect aversive solutions, but rather may be due to an immaturity in pups younger than 12 days of the neural substrates subserving these aversive responses. The emergence of chin scraping and paw treading to strong acid or quinine solutions was not influenced to any marked extent by either the deprivational state of the pup (Experiment 2a) or the site of the infusion (Experiment 2b).

Neonatal hyperthyroidism in the rat: thyroxine accelerates the development of unconditioned but not learned responses to tastes

The effects of neonatal hyperthyroidism induced by thyroxine injection on the development of unconditioned and learned behaviors mediated by the gustatory system of the rat were investigated. The development of unconditioned ingestive responses evoked by 10% sucrose and 0.1% HCl taste solutions was advanced several days by thyroxine treatment. However, the emergence of taste aversion learning involving 10% sucrose and LiCl injection was not advanced and may have been slightly delayed. Thus, the ontogenesis of unconditioned and learned behaviors mediated by the gustatory system was not influenced uniformly by neonatal thyroxine treatment.