Dopamine and sodium appetite: antagonists suppress sham drinking of NaCl solutions in the rat

Sodium (Na) ingestion in rats depleted of Na is a strong, motivated behavior that is enhanced further when depleted rats are sham drinking. Dopamine plays a critical role in motivation, including reward associated with consumption of palatable tastes. The present studies assessed the role of dopamine in real and sham drinking of NaCl solutions after Na depletion with the diuretic furosemide (10 mg/kg). Dopamine (D2) receptor antagonists were evaluated (Haloperidol [0.1 mg/kg] and raclopride [0.2 mg/kg]), for their effects on sham and real drinking of 0.3 M NaCl. Sham drinking was markedly reduced by both antagonists whereas real drinking was unaffected. These effects did not appear to be due to malaise or suppression of motor behavior because drug-treated animals were able to increase ingestion substantially when offered less concentrated NaCl (0.1 M). These results suggest that the positive motivating properties of NaCl stimulation in depleted, sham-drinking rats are mediated by central D2 receptors.

Development of the enhanced neural response to NaCl in Fischer 344 rats

To examine the development of strain differences in the response of the chorda tympani nerve (CT) to NaCl, integrated CT responses to NaCl were recorded from Wistar and F344 rats between 15 and 36 days old. NaCl responses before and after the application of amiloride were expressed relative to 0.5 M NH4Cl as a standard. At 15-17 days old, there were no significant strain differences in the magnitude of NaCl responses. Strain differences emerged at 21-23 days old with responses of F344 rats significantly higher than those of Wistars; these differences became more pronounced at 34-36 days old. Amiloride significantly reduced responses to NaCl at all ages and eliminated any differences between the strains. Changes in neural responsiveness correspond with the emergence of the behavioral expression of F344 salt aversion.

Forebrain contribution to the induction of a brainstem correlate of conditioned taste aversion: I. The amygdala

The induction of c-Fos-like immunoreactivity (c-FLI) in the intermediate division of the nucleus of the solitary tract (iNTS) has been shown to be a reliable cellular correlate of the acquisition and/or behavioral expression of a conditioned taste aversion (CTA). To begin to define neuroanatomical structures and pathways that contribute to this cellular response and to CTA learning in general, electrolytic lesions of the amygdala were combined with immunostaining for c-FLI. Rats were given either unilateral or bilateral electrolytic lesions of the amygdala or "sham' operations. Following surgery "paired' animals were given a single conditioning trial consisting of intraoral infusion of 5 ml 0.15% sodium-saccharin followed by injection with LiCl (0.15 M, 20 ml/kg, i.p.) while "unpaired' controls received a non-contingent saccharin-LiCl presentation. When tested, unilateral-lesioned rats displayed a CTA by rejecting the saccharin, but increases in c-FLI were evident only on the side of the iNTS contralateral to the lesion. Rats with bilateral lesions showed no evidence of having acquired a CTA and no increase in c-FLI in iNTS relative to unpaired controls. These findings support involvement of amygdala in CTA learning and suggest that a lateralized connection between amygdala and iNTS is necessary for the conditioned c-FLI which is induced by exposure to a conditioned aversive taste.

Taste aversion learning in fyn mutant mice

Conditioned taste aversion (CTA) learning is a robust form of classical conditioning in which animals rapidly associate a flavor with aversive internal symptoms. The present study assessed CTA learning in transgenic mice deficient in a specific nonreceptor tyrosine kinase (the fyn mutant). Fyn mutants show impaired long-term potentiation and marked deficits in acquisition of spatial learning tasks. To assess whether they are also impaired in CTA learning, fyn mutant and wild-type mice received 2 conditioning trials consisting of access to a flavored solution followed by administration of LiCl. Fyn mutant mice acquired significant CTAs following a single conditioning trial and these aversions were comparable to those seen in wild-type mice. These results indicate that the fyn mutation does not interfere with the acquisition of CTAs and hence that this mutation is not associated with a global learning deficit.

Forebrain contribution to the induction of a cellular correlate of conditioned taste aversion in the nucleus of the solitary tract

A conditioned taste aversion (CTA) is a form of classical conditioning in which animals avoid a taste (conditioned stimulus; CS) which has been previously paired with a treatment (unconditioned stimulus; US) that produces transient illness. Recently, a reliable cellular correlate of the behavioral expression of a CTA was identified using c-Fos immunostaining as a marker of neuronal activation. Exposure to a saccharin solution (CS) which had previously been paired with lithium chloride (LiCl; US) induced significant c-Fos-like immunoreactivity (c-FLI) in the intermediate zone of the nucleus of the solitary tract (NTS), a response that was quite similar to that displayed following administration of LiCl alone. The present studies employed a variant of the chronic decerebrate rat preparation to explore whether circuitry intrinsic to the brainstem is sufficient for the induction of c-FLI in both as an unconditioned response to the LiCl and as a conditioned response to the saccharin. Using chronic hemidecerebrate rats, which have a unilateral brain transection at the level of the superior colliculus, we found that the unconditioned c-FLI to LiCI was unaltered by the transection, while the conditioned expression of c-FLI to the CS taste was evident only on the side of the NTS which retained neural connections with the forebrain. These findings strongly implicate forebrain input in this cellular correlate of CTA learning and also indicate that the pathways mediating the response to the US (LiCl) and the CS (saccharin) differ.

The CS-US interval and taste aversion learning: a brief look

Temporal parameters of taste aversion learning are known to differ markedly from other learning paradigms in that acquisition occurs despite lengthy delays between exposure to conditioned (CS) and unconditioned stimulus (US). Far less consideration has been paid to very brief CS-US intervals and the possibility that this learning may also be distinguished by an ineffectiveness of close temporal contiguity between CS and US. The effectiveness of a very brief CS-US interval (10 s) was compared with that of 2 lengthier intervals (15 and 30 min). Temporal control of CS delivery (0.15% saccharin solution) into the oral cavity and US delivery (7.5 mg/kg apomorphine hydrochloride) into circulation involved infusion pumps and indwelling catheters. Using a 1-trial learning paradigm, CS-US delays of 15 and 30 min led to significant aversions whereas the 10-s CS-US interval did not, suggesting that close temporal contiguity between CS and US is neither necessary nor sufficient for conditioned taste aversion acquisition.

c-Fos induction in response to taste stimuli previously paired with amphetamine or LiCl during taste aversion learning

Amphetamine and lithium chloride (LiCl) are both effective unconditioned stimuli (USs) in the establishment of conditioned taste aversions (CTA) in the rat. However, the mechanism of action of these drugs is quite different with the area postrema and related emetic circuitry critical to the response to LiCl but not amphetamine. c-Fos immunohistochemistry was used to define brain regions activated during drug administration and during expression of a CTA using either amphetamine or LiCl as the US drug. Administration of LiCl induced dense c-Fos-like immunoreactivity (c-FLI) in the nucleus of the solitary tract (NTS) while amphetamine induced only light staining in this area. A conditioned stimulus (CS) saccharin solution paired with amphetamine, however, was associated with c-FLI in NTS in a pattern quite similar to that seen to a LiCl-paired CS. This suggests that the pattern of c-Fos expression to a taste CS after conditioning is characteristic of aversion conditioning, in general, and appears not to represent a matching of the conditioned response to specific unconditioned effects of the drug. To examine this conditioned response further, c-FLI to the aversive saccharin CS was compared to the response to quinine hydrochloride, which is innately aversive. Although behaviorally the animals' ingestive responses were quite similar, the saccharin CS induced significant elevations of c-FLI in NTS whereas the quinine did not. Thus, a taste which had become aversive by virtue of conditioning induced c-FLI expression in NTS while a taste which was inherently aversive did not.