Ethanol-induced taste aversions: lack of involvement of acetaldehyde and the area postrema

Estradiol accelerates extinction of lithium chloride-induced conditioned taste aversions through its illness-associated properties

Estradiol accelerates extinction of LiCl-induced conditioned taste aversions when it is present during a period that starts 2-3 days after acquisition and extends throughout extinction (before and during extinction). It has been suggested that estradiol acts before, not during, extinction and that its effect on extinction is associated with its illness-inducing properties. This hypothesis is based on previous work which shows an attenuation of conditioned taste aversion learning when rats are exposed to illness-inducing agents during a period that starts 2 days after acquisition and ends 2 days before extinction trials are initiated. Four experiments were designed to test elements of this hypothesis. The first two experiments demonstrated that if an estradiol-filled Silastic capsule is implanted before extinction of a LiCl-induced aversion, when the conditioned taste is not present, it accelerates extinction, but if it is implanted during extinction, when the conditioned taste is present, it prolongs extinction. The third experiment showed that the same dose of estradiol that accelerates extinction of a LiCl-induced aversion was effective in producing a conditioned taste aversion when it was present for 18 h after consumption of a novel sucrose solution. The fourth experiment indicated that serum levels of estradiol were elevated during the 18 h. These results are consistent with the hypothesis that the acceleration of extinction by estradiol is associated with its illness-inducing properties. It is suggested that estradiol acts on neural areas that mediate illness information and that one of these areas, the area postrema is necessary for estradiol to accelerate extinction of a LiCl-induced aversion.

Forced-choice discrimination of equimolar NaCl and LiCl solutions in rats: effects of ablating the chemosensitive area postrema on acquisition and retention

The area postrema (AP), a chemosensitive organ located in the fourth ventricle, has been shown to mediate the formation of a lithium-induced conditioned taste avoidance (CTA) in rats. The present experiments examined the role of the AP in the discrimination between two equimolar solutions of sodium chloride (NaCl) and lithium chloride (LiCl). In the first experiment adult male rats were trained to discriminate between equimolar (0.12 M) solutions of NaCl and LiCl in a forced-choice procedure over a 10-day acquisition phase. Subsequently half of the rats (n = 7) received AP lesions (APX) and the other half (n = 7) were given sham lesions (SHAM). In the retention phase all animals were again exposed to the same salt solutions over a 10-day period. Good discrimination (P < 0.001) between the two salt solutions was demonstrated by the end of the acquisition phase and both the APX and SHAM groups exhibited robust retention (P < 0.01) of this discrimination in the second phase. However, when only a LiCl solution was available the APX group ingested significantly more (P < 0.01) than the SHAM rats. No significant group difference emerged when only NaCl was available. In the second experiment rats received ablations of AP or sham lesions and were then trained to discriminate between 0.12 M NaCl and LiCl solutions in a forced-choice procedure over a 10-day period. Both groups exhibited a clear discrimination (P < 0.01) between the two solutions by the end of the acquisition phase. APX rats ingested significantly more LiCl (P < 0.01) than did the SHAM group when this was the only type of fluid available. Again, no such difference was evident when only NaCl was available. These experiments demonstrate that the AP is not necessary for either the acquisition or retention of a discrimination between equimolar solutions of NaCl and LiCl in a forced-choice procedure and that this discrimination is not mediated by a conditioned taste aversion to the LiCl solution.

c-Fos induction in rat brainstem in response to ethanol- and lithium chloride-induced conditioned taste aversions

When consumption of a novel taste (conditioned stimulus; CS) is followed by exposure to a toxin, organisms will avoid consumption of that taste in the future. This learned response, known as a conditioned taste aversion (CTA), can be demonstrated using a variety of drugs, including lithium chloride (LiCl) and ethanol. c-Fos immunohistochemistry was used to examine neural activation in the rat brainstem associated with drug administration and with a CS taste previously paired with these drugs. Relative to saline controls, animals injected with either LiCl (76 mg/kg) or ethanol (3.5 g/kg) displayed greater c-Fos expression in area postrema, nucleus of the solitary tract (NTS), and lateral parabrachial nucleus. At these doses, LiCl- and ethanol-injected groups did not differ from each other. For establishing a CTA, intraoral infusion of a 0.15% saccharin solution was followed by injection of either LiCl or ethanol. Both LiCl and ethanol produced quantitatively similar CTAs. Relative to unpaired control groups, saccharin paired with either drug induced significant c-Fos expression in NTS. Thus, like LiCl, ethanol and tastes that have become aversive by virtue of their association with ethanol activate brainstem regions hypothesized to play a role in CTA learning.

Sex- and strain-related differences in first-pass alcohol metabolism in mice

Adult males and females of three strains of mice, C57BL/10J, C57BL/6J and DBA/2J, were intubated or injected intraperitoneally with 0.02 ml/g body weight of a 25% alcohol solution. Thirty minutes later, their blood alcohol levels (BAL) were measured. Another group of mice, including both sexes, representative of the three strains, was fasted for 12 h and sacrificed; their stomachs were removed, homogenized, and assayed for gastric alcohol dehydrogenase (GAD) activity. Higher BALs were found in all intubated females compared to the intubated males. The reverse was observed in the injected group, which showed the males with the highest BAL values. GAD activity was evidenced in both sexes of the three strains and it was highest in the males. Strain-related differences were evident in the intubated groups and not in the injected groups. Intubated DBA animals had the lowest BALs as well as the highest GAD values. The results provide evidence for first-pass alcohol metabolism in mice and show the effects of sex and strain on gastric oxidation of alcohol.

Toxin-induced conditioned changes in taste reactivity and the role of the chemosensitive area postrema

Conditioned taste avoidances (CTAs) are an important component of behavioral regulation of ingestion. In the laboratory CTAs can be produced by pairing a novel taste stimulus with the physiological feedback produced by a toxin, such as lithium. Such toxins putatively activate a chemosensitive brainstem structure, the area postrema, which ultimately results in the production of a CTA. The present review describes a series of studies which examined conditioned changes in taste reactivity responses (TRRs) when a novel intraoral sucrose taste was paired with the effects of an intraperitoneal (IP) injection of LiCl, and the role of the area postrema in the formation of conditioned palatability shifts. It was first of all necessary to examine the effects of area postrema ablations on TRRs to a range of intraoral sucrose and quinine stimulus intensities. In the first study area postrema lesioned rats exhibited concentration dependent changes in TRRs to these taste stimuli that were very similar to those exhibited by sham lesioned rats. The second study demonstrated that 30 s intraoral infusions of sucrose (0.3 M), presented at 5 or 10 min intervals following an IP injection of LiCl (3.0 meq), resulted in conditioned changes in TRRs. These were characterized by orderly, gradual reductions in ingestive responses and increases in aversive responses. Finally, when area postrema lesioned rats (Study 3) were subjected to this conditioning procedure (brief sucrose presentations paired with the effects of LiCl) no evidence for conditioned or unconditioned changes in TRRs to sucrose were obtained. Lesioned rats injected with LiCl behaved similarly to sham lesioned rats injected with NaCl. These series of studies provide evidence indicating that the chemosensitive area postrema mediates the formation of conditioned palatability shifts induced by treatment with a toxin such as lithium.

Novel diet consumption and body weight gain are reduced in rats chronically infused with lithium chloride: mediation by the chemosensitive area postrema

The effects of chronic lithium chloride infusions on consumption of, and subsequent preferences for, a novel diet were examined in rats with ablations of the area postrema (AP) and sham-lesioned control rats. Osmotic minipumps (Alza), filled with a saturated aqueous solution of LiCl (63 g/100 ml), were implanted in the peritoneal cavity of half of the lesioned rats (n = 9) and half of the control rats (n = 8). The remaining rats received empty pumps (n = 9 and n = 7 for lesioned and controls, respectively). The LiCl or sham drug phase was paired with free access to a highly palatable novel diet (AIN diet) during a 7-day conditioning period. Subsequent preferences for the novel diet relative to a familiar diet (ground Purina lab pellets) were determined using a two-food choice procedure. The only group to show a persistent and significant reduction in novel food consumption during the conditioning phase was the sham-lesioned group infused with LiCl (p < 0.01). This group also exhibited a marked aversion for the novel diet, indicative of a conditioned food aversion (CFA), during the preference tests. No significant differences in novel diet consumption or in novel diet preference were found between the two AP-lesioned groups. This study provides evidence that anorexia and CFAs to a novel diet, induced with chronic infusions of lithium, are abolished by destruction of the chemosensitive area postrema.

Effects of acute swim stress on LiCl-induced conditioned taste aversions

The present study examined the effects of a 5-min period of swim stress experienced between a flavor (saccharin) and illness (LiCl) on conditioned taste aversion learning. Experiment 1 obtained a stress-induced attenuation of learning. Experiment 2 replicated the findings of Experiment 1, and also obtained a similar attenuation when stress was administered 30 min prior to the saccharin presentation. Experiment 3 examined the effects of swim stress either 15 min or 90 min after the LiCl had been administered. It was found that swim stress 15 min after LiCl significantly attenuated CTA, but swim stress 90 min after LiCl did not. These results are discussed with regard to current views of the relationship between external events and conditioned taste aversions.

Ethanol tolerance developed during intoxicated operant performance in rats prevents subsequent ethanol-induced conditioned taste aversion

Four groups of Sprague-Dawley rats (n = 10 per group) were trained in a two-phase conditioning experiment. All rats were initially trained in an FR30 operant task (phase 1), and subsequently trained in a conditioned taste aversion (CTA) task. The groups of rats differed in their ETOH exposure. All rats received 2-week chronic exposure in phase 1. Two groups received chronic presession ETOH and, therefore, the opportunity for intoxicated practice; another group, yoked to this latter group, received postsession ETOH; the final group received presession saline injections. The presession ETOH groups were conditioned in the CTA task with either ETOH or saline; both increased their intakes of the conditioned tastant. The presession saline and the postsession ETOH groups received ETOH CTA; both developed a robust CTA. Thus, prior history of intoxicated practice under the operant task prevented the development of ETOH-induced CTA. We argue that ETOH exposure may be a necessary but not sufficient condition for tolerance to develop to the aversive attributes of ETOH.

Nicotine-induced conditioned taste aversions are enhanced in rats with lesions of the area postrema

Lesions, which destroy the area postrema and damage the adjacent nucleus of the solitary tract, attenuate or abolish conditioned taste aversions (CTA) induced by a variety of pharmacological agents. In the present experiment 2 groups of male rats received lesions of the area postrema and 2 groups were given sham lesions. One lesioned group and one sham-lesioned group were twice conditioned with 30-min access to a novel 0.15% saccharin solution followed by injection of nicotine (1 mg/kg, IP). The other 2 groups were similarly conditioned with saccharin followed by saline injections. In subsequent two-bottle choice tests (saccharin vs. water), the saline-injected rats exhibited strong preferences for saccharin, the sham-lesioned rats injected with nicotine showed a weak but significant (p less than 0.05) aversion to saccharin, and the area postrema-lesioned rats injected with nicotine displayed a significantly (p less than 0.05) stronger CTA than the drug-injected sham-lesioned animals. In Phase 2 all rats were given novel chocolate metrecal (30 min) followed by injection of scopolamine HCl (1 mg/kg, IP). The area postrema-lesioned rats showed significant (p less than 0.01) preference for the chocolate taste relative to the aversions shown by the sham-lesioned animals. Thus, area postrema lesions attenuated a scopolamine-induced CTA, but enhanced a nicotine-induced aversion. These results suggest that nicotine and scopolamine act at different neural sites in producing CTAs.

Effects of medullary afferent vagal axotomy and area postrema lesions on short-term and long-term NaCl-induced taste aversion learning

This series of experiments demonstrates a functional dissociation between the area postrema (AP) and the vagus nerve in short-term taste aversion learning (TAL). Although medullary axotomy of the afferent component of the vagus disrupted the learning observed with NaCl-induced short-term (nondelayed) TAL, lesioning the AP failed to interfere with the discriminative process employed by the animals under the same conditions. However, involvement of neither the vagus nerve nor the AP seemed to be indispensable for learning in NaCl-induced long-term (delayed) TAL. The possibility that the vagus nerve and the AP are involved in temporally distinct visceral processing is discussed.

Attenuation and cross-attenuation in taste aversion learning in the rat: studies with ionizing radiation, lithium chloride and ethanol

The preexposure paradigm was utilized to evaluate the similarity of ionizing radiation, lithium chloride and ethanol as unconditioned stimuli for the acquisition of a conditioned taste aversion. Three unpaired preexposures to lithium chloride (3.0 mEq/kg, IP) blocked the acquisition of a taste aversion when a novel sucrose solution was paired with either the injection of the same dose of lithium chloride or exposure to ionizing radiation (100 rad). Similar pretreatment with radiation blocked the acquisition of a radiation-induced aversion, but had no effect on taste aversions produced by lithium chloride (3.0 or 1.5 mEq/kg). Preexposure to ethanol (4 g/kg, PO) disrupted the acquisition of an ethanol-induced taste aversion, but not radiation- or lithium chloride-induced aversions. In contrast, preexposure to either radiation or lithium chloride attenuated an ethanol-induced taste aversion in intact rats, but not in rats with lesions of the area postrema. The results are discussed in terms of relationships between these three unconditioned stimuli and in terms of implications of these results for understanding the nature of the proximal unconditioned stimulus in taste aversion learning.

Attenuation of a radiation-induced conditioned taste aversion after the development of ethanol tolerance

An attempt to reduce a radiation-induced conditioned taste aversion (CTA) was undertaken by rendering animals tolerant to ethanol. Ethanol tolerance, developed over 5 days, was sufficient to block a radiation-induced taste aversion, as well as an ethanol-induced CTA. Several intermittent doses of ethanol, which did not induce tolerance but removed the novelty of the conditioning stimulus, blocked an ethanol-induced CTA but not the radiation-induced CTA. A CTA induced by doses of radiation up to 500 rads was attenuated. These data suggest that radioprotection developing in association with ethanol tolerance is a result of a physiological response to the chronic presence of ethanol not to the ethanol itself.