Effects of midazolam on acquisition and extinction of conditioned taste aversion memory in rats

Some intravenous anesthetic agents such as midazolam are known to induce anterograde and retrograde amnesia. We analyzed the effect of midazolam by the conditioned taste aversion (CTA) acquisition and retention. After the rats were offered 0.1% sodium saccharin (Sac) as conditioned stimulus (CS), an intraperitoneal (i.p.) injection of several concentrations (5–30 mg/kg) of midazolam was followed by an i.p. injection of 0.15 M LiCl (2% of body weight) as unconditioned stimulus (US). The rats, which acquired CTA by every CS–US paradigm, strongly avoided Sac on the 1st test day after conditioning and maintained the avoidance for 3 days. We have already reported that Sac intake abruptly increased on the 2nd test day and the almost complete extinction occurred on the 3rd test day after conditioning by injection of subhypnotic dose of propofol before LiCl-injection. In contrast, we found that subhypnotic dose of midazolam suppressed not only CTA acquisition, but also CTA retention. On the other hand, an α2-adrenergic blocker, yohimbin (1 mg/kg) suppressed only the CTA retention. These results suggest that the subhypnotic doses of midazolam firstly affect the acquisition mechanism of the CTA memory (CTAM), resulting the suppression of the retention of CTAM.     

The anterograde effect of ECS on the acquisition, retrieval and extinction of conditioned taste aversion.

Conditioned taste aversion was established in rats by spontaneous or forced drinking of saccharin (0.1%, CS) followed after 30 min by poisoning (0.15 M LiCl, 2% body weight, US) and retention was tested by offering the animals saccharin 1 or 2 days later. ECS (50 Hz, 50 mA, 0.5 s) applied 120, 60, 30 or 15 min before saccharin administration elicited an increasing disruption of CTA acquisition which was complete with the 15-min ECS-CS interval. On the other hand, ECS applied 2, 15 or 30 min after saccharin drinking caused either no or only moderate CTA disruption. ECS applied 30 min before retention testing reduced the fluid intake but did not prevent CTA retrieval. Forced feeding saccharin to rats with a strong CTA, established by repeated saccharin-LiCl pairing, caused CTA extinction which was only slightly decreased by ECS applied 10 or 60 min before the extinction trial. The proactive ECS effects prevent formation of the short-term gustatory trace but do not seriously disrupt the gustatory trace-poisioning association and leave the CTA retrival and extinction unaffected. In this respect ECS resembles other procedures that differentially interfere with CTA acquisition and retrieval, particularly the effect of paradoxical sleep deprivation.     

Early learning and retention of a conditioned taste aversion

Weanling rats were trained on a taste aversion task and tested 60 days later using the single bottle and preference methods for assessing the retention of a taste aversion, and a competitive situation in which the CS was eventually substituted for plain water. Only the competitive situation showed the effects of the early taste aversion training. The results are discussed in terms of the conditioning and reactivation of adrenocortical steroid elevations and how these might affect subsequent retrieval.     

Impaired conditioned taste aversion learning in APP transgenic mice

Cognition in transgenic mouse models of Alzheimer's disease (AD) has been predominantly characterized in explicit spatial orientation tasks. However, dementia in AD encompasses also implicit memory systems. In the present study a line of transgenic mice (TgCRND8) encoding a double mutated allele of the human amyloid precursor protein (APP) genes was evaluated in an implicit associative learning task of conditioned taste aversion (CTA). CTA is a form of Pavlovian classical conditioning, in which a mouse learns to avoid a novel taste of saccharine (conditioned stimulus) paired with an experimentally induced (systemic injection of lithium chloride) nausea (unconditioned stimulus). In contrast to conditioned non-Tg mice, TgCRND8 APP mice developed weaker aversion against saccharine and quickly increased its consumption in repeated tests. These results indicate that TgCRND8 mice show a significant impairment not only in explicit spatial memory, as has been previously shown [Nature 408 (2000) 979], but also in implicit memory. Control experiments confirmed that TgCRND8 and non-Tg mice had comparable taste sensitivities in response to appetitive as well as aversive tastes. The study suggests that the CTA paradigm can be a sensitive tool to evaluate deficits in implicit associative learning in APP transgenic mouse models of AD.     

Disruption of conditioned taste aversion: the effect of ECS after the taste-illness interval

Rats were taught a conditioned taste aversion (CTA) by pairing a 10% sucrose solution (CS) with lithium chloride-induced poisoning (UCS) 30 min later. The extent to which electroconvulsive shock (ECS) (80 mA for 600 msec) impaired the acquisition of the CTA was studied by either interpolating the ECS within the CS-UCS interval or by administering it at various times following the UCS (0, 5, 10, 15, 30, 60 and 120 min). There was a pronounced although limited loss of learning among all groups when ECS was delivered no later than 10 min after the UCS. When ECS was administered between 15 and 120 min, the overall aversion acquired did not differ significantly from that of the poisoned controls. It is concluded that while the stability of CTA seems to increase at about the same time the behavioral concomitants of lithium toxicity become apparent (10 min), this phenomenon does not necessarily imply that the neural trace underlying CTA has consolidated into a less labile state.     

The effect of CS-preexposure on conditioned taste aversion in young and adult rats

Young and adult rats were given nonreinforced exposures to the flavor-CS prior to a taste aversion conditioning session. CS-preexposure prevented the conditioning of a taste aversion in young rats but only attenuated conditioning in adults. These results suggest that an enhanced CS-preexposure effect may account for the previously reported weaker conditioned taste aversion observed in young rats, as compared to adults, when there is a protracted interstimulus interval.     

Postconditioning recovery from the latent inhibition effect in conditioned taste aversion

Two experiments were conducted examining the effects of flavor (CS) preexposure on the retention of conditioned taste aversion. In Experiment 1, rats received preexposure to sucrose solution followed by a sucrose-illness pairing. The expected “latent inhibition” effect was obtained when testing occurred after a two-day but not an eleven-day training-to-test interval. Experiment 2 extended these results by employing five- and twenty-one-day training-to-test interval parameters and provided evidence that the stronger taste aversion displayed by preexposed subjects following long retention intervals is not attributable to differences in training consumption of sucrose solution. This posttraining increase in conditioned taste aversion (CTA) suggests that preexposure blocks expression of memory.     

Rapid extinction of a conditioned taste aversion following unreinforced intraperitoneal injection of the fluid CS

After drinking a novel, saccharin-cyclamate solution, two groups of rats were poisoned with LiCl injection in order to establish a conditioned taste aversion, while a control group was not poisoned. Eighteen hours later, one of the poisoned groups of rats received differential unreinforced exposure to the sweet solution via an intraperitoneal injection. The group injected with the concentrated form of the saccharin-cyclamate solution showed subsequent rapid extinction of the conditioned taste aversion. An analogy was made to the technique of flooding (response prevention) used to hasten extinction of active, shock-motivated avoidance behavior.     

Hzf-3 expression in the amygdala after establishment of conditioned taste aversion

We studied the regulation of the expression of the inducible orphan nuclear receptor known as HZF-3 (or Nurr1) in acquisition of conditioned taste aversion in rats. Our results show that HZF-3 expression in the lateral/basolateral (LA/BLA) amygdala complex was significantly up-regulated when both the conditioned and the unconditioned stimuli were paired, but not when either of the stimuli was presented alone. Induction in the LA/BLA had a faster onset than induction in the central nucleus of the amygdala. The results implicate HZF-3 in the acquisition of associative aversive experiences.     

Failure of ACTH to prolong extinction of a conditioned taste aversion in the absence of the testes

Both corticotropin (ACTH) and testosterone prolong the extinction of a conditioned taste aversion in water-deprived intact male rats. An investigation was made to determine whether ACTH affects extinction in the absence of the testes and also to determine the effect of ACTH on serum testosterone levels. Water-deprived intact males showed prolonged extinction after ACTH injections; water-deprived gonadectomized males and intact females did not. All three of these groups showed elevated testosterone levels after ACTH administration, but testosterone levels were higher in the intact males than in the gonadectomized males or intact females. These results clearly show that in the absence of the testes ACTH is unable to prolong extinction. It is proposed that the increased level of testosterone following ACTH injection in water-deprived intact males is responsible for the prolonged extinction of a conditioned taste aversion. Although testosterone levels may increase in females and castrated males following ACTH injection, the increase is not sufficient to prolong extinction in these water-deprived animals.     

Cycloheximide impairs reconsolidation of a contextually reactivated memory in a conditioned taste aversion paradigm

Rats were used to examine the impact of systemic protein synthesis inhibition (PSI) on the reconsolidation of a contextually reactivated memory of conditioned taste aversion (CTA). Rats were administered intraperitoneal injections of saline or lithium chloride (LiCl; .15 M) following exposure to a novel sucrose solution in a unique context. Seven days later, rats were injected subcutaneously with saline or cycloheximide (CXM; 1 mg/kg) and returned to their home cage or placed into the CTA training context in the absence of the target conditioned stimulus to reactivate the training memory. At testing, LiCl-trained rats that had been given CXM at reactivation had significantly greater difference scores (sucrose-water) in comparison with LiCl/CXM rats that had not been given a reactivation treatment and LiCl/saline memory-reactivated rats. These results suggest that context re-exposure effectively reactivates memory of CTA training that may be weakened through PSI. Extinction tests revealed rapid attenuation of taste aversions in all of the LiCl-injected groups. The involvement of taste-potentiated aversions and the role of the context in taste aversion conditioning are discussed.     

Odor of taste stimuli in conditioned "taste" aversion learning

The present research addresses whether rats can express odor aversions to the odor of taste stimuli. In Experiment 1, saccharin or salt were either mixed in distilled water, so the rats could taste and smell them, or presented on disks attached to the tubes' metal spouts so the rats could only smell them. Aversions were established to taste stimuli under both conditions. The results of Experiment 2 indicate that conditioning was to the odor of the tastes when they were presented on disks in Experiment 1, hence both taste and odor aversions were established by means of "taste" stimuli. Taste aversion learning thus may more properly be termed flavor aversion learning, with flavor referring to both taste and odor components.     

The brain mapping of the retrieval of conditioned taste aversion memory using manganese-enhanced magnetic resonance imaging in rats

Manganese-enhanced MRI (MEMRI) is a newly developed noninvasive imaging technique of brain activities. The signal intensity of MEMRI reflects cumulative activities of the neurons. To validate the use of MEMRI technique to investigate the neural mechanisms of learning and memory, we tried to map brain areas involved in the retrieval of conditioned taste aversion (CTA) memory. CTAs were established to saccharin (conditioned stimulus: CS) by pairing its ingestion with an i.p. injection of LiCl (unconditioned stimulus: US). LiCl solutions (as a robust aversion chemical) of 0.15 M were injected i.p. 15 min after drinking the saccharine solution (CS). After the two times conditionings, these rats showed a robust aversion to the saccharine solution (CS). Rats of the control group were injected saline i.p. instead of LiCl solutions. The MRI signal intensities at the gustatory cortex (GC), the core subregion of the nucleus accumbens (NAcC), the shell subregion of the nucleus accumbens (NAcSh), the ventral pallidum (VP), the central nucleus of amygdala (CeA), the lateral hypothalamus (LH), and the basolateral nucleus of amygdala (BLA) of the conditioned group were higher than those of the control group. There were no significant differences between the conditioned and the control groups in the intensities for other regions, such as the striatum area, motor cortex, cingulate cortex, interstitial nucleus of the posterior limb of the anterior commissure and hippocampus. These indicate that the GC, NAcC, NAcSh, VP, CeA, LH and BLA have important roles in the memory retrieval of CTA.     

Further examination of ontogenetic limitations on conditioned taste aversion

This study was to resolve a discrepancy in the literature as to the capability of infant rats in acquiring conditioned taste aversion. Previous studies had indicated that during the 1st postnatal week, an aversion to saccharin could be conditioned when paired with lithium chloride (LiCl). Analogous conditioning with sucrose did not seem to occur until the end of the 2nd postnatal week, however, even though sucrose is discriminated from water and preferred before then. We observed that 5- and 9-day old pups express conditioned taste aversion to both saccharin and sucrose flavors that previously were paired with illness induced by LiCl. This learning occurred only when several hours separated cannulation and conditioning. A number of other factors that seemed likely to determine this early learning were found to have no effect. Thus it appears that rats can learn taste aversions very early in life, but only under certain circumstances. The results are discussed with reference to Vogt and Rudy's (1984) conclusions on the ontogeny of taste guided behaviors in the rat.     

Blockage of the effects of testosterone on extinction of a conditioned taste aversion by estradiol: time of action

Testosterone (T) prolongs the extinction of a conditioned taste aversion only if it is present during extinction. Experiments were conducted to determine whether estradiol (E) blocks the effects of T by acting during acquisition or extinction. In the first experiment, gonadectomized male and female rats injected with estradiol dipropionate (EP) and testosterone propionate (TP) during extinction had significantly faster extinction rates than those only injected with TP. Treating gonadectomized rats with TP prior to as well as during extinction did not prevent EP from blocking the effects of T. In Experiment 2, E was equally effective in preventing T from prolonging extinction when it was implanted in gonadectomized males during acquisition, extinction, or both acquisition and extinction. Thus, E does not have to be present concurrently with T during extinction to be effective. This suggests that E does not act on a T-related mechanism but rather acts independently of T.     

Early handling effects on neophobia and conditioned taste aversion

The purpose of this study was to determine whether early handling, a manipulation which affects both behavioral and pituitary-adrenal responsiveness to novel and aversive situations, will affect responses in adult rats confronted with novel substances (neophobia) or with substances associated with illness (conditioned taste aversion). We found that (1) early handling reduces the neophobia shown by adult animals and that handled animals appear better able to distinguish between a preferred and a nonpreferred substance. (2) Handling reduces the magnitude of the initial taste aversion and also increases the rate of recovery of drinking to pretoxicosis levels. (3) These behavioral differences between handled and nonhandled animals are not due to differential pituitary-adrenal responses to LiCl. (4) Early handling does affect the conditioned elevation of plasma corticoids upon reexposure to milk under a Forced Extinction procedure. In this situation nonhandled animals show greater corticoid elevations than handled animals. (5) Manipulation of the number of exposures to a substance prior to pairing that substance with LiCl affects the magnitude of both the aversion and the elevation of plasma corticoids which are produced upon reexposure. As number of preexposures increase, both the magnitude of the aversion and the elevation of corticoids decrease.