Isabel de Brugada

Tests for inhibition after extinction of a conditioned stimulus in the flavour aversion procedure.

In four experiments, rats received flavour aversion conditioning followed by extinction. The flavour was then subjected to retardation and summation tests. Experiment 1 showed that reacquisition of an extinguished flavour aversion was retarded with respect to the performance shown by rats for whom the flavour was novel. No retardation was found, however, with respect to a control group that had been given non-reinforced pre-exposure to the flavour. Experiment 2 demonstrated that extinction showed the same sensitivity to the effects of a retention interval as did latent inhibition, consistent with the view that the retardation effect was a consequence of the occurrence of latent inhibition during extinction. An extinguished stimulus was also found to alleviate the response governed by a separately trained excitor in a summation test (Experiments 3 and 4), but the size of this effect did not exceed that produced by a control stimulus when the procedure used ensured an equivalent aversion to the test excitor in the two cases. These results challenge the proposal that extinction can turn a stimulus into a net inhibitor.

The US-Preexposure Effect in Lithium-Induced Flavor-Aversion Conditioning Is a Consequence of Blocking by Injection Cues

In 2 experiments, rats received flavor-aversion conditioning in which the unconditioned stimulus (US) was an orally consumed solution of lithium chloride (LiCl). The resulting aversion was not attenuated by giving preexposure to injections of LiCl, although such preexposure did attenuate aversions established using injected LiCl as the US (Experiment 1). This outcome suggests that blocking by injection-related cues is responsible for the US-preexposure effect observed in this situation. Experiment 2 confirmed this interpretation by showing that presenting such cues (by giving an injection of saline) at the time that the LiCl was drunk resulted in an attenuation of conditioning in animals preexposed to injections of LiCl. The US-preexposure effect obtained in these experiments can be explained solely in terms of blocking by injection cues, although other mechanisms may contribute to the effect seen in other flavor-aversion paradigms.

Safety signals from avoidance learning but not from yoked classical conditioning training pass both summation and retardation tests for inhibition

In one experiment half of the animals were trained to avoid a signaled footshock by jumping (30 or 160 trials), whereas the rest of the animals received the same events as yoked. For all of them the termination of the warning signal and of the shock was followed by a safety signal. Several tests were conducted to assess the ability of the stimuli to suppress licking by measuring the latency in completing 25 consecutive licks in the presence of the stimuli. Fear of the warning signal and inhibitory properties of the safety signal (summation and retardation tests) were measured. The results showed that there were no differences in fear to the warning signal, and that the safety signal behaves as a conditioned inhibitor only for animals trained with a long avoidance procedure, but not in the yoked (classical conditioning) procedure. These results highlight the role played by the avoidance response and its consequences in avoidance learning.

The role of habituation of the response to LiCl in the US-preexposure effect

In two experiments, rats received preexposure consisting of six intraperitoneal injections of lithium chloride (LiCl). This treatment reduced the magnitude of the unconditioned response (UR; suppressed consumption of a novel flavor) evoked by an additional injection (Experiment 1) or by oral consumption (Experiment 2) of LiCl. In both experiments, preexposure also attenuated the acquisition of a conditioned aversion with an LiCl injection as the unconditioned stimulus (US) but had no effect on the aversion produced when the US was oral consumption of LiCl (Experiment 2). These results are consistent with the view that the reduced ability of the preexposed US to serve as a reinforcer depends on blocking by injection-related cues and is independent of habituation of the UR recorded in the present study. Possible interpretations of this dissociation are discussed.

The role of injection cues in the associative control of the US pre-exposure effect in flavour aversion learning

Two experiments, using rats as subjects, examined the role of contextual cues in producing the unconditioned stimulus (US) pre-exposure effect in conditioned taste aversion. Experiment 1 showed a significant US pre-exposure effect, when the pre-exposure was conducted in a familiar context, and that a change of context between the pre-exposure and conditioning phases did not attenuate this effect. Experiment 2 demonstrated that extinction of injection-related cues after the pre-exposure stage attenuated the US pre-exposure effect, when the pre-exposure was conducted in either a familiar or a novel context. Taken together, these results support the associative explanation of the US pre-exposure effect in terms of blocking, incorporating a role for injection-related cues in the context blocking analysis of the US pre-exposure effect.

Repeated administration of LiCl produces an unconditioned stimulus preexposure effect in backward excitatory CTA but not habituation of the unconditioned increment in neophobia.

In one experiment using conditioned taste aversion and the unconditioned stimulus (US) preexposure procedure, one group of rats was given LiCl exposure for 3 days, whereas two other groups received saline. Following this phase, all groups were given a novel flavour (saccharine) to drink following either LiCl (group preexposed and one of the control groups) or saline injections (the remaining control group) and the consumption of the flavour was assessed. After this neophobia test, the acquired saccharine aversion was evaluated. The results show that three LiCl injections are enough to produce a US preexposure effect on backward excitatory taste aversion conditioning, whereas this number of injections procedure does not produce habituation of the increment in neophobia, an unconditioned response to the LiCl. The results are discussed taking into account different mechanisms involved in US preexposure effect.

The role of injection cues in the production of the morphine preexposure effect in taste aversion learning

The attenuation of an LiCl-induced conditioned taste aversion (CTA) by LiCl preexposure is mediated primarily by associative blocking via injection-related cues. Given that preexposure to morphine attenuates morphine-induced CTAs, it was of interest to determine whether injection cues also mediate this effect. Certain morphine-induced behaviors such as analgesic tolerance are controlled associatively, via injection-related cues. Accordingly, animals in the present experiments were preexposed to morphine (or vehicle) every other day for five total exposures, followed by an extinction phase, in which the subjects were given saline injections (or no treatment) for 8 (Experiment 1) or 16 (Experiment 2) consecutive days. All of the animals then received five CTA trials with morphine (or vehicle). The morphine-preexposed animals in Experiment 1 displayed an attenuation of the morphine CTA that was unaffected by extinction saline injections, suggesting that blocking by injection cues during morphine preexposure does not mediate this effect. All of the morphine-preexposed subjects in Experiment 2 displayed a weakened preexposure effect, an effect inconsistent with a selective extinction of drug-associated stimuli. The attenuating effects of morphine preexposure in aversion learning are most likely controlled by nonassociative mechanisms, like drug tolerance.

Long-lasting effects of prenatal dietary choline availability on object recognition memory ability in adult rats

Abstract Choline is an essential nutrient required for early development. Previous studies have shown that prenatal choline availability influences adult memory abilities depending on the medial temporal lobe integrity. The relevance of prenatal choline availability on object recognition memory was assessed in adult Wistar rats. Three groups of pregnant Wistar rats were fed from E12 to E18 with choline-deficient (0 g/kg choline chloride), standard (1.1 g/kg choline chloride), or choline-supplemented (5 g/kg choline chloride) diets. The offspring was cross-fostered to rat dams fed a standard diet during pregnancy and tested at the age of 3 months in an object recognition memory task applying retention tests 24 and 48 hours after acquisition. Although no significant differences have been found in the performance of the three groups during the first retention test, the supplemented group exhibited improved memory compared with both the standard and the deficient group in the second retention test, 48 hours after acquisition. In addition, at the second retention test the deficient group did not differ from chance. Taken together, the results support the notion of a long-lasting beneficial effect of prenatal choline supplementation on object recognition memory which is evident when the rats reach adulthood. The results are discussed in terms of their relevance for improving the understanding of the cholinergic involvement in object recognition memory and the implications of the importance of maternal diet for lifelong cognitive abilities.

Chronic dietary choline supplementation modulates attentional change in adult rats.

In two experiments adult rats were maintained on a diet enriched with added choline for 12 weeks prior to behavioral testing; control rats remained on the standard diet during this time. In Experiment 1 all rats received training in the Hall-Pearce negative transfer paradigm in which prior training with a conditioned stimulus (CS) paired with a small reinforcer retards further learning when the size of the reinforcer is increased. This effect, which has been attributed to a loss of associability by the CS, was obtained in control subjects but not in those given the supplement. Experiment 2 investigated the effect of prior nonreinforced exposure of the to-be-CS (latent inhibition). Such exposure retarded subsequent learning in control subjects, but latent inhibition was not obtained in those given the supplement. We conclude that the mechanism that reduces the attention paid to a stimulus that accurately predicts its consequences does not operate effectively after choline supplementation. These results are consistent with a role for the cholinergic system of the basal forebrain in modulation of attention.

Choline dietary supplementation improves LiCl-induced context aversion retention in adult rats.

Previous studies have demonstrated that choline is an essential nutrient during prenatal and early postnatal developmental periods. Thus, the availability of choline during these periods produces some beneficial effects on hippocampal-dependent learning and memory in rats. However, research on the effect of adult choline supplementation on learning and memory abilities is scarce. In the present study, 3-4 month-old male Wistar rats receiving a 7-week choline-supplemented diet (4.5 fold that of a standard diet) and control rats receiving a standard diet were trained in a LiCl-induced contextual aversion task. Short and long-term context aversion retention was assessed by recording the consumption of a flavoured solution in the aversive and safe contexts over two subsequent tests. Statistical analysis showed that the supplemented group exhibited greater intake suppression in the aversive context than in the safe context when two retention tests were applied 3 and 15 days after conditioning. These results suggest that increasing dietary choline availability during adulthood may favour the retention of a context aversion.