Frontal lesions do not alter the differential extinction of taste aversion conditioning in rats, when using two methods of sucrose delivery

Dorsal medial prefrontal cortex contributes to conditioned taste aversion memory consolidation and retrieval

The medial prefrontal cortex (mPFC) is known for its role in decision making and memory processing, including the participation in the formation of extinction memories. However, little is known regarding its contribution to aversive memory consolidation. Here we demonstrate that neural activity and protein synthesis are required in the dorsal mPFC for memory formation of a conditioned taste aversion (CTA) task and that this region is involved in the retrieval of recent and remote long-term CTA memory. In addition, both NMDA receptor and CaMKII activity in dorsal mPFC are needed for CTA memory consolidation, highlighting the complexity of mPFC functions.

Adrenergic drugs modify the level of noradrenaline in the insular cortex and alter extinction of conditioned taste aversion in rats

We compared the effect of conditioned taste aversion in rats by measuring the amount of sucrose that they drunk after conditioning, which differed according to whether rats had drunk the sucrose freely (SD: self drinking) during the conditioning session, or had been forced to drink it (IO: intra-oral administration through a chronically implanted cannula). The SD procedure delayed the extinction of conditioned taste aversion. Enhanced arousal, alertness, awareness or attention in the SD condition may have strengthened the memory of the taste. Brain noradrenergic networks are involved in such processes. We administered two noradrenergic drugs that produce opposite effects on noradrenaline release in the brain, methoxy-idazoxan, RX821002 (1mg/kg, i.p.), and guanfacine (0.12mg/kg, i.p.). We evaluated their effect (i) on the level of noradrenaline in the gustatory cortex using microdialysis, (ii) on glycaemia that is an essential factor of taste learning and (iii) on the comparative SD versus IO conditioned taste aversion protocol mentioned above. Injecting RX821001 increased the level of noradrenaline in the gustatory cortex up to two-fold of the baseline. This effect lasted 1h. The same dose of RX821002 did not elicit any alteration of glycaemia. It enhanced extinction of conditioned taste aversion in the SD group of rats. Injecting 0.12mg/kg of guanfacine produced the opposite effect. The noradrenaline level of the gustatory cortex decreased, but only down to 20% of the baseline. This decrease lasted 2h. Guanfacine increased glycaemia. Extinction of conditioned taste aversion was only marginally decreased by guanfacine in the SD group of rats. These results fit with Aston-Jones' point of view that the role of the noradrenergic coeruleo-cortical system may be to enhance arousal, alertness, awareness or attention to an event by a transient increase of cortical noradrenaline.

There is a time and a place for everything: bidirectional modulations of latent inhibition by time-induced context differentiation

Latent inhibition (LI) is defined as poorer evidence of learning with a stimulus that previously was presented without consequence, as compared with a novel or previously attended stimulus. The present article reviews the evidence, mostly from three-stage conditioned taste aversion studies (preexposure, conditioning, and test), that LI can be either attenuated or enhanced depending on the length of the retention interval between conditioning and test and where that interval was spent. Time-induced reduction in LI is observed when the interval context is the same as that of the preexposure, conditioning, and test stages. Super-LI is obtained when a long retention interval is spent in a context that is different from that of the other stages. The differential modulations of LI appear to be the result of the strengthening of primacy effects (i.e., first training disproportionately stronger than subsequent training) by long-interval different contexts, thereby producing super-LI, and the reversal of this effect by long-interval same contexts, thereby producing attenuated LI. The bidirectional effects of time/ context modulations on LI, unaccounted for by current learning theories, are explained, in part, by a time-induced context differentiation process. Implications for theories of LI, learning, and, memory are discussed.

Extinction of conditioned taste aversion depends on functional protein synthesis but not on NMDA receptor activation in the ventromedial prefrontal cortex

We investigated the role of the ventromedial prefrontal cortex (vmPFC) in extinction of conditioned taste aversion (CTA) by microinfusing a protein synthesis inhibitor or N-methyl-d-asparate (NMDA) receptors antagonist into the vmPFC immediately following a non-reinforced extinction session. We found that the protein synthesis blocker anisomycin, but not the NMDA receptors antagonist D,L-2-amino-5-phosphonovaleric acid, impaired CTA extinction in the vmPFC. Anisomycin microinfusion into vmPFC had no effect on CTA acquisition and by itself did not induce CTA. These findings show the necessary role functional protein synthesis is playing in the vmPFC during the learning of CTA extinction.

A role for prefrontal cortex in the extinction of a conditioned taste aversion

This study used immunohistochemical methods to determine if the medial prefrontal cortex (mPFC) is involved in the extinction of a conditioned taste aversion (CTA). As rats reached 90% reacceptance of a tastant (saccharin: SAC) that had previously been associated with lithium chloride-induced malaise, c-Fos protein expression increased dramatically as compared to animals with active CTAs, animals without CTAs (i.e., explicitly unpaired CS-US exposures) or animals drinking SAC for the first time. These data indicate a role for mPFC (prelimbic and infralimbic cortex) in the formation of a CTA extinction memory.

Insular cortex lesions alter conditioned taste avoidance in rats differentially when using two methods of sucrose delivery

The insular gustatory cortex may be essential for the evaluation of saliency and representation of the incentive values of tastes. Gustatory cortex lesions should interfere with conditioned taste avoidance according to these factors, which depend on the conditioned taste avoidance protocol used. The present study was aimed at investigating the effects of bilateral lesions of the gustatory cortex-focusing on electrolytic and excitotoxic lesions. Lesioned and sham-operated male Long-Evans rats were intoxicated using LiCl after drinking sucrose from a tube (SD) or having the same amount of sucrose fed directly into their mouths through a chronically implanted intra-oral (IO) cannula. Every aspect of the experiment was carefully counterbalanced between the experimental groups. In the control groups, the acquired avoidance towards sucrose was strongly preserved over eight extinction test days in SD rats but not in IO rats, in which a progressive decline was recorded. Electrolytic gustatory cortex lesions impaired but did not suppress conditioned taste avoidance in both protocols. Excitotoxic lesions tend to impair CTA also, but differentially according to the SD or IO protocols. Extinction of CTA was selectively impaired in the SD protocol by small lesions destroying the anterior insular cortex.

Prefrontal executive and cognitive functions in rodents: neural and neurochemical substrates

The prefrontal cortex has been implicated in a variety of cognitive and executive processes, including working memory, decision-making, inhibitory response control, attentional set-shifting and the temporal integration of voluntary behaviour. This article reviews current progress in our understanding of the rodent prefrontal cortex, especially evidence for functional divergence of the anatomically distinct sub-regions of the rat prefrontal cortex. Recent findings suggest clear distinctions between the dorsal (precentral and anterior cingulate) and ventral (prelimbic, infralimbic and medial orbital) sub-divisions of the medial prefrontal cortex, and between the orbitofrontal cortex (ventral orbital, ventrolateral orbital, dorsal and ventral agranular cortices) and the adjacent medial wall of the prefrontal cortex. The dorso-medial prefrontal cortex is implicated in memory for motor responses, including response selection, and the temporal processing of information. Ventral regions of the medial prefrontal cortex are implicated in interrelated 'supervisory' attentional functions, including attention to stimulus features and task contingencies (or action-outcome rules), attentional set-shifting, and behavioural flexibility. The orbitofrontal cortex is implicated in lower-order discriminations, including reversal of stimulus-reward associations (reversal learning), and choice involving delayed reinforcement. It is anticipated that a greater understanding of the prefrontal cortex will come from using tasks that load specific cognitive and executive processes, in parallel with discovering new ways of manipulating the different sub-regions and neuromodulatory systems of the prefrontal cortex.