Medial prefrontal cortex and precision of temporal discrimination: a lesion, microinjection, and microdialysis study

Interoceptive accuracy correlates with precision of time perception in the millisecond range

It has been proposed that accuracy in time perception is related to interoceptive accuracy and vagal activity. However, studies investigating time perception in the supra-second range have provided mixed results, and few studies have investigated the sub-second range. Moreover, there is a lack of studies investigating the relationship between precision in time perception and interoceptive accuracy. A recent meta-analytic review of neuroimaging studies proposed a dynamic interaction between two types of timing processing-an endogenous time keeping mechanism and the use of exogenous temporal cues. Interoceptive accuracy may affect both accuracy and precision of primary temporal representations, as they are generated based on the endogenous time keeping mechanism. Temporal accuracy may vary when adapted to the environmental context. In contrast, temporal precision contains some constant noise, which may maintain the relationship with interoceptive accuracy. Based on these assumptions, we hypothesized that interoceptive accuracy would be associated with temporal precision in the sub-second range, while vagal activity would be associated with temporal accuracy. We used the temporal generalization task, which allowed us to calculate the indices of temporal accuracy and temporal precision in line with the existing research, and also compute the index of participants' sensitivity according to the signal detection theory. Specifically, we investigated whether (1) interoceptive accuracy would correlate with temporal accuracy, temporal precision, or sensitivity and (2) resting-state vagal activity would correlate with temporal accuracy, temporal precision, or sensitivity. The results indicated that interoceptive accuracy was positively correlated with temporal precision as well as sensitivity, but not with temporal accuracy, in the sub-second range time perception. Vagal activity was negatively correlated only with sensitivity. Furthermore, we found a moderation effect of sensitivity on the relationship between vagal activity and perceived duration, which affected the association between vagal activity and temporal accuracy. These findings suggest the importance of precision as an aspect of time perception, which future studies should further explore in relation to interoception and vagal activity, and of the moderation effects of factors such as participants' sensitivity in this context.

Overlapping striatal sites mediate scopolamine-induced feeding suppression and mu-opioid-mediated hyperphagia in the rat

RATIONALE

Intra-striatal infusions of the muscarinic antagonist, scopolamine, markedly suppress feeding; however, the underlying mechanisms are unclear. Recent findings suggest that scopolamine influences opioid-dependent mechanisms of feeding modulation. Robust mu-opioid-mediated feeding responses are obtained in anterior, ventral sectors of the striatum with progressively weaker effects posteriorly and dorsally. One might therefore expect the effects of scopolamine to conform to similar boundaries, but a systematic mapping of scopolamine-induced feeding suppression has not yet been undertaken.

OBJECTIVE

This study aimed to assess the overlap between the striatal sites mediating scopolamine-induced feeding suppression and mu-opioid-induced hyperphagia.

METHODS

Dose-effect functions for scopolamine (0, 1, 5, and 10 μg) were obtained in the nucleus accumbens (Acb), anterior dorsal striatum (ADS), and posterior dorsal striatum (PDS) in three different groups of rats. In the same subjects, the mu-opioid receptor agonist (D-Ala2-N-MePhe4, Glyol)-enkephalin (DAMGO; 0.25 μg) was infused on a separate test day. The dependent variables were food and water intake, ambulation, and rearing.

RESULTS

The greatest dose sensitivity for scopolamine-induced feeding suppression was observed in the Acb. Only the highest dose was effective in the ADS, and no effects were seen in the PDS. Water intake and general motor activity were not altered by scopolamine in any site. DAMGO infusions produced hyperphagia only in the Acb.

CONCLUSIONS

These results support a model in which the behavioral effects of muscarinic blockade are limited by the same anatomical constraints that govern mu-opioid receptor-mediated control of feeding. These constraints are likely imposed by the topographic arrangement of feeding-related afferent inputs and efferent projections of the striatum.

Participation of the dorsal hippocampus in stimulus discrimination with scopolamine

Stimulus discrimination is the capacity of an organism to differentiate between stimuli and emit associated responses. The administration of the muscarinic antagonist scopolamine can be used as a stimulus by mammals in a discrimination task. The present study analyzes the contribution of the hippocampus in scopolamine discrimination and generalization. Male Wistar rats, weighing 250-300 g at the beginning of the experiment, were trained to discriminate between scopolamine (1.0 mg/kg, i.p.) and saline administration using a two-lever operant task; rats had to respond differentially to each lever depending on the preceding drug or saline administration. Once stimulus control was attained, rats were tested with different scopolamine doses (0.0, 0.056, 0.091, 0.16, 0.31 and 1.0 mg/kg, i.p.) in order to obtain generalization curves. After generalization the rats were randomly assigned to hippocampal CA1 lesion or control groups. Hippocampus impairment produced a transient decrease in the capacity to discriminate between scopolamine and saline conditions; nonetheless, scopolamine correct responses were rapidly recovered after a few sessions and even maintained after 90 days. Correct responses for saline condition were never recovered. The generalization curve obtained after hippocampus lesion showed a response gradient severely flattened. Results suggest that the hippocampus participates as a neural system supporting the sensitivity to detect discrete changes in stimulus properties and relational memory, more than on the capacity to recall for simple associative responses.

Relationship between impulsivity, hyperactivity and working memory: a differential analysis in the rat

BACKGROUND

Impulsivity is a behavioural trait that comprises several distinct processes. It is a key feature of many psychopathologies such as mania, addictive disorders or attention deficit-hyperactivity disorders. To date, the aspects of impulsiveness involved in these pathologies have not yet been explicitly defined. In these disorders, sensation or drug seeking and cognitive deficits are closely related, but the nature of these relationships remains largely unknown. A new animal model of impulsiveness based on spontaneous inter-individual differences is proposed here to help clarify the relationship between characteristic aspects of impulsive-related pathologies.

METHODS

Rats were divided into sub-groups according to their scores in three operant tasks with varying degrees of behavioural inhibition, timing and motor vs. cognitive impulsivity demands. These tasks included a fixed consecutive number schedule (ability to complete an action to receive a reinforcer), a multiple fixed-interval/extinction schedule of reinforcement (high level of responding), and a delayed reward task (delay discounting). In addition, measurements of locomotor responses to novelty and to amphetamine in a circular corridor, and working memory in an 8-arm radial maze were obtained.

RESULTS

Substantial behavioural inter-individual differences were observed in each task, whereas few inter-task relationships were found. Impulsive rats, as defined in a task requiring inhibition of premature responses, presented a higher increase in amphetamine-induced locomotion. Reduced working memory performance was only observed in hyperactive rats in an extinction schedule.

CONCLUSION

This novel approach shows that distinct aspects of impulsiveness and hyperactivity can be expressed based on large inter-individual differences that vary from poorly to highly adapted behaviours ones in a normal population of rats. Inhibitory deficit was related to a higher response to psychostimulants a characteristic of rats predisposed to amphetamine self-administration and related to higher limbic dopaminergic activity, whereas working memory capacity was only related to hyperactivity. This approach allows for the identification of particular individuals presenting distinct behavioural characteristics of impulsive-related psychopathologies. These individuals may be of great interest in the modelling of these disorders and the exploration of their neurobiological bases.

Morphological correlates of corticosteroid-induced changes in prefrontal cortex-dependent behaviors

Imbalances in the corticosteroid milieu have been implicated in several neuropsychiatric disorders, including depression and schizophrenia. Prefrontal cortex (PFC) dysfunction is also a hallmark of these conditions, causing impairments in executive functions such as behavioral flexibility and working memory. Recent studies have suggested that the PFC might be influenced by corticosteroids released during stress. To test this possibility, we assessed spatial working memory and behavioral flexibility in rats submitted to chronic adrenalectomy or treatment with corticosterone (25 mg/kg) or the synthetic glucocorticoid dexamethasone (300 microg/kg); the behavioral analysis was complemented by stereological evaluation of the PFC (prelimbic, infralimbic, and anterior cingulate regions), the adjacent retrosplenial and motor cortices, and the hippocampal formation. Dexamethasone treatment resulted in a pronounced impairment in working memory and behavioral flexibility, effects that correlated with neuronal loss and atrophy of layer II of the infralimbic, prelimbic, and cingulate cortices. Exposure to corticosterone produced milder impairments in behavioral flexibility, but not in working memory, and reduced the volume of layer II of all prefrontal areas. Interestingly, adrenalectomy-induced deleterious effects only became apparent on the reverse learning task and were not associated with structural alterations in the PFC. None of the experimental procedures influenced the morphology of retrosplenial or motor cortices, but stereological measurements confirmed previously observed effects of corticosteroids on hippocampal structure. Our results describe, for the first time, that imbalances in the corticosteroid environment can induce degeneration of specific layers of the PFC; these changes appear to be the morphological correlate of corticosteroid-induced impairment of PFC-dependent behavior(s).