Representation-mediated Aversion as a Model to Study Psychotic-like States in Mice

Lack of interactions between prenatal immune activation and Δ9-tetrahydrocannabinol exposure during adolescence in behaviours relevant to symptom dimensions of schizophrenia in rats

The causality in the association between cannabis use and the risk of developing schizophrenia has been the subject of intense debate in the last few years. The development of animal models recapitulating several aspects of the disease is crucial for shedding light on this issue. Given that maternal infections are a known risk for schizophrenia, here, we used the maternal immune activation (MIA) model combined with THC exposure during adolescence to examine several behaviours in rats (working memory in the Y maze, sociability in the three-chamber test, sucrose preference as a measure, prepulse inhibition and formation of incidental associations) that are similar to the different symptom clusters of the disease. To this end, we administered LPS to pregnant dams and when the offspring reached adolescence, we exposed them to a mild dose of THC to examine their behaviour in adulthood. We also studied several parameters in the dams, including locomotor activity in the open field, elevated plus maze performance and their response to LPS, that could predict symptom severity of the offspring, but found no evidence of any predictive value of these variables. In the adult offspring, MIA was associated with impaired working memory and sensorimotor gating, but surprisingly, it increased sociability, social novelty and sucrose preference. THC, on its own, impaired sociability and social memory, but there were no interactions between MIA and THC exposure. These results suggest that, in this model, THC during adolescence does not trigger or aggravate symptoms related to schizophrenia in rats.

Dietary vitamin A supplementation prevents early obesogenic diet-induced microbiota, neuronal and cognitive alterations

BACKGROUND

Early consumption of obesogenic diets, rich in saturated fat and added sugar, is associated with a plethora of biological dysfunctions, at both peripheral and brain levels. Obesity is also linked to decreased vitamin A bioavailability, an essential molecule for brain plasticity and memory function.

METHODS

Here we investigated in mice whether dietary vitamin A supplementation (VAS) could prevent some of the metabolic, microbiota, neuronal and cognitive alterations induced by obesogenic, high-fat and high-sugar diet (HFSD) exposure from weaning to adulthood, i.e. covering periadolescent period.

RESULTS

As expected, VAS was effective in enhancing peripheral vitamin A levels as well as hippocampal retinoic acid levels, the active metabolite of vitamin A, regardless of the diet. VAS attenuated HFSD-induced excessive weight gain, without affecting metabolic changes, and prevented alterations of gut microbiota α-diversity. In HFSD-fed mice, VAS prevented recognition memory deficits but had no effect on aversive memory enhancement. Interestingly, VAS alleviated both HFSD-induced higher neuronal activation and lower glucocorticoid receptor phosphorylation in the hippocampus after training.

CONCLUSION

Dietary VAS was protective against the deleterious effects of early obesogenic diet consumption on hippocampal function, possibly through modulation of the gut-brain axis.

Assessing Reality Testing in Mice Through Dopamine-Dependent Associatively Evoked Processing of Absent Gustatory Stimuli

Impairments in reality testing are core features of numerous neuropsychiatric conditions. However, relatively few animal models have been developed to assess this critical facet of neuropsychiatric illness, thus impeding our understanding of the underlying central systems and circuits. Using mice in which dominant-negative Disrupted-in-Schizophrenia-1 is expressed throughout central nervous system circuitry (DN-DISC1-PrP), the capacity for an auditory conditioned stimulus (CS) to evoke perceptual processing of an absent sucrose solution was examined. At test, during CS presentations, DN-DISC1-PrP mice consumed more water and displayed a licking profile that is more typically revealed while ingesting a sweet-tasting solution. DN-DISC1-PrP mice also displayed greater c-fos expression in the insular (gustatory) cortex when consuming water in the presence of the CS. This capacity for the CS to more readily substitute for the taste features of the absent sucrose solution in DN-DISC1-PrP mice was attenuated following systemic treatment with the antipsychotic haloperidol. Conversely, social isolation during adolescence promoted the manifestation of these effects. These results provide strong validation for using associative learning procedures to examine dopamine-mediated reality testing associated with insular cortex activation.

CB1 Receptors in the Anterior Piriform Cortex Control Odor Preference Memory

The retrieval of odor-related memories shapes animal behavior. The anterior piriform cortex (aPC) is the largest part of the olfactory cortex, and it plays important roles in olfactory processing and memory. However, it is still unclear whether specific cellular mechanisms in the aPC control olfactory memory, depending on the appetitive or aversive nature of the stimuli involved. Cannabinoid-type 1 (CB1) receptors are present in the aPC (aPC-CB1), but their potential impact on olfactory memory was never explored. Here, we used a combination of behavioral, genetic, anatomical, and electrophysiological approaches to characterize the functions of aPC-CB1 receptors in the regulation of appetitive and aversive olfactory memory. Pharmacological blockade or genetic deletion of aPC-CB1 receptors specifically impaired the retrieval of conditioned odor preference (COP). Interestingly, expression of conditioned odor aversion (COA) was unaffected by local CB1 receptor blockade, indicating that the role of aPC endocannabinoid signaling is selective for retrieval of appetitive memory. Anatomical investigations revealed that CB1 receptors are highly expressed on aPC GABAergic interneurons, and ex vivo electrophysiological recordings showed that their pharmacological activation reduces miniature inhibitory post-synaptic currents (mIPSCs) onto aPC semilunar (SL), but not pyramidal principal neurons. COP retrieval, but not COA, was associated with a specific CB1-receptor-dependent decrease of mIPSCs in SL cells. Altogether, these data indicate that aPC-CB1 receptor-dependent mechanisms physiologically control the retrieval of olfactory memory, depending on odor valence and engaging modulation of local inhibitory transmission.

Hallucinations and Strong Priors

Hallucinations, perceptions in the absence of objectively identifiable stimuli, illustrate the constructive nature of perception. Here, we highlight the role of prior beliefs as a critical elicitor of hallucinations. Recent empirical work from independent laboratories shows strong, overly precise priors can engender hallucinations in healthy subjects and that individuals who hallucinate in the real world are more susceptible to these laboratory phenomena. We consider these observations in light of work demonstrating apparently weak, or imprecise, priors in psychosis. Appreciating the interactions within and between hierarchies of inference can reconcile this apparent disconnect. Data from neural networks, human behavior, and neuroimaging support this contention. This work underlines the continuum from normal to aberrant perception, encouraging a more empathic approach to clinical hallucinations.