Altered perirhinal cortex activity patterns during taste neophobia and their habituation in aged rats

Chronic-Antibiotics Induced Gut Microbiota Dysbiosis Rescues Memory Impairment and Reduces β-Amyloid Aggregation in a Preclinical Alzheimer's Disease Model

Alzheimer's disease (AD) is a multifactorial pathology characterized by β-amyloid (Aβ) deposits, Tau hyperphosphorylation, neuroinflammatory response, and cognitive deficit. Changes in the bacterial gut microbiota (BGM) have been reported as a possible etiological factor of AD. We assessed in offspring (F1) 3xTg, the effect of BGM dysbiosisdysbiosis in mothers (F0) at gestation and F1 from lactation up to the age of 5 months on Aβ and Tau levels in the hippocampus, as well as on spatial memory at the early symptomatic stage of AD. We found that BGM dysbiosisdysbiosis with antibiotics (Abx) treatment in F0 was vertically transferred to their F1 3xTg mice, as observed on postnatal day (PD) 30 and 150. On PD150, we observed a delay in spatial memory impairment and Aβ deposits, but not in Tau and pTau protein in the hippocampus at the early symptomatic stage of AD. These effects are correlated with relative abundance of bacteria and alpha diversity, and are specific to bacterial consortia. Our results suggest that this specific BGM could reduce neuroinflammatory responses related to cerebral amyloidosis and cognitive deficit and activate metabolic pathways associated with the biosynthesis of triggering or protective molecules for AD.

Effect of hippocampal 6-OHDA lesions on the contextual modulation of taste recognition memory

Taste recognition memory is evident in rodents because the initial neophobia to novel tastes attenuates across exposures as the taste becomes familiar and safe. This attenuation of taste neophobia (AN) is context-dependent and an auditory background change could induce the recovery of the neophobic response. The AN auditory context-dependency requires the hippocampal integrity but the neurochemical mechanisms underlying the interaction with the taste memory circuit remain unexplored. We have applied pharmacological intervention by 6-hidroxydopamine (6-OHDA) hippocampal lesion for assessing the role of catecholamines in the hippocampal system to Wistar rats that drank a novel 3% vinegar solution for several consecutive days. Additionally, we manipulated the auditory background as a context that could either change or remain constant across all the drinking sessions. We found that a disruption of the context-dependent AN was induced by intracerebral administration of 6-OHDA targeted to the ventral CA1 hippocampus (vCA1). We conclude that the ability of the auditory context to modulate taste recognition memory involves the catecholaminergic activity in the ventral hippocampal circuit for the proper acquisition of safe taste memory.

Prefrontal cortex activity patterns during taste neophobia habituation in adult and aged rats

Age-related memory decline has been associated with changes in the medial prefrontal cortex (mPFC) function. In order to explore the role of mPFC in taste recognition memory, we have assessed mPFC c-Fos immunoreactivity in adult (5-month-old) and aged (24-month-old) male Wistar rats during the first (Novel), second (Familiar I), and sixth (Familiar II) exposure to a cider vinegar solution. Adult brains showed higher c-Fos expression in the ventral but not the dorsal region of mPFC during the second taste exposure. Interestingly, old brains exhibited an altered activity pattern selectively in the dorsal peduncular cortex (DP) which can be associated with a delayed attenuation of vinegar neophobia in this group. These results support the involvement of this area in the formation of safe taste memory. Further research is needed for understanding the role of DP in taste recognition memory and the impact of aging on it.

Accumbens and amygdala in taste recognition memory: The role of d1 dopamine receptors

The attenuation of taste neophobia (AN) is a good model for studying the structural and neurochemical mechanisms of the emotional component of memory because taste recognition memory exhibits the unique feature of being necessarily linked to hedonic properties. Whilst novel tastes elicit cautious neophobic responses, taste exposures which are not followed by aversive consequences attenuate neophobia as the taste becomes safe and palatable. Given the involvement of the nucleus accumbens in reward and of the amygdala in emotional memories, we applied c-Fos immunohistochemistry as an index of neural activity in Wistar rats that were exposed to a vinegar solution for one, two or six days. An inverse pattern of accumbens nucleus vs amygdala activity was found on the second exposure day on which AN occurred. The number of c-Fos positive cells in the nucleus accumbens shell increased whilst the number of c-Fos positive cells in the basolateral amygdala decreased. Further analyses revealed a positive correlation between AN and the number of c-Fos positive cells in the accumbens shell but a negative correlation in the basolateral amygdala. Furthermore the accumbens-amygdala interplay relevant for AN seems to be mediated by dopamine D1 receptors (D1DR). The injection of SCH23390 (D1DR antagonist) in both the accumbens shell and the basolateral amygdala on the second taste exposure resulted in selectively impaired AN but had opposite long term effects. This finding supports the relevance of a dopaminergic network mediated by D1DRs in the nucleus accumbens shell and basolateral amygdala which is critical for adding the emotional component during the formation of taste memory.

Dorsal hippocampal damage disrupts the auditory context-dependent attenuation of taste neophobia in mice

Rodents exhibit neophobia for novel tastes, demonstrated by an initial reluctance to drink novel-tasting, potentially-aversive solutions. Taste neophobia attenuates across days if the solution is not aversive, demonstrated by increased consumption as the solution becomes familiar. This attenuation of taste neophobia is context dependent, which has been demonstrated by maintained reluctance to drink the novel tasting solution if the subject has to drink it after being brought to a novel environment. This spatial context-dependent attenuation of taste neophobia has been described and likely depends on the integrity of the dorsal hippocampus because this brain area is crucial for representing space and spatial context associations, but is unnecessary for processing taste memories per se. Whether changing the non-spatial auditory context causes a similar effect on attenuation of taste neophobia and the potential role of the dorsal hippocampus in processing this decidedly non-spatial information has not been determined. Here we demonstrate that changing the non-spatial auditory context affects the attenuation of taste neophobia in mice, and investigate the consequence of hippocampal lesion. The results demonstrate that the non-spatial auditory context-dependent attenuation of taste neophobia in mice is lost following NMDA excitotoxic lesions of the CA1 region of the dorsal hippocampus. These findings demonstrate that the dorsal hippocampus is crucial for the modulation non-associative taste learning by auditory context, neither of which provide information about space.

Flavor recognition memory related activity of the posterior piriform cortex in adult and aged rats

The relationship between the piriform cortex and flavor recognition memory was investigated in adult and aged rats. By using c-Fos immunohistochemistry, we assessed the piriform cortex activity induced by flavor familiarity. The results indicated increased activity in the rostral region of the posterior piriform cortex elicited by the most familiar cider vinegar solution after six exposures. Aged rats exhibited overall increased activity in the posterior, but not the anterior piriform cortex, which was not related to flavor familiarity. This suggests that the posterior piriform cortex is related to flavor recognition memory and that aging modifies its activity pattern which might underlie their slower attenuation of flavor neophobia.

Insights on consciousness from taste memory research

Taste research in rodents supports the relevance of memory in order to determine the content of consciousness by modifying both taste perception and later action. Associated with this issue is the fact that taste and visual modalities share anatomical circuits traditionally related to conscious memory. This challenges the view of taste memory as a type of non-declarative unconscious memory.

Shared Functions of Perirhinal and Parahippocampal Cortices: Implications for Cognitive Aging

A predominant view of perirhinal cortex (PRC) and postrhinal/parahippocampal cortex (POR/PHC) function contends that these structures are tuned to represent objects and spatial information, respectively. However, known anatomical connectivity, together with recent electrophysiological, neuroimaging, and lesion data, indicate that both brain areas participate in spatial and nonspatial processing. Instead of content-based organization, the PRC and PHC/POR may participate in two computationally distinct cortical-hippocampal networks: one network that is tuned to process coarse information quickly, forming gist-like representations of scenes/environments, and a second network tuned to process information about the specific sensory details that are necessary for discrimination across sensory modalities. The available data suggest that the latter network may be more vulnerable in advanced age.