Differential activity pattern of c-Fos in the nucleus accumbens between adult and aged rats during flavor recognition memory

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

Increased basolateral amygdala metabolic activity during flavor familiarization: an experimental study

BACKGROUND

Novel flavors elicit a cautious neophobic response which is attenuated as the flavor becomes familiar and safe. The attenuation of neophobia reveals the formation of a safe memory. Previous lesion studies in rats have reported that basolateral amygdala integrity is required for taste neophobia, but not neophobia to flavor, i.e., taste linked to an odorous component. Accordingly, immunohistochemical analyses show that novel tastes induced higher basolateral amygdala activity when compared to familiar ones. However, a different role of basolateral amygdala in flavor attenuation of neophobia is suggested by lesion studies using a vinegar solution. Studies assessing basolateral amygdala activity during flavor attenuation of neophobia are lacking. Thus, we quantified cytochrome oxidase as an index of basolateral amygdala activity along the first and second vinegar exposures in order to assess flavor neophobia and attenuation of neophobia.

METHODS

We exposed adult male Wistar rats either once or twice to a 3% cider vinegar solution or water, and compared the basolateral amygdala, piriform cortex and caudate putamen brain metabolic activity using cytochrome c-oxidase histochemistry.

RESULTS

We found increased flavor intake and cytochrome c-oxidase histochemistry activity during the second exposure in basolateral amygdala, but not in the piriform cortex and caudate/putamen.

CONCLUSIONS

The main finding of the study is that BLA metabolic activity was higher in the group exposed to a familiar vinegar solution than in the groups exposed to either water or a novel vinegar solution.

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.

Sleep, brain vascular health and ageing

Sleep maintains the function of the entire body through homeostasis. Chronic sleep deprivation (CSD) is a prime health concern in the modern world. Previous reports have shown that CSD has profound negative effects on brain vasculature at both the cellular and molecular levels, and that this is a major cause of cognitive dysfunction and early vascular ageing. However, correlations among sleep deprivation (SD), brain vascular changes and ageing have barely been looked into. This review attempts to correlate the alterations in the levels of major neurotransmitters (acetylcholine, adrenaline, GABA and glutamate) and signalling molecules (Sirt1, PGC1α, FOXO, P66^shc, PARP1) in SD and changes in brain vasculature, cognitive dysfunction and early ageing. It also aims to connect SD-induced loss in the number of dendritic spines and their effects on alterations in synaptic plasticity, cognitive disabilities and early vascular ageing based on data available in scientific literature. To the best of our knowledge, this is the first article providing a pathophysiological basis to link SD to brain vascular ageing.