Inhibition of PACAP/PAC1/VPAC2 signaling impairs the consolidation of social recognition memory and nitric oxide prevents this deficit

Mismatch novelty exploration training shifts VPAC1 receptor-mediated modulation of hippocampal synaptic plasticity by endogenous VIP in male rats

Novelty influences hippocampal-dependent memory through metaplasticity. Mismatch novelty detection activates the human hippocampal CA1 area and enhances rat hippocampal-dependent learning and exploration. Remarkably, mismatch novelty training (NT) also enhances rodent hippocampal synaptic plasticity while inhibition of VIP interneurons promotes rodent exploration. Since VIP, acting on VPAC1 receptors (Rs), restrains hippocampal LTP and depotentiation by modulating disinhibition, we now investigated the impact of NT on VPAC1 modulation of hippocampal synaptic plasticity in male Wistar rats. NT enhanced both CA1 hippocampal LTP and depotentiation unlike exploring an empty holeboard (HT) or a fixed configuration of objects (FT). Blocking VIP VPAC1Rs with PG 97269 (100 nM) enhanced both LTP and depotentiation in naïve animals, but this effect was less effective in NT rats. Altered endogenous VIP modulation of LTP was absent in animals exposed to the empty environment (HT). HT and FT animals showed mildly enhanced synaptic VPAC1R levels, but neither VIP nor VPAC1R levels were altered in NT animals. Conversely, NT enhanced the GluA1/GluA2 AMPAR ratio and gephyrin synaptic content but not PSD-95 excitatory synaptic marker. In conclusion, NT influences hippocampal synaptic plasticity by reshaping brain circuits modulating disinhibition and its control by VIP-expressing hippocampal interneurons while upregulation of VIP VPAC1Rs is associated with the maintenance of VIP control of LTP in FT and HT animals. This suggests VIP receptor ligands may be relevant to co-adjuvate cognitive recovery therapies in aging or epilepsy, where LTP/LTD imbalance occurs.

Enhancing long-term memory through strength training: An experimental study in adult and middle-aged rats

The study aimed to explore the impact of strength training on long-term memory in adult and middle-aged rodents, specifically male Wistar rats aged 9 and 20 months. These rats were divided into two groups: one sedentary (SED) and the other trained (ST) for a period of 12 weeks. The strength training involved squatting exercises using adapted equipment, while the sedentary group maintained their regular, non-exercised routine. Behavioral tasks assessing mobility, anxiety, and multiple facets of memory, such as object recognition memory (ORM), social recognition memory (SRM), and object location memory (OLM), were conducted post-training. The findings were promising, revealing a generally beneficial impact of strength training on memory tasks across both age groups. Specifically, the ORM tasks showed facilitated and improved learning in both adult and middle-aged rats that underwent training. In contrast, OLM displayed only a facilitatory effect in both age groups, meaning that while the trained rats learned the task, they did not outperform the sedentary group. For SRM, a facilitatory effect was observed only in the adult group. In addition to the cognitive benefits, strength training was found to have an anxiolytic effect in the 9-month-old rats and positively affected body mass and adipose tissue composition. Notably, the study correlated the strength gains from the training with improved performance in memory tasks. These outcomes provide crucial insights into the potential of exercise-based interventions to bolster cognitive health and mitigate age-related cognitive decline.

Participation of Hippocampal 5-HT5A, 5-HT6 and 5-HT7 Serotonin Receptors on the Consolidation of Social Recognition Memory

Social recognition is the ability of animals to identify and recognize a conspecific. The consolidation of social stimuli in long-term memory is crucial for the establishment and maintenance of social groups, reproduction and species survival. Despite its importance, little is known about the circuitry and molecular mechanisms involved in the social recognition memory (SRM). Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator, which plays a key role in learning and memory. Focusing on the more recently described 5-HT receptors, we investigated in the CA1 region of the dorsal hippocampus the participation of 5-HT_5A, 5-HT₆ and 5-HT₇ receptors in the consolidation of SRM. Male Wistar rats cannulated in CA1 were subjected to a social discrimination task. In the sample phase the animals were exposed to a juvenile conspecific for 1 h. Immediately after, they received different pharmacological treatments. Twenty-four hours later, they were submitted to a 5 min retention test in the presence of the previously presented juvenile (familiar) and a novel juvenile. The animals that received infusions of 5-HT_5A receptor antagonist SB-699551 (10 µg/µL), 5-HT₆ receptor agonist WAY-208466 (0.63 µg/µL) or 5-HT₇ receptor agonist AS-19 (5 µg/µL) intra-CA1 were unable to recognize the familiar juvenile. This effect was blocked by the coinfusion of WAY-208466 plus 5-HT₆ receptor antagonist SB-271046 (10 µg/µL) or AS-19 plus 5-HT₇ receptor antagonist SB-269970 (5 µg/µL). The present study helps to clarify the neurobiological functions of the 5-HT receptors more recently described and extends our knowledge about mechanisms underlying the SRM.