Social interaction masking contributes to changes in the activity of the suprachiasmatic nucleus and impacts on circadian rhythms.

Light is the most powerful temporal cue that entrains physiology and behavior through modulation of the suprachiasmatic nucleus (SCN) of the hypothalamus. However, on a daily basis, individuals face a combination of light and several non-photic cues, such as social interaction. In order to investigate whether SCN activity and SCN-driven rhythms are altered by social interaction, adult male C57BLJ/6 mice were maintained in groups of 3-4 animals per cage or 1 animal per cage (socially isolated) under 12:12 h / light:dark (LD) cycles or constant darkness (DD). Analysis of the two anatomical subdivisions (ventral, v and dorsal, d) of the medial SCN revealed an effect of housing conditions on the d-SCN but not on the v-SCN on the number of c-Fos immunoreactive (ir) neurons. As such, 2 h after the light-phase onset d-SCN c-Fos-ir number was lower in single-housed mice under LD. Importantly, under DD there were no effect of housing conditions in the number of c-Fos-ir SCN neurons. Social isolation increased the amplitude and strength of SCN-driven rhythm of body temperature (Tc) entrained to LD and it advanced its onset, uncoupling with spontaneous locomotor activity (SLA) rhythm, without altering endogenous Tc and SLA rhythms expressed under DD. Associated with reduced Tc in the light phase, single-housed mice showed reduced body weight. However, these phenotypes were not accompanied by changes in the number of c-Fos-ir neurons in the preoptic area (POA), which are known to regulate energy metabolism and Tc. Altogether, these results imply that the social interaction masking effect on the d-SCN is added to that of light stimulus, in order to achieve full c-Fos expression in the SCN, which, in turn seems to be required to maintain daily-phase coherence between the photo-entrained rhythms of Tc and SLA. There might be an inter-relationship between masking (social interaction) and entrainment stimulus (light) that impacts the circadian parameters of the photo-entrained Tc rhythm. As such, in the absence of social interactions a more robust Tc rhythm is shown. This inter-relationship seems to occur in the dorsal subdivision of the SCN but not in the POA.

Vesicular acetylcholine transporter knock down-mice are more susceptible to inflammation, c-Fos expression and sickness behavior induced by lipopolysaccharide.

In addition to the well-known functions as a neurotransmitter, acetylcholine (ACh) can modulate of the immune system. Nonetheless, how endogenous ACh release inflammatory responses is still not clear. To address this question, we took advantage of an animal model with a decreased ACh release due a reduction (knockdown) in vesicular acetylcholine transporter (VAChT) expression (VAChT-KD(HOM)). These animals were challenged with lipopolysaccharide (LPS). Afterwards, we evaluated sickness behavior and quantified systemic and cerebral inflammation as well as neuronal activation in the dorsal vagal complex (DVC). VAChT-KD(HOM) mice that were injected with LPS (10mg/kg) showed increased mortality rate as compared to control mice. In line with this result, a low dose of LPS (0.1mg/kg) increased the levels of pro-inflammatory (TNF-α, IL-1ß, and IL-6) and anti-inflammatory (IL-10) cytokines in the spleen and brain of VAChT-KD(HOM) mice in comparison with controls. Similarly, serum levels of TNF-α and IL-6 were increased in VAChT-KD(HOM) mice. This excessive cytokine production was completely prevented by administration of a nicotinic receptor agonist (0.4mg/kg) prior to the LPS injection. Three hours after the LPS injection, c-Fos expression increased in the DVC region of VAChT-KD(HOM) mice compared to controls. In addition, VAChT-KD(HOM) mice showed behavioral changes such as lowered locomotor and exploratory activity and reduced social interaction after the LPS challenge, when compared to control mice. Taken together, our results show that the decreased ability to release ACh exacerbates systemic and cerebral inflammation and promotes neural activation and behavioral changes induced by LPS. In conclusion, our findings support the notion that activity of cholinergic pathways, which can be modulated by VAChT expression, controls inflammatory and neural responses to LPS challenge.