Electrophysiological and neuroanatomical evidence for reciprocal connections with the medial prefrontal cortex (mPFC) and the hippocampus make the reuniens and rhomboid (ReRh) thalamic nuclei a putatively major functional link for regulations of corticohippocampal interactions. In a first experiment using a new water escape device for rodents, the double-H maze, we demonstrated in rats that a bilateral muscimol (MSCI) inactivation (0.70 vs 0.26 and 0 nmol) of the mPFC or dorsal hippocampus (dHip) induces major deficits in a strategy shifting/spatial memory retrieval task. By way of comparison, only dHip inactivation impaired recall in a classical spatial memory task in the Morris water maze. In the second experiment, we showed that ReRh inactivation using 0.70 nmol of MSCI, which reduced performance without obliterating memory retrieval in the water maze, produces an as large strategy shifting/memory retrieval deficit as mPFC or dHip inactivation in the double-H maze. Thus, behavioral adaptations to task contingency modifications requiring a shift toward the use of a memory for place might operate in a distributed circuit encompassing the mPFC (as the potential set-shifting structure), the hippocampus (as the spatial memory substrate), and the ventral midline thalamus, and therein the ReRh (as the coordinator of this processing). The results of the current experiments provide a significant extension of our understanding of the involvement of ventral midline thalamic nuclei in cognitive processes: they point to a role of the ReRh in strategy shifting in a memory task requiring cortical and hippocampal functions and further elucidate the functional system underlying behavioral flexibility.

Restricted feeding during the resting period causes pronounced shifts in a number of peripheral clocks, but not the central clock in the suprachiasmatic nucleus (SCN). By contrast, daily caloric restriction impacts also the light-entrained SCN clock, as indicated by shifted oscillations of clock (PER1) and clock-controlled (vasopressin) proteins. To determine if these SCN changes are due to the metabolic or timing cues of the restricted feeding, mice were challenged with an ultradian 6-meals schedule (1 food access every 4 h) to abolish the daily periodicity of feeding. Mice fed with ultradian feeding that lost <10% body mass (i.e. isocaloric) displayed 1.5-h phase-advance of body temperature rhythm, but remained mostly nocturnal, together with up-regulated vasopressin and down-regulated PER1 and PER2 levels in the SCN. Hepatic expression of clock genes (Per2, Rev-erbα, and Clock) and Fgf21 was, respectively, phase-advanced and up-regulated by ultradian feeding. Mice fed with ultradian feeding that lost >10% body mass (i.e. hypocaloric) became more diurnal, hypothermic in late night, and displayed larger (3.5 h) advance of body temperature rhythm, more reduced PER1 expression in the SCN, and further modified gene expression in the liver (e.g. larger phase-advance of Per2 and up-regulated levels of Pgc-1α). While glucose rhythmicity was lost under ultradian feeding, the phase of daily rhythms in liver glycogen and plasma corticosterone (albeit increased in amplitude) remained unchanged. In conclusion, the additional impact of hypocaloric conditions on the SCN are mainly due to the metabolic and not the timing effects of restricted daytime feeding.

Summary Background Melatonin is a hormone synthesized mainly by the pineal gland, and secreted only at night. Melatonin has been proposed as a modulator of glucose metabolism. Methods Here we studied the metabolic effects of melatonin administration alone (s.c. 10 mg/kg) or in combination with metformin (p.o. 300 mg/kg), a widely used anti‐diabetic drug. These treatments were tested on glucose tolerance, insulin sensitivity and food intake in Zucker fatty rats (i.e., bearing a missense mutation in the leptin receptor gene) and high‐fat fed Sprague‐Dawley rats. Results Melatonin alone or in combination did not significantly modify glucose tolerance in either model. Melatonin alone in high‐fat fed Sprague‐Dawley improved insulin sensitivity to the level of metformin. In addition, combined treatment further ameliorated insulin sensitivity (+13%), especially during the late phase of rising glycemia. The lack of similar effects in Zucker rats suggests an involvement of leptin signaling in mediating the positive effects of melatonin. Body mass gain in Sprague‐Dawley rats was decreased by both metformin, and combined metformin and melatonin. While melatonin alone did not markedly affect food intake, its combination with metformin led to a more pronounced anorexia (‐17% food intake during the last week), as compared to metformin alone. Conclusions Melatonin improves the beneficial effects of metformin on insulin sensitivity and body mass gain in high‐fat fed Sprague‐Dawley rats. Therefore, the combination of melatonin and metformin could be beneficial to develop dual therapies to treat or delay type 2 diabetes associated with obesity.