Ghrelin-deficient mice have fewer orexin cells and reduced cFOS expression in the mesolimbic dopamine pathway under a restricted feeding paradigm.

Ghrelin is an orexigenic stomach peptide previously found to be important for the full display of anticipatory locomotor activity and hypothalamic neuronal activation that precedes a daily scheduled meal in mice. Ghrelin is also important for food-related motivation and seems to have direct effects in the mesocorticolimbic dopamine reward system. Here we hypothesized that neuronal activation in reward-related areas in anticipation of a scheduled meal could be mediated by elevated ghrelin induced by scheduled feeding, and therefore this would be attenuated in ghrelin receptor knock-out (GHSR KO) animals. We found that this was indeed the case for the ventral tegmental area and the shell, but not the core, of the nucleus accumbens. In addition, our results show a reduction in the proportion of activated orexin-immunoreactive (IR) neurons in GHSR KO animals in anticipation of the scheduled meal in comparison to the proportion of activated orexin neurons in wild type (WT) mice. Interestingly we observed that both GHSR and ghrelin KO mice had fewer orexin-IR cells than their WT littermates suggesting that lack of ghrelin or sensitivity to ghrelin may play a role in the development of the orexin system. Our data also suggest that ghrelin may mediate food anticipation, in part, by stimulating both the orexin system and the mesolimbic reward system.

Reduced anticipatory locomotor responses to scheduled meals in ghrelin receptor deficient mice.

Ghrelin, an orexigenic hormone produced by the stomach, is secreted in anticipation of scheduled meals and in correlation with anticipatory locomotor activity. We hypothesized that ghrelin is directly implicated in stimulating locomotor activity in anticipation of scheduled meals. To test this hypothesis, we observed 24 h patterns of locomotor activity in mice with targeted mutations of the ghrelin receptor gene (GHSR KO) and wild-type littermates, all given access to food for 4 h daily for 14 days. While wild type (WT) and GHSR KO mice produced increases in anticipatory locomotor activity, anticipatory locomotor activity in GHSR KO mice was attenuated (P<0.05). These behavioral measures correlated with attenuated levels of Fos immunoreactivity in a number of hypothalamic nuclei from GHSR KO placed on the same restricted feeding schedule for 7 days and sacrificed at ZT4. Interestingly, seven daily i.p. ghrelin injections mimicked hypothalamic Fos expression patterns to those seen in mice under restricted feeding schedules. These data suggest that ghrelin acts in the hypothalamus to augment locomotor activity in anticipation of scheduled meals.

Daily restricted feeding rescues a rhythm of period2 expression in the arrhythmic suprachiasmatic nucleus.

Second only to light, daily restricted feeding schedules can entrain circadian rhythms in mammals [Neurosci Biobehav Rev 4 (1980) 119; J Biol Rhythms 17 (2002) 284]. Contrary to light, however, such feeding schedules have been found not to affect the master circadian clock in the suprachiasmatic nucleus (SCN) [Genes Dev 14 (2000) 2950; Eur J Neurosci 13 (2001) 1190]. Here, we show that in rats that are arrhythmic as a consequence of prolonged housing in constant light, a daily restricted feeding schedule not only restores behavioral rhythmicity, as previously shown [Physiol Behav 53 (1993) 509], but in addition, induces a rhythm of the clock protein, Period2 in the SCN. These findings challenge the idea that the SCN is invulnerable to feeding schedules and call for a reevaluation of the role of the SCN clock in the circadian effects of such schedules.

Infusion of the dopamine D1 receptor antagonist SCH 23390 into the amygdala blocks fear expression in a potentiated startle paradigm.

Dopamine (DA) D1 receptors are distributed in the nucleus accumbens and the amygdala, two regions of the mesocorticolimbic DA system known to be activated by aversive environmental stimuli. The objective of the present study was to determine the contribution of D1 receptors in these brain regions to the expression of a fear-motivated behavior, notably, potentiated startle in rats. Injection of the DA D1 receptor antagonist SCH 23390 into the amygdala blocked the ability of a conditioned light stimulus previously paired with footshock to enhance acoustic startle amplitudes. Bilateral intracerebral administration of SCH 23390 into the nucleus accumbens had no effect on fear-potentiated startle. The observed opposing effects of amygdaloid DA D1 receptor antagonism on fear expression, along with earlier research demonstrating the involvement of ventral tegmental area (VTA) DA neurons on fear-potentiated startle, suggest a role for mesoamygdaloid activity in conditioned excitatory fear reactions.