The neuropeptides pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are implicated in the photic entrainment of circadian rhythms in the suprachiasmatic nuclei (SCN). We now report that mice carrying a null mutation of the VPAC(2) receptor for VIP and PACAP (Vipr2(-/-)) are incapable of sustaining normal circadian rhythms of rest/activity behavior. These mice also fail to exhibit circadian expression of the core clock genes mPer1, mPer2, and mCry1 and the clock-controlled gene arginine vasopressin (AVP) in the SCN. Moreover, the mutants fail to show acute induction of mPer1 and mPer2 by nocturnal illumination. This study highlights the role of intercellular neuropeptidergic signaling in maintenance of circadian function within the SCN.

In humans, nicotine has been shown to improve attention in both normal and impaired individuals. Observations in rats reflect some, but not all aspects of the nicotine-induced improvements in humans. To date these findings have not been replicated in mice. To examine the effect of nicotine on sustained attention in mice, we have established a version of the 5-choice serial reaction-time (5-CSR) task with graded levels of difficulty, based upon spatial displacement and a variable intertrial interval. Using this paradigm, microgram doses of nicotine produced a consistent reduction in the level of omissions and an improvement in proportion correct in normal mice. This improvement in sustained attention was made irrespectively of whether mice had previously received nicotine. In an attempt to elucidate which nicotinic acetylcholine receptor (nAChR) subtype(s) mediate this effect, we examined the performance of a7 nAChR knockout (KO) mice in the 5-CSR task. a7 nAChR KO mice not only acquired the task more slowly than their wild-type littermates, but on attaining asymptotic performance, they exhibited a higher level of omissions. In conclusion, by increasing the level of task difficulty, the performance of mice was maintained at sufficiently low levels to allow a demonstrable improvement in performance upon nicotine administration. Furthermore, as a7 KO mice are clearly impaired in the acquisition and asymptotic performance of this task, the a7 nAChR may be involved in mediating these effects of nicotine.

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The neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) belong to a superfamily of structurally related peptide hormones that includes glucagon, glucagon-like peptides, secretin, and growth hormonereleasing hormone. Microinjection of VIP or PACAP into the rodent suprachiasmatic nucleus (SCN) phase shifts the circadian pacemaker and VIP antagonists, and antisense oligodeoxynucleotides have been shown to disrupt circadian function. VIP and PACAP have equal potency as agonists of the VPAC 2 receptor (VPAC2R), which is expressed abundantly in the SCN, in a circadian manner. To determine whether manipulating the level of expression of the VPAC 2R can influence the control of the circadian clock, we have created transgenic mice overexpressing the human VPAC 2R gene from a yeast artificial chromosome (YAC) construct. The YAC was modified by a strategy using homologous recombination to introduce (i) the HA epitope tag sequence (from influenza virus hemagglutinin) at the carboxyl terminus of the VPAC 2R protein, (ii) the lacZ reporter gene, and (iii) a conditional centromere, enabling YAC DNA to be amplified in culture in the presence of galactose. High levels of lacZ expression were detected in the SCN, habenula, pancreas, and testis of the transgenic mice, with lower levels in the olfactory bulb and various hypothalamic areas. Transgenic mice resynchronized more quickly than wild-type controls to an advance of 8 h in the light-dark (LD) cycle and exhibited a significantly shorter circadian period in constant darkness (DD). These data suggest that the VPAC 2R can influence the rhythmicity and photic entrainment of the circadian clock. E vidence is accumulating to suggest that the neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) may play important roles in the control of the circadian clock in the suprachiasmatic nucleus (SCN). VIP is synthesized in cells of the retinorecipient region of the rodent SCN (1, 2), at least some of which may have a direct retinal input (3). These cells form synapses with vasopressin-containing cells in the dorsomedial SCN as well as contacts with other VIP cells (4). There is diurnal variation of VIP immunoreactivity (5) and prepro VIP mRNA (6, 7) in the SCN. Exogenous VIP can mimic the phase-advancing and -delaying effects of light pulses (8, 9). VIP antagonists (10) and VIP antisense oligodeoxynucleotides (11) have been shown to disrupt circadian function. Elevated SCN levels of VIP mRNA have been found in two mutant rodent strains [the tau mutant hamster (12) and the spontaneously hypertensive (SHR) rat (13)] that exhibit a significantly shorter free-running period in constant darkness (DD), consistent with an involvement of VIP in establishing the free-running period of the circadian clock.In the SCN, PACAP originates almost exclusively from a subpopulation of glutamate-containing retinal ganglion cells (14, 15) that terminate in the retinorecipient region of the SC...