Vasoactive intestinal peptide and the mammalian circadian system

Abstract

In mammals, the circadian oscillators that drive daily behavioral and endocrine rhythms are located in the hypothalamic suprachiasmatic nucleus (SCN). While the SCN is anatomically well-situated to receive and transmit temporal cues to the rest of the brain and periphery, there are many holes in our understanding of how this temporal regulation occurs. Unanswered questions include how cell autonomous circadian oscillations within the SCN remain synchronized to each other as well as communicate temporal information to downstream targets. In recent years, it has become clear that neuropeptides are critically involved in circadian timekeeping. One such neuropeptide, vasoactive intestinal peptide (VIP), defines a cell population within the SCN and is likely used as a signaling molecule by these neurons. Converging lines of evidence suggest that the loss of VIP or its receptor has a major influence on the ability of the SCN neurons to generate circadian oscillations as well as synchronize these cellular oscillations. VIP, acting through the VPAC(2) receptor, exerts these effects in the SCN by activating intracellular signaling pathways and, consequently, modulating synaptic transmission and intrinsic membrane currents. Anatomical evidence suggests that these VIP expressing neurons connect both directly and indirectly to endocrine and other output targets. Striking similarities exist between the role of VIP in mammals and the role of Pigment Dispersing Factor (PDF), a functionally related neuropeptide, in the Drosophila circadian system. Work in both mammals and insects suggests that further research into neuropeptide function is necessary to understand how circadian oscillators work as a coordinated system to impose a temporal structure on physiological processes within the organism.

Figure 1

illustrates a simplified model of a circuit within the SCN. Light signals are detected and transmitted through neurons in the retina directly to the SCN via the retinohypothalamic tract (RHT). These neurons use glutamate (Glut) and PACAP as transmitters. The neurons in the ventral region of the SCN receive most of this retinal information along with information from the raphe nuclei, carried by serotonin (5HT), and the intergeniculate leaflet of the thalamus (IGL), carried by neuropeptide Y (NPY). These ventral neurons, in turn, use GABA and VIP to communicate with other neurons within the different SCN cell populations, including the vasopressin (VP) expressing neurons of the dorsal SCN. Neural outputs from the SCN can arise from the ventral or dorsal cell populations, but most projections are to a few key relay nuclei within the hypothalamus. While there are a few reported direct connections from the SCN to some neuroendocrine outputs, strong evidence exists for an indirect endocrine output pathway that utilizes these hypothalamic relay nuclei and in some cases, the autonomic nervous system as well. 3V, third ventricle; OC, optic chiasm; CRH, corticotrophin-releasing hormone; TRH, thyrotropin-releasing hormone; GnRH, gonadotropin releasing hormone; DMH, dorsomedial nucleus of the hypothalamus; SPZ, subparaventricular zone; PVN, paraventricular nucleus of the hypothalamus; MPOA, medial preoptic area; PNS, parasympathetic nervous system; SNS, sympathetic nervous system.

Figure 2

illustrates the putative deficits in the SCN of VIP-deficient mice. A) Based on the published results with Vipr2 −/− mice (Harmar et al, 2002) as well as our own unpublished data, we expect that the rhythms in extracellularly recorded multi-unit activity (MUA) and clock gene expression will be reduced in the mutant mice. B) In this schematic, we seek to illustrate two explanations for the loss of rhythms in the SCN cell population of mutant (solid lines) compared to WT (dashed lines) mice. The sine waves represent rhythmic cells while the straight line represents arrhythmic cells. One possibility is that the single cell oscillators may lose their ability to generate oscillations (proposed by Harmar et al., 2002). Alternatively, the single cell oscillators may lose their synchrony (proposed by Colwell et al., 2003). There is evidence that suggests that both factors are at work in the Vip −/− and Vipr2 −/− mice (Aton et al., 2005; Maywood et al., 2006; Brown et al., 2007).

Figure 3

illustrates the key behavioral deficits exhibited by the VIP-deficient mice (see Colwell et al., 2003). The black bars represent the daily wheel running activity with successive days plotted one on top of the other. WT mice (left panel) exhibit stable activity patterns in an LD cycle. When released into DD on day 8, the mice exhibit a period of ≈23.5 hrs. Exposure to a brief light pulse at CT 16 (asterisk) will reset the phase of the activity cycle. VIP KO mice (right panel) in a LD cycle are entrained though activity onset is variable compared to controls. When placed into DD, the mice start their activity from a phase 8 to 10 hrs earlier compared to WT. The period of the activity rhythm is ≈22.5 hrs. Many, but not all of the Vip −/− mice, eventually become arrhythmic in DD. The Vipr2 −/− mice exhibit a similar phenotype (see Harmar et al., 2002). Furthermore, VIP KO mice fail to shift their behavioral rhythms in response to a phase-delaying light pulse during early subjective night, as indicated by the yellow asterisk.

Figure 4

illustrates the possible mechanisms that underlie VIP’s actions in SCN neurons. In all cases, we expect that VIP would activate VPAC₂R to stimulate the AC/PKA signaling pathway. This VIP-activated cascade has at least three actions within the SCN including modulation of intrinsic voltage gated channels, presynaptic regulation of GABA release, and alterations in transcription via CREB.