Hypothalamus Suprachiasmatic nucleus

The effects of VIP and PACAP are mediated by three GPCR subtypes, VIP, VIP2, and PACAP receptor, coupled to the activation of adenjiate cyclase (54). The VIP subtype is localized ia the lung, Hver, and iatestiae, and the cortex, hippocampus, and olfactory bulb ia the CNS. The VIP2 receptor is most abundant ia the CNS, ia particular ia the thalamus, hippocampus, hypothalamus, and suprachiasmatic nucleus. PACAP receptors have a wide distribution ia the CNS with highest levels ia the olfactory bulb, the dentate gyms, and the cerebellum (84). The receptor is also present ia the pituitary. The VIP and PACAP receptors have been cloned. 

Figure 22.1 Pathways projecting to and from the suprachiasmatic nucleus (SCN). Inputs from photoreceptors in the retina help to reset the circadian clock in response to changes in the light cycle. Other inputs derive from the lateral geniculate complex and the serotonergic, Raphe nuclei and help to reset the SCN in response to non-photic stimuli. Neurons in the SCN project to the hypothalamus, which has a key role in the regulation of the reproductive cycle, mood and the sleep-waking cycle. These neurons also project to the pineal gland which shows rhythmic changes in the rate of synthesis and release of the hormone, melatonin...

Figure 22.3 Possible links in the induction of circadian rhythm between daylight, the suprachiasmatic nucleus and melatonin release from the pineal gland. Some fibres in the optic nerve, projecting from the eye to the lateral geniculate nucleus (LGN) in the thalamus, innervate the suprachiasmatic nucleus (SCN) in the anterior hypothalamus, via the retinohypothalamic tract (RHT). Others project to the SCN from the LGN in the geniculohypothalamic tract (GHT). The release of melatonin into the circulation from the pineal gland (PG) is maximal at night and appears to be controlled partly by noradrenaline released from sympathetic nerves originating in the superior cervical ganglion (SCG). Melatonin receptors are found in the SCN, the removal of which dampens melatonin secretion...

Which of these metaphorical roles does the suprachiasmatic nucleus of the hypothalamus (SCN) play as it influences rhythms in the rest of the body The answer hinges upon the interpretation of the effects of SCN lesions and on the rhythmic behaviour of tissues and organs isolated from SCN influences. 

The circadian clock driving locomotor activity and other circadian behaviours, such as the sleep—wake cycle, is located within the suprachiasmatic nucleus (SCN) of the hypothalamus (Klein et al 1991). 

The 5-HT7 receptors are also positively coupled to adenylate cyclase. Since their discovery in 1993 (229-233), they have been identified in numerous mammalian species, including human. Different splice variants have been detected in the rat compared to the human, but do not seem to differ in functional properties (234-236). Early and recent in situ hybridization studies in the guinea pig and rat have demonstrated 5-HT7 mRNAs to be most abundant in the tenia tecta and piriform cortex, hippocampus, thalamus, and hypothalamus, including the suprachiasmatic nucleus (230,237-239). 

Both early and more recent studies have used in situ hybridization to determine with increasing resolution where the 5-HT7 receptor gene is expressed (4,51-53). The studies consistently show that 5-HT7 receptor mRNA is most abundantly present in the thalamus, hippocampus, and hypothalamus. It should especially be noted that receptor mRNA is present in all of the CA fields of the hippocampus and in the suprachiasmatic nucleus (SCN) of the hypothalamus. In situ hybridization has also shown that, exclusively within the ventral CA3 region of the rat hippocampus, the 5-HT7 mRNA levels decrease by 30% between young and middle-aged animals, with no further decrease between middle and old age (54). 

Di receptor mRNA is most abundant in the olfactory tubercle, NAc and CPu. Cells expressing Di receptor mRNA were also found in the neocortex, lateral septal nuclei, amygdala, hypothalamus (and in particular in the suprachiasmatic nucleus), in the thalamus and in the retina. Di receptor mRNA was instead absent in the VTA, GP, SNr, SNc and EP (Mansour et al., 1990, 1992 Fremeau et al., 1991 Le Moine et al., 1991 Meador-Woodroff et al., 1991b Mengod et al., 1991 Weiner et al., 1991 Gaspar et al., 1995). 

Preoptic region and hypothalamus Lateral preoptic area Medial preoptic area Medial preoptic nucleus Magnocellular preoptic nucleus Suprachiasmatic nucleus Supraoptic nucleus Anterior hypothalamic area Lateral hypothalamic area Periventricular nucleus Paraventricular hypothalamic nucleus Arcuate nucleus Dorsal hypothalamic area Ventromedial hypothalamic nucleus Dorsomedial hypothalamic nucleus Compact part Tuber cinereum Posterior hypothalamic area Premammillary nucleus Supramammillary nucleus Medial mammillary nucleus Lateral mammillary nucleus... 

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