Geniculate nucleus

Two AT-II receptors, AT and AT2 are known and show wide distribution (27). The AT receptor has been cloned and predominates ia regions iavolved ia the regulation of blood pressure and water and sodium retention, eg, the aorta, Hver, adrenal cortex, and ia the CNS ia the paraventricular nucleus, area postrema, and nucleus of the soHtary tract. AT2 receptors are found primarily ia the adrenal medulla, utems, and ia the brain ia the locus coeruleus and the medial geniculate nucleus. AT receptors are GCPRs inhibiting adenylate cyclase activity and stimulating phosphoHpases C, A2, and D. AT2 receptors use phosphotyrosiae phosphatase as a transduction system. 

At its most fundamental level, the circadian cycle rests on the influence of so-called clock genes . These genes have been studied most extensively in insects but they have also been found in humans. Their protein products enter the cell nucleus and regulate their own transcription. This feedback process is linked to exposure to light and so it is not surprising that visual inputs are important for maintenance of circadian rhythms. However, it is not the reception of specific visual information, transmitted in the optic nerve to the lateral geniculate nucleus (LGN) and visual cortex (i.e. visual discrimination), that is responsible for the rhythm but the more simple, almost subconscious, reception of light. 

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...

The second cluster of neurons lies more caudally, near the pons, in the pedunculo-pontine (PPT) and laterodorsal tegmental (LDT) nuclei (see Fig. 22.6) and could be regarded as part of the ARAS (see McCormick 1992). It innervates the non-specific thalamic nuclei as well as some more specific ones like the lateral geniculate nucleus (visual pathway), the pontine reticular formation and occipital cortex. Because long... 

Geniculate nucleus The portion of the brain that processes visual information from the optic nerve and relays it to the cerebral cortex. 

Albrecht D., Quaschling U., Zippel U., Davidowa H. (1996). Effects of dopamine on neurons of the lateral geniculate nucleus an iontophoretic study. Synapse. 23, 70-8. 

Govindaiah G., Cox C. (2006). Depression of retinogeniculate synaptic transmission by presynaptic D(2)-like dopamine receptors in rat lateral geniculate nucleus. Eur. J. Neurosci. 23, 423-34. 

C.K. Tong and M. Chesler, Activity-evoked extracellular pH shifts in slices of rat dorsal lateral geniculate nucleus. Brain Res. 815, 373-381 (1999). 

Uhlrich, D. J., Manning, K. A. and Xue, J. T. Effects of activation of the histaminergic tuberomammiUary nucleus on visual responses of neurons in the dorsal lateral geniculate nucleus. /. Neurosci. 22 1098-1107, 2002. 

The mechanisms of flashbacks are probably mixed. Some cases may be similar to post-traumatic stress disorder induced by a bad trip (Paton et al., 1973). Abraham (1983) suggested that some of the visual phenomena, such as trailing and after-images, were due to failure of inhibition in visual pathways, possibly mediated in the lateral geniculate nucleus which (in the macaque monkey) contains on-off colour neurons with receptor fields similar to those described in flashbacks. The neurochemical causes of such flashbacks, which can be very disturbing, remains elusive and attempts at treatment are usually ineffective. 

Upton AL, Ravary A, Salichon N, Moessner R, Lesch KP, Hen R, Seif 1, Gaspar P (2002) Lack of 5-HT(lB) receptor and of serotonin transporter have different effects on the segregation of retinal axons in the lateral geniculate nucleus compared to the superior colliculus. Neuroscience 111 597-610... 

Figure 3. Influence of LSD-25 on synaptic transmission in lateral geniculate body of the cat. The optic nerve is electrically stimulated about 250 times per second, leading to the repetitive complex response seen in A and B. T points to the presynaptic wave arriving in the geniculate nucleus. After a delay of less than one millisecond, the nerve cells are synaptically excited and produce spike S. The deflections occurring prior to wave T are the so-called stimulus artifacts, i.e., the electrically conducted disturbance elicited by the electrical stimulus they have no biological significance. A control record of five responses out of a larger series. B after 15 jjg of LSD-25 had been given via the carotid artery. Note drastic reduction of postsynaptic spike, S, signaling impairment of synaptic transmission. (See Evarts et al., 1955-)...

Figure 2.9 Path from the retina to the visual cortex. On its way to the lateral geniculate nucleus, some axons cross at the optic chiasm. Information from the right half of the visual field is processed inside the left hemisphere of the brain. Information from the left half of the visual field is processed inside the right hemisphere. (Reproduced from Semir Zeki. A Vision of the Brain. Blackwell Science, Oxford, 1993, by permission of Blackwell Science, UK.)...

Figure 2.11 A simple cell responds to lines or edges oriented at a particular orientation. It may be constructed by receiving input from several of the center-surround neurons found in the lateral geniculate nucleus. (Reproduced from D. H. Hubei and T. N. Wiesel. Receptive fields, binocular interaction and functional architecture in the cat s visual cortex. Journal of Physiology, Vol. 160, pp. 106-154, 1962, by permission of Blackwell Publishing, UK.)...

De Valois RL and Pease PL 1971 Contours and contrast responses of monkey lateral geniculate nucleus cells to luminance and color figures. Science 171, 694-696. 

Posterior nucleus Ventral posterior nucleus Medial geniculate nucleus... 

Davidson C, Stamford JA. Serotonin efflux in the rat ventral lateral geniculate nucleus assessed by fast cyclic voltammetry is modulated by 5-HT1B and 5-HTm autoreceptors. Neuropharmacology 1996 35 1627-1634. 

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