Lateral geniculate

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 22.1 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...

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... 

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). 

Frontal cortex, superior colliculus, lateral geniculate, deep nuclei of the cerebellum... 

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. 

Marenco et al. (2000) Rats - chronically nic exposed vs, nic naive 2-Deoxy-D-[l- C] glucose autoradiography SC nic (0.4 mg kg- ) vs. saline t Thai, superior colliculus in chronically exposed f thal, superior colliculus, medial habenula, and dorsal lateral geniculate in nic naive... 

Dopamine receptors include at least 4 subtypes which are concentrated in the striatum where D1 and D2 are evenly distributed and D3 is concentrated in the limbic portion, nucleus accumbens (Herroelen et al., 1994). D2 but not D1 receptors also occur throughout the cerebral cortex particularly temporal lobe, and D3 receptors are also present in lower densities in hippocampus and amygdala. D3 receptors are localised in several thalamic nuclei including the lateral geniculate, mediodorsal and anteroventral. 

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... 

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