Intergeniculate Leaflet And Photoperiodic Information

The IGL receives retinal projections and sends NPY—containing fibers to the SCN. Although the role of the IGL in photic entrainment has been the subject of extensive investigation, its exact function is still unknown. The IGL does not appear to be essen— tial for photic—entrainment since, in a number of studies, entrainment was not affected by lesion of the IGL (48,55). However, other data indicate that the IGL may play a role, micro—injections of NPY into the SCN or activation of the IGL by electrical or chemical stimulation inducing either phase advances or phase delays during the subjective night (1,20,34,54).

The IGL is considered to be a component of the circadian system. Whether it also represents a component of the photoperiodic system enabling, for example, the SCN to integrate photoperiodic information is not known.

In the jerboa (39) NPY immunolabeling in the SCN was higher in animals killed in autumn (SP, period of sexual quiescence) than in those killed in the spring to middle of summer (period of sexual activity) (39). These photoperiod—dependent changes are independent of the photoperiod—induced changes in circulating levels of gonadal steroids (39). In the rat, the density of NPY fibers within the SCN exhibits a daily rhythm characterized by an increase at the light/dark transition (8,33) and this rhythm is known to be drastically modulated by photic stimulation and to disappear in con— stant conditions (66). Moreover, like SCN neurons, specific neurons of the IGL that project to the SCN are responsive to tonic changes in light stimulation (45,81).All these data suggest that the increase in NPY immunoreactivity observed in autumn in the jerboa SCN or under SP in the Siberian hamster might be a consequence of the increased duration of the dark phase. NPY might thus play an important role in the integration of photoperiodic information by the SCN. Are these NPY fibers involved in the photoperiodic—induced changes in the duration of the nocturnal light sensitive period of the SCN?

An increase in the number of NPY mRNA containing neurons in the IGL of the Syrian hamster has been observed after 8 weeks of exposure to SP (31). A densito— metric analysis of the concentration of NPY mRNA in each neuron did not show a par— allel increase. This findings suggests that SP activates a population of quiescent cells which do not express NPY mRNA in LP. A similar phenomenon has already been described in the human SCN, in which an additional population of neurons express vasopressin in the summer time (23). This SP—induced increase in the number of NPY— neurons indicates that photoperiod controls or modulates the synthesis of NPY within some neurons of the IGL. What could the physiological consequence of this phenom— enon be?

Clearly a photoperiodic message is integrated and expressed at the level of the IGL. It may be that it is through this photoperiod dependent change in NPY content in IGL afferent fibers that the circadian clock integrates photoperiodic information. The geniculo—hypothalamic tract (GHT) would then be directly involved in the pho— toperiodic—induced changes in the duration of the circadian light sensitive period of the SCN. Preliminary data from the present laboratory support such an interpretation. In

IGL-lesioned hamsters, 3 weeks after transfer to SP (a condition in which in non lesioned animals a full extension of the light sensitive period is observed), the number of cells expressing Fos-ir after light exposure was reduced compared to intact animals. This findings indicates that the GHT is involved in the photoperiod-induced SCN changes. However, as the amount of Fos cells in lesioned animals was higher than in the control animals kept under LP, it is also evident that even in the absence of the IGL, the photoperiodic message has been, at least partly, integrated.

A clear photoperiodic message is thus expressed at the level of the IGL but its origin remains to be determined. It could originate directly from the retina, variation in the duration of the night or day being directly transmitted through retinal projections to the IGL. The observation that the increase in number of NPY-neurons is observed 4 days only after the transfer from LP to SP (Figure 1) deeply supports this interpretation. This observation, although melatonin receptors have been described in the IGL of some mammalian species (41), permits to exclude a role of melatonin (3 weeks are needed to obtain the maximal extension of the nocturnal melatonin peak). Since six to eight weeks are needed for the SP to induce gonadal atrophy, it permits also to exclude a role of sex steroids in such phenomenon.

Neurons in the IGL are activated by both photic (55) and non-photic stimuli (32,42,50). Both stimuli involve activation of an immediate early gene, c-fos. Non-photic stimulation (associated with locomotor activity) induces Fos-ir primarily in the NPY neurons. After transfer from LP to SP the locomotor activity period in the hamster is extended. The increase in the number of NPY-mRNA containing neurons in the IGL after exposure to SP may also be the consequence of the photoperiod-dependent increase in the duration of the activity period. Experiments are in progress to test these different hypotheses.

Although it is not a dense tract, SCN neurons also project to the IGL (47). Considering the model proposed by Pittendrigh and Daan (58) and the interpretation of the data by Illnerova's group (see above) and that after an IGL lesion a photope-riodic message can be, at least partly, integrated by the SCN, it might also be that

Figure 1. Graph showing the number of NPY mRNA containing cells per section of mid-part IGL of hamsters kept under LP or SP for 25 days. Note that 4 days after transfert under SP the increase in number is already maximal. Means ± SEM. *Significant difference between the two groups (P < 0.001, student t-test).

Figure 1. Graph showing the number of NPY mRNA containing cells per section of mid-part IGL of hamsters kept under LP or SP for 25 days. Note that 4 days after transfert under SP the increase in number is already maximal. Means ± SEM. *Significant difference between the two groups (P < 0.001, student t-test).

Figure 2. Through its efferent fibers IGL is able to distribute photoperiodic information to different brain structures including the SCN. Whether the photoperiodic signal is directly integrated by the IGL through retinal fibers or originates from the SCN is not known.

the photoperiodic information originates in the SCN itself. If this is true the IGL may be used by the SCN to transmit the photoperiodic message to IGL-connected structures.

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