INTERGENICULATE LEAFLET OF THE THALAMUS

The IGL of the thalamus is the other (in addition to the SCN) structurally important element of the mechanism of mammalian biological clock.

Anatomy

The IGL belongs to the lateral geniculate nucleus (LGN) complex of the thalamus, an extremely important and interesting structure of the optic pathway. It is located between the dorsal lateral geniculate (DLG) and ventral lateral geniculate (VLG) parts of the lateral geniculate body. For a long time, the IGL was indistinguishable from the rest of the lateral geniculate body and was classified as in its ventral part. Only immunohistochemical and autoradiographic methods helped to demonstrate its complete anatomical identity and to define the limits of its occurrence and its connections with other brain structures [26, 27] (Fig. 3).

The IGL is composed of small and medium-size multipolar interneurons with a dendritic zone that is limited to the area occupied by this structure, over which it differs anatomically from the remainder of the lateral geniculate body. This structure is clearly identifiable in rodents, whereas its homologs have so far been barely recognized in other species. In cats, such a homolog is the medial part of the ventral LGN, while in monkeys and humans it is the preginiculate nucleus [28]. The IGL receives strong innervation from retinal ganglionic cells of both eyes, branching off the RHT pathway running to the SCN. The projection to the contralaterally situated IGL is twice as large

IGL

DLG

VLG IGL

VLG

500 m 100 m Bregma - 4.70mm

Fig. 3. The coronal section through the rat brain illustrating (left) the localization of the intergeniculate leaflet (IGL). A photomicrograph (right) of a rat brain coronal section across the geniculate complex with dark labeling of neuropeptide Y (NPY) immunoreactive neurons within the IGL area. DLG dorsal lateral geniculate nucleus, IGL intergeniculate leaflet, VLG ventral lateral geniculate nucleus.

as that to the IGL situated ipsilaterally [29]. The above-mentioned leaflet stretches along the whole horizontal length of the LGN, occupying a section of about 2 mm in the rat and 2.2 mm in the hamster [27]. The majority of leaflet neurons reach the ventrolateral area of the SCN, bypassing—via the geniculohypothalamic tract (GHT)—neurons that form the RHT.

The body of anatomic evidence showing the connection between the IGL and many other brain areas, especially those involved in the process of seeing, has been growing [30–34]. These relationships are generally bilateral and often reciprocal, being clear proof of a functional connection between the IGL and structures participating in the process of seeing, representing mainly the visuomotor function. This phenomenon was first investigated in 1994 by Morin [35], who confirmed it by additional proof, based mostly on his own anatomical studies published in an extremely interesting review, called most accurately, “The Circadian Visual System, 2005” [36]. It is noteworthy that there also exists a reciprocal bilateral connection between the two leaflets via the supraoptic commissure, which is limited to the IGL only, and therein to a population of neurons that do not project to the SCN. The role of this bilateral connection between the leaflets via the geniculogeniculate pathway is not entirely clear. It is thus assumed, although not yet conclusively proven, that both leaflets are reciprocally synchronized by means of this connection [37]. This synchronization seems to be particularly important with respect to the potential involvement of the IGL in the process of seeing, which requires synchronic activity on the part of all elements forming the visual system for the normal perception of an image. Possibly, the reciprocal connection between these two leaflets is another example of the integratory activity of brain.

The Pharmacology of the IGL

The total number of nerve cells forming the IGL ranges between 1,800 and 2,000 [27]. They are mostly GABAergic neurons that constitute a basic population of cells building up the neuronal mechanism of mammalian biological clock [38]. The pharmacological property by which the IGL differs from the remainder of the LGN is the presence of neuropeptide Y (NPY) (Fig. 3), which can be found mostly among GABAergic neurons projecting to the SCN [27, 39, 40]. Thus, NPY is regarded as the main transmitter of nonphotic information from the IGL to the SCN. Another neuropeptide present in the IGL is enkephalin (ENK), which in the majority of cases participates in the projection to the IGL situated opposite it. Also, neurotensin, which occurs in the vicinity of the majority of neurons with NPY, is a pharmacological marker of IGL neurons.

Regarding the role of nonphotic information in the regulation of the mechanism of the biological clock, another very important IGL neuropeptide is relaxin 3. The extraordinarily rich nerve fibers containing this neuropeptide are present in numerous brain structures, yet above all in the IGL, being definitely less abundant in the SCN [41]. Relaxin probably participates in the modulation of behavioral patterns by adapting animals to environmental stressful conditions, which may constitute another nonphotic factor affecting the mechanism of the biological clock. The involvement of relaxin in the control of a visuomotor response has also been postulated [41], which in turn may confirm the engagement of the circadian system, above all the IGL, in visual processes.

Chronobiology

A vast body of anatomical evidence but above all the results of physiological and behavioral studies confirmed the involvement of the IGL in the regulation of biological rhythms. Electric stimulation of the IGL, or such nonphotic information as benzodiazepine treatment or novel wheel activity, produces a shift in the rhythm phase during the subjective day and a reduction in the expression of Period genes in the SCN [42], while light affects the circadian system during the subjective night, simultaneously bringing about an increase in the expression of the Period gene in the SCN. Electric lesion of both the IGLs always causes desynchronization of the rhythm of mouse locomotor activity [43]. The last effect may be explained by the lack of nonphotic synchronization in the circadian system after IGL lesion [44]. The observation that IGL lesion does not disturb the course of circadian rhythms in standard laboratory conditions under a specific permanent light regime (12 h of light/12 h of darkness) is of utmost importance. With limited access to other nonphotic stimuli, the strong light/darkness stimulus— always administered at the same time intervals—is a dominating signal that synchronizes rhythmic processes. However, it should be borne in mind that laboratory regimens are distant from the real conditions in which the majority of organisms live, above all humans. Therefore, the presence and significance of the structure that receives and integrates nonphotic information with photic information are of paramount importance. It is the IGL that fulfills this task.

The Electrophysiology of the IGL

The IGL neurons reveal an extremely interesting pattern of electric activity, characteristic only of this structure of the whole LGN complex. They generate action potentials in rhythmically repeated firing bursts with a constant interburst interval lasting several hours, defined as isoperiodic oscillations (Fig. 4).

The mean time in which IGL cells change the level of their activity amounts to 124 ± 7s [45]. Interestingly, such a pattern of activity is revealed by leaflet neurons only and is lacking in the dorsal and ventral parts of the LGN. On the other hand, these oscillations can be observed in the SCN of the hypothalamus [46, 47] and in the activity of cells of the pineal gland [48], to which the IGL has an additional projection in rats [49] and in

Fig. 4. Firing rate histogram showing rhythmic slow bursting activity (isoperiodic, ultradian oscillation) of intergeniculate leaflet (IGL) neurons. Bin size 1 s.

Mongolian gerbils [50]. The same pattern of cellular activity in the two most important structures (SCN and IGL) of the neuronal mechanism of mammalian biological clock permits an assumption that it constitutes a natural, extremely important basal rhythm characteristic of the work of not only these two structures, but also the whole mechanism of the mammalian biological clock. The absence of such oscillations in an in vitro preparation may suggest that they are of exogenous origin [51]. On the basis of the experiments conducted so far, in all probability it may be concluded that a photic signal from the retina is necessary for their development. Like the blockade of sodium conduction by means of TTX (tetrodotoxin citrate) administration to the eyeball, switching off the light inhibits the pattern of the oscillatory activity of IGL neurons [45, 52]. In this activity, mainly GABAergic neurons are involved; the blockade of their receptors leads to temporary disappearance of the oscillatory activity of the IGL [53]. It has also been ascertained that a reciprocal connection between both leaflets is not necessary for the occurrence of oscillatory activity in the opposite leaflet. The lesion or pharmacological blockade of one leaflet does not affect the oscillatory activity of the other [54].

Electric stimulation of the dorsal raphe nuclei—the main source of the serotoninergic projection to the IGL—causes a temporary decrease in the level of the oscillatory activity of neurons, while electric lesion of this projection results in a pronounced increase in this activity [55]. Our most recent studies with lesioned terminals of serotoninergic fibers in the IGL have confirmed the direct, distinct, modulatory role of the 5 hydroxytryptamine (5-HT) projection from the dorsal raphe nuclei in the oscillatory activity of IGL neurons [56]. A similar modulating effect is also observed in the case of two other nonspecific projections of the brain: cholinergic from the laterodorsal tegmental nuclei and noradrenergic from the locus coeruleus. Electrophysiological in vivo studies into the rat circadian system have shown an inhibitory effect of electric stimulation of the nonspecific projections of the brain stem on the potential induced in the IGL [57]. However, the influence of these two projections on the oscillatory activity of IGL neurons is not as apparent as that of the serotoninergic projection, which plays a dominating role in the mechanism of the mammalian biological clock.

All the same, the crucial question about the role and significance of these short (rapid) oscillations for the functioning of the mammalian biological clock still remains unanswered. The physiological importance and connection of this ultradial rhythmicity of IGL and SCN neurons with long, commonly observed, and recorded circadian rhythms need to be elucidated.

It is not easy to give an answer to this question at the present stage of knowledge. First, one should reflect on the issue whether this type of activity is limited exclusively to the mechanism of the biological clock. A similar activity, recorded by us lately in the pretectum (OPT) [58], permits us to assume that it may also be engaged in visual processes, in particular in visuomotor functions. This would provide us with another line of evidence for a close connection between the visual processes and regulation of biological rhythms and might account for the anatomical localization of the IGL in the structure involved in the process of seeing. However, the presence of oscillatory activity in SCN neurons—a structure engaged in the mechanism of the biological clock only—makes the involvement of oscillations of this type exclusively in visual processes questionable. One of the hypotheses (fairly universal, in my opinion) that explain the role of this rapid neuronal