Intergeniculate leaflet: an anatomically and functionally distinct subdivision of the lateral geniculate complex

GABAA-receptor subunit composition in the circadian timing system

In the present study, the distribution of GABAA-receptor alpha 1-, alpha 2-, alpha 3-, alpha 5-, beta 2.3- and gamma 2-subunits were localized immunohistochemically with subunit specific antibodies in the rat circadian timing system (CTS). The areas examined include the principal circadian pacemaker, the suprachiasmatic nucleus (SCN), and areas that receive important SCN input including the intergeniculate leaflet (IGL), subparaventricular zone (SPVZ), paraventricular hypothalamic nucleus (PVH), the retrochiasmatic area (RCh) and the paraventricular nucleus of the thalamus (PVT). The SCN has an unusual pattern with immunoreactivity for the alpha 2-, alpha 3-, alpha 5-, and gamma 2-subunits but not for the commonly expressed alpha 1- and beta 2.3-subunits. In all of the areas receiving SCN efferent input (SPVZ, PVH, RCh, PVT and IGL), staining is present either for all six subunits or for the three common subunits, alpha 1-, beta 2.3-, and gamma 2. There is some evidence for a differential distribution of subunits at the cellular level. The alpha 2-, and beta 2.3-subunits are predominantly expressed in neuropil, the alpha 3-, alpha 5- and gamma 2-subunits are predominantly expressed over perikarya and the alpha 1-subunit is expressed over both neuropil and perikarya in the areas in which subunit immunoreactivity is found. The demonstration of this regional and cellular expression of GABAA-receptor subunits should contribute to our understanding of GABAergic neurotransmission in the CTS.

A confocal approach to the morphofunctional characterization of the transient tyrosine hydroxylase system in the rat suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the neonatal rat is transiently innervated by tyrosine hydroxylase (TH) fibers of unknown origin and whose catecholaminergic nature is rather doubtful. In order to characterize this system morphofunctionally, immunocytochemical double labelling and confocal laser scanning microscopy analysis were employed on cryostat brain sections of 10-day-old rats. Simultaneous stainings for neuropeptide Y (NPY) and tyrosine hydroxylase (TH) immunoreactivity showed that they are not colocalized, neither in the SCN fibers nor in the intergeniculate leaflet (IGL) neurons, site of origin of the NPY projection to the SCN. Therefore, the possibility that SCN transient TH fiber system originates from the IGL could be excluded. Double labelling for TH and aromatic L-aminoacid decarboxylase (AADC) demonstrated that transient SCN TH immunoreactive (IR) fibers are AADC negative, thus supporting the hypothesis of their non-catecholaminergic nature. Moreover two new group of cells which are TH positive and AADC negative were found: one in the SCN and the other in the periventricular hypothalamic nucleus (PHN). The presence of somatostatin (SRIF) and TH in PHN neurons and SCN fibers suggested their possible colocalization, but double immunolabellings gave negative results. Simultaneous immunocytochemical staining for vasoactive intestinal polypeptide (VIP) and TH showed that TH fibers may interact with ventrolateral SCN VIP neurons. This result suggests a possible involvement of TH fibers in regulating VIP cells activity in the entrainment of circadian rhythms.

The effects of lesions of the thalamic intergeniculate leaflet on the pineal metabolism

The aim of the present work was to study, in rats, the effects of lesions of the thalamic intergeniculate leaflet (IGL) and the deep pineal/lamina intercalaris region (DP) on the diurnal profile of N-acetylserotonin (NAS) and on the nocturnal pineal reactivity to acute retinal light stimulation (1 or 15 min). The 24-h experiment shows that there is no phase-shifting on the diurnal NAS curve of groups of rats with bilateral IGL lesion compared to the controls. On the other hand there is a significant reduction on the amplitude of pineal NAS content observed in every nocturnal point of the curve. The pineal glands of IGL-lesioned rats, after 1 min of retinal light stimulation, keep their NAS content equal to the lesioned dark-killed rats. Nonetheless, after 15 min of photostimulation, the pineal NAS content is reduced to nearly zero equally to the control animals. DP lesion does not modify the content of NAS in the pineal gland of rats killed in the dark. However, the pineal photo-inhibition process induced by 1 min of light exposure is impaired. These results suggest that: (1) the intergeniculate leaflet has a role in regulating the amplitude of the diurnal rhythm of pineal NAS production rather than its phase entrainment to light-dark cycle. This effect is not dependent on the direct geniculo-pineal connections. (2) The nocturnal pineal photo-inhibition phenomenon could be decomposed in two processes. One, triggered by short pulses of light and totally dependent on the IGL and partially dependent on the direct monosynaptic pathway between this structure and the pineal gland.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-photic manipulations induce expression of Fos protein in the suprachiasmatic nucleus and intergeniculate leaflet in the rat

Expression of Fos protein in the suprachiasmatic nucleus (SCN) and intergeniculate leaflet (IGL) is considered a cellular correlate of light-induced phase-shift of circadian rhythms in rodents. Non-photic stimuli also induce phase shifts, but their effects on Fos expression have not been established. We examined induction of Fos protein in SCN and IGL regions, in response to cage change, intraperitoneal saline injection, and restraint stress. Fos immunoreactivity was observed in SCN and IGL regions, with greater expression observed in IGL during the light phase of the light-dark cycle. Results suggest that cells in SCN and IGL respond to several types of non-photic manipulations and that expression of Fos in these regions is not light-specific.

Efferent projections of the olivary pretectal nucleus in the albino rat subserving the pupillary light reflex and related reflexes. A light microscopic tracing study

The olivary pretectal nucleus is a primary visual centre sensitive to luminance changes. It is involved in the pupillary light reflex, the consensual pupillary light reflex and related reflexes, such as the lid closure reflex whereby pupillary constriction takes place. Since the olivary pretectal nucleus is a small nucleus, previous studies using degeneration, horseradish peroxidase and radioactive amino acid tracing were limited regarding to the exclusiveness of the projections from the olivary pretectal nucleus. In the present study the position of the olivary pretectal nucleus in the rat was first localized by physiological recording of the neurons upon luminance stimulation. Subsequently, an anterograde tracer Phaseolus vulgaris leucoagglutinin was injected iontophoretically. This allows a much more precise localization of the olivary pretectal nucleus projections. Ascending and descending pathways originating from the olivary pretectal nucleus were observed. Ascending fibres project bilaterally to the intergeniculate leaflet, the ventral part of the lateral geniculate nucleus and ipsilaterally to the anterior pretectal nucleus. In addition, contralateral projections were observed to the zona incerta and the fields of Forel. Descending fibres project bilaterally to the periaqueductal gray, the nucleus of Darkschewitsch, the interstitial nucleus of Cajal, the Edinger-Westphal nucleus and the intermediate gray layer of the superior colliculus. Also a contralateral projection to the oculomotor nucleus and an ipsilateral projection to the pontine nucleus and the nucleus of the optic tract were found. Furthermore, the contralateral olivary pretectal nucleus received a small projection. Retrograde tracing experiments using two fluorescent dyes revealed that the fibres projecting to the contralateral olivary pretectal nucleus and to the contralateral interstitial nucleus of Cajal are collaterals. The projection from the olivary pretectal nucleus to the facial nucleus which has been described to receive an input in cats could not be confirmed for the rat. The fact that the Edinger-Westphal nucleus, the interstitial nucleus of Cajal and the superior colliculus receive an input from the olivary pretectal nucleus suggests that this primary visual centre is not only involved in the pupillary light reflex, but also in controlling eye and head position and saccadic eye movements. Although visual acuity largely depends on receptive field sizes of retinal ganglion cells and their central connections, the stronger sympathetic influence during the pupillary light reflex in animals with frontally placed eyes compared to animals with laterally placed eyes may also contribute to the higher visual acuity in animals with frontally placed eyes.

A putative retinohypothalamic projection containing substance P in the human

The retinohypothalamic tract (RHT) is the principal pathway mediating the entraining effects of light on the circadian pacemaker, the suprachiasmatic nucleus (SCN). In the rat, the RHT has two components, one which projects to the SCN and the intergeniculate leaflet of the thalamus and has no known peptide content and one which projects to the SCN and, perhaps, to the olivary pretectal nucleus and contains substance P (SP). Both terminate predominantly in a zone of the SCN that contains vasoactive intestinal polypeptide (VIP)-producing neurons. In the human, there is a similar dense axonal plexus of SP-immunoreactive axons in the SCN located largely in the area occupied by VIP-immunoreactive neurons and distinct from other SP-immunoreactive axons in the area. We propose that this SP plexus represents a component of the RHT in the human brain.