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Key Points: Neuronal oscillations observed in sensory systems are physiological carriers of information about stimulus features. Rhythm in the infra-slow range, originating from the retina, was previously found in the firing of subcortical visual system nuclei involved in both image and non-image forming functions. The present study shows that the firing of neurons in the lateral geniculate nucleus is also governed by gamma oscillation (∼35 Hz) time-locked to high phase of infra-slow rhythm that codes the intensity of transient light stimulation. We show that both physiological rhythms are synchronized within and between ipsilateral nuclei of the subcortical visual system and are dependent on retinal activity. The present study shows that neurophysiological oscillations characterized by various frequencies not only coexist in the subcortical visual system, but also are subjected to complex interference and synchronization processes., Abstract: The physiological function of rhythmic firing in the neuronal networks of sensory systems has been linked with information coding. Also, neuronal oscillations in different frequency bands often change as a signature of brain state or sensory processing. Infra-slow oscillation (ISO) in the neuronal firing dependent on the retinal network has been described previously in the structures of the subcortical visual system. In the present study, we show for the first time that firing of ISO neurons in the lateral geniculate nucleus is also characterized by a harmonic discharge pattern (i.e. action potentials are separated by the intervals governed by fundamental frequency in the gamma range: ∼35 Hz). A similar phenomenon was recently described in the suprachiasmatic nuclei of the hypothalamus: the master biological clock. We found that both gamma and ISO rhythms were synchronized within and between ipsilateral nuclei of the subcortical visual system and were dependent on the retinal activity of the contralateral eye. These oscillatory patterns were differentially influenced by transient and prolonged light stimulation with respect to both frequency change direction and sustainability. The results of the present study show that the firing pattern of neurons in the subcortical visual system is shaped by oscillations from infra-slow and gamma frequency bands that are plausibly generated by the retinal network. Additionally, the results demonstrate that both rhythms are not a distinctive feature of image or non-image forming visual systems but, instead, they comprise two channels carrying distinctive properties of photic information., (© 2018 The Authors. The Journal of Physiology © 2018 The Physiological Society.)

The intergeniculate leaflet (IGL) of the lateral geniculate body in the rat is a population of GABAergic neurons that can be divided into two, anatomically and neurochemically distinct populations. One population comprises neuropeptide-Y (NPY)-positive neurons that form the geniculohypothalamic tract innervating the suprachiasmatic nuclei (SCN) and the other population comprises enkephalin-positive (ENK) neurons giving rise to the geniculo-geniculate tract innervating the contralateral IGL (cIGL). Previous electrophysiological studies have observed various patterns of firing and different responses to changes in lighting conditions of IGL neurons in vitro and in vivo. The aim of the present study was to determine if these distinct properties could be ascribed to differentially projecting IGL neurons. Neuron activity was recorded extracellularly in the IGL of anaesthetised rats under different lighting conditions (i.e. light/dark). Antidromic activation was used to identify recorded cells as projecting to the SCN or the contralateral IGL. All IGL neurons identified as projecting to the contralateral IGL displayed infra-slow oscillatory activity (ISO; i.e. slow rhythmic bursts of action potentials). ISO of these neurons was sustained in the light and was diminished in the darkness. In contrast, all IGL neurons identified as projecting to the SCN displayed a low level of firing in the light and a majority of these cells increased firing in the darkness. All IGL neurons projecting to the SCN were characterised by an irregular pattern of firing in the light and dark. These data are the first to demonstrate that differentially projecting rat intergeniculate leaflet neurons are characterised by distinct firing patterns and opposite responses to light and dark conditions., (Copyright © 2012 IBRO. Published by Elsevier Ltd. All rights reserved.)

Orexins influence various physiological processes associated with feeding behaviour, endocrine functions and wakefulness. One component of mammalian circadian timing systems, intergeniculate leaflet (IGL) of the lateral geniculate nucleus, is thought to contribute to circadian entrainment by processing photic and non-photic/arousal-related signals. Because the IGL is possibly innervated by the orexinergic system, using in vitro extracellular recording techniques we evaluated the influence of orexin A (OXA) and orexin B (OXB) on the rate and pattern of neuronal firing in this structure. Significant increases in the activity of 33 and 28% of IGL cells were observed after locally applied OXA (1 μm) and OXB (1 μm), respectively. In the great majority of neurons responses to OXA were maintained in the presence of orexin-1 receptor OX1R antagonist, SB 334867 (10 μm). Additionally, 75% of the OXB-responsive neurons were also sensitive to an orexin-2 receptor (OX2R)-selective agonist, [Ala11, D-Leu15]-OXB (1 μm). Immunohistochemical stainings showed putative synaptic contacts between OXA- and OXB-immunoreactive fibres and neuropeptide Y, and enkephalin-positive neurons in the investigated area. The outcome of our experiments reinforces previous reports indicating the possible linkage between the orexinergic and circadian systems. To our knowledge the presented findings are the first showing the direct influence of orexins on the IGL activity, mostly through activation of OX2R., (© 2011 The Authors. European Journal of Neuroscience © 2011 Federation of European Neuroscience Societies and Blackwell Publishing Ltd.)

Electrophysiological studies combined with local neurotoxic lesions were conducted on anaesthetized rats in order to determine whether the dorsal raphe nucleus (DRN) inhibits the intergeniculate leaflet (IGL) of the lateral geniculate nucleus by means of innervation by serotonin-containing fibres. In the control animals, electrical stimulation of the DRN induced the long-latency and long-lasting inhibition of the neuronal firing of the IGL cells that are characterized by rhythmic, slow-bursting activity in light conditions. The electrical destruction of the DRN resulted in an increase in the firing rate of the recorded IGL cells, whilst at the same time not affecting the rhythmic, bursting pattern of the activity. In the second group of animals, local neurotoxic lesion of serotonergic fibres was performed by injection of the toxin 5,7-dihydroxytryptamine into the IGL. After 10 days of postoperative recovery, electrophysiological experiments were performed on the toxin-treated rats. In these animals, electrical stimulation as well as electrical lesion of the DRN did not induce any change in the firing of the slowly bursting cells in the 5,7-dihydroxytryptamine-injected IGL. The results obtained provide evidence that inhibition of the IGL slowly bursting cells, by innervation from the dorsal raphe, is mediated by the release of serotonin. Furthermore, the observed serotonergic inhibition of the light-dependent activity of slowly bursting cells can contribute to the neuronal mechanism gating the information that flows through this nucleus to the vestibular, visuomotor, circadian and sleep/arousal systems, with which the IGL is strongly interconnected.