Retinal gap junctions are involved in rhythmogenesis of neuronal activity at remote locations – Study on infra-slow oscillations in the rat olivary pretectal nucleus

Short Wavelengths Contribution to Light-induced Responses and Irradiance Coding in the Rat Dorsal Lateral Geniculate Nucleus - An In vivo Electrophysiological Approach

The dorsal lateral geniculate nucleus (dLGN) is the main neuronal station en route to higher visual areas. It receives information about environmental light from retinal photoreceptors whose sensitivity peaks are distributed across a visible spectrum. Here, using electrophysiological multichannel recordings in vivo combined with different light stimulations, we investigated short wavelength contribution to the dLGN responses to light and irradiance coding. The results showed that the majority of dLGN cells responded evenly to almost all wavelengths from the 340 to 490 nm spectrum; however, some cells representing extremes of unimodal distribution of Blue-UV index were specialised in the reception of blue or UV light. Moreover, by using alternate yellow and monochromatic light stimuli from blue - UV range, we also assessed the relative spectral contribution to rat dLGN responses to light. Finally, we observed no clear changes in the irradiance coding property of short wavelength-deficient light stimuli, however we noticed a distortion of the coding curves manifested by a significant drop in measure of fit after using short wavelength blocking filter. In conclusion, our data provide the first electrophysiological report on dLGN short wavelength-induced responses under changing light conditions and suggest the presence of colour opponent cells in the rat dLGN.

Infra-slow modulation of fast beta/gamma oscillations in the mouse visual system

KEY POINTS

Neurophysiological activity in the subcortical visual system fluctuates in both infra-slow and fast oscillatory ranges, but the level of co-occurrence and potential functional interaction of these rhythms is unknown. Analysing dark-adapted spontaneous activity in the mouse subcortical visual system, we find that these two types of oscillation interact uniquely through a population of neurons expressing both rhythms. Genetic ablation of rod/cone signalling potentiates infra-slow and abolishes fast beta/gamma oscillations while genetic ablation of melanopsin substantially diminishes the interaction between these two rhythms. Our results indicate that in an intact visual system the phase of infra-slow modulates fast beta/gamma oscillations. Thus one possible impact of infra-slow oscillations in vision is to guide visual processing by interacting with fast narrowband oscillations.

ABSTRACT

Infra-slow (<0.02 Hz) and fast beta/gamma (20-100 Hz) oscillations in neurophysiological activity have been widely found in the subcortical visual system. While it is well established that fast beta/gamma oscillations are involved in visual processing, the role (if any) of infra-slow oscillations is currently unknown. One possibility is that infra-slow oscillations exert influence by modulating the amplitude of fast oscillations, yet the extent to which these different oscillations arise independently and interact remains unknown. We addressed these questions by recording in vivo spontaneous activity from the subcortical visual system of visually intact mice, and animals whose retinal network was disrupted by advanced rod/cone degeneration (rd/rd cl) or melanopsin loss (Opn4^-/- ). We found many neurons expressing only one type of oscillation, and indeed fast oscillations were absent in rd/rd cl. Conversely, neurons co-expressing the two oscillations were also common, and were encountered more often than expected by chance in visually intact but not Opn4^-/- mice. Finally, where they co-occurred we found that beta/gamma amplitude was modulated by the infra-slow rhythm. Our data thus reveal that: (1) infra-slow and beta-gamma oscillations are separable phenomena; and (2) that they actively co-occur in a subset of neurones in which the phase of infra-slow oscillations defines beta-gamma oscillations amplitude. These findings suggest that infra-slow oscillations could influence vision by modulating beta-gamma oscillations, and raise the possibility that disruptions in these oscillatory behaviours contribute to vision dysfunction in retinal dystrophy.

Gamma and infra-slow oscillations shape neuronal firing in the rat subcortical visual system

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.