Visual response properties of ventral lateral geniculate nucleus cells projecting to the pretectum and superior colliculus in the cat

Distinct roles of metabotropic glutamate receptor activation on inhibitory signaling in the ventral lateral geniculate nucleus

The ventral lateral geniculate nucleus (vLGN) has been implicated in numerous functions including circadian rhythms, brightness discrimination, pupillary light reflex, and other visuomotor functions. The contribution of inhibitory mechanisms in the regulation of vLGN neuron excitability remains unexplored. We examined the actions of metabotropic glutamate receptor (mGluR) activation on the intrinsic excitability and inhibitory synaptic transmission in different lamina of vLGN. Activation of mGluRs exerts distinct pre- and postsynaptic actions in vLGN neurons. In the lateral magnocellular subdivision of vLGN (vLGNl), the general mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) enhanced the frequency of GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSC) that persisted in the presence of sodium channel blocker tetrodotoxin (TTX) in a subpopulation of neurons (TTX insensitive). This increase is attributed to the increased output of dendritic GABA release from vLGN interneurons. In contrast, in the medial subdivision of vLGN (vLGNm), the mGluR agonist-mediated increase in sIPSC frequency was completely blocked by TTX. The selective Group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) increased sIPSC frequency, whereas the selective Group II mGluR agonist (2R, 4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) significantly decreased sIPSC frequency in vLGNl neurons. Optic tract stimulation also produced an mGluR-dependent increase in sIPSC frequency in vLGNl neurons. In contrast, we were unable to synaptically evoke alterations in sIPSC activity in vLGNm neurons. In addition to these presynaptic actions, DHPG depolarized both vLGNl and vLGNm neurons. In vLGN interneurons, mGluR activation produced opposing actions: APDC hyperpolarized the membrane potential, whereas DHPG produced a membrane depolarization. The present findings demonstrate diverse actions of mGluRs on vLGN neurons localized within different vLGN lamina. Considering these different lamina are coupled with distinct functional roles, thus these diverse actions may be involved in distinctive forms of visual and visuomotor information processing.

Cytoarchitectonic and connectional organization of the ventral lateral geniculate nucleus in the cat

The ventral lateral geniculate nucleus is a small extrageniculate visual structure that has a complex cytoarchitecture and diverse connections. In addition to small-celled medial and lateral divisions, we cytoarchitectonically defined a small-celled dorsal division. A large-celled intermediate division intercalated between the three small-celled divisions, which we divided into medial and lateral intermediate subdivisions. In WGA-HRP injection experiments, the different cytoarchitectonic divisions were shown to have connections with different nuclei. The medial division was reciprocally connected to the pretectum and projected to the superficial layers of the superior colliculus and the intralaminar nuclei. The medial intermediate division received projections from the intermediate layer of the superior colliculus and the lateral and interpositus posterior cerebellar nuclei, and projected to the intermediate layer of the superior colliculus, the periaqueductal gray of midbrain, and the intralaminar nuclei. The lateral intermediate divisions received projections from the pretectum, the intermediate layer of the superior colliculus, and the lateral and interpositus posterior cerebellar nuclei, and projected to the pretectum, superficial layers of the superior colliculus, and the pulvinar. The lateral division received projections from superficial layers of the superior colliculus and had reciprocal connections with the pretectum. The dorsal division received projections from the pretectum and had reciprocal connections with the periaqueductal gray of midbrain. The different cytoarchitectonic divisions of the ventral lateral geniculate nucleus are thus suggested to play different functional roles related to vision, eye and head movements, attention, and defensive reactions.

Visual-ocular motor activity in the macaque pregeniculate complex

The anatomical connections of the pregeniculate complex (PrGC) with components of the visual-ocular motor system suggested its contribution to ocular motor behavior. Subsequent studies reported saccade-related activity in the primate PrGC. To determine its contribution, we characterized pregeniculate units (n = 128) in alert macaques during ocular motor tasks and visual stimulation. We found that 36/109 saccade-related units exhibited postsaccadic bursts or pauses in tonic discharge for saccades of any amplitude or direction. In contrast to previous results, 46/109 responses preceded or coincided with the saccade, while 47/109 responses were directionally tuned. Pregeniculate units were modulated not only in association with saccades (109/128) but also with smooth eye movements and visual motion (20/128) or eye position (23/128). Multiple ocular motor signals were recorded from 19% of the units, indicating signal convergence on individual neurons. Visual responses were demonstrated in 51% of PrGC units: visual field illumination modulated the resting discharge of 33 units; the responses of 37 saccade-related units and all 23 position-dependent units were modulated by visual stimulation. Early saccadic activity in the PrGC suggests that it contributes more to gaze than postsaccadic modulation of visual or ocular motor activity. The patterns of saccadic responses and the modulation of PrGC activity in association with a variety of visual-ocular motor behaviors suggest its potential role as a relay between the parietal cortex and elements of the brain stem ocular motor pathways, such as the superior colliculus and pretectal nucleus of the optic tract.

Physiological Characterization of Pretectal Neurons Projecting to the Láteral Geniculate Nucleus in the Cat

Single neurons in the pretectal nucleus of the optic tract and posterior pretectal nucleus were extracellularly recorded in anaesthetized cats and tested for antidromic activation after electrical stimulation of the ipsilateral dorsal lateral geniculate nucleus. Cells were further characterized by their response latencies to electrical stimulation of the optic nerve head and the optic chiasm, and by responses to various visual stimuli. 46 out of 188 neurons (24%) were antidromically activated from the lateral geniculate nucleus at response latencies of 0.6 - 2.6 ms. They had low spontaneous activities and preferred fast-moving visual stimuli. 29 of the antidromically activated neurons (63%) could be activated from the optic chiasm with response latencies of 4 - 10 ms. Together with the mean conduction time of 0.8 ms between the optic nerve head and the optic chiasm, this indicates that they receive an indirect retinal input via fast-conducting Y-fibres. Sometimes antidromically activated neurons spontaneously showed irregular burst activity. During unidirectional stimulation with a large moving visual stimulus, burst activity became more regular, and interburst intervals and the duration of single bursts decreased. After the stimulus was stopped, interburst intervals slowly increased until prestimulation activity was restored. The response properties of these neurons could reflect the transfer of saccade-related visual as well as oculomotor signals through the pretectum to the visual thalamus.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

Response properties of pretectal omnidirectional pause neurons in the behaving primate

We have identified a region in the pretectum of rhesus monkeys (Macaca mulatta) that contains units that evince a complete cessation in firing immediately after saccades. The pause occurs for saccades to target steps and catch up saccades during smooth pursuit, spontaneously in complete darkness or after quick phases of nystagmus. Because the pause in unit firing always follows saccade onset, we call these neurons following omnidirectional pause neurons (FOPNs). Because the pause also occurs with saccades in the dark, it is related to the saccade per se and is not a visually contingent response. The duration of the pause in firing exceeded the duration of all saccades up to 40 deg. For targeting saccades, the start of the pause was locked rather tightly to the beginning of the saccade but began an average of 51 ms after the saccade did. The end of the pause was linked only loosely to either the beginning or end of the saccade. About half (54%) of our 59 FOPNs also discharged a distinct burst of firing that preceded the pause. In different units, the burst preceded saccade onset by from 0 to 20 ms with an average of 11 ms and therefore could signal the occurrence of an impending saccade. The presaccadic burst was not correlated with any parameter of the saccade. Most FOPNs were found 278 microns, on average, dorsal to the direction-selective units characteristic of the pretectal nucleus of the optic tract (NOT) and occasionally slightly beyond the anterior-posterior and medial-lateral borders of the NOT. The FOPN region does not coincide with any known anatomically or functionally delineated pretectal nucleus. Because the characteristics of the FOPN pause are not reflected in the characteristics of the saccade and the FOPN pause occurs well after the saccade is over, it is unlikely that the pause in pretectal FOPNs is involved with saccade generation. On the other hand, the leading burst exhibited by the majority of FOPNs reliably signals that a saccade is occurring but neither its size nor direction. Perhaps this signal indicating the occurrence of all saccades is routed to visual relay neurons to effect saccadic modification of visual pathways. The substantial efferent connections of the FOPN/NOT region to the pregeniculate nucleus and the saccadic discharge or pregeniculate cells are discussed in the context of this suggestion.

Modulation of visually evoked responses in units of the ventral lateral geniculate nucleus of the rat by somatic stimuli

Single unit activity was recorded from the ventral part of the lateral geniculate nucleus (vLGN) in rats anaesthetized with urethane. Most of the cells located laterally in the nucleus were excited by light. The studied vLGN neurones did not respond to electrical stimulation of the tail, but about half of them changed their response to light significantly when the light flash was paired with the electrical stimulation. When the tail stimulus preceded the light, the changes consisted in a pronounced facilitation of flash-evoked activity. When the electrical stimulus was applied after the flash in a forward conditioning paradigm, facilitations were less pronounced and responses of some neurones were suppressed. These results are in contrast to those of similar experiments on the dorsal LGN, neurones of which were mainly facilitated by the conditioning paradigm. Thus, light-evoked activity of ventral geniculate cells can be enhanced by arousal-related processes.

Luminance coding properties of intergeniculate leaflet neurons in the golden hamster and the effects of chronic clorgyline

Cells in the intergeniculate leaflet (IGL) project to the suprachiasmatic nuclei, a mammalian circadian pacemaker. Chronic treatment with clorgyline alters hamster circadian rhythms in ways similar to alterations seen after ablation of the IGL. Chronic clorgyline also alters the light intensity dependence of phase-shifting. In this study luminance coding properties of IGL cells were measured in control hamsters and in hamsters chronically treated with clorgyline. In control animals three patterns of response to increasing and decreasing luminance were observed. Type I cells showed a monotonic pattern. Type II cells were similar to Type I with additional increases in firing rate at several specific luminance levels. Type III cells only coded increases in luminance. Cells from clorgyline-treated animals did not differ from those from control animals in the pattern of luminance response but IGL cells from these animals showed decreased firing rate in both light and dark conditions. These results suggest that the effects of clorgyline on the photic sensitivity of circadian rhythms may be related to a clorgyline-induced decrease in firing rate of IGL cells. They also indicate that some IGL cells show complex patterns of response to luminance changes in addition to those showing simple monotonic responses.

Retinal X-afferents bifurcate to lateral geniculate X-cells and to the pretectum or superior colliculus in cats

The question of whether retinal X-type ganglion cell axons project via axonal bifurcation to both the dorsal lateral geniculate nucleus (LGN) and the pretectum (PT) or the superior colliculus (SC) in the cat, was studied by examining the effects of PT and/or SC stimulation on the LGN cells. X-cells that responded monosynaptically to PT or SC stimulation were encountered as follows: 29%, 26%, and 4% of the tested X-cells responded to stimulation of PT, SC, and both, respectively. For the X-cells activated from the PT or SC, the latency tended to be a little longer than optic chiasm latency. The receptive field centers of the X-cells were located within the receptive fields of the multiple units from the SC whose stimulation could activate the corresponding X-cells. The present results demonstrate that a substantial proportion of the X-type LGN cells receive excitatory inputs from the retinal X-type ganglion cell axons that branch to the PT or the SC.

The pupillary light reflex in normal and innate microstrabismic cats, II: Retinal and cortical input to the nucleus praetectalis olivaris

The anatomical substrate of the pupillary light reflex was investigated in normal and innate microstrabismic cats using anatomical methods as well as electrical stimulation. The bilateral retinal input to the nucleus praetectalis olivaris (NPO), the pretectal relay station in the subcortical pupilloconstrictor pathway, was identified to come from the ventral retina were the upper visual field is represented. Orthodromic electrical stimulation revealed that retinal information is transmitted to on-tonic neurons in the NPO mainly via slowly conducting axons probably originating from W- and X-type retinal ganglion cells. For the first time, a direct cortical input to on-tonic neurons in the NPO could be demonstrated. This cortical input originates from caudolateral parts of the occipital cortex. Putative input structures are those subdivisions of areas 19 and 20a where the upper part of the visual field is represented. A direct, predominantly contralateral projection with a weak ipsilateral component from NPO to the nucleus of Edinger-Westphal, and an interhemispheric connection between the NPOs could be demonstrated. With respect to the anatomical connections as described in this study, no differences between normal and innate microstrabismic cats could be found. The results are discussed with respect to the binocular summation of the pupillary light reflex and its reduction in subjects with impaired binocular vision.

Photic responses of geniculo-hypothalamic tract neurons in the Syrian hamster

The putative neural pacemaker controlling circadian rhythms in mammals is contained in the suprachiasmatic nuclei of the hypothalamus. These nuclei receive a projection, the geniculo-hypothalamic tract (GHT), from neurons in the intergeniculate leaflet (IGL) and portions of the ventral lateral geniculate nucleus (vLGN) of the thalamus. We examined the responses of putative GHT neurons to diffuse illumination using extracellular electrophysiological recordings. The great majority of IGL neurons showed sustained ON responses to diffuse retinal illumination; vLGN neurons showed more variation in their responses. Discharge rates of sustained ON neurons increased monotonically as light intensity was increased and saturated over 2-3 log units of intensity changes. Many IGL neurons had binocular input, and input from the ipsilateral eye was often inhibitory. These results indicate that GHT neurons may provide information about ambient light intensity to the suprachiasmatic nuclei.

Identification of ventral lateral geniculate nucleus cells projecting to the pretectum and superior colliculus in the cat

Among 235 histologically identified cells of the ventral lateral geniculate nucleus (LGV) in the cat, 66 responded antidromically to electrical stimulation of the pretectum (PT) and/or superior colliculus (SC): 22 projected to PT, 22 to SC and 22 to both sites. The LGV cells were innervated by optic tract fibers corresponding to axons of X- as well as W-type retinal ganglion cells.