Ferret Lateral Geniculate Nucleus Research Articles

Physiological characterization of a rare subpopulation of doublet-spiking neurons in the ferret lateral geniculate nucleus.

Interest in exploring homologies in the early visual pathways of rodents, carnivores, and primates has recently grown. Retinas of these species contain morphologically and physiologically heterogeneous retinal ganglion cells that form the basis for parallel visual information processing streams. Whether rare retinal ganglion cells with unusual visual response properties in carnivores and primates project to the visual thalamus and drive unusual visual responses among thalamic relay neurons is poorly understood. We surveyed neurophysiological responses among hundreds of lateral geniculate nucleus (LGN) neurons in ferrets and observed a novel subpopulation of LGN neurons displaying doublet-spiking waveforms. Some visual response properties of doublet-spiking LGN neurons, like contrast and temporal frequency tuning, were intermediate to those of X and Y LGN neurons. Interestingly, most doublet-spiking LGN neurons were tuned for orientation and displayed direction selectivity for horizontal motion. Spatiotemporal receptive fields of doublet-spiking neurons were diverse and included center/surround organization, On/Off responses, and elongated separate On and Off subregions. Optogenetic activation of corticogeniculate feedback did not alter the tuning or spatiotemporal receptive fields of doublet-spiking neurons, suggesting that their unusual tuning properties were inherited from retinal inputs. The doublet-spiking LGN neurons were found throughout the depth of LGN recording penetrations. Together these findings suggest that while extremely rare (<2% of recorded LGN neurons), unique subpopulations of LGN neurons in carnivores receive retinal inputs that confer them with nonstandard visual response properties like direction selectivity. These results suggest that neuronal circuits for nonstandard visual computations are common across a variety of species, even though their proportions vary.NEW & NOTEWORTHY Interest in visual system homologies across species has recently increased. Across species, retinas contain diverse retinal ganglion cells including cells with unusual visual response properties. It is unclear whether rare retinal ganglion cells in carnivores project to and drive similarly unique visual responses in the visual thalamus. We discovered a rare subpopulation of thalamic neurons defined by unique spike shape and visual response properties, suggesting that nonstandard visual computations are common to many species.

Cell type-specific expression of FoxP2 in the ferret and mouse retina

Although the anatomical and physiological properties of subtypes of retinal ganglion cells (RGCs) have been extensively investigated, their molecular properties are still unclear. Here, we examined the expression patterns of FoxP2 in the retina of ferrets and mice. We found that FoxP2 was expressed in small subsets of neurons in the adult ferret retina. FoxP2-positive neurons in the ganglion cell layer were divided into two groups. Large FoxP2-positive neurons expressed Brn3a and were retrogradely labeled with cholera toxin subunit B injected into the optic nerve, indicating that they are RGCs. The soma size and the projection pattern of FoxP2-positive RGCs were consistent with those of X cells. Because we previously reported that FoxP2 was selectively expressed in X cells in the ferret lateral geniculate nucleus (LGN), our findings indicate that FoxP2 is specifically expressed in the parvocellular pathway from the retina to the LGN. Small FoxP2-positive neurons were positive for GAD65/67, suggesting that they are GABAergic amacrine cells. Most Foxp2-positive cells were RGCs in the adult mouse retina. Dendritic morphological analyses suggested that Foxp2-positive RGCs included direction-selective RGCs in mice. Thus, our findings suggest that FoxP2 is expressed in specific subtypes of RGCs in the retina of ferrets and mice.

Rapid Plasticity of Visual Responses in the Adult Lateral Geniculate Nucleus

Compared to the developing visual system, where neuronal plasticity has been well characterized at multiple levels, little is known about plasticity in the adult, particularly within subcortical structures. We made intraocular injections of 2-amino-4-phosphonobutyric acid (APB) in adult cats to block visual responses in On-center retinal ganglion cells and examined the consequences on visual responses in the lateral geniculate nucleus (LGN) of the thalamus. In contrast to current views of retinogeniculate organization, which hold that On-center LGN neurons should become silent with APB, we find that ∼50% of On-center neurons rapidly develop Off-center responses. The time course of these emergent responses and the actions of APB in the retina indicate the plasticity occurs within the LGN. These results suggest there is greater divergence of retinogeniculate connections than previously recognized and that functionally silent, nonspecific retinal inputs can serve as a substrate for rapid plasticity in the adult.

The Developmental Remodeling of Eye-Specific Afferents in the Ferret Dorsal Lateral Geniculate Nucleus

Ferret vision mandala. This photo was created by overlapping multiple images of a horizontal section through the central portion of the adult ferret lateral geniculate nucleus and superior colliculus. The retinofugal projections from one eye were labeled with a green fluorescent tracer and those from the other eye with a red fluorescent tracer. See Speer, et al., on page 1, in this issue.

Selective GABAergic Control of Higher-Order Thalamic Relays

GABAergic signaling is central to the function of the thalamus and has been traditionally attributed primarily to the nucleus reticularis thalami (nRT). Here we present a GABAergic pathway, distinct from the nRT, that exerts a powerful inhibitory effect selectively in higher-order thalamic relays of the rat. Axons originating in the anterior pretectal nucleus (APT) innervated the proximal dendrites of relay cells via large GABAergic terminals with multiple release sites. Stimulation of the APT in an in vitro slice preparation revealed a GABA(A) receptor-mediated, monosynaptic IPSC in relay cells. Activation of presumed single APT fibers induced rebound burst firing in relay cells. Different APT neurons recorded in vivo displayed fast bursting, tonic, or rhythmic firing. Our data suggest that selective extrareticular GABAergic control of relay cell activity will result in effective, state-dependent gating of thalamocortical information transfer in higher-order but not in first-order relays.

Molecular Organization of the Ferret Visual Thalamus

The visual system encodes and deciphers information using parallel, anatomically segregated, processing streams. To reveal patterns of gene expression in the visual thalamus correlated with physiological processing streams, we designed a custom ferret cDNA microarray. By isolating specific subregions and layers of the thalamus, we identified a set of transcription factors, including Zic2, Islet1, and Six3, the unique distribution profiles of which differentiated the lateral geniculate nucleus (LGN) from the associated perigeniculate nucleus. Within the LGN, odd homeobox1 differentiated the A layers, which contain X cells and Y cells, from the C layers. One neuron-specific protein, Purkinje cell protein 4 (PCP4), was strongly expressed in Y cells in the ferret LGN and in the magnocellular layers of the primate LGN. In the ferret LGN, PCP4 expression began as early as postnatal day 7 (P7), suggesting that Y cells are already specified by P7. These results reveal a rich molecular repertoire that correlates with functional divisions of the LGN.

Brain nitric oxide synthase expression in the developing ferret lateral geniculate nucleus: analysis of time course, localization, and synaptic contacts.

Nitric oxide (NO) is a diffusible neurotransmitter that has been implicated in key developmental events, including the refinement of retinogeniculate axons into ON/OFF sublayers in the ferret lateral geniculate nucleus (LGN), and in the formation of eye-specific laminae in other species. To understand the role of NO in the LGN, it is critical to fully characterize the pattern of brain nitric oxide synthase (bNOS) expression within the nucleus, including the phenotype of the neural elements that express it. We have examined the temporal and spatial pattern of bNOS expression in the ferret LGN during the first 6 weeks of postnatal development, and in the adult, by detecting bNOS with a monoclonal antibody as well as beta-nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry. We have found that bNOS is expressed in neurons in the A laminae of the LGN as early as postnatal day 7 (P7), a time coincident with eye-specific segregation of retinal axons. This expression continues through P35, with peak somatodendritic expression at P21. Fluorescent double labeling using antibodies to bNOS and glutamic acid decarboxylase indicate that bNOS is expressed in gamma-aminobutyric acid-ergic interneurons within the A laminae. Electron microscopic examination of bNOS-labeled cells showed synaptic contacts from terminals with two distinct morphologic profiles. Expression of bNOS within interneurons that receive contacts from multiple sources indicates that the synaptic circuitry associated with bNOS activation and the potential targets of NO may be more complex than originally thought and supports a potential new role for interneurons as cellular intermediaries in the refinement of pathways in the LGN. Our findings broaden the window of time that bNOS may be active within the developing LGN, suggesting an expanded role for NO during early postnatal development.

Role of retinal afferents in regulating growth and shape of the lateral geniculate nucleus.

Segregated binocular maps in the dorsal lateral geniculate nucleus (LGN) develop from a stage where they initially overlap. Sophisticated computational models have been used to describe the dynamics of three-dimensional LGN shape changes that play a role in segregation. These models have revealed specific nuclear growth vectors associated with the process of ocular segregation in the LGN (Williams and Jeffery [2001] J Comp Neurol 430:332-342.). In this study, we use similar techniques to determine whether retinal innervation contributes to the dynamics of shape maturation in the ferret LGN. In this animal, 90% of the retinal innervation of the mature LGN comes from the contralateral eye. If one eye is removed before segregation, the projection from the remaining eye remains diffuse and nuclear growth is stunted. Here, we quantify this effect and show that removing the contralateral projection before segregation has a profound impact on LGN size but changes its ultimate shape by only 12%. The impact on shape on the other side of the brain where the ipsilateral projection is removed, which accounts for only 10% of its innervation in maturity, is less than 2%. Hence, retinal innervation plays a minor role in determining mature LGN shape. Although in both hemispheres, the ultimate shape of the nucleus is close to normal, removal of the larger projection disrupts normal growth vectors, with almost none being present in the 5 days after enucleation, when the normal nucleus expands markedly. Hence, the effect of enucleation is to delay shape maturation. Growth vectors absent after removal of the smaller projection are mainly confined to those in what would be the binocular region.

Early development of ocular dominance columns. Crowly JC, Katz LC. Science 2000;290:1321–1324.

The segregation of lateral geniculate nucleus (LGN) axons into ocular dominance columns is believed to involve a prolonged, activity-dependent sorting process. However, visualization of early postnatal ferret LGN axons by direct LGN tracer injections revealed segregated ocular dominance columns less than seven days after innervation of layer four. These early columns were unaffected by experimentally induced imbalances in retinal activity, implying that different mechanisms govern initial column formation and their modification during the subsequent critical period. Instead of activity-dependent plasticity, the authors propose that ocular dominance column formation relies on the targeting of distinct axonal populations to defined locales in cortical layer four. This study shows that ocular dominance columns in ferrets appear long before the columns can be modified by visual experience. The data presented suggests that neural activity from the eyes has little influence over the formation of the columns, questioning the earlier work of Hubel and Wiesel.—Hans E. Grossniklaus

Growth dynamics of the developing lateral geniculate nucleus

Segregated binocular maps in the lateral geniculate nucleus (LGN) develop from stages where they initially completely overlap. Here, we show that segregation occurs at different rates across the depth of the nucleus and that the volume of the ipsilateral projection does not decrease significantly during this period, rather LGN volume expands markedly and its shape changes. Hence, we have examined the differential growth of the ferret LGN during the process of segregation by using novel shape modelling techniques. These have facilitated quantification of its three-dimensional structure at successive developmental stages as well as the definition of growth vectors which illustrate shape change. This has been undertaken in direct representations of the LGN and those normalised for size and orientation. Spatiotemporal aspects of shape change have then been compared with different measurements of its cellular population. Initial stages of segregation are associated with a large expansion of the rostrocaudal axis of the nucleus along which segregation takes place, and an expansion of caudal regions that will eventually contain the binocular representation. Later stages are associated with dorsoventral expansions and a consolidation of the rostrocaudal axis. The pace of shape change peaks toward the end of the period of segregation when the nucleus has adopted ∼50% of its adult shape. After segregation, nuclear growth is mainly isotropic. The mature shape of the nucleus is achieved before it reaches its full size and while cell density and cell sizes are still changing. J. Comp. Neurol. 430:332–342, 2001. © 2001 Wiley-Liss, Inc.

Growth dynamics of the developing lateral geniculate nucleus

Segregated binocular maps in the lateral geniculate nucleus (LGN) develop from stages where they initially completely overlap. Here, we show that segregation occurs at different rates across the depth of the nucleus and that the volume of the ipsilateral projection does not decrease significantly during this period, rather LGN volume expands markedly and its shape changes. Hence, we have examined the differential growth of the ferret LGN during the process of segregation by using novel shape modelling techniques. These have facilitated quantification of its three-dimensional structure at successive developmental stages as well as the definition of growth vectors which illustrate shape change. This has been undertaken in direct representations of the LGN and those normalised for size and orientation. Spatiotemporal aspects of shape change have then been compared with different measurements of its cellular population. Initial stages of segregation are associated with a large expansion of the rostrocaudal axis of the nucleus along which segregation takes place, and an expansion of caudal regions that will eventually contain the binocular representation. Later stages are associated with dorsoventral expansions and a consolidation of the rostrocaudal axis. The pace of shape change peaks toward the end of the period of segregation when the nucleus has adopted approximately 50% of its adult shape. After segregation, nuclear growth is mainly isotropic. The mature shape of the nucleus is achieved before it reaches its full size and while cell density and cell sizes are still changing.

Early development of ocular dominance columns.

The segregation of lateral geniculate nucleus (LGN) axons into ocular dominance columns is believed to involve a prolonged, activity-dependent sorting process. However, visualization of early postnatal ferret LGN axons by direct LGN tracer injections revealed segregated ocular dominance columns <7 days after innervation of layer 4. These early columns were unaffected by experimentally induced imbalances in retinal activity, implying that different mechanisms govern initial column formation and their modification during the subsequent critical period. Instead of activity-dependent plasticity, we propose that ocular dominance column formation relies on the targeting of distinct axonal populations to defined locales in cortical layer 4.

Activity-Dependent Patterning of Retinogeniculate Axons Proceeds with a Constant Contribution from AMPA and NMDA Receptors

Neural activity is critical for the refinement of neural circuitry during development, although the mechanisms involved in stabilizing appropriate connectivity remain unclear. It has been proposed that the insertion of AMPA receptors at synapses with only NMDA receptors ("silent synapses") mediates this stabilization, leading to an increasing contribution from AMPA receptors as development proceeds. Here we show in a mammalian system known to undergo activity-dependent development [the segregation of retinal afferents into ON/OFF sublaminae in the ferret lateral geniculate nucleus (LGN)] that the refinement of the neural circuitry occurs in the presence of a constant functional contribution from AMPA and NMDA receptors. Although we detected a small number of silent synapses on LGN cells, their proportion did not decrease with age. The size and kinetics of NMDA-mediated spontaneous EPSCs (sEPSCs) were also stable over this period. Together with previous results reporting the stability of unitary AMPA-mediated EPSCs, the constancy of NMDA-mediated sEPSCs indicates an unchanging AMPA/NMDA contribution. Additionally, the CNQX-sensitivity did not increase for either sEPSCs or optic tract-evoked EPSCs. Likewise, the anatomical AMPA/NMDA ratio, as assayed by quantifying the colocalized expression of AMPA and NMDA receptor subunits, was fixed throughout ON/OFF sublamination. In particular, the colocalization of AMPA receptor subunits (GluR1 or GluR4) and NMDA receptor subunit NR1 opposite identified retinogeniculate terminals was stable during this period. These results add to the view of the population of retinogeniculate synapses as robustly stable or normalized during a period when retinogeniculate axons are undergoing dramatic activity-dependent refinement.

Corticothalamic Inputs Control the Pattern of Activity Generated in Thalamocortical Networks

Absence seizures (3-4 Hz) and sleep spindles (6-14 Hz) occur mostly during slow-wave sleep and have been hypothesized to involve the same corticothalamic network. However, the mechanism by which this network transforms from one form of activity to the other is not well understood. Here we examine this question using ferret lateral geniculate nucleus slices and stimulation of the corticothalamic tract. A feedback circuit, meant to mimic the cortical influence in vivo, was arranged such that thalamic burst firing resulted in stimulation of the corticothalamic tract. Stimuli were either single shocks to mimic normal action potential firing by cortical neurons or high-frequency bursts (six shocks at 200 Hz) to simulate increased cortical firing, such as during seizures. With one corticothalamic stimulus per thalamic burst, 6-10 Hz oscillations resembling spindle waves were generated. However, if the stimulation was a burst, the network immediately transformed into a 3-4 Hz paroxysmal oscillation. This transition was associated with a strong increase in the burst firing of GABAergic perigeniculate neurons. In addition, thalamocortical neurons showed a transition from fast (100-150 msec) IPSPs to slow ( approximately 300 msec) IPSPs. The GABA(B) receptor antagonist CGP 35348 blocked the slow IPSPs and converted the 3-4 Hz paroxysmal oscillations back to 6-10 Hz spindle waves. Conversely, the GABA(A) receptor antagonist picrotoxin blocked spindle frequency oscillations resulting in 3-4 Hz oscillations with either single or burst stimuli. We suggest that differential activation of thalamic GABA(A) and GABA(B) receptors in response to varying corticothalamic input patterns may be critical in setting the oscillation frequency of thalamocortical network interactions.

Diverse receptive fields in the lateral geniculate nucleus during thalamocortical development.

Most models of thalamocortical development in the visual system assume a homogeneous population of thalamic inputs to the cortex, each with concentric on- or off-center receptive fields. To test this, we made high-resolution spatial maps of receptive fields in the developing ferret lateral geniculate nucleus (LGN). Developing receptive fields (RFs), had a variety of shapes: some concentric, others elongated (like adult cortical receptive fields) and some with 'hot spots' of sensitivity. These receptive fields seemed to arise from convergence of multiple retinal afferents onto LGN neurons. We present a Hebbian model whereby imprecise retinogeniculate connections help refine geniculocortical connections, sharpening both thalamocortical topography and perhaps orientation selectivity.

Necessity for afferent activity to maintain eye-specific segregation in ferret lateral geniculate nucleus.

In the adult mammal, retinal ganglion cell axon arbors are restricted to eye-specific layers in the lateral geniculate nucleus. Blocking neuronal activity early in development prevents this segregation from occurring. To test whether activity is also required to maintain eye-specific segregation, ganglion cell activity was blocked after segregation was established. This caused desegregation, so that both eyes' axons became concentrated in lamina A, normally occupied only by contralateral afferents. These results show that an activity-dependent process is necessary for maintaining eye-specific segregation and suggest that activity-independent cues may favor lamina A as the target for arborization of afferents from both eyes.

Erratum

Hahm J-O, Cramer KS, Sur M. 1999. Pattern formation by retinal afferents in the ferret lateral geniculate nucleus: developmental segregation and the role of N-methyl-D-aspartate receptors. J Comp Neurol 411:327-345. Due to a printer's error, Figures 6 and 12 were transposed after the proofs of the article had been approved by the author. The correct figures and legends are reprinted on the following pages. The publisher regrets the error.

Pattern formation by retinal afferents in the ferret lateral geniculate nucleus: Developmental segregation and the role of N-methyl-D-aspartate receptors

Pattern formation by retinal afferents in the ferret lateral geniculate nucleus: Developmental segregation and the role of N-methyl-D-aspartate receptors

The neuronal form of nitric oxide synthase is required for pattern formation by retinal afferents in the ferret lateral geniculate nucleus

The ferret retinogeniculate projection undergoes activity-dependent refinement of connections that become restricted to eye specific laminae and On/Off sublaminae in the lateral geniculate nucleus (LGN). We have previously shown that the developmental process by which On/Off sublaminae form requires N-methyl- d-aspartate (NMDA) receptors and nitric oxide (NO). In this study, we investigate the role of the neuronal form of NO synthase (nNOS) in sublaminar refinement. This isoform of NOS may be coupled with NMDA receptors at postsynaptic sites. We found that nNOS is present in the developing LGN, and that blocking nNOS during development disrupts the formation of On/Off sublaminae. Endothelial NOS (eNOS) is not expressed in the LGN until after sublaminae have formed. These results suggest that the nNOS isoform is the predominant contributor of NO during development, and support the hypothesis that NO acts downstream of NMDA receptor activation to mediate activity-dependent changes in the patterning of connections in the LGN.

Pattern formation by retinal afferents in the ferret lateral geniculate nucleus: developmental segregation and the role of N-methyl-D-aspartate receptors.

The projection from the retina to the lateral geniculate nucleus (LGN) in ferrets segregates during development into eye-specific layers and ON/OFF sublayers. The projection pattern and the morphology of single axons was examined at several postnatal ages. The axons progress from a simple, sparsely branched morphology at birth to crude arbors at postnatal day 7 (P7). At P14-P15, axons have terminal arbors that span one eye-specific layer. By P19-P21, retinal afferents in the A layers have segregated into inner and outer sublaminae that correspond to ON- and OFF-center cells. Sublaminae form mainly by directed growth of terminal arbors in appropriately positioned regions of the LGN, along with elimination of extraneous branches in inappropriate regions. From P28 to P35, the LGN assumes an adult-like shape, and retinogeniculate axons form terminal boutons on branch endings. During the period between P14 and P21, when retinogeniculate axons segregate into ON/OFF sublaminae, N-methyl-D-aspartate (NMDA) receptors were blocked with chronic infusion of specific antagonists into the LGN. NMDA receptor blockade prevents the retinal afferent segregation into ON/OFF sublaminae. Some individual retinogeniculate axons have arbors that are not restricted appropriately, and most are restricted in size but are located inappropriately within the eye-specific laminae. Thus, NMDA receptor blockade prevents the positioning of retinogeniculate arbors that lead to the formation of ON/OFF sublaminae in the LGN. These results indicate that the activity of postsynaptic cells, and the activation of NMDA receptors in particular, can influence significantly the patterning of inputs and the structure of presynaptic afferents during development.