Maturation of visual receptive field properties in the rat superior colliculus

Formation of the Looming-evoked Innate Defensive Response during Postnatal Development in Mice

Environmental threats often trigger innate defensive responses in mammals. However, the gradual development of functional properties of these responses during the postnatal development stage remains unclear. Here, we report that looming stimulation in mice evoked flight behavior commencing at P14-16 and had fully developed by P20-24. The visual-evoked innate defensive response was not significantly altered by sensory deprivation at an early postnatal stage. Furthermore, the percentages of wide-field and horizontal cells in the superior colliculus were notably elevated at P20-24. Our findings define a developmental time window for the formation of the visual innate defense response during the early postnatal period and provide important insight into the underlying mechanism.

Visual Function, Organization, and Development of the Mouse Superior Colliculus

The superior colliculus (SC) is the most prominent visual center in mice. Studies over the past decade have greatly advanced our understanding of the function, organization, and development of the mouse SC, which has rapidly become a popular model in vision research. These studies have described the diverse and cell-type-specific visual response properties in the mouse SC, revealed their laminar and topographic organizations, and linked the mouse SC and downstream pathways with visually guided behaviors. Here, we summarize these findings, compare them with the rich literature of SC studies in other species, and highlight important gaps and exciting future directions. Given its clear importance in mouse vision and the available modern neuroscience tools, the mouse SC holds great promise for understanding the cellular, circuit, and developmental mechanisms that underlie visual processing, sensorimotor transformation, and, ultimately, behavior.

Retinal origin of direction selectivity in the superior colliculus

Detecting visual features in the environment such as motion direction is crucial for survival. The circuit mechanisms that give rise to direction selectivity in a major visual center, the superior colliculus (SC), are entirely unknown. Here, we optogenetically isolate the retinal inputs that individual direction-selective SC neurons receive and find that they are already selective as a result of precisely converging inputs from similarly-tuned retinal ganglion cells. The direction selective retinal input is linearly amplified by the intracollicular circuits without changing its preferred direction or level of selectivity. Finally, using 2-photon calcium imaging, we show that SC direction selectivity is dramatically reduced in transgenic mice that have decreased retinal selectivity. Together, our studies demonstrate a retinal origin of direction selectivity in the SC, and reveal a central visual deficit as a consequence of altered feature selectivity in the retina.

The role of sensory modality in prepulse inhibition: An ontogenetic study

Deficits in prepulse inhibition (PPI), a measure of sensorimotor gating, are observed in neurodevelopmental and neuropsychiatric disorders. Despite the large PPI literature, the majority of studies characteristically employ tests with one interstimulus interval (ISI), of one modality, at one age. In the context of the auditory startle response (ASR), the present study examined (1) the profile for the ontogeny of PPI through adulthood in Long-Evans hooded rats with a reasonably comprehensive ISI function, (2) whether the ontogenetic profile for PPI is sensitive to modality of the prepulse stimulus, as a within-session variable, and (3) whether the maturation of PPI differs for males and females. Despite the basic effect of more pronounced PPI in adult relative to preweanling animals, each sensory modality displayed a unique ontogenetic profile for PPI, without any compelling evidence for major differences between males and females, in accordance with the known temporal course of peripheral and central maturational profiles of sensory systems in the rat. The context for assessing auditory PPI (auditory and tactile vs. auditory and visual prepulses) influenced the overall startle response, i.e., a shift in the height of the entire profile, but did not significantly impact the auditory PPI profile per se. The translational relevance of preclinical sensorimotor assessments to patients with neurodevelopmental and/or neuropsychiatric disorders depends partly on an understanding of the ontogeny of sensorimotor gating in different sensory systems, and can be strengthened with the use of a reasonably comprehensive number of ISIs to provide relatively precise and defined response functions.

Spatial synchronization of visual stimulus-evoked gamma frequency oscillations in the rat superior colliculus

In the superior colliculus, visual stimuli can induce gamma frequency oscillations of neuronal activity. It has been shown that in cats, these oscillations are synchronized over distances of greater than 300 μm that may contribute toward visual information processing. We investigated the spatial properties of such oscillations in a rodent because the availability of molecular tools could enable future studies on the role of these oscillations in visual information processing. Using extracellular electrode array recordings in anesthetized rats, we found that visual stimuli-induced gamma and eta frequency (30-115 Hz) oscillations of the local field potential that were synchronized over distances of ∼ 600 μm. Multiple-unit events were phase locked to the local field potential signal and showed prominent oscillations during OFF responses. The rate of lower than 5 ms cross-electrode coincidences was in line with the response-corrected predictions for each electrode. These data suggest that the synchronized superior colliculus neuronal activity is largely network driven, whereas common synaptic inputs play a minor role.

Neuronal correlates of attention and its disengagement in the superior colliculus of rat

Orienting attention to a new target requires prior disengagement of attention from the current focus. Previous studies indicate that the superior colliculus (SC) plays an important role in attention. However, recordings of responses of SC neurons during attentional disengagement have not yet been reported. Here, we analyzed rat SC neuronal activity during performance of an attention-shift task with and without disengagement. In this task, conditioned stimuli (CSs; right and/or left light-flash or sound) were sequentially presented. To obtain an intracranial self-stimulation reward, rats were required to lick a spout when an infrequent conditioned stimulus appeared (reward trials). In the disengagement reward trials, configural stimuli consisting of an infrequent stimulus and frequent stimulus in the former trials were presented; in the non-disengagement reward trials, only an infrequent stimulus was presented. Of the 186 SC neurons responding to the CSs, 41 showed stronger responses to the CSs in the disengagement reward trials than in the non-disengagement reward trials (disengagement-related neurons). Furthermore, lick latencies in the disengagement reward trials were negatively correlated with response magnitudes to the CSs in half of the disengagement-related neurons. These disengagement-related neurons were located mainly in the deep layers of the SC. Another 70 SC neurons responded to the CSs in both disengagement and non-disengagement reward trials, suggesting that these neurons were involved in attention engagement. Our results suggest complementary mechanisms of attentional shift based on two subpopulations of neurons in the SC.

Refinement but not maintenance of visual receptive fields is independent of visual experience

Visual deprivation is reported to prevent or delay the development of mature receptive field (RF) properties in primary visual cortex (V1) in several species. In contrast, visual deprivation neither prevents nor delays refinement of RF size in the superior colliculus (SC) of Syrian hamsters, although vision is required for RF maintenance in the SC. Here, we report that, contrary to expectation, visual cortical RF refinement occurs normally in dark-reared animals. As in the SC, a brief period of visual experience is required to maintain V1 RF refinement in adulthood. Whereas in the SC, 3 days of visual experience within a sensitive period (P37-40) was sufficient to protect RFs from deprivation-induced enlargement in adulthood, 7 days (P33-40) were required for RF size maintenance in V1. Thus, spontaneous activity is sufficient for RF refinement at these 2 levels of the visual pathway, and visual input is necessary only to prevent deprivation-induced RF enlargement in adulthood. These studies show that sensory experience during a late juvenile sensitive period protects the visual pathway against sensory deprivation in adulthood, and suggest that more importance may have been placed on the role of early visual experience in visual RF development than is warranted.

Visual receptive field properties of neurons in the superficial superior colliculus of the mouse

The mouse is a promising model in the study of visual system function and development because of available genetic tools. However, a quantitative analysis of visual receptive field properties had not been performed in the mouse superior colliculus (SC) despite its importance in mouse vision and its usefulness in developmental studies. We have made single-unit extracellular recordings from superficial layers of the SC in urethane-anesthetized C57BL/6 mice. We first map receptive fields with flashing spot stimuli and show that most SC neurons have spatially overlapped ON and OFF subfields. With drifting sinusoidal gratings, we then determine the tuning properties of individual SC neurons, including selectivity for stimulus direction and orientation, spatial frequency tuning, temporal frequency tuning, response linearity, and size preference. A wide range of receptive field sizes and selectivity are observed across the population and in various subtypes of SC neurons identified morphologically. In particular, orientation-selective responses are discovered in the mouse SC, and they are not affected by cortical lesion or long-term visual deprivation. However, ON/OFF characteristics and spatial frequency tuning of SC neurons are influenced by cortical inputs and require visual experience during development. Together, our results provide essential information for future investigations on the functional development of the superior colliculus.

Direction-specific disruption of subcortical visual behavior and receptive fields in mice lacking the beta2 subunit of nicotinic acetylcholine receptor

Retinotopic mapping is a basic feature of visual system organization, but its role in processing visual information is unknown. Mutant mice lacking the beta2 subunit of nicotinic acetylcholine receptor have imprecise maps in both visual cortex (V1) and the superior colliculus (SC) due to the disruption of spontaneous retinal activity during development. Here, we use behavioral and physiological approaches to study their visual functions. We find that beta2-/- mice fail to track visual stimuli moving along the nasotemporal axis in a subcortical optomotor behavior, but track normally along the dorsoventral axis. In contrast, these mice display normal acuity along both axes in the visual water task, a behavioral test of cortical functions. Consistent with the behavioral results, we find that V1 neurons in beta2-/- mice have normal response properties, while SC neurons have disrupted receptive fields, including enlarged structure and decreased direction and orientation selectivity along the nasotemporal axis. The subcortical-specific deficits indicate that retinotopic map disruption has different impacts on the development of functional properties in V1 and the SC.

Functional MRI of postnatal visual development in normal and hypoxic-ischemic-injured superior colliculi

The superior colliculus (SC) is a laminated subcortical structure in the mammalian midbrain, whose superficial layers receive visual information from the retina and the visual cortex. To date, its functional organization and development in the visual system remain largely unknown. This study employed blood oxygenation level-dependent (BOLD) functional MRI to evaluate the visual responses of the SC in normally developing and severe neonatal hypoxic-ischemic (HI)-injured rat brains from the time of eyelid opening to adulthood. MRI was performed to the normal animals (n=7) at postnatal days (P) 14, 21, 28 and 60. In the HI-injured group (n=7), the ipsilesional primary and secondary visual cortices were completely damaged after unilateral ligation of the left common carotid artery at P7 followed by hypoxia for 2 h, and MRI was performed at P60. Upon unilateral flash illumination, the normal contralateral SC underwent a systematic increase in BOLD signal amplitude with age especially after the third postnatal week. However, no significant difference in BOLD signal increase was found between P14 and P21. These findings implied the presence of neurovascular coupling at the time of eyelid opening, and the progressive development of hemodynamic regulation in the subcortical visual system. In the HI-injured group at P60, the BOLD signal increases in both SC remained at the same level as the normal group at P28 though they were significantly lower than the normal group at P60. These observations suggested the residual visual functions on both sides of the subcortical brain, despite the damages to the entire ipsilesional visual cortex. The results of this study constitute important evidence on the progressive maturation of visual functions and hemodynamic responses in the normal subcortical brain, and its functional plasticity upon neonatal HI injury.

Inhibitory plasticity facilitates recovery of stimulus velocity tuning in the superior colliculus after chronic NMDA receptor blockade

The developing nervous system is shaped in important ways by spontaneous and stimulus-driven neural activity. Perturbation of normal activity patterns can profoundly affect the development of some neural response properties, whereas others are preserved through mechanisms that either compensate for or are unaffected by the perturbation. Most studies have examined the role of excitation in activity-dependent plasticity of response properties. Here, we examine the role of inhibition within the context of response selectivity for moving stimuli. The spatial extent of retinal input to the developing hamster superior colliculus (SC) can be experimentally increased by chronic NMDA receptor (NMDAR) blockade. Remarkably, stimulus velocity tuning is intact despite the increase in excitatory inputs. The goal of this study was to investigate whether plasticity in surround inhibition might provide the mechanism underlying this preservation of velocity tuning. Surround inhibition shapes velocity tuning in the majority of superficial layer SC neurons in normal hamsters. We show that despite the NMDAR blockade-induced increase in feedforward excitatory convergence from the retina, stimulus velocity tuning in the SC is maintained via compensatory plasticity in surround inhibition. The inhibitory surround increased in strength and spatial extent, and surround inhibition made a larger contribution to velocity tuning in the SC after chronic NMDAR blockade. These results show that inhibitory plasticity can preserve the balance between excitation and inhibition that is necessary to preserve response properties after developmental manipulations of neural activity. Understanding these compensatory mechanisms may permit their use to facilitate recovery from trauma or sensory deprivation.

Most superficial sublamina of rat superior colliculus: neuronal response properties and correlates with perceptual figure-ground segregation

The uppermost layer (stratum griseum superficiale, SGS) of the superior colliculus (SC) provides an important gateway from the retina to the visual extrastriate and visuomotor systems. The majority of attention has been given to the role of this "visual" SC in saccade generation and target selection and it is generally considered to be less important in visual perception. We have found, however, that in the rat SGS1, the most superficial division of the SGS, the neurons perform very sophisticated analysis of visual information. First, in studying their responses with a variety of flashing stimuli we found that the neurons respond not to brightness changes per se, but to the appearance and/or disappearance of visual shapes in their receptive fields (RFs). Contrary to conventional RFs of neurons at the early stages of visual processing, the RFs in SGS1 cannot be described in terms of fixed spatial distribution of excitatory and inhibitory inputs. Second, SGS1 neurons showed robust orientation tuning to drifting gratings and orientation-specific modulation of the center response from surround. These are features previously seen only in visual cortical neurons and are considered to be involved in "contour" perception and figure-ground segregation. Third, responses of SGS1 neurons showed complex dynamics; typically the response tuning became progressively sharpened with repetitive grating periods. We conclude that SGS1 neurons are involved in considerably more complex analysis of retinal input than was previously thought. SGS1 may participate in early stages of figure-ground segregation and have a role in low-resolution nonconscious vision as encountered after visual decortication.

Early visual experience prevents but cannot reverse deprivation-induced loss of refinement in adult superior colliculus

The role of sensory experience in the development and plasticity of the visual system has been widely studied. It has generally been reported that once animals reach adulthood, experience-dependent visual plasticity is reduced. We have found that visual experience is not needed for the refinement of receptive fields (RFs) in the superior colliculus (SC) but instead is necessary to maintain them in adulthood (Carrasco et al., 2005). Without light exposure, RFs in SC of hamsters refine by postnatal day 60 as usual but then enlarge, presumably reducing visual acuity. In this study we examine whether a brief period of light exposure during early postnatal development would be sufficient to prevent RF enlargement in adulthood, and whether prolonged light exposure in adulthood could reverse the deprivation-induced increase in RF size. We found that an early postnatal period of at least 30 days of visual experience was sufficient to maintain refined RFs in the adult SC. Prolonged visual experience in adulthood could not reverse the RF enlargement resulting from long-term dark rearing, reflecting a loss of plasticity at this age. Our results suggest that, unlike in visual cortex, dark rearing does not indefinitely extend the critical period of plasticity in SC. Rather, there is a limited time window when early experience can protect RFs from the detrimental effects of visual deprivation in adulthood. These results contribute to understanding adult brain plasticity and argue for the importance of early visual experience in protecting the adult visual system.

Spatio-temporal receptive field properties of cells in the rat superior colliculus

Although the rat is widely used in neurobehavioural research, the spatio-temporal receptive field properties of neurons in superficial layers of the superior colliculus are relatively unknown. Extracellular recordings were carried out in anesthetized Long Evans rats. Neurons in these layers had simple-like and complex-like receptive fields (RFs). Most cells (67%) had RFs showing band-pass and low-pass spatial frequency (SF) tuning profiles. Spatial band-pass profiles showed low optimal SF (mean=0.03 c/deg), low spatial resolution (mean=0.18 c/deg) and large spatial bandwidths (mean=2.3 octaves). More than two-thirds of the RFs (71%) were selective to orientation and only 11% were clearly direction selective. Nearly two-thirds of cells (68%) had band-pass temporal frequency (TF) tuning profiles with narrow bandwidths (mean=1.7 oct.) whereas the others showed low-pass TF tuning profiles. Temporal band-pass profiles had low optimal TFs (mean=3.5 c/s). Although some cells showed relatively low contrast thresholds (6%), most cells only responded to high contrast values (mean=38.2%). These results show that the spatial resolution of collicular cells is poor and that they respond mainly to highly contrasted moving stimuli.

Recruitment of local excitatory circuits in the superior colliculus following deafferentation and the regeneration of retinocollicular inputs

The local synaptic connectivity in the superficial gray layer of the superior colliculus (SC) was assessed following retinal ganglion cell axonal regeneration through a peripheral nerve graft into the SC of Lister Hooded rats, using in vitro brain slice techniques. Repair was effected between the ipsilateral eye and SC, following bilateral lesion of optic nerves and ablation of ipsilateral occipital cortex. Deafferentation surgery alone resulted in a complete loss of synaptic potentials of extrinsic origin, once both retinal and cortical inputs were removed. Stimulation of graft insertion sites elicited synaptic responses comprising monosynaptic and network-mediated depolarising events. This activity, together with similar spontaneous bursts of depolarising events and action potential firing, was generated by the activation of non-N-methyl-D-aspartate glutamate receptors. This behaviour may reflect the development of a local recurrent synaptic connectivity following the repair surgery, as both evoked and spontaneous responses developed into large long-lasting bursts of excitatory activity when inhibition mediated by GABA receptors was blocked. These results suggest that the ultrastructural changes in the superficial layers of the SC resulting from deafferentation are reflected functionally at the synaptic level in the target structure even after repair. Such changes are likely to compromise the ability of the target structure to function normally during information processing. Therefore, although axons regenerating along peripheral nerve grafts can make functional synaptic connections, their efficacy in activating the target structure will probably be compromised by local changes in synaptic connectivity.

Visual Experience Is Necessary for Maintenance But Not Development of Receptive Fields in Superior Colliculus

Sensory deprivation is thought to have an adverse effect on visual development and to prolong the critical period for plasticity. Once the animal reaches adulthood, however, synaptic connectivity is understood to be largely stable. We reported previously that N-methyl-d-aspartate (NMDA) receptor blockade in the superior colliculus of the Syrian hamster prevents refinement of receptive fields (RFs) in normal or compressed retinotopic projections, resulting in target neurons with enlarged RFs but normal stimulus tuning. Here we asked whether visually driven activity is necessary for refinement or maintenance of retinotopic maps or if spontaneous activity is sufficient. Animals were deprived of light either in adulthood only or from birth until the time of recording. We found that dark rearing from birth to 2 mo of age had no effect on the timing and extent of RF refinement as assessed with single unit extracellular recordings. Visual deprivation in adulthood also had no effect. Continuous dark rearing from birth into adulthood, however, resulted in a progressive loss of refinement, resulting in enlarged, asymmetric receptive fields and altered surround suppression in adulthood. Thus unlike in visual cortex, early visually driven activity is not necessary for refinement of receptive fields during development, but is required to maintain refined visual projections in adulthood. Because the map can refine normally in the dark, these results argue against a deprivation-induced delay in critical period closure, and suggest instead that early visual deprivation leaves target neurons more vulnerable to deprivation that continues after refinement.

Neural mechanisms of stimulus velocity tuning in the superior colliculus

Superior colliculus (SC)-mediated control of visuomotor behavior depends on neuronal selectivity for stimulus velocity. However, the mechanism responsible for velocity tuning in SC neurons is unclear. It was shown in a previous study of anesthetized, decorticate hamsters that the number and distribution of feed-forward retinal inputs are not critical for velocity tuning. Here the alternate hypothesis that inhibition from the surround determines velocity tuning of SC neurons was tested. Surround inhibition was present in 65% (43/66) of SC neurons recorded in the superficial gray layer. Neurons within this group that were selective for slowly moving stimuli exhibited spatially asymmetric surround inhibition, and their velocity tuning arose by preferential suppression of responses to rapidly moving stimuli. In the other 35% (23/66) of SC neurons recorded, surround inhibition was weak or absent and did not play a role in velocity tuning. Most neurons with surround inhibition were nonselective for the duration of stationary flashed stimuli, whereas neurons without surround inhibition were selective for stimulus duration. The majority of neurons that preferred intermediate or rapidly moving stimuli exhibited spatially symmetric surround inhibition. In these neurons, occluding the surround reduced velocity selectivity by enhancing responses to slowly moving stimuli. Based on these data, a model is proposed suggesting spatiotemporal interactions between inhibition and excitation that could underlie velocity tuning.

Postnatal expression of the plasticity-related nerve growth factor-induced gene A (NGFI-A) protein in the superficial layers of the rat superior colliculus: Relation to N-methyl-d-aspartate receptor function

Immediate early gene expression in the CNS is induced by sensory stimulation and seems to be involved in long-term synaptic plasticity. We have used an immunohistochemical method to detect the nerve growth factor-induced gene A (NGFI-A) protein expression in the superficial layers of the rat superior colliculus during postnatal development. Our goal was to correlate the expression of this candidate plasticity protein with developmental events, especially the activity-dependent refinement of the retinocollicular and corticocollicular pathways. We have also investigated the N-methyl-D-aspartate (NMDA)-receptor dependence of the NGFI-A expression. Animals of various postnatal ages were used. Postnatal day (P) 12 and older animals were submitted to a protocol of dark adaptation followed by light stimulation. NGFI-A expression was never observed during the first 2 postnatal weeks. The first stained cells were observed at P15, 2 days after eye opening (P13). The highest number of stained cells was observed at the end of the third postnatal week (P22). Adult-like level of expression was reached at P30, since at this age, the number of stained cells was comparable to that found in adult rats (P90). Both P22 animals submitted to an acute treatment with MK-801 (i.p. injection) and adult animals submitted to chronic intracranial infusion of a MK-801 presented a clear decrease in the NGFI-A expression in response to light stimulation. These results suggest that the NGFI-A expression is dependent on the NMDA receptor activation, and the observed pattern of expression is in close agreement with previous descriptions of the changes in the NMDA receptor-mediated visual activity in the developing rat superior colliculus (SC). Our results suggest that the plasticity-related NGFI-A protein might play a role in the developmental plasticity of the superficial layers of the rat SC after eye opening.

NMDA receptor blockade in the superior colliculus increases receptive field size without altering velocity and size tuning

Neonatal brain injury triggers compensatory processes that can be adaptive or detrimental, but little is known about the mechanisms of compensation or how they might affect the response properties of neurons within the injured region. We have studied this issue in a rodent model. Partial ablation of the hamster superior colliculus (SC) at birth results in a compressed but complete visual field map in the remaining SC and a compensatory conservation of receptive field (RF) size and stimulus velocity and size tuning. The circuit underlying stimulus tuning in this system or its preservation after brain lesions is not known. Our previous work has shown that N-methyl-d-aspartate (NMDA) receptors are necessary for the development and conservation of RF size after partial SC ablation. In this study, we examined whether NMDA receptor function is also necessary for the development and conservation of stimulus velocity and size tuning. We found that velocity and size tuning were unaffected by chronic postnatal blockade of NMDA receptors and the resulting increases in RF size. Thus NMDA receptors in the SC are not necessary for the development of stimulus velocity and size tuning or in the compensatory maintenance of these properties following brain damage. These results suggest that stimulus velocity and size tuning may arise in the retina or from NMDA receptor-independent circuitry intrinsic to SC. The lack of conflict between NMDA receptor activity-dependent and -independent processes may allow conservation of some RF properties while others change during injury-induced or evolutionary changes in afferent/target convergence.

Eye opening induces a rapid dendritic localization of PSD-95 in central visual neurons

The membrane-associated guanylate kinase PSD-95 scaffolds N-methyl-d-aspartate receptors to cytoplasmic signaling molecules, and associates with other glutamate receptors at central synapses. However, regulation of PSD-95 in vivo is poorly understood. We provide evidence of an activity-dependent redistribution of PSD-95 to dendrites in central visual neurons that is tied to eye opening. Six hours after eye opening, increased dendritic PSD-95 coimmunoprecipitates with the same proportions of stargazin, increased proportions of the N-methyl-d-aspartate receptor subunit NR2A, and decreased proportions of NR2B. Sustained high levels of PSD-95 in dendrites are dependent on continued pattern vision in juvenile but not mature animals, suggesting that the stabilization of PSD-95 at synapses may be involved in the control of developmental plasticity.

Functionally different AMPA-type glutamate receptors in morphologically identified neurons in rat superficial superior colliculus

In the superficial superior colliculus, a center of sensory processing related to visual salience, glutamate is used as a major excitatory neurotransmitter. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors include a Ca(2+)-impermeable, outwardly rectifying type (type I) and a Ca(2+)-permeable, inwardly rectifying type (type II). To study the contribution of these AMPA receptor subtypes to visual sensory processing in the superior colliculus, we investigated the expression of these two types of AMPA receptors in six morphologically identified subgroups of neurons in the superficial superior colliculus by whole-cell recording using slice preparations of young (17-23 days old) and adult (60-68 days old) rats. Both outwardly and inwardly rectifying current responses were observed to pressure applied 10 mM kainate, a non-desensitizing AMPA receptor agonist. These currents were completely abolished by the selective AMPA receptor antagonist 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine (100 microM). The type II receptor antagonist spermine (1 mM) suppressed inwardly rectifying responses. The degree of inward rectification was correlated with the ratio of suppression by spermine, and inversely correlated with estimated Ca(2+) permeability, indicating that the degree of rectification reflects the relative amount of co-expressed type I and type II receptors. An inwardly rectifying and spermine-sensitive AMPA component of excitatory postsynaptic currents was observed, suggesting involvement of type II receptors in synaptic transmission. Morphological analysis revealed that a substantial population of horizontal cells in both young and adult rats (n=31/53 and 15/17, respectively) and all wide field multipolar cells in adult rats (n=6) showed inwardly rectifying AMPA receptor responses. From these results we suggest that type I and type II AMPA receptors are co-expressed with varying ratios in individual neurons in the rat superficial superior colliculus, and that type II receptors are abundantly expressed in most horizontal cells and wide field multipolar cells. Since these neurons are putatively GABAergic inhibitory neurons and have wide dendritic trees, type II receptors may contribute to the regulation of remote inhibitory interaction in the visual field map in the the superficial superior colliculus.

NMDA antagonists in the superior colliculus prevent developmental plasticity but not visual transmission or map compression

Partial ablation of the superior colliculus (SC) at birth in hamsters compresses the retinocollicular map, increasing the amount of visual field represented at each SC location. Receptive field sizes of single SC neurons are maintained, however, preserving receptive field properties in the prelesion condition. The mechanism that allows single SC neurons to restrict the number of convergent retinal inputs and thus compensate for induced brain damage is unknown. In this study, we examined the role of N-methyl-D-aspartate (NMDA) receptors in controlling retinocollicular convergence. We found that chronic 2-amino-5-phosphonovaleric acid (APV) blockade of NMDA receptors from birth in normal hamsters resulted in enlarged single-unit receptive fields in SC neurons from normal maps and further enlargement in lesioned animals with compressed maps. The effect was linearly related to lesion size. These results suggest that NMDA receptors are necessary to control afferent/target convergence in the normal SC and to compensate for excess retinal afferents in lesioned animals. Despite the alteration in receptive field size in the APV-treated animals, a complete visual map was present in both normal and lesioned hamsters. Visual responsiveness in the treated SC was normal; thus the loss of compensatory plasticity was not due to reduced visual responsiveness. Our results argue that NMDA receptors are essential for map refinement, construction of receptive fields, and compensation for damage but not overall map compression. The results are consistent with a role for the NMDA receptor as a coincidence detector with a threshold, providing visual neurons with the ability to calculate the amount of visual space represented by competing retinal inputs through the absolute amount of coincidence in their firing patterns. This mechanism of population matching is likely to be of general importance during nervous system development.

Developmental depression of glutamate neurotransmission by chronic low-level activation of NMDA receptors

Slabs of slow-release plastic (Elvax) containing NMDA or solvent were implanted over the rat colliculus beginning on postnatal day 8 (P8). Whole-cell patch clamping in the superficial superior collicular layers (sSCs) from P10 to P21 demonstrated a severe decrease in spontaneous EPSC frequency after chronic NMDA treatment. The decrease was not attributable to an increase in GABA(A) receptor-mediated inhibition and was present only when NMDA receptor (NMDAR) current was blocked by Mg(2+). Analysis of miniature EPSCs indicated that many active sites on NMDA-treated neurons lacked functional AMPA and kainate receptor (AMPA/KAR) currents, and AMPA/KAR:NMDAR current ratios of evoked EPSCs were also significantly reduced. In addition, the normal downregulation of NMDAR decay time in sSC neurons at P11 was absent after NMDA treatment. Nevertheless, neither AMPA nor NMDA receptor subunit expression was altered by NMDA treatment, and experiments with the NMDAR antagonist ifenprodil suggested that incorporation of NR2A-containing NMDARs at the sSC synapses was unperturbed. Thus, disrupting but not blocking NMDARs suppresses the development of AMPA/KAR currents. The absence of the P11 NMDAR current downregulation is likely a secondary effect resulting from the reduction of AMPA/KAR function. Chronic agonist application reduces but does not eliminate NMDAR conductances. Therefore these data support an active role for NMDAR currents in synaptic development. Prolonged NMDA treatment in vivo, which couples reduced postsynaptic Ca(2+) responses with normally developing afferent activity, produces a long-lasting synaptic depression and stalls glutamatergic synaptogenesis, suggesting that the correlation between robust NMDAR activation and afferent activity is an essential component during normal development.

Activity-dependent induction of tonic calcineurin activity mediates a rapid developmental downregulation of NMDA receptor currents

Whole-cell recording in the superficial layers of the developing superior colliculus (sSC) reveals a large drop in NMDA receptor (NMDAR) current decay time synchronized across all neurons and occurring consistently between P10 and P11. We show that blocking the Ca2+/calmodulin-dependent phosphatase calcineurin (CaN) in the postsynaptic neuron can abolish this drop. The regulation is induced prematurely by 1-2 hr of electrical stimulation in P10 collicular slices only if CaN and NMDAR currents can be activated in the neuron. These data suggest that a long-lasting, CaN-mediated control of NMDAR kinetics is rapidly initiated by heightened activity of the NMDAR itself and demonstrate a novel developmental and tonic function of CaN that can play an important role in modulating the plasticity of the developing CNS.