The role of glutamate in neuronal ion homeostasis: A case study of spreading depolarization.

Simultaneous changes in ion concentrations, glutamate, and cell volume together with exchange of matter between cell network and vasculature are ubiquitous in numerous brain pathologies. A complete understanding of pathological conditions as well as normal brain function, therefore, hinges on elucidating the molecular and cellular pathways involved in these mostly interdependent variations. In this paper, we develop the first computational framework that combines the Hodgkin-Huxley type spiking dynamics, dynamic ion concentrations and glutamate homeostasis, neuronal and astroglial volume changes, and ion exchange with vasculature into a comprehensive model to elucidate the role of glutamate uptake in the dynamics of spreading depolarization (SD)-the electrophysiological event underlying numerous pathologies including migraine, ischemic stroke, aneurysmal subarachnoid hemorrhage, intracerebral hematoma, and trauma. We are particularly interested in investigating the role of glutamate in the duration and termination of SD caused by K+ perfusion and oxygen-glucose deprivation. Our results demonstrate that glutamate signaling plays a key role in the dynamics of SD, and that impaired glutamate uptake leads to recovery failure of neurons from SD. We confirm predictions from our model experimentally by showing that inhibiting astrocytic glutamate uptake using TFB-TBOA nearly quadruples the duration of SD in layers 2-3 of visual cortical slices from juvenile rats. The model equations are either derived purely from first physical principles of electroneutrality, osmosis, and conservation of particles or a combination of these principles and known physiological facts. Accordingly, we claim that our approach can be used as a future guide to investigate the role of glutamate, ion concentrations, and dynamics cell volume in other brain pathologies and normal brain function.

WONOEP appraisal: molecular and cellular imaging in epilepsy.

Great advancements have been made in understanding the basic mechanisms of ictogenesis using single-cell electrophysiology (e.g., patch clamp, sharp electrode), large-scale electrophysiology (e.g., electroencephalography [EEG], field potential recording), and large-scale imaging (magnetic resonance imaging [MRI], positron emission tomography [PET], calcium imaging of acetoxymethyl ester [AM] dye-loaded tissue). Until recently, it has been challenging to study experimentally how population rhythms emerge from cellular activity. Newly developed optical imaging technologies hold promise for bridging this gap by making it possible to simultaneously record the many cellular elements that comprise a neural circuit. Furthermore, easily accessible genetic technologies for targeting expression of fluorescent protein-based indicators make it possible to study, in animal models of epilepsy, epileptogenic changes to neural circuits over long periods. In this review, we summarize some of the latest imaging tools (fluorescent probes, gene delivery methods, and microscopy techniques) that can lead to the advancement of cell- and circuit-level understanding of epilepsy, which in turn may inform and improve development of next generation antiepileptic and antiepileptogenic drugs.

Oxygen and seizure dynamics: I. Experiments.

We utilized a novel ratiometric nanoquantum dot fluorescence resonance energy transfer (NQD-FRET) optical sensor to quantitatively measure oxygen dynamics from single cell microdomains during hypoxic episodes as well as during 4-aminopyridine (4-AP)-induced spontaneous seizure-like events in rat hippocampal slices. Coupling oxygen sensing with electrical recordings, we found the greatest reduction in the O2 concentration ([O2]) in the densely packed cell body stratum (st.) pyramidale layer of the CA1 and differential layer-specific O2 dynamics between the st. pyramidale and st. oriens layers. These hypoxic decrements occurred up to several seconds before seizure onset could be electrically measured extracellularly. Without 4-AP, we quantified a narrow range of [O2], similar to the endogenous hypoxia found before epileptiform activity, which permits a quiescent network to enter into a seizure-like state. We demonstrated layer-specific patterns of O2 utilization accompanying layer-specific neuronal interplay in seizure. None of the oxygen overshoot artifacts seen with polarographic measurement techniques were observed. We therefore conclude that endogenously generated hypoxia may be more than just a consequence of increased cellular excitability but an influential and critical factor for orchestrating network dynamics associated with epileptiform activity.

Purinergic control of hippocampal circuit hyperexcitability in Dravet syndrome.

OBJECTIVE: Severe myoclonic epilepsy in infancy (SMEI) or Dravet syndrome is one of the most devastating childhood epilepsies. Children with SMEI have febrile and afebrile seizures (FS and aFS), ataxia, and social and cognitive dysfunctions. SMEI is pharmacologically intractable and can be fatal in 10-20% of patients. It remains to be elucidated how channelopathies that cause SMEI impact synaptic activities in key neural circuits, and there is an ongoing critical need for alternative methods of controlling seizures in SMEI. Using the SCN1A gene knock-in mouse model of SMEI (mSMEI), we studied hippocampal cell and circuit excitability, particularly during hyperthermia, and tested whether an adenosine A1 receptor (A1R) agonist can reliably control hippocampal circuit hyperexcitability. METHODS: Using a combination of electrophysiology (extracellular and whole-cell voltage clamp) and fast voltage-sensitive dye imaging (VSDI), we quantified synaptic excitation and inhibition, spatiotemporal characteristics of neural circuit activity, and hyperthermia-induced febrile seizure-like events (FSLEs) in juvenile mouse hippocampal slices. We used hyperthermia to elicit FSLEs in hippocampal slices, while making use of adenosine A1R agonist N6-cyclopentyladenosine (CPA) to control abnormally widespread neural activity and FSLEs. RESULTS: We discovered a significant excitation/inhibition (E/I) imbalance in mSMEI hippocampi, in which inhibition was decreased and excitation increased. This imbalance was associated with an increased spatial extent of evoked neural circuit activation and a lowered FSLE threshold. We found that a low concentration (50 nm) of CPA blocked FSLEs and reduced the spatial extent of abnormal neural activity spread while preserving basal levels of excitatory synaptic transmission. SIGNIFICANCE: Our study reveals significant hippocampal synapse and circuit dysfunctions in mSMEI and demonstrates that the A1R agonist CPA can reliably control hippocampal hyperexcitability and FSLEs in vitro. These findings may warrant further investigations of purinergic agonists as part of the development of new therapeutic approaches for Dravet syndrome.

Seizures as imbalanced up states: excitatory and inhibitory conductances during seizure-like events.

Precisely timed and dynamically balanced excitatory (E) and inhibitory (I) conductances underlie the basis of neural network activity. Normal E/I balance is often shifted in epilepsy, resulting in neuronal network hyperexcitability and recurrent seizures. However, dynamics of the actual excitatory and inhibitory synaptic conductances (ge and gi, respectively) during seizures remain unknown. To study the dynamics of E and I network balance, we calculated ge and gi during the initiation, body, and termination of seizure-like events (SLEs) in the rat hippocampus in vitro. Repetitive emergent SLEs in 4-aminopyridine (100 µM) and reduced extracellular magnesium (0.6 mM) were recorded in the identified CA1 pyramidal cells (PC) and oriens-lacunosum moleculare (O-LM) interneurons. Calculated ge/gi ratio dynamics showed that the initiation stage of the SLEs was dominated by inhibition in the PCs and was more balanced in the O-LM cells. During the body of the SLEs, the balance shifted toward excitation, with ge and gi peaking in both cell types at nearly the same time. In the termination phase, PCs were again dominated by inhibition, whereas O-LM cells experienced persistent excitatory synaptic barrage. In this way, increased excitability of interneurons may play roles in both seizure initiation (ziburkus J, Cressman JR, Barreto E, Schiff SJ. J Neurophysiol 95: 3948-3954, 2006) and in their termination. Overall, SLE stages can be characterized in PC and O-LM cells by dynamically distinct changes in the balance of ge and gi, where a temporal sequence of imbalance shifts with the changing firing patterns of the cellular subtypes comprising the hyperexcitable microcircuits.

ß-Adrenergic modulation of spontaneous spatiotemporal activity patterns and synchrony in hyperexcitable hippocampal circuits.

A description of healthy and pathological brain dynamics requires an understanding of spatiotemporal patterns of neural activity and characteristics of its propagation between interconnected circuits. However, the structure and modulation of the neural activation maps underlying these patterns and their propagation remain elusive. We investigated effects of ß-adrenergic receptor (ß-AR) stimulation on the spatiotemporal characteristics of emergent activity in rat hippocampal circuits. Synchronized epileptiform-like activity, such as interictal bursts (IBs) and ictal-like events (ILEs), were evoked by 4-aminopyridine (4-AP), and their dynamics were studied using a combination of electrophysiology and fast voltage-sensitive dye imaging. Dynamic characterization of the spontaneous IBs showed that they originated in dentate gyrus/CA3 border and propagated toward CA1. To determine how ß-AR modulates spatiotemporal characteristics of the emergent IBs, we used the ß-AR agonist isoproterenol (ISO). ISO significantly reduced the spatiotemporal extent and propagation velocity of the IBs and significantly altered network activity in the 1- to 20-Hz range. Dual whole cell recordings of the IBs in CA3/CA1 pyramidal cells and optical analysis of those regions showed that ISO application reduced interpyramidal and interregional synchrony during the IBs. In addition, ISO significantly reduced duration not only of the shorter duration IBs but also the prolonged ILEs in 4-AP. To test whether the decrease in ILE duration was model dependent, we used a different hyperexcitability model, zero magnesium (0 Mg(2+)). Prolonged ILEs were readily formed in 0 Mg(2+), and addition of ISO significantly reduced their durations. Taken together, these novel results provide evidence that ß-AR activation dynamically reshapes the spatiotemporal activity patterns in hyperexcitable circuits by altering network rhythmogenesis, propagation velocity, and intercellular/regional synchronization.

Neuromodulators control the polarity of spike-timing-dependent synaptic plasticity.

Near coincidental pre- and postsynaptic action potentials induce associative long-term potentiation (LTP) or long-term depression (LTD), depending on the order of their timing. Here, we show that in visual cortex the rules of this spike-timing-dependent plasticity are not rigid, but shaped by neuromodulator receptors coupled to adenylyl cyclase (AC) and phospholipase C (PLC) signaling cascades. Activation of the AC and PLC cascades results in phosphorylation of postsynaptic glutamate receptors at sites that serve as specific "tags" for LTP and LTD. As a consequence, the outcome (i.e., whether LTP or LTD) of a given pattern of pre- and postsynaptic firing depends not only on the order of the timing, but also on the relative activation of neuromodulator receptors coupled to AC and PLC. These findings indicate that cholinergic and adrenergic neuromodulation associated with the behavioral state of the animal can control the gating and the polarity of cortical plasticity.

LTD and LTP at the developing retinogeniculate synapse.

The purpose of the present study was to determine whether retinal activity can support long-term changes in synaptic strength in the developing dorsal lateral geniculate nucleus (LGN) of thalamus. To test for this we made use of a rodent in vitro explant preparation in which retinal afferents and the intrinsic circuitry of the LGN remain intact. We repetitively stimulated the optic tract with a tetanus protocol that approximated the temporal features of spontaneous retinal waves. We found the amplitude of extracellular field potentials evoked by retinal stimulation changed significantly after tetanus and that the polarity of these alterations was related to postnatal age. At a time when substantial pruning of retinal connections occurs (postnatal day 1 [P1] to P14), high-frequency stimulation led to an immediate and long-term depression (LTD). However, at times when pruning wanes and adult-like patterns of connectivity are stabilizing (P16 to P30), the identical form of stimulation produced a modest form of potentiation (long-term potentiation [LTP]). The LTD was unaffected by the bath application of gamma-aminobutyric acid type A and N-methyl-D-aspartate receptor antagonists. However, both LTD and LTP were blocked by L-type Ca(2+)-channel antagonists. Thus the Ca(2+) influx associated with L-type channel activation mediates the induction of synaptic plasticity and may signal the pruning and subsequent stabilization of developing retinogeniculate connections.

The influence of sodium and potassium dynamics on excitability, seizures, and the stability of persistent states: I. Single neuron dynamics.

In these companion papers, we study how the interrelated dynamics of sodium and potassium affect the excitability of neurons, the occurrence of seizures, and the stability of persistent states of activity. In this first paper, we construct a mathematical model consisting of a single conductance-based neuron together with intra- and extracellular ion concentration dynamics. We formulate a reduction of this model that permits a detailed bifurcation analysis, and show that the reduced model is a reasonable approximation of the full model. We find that competition between intrinsic neuronal currents, sodium-potassium pumps, glia, and diffusion can produce very slow and large-amplitude oscillations in ion concentrations similar to what is seen physiologically in seizures. Using the reduced model, we identify the dynamical mechanisms that give rise to these phenomena. These models reveal several experimentally testable predictions. Our work emphasizes the critical role of ion concentration homeostasis in the proper functioning of neurons, and points to important fundamental processes that may underlie pathological states such as epilepsy.

Three-dimensional GPU-accelerated active contours for automated localization of cells in large images.

Cell segmentation in microscopy is a challenging problem, since cells are often asymmetric and densely packed. Successful cell segmentation algorithms rely identifying seed points, and are highly sensitive to variablility in cell size. In this paper, we present an efficient and highly parallel formulation for symmetric three-dimensional contour evolution that extends previous work on fast two-dimensional snakes. We provide a formulation for optimization on 3D images, as well as a strategy for accelerating computation on consumer graphics hardware. The proposed software takes advantage of Monte-Carlo sampling schemes in order to speed up convergence and reduce thread divergence. Experimental results show that this method provides superior performance for large 2D and 3D cell localization tasks when compared to existing methods on large 3D brain images.

Robust Cell Detection for Large-Scale 3D Microscopy Using GPU-Accelerated Iterative Voting.

High-throughput imaging techniques, such as Knife-Edge Scanning Microscopy (KESM),are capable of acquiring three-dimensional whole-organ images at sub-micrometer resolution. These images are challenging to segment since they can exceed several terabytes (TB) in size, requiring extremely fast and fully automated algorithms. Staining techniques are limited to contrast agents that can be applied to large samples and imaged in a single pass. This requires maximizing the number of structures labeled in a single channel, resulting in images that are densely packed with spatial features. In this paper, we propose a three-dimensional approach for locating cells based on iterative voting. Due to the computational complexity of this algorithm, a highly efficient GPU implementation is required to make it practical on large data sets. The proposed algorithm has a limited number of input parameters and is highly parallel.

Effects of experimental traumatic brain injury and impaired glutamate transport on cortical spreading depression.

Cortical spreading depression (CSD) is associated with traumatic brain injury (TBI), stroke, migraines, and seizures. Typically, following TBIs and other insults, neuronal excitability in and around the area of the injury is affected, with reported increases in local glutamate signaling. Astrocytic glutamate transporters are critical for precise regulation of the extracellular glutamate availability. However, it remains unclear how impaired astrocytic glutamate transport or an acute TBI affect characteristics of the CSD. We quantified the properties of CSD using whole-cell and extracellular electrophysiological recordings, and voltage-sensitive dye imaging (VSDI) in rat visual cortex in vitro. To model impaired astrocytic glutamate transport, we used astrocytic glutamate transporter blocker (2S, 3S)-3-[3-[4-(trifluoromethyl) benzoylamino] benzyloxy] aspartate (TFB-TBOA). In addition, an acute incision through the superficial cortical layers was used to model the effects of acute traumatic brain injury (TBI) on CSD characteristics. Both manipulations; impaired glutamate cycling and acute cut profoundly affected the physiological properties of cell firing, latency to CSD formation, and its frequency of occurrence. VSD imaging analysis revealed significant changes in spatiotemporal dynamics and propagation of the CSD, suggesting that the cut itself may not initiate CSD depolarizing waves, but rather attract them. Blockade of GLT-1 caused significant reduction in whole-cell sodium currents and changes in CSD wave spatiotemporal characteristics as well, slowing it or even 'trapping' its propagation. Our results reveal new information about CSD properties in these pathological conditions and demonstrate an important role of GLT-1 in regulation of CSD.

Analyzing and Modeling the Dysfunction of Inhibitory Neurons in Alzheimer's Disease.

Alzheimer's disease (AD) is characterized by the abnormal proteolytic processing of amyloid precursor protein, resulting in increased production of a self-aggregating form of beta amyloid (Aß). Several lines of work on AD patients and transgenic mice with high Aß levels exhibit altered rhythmicity, aberrant neuronal network activity and hyperexcitability reflected in clusters of hyperactive neurons, and spontaneous epileptic activity. Recent studies highlight that abnormal accumulation of Aß changes intrinsic properties of inhibitory neurons, which is one of the main reasons underlying the impaired network activity. However, specific cellular mechanisms leading to interneuronal dysfunction are not completely understood. Using extended Hodgkin-Huxley (HH) formalism in conjunction with patch-clamp experiments, we investigate the mechanisms leading to the impaired activity of interneurons. Our detailed analysis indicates that increased Na+ leak explains several observations in inhibitory neurons, including their failure to reliably produce action potentials, smaller action potential amplitude, increased resting membrane potential, and higher membrane depolarization in response to a range of stimuli in a model of APPSWE/PSEN1DeltaE9 (APdE9) AD mice as compared to age-matched control mice. While increasing the conductance of hyperpolarization activated cyclic nucleotide-gated (HCN) ion channel could account for most of the observations, the extent of increase required to reproduce these observations render such changes unrealistic. Furthermore, increasing the conductance of HCN does not account for the observed changes in depolarizability of interneurons from APdE9 mice as compared to those from NTG mice. None of the other pathways tested could lead to all observations about interneuronal dysfunction. Thus we conclude that upregulated sodium leak is the most likely source of impaired interneuronal function.

From Molecular Circuit Dysfunction to Disease: Case Studies in Epilepsy, Traumatic Brain Injury, and Alzheimer's Disease.

Complex circuitry with feed-forward and feed-back systems regulate neuronal activity throughout the brain. Cell biological, electrical, and neurotransmitter systems enable neural networks to process and drive the entire spectrum of cognitive, behavioral, and motor functions. Simultaneous orchestration of distinct cells and interconnected neural circuits relies on hundreds, if not thousands, of unique molecular interactions. Even single molecule dysfunctions can be disrupting to neural circuit activity, leading to neurological pathology. Here, we sample our current understanding of how molecular aberrations lead to disruptions in networks using three neurological pathologies as exemplars: epilepsy, traumatic brain injury (TBI), and Alzheimer's disease (AD). Epilepsy provides a window into how total destabilization of network balance can occur. TBI is an abrupt physical disruption that manifests in both acute and chronic neurological deficits. Last, in AD progressive cell loss leads to devastating cognitive consequences. Interestingly, all three of these neurological diseases are interrelated. The goal of this review, therefore, is to identify molecular changes that may lead to network dysfunction, elaborate on how altered network activity and circuit structure can contribute to neurological disease, and suggest common threads that may lie at the heart of molecular circuit dysfunction.

Inhibitory neuron and hippocampal circuit dysfunction in an aged mouse model of Alzheimer's disease.

In Alzheimer's disease (AD), a decline in explicit memory is one of the earliest signs of disease and is associated with hippocampal dysfunction. Amyloid protein exerts a disruptive impact on neuronal function, but the specific effects on hippocampal network activity are not well known. In this study, fast voltage-sensitive dye imaging and extracellular and whole-cell electrophysiology were used on entorhinal cortical-hippocampal slice preparations to characterize hippocampal network activity in 12-16 month old female APPswe/PSEN1DeltaE9 (APdE9 mice) mice. Aged APdE9 mice exhibited profound disruptions in dentate gyrus circuit activation. High frequency stimulation of the perforant pathway in the dentate gyrus (DG) area of APdE9 mouse tissue evoked abnormally large field potential responses corresponding to the wider neural activation maps. Whole-cell patch clamp recordings of the identified inhibitory interneurons in the molecular layer of DG revealed that they fail to reliably fire action potentials. Taken together, abnormal DG excitability and an inhibitory neuron failure to generate action potentials are suggested to be important contributors to the underlying cellular mechanisms of early-stage Alzheimer's disease pathophysiology.

Loss of binocular responses and reduced retinal convergence during the period of retinogeniculate axon segregation.

In the developing mammalian visual system, axon terminals from the two eyes overlap in the dorsal lateral geniculate nucleus (LGN) but then undergo a period of refinement and segregate to form distinct eye-specific domains. We report on the changes in synaptic transmission that occur in rodent LGN during the period of retinogeniculate axon segregation by using anterograde labeling techniques in conjunction with an in vitro preparation where large segments of each optic nerve are preserved. Anterograde labeling of retinal projections in early postnatal day (P) rats with cholera toxin beta subunit indicated an age-related recession in uncrossed retinal projections. Between P2 and P5 uncrossed projections occupied as much as 50% of the LGN and overlapped substantially with crossed projections. Between the first and second postnatal week uncrossed projections receded, so by P14 they assumed an adultlike profile occupying 15-20% of LGN and showed little or no overlap with crossed projections. The postsynaptic responses of LGN cells evoked by the separate stimulation of each optic nerve indicated that before P14, many relay cells were binocularly innervated and received at least four to six inputs from each eye. However, these features of retinogeniculate connectivity were transient and their attrition occurred in concert with a retraction of retinal arbors into nonoverlapping, eye-specific regions. By P18 cells were monocularly innervated and received input from one to three retinal ganglion cells. These results provide a better understanding of the underlying changes in synaptic circuitry that occur during the anatomical segregation of retinal inputs into eye-specific territories.

Interneuron and pyramidal cell interplay during in vitro seizure-like events.

Excitatory and inhibitory (EI) interactions shape network activity. However, little is known about the EI interactions in pathological conditions such as epilepsy. To investigate EI interactions during seizure-like events (SLEs), we performed simultaneous dual and triple whole cell and extracellular recordings in pyramidal cells and oriens interneurons in rat hippocampal CA1. We describe a novel pattern of interleaving EI activity during spontaneous in vitro SLEs generated by the potassium channel blocker 4-aminopyridine in the presence of decreased magnesium. Interneuron activity was increased during interictal periods. During ictal discharges interneurons entered into long-lasting depolarization block (DB) with suppression of spike generation; simultaneously, pyramidal cells produced spike trains with increased frequency (6-14 Hz) and correlation. After this period of runaway excitation, interneuron postictal spiking resumed and pyramidal cells became progressively quiescent. We performed correlation measures of cell-pair interactions using either the spikes alone or the subthreshold postsynaptic interspike signals. EE spike correlation was notably increased during interneuron DB, whereas subthreshold EE correlation decreased. EI spike correlations increased at the end of SLEs, whereas II subthreshold correlations increased during DB. Our findings underscore the importance of complex cell-type-specific neuronal interactions in the formation of seizure patterns.

Synaptic mechanisms regulating the activation of a Ca(2+)-mediated plateau potential in developing relay cells of the LGN.

Using intracellular recordings in an isolated (in vitro) rat brain stem preparation, we examined the synaptic responses of developing relay neurons in the dorsal lateral geniculate nucleus (LGN). In newborn rats, strong stimulation of the optic tract (OT) evoked excitatory postsynaptic potentials (EPSPs) that gave rise to a sustained (300-1,300 ms), slow-decaying (<0.01 mV/s), depolarization (25-40 mV). Riding atop this response was a train of spikes of variable amplitude. We refer to this synaptically evoked event as a plateau potential. Pharmacology experiments indicate the plateau potential was mediated by the activation of high-threshold L-type Ca(2+) channels. Synaptic activation of the plateau potential relied on N-methyl-D-aspartate (NMDA) receptor-mediated activity and the spatial and/or temporal summation of retinally evoked EPSPs. Inhibitory postsynaptic responses (IPSPs) did not prevent the expression of the plateau potential. However, GABA(A) receptor activity modulated the intensity of optic tract stimulation needed to evoke the plateau potential, while GABA(B) receptor activity affected its duration. Expression of the plateau potential was developmentally regulated, showing a much higher incidence at P1-2 (90%) than at P19-20 (1%). This was in part due to the fact that developing relay cells show a greater degree of spatial summation than their mature counterparts, receiving input from as many as 7-12 retinal ganglion cells. Early spontaneous retinal activity is also likely to trigger the plateau potential. Repetitive stimulation of optic tract in a manner that approximated the high-frequency discharge of retinal ganglion cells led to a massive temporal summation of EPSPs and the activation of a sustained depolarization (>1 min) that was blocked by L-type Ca(2+) channel antagonists. These age-related changes in Ca(2+) signaling may contribute to the activity-dependent refinement of retinogeniculate connections.

Nature of inhibitory postsynaptic activity in developing relay cells of the lateral geniculate nucleus.

Using intracellular recordings in an isolated (in vitro) brain stem preparation, we examined the inhibitory postsynaptic responses of developing neurons in the dorsal lateral geniculate nucleus (LGN) of the rat. As early as postnatal day (P) 1-2, 31% of all excitatory postsynaptic (EPSP) activity evoked by electrical stimulation of the optic tract was followed by inhibitory postsynaptic potentials (IPSPs). By P5, 98% of all retinally evoked EPSPs were followed by IPSP activity. During the first postnatal week, IPSPs were mediated largely by GABA(A) receptors. Additional GABA(B)-mediated IPSPs emerged at P3-4 but were not prevalent until after the first postnatal week. Experiments involving the separate stimulation of each optic nerve indicated that developing LGN cells were binocularly innervated. At P11-14, it was common to evoke EPSP/IPSP pairs by stimulating either the contralateral or ipsilateral optic nerve. During the third postnatal week, binocular excitatory responses were encountered far less frequently. However, a number of cells still maintained a binocular inhibitory response. These results provide insight about the ontogeny and nature of postsynaptic inhibitory activity in the LGN during the period of retinogeniculate axon segregation.