Synaptic plasticity in the trigeminal principal nucleus during the period of barrelette formation and consolidation

We examined whether the postsynaptic responses of cells in the principal sensory nucleus of the trigeminal nerve (PrV) are subject to long-term changes in synaptic strength, and if such changes were correlated the whisker-specific patterning during and just after the critical period for pattern formation. We used an in vitro brainstem preparation in which the trigeminal ganglion (TG) and PrV remained attached. By electrically activating TG afferents, we evoked large-amplitude extracellular field potentials. These responses were postsynaptic in origin and blocked by the glutamate antagonist, DNQX. At P1, a time when barrelettes are consolidating, high frequency stimulation of their afferents led to an immediate (<1 min) and long-lasting (> or =90 min) reduction (35%) in the amplitude of the evoked response. At P3-7, when the pattern of barrelettes have stabilized, the same form of tetanus led to an immediate and long-lasting increase (40%) in the amplitude of the response. Both forms of synaptic plasticity were mediated by the activation of L-type Ca(2+) channels. Application of the L-type channel blocker, nitrendipine, led to a complete blockade of any the tetanus induced changes. These associative processes may regulate the patterning and maintenance of whisker-specific patterns in the brainstem trigeminal nuclei.

Prenatal and postnatal expression of nitric oxide in the developing kitten superior colliculus revealed with NADPH diaphorase histochemistry

Nitric oxide (NO) is a neuronal messenger molecule that mediates pathway refinement in some brain regions. We used nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry to examine the development of NO expression in the superior colliculus (SC) of kittens aged E28-E58 and P2-P57 and adults in order to determine if NO expression is correlated with pathway refinement. At E28, labeled cells were seen only within the subventricular zone (SVZ). At E36-E41, labeled cells were also found within the deep gray layer (DGL) of SC. At E51 and E58, a few labeled neurons were also present in the intermediate gray layer (IGL). These neurons already had extensive dendritic fields and well-developed morphologies at the time that they first expressed nitric oxide synthase (NOS). The number of neurons labeled in the DGL and IGL increased postnatally, reaching a peak density between P14 and P35. Neurons within the optic (OL) and superficial gray layers (SGL) were first visible at P7 and increased slightly in number until adulthood. However, SGL-labeled neurons were relatively limited in number and lightly labeled at all ages examined. We conclude that (1) NADPHd expression occurs in SC beginning in the second trimester in kittens and progresses in a ventral to dorsal pattern between E36-P35; (2) few neurons in kitten SGL are labeled by NADPHd and these appear relatively late in postnatal development; and (3) there is no correlation between NOS expression and retinocollicular pathway refinement in kittens, a result different from that seen in rodents.

Burst and tonic response modes in thalamic neurons during sleep and wakefulness

Thalamic neurons can exhibit two distinct firing modes: tonic and burst. In the lateral geniculate nucleus (LGN), the tonic mode appears as a relatively faithful relay of visual information from retina to cortex. The function of the burst mode is less understood. Its prevalence during slow-wave sleep (SWS) and linkage to synchronous cortical electroencephalogram (EEG) suggest that it has an important role during this form of sleep. Although not nearly as common, bursting can also occur during wakefulness. The goal of this study was to identify conditions that affect burst probability, and to compare burst incidence during sleeping and waking. LGN neurons are extraordinarily heterogenous in the degree to which they burst, during both sleeping and waking. Some LGN neurons never burst under any conditions during wakefulness, and several never burst during slow-wave sleep. During wakefulness, <1% of action potentials were associated with bursting, whereas during sleep this fraction jumps to 18%. Although bursting was most common during slow-wave sleep, more than 50% of the bursting originated from 14% of the LGN cells. Bursting during sleep was largely restricted to episodes lasting 1-5 s, with approximately 47% of these episodes being rhythmic and in the delta frequency range (0.5-4 Hz). In wakefulness, although visual stimulation accounted for the greatest number of bursts, it was still a small fraction of the total response (4%, 742 bursts/17,744 cycles in 93 cells). We identified two variables that appeared to influence burst probability: size of the visual stimuli used to elicit responses and behavioral state. Increased stimulus size increased burst probability. We attribute this to the increased influence large stimuli have on a cell's inhibitory mechanisms. As with sleep, a large fraction of bursting originated from a small number of cells. During visual stimulation, 50% of bursting was generated by 9% of neurons. Increased vigilance was negatively correlated with burst probability. Visual stimuli presented during active fixation (i.e., when the animal must fixate on an overt fixation point) were less likely to produce bursting, than when the same visual stimuli were presented but no fixation point present ("passive" fixation). Such observations suggest that even brief departures from attentive states can hyperpolarize neurons sufficiently to de-inactivate the burst mechanism. Our results provide a new view of the temporal structure of bursting during slow-wave sleep; one that supports episodic rhythmic activity in the intact animal. In addition, because bursting could be tied to specific conditions within wakefulness, we suggest that bursting has a specific function within that state.

A novel means of Y cell identification in the developing lateral geniculate nucleus of the cat

We examined the postnatal development of putative Y cells in the dorsal lateral geniculate nucleus (dLGN) using the SMI-32 antibody, which has been demonstrated in the adult cat to stain cells with Y cell morphology. At birth, SMI-32 stained cells were concentrated in the interlaminar zones. During postnatal development, the SMI-32 staining gradually becomes more disperse and by P21 stained cells are found throughout the A and magnocellular C laminae. By the end of the first postnatal week, and in all later ages examined, the SMI-32 stained cells were significantly larger than the overall population of Nissl stained cells and interneurons (stained with an antibody against glutamic acid decarboxylase). Postnatal SMI-32 staining revealed a dramatic increase in soma sizes and the expansion of putative geniculate Y cell dendritic arbors that continued past the second postnatal month. In contrast, the growth of interneurons appeared to be complete by 3-4 postnatal weeks, at which time cell somas stained with SMI-32 have only reached a little over one half of their adult size. Similar to the adult cat, SMI-32 appears to selectively stain the Y cell population during development and may provide a useful morphological marker to examine the participation of Y cells in the developing postnatal circuitry of the dLGN. This further establishes the cat dLGN as a novel model system to study the normal function and pathological reorganization of neurofilaments.

Nitric oxide synthase distribution in the cat superior colliculus and co-localization with choline acetyltransferase

Nitric oxide and acetylcholine are important neuromodulators implicated in brain plasticity and disease. We have examined the cellular and fiber localization of nitric oxide in the cat superior colliculus (SC) and its degree of co-localization with ACh using nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry and an antibody to neuronal nitric oxide synthase. ACh was localized using an antibody against choline acetyltransferase. We also made injections of biocytin into the region of the parabrachial brainstem to confirm that this region is a source of nitric oxide containing fibers in SC. NADPHd labeled neurons within the superficial layers of the superior colliculus included pyriform, vertical fusiform, and horizontal morphologies. Labeled neurons in the intermediate gray layer were small to medium in size, and mostly of stellate morphology. Neurons in the deepest layers had mostly vertical or stellate morphologies. NADPHd labeled fibers formed dense patches of terminal boutons within the intermediate gray layer and streams of fibers within the deepest layers of SC. Choline acetyltransferase antibody labeling in adjacent sections indicated that many fibers must contain both labels. Over 94% of neurons in the pedunculopontine tegmental and lateral dorsal tegmental nuclei were also labeled by both NADPHd and choline acetyltransferase. In addition, biocytin labeled fibers from this region were localized in the NADPHd labeled patches. We conclude that nitric oxide is contained in a variety of cell types in SC and that both nitric oxide and ACh likely serve as co-modulators in this midbrain structure.

NMDAR-1 staining in the lateral geniculate nucleus of normal and visually deprived cats

In normal adult cats, a monoclonal antibody directed toward the NR-1 subunit of the N-methyl-D-aspartate (NMDA) receptor (Pharmingen, clone 54.1) produced dense cellular and neuropil labeling throughout all layers of the lateral geniculate nucleus (LGN) and adjacent thalamic nuclei, including the thalamic reticular, perigeniculate, medial intralaminar, and ventral lateral geniculate nuclei. Cellular staining revealed well-defined somata, and in some cases proximal dendrites. NMDAR-1 cell labeling was also evident in the LGN of early postnatal kittens, suggesting that developing LGN cells possess this receptor subunit at or before eye opening. Within the A-layers of the adult LGN, staining encompassed a wide range of soma sizes. Soma size comparisons of NMDAR-1 stained cells with those stained with an antibody directed toward a nonphosphorylated neurofilament protein (SMI-32), which selectively stains Y-relay cells (Bickford et al., 1998), or an antibody to glutamic acid decarboxylase (GAD), which stains for GABAergic interneurons, suggested that NMDA receptors are utilized by relay cells and interneurons. NMDAR-1 staining was also observed in the LGN of cats with early monocular lid suture. Although labeling was apparent in both deprived and nondeprived A-layers of LGN, the distribution of soma sizes was significantly different. In the deprived A-layers of LGN, staining was limited to small- and medium-sized cells. Cells with relatively large soma were lacking. However, cell density measurements as well as soma size comparisons with cells stained for Nissl substance suggested these differences were due to deprivation-induced cell shrinkage and not to a loss of NMDAR-1 staining in Y-cells. Taken together, these results suggest that NMDA receptors are utilized by both relay cells and interneurons in LGN and that alterations in early visual experience do not necessarily affect the expression of NMDA receptors in the LGN.

Development of the cholinergic, nitrergic, and GABAergic innervation of the cat dorsal lateral geniculate nucleus

Cholinergic projections from the brainstem have been shown to be important modulators of visual activity in the dorsal lateral geniculate nucleus (dLGN) of the adult, but little is known about the role of these modulatory inputs during development. We examined the postnatal development of the cholinergic innervation of the dLGN by using an monoclonal antibody against choline acetyl transferase (ChAT). We also investigated the development of GABAergic interneurons in the dLGN by using an antibody against glutamic acid decarboxylase (GAD), and the developmental expression of brain nitric oxide synthase (BNOS) by using an antibody against this enzyme. We found that brainstem cells surrounding the brachium conjunctivum express ChAT at birth, although axons in the dLGN do not express ChAT until the end of the first postnatal week. Cholinergic synaptic contacts were observed as early as the second postnatal week. The number of axons stained with the ChAT antibody increased slowly during the subsequent weeks in the dLGN and reached adult levels by the eighth postnatal week. GABAergic interneurons were present at birth and reached their adult soma size by the third postnatal week. GABAergic fibers are dense at birth but change during development from a diffuse pattern to clustered arrangements that can be recognized as distinct rings of GAD staining by P35. Cellular expression of BNOS was seen within all dLGN laminae during development. The BNOS-stained cells are tentatively identified as interneurons because their soma sizes were similar to those of GAD-stained cells. Although cellular BNOS staining remained robust in the C1-3 laminae through adulthood, cellular expression of BNOS in the A laminae declined during the first five postnatal weeks and remains sparse in the adult. As cellular BNOS staining declined, there was a steady increase in BNOS-stained fibers, which paralleled the increase of ChAT-stained fibers that are known to colocalize BNOS in the adult. Our results emphasize the continued transformations of intrinsic as well as extrinsic innervation patterns that occur during the development, of the dLGN.

Electrophysiological properties and synaptic responses of cells in the trigeminal principal sensory nucleus of postnatal rats

In the rodent brain stem trigeminal complex, select sets of neurons form modular arrays or "barrelettes," that replicate the patterned distribution of whiskers and sinus hairs on the ipsilateral snout. These cells detect the patterned input from the trigeminal axons that innervate the whiskers and sinus hairs. Other brain stem trigeminal cells, interbarrelette neurons, do not form patterns and respond to multiple whiskers. We examined the membrane properties and synaptic responses of morphologically identified barrelette and interbarrelette neurons in the principal sensory nucleus (PrV) of the trigeminal nerve in early postnatal rats shortly after whisker-related patterns are established. Barrelette cell dendritic trees are confined to a single barrelette, whereas the dendrites of interbarrelette cells span wider territories. These two cell types are distinct from smaller GABAergic interneurons. Barrelette cells can be distinguished by a prominent transient A-type K(+) current (I(A)) and higher input resistance. On the other hand, interbarrelette cells display a prominent low-threshold T-type Ca(2+) current (I(T)) and lower input resistance. Both classes of neurons respond differently to electrical stimulation of the trigeminal tract. Barrelette cells show either a monosynaptic excitatory postsynaptic potential (EPSP) followed by a large disynaptic inhibitory postsynaptic potential (IPSP) or just simply a disynaptic IPSP. Increasing stimulus intensity produces little change in EPSP amplitude but leads to a stepwise increase in IPSP amplitude, suggesting that barrelette cells receive more inhibitory input than excitatory input. This pattern of excitation and inhibition indicates that barrelette cells receive both feed-forward and lateral inhibition. Interbarrelette cells show a large monosynaptic EPSP followed by a small disynaptic IPSP. Increasing stimulus intensity leads to a stepwise increase in EPSP amplitude and the appearance of polysynaptic EPSPs, suggesting that interbarrelette cells receive excitatory inputs from multiple sources. Taken together, these results indicate that barrelette and interbarrelette neurons can be identified by their morphological and functional attributes soon after whisker-related pattern formation in the PrV.

Neurofilament proteins in Y-cells of the cat lateral geniculate nucleus: normal expression and alteration with visual deprivation

We examined neurofilament staining in the normal and visually deprived lateral geniculate nucleus (LGN), using the SMI-32 antibody. This antibody preferentially stains LGN cells that display the morphological characteristics of Y-cells. The soma sizes of SMI-32-stained cells were consistent with those of the overall population of Y-cells, and the Golgi-like staining of their dendrites revealed a radial distribution that often crossed laminar boundaries. Labeled cells were distributed within the A laminae (primarily near laminar borders), the magnocellular portion of the C laminae, and the medial intralaminar nucleus, but they were absent in the parvocellular C laminae. Electron microscopic examination of SMI-32-stained tissue revealed that staining was confined to somata, dendrites, and large myelinated axons. Retinal synapses on SMI-32-labeled dendrites were primarily simple axodendritic contacts; few triadic arrangements were observed. In the LGN of cats reared with monocular lid suture, SMI-32 staining was decreased significantly in the A laminae that received input from the deprived eye. Dephosphorylation of the tissue did not alter the cellular SMI-32 staining patterns. Analysis of staining patterns in the C laminae and monocular zone of the A laminae suggests that changes in the cytoskeleton after lid suture reflect cell class and not binocular competition. Taken together, the results from normal and lid-sutured animals suggest that the cat LGN offers a unique model system in which the cytoskeleton of one class of cells can be manipulated by altering neuronal activity.

Response latencies of cells in the cat's lateral geniculate nucleus are less variable during burst than tonic firing

We measured the variability in latency of the first spike seen in cells of the cat's lateral geniculate nucleus following the onset of a visual stimulus. We found that, in each of the 11 cells tested, this variability was significantly lower during burst than during tonic firing. We suggest that this difference confers an advantage in signal detection during burst compared to tonic firing. This complements other reported advantages of burst firing for signal detection seen in signal-to-noise ratios and in the ability to efficiently drive postsynaptic cells.

Developmental changes in the electrophysiological properties of brain stem trigeminal neurons during pattern (barrelette) formation

In the brain stem trigeminal nuclei of rodents there is a patterned representation of whiskers and sinus hairs. The subnucleus interpolaris (SPI) contains the largest and the most conspicuous whisker patterns (barrelettes). Although neural activity plays a role in pattern formation, little is known about the electrophysiological properties of developing barrelette neurons. Here we examined the functional state of early postnatal SPI neurons during and after the consolidation of patterns by using in vitro intracellular recording techniques. After the consolidation of barrelettes [>/= postnatal day (P)4], responses to intracellular current injection consistently reflected the activation of a number voltage-dependent conductances. Most notable was a mixed cation conductance (IH) that prevented strong hyperpolarization and a large low-threshold Ca2+ conductance, which led to Ca2+ spikes and burst firing. At the oldest ages tested (P11-P14) some cells also exhibited an outward K+ conductance (IA), which led to significant delays in action-potential firing. Between P0-3, a time when the formation of barrelettes in the brain stem is still susceptible to damage of the sensory periphery, cells responded linearly to intracellular current injection, indicating they either lacked such voltage-gated properties or weakly expressed them. At all ages tested (P0-14), SPI cells were capable of generating trains of action potentials in response to intracellular injection of depolarizing current pulses. However, during the first few days of postnatal life, spikes were shorter and longer. Additionally, spike trains rose more linearly with stimulus intensity and showed frequency accommodation at early ages. Taken together, these results indicate that the electrophysiological properties of SPI neurons change markedly during the period of barrelette consolidation. Moreover, the properties of developing SPI neurons may play a significant role in pattern formation by minimizing signal distortion and ensuring that excitatory responses from sensory periphery are accurately received and transmitted according to stimulus strength.

Developmental changes in the pattern of NADPH-diaphorase staining in the cat's lateral geniculate nucleus

We examined the pattern of NADPH-diaphorase (NADPH-d) staining in the lateral geniculate nucleus (LGN) of dorsal thalamus in fetal and newborn kittens, and adult cats. This staining visualizes the synthesizing enzyme of nitric oxide (NO), a neuromodulator associated with central nervous system (CNS) development and synaptic plasticity. In the adult, very few LGN cells stained for NADPH-d, and these were restricted to interlaminar zones and ventral C layers. NADPH-d labeled a dense network of fibers and axon terminals throughout the LGN and adjacent thalamic nuclei. The source of such labelling has been reported to be cholinergic neurons from the parabrachial region of the brain stem (Bickford et al., 1993). A very different pattern of staining was observed in prenatal and early postnatal kittens. Between embryonic (E) day 46-57, lightly stained cells appeared throughout the LGN. From this age, through about the first month of life, the number of stained cells in the LGN rose rapidly. The density (cells/mm2) of labeled cells peaked at postnatal day (P) 28 (P28), and was about 150 times greater than the level measured in the adult LGN. After P28, cell staining declined rapidly, and fell to adult levels at P41. The reduction in cell staining that occurred between P35-41 was accompanied by the appearance of fine-caliber fiber staining, similar to that observed in the adult LGN. NADPH-d staining, which reveals the presence of nitric oxide synthase (NOS), and thus NO activity, may reflect two processes. In the adult LGN, the labeling of cholinergic axons arising from the brain-stem parabrachial region coupled with a paucity of the LGN cellular staining suggests that NO operates in an orthograde manner, being co-released with ACh to influence the gain and efficacy of retinogeniculate transmission. By contrast, in developing kitten, NADPH-d staining of LGN cells suggests that NO acts in a retrograde fashion, perhaps playing a role in maintaining associative processes underlying activity-dependent refinement of retinogeniculate connections.

Functional connectivity in the rodent trigeminal pathway grown in vitro

In explant cocultures of the rat trigeminal pathway, embryonic trigeminal ganglion cells grow their axons into peripheral cutaneous and central nervous system targets (R.S. Erzurumlu, S. Jhaveri, Target influences on the morphology of trigeminal axons, Exp. Neurol, 135 (1995) 1-16; R.S. Erzurumlu, S. Jhaveri, H. Takahashi, R.D.G. McKay, Target-derived influences on axon growth modes in explant cocultures of trigeminal neurons, Proc. Natl. Acad. Sci. USA 90 (1993) 7235-7239). In heterochronic cocultures, composed of embryonic trigeminal ganglion, embryonic whisker pad and postnatal brainstem slice, trigeminal axons develop arbors and terminal boutons in the brainstem trigeminal nuclei. To determine whether these terminal arbors establish functional connections with the brainstem neurons, we examined the electrophysiological properties of brainstem neurons and their responsiveness to trigeminal ganglion stimulation. Intracellular recordings were done in vitro on cells of the trigeminal subnucleus interpolaris (SPI) in trigeminal pathway cocultures (E15 whisker pad, E15 trigeminal ganglion, and postnatal day (PND) 0-2 brainstem slice) or in the SPI of acutely prepared brainstem slices. Electrophysiological properties of SPI cells in both preparations were virtually identical. The voltage responses of SPI neurons to intracellular current injection were highly linear suggesting they lacked a number of voltage-dependent conductances. Depolarizing current injection produced trains of action potentials with a frequency that varied with stimulus intensity. In explant cocultures, electrical activation of the trigeminal ganglion evoked EPSPs, and EPSPs coupled with IPSPs in SPI cells. Bicuculline blockade of IPSP activity resulted in long lasting EPSPs whose duration increased with membrane depolarization. These results show that brainstem trigeminal neurons can retain their functional properties in culture and establish functional connections with primary sensory afferents.

An in vitro model of the kitten retinogeniculate pathway

An organotypic explant coculture method is described for the developing retinogeniculate pathway of the cat. Retinal explants and thalamic slices containing the dorsal lateral geniculate nucleus (LGN), derived from early postnatal kittens, can be grown in serum-free culture medium for several days. In such cultures, retinal ganglion cells (RGCs) and LGN neurons retained their age-specific morphological features and developed functional connections. Labeling of RGCs and their processes with DiI showed that all three major classes of RGCs (alpha/Y, beta/X, gamma/W) were present in cocultured retinal explants. Retinal axons readily regenerated into thalamic slices and, over time, developed arbors within the LGN. Retrograde labeling from the LGN traced the origin of these axons almost exclusively to alpha-cells in the retina. In vitro intracellular recordings indicated that LGN cells maintained their basic electrophysiological properties in coculture. Current injection generated action potentials, and, at hyperpolarized levels, it led to low-threshold Ca2+ spiking. Regenerated retinal axons also formed functional connections with LGN neurons. Electrical stimulation of the retinal explant elicited excitatory postsynaptic responses (EPSPs) in LGN cells. Drop application of specific glutamate antagonists indicated that EPSPs had both N-methyl-D-aspartate (NMDA) and non-NMDA receptor components. The morphology of the LGN neurons was examined after intracellular injections of biocytin during recording. Labeled cells were very similar to those of early postnatal kittens. Although, in general, they had relatively small soma and simple dendritic branching patterns, a few could be recognized as X- or Y-cells. Thus the coculture model can be used to assay the regenerative propensity of different types of RGCs during development.

Burst responses in thalamic relay cells of the awake behaving cat

1. Relay cells of the dorsal lateral geniculate nucleus (LGN) respond in one of two modes: tonic or burst. The purpose of this study was to determine whether, and under what conditions, burst responses occur in LGN cells of an awake, behaving animal. 2. We recorded the extracellular responses of cells located in the A layers of LGN in two cats trained to perform a simple fixation task. In our paradigm, head position was fixed and gaze monitored with the use of the scleral search coil technique. They were trained to fixate on a small target light while we probed the receptive field of an isolated LGN cell with drifting gratings. 3. Many LGN cells (37/48; 77%) recorded in the awake cat showed some form of burst firing. However, the degree of burstiness a given cell displayed was relatively low. The probability of recording a burst response during any second within the fixation task was 0.09. This reflects the fact that during wakefulness, LGN cells are generally in a state of relative depolarization and the low-threshold Ca2+ conductance underlying burst firing is inactivated. 4. The majority of bursts occurred (262/377; 71%) during visual stimulation. These comprised the initial response to the grating and were confined to the early phase of fixation. As the cat continued to hold fixation and as subsequent cycles of the grating passed through the cell's receptive field, the response shifted from a burst to a tonic firing pattern. Some bursts (67/377, 18%) were related to eye movements. All of these were postsaccadic, and most occurred > 150 ms from the onset of the eye movement. Finally, some bursts were neither visually driven nor related to eye movements. However, these spontaneous bursts were infrequent (41/377; 11%) and never rhythmic. 5. Burst firing in LGN cells has a dual purpose. During quiescent states such as slow-wave sleep, they support the decoupling of retinal signals from LGN. During the waking state, bursts can facilitate signal transmission during target acquisition and early phases of fixation.

Receiver operating characteristic (ROC) analysis of neurons in the cat's lateral geniculate nucleus during tonic and burst response mode

Relay cells of the lateral geniculate nucleus respond to visual stimuli in one of two modes: burst and tonic. The burst mode depends on the activation of a voltage-dependent, Ca2+ conductance underlying the low threshold spike. This conductance is inactivated at depolarized membrane potentials, but when activated from hyperpolarized levels, it leads to a large, triangular, nearly all-or-none depolarization. Typically, riding its crest is a high-frequency barrage of action potentials. Low threshold spikes thus provide a nonlinear amplification allowing hyperpolarized relay neurons to respond to depolarizing inputs, including retinal EPSPs. In contrast, the tonic mode is characterized by a steady stream of unitary action potentials that more linearly reflects the visual stimulus. In this study, we tested possible differences in detection between response modes of 103 geniculate neurons by constructing receiver operating characteristic (ROC) curves for responses to visual stimuli (drifting sine-wave gratings and flashing spots). Detectability was determined from the ROC curves by computing the area under each curve, known as the ROC area. Most cells switched between modes during recording, evidently due to small shifts in membrane potential that affected the activation state of the low threshold spike. We found that the more often a cell responded in burst mode, the larger its ROC area. This was true for responses to optimal and nonoptimal visual stimuli, the latter including nonoptimal spatial frequencies and low stimulus contrasts. The larger ROC areas associated with burst mode were due to a reduced spontaneous activity and roughly equivalent level of visually evoked response when compared to tonic mode. We performed a within-cell analysis on a subset of 22 cells that switched modes during recording. Every cell, whether tested with a low contrast or high contrast visual stimulus exhibited a larger ROC area during its burst response mode than during its tonic mode. We conclude that burst responses better support signal detection than do tonic responses. Thus, burst responses, while less linear and perhaps less useful in providing a detailed analysis of visual stimuli, improve target detection. The tonic mode, with its more linear response, seems better suited for signal analysis rather than signal detection.

Latency variability of responses to visual stimuli in cells of the cat's lateral geniculate nucleus

We constructed average histograms from responses evoked by flashing stimuli and noted previously described variations in the shape of the response profile, particularly with respect to sharpness of the peak. To express this variable, we measured the half-rise latency, which is the latency from stimulus onset required to reach half the maximum response. A short half-rise latency, which is characteristic of nonlagged cells, is associated with a brisk response and sharp peak; a long half-rise latency, characteristic of lagged cells, is associated with a sluggish response and broad peak. Nonlagged cells were readily seen; we attempted to identify cells with long latencies as lagged, but we were unable to do so unambiguously due to failure to observe lagged properties other than latency. We thus refer to these latter cells as having "lagged-like" responses to indicate that we are not certain whether these are indeed lagged cells. In addition to the histograms, we analyzed the individual response trials that were summed to create each histogram, and we used spike density analysis to estimate the initial response latency to the flashing spot for each trial. We found that lagged-like responses were associated with more variability in initial response latency than were nonlagged responses. We then employed an alignment procedure to eliminate latency variation from individual trials; that is, responses during individual trials were shifted in time as needed so that each had a latency equal to the average latency of all trials. We used these "aligned" trials to create a second, "aligned" response histogram for each cell. The alignment procedure had little effect on nonlagged responses, because these were already well aligned due to consistent response latencies amongst trials. For lagged-like responses, however, the alignment made a dramatic difference. The aligned histograms looked very much like those for nonlagged responses: the responses appeared brisk, with a sharply rising peak that was fairly high in amplitude. We thus conclude that the slow build up to a relatively low peak of firing of the lagged-like response histogram is not an accurate reflection of responses on single trials. Instead, the sluggishness of lagged-like responses inferred from average response histograms results from temporal smearing due to latency variability amongst trials. We thus conclude that there is relatively little difference in briskness between nonlagged and lagged-like responses to single stimuli.

Cellular bases for the control of retinogeniculate signal transmission

The dorsal lateral geniculate nucleus (LGN) is the major thalamic relay for retinal signals en route to cortex. However, LGN cells operate as more than just a simple relay of their retinal inputs. Rather, they function as a variable gate, determining what, when, and how much retinal information gets passed to visual cortex. Two factors that are key to this control are the innervation patterns and electrophysiological membrane properties of geniculate cells. This paper discusses three active membrane properties and the manner in which they modulate the transfer of retinal signals to cortex. They are the low threshold calcium (Ca2+) conductance, a transient potassium (K+) conductance, and NMDA receptor-mediated excitatory postsynaptic potentials (EPSPs). The low-threshold Ca2+ conductance transforms a geniculate cell from a state of single spike activity to one of bursting discharge, the potassium current leads to a delay in membrane depolarization to reach spike threshold, and NMDA receptor activity modulates EPSP amplitude and duration near spike threshold. Additionally, we consider how nonretinal inputs, such as the ascending cholinergic pathway from the brainstem parabrachial region and the descending pathway from layer VI of visual cortex, influence the expression of these membrane properties through their control of membrane potential.

Evidence that cholinergic axons from the parabrachial region of the brainstem are the exclusive source of nitric oxide in the lateral geniculate nucleus of the cat

We investigated the source of axons and terminals in the cat's lateral geniculate nucleus that stain positively for NADPH-diaphorase. The functional significance of such staining is that NADPH-diaphorase is identical to the enzyme nitric oxide synthetase, and thus it is thought to reveal cells and axons that use nitric oxide as a neuromodulator. Within the lateral geniculate and adjacent perigeniculate nuclei, a dense network of axons and terminals is labeled for NADPH-diaphorase. The pattern of NADPH-diaphorase staining here is remarkably similar to that of choline acetyltransferase (ChAT) staining, suggesting that the source of these axons and terminals might be the parabrachial region of the brainstem because this provides the major cholinergic input to the lateral geniculate nucleus. In other areas of the brain to which parabrachial axons project, there is also a similar staining pattern for NADPH-diaphorase and ChAT. Furthermore, the patterns of cell staining within the parabrachial region for NADPH-diaphorase and ChAT are virtually identical. However, the relationship between ChAT and NADPH-diaphorase staining for the parabrachial region is not a general property of cholinergic neurons. Other cholinergic cells and axons, such as the trochlear nerve, the oculomotor nerve and nucleus, and the parabigeminal nucleus, which all label densely for ChAT, stain poorly or not at all for NADPH-diaphorase. It is significant that the parabigeminal nucleus, which provides a cholinergic input to the lateral geniculate nucleus, has no cells that label for NADPH-diaphorase. We used double labeling methods to identify further the source of NADPH-diaphorase staining in the lateral geniculate nucleus. We found that parabrachial cells co-localize NADPH-diaphorase and ChAT. Noradrenergic and serotoninergic cells in the brainstem also innervate the lateral geniculate nucleus, but we found that none of these co-localize NADPH-diaphorase. Finally, by combining NADPH-diaphorase histochemistry with retrograde labeling of cells that project to the lateral geniculate nucleus, we found that the cholinergic cells of the parabrachial region are essentially the sole source of NADPH-diaphorase in the lateral geniculate nucleus. We thus conclude that cells from the parabrachial region that innervate the lateral geniculate nucleus use both acetylcholine and nitric oxide for neurotransmission, and that this is virtually the only afferent input to this region that uses nitric oxide.

The brain-stem parabrachial region controls mode of response to visual stimulation of neurons in the cat's lateral geniculate nucleus

We recorded the responses of neurons from the cat's lateral geniculate nucleus to drifting sine-wave grating stimuli both before and during electrical stimulation of the parabrachial region of the midbrain. The parabrachial region provides a mostly cholinergic input to the lateral geniculate nucleus, and our goal was to study its effect on responses of geniculate cells to visual stimulation. Geniculate neurons respond to visual stimuli in one of two modes. At relatively hyperpolarized membrane potentials, low threshold (LT) Ca2+ spikes are activated, leading to high-frequency burst discharges (burst mode). At more depolarized levels, the low threshold Ca2+ spike is inactivated, permitting a more tonic response (relay or tonic mode). During our intracellular recordings of geniculate cells, we found that, at initially hyperpolarized membrane potentials, LT spiking in response to visual stimulation was pronounced, but that parabrachial activation abolished this LT spiking and associated burst discharges. Coupled with the elimination of LT spiking, parabrachial activation also led to a progressive increase in tonic responsiveness. Parabrachial activation thus effectively switched the responses to visual stimulation of geniculate neurons from the burst to relay mode. Accompanying this switch was a gradual depolarization of resting membrane potential by about 5-10 mV and a reduction in the hyperpolarization that normally occurs in response to the inhibitory phase of the visual stimulus. Presumably, the membrane depolarization was sufficient to inactivate the LT spikes. We were able to extend and confirm our intracellular observations on the effects of parabrachial activation to a sample of cells recorded extracellularly. This was made possible by adopting empirically determined criteria to distinguish LT bursts from tonic responses solely on the basis of the temporal pattern of action potentials. During parabrachial activation, every cell responded only in the relay mode, an effect that corresponds to our intracellular observations. We quantified the effects of parabrachial activation on various response measures. The fundamental Fourier response amplitude (F1) was calculated separately for the total response, the tonic response component, and the LT burst component. Parabrachial activation resulted in an increased F1 amplitude for the total response. This increase was due to an increase in the tonic response component. For a subset of cells showing epochs of LT bursting, parabrachial activation concurrently reduced LT bursting and increased the amplitude of the tonic response. Parabrachial activation, by eliminating LT bursting, also caused cells to respond with more linearity.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of membrane voltage on receptive field properties of lateral geniculate neurons in the cat: contributions of the low-threshold Ca2+ conductance

1. Thalamic relay cells, including those of the lateral geniculate nucleus, display a low-threshold spike (LT spike), which is a large depolarization due to an increased Ca2+ conductance. Typically riding the crest of each LT spike is a burst of from two to seven action potentials, which we refer to as the LT burst. The LT spike is voltage dependent, because if the cell's resting membrane potential is more depolarized than roughly -60 mV, the LT spike is inactivated, but if more hyperpolarized, the spike is deinactivated and can be activated by a depolarization, such as from an afferent excitatory postsynaptic potential (EPSP). Thalamic relay cells thus display two response modes: a relay or tonic mode, when the cell is depolarized and LT spikes are inactivated, leading to tonic firing of action potentials; and a burst mode, when the cell is hyperpolarized and tends to respond with LT spikes and their associated bursts of action potentials. 2. We were interested in the contribution of the LT spike on the transmission of visually evoked signals through geniculate relay cells to visual cortex. We recorded intracellularly from geniculate cells in an anesthetized, paralyzed, in vivo cat preparation to study the effects of membrane voltage, and thus the presence or absence of LT spikes, on responses to drifting sine-wave gratings. We monitored the visually evoked responses of 14 geniculate neurons (6 X, 7 Y, and 1 unclassified) at different membrane potentials at which LT spikes were inactivated or deinactivated. 3. Changing membrane voltage during visual stimulation switched the response mode of every cell between the relay and burst modes. In the burst mode, LT spikes occurred in phase with the visual stimulus and not at rhythmic intervals uncorrelated to visual stimuli. To any given stimulus cycle, the cell responded usually with an LT burst or a tonic response, and rarely was more than one LT burst evoked by a stimulus cycle. Occasionally a single cycle evoked both an LT burst and tonic response, but always the LT burst occurred first. 4. The spatial tuning characteristics of the cells did not differ dramatically as a function of membrane potential, because the tuning of the LT bursts was quite similar to that of the tonic response component. Although we did not obtain complete temporal tuning properties, we did note that hyperpolarized cells responded reliably with LT bursts at several temporal frequencies. 5. A consistent difference was seen between the LT burst and tonic response components in terms of response linearity.(ABSTRACT TRUNCATED AT 400 WORDS)

Relative contributions of burst and tonic responses to the receptive field properties of lateral geniculate neurons in the cat

1. In an anesthetized, paralyzed in vivo preparation, we recorded extracellular responses of 61 geniculate neurons (2 W, 25 X, 33 Y, and 1 mixed) to drifting sine-wave gratings of various spatial frequency, temporal frequency, and contrast. Our goal was to study the differential contributions to these visual responses of bursting caused by voltage dependent, low-threshold (LT) Ca2+ spikes and of purely tonic responses unrelated to LT spikes. Cells responding with LT spikes are said to be in the burst firing mode and those responding in a purely tonic fashion to be in the relay or tonic firing mode. We separated the total visual response into LT burst and tonic components by use of the empirical criteria set forth in our intracellular study described in the previous paper (Lu et al. 1992). A response component was considered to be an LT burst if its action potentials displayed interspike intervals < or = 4 ms and if the first spike in the burst episode occurred after a silent period of > or = 100 ms (or > or = 50 ms when the neuron responds to visual stimuli at temporal rates > or = 8 Hz). All other activity is considered to be part of the tonic response. 2. In addition to LT bursts, we recognized another type of burst response, the high-threshold (HT) burst. These also have clusters of action potentials with interspike intervals < or = 4 ms. However, HT bursts, unlike LT bursts, lack a preburst silent period. HT bursts are part of the tonic response component and merely reflect the gradual decrease in interspike intervals that occurs as the cell becomes more depolarized and thus more responsive. Thus interspike interval is a necessary but insufficient criterion to identify LT bursts. 3. Visually evoked LT bursts were recorded among W, X, and Y cells. When evoked, LT bursts occurred in phase with drifting sine-wave grating stimuli at a rate never exceeding one per stimulus cycle. In response to individual cycles of the visual stimulus, LT bursts could comprise the total response, a tonic component could comprise the total response, or an LT burst and tonic component could be mixed. When a stimulus evoked a mixture of LT bursts and tonic response components, LT bursts were always the first response. 4. Of the 61 cells tested with grating stimuli, 47 exhibited LT bursts and 14 did not. Those that did exhibited varying amounts of burstiness.(ABSTRACT TRUNCATED AT 400 WORDS)

How complete is physiological compensation in extrastriate cortex after visual cortex damage in kittens?

Previous studies indicate that neurons in the cat's posteromedial lateral suprasylvian (PMLS) visual area of cortex show physiological compensation after neonatal but not adult damage to areas 17, 18, and 19 of the visual cortex (collectively, VC). Thus, VC damage in adults produces a loss of direction selectivity and a decrease in response to the ipsilateral eye among PMLS cells, but these changes are not seen in adult cats that received VC damage as kittens. This represents compensation for early VC damage in the sense that PMLS neurons develop properties they would have had if there had been no brain damage. However, this is only a partial compensation for the effects of VC damage. A full compensation would involve development of properties of the VC cells that were removed in the damage. The present study investigated whether this type of compensation occurs for detailed spatial- and temporal-frequency processing. Single-cell recordings were made in PMLS cortex of adult cats that had received a VC lesion on the day of birth or at 8 weeks of age. Responses to sine-wave gratings that varied in spatial frequency, contrast, and temporal frequency were assessed quantitatively. We found that the spatial- and temporal-frequency processing of PMLS cells in adult cats that had neonatal VC damage were not significantly different from PMLS cells in normal cats. Therefore, there was no evidence that PMLS cells can compensate for VC damage by developing properties that are better than normal and like those of the striate cortex cells that were damaged. We also assessed the effects of long-term VC damage in adult cats to determine whether the normal properties seen in cats with neonatal VC damage represent a compensation for abnormalities in PMLS cortex present after adult damage. In a previous study, we found that acute VC damage in adult cats has small but reliable effects on maximal response amplitude, maximal contrast sensitivity, and spatial resolution (Guido et al. 1990b). In the present study, we found that long-term VC damage in adult cats does not increase these abnormalities as a result of secondary degenerative changes. In fact, the minor abnormalities that were present after an acute VC lesion were virtually absent following a long-term adult lesion, perhaps because they were due to transient traumatic effects. Therefore, there was little evidence for abnormalities in spatial- or temporal-frequency processing following long-term adult VC damage for which PMLS cells might show compensation following long-term neonatal damage.(ABSTRACT TRUNCATED AT 400 WORDS)

Binocular interactions in the cat's dorsal lateral geniculate nucleus, II: Effects on dominant-eye spatial-frequency and contrast processing

The present study tested the hypothesis that nondominant-eye influences on lateral geniculate nucleus (LGN) neurons affect the processing of spatial and contrast information from the dominant eye. To do this, we determined the effects of stimulating the nondominant eye at its optimal spatial frequency on the responses of LGN cells to sine-wave gratings of different spatial frequency and contrast presented to the dominant eye. Detailed testing was carried out on 49 cells that had statistically significant responses to stimulation of the nondominant eye alone. Spatial-frequency response functions to nondominant-eye stimulation indicated that the responses were spatially tuned, as reported previously (Guido et al., 1989). Optimal spatial frequencies through the nondominant eye were significantly correlated with the optimal spatial frequencies through the dominant eye (r = 0.54; P less than 0.0001), and the optimal spatial frequencies were fairly similar for the two eyes. Nondominant-eye stimulation changed the maximal amplitude of the fundamental (F1) response to dominant-eye stimulation for only about 45% (22 of 49) of the cells that responded to nondominant-eye stimulation alone. The response vs. contrast function through the dominant eye was altered for 73% of the cells (51% independent of spatial frequency). Three types of effects were observed: a change in the initial slope of the response vs. contrast function (contrast gain), a change in the response amplitude at which saturation occurred, or an overall change in response at all contrasts. The incidence of these changes was similar for X and Y cells in LGN layers A, A1, and C (only four W cells were tested). Nondominant-eye stimulation had little or no effect on the sizes or sensitivities of the receptive-field centers or surrounds for the dominant eye. In addition, nondominant-eye stimulation had little or no effect on optimal spatial frequency, spatial resolution, or the bandwidth of spatial-frequency contrast sensitivity curves for the dominant eye. Possible functions of binocular interactions in the LGN are considered. The present results suggest a role in interocular contrast-gain control. Interocular contrast differences can occur before the acquisition of binocular fusion, when the two eyes are viewing different aspects of a visual stimulus. Psychophysical and physiological studies suggest that an interocular mechanism exists to maintain relatively constant binocular interactions despite differences in interocular contrast. The present results suggest that at least part of this mechanism occurs in the LGN.

Functional compensation in the lateral suprasylvian visual area following bilateral visual cortex damage in kittens

Previous studies have shown that functional compensation is present in the cat's posteromedial lateral suprasylvian (PMLS) area of cortex after damage to areas 17, 18, and 19 (visual cortex) early in life but not after damage in adults. These studies all have investigated animals with a unilateral visual cortex lesion, whereas all behavioral studies of compensation for early visual cortex damage have investigated animals with a bilateral lesion. In the present experiment, we investigated whether functional compensation also is present in PMLS cortex after a bilateral visual cortex lesion early in life. We recorded from single neurons in the PMLS cortex of adult cats that had received a bilateral lesion of areas 17, 18, and 19 on the day of birth or at 8 weeks of age. We found that PMLS cells in both groups of cats had functional compensation (normal direction selectivity and ocular dominance) similar to that seen after a unilateral lesion at the same ages. These results are consistent with the hypothesis that PMLS cortex is involved in the behavioral compensation seen after early visual cortex damage. In addition, the results indicate that inputs from contralateral visual cortex are not necessary for the development of functional compensation seen in PMLS cortex.

Afferent bases of spatial- and temporal-frequency processing by neurons in the cat's posteromedial lateral suprasylvian cortex: effects of removing areas 17, 18, and 19

1. We investigated whether spatial- and temporal-frequency processing by neurons in the cat's posteromedial lateral suprasylvian (PMLS) extrastriate cortex depends on inputs from ipsilateral areas 17, 18, and 19 (visual cortex; VC) or occurs in parallel with those cortical areas. 2. Single neurons were recorded in PMLS cortex of normal adult cats and adult cats that had ipsilateral VC removed within 24 h before recording. Receptive-field properties were characterized, and responses to sine-wave gratings of different spatial frequencies, contrasts, and temporal frequencies were measured and Fourier analyzed. 3. As in previous studies, removing inputs from VC led to a reduction in the proportion of direction-selective PMLS cells. In addition there were statistically significant reductions in response amplitude and variability, although signal-to-noise ratios were unchanged. Contrast sensitivity also was reduced at all spatial frequencies. Spatial resolution was reduced slightly; however, this reduction appears to be secondary to the overall reduction in response amplitude and sensitivity. 4. The shape of the spatial-frequency contrast-sensitivity functions and the distribution of optimal spatial frequencies were unaffected by removing inputs from VC. In addition, once response threshold was reached, the slope of the contrast-response function (contrast gain) at the optimal spatial frequency was similar for PMLS cells in normal cats and cats with a VC lesion. 5. When tested at the optimal spatial frequency, temporal-frequency bandwidths, high and low temporal-frequency cutoffs, and optimal temporal frequencies were similar for PMLS cells in normal cats and cats with VC removed. 6. The results thus indicate that inputs from VC are important for the elaboration of direction selectivity and affect response amplitude and contrast sensitivity among PMLS neurons. However, visual-cortical inputs have little or no influence on spatial- and temporal-frequency processing by PMLS neurons. These properties depend on inputs from other cortical areas or the thalamus and are processed in parallel with areas 17, 18, and 19.

N-methyl-D-aspartate receptors contribute to excitatory postsynaptic potentials of cat lateral geniculate neurons recorded in thalamic slices

Neurons of the cat's dorsal lateral geniculate nucleus were recorded intracellularly to study the contribution of N-methyl-D-aspartate (NMDA) receptors to excitatory postsynaptic potentials (EPSPs) and low-threshold calcium spikes. EPSPs were evoked by stimulation of retinogeniculate axons in the optic tract and/or corticogeniculate axons in the optic radiations; EPSPs from both sources were similar. These EPSPs had one or two components, and the second component had several characteristics of NMDA receptor-mediated events. For example, EPSP amplitude decreased when neurons were hyperpolarized and increased when stimulus frequency was increased; these EPSPs could also be blocked reversibly by application of the selective NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV). We also studied the influence of NMDA receptors on low-threshold calcium spikes, which are large, voltage- and calcium-dependent depolarizations that are often accompanied by high-frequency action potential discharge. APV blocked synaptically activated low-threshold calcium spikes, but APV had no effect on low-threshold calcium spikes that were elicited by current injection. Therefore, APV does not appear to have a direct effect on the T-type calcium channel that is involved in generation of low-threshold calcium spikes. The voltage and frequency dependence of the NMDA receptor-mediated component of the EPSPs, as well as its ability to trigger low-threshold calcium spikes, provide for complex signal processing in the lateral geniculate nucleus.

6-Hydroxydopamine treatment blocks the effects of chronic monocular paralysis in the cat's lateral geniculate nucleus

The abnormal patterns of binocular stimulation produced by unilateral eye immobilization (monocular paralysis) alter the physiology of the lateral geniculate nucleus (LGN), shifting the LGN X/Y ratio in such a way that X cells are encountered far less frequently than Y cells. These changes are not observed in cats treated with intraventricular injections of the neurotoxin, 6-hydroxydopamine (6-OHDA) during the period of chronic monocular paralysis. Additional experiments indicated that the blockade was not due to any non-specific effects associated with the injection procedures, nor to any direct effects the drug itself may have had on LGN cell recording. These results suggest that the neuronal mechanism mediating the shift in the X/Y ratio produced by monocular paralysis contains elements that are sensitive to 6-OHDA.

Binocular interactions in the cat's dorsal lateral geniculate nucleus. I. Spatial-frequency analysis of responses of X, Y, and W cells to nondominant-eye stimulation

1. X, Y, and W cells in the A and C layers of the cat's dorsal lateral geniculate nucleus (LGN) were tested for responses to stimulation of the nondominant eye. The main purpose was to determine the incidence of nondominant-eye excitation and inhibition among different classes of cells and to examine the spatial-frequency tuning of responses to the nondominant eye. 2. Of 198 cells that were tested with drifting sine-wave gratings presented to the nondominant eye, 109 (55%) showed statistically significant responses. Four types of responses were observed: an increase in the mean discharge rate (F0 excitation), a decrease in the mean discharge rate (F0 inhibition), an increased modulation at the fundamental frequency of the grating (F1 excitation), and a decreased modulation at the fundamental frequency of the grating (F1 inhibition). Overall, 29% of the cells responded with inhibition, 24% responded with excitation, and 2% showed both excitation and inhibition, depending upon the spatial frequency and/or the harmonic response component. The relative incidence of excitation and inhibition was similar for X, Y, and W cells, for cells with on-center and off-center receptive fields, for cells with different receptive-field eccentricities, and for cells in each LGN layer. In addition, within layers A and A1, responses were similar for cells at different distances from the laminar borders. 3. Spatial-frequency response functions indicated that cells could have band-pass or low-pass spatial-frequency tuning through the nondominant eye. Band-pass cells tended to be narrowly tuned (less than or equal to 1 octave), and low-pass cells responded to a broader range of spatial frequencies. These properties were similar for X, Y, and W cells. Spatial resolution tended to be low (less than or equal to 0.8 c/deg for most cells), although a few cells responded to the highest spatial frequency tested (5.4 c/deg). Likewise, optimal spatial frequency was low (less than or equal to 0.2 c/deg) for most cells. These properties were similar for X and Y cells, and there was a weak tendency for X and Y cells to have higher optimal spatial frequencies and spatial resolutions than W cells. 4. In terms of absolute change in activity, responses to drifting gratings were weak. However, cells that were inhibited generally showed 20-60% decreases in activity to the optimal spatial frequency, and cells that were excited generally showed 40-100% increases. Response amplitudes were similar for X, Y, and W cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of lateral geniculate neurons that survive long-term visual cortex damage in kittens and adult cats

Damage to visual cortex (areas 17-19) in kittens or adult cats produces severe retrograde degeneration of neurons in the dorsal lateral geniculate nucleus (LGN). However, some neurons survive in otherwise degenerated portions of the LGN after a visual cortex lesion at any age. Previous studies have shown that there are well-defined differences in potential retinal inputs, soma size, synaptic connections, outputs, and physiological properties of output targets of the surviving LGN cells in cats that received visual cortex damage at different ages. The present experiment investigated the relationships between these differences and the responses of surviving LGN neurons to visual stimulation. Recordings were made from surviving neurons in the degenerated A- and C-layers of the LGN in cats that had received a visual cortex lesion on the day of birth, at 8 weeks of age, or as adults (survival was 11.5-36 months). Normal adult cats were studied for comparison. The visual receptive field was mapped, and tests were carried out to classify each cell as X, Y, or W. In addition, quantitative methods were used to assess response amplitude, strength of receptive-field surround inhibition, spatial-frequency tuning to drifting or counterphased sine-wave gratings, and response to nondominant-eye stimulation for each cell. We found that surviving cells in all LGN layers respond to light, have normal receptive-field organization, and have normal eye dominance following a lesion at any age tested. In addition, gross retinotopic organization of the LGN is normal. However, 2 main abnormalities were observed following a lesion at all 3 ages. First, there is a reduction in the percentage of X cells in the A layers, from 62% in normal LGNs to about 15% in degenerated LGNs. Second, many surviving cells in both the A- and C-layers have abnormally large receptive-field centers. Other differences that were observed between normal A-layer cells and surviving A-layer cells could be attributed to the loss of X cells. These results indicate that cells within a structure that shows severe retrograde degeneration after brain damage can maintain relatively normal function and can take part in potentially important residual neural pathways. Previous studies indicate that these residual pathways can show both anatomical and physiological compensation for the brain damage, and the present findings bear on the consequences and mechanisms of this compensation.

The influence of anesthesia upon binocular processes controlling the recordability of X- and Y-cells in the lateral geniculate nucleus of the cat

The interaction between anesthesia and binocular physiology was explored using chronic monocular paralysis. Monocular paralysis allows analysis and classification of lateral geniculate nucleus (LGN) cells without systemic paralysis and anesthesia and also produces a tonic bias in binocular mechanisms which control the relative recordability of X- and Y-cells (i.e. the LGN X/Y ratio). This effect appears to be reversed by the induction of anesthesia. In this study we (1) assessed the effects of anesthesia induction and withdrawal upon the X/Y ratio in a large number of chronic monocularly paralyzed cats, and (2) evaluated the degree to which a change in excitability versus a change in functional identity in individual LGN cells may contribute to these anesthesia-induced shifts in the X/Y ratio. Although anesthesia induction invariably increased the X/Y ratio (which is typically quite low in chronic monocular paralysis), it never caused a reliable shift between X- and Y-categories in any cell. Congruent with its effects upon the X/Y ratio, however, anesthesia induction increased excitability in 73% of X-cells and decreased excitability in 55% of Y-cells. Control experiments indicated that these systematic effects of anesthesia are not characteristic of normal animals but are specific to those with chronic monocular paralysis. Thus, the induction of anesthesia does reverse the effects of chronic monocular paralysis upon the LGN X/Y ratio apparently by inducing reciprocal changes in X- and Y-excitability. Further, while we find no evidence that anesthesia produces a qualitative distortion in the monocular properties of LGN cells, the induction and withdrawal of anesthesia does appear to modulate the operation of binocular processes controlling the recordability of LGN X- and Y-cells.

Binocular interactions in the dorsal lateral geniculate nucleus of monocularly paralyzed cats: extraretinal and retinal influences

Prolonged periods of monocular paralysis alter the physiology of the dorsal lateral geniculate nucleus (LGN), shifting the X/Y cell ratio so that X cells are encountered less frequently than Y cells. The shift in the LGN X/Y cell ratio is observed in both the A-layers of both geniculates whether the innervating eye is paralyzed or mobile. This change in the LGN has been attributed to a mechanism that is sensitive to disruptions in binocular cues. The effects of monocular paralysis in the LGN were used to demonstrate that LGN cells possess a sensitivity to binocular cues of an extraretinal and retinal source. The removal of extraretinal signals, in the form of proprioceptive feedback from the extraocular muscles of the mobile eye, by section of the ophthalmic branch of the Vth cranial nerve, resulted in an immediate and long-lasting reversal in the effects of monocular paralysis. The LGN X/Y ratio was restored to a normal value in the layers innervated by the eye with intact proprioceptive inputs as well as in the layers innervated by the eye in which proprioceptive inputs were removed. In contrast to this, the removal of proprioceptive inputs from the paralyzed eye had no effect on the LGN X/Y ratio. The removal of visual inputs from the mobile eye by section of the optic nerve resulted in an immediate, but somewhat transient reversal in the effects of monocular paralysis. Within the first 25 h after optic nerve section, the LGN X/Y ratio was restored to a normal value in the layers innervated by the eye with intact visual inputs. A transient reversal was also observed when both visual and proprioceptive inputs from the mobile eye were removed. These results are consistent with the belief that the LGN is one site in the visual pathway where proprioceptive and visual signals from the two eyes converge.

The shift in X/Y ratio after chronic monocular paralysis: a binocularly mediated, barbiturate-sensitive effect in the adult lateral geniculate nucleus

Adult-onset stimulus modifications, such as monocular paralysis, alter the physiology of the lateral geniculate nucleus (LGN), reducing the encounter rate for X-latency cells in all of the principal layers of both LGNs whether the innervating eye is paralyzed or mobile. These reductions in encounter rate for X-latency cells are confined to those portions of the LGN representing central binocular visual space and are sensitive to the level of anesthesia in that, while these effects are evident in subjects sedated during recording, no such reductions are found when subjects are anesthetized with sodium pentobarbital during recording. Finally, conduction velocity and receptive field classification data from these experiments confirm, as the shifts in OX latency distributions would indicate, that chronic monocular paralysis does have a selective impact upon the recordability of LGN X-cells. These observations together with earlier ones involving monocular paralysis suggest that this adult-onset modification reduces the encounter rate for X-cells by disrupting a binocular mechanism which controls the relative excitability of X- and Y-cells which represent central visual space.