Max S. Cynader

Sensitivity of cat primary auditory cortex (Al) neurons to the direction and rate of frequency modulation

Responses of 65 single auditory cortex (AI) neurons to frequency-modulated (FM) sweeps with different rates and direction of frequency change were examined quantitatively. Most units responded differentially depending on the characteristics of the FM sweep stimulus. Sixty-five percent of the units encountered responded at least twice as well for one direction of the FM sweep as for the other direction. Of these direction selective neurons, 67% preferred downward-directed FM sweeps (i.e. changing from high to low frequencies) while only 33% preferred upward-directed FM sweeps. The preference for downward-directed FM sweeps was especially clear in EI cells. In addition, cortical neurons often displayed sensitivity to the rate of frequency modulation (speed sensitivity).

Comparison of receptive‐field organization of the superior colliculus in Siamese and normal cats

1. The superior colliculus has been studied in Siamese and normal cats by recording the responses of single tectal units to visual stimuli.

Neurones in cat parastriate cortex sensitive to the direction of motion in three-dimensional space

1. On psychophysical grounds, Beverley & Regan suggested that in man different neural mechanisms mediate the binocular perception of movement in depth and the binocular perception of positional (static) depth. They proposed that the human visual pathway contains several neural mechanisms, each sensitive to a different direction of motion in space. These mechanisms compute the direction of motion from the relative speeds and directions of movement of the left and right retinal images.

Recovery of function in cat visual cortex following prolonged deprivation

SummaryEvidence that there is a critical period during which response characteristics of neurons in visual cortex of the cat may be influenced has been provided in several studies, which suggest that the period of influence is restricted to the first few months of life. Using a somewhat different experimental procedure, we have obtained evidence that cortical units retain plasticity long after the end of this period has passed. In our procedure prolonged visual deprivation was followed by exposure in a normal visual environment. The animals were maintained throughout the first year of life either in total darkness or in an enclosure illuminated intermittently by a strobe light. Following the period of deprivation, electrophysiologic recordings were taken from some of these animals. The remaining cats were permitted 6–12 months in a normally-illuminated environment prior to recording. Cats of the same age reared from birth in a normally lit environment were also recorded.Cortical neurons in cats deprived of any normal visual experience rarely show orientation selective responses. In animals allowed subsequent normal visual experience about one-half of the units studied exhibited this property. This level of response specificity is intermediate between that of normally-reared and recently-deprived animals. While most cortical units in normally-reared cats exhibit direction selectivity, this property is rarely observed in the “recovery” cats. A number of unit types which are rarely observed in either normal or totally deprived animals were encountered in cats that had normal exposure following prolonged deprivation. A convergent strabismus was observed, in contrast with the divergent strabismus often shown by cats immediately following prolonged visual deprivation. This shows that ocular alignment as well as cortical unit properties can remain plastic in the adult.

Enrichment of glutamate in zinc-containing terminals of the cat visual cortex

The presence of glutamate and GABA was examined in zinc-containing terminals of the cat visual cortex using a post-embedding immunogold method. The surface density of immunogold-labelling was also evaluated in morphologically defined ultrastructural elements, namely terminals having round synaptic vesicles and making asymmetrical synapses (RA boutons), terminals with flat vesicles and symmetrical synapses (FS) and glial cell processes. Glutamate immunoreactivity was highest in RA terminals and in zinc-containing boutons. It was lower in FS terminals and lowest in glial cell processes. GABA immunoreactivity was highest in FS terminals and low in all other ultrastructural elements analysed, including zinc-containing terminals. Therefore, zinc-containing terminals show an enrichment of glutamate and they are likely to use this amino acid as their neurotransmitter. Moreover, the fact that many RA terminals that are negative for zinc show an enrichment of immunoreactive glutamate suggests that zinc-containing fibres represent a subpopulation of the glutamate axonal network.

Endstopping and curvature

Hypercomplex or endstopped visual cortical neurons are usually supposed to be concerned with length or end point analysis. However, recent evidence demonstrates that endstopped neurons are curvature-selective, a connection that we explore here in some detail. A model of endstopped simple cells is developed and a variety of computational simulations examine the connection of the model to the reported length and orientation responses of endstopped neurons. Even and odd versions of the model are described, both of which are shown to be curvature-selective. Even-symmetric instances of the model respond well to thin curves over a range of curve orientation and curvature, independent of sign of curvature. In contrast, odd-symmetric instances respond to both thin and thick curves while exhibiting a more complex curvature-sign dependence--responding in a sign-selective fashion to curved lines but not to curved edges. Finally, the response of the endstopped model to curve singularities is explored, and the possible role of nonendstopped and endstopped cells in building curve descriptions is discussed.

Bilirubin as a potent antioxidant suppresses experimental autoimmune encephalomyelitis: implications for the role of oxidative stress in the development of multiple sclerosis

Increasing evidence shows that oxidative stress plays an important role in the pathogenesis of multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). In recent years, bilirubin has been demonstrated to be a potent antioxidant in vitro. In this study, we administered bilirubin to rats with acute and chronic EAE. Bilirubin prevented both acute and chronic EAE effectively. More significantly, bilirubin suppressed ongoing clinical EAE and halted EAE progression when given after disease onset. Subsequent histological examination showed that if administered to rats before the onset of EAE, bilirubin interfered with the invasion of inflammatory cells into the central nervous system (CNS) because it protected the blood-brain barrier (BBB) from free radical-induced permeability changes. However, in some cases, inflammation still occurred even when no clinical illness was observed. In rats with treatment initiated after the onset of EAE, despite the clinical improvements, treatment with bilirubin did not reduce the degree of CNS inflammation, or change cytokine expression in CNS lesions, indicating a lack of immunosuppressive effect of this treatment. By contrast, bilirubin treatment significantly alleviated oxidative damage in the spinal cord, and the clinical signs of EAE correlated well with the degree of oxidative injury in the lesions. Our results suggest that free radicals play an important role in the final effector stages of EAE, and that antioxidant therapies may have potential for the treatment of MS.

Neuronal activity in primate visual cortex assessed by immunostaining for the transcription factor Zif268

It is now well established that environmental signals mediated via neurotransmitters and hormones can induce responses in cells which involve a cascade of receptors, G proteins, and second messengers. These in turn can induce transcription factors which regulate long-term changes in gene expression. It has been proposed that the stimulus-transcription coupling properties of these DNA-binding proteins can be exploited to visualize activated neurons by way of immunostaining. We have used standard immunohistochemical techniques to detect the expression of one specific transcription factor, Zif268, in the visual cortex (area 17, V1) of vervet monkeys (Cercopithecus aethiops). Immunopositive neurons were present in large numbers throughout the visual cortex of the normal animal, being concentrated in layers 2/3 and 6 and at moderate levels in 4C beta and 5. To determine if Zif268 expression was affected by visual stimulation in the monkey, we restricted light input to one eye with the aim of revealing ocular-dominance columns in striate cortex. We found that short-term monocular deprivation induced either by enucleation, intravitreal TTX injection, or eyelid suturing resulted in dramatic changes in Zif268 levels, revealing vertically oriented columns of reduced Zif268 staining interdigitated with columns of normal expression. Furthermore, these columns were discernible after just 2 h of monocular blockade. A comparison of the ocular-dominance pattern obtained with Zif268 immunostaining and cytochrome oxidase histochemistry in long-term monocularly deprived animals showed a coincident reduction of both markers along columns that were precisely aligned in adjacent sections, indicating that Zif268 expression is restricted to cortical regions of high metabolic activity. Simultaneous immunostaining for Zif268 and the calcium-binding proteins calbindin and parvalbumin showed a negative correlation, suggesting that the Zif268 protein may be expressed selectively within excitatory neurons. A similar approach with immunostaining for neurofilament and microtubule-associated proteins (SMI-32 and MAP2) revealed pyramidal neurons which were regularly found to contain a Zif268-positive nucleus. Furthermore, confocal images of lucifer yellow filled neurons possessing Zif268-positive nuclei all showed pyramidal morphology. Taken together, these results point to activity-dependent expression of Zif268 within a subset of excitatory neurons.

Dendritic and Synaptic Pathology in Experimental Autoimmune Encephalomyelitis

Evidence has shown that excitotoxicity may contribute to the loss of central nervous system axons and oligodendrocytes in multiple sclerosis and experimental autoimmune encephalomyelitis (EAE). Because dendrites and synapses are vulnerable to excitotoxicity, we examined these structures in acute and chronic models of EAE. Immunostaining for microtubule-associated protein-2 showed that extensive dendritic beading occurred in the white matter of the lumbosacral spinal cord (LSSC) during acute EAE episodes and EAE relapses. Retrograde labeling confirmed that most motoneuron dendrites were beaded in the white matter of the LSSC in acute EAE. In contrast, only mild swelling was observed in the gray matter of the LSSC. Dendritic beading showed marked recovery during EAE remission and after EAE recovery. In addition, synaptophysin, synapsin I, and PSD-95 immunoreactivities were significantly reduced in both the gray and white matter of the LSSC during acute EAE episodes and EAE relapses, but showed partial recovery during EAE remission and after EAE recovery. Pathologically, both dendritic beading and the reduction in synaptic protein immunoreactivity were well correlated with inflammatory cell infiltration in the LSSC at different EAE stages. We propose that dendritic and synaptic damage in the spinal cord may contribute to the neurological deficits in EAE.

Bilirubin Possesses Powerful Immunomodulatory Activity and Suppresses Experimental Autoimmune Encephalomyelitis

Bilirubin, an abundant bile pigment in mammalian serum, was once considered a toxic waste product and has more recently been recognized as a potent antioxidant of physiological importance. However, its potential biological functions in other fields are not well understood. Herein we show that bilirubin is also a powerful immunomodulatory agent. Bilirubin significantly inhibited Ag-specific and polyclonal T cell responses, while other similar antioxidants completely lacked this effect. Bilirubin suppressed CD4+ T cell responses at multiple steps. High levels of bilirubin could induce apoptosis in reactive CD4+ T cells. Bilirubin at nonapoptotic concentrations suppressed CD4+ T cell reactivity through a wide range of actions, including inhibition of costimulator activities, suppression of immune transcription factor activation, and down-regulation of inducible MHC class II expression. Further studies suggest that bilirubin actions were direct, rather than via induction of immune deviation or regulatory T cells. In vivo, treatment with bilirubin effectively suppressed experimental autoimmune encephalomyelitis in SJL/J mice. In contrast, depletion of endogenous bilirubin dramatically exacerbated this disease. In summary, our results identify bilirubin as an important immunomodulator that may protect mammals against autoimmune diseases, thereby indicating its potential in the treatment of multiple sclerosis and other immune disorders.