O. Fehér

Neuronal plasticity induced by neonatal monocular (and binocular) enucleation

Monocular (ME) and binocular enucleation has become a useful experimental tool for analyzing the mechanisms of neural plasticity. ME when performed during an early postnatal period (up to 15 days after birth) initiates a series of adaptive reactions in the visual (and other sensory) system(s) which tend to compensate for the lost sensory capacity. Extirpation of one eye (usually the right) destroys afferents to both lateral geniculate bodies dorsal nucleus (CGLd) and superior colliculi (CS), being severely impaired by the degeneration of retino-geniculate and collicular synapses. The sprouting of retinogeniculate fibers coming from the remaining eye replaces these synapses in both CGLds. Ipsilateral representation of the remaining eye (usually of minor significance) becomes extended in the left CGLd and consequently in the left visual area, just as in the superior colliculi. A similar but somewhat smaller extension takes place in the contralateral CGLd and visual cortex. The strengthening of commissural connections results in a remarkable extension of callosally connected stripes and patches in both hemispheres. After ME in the critical period, the control over behavior is taken over by the remaining eye. Its power of resolution is improved because of the higher survival of (mainly ipsilaterally projecting) ganglion cells. Therefore, both hemispheres are still available for storing visual information. In ME rats the learning of visual tasks requires both hemispheres, but relearning is still possible after extirpation of the contralateral one. The possible two main mechanisms of adaptive plastic changes are: (i) replacement of degenerated synapses by sprouting collaterals of ingrowing foreign fibers, and (ii) neurons having morphologically intact but inactive synapses establishing connections with afferent fibers other than the usual. The same mechanism is seen operating in cross-modal adaptive reactions as well.

Neonatal monocular enucleation-induced cross-modal effects observed in the cortex of adult rat

The cortices of neonatally enucleated rats were explored for somatosensory responses with special reference to an extension into the occipital cortex. Monocular enucleation was performed on rats at birth. The animals were raised and from the age of three months the activity evoked by either electric stimulation of the vibrissa pad or bending of the vibrissae was tested in the contralateral cortex by electric recording and autoradiography. It was found that early enucleation caused an expansion of the somatosensory responses, among others into the visual area. Neurons responsive to visual and somatosensory stimuli were demonstrated in the anterior part of the primary and secondary visual areas, contralateral to the enucleation. Electrophysiological and autoradiographic studies unambiguously proved that early enucleation exerted a significant cross-modal effect on the somatosensory responsive area.

γ-aminobutyric acid enables synaptogenesis in the intact superior cervical ganglion of the adult rat

Local gamma-aminobutyric acid (GABA) application into the intact superior cervical ganglion (SCG) of the adult rat allows active innervation of a surgically implanted hypoglossal nerve in addition to the normal nerve supply of the ganglion. In GABA-treated SCG of the adult rat, action potentials could be obtained on stimulation of both the preganglionic nerve trunk and the implanted hypoglossal nerve. Both action potentials were reversibly sensitive to hexamethonium bromide indicating new cholinergic synapses established between axons in the hypoglossal nerve and principal sympathetic neurons. If GABA treatment of the ganglion was omitted, the double innervation did not develop after hypoglossal nerve implantation.

Modified distribution patterns of responses in rat visual cortex induced by monocular enucleation

Monocular enucleation was performed on rats at birth or on the 8th or 15th day, respectively. The animals were raised and from the age of 3 months the evoked activity was tested in the visual cortex. It was found that monocular enucleation changed the distribution of visually evoked responses. In the right hemisphere (contralateral to the preserved eye) two focuses of evoked potentials appeared with large amplitudes shifted to both sides, towards the lateral and medial borders of the primary visual area. In the left hemisphere the focus of the evoked potentials was shifted slightly laterally and posteriorly. Early enucleation caused an expansion of the somatosensory responses into the visual area. Bimodal neurons (responsive to visual and somatosensory stimuli) were observed in the anterior part of the primary and secondary visual areas contralateral to the enucleation. These changes became moderate with increasing age at which enucleation was performed and were not found in animals enucleated on the 15th day after birth.

Acoustic sensitivity and bimodal properties of cells in the anterior suprasylvian gyrus of the cat.

SummaryThe properties of acoustically responsive neurons were studied in the anterior part of the gyrus suprasylvius (ASG) of the cat. The most important features of the responses given to pure tones of different frequencies were: short latency, sharp tuning curves with well definable best frequency (BF). In these respects the cells showed a close resemblance to those of primary acoustic area (AI), and at the same time they proved to be true bimodal cells (responding to somatosensory stimuli, too) like those of the associative areas.

Sodium bromide treatment influences the plasticity of somatosensory responses in the rat cortex as induced by enucleation

Effects of sodium bromide were studied on central neuroplasticity induced by early binocular enucleation. It has previously been found that enucleation on the day of birth, but not later than the first postnatal week, resulted in changes in the occipital cortex, such as the invasion of somatosensory evoked activity into the visual cortex areas. The present results showed that sodium bromide treatment extended at least up to 15 days after birth, the critical period during which somatosensory projections could be modified by visual deafferentation. Together with observations of Frost [J. comp. Neurol. (1981) 203, 227-256; Devl Brain Res. (1982) 3, 627-636], the present results suggest a mutual dependency of visual and somatosensory projection development. The present study is the first demonstration that the critical period of development, during which a specific type of neural plasticity can be induced, may be prolonged by pharmacological means, i.e. by chronic treatment with sodium bromide.

Spatial correlation between sensory regions and the drainage fields of pial veins in rat cerebral cortex

SummaryVisual and somatosensory evoked potentials were mapped in the cerebral cortex of adult rats and, after filling the cerebral arteries and veins with dye, the mappings were then compared to the distribution of pial veins. A close relationship was found between the position, size and shape of the occipital venous drainage field and the distribution of visual evoked potentials with high amplitudes and short latencies. Accordingly, such potentials evoked by stimulation of the forepaw and the tailroot were confined to the fronto-parietal drainage field. In the case of individual variations in the expansion and shape of sensory areas, the medial and lateral borders of the occipital drainage field and the medial border of the fronto-parietal drainage field covaried. Only at the common border between these two drainage fields, visual evoked potentials with small amplitudes and long latencies extended into the parietal drainage field and overlapped with somatosensory evoked potentials. This overlapping area corresponds in position to the anterior part of the peristriate cortex. A comparison between the vascular organization and cytoarchitectonic maps of the rat cortex indicates that other parts of the characteristic pattern of venous drainage fields may also correlate with the cytoarchitectonic and functional organization of the cerebral cortex. These observations suggest that during morphogenesis the formation of sensory projections to the cerebral cortex may interact with the angiogenesis, mainly with the development of veins.

Two different polysensory systems in the suprasylvian gyrus of the cat. An electrophysiological and autoradiographic study

Abstract The suprasylvian gyrus of cats was systematically mapped by recording acoustic and somatosensory-evoked potentials under pentobarbital and chloralose anaesthesia. In this way two polysensory systems could be differentiated. One of them operates only under chloralose anaesthesia and proved to be identical with the long latency association fields: the anterior middle suprasylvian association area and the posterior middle suprasylvian association area, discovered by Thompson , Johnson & Hopes (1963). The other one is localized to the anterior suprasylvian gyrus and exhibits short latency-evoked potentials and is also active under pentobarbital anaesthesia. The incorporation of [ 3 H]-glycine, as demonstrated by autoradiography, showed good correspondence with the electrophysiological findings and so provides a morphological way of localising the two polysensory systems.

Promotion by sodium bromide of functional synapse formation from foreign nerves in the superior cervical ganglion of adult rat with intact preganglionic nerve supply

The possible effect of sodium bromide (NaBr) (a substance with known inhibitory action on synaptic transmission) was studied on synapse formation with foreign nerves, implanted into the superior cervical ganglion of adult rats. It was found that in spite of the presence of preganglionic nerve supply, both implanted nerves (n.XII and n.X, respectively) were enabled to establish functional synapses with the principal ganglion cells in NaBr-treated animals. In contrast, synapse formation was almost absent in ganglia of sodium chloride drinking (control) rats with intact preganglionic nerve supply. This effect of NaBr is considered to be analogous to that of GABA, whose promoting action on synaptogenesis in adult rat superior cervical ganglion has been previously described.

Dynamic interactions of evoked potentials in a polysensory cortex of the cat

Interactions of acoustic and somatosensory evoked potentials were studied in the anterior suprasylvian gyrus of the cat. Two kinds of interaction could be observed: occlusion or facilitation. In most cases occlusion was observed. The interactions showed dynamic changes and were susceptible to different kinds of influences. After having determined the control values of interaction over a period of several minutes, reversible enhancement of occlusion was observed after synchronous activation of the acoustic and somatosensory inputs with 2 Hz frequency. The same effect could be observed during stimulation of the mesencephalic reticular formation with 200 Hz frequency. The interactions could also be influenced by amphetamine and gamma-glutamyltaurine, known as drugs capable of influencing the arousal level of the brain. During treatment with amphetamine the interaction was shifted in the facilitatory direction. The antagonists of amphetamine (haloperidol and reserpine) prevented this effect. The authors suggest that the interactions of acoustic and somatosensory responses are mediated by interneurons (inhibitory and/or excitatory) and particular stimulus situations and drugs are able to modify the equilibrium between excitatory and inhibitory subsystems.