Absence of sprouting by retinogeniculate axons after chronic focal lesions in the adult cat retina

Effect of unilateral partial cochlear lesions in adult cats on the representation of lesioned and unlesioned cochleas in primary auditory cortex

We examined the effect of unilateral restricted cochlear lesions in adult cats on the topographic representations ("maps") of the lesioned and unlesioned cochleas in the primary auditory cortex (AI) contralateral to the lesioned cochlea. Frequency (tonotopic) maps were derived by conventional multineuron mapping procedures in anesthetized animals. In confirmation of a study in adult guinea pigs (Robertson and Irvine [1989] J. Comp. Neurol. 282:456-471), we found that 2-11 months after the unilateral cochlear lesion the map of the lesioned cochlea in the contralateral AI was altered so that the AI region in which frequencies with lesion-induced elevations in cochlear neural sensitivity would have been represented was occupied by an enlarged representation of lesion-edge frequencies (i.e., frequencies adjacent to those with elevated cochlear neural sensitivity). Along the tonotopic axis of AI the total representation of lesion-edge frequencies could extend up to approximately 2.6 mm rostal to the area of normal representation of these frequencies. There was no topographic order within this enlarged representation. Examination of threshold sensitivity at the characteristic frequency (CF, frequency to which the neurons were most sensitive) in the reorganized regions of the map of the lesioned cochlea established that the changes in the map reflected a plastic reorganization rather than simply reflecting the residue of prelesion input. In contrast to the change in the map of the lesioned contralateral cochlea, the map of the unlesioned ipsilateral cochlea did not differ from those in normal animals. Thus, in contrast to the normal very good congruency between ipsilateral and contralateral AI maps, in the lesioned animals ipsilateral and contralateral maps differed in the region of AI in which there had been a reorganization of the map of the lesioned cochlea. Outside the region of contralateral map reorganization, ipsilateral and contralateral AI maps remained congruent within normal limits. The difference between the two maps in the region of contralateral map reorganization suggested, in light of the physiology of binaural interactions in the auditory pathway, that the cortical reorganization reflected subcortical changes. Finally, response properties of neuronal clusters within the reorganized map of the lesioned cochlea were compared to normative data with respect to threshold sensitivity at CF, the size of frequency "response areas," and response latencies. In the majority of cases, CF thresholds were similar to normative data. The frequency "response areas" were slightly less sharply tuned than normal, but not significantly. Response latencies were significantly shorter than normal in three animals and significantly longer in one animal.

Effect of retinal impulse blockage on cytochrome oxidase-rich zones in the macaque striate cortex: II. Quantitative electron-microscopic (EM) analysis of neuropil

Unilateral retinal impulse blockage with tetrodotoxin (TTX) induces reversible shrinkage and decreased cytochrome oxidase (CO) activity in alternate rows of supragranular, CO-rich puffs in the adult macaque striate cortex (Wong-Riley & Carroll, 1984b: Carroll & Wong-Riley, 1987). The present study extended the findings to the electron-microscopic (EM) level to determine if various neuropil profiles in control puffs exhibit heterogeneous levels of CO activity, and whether specific processes were more susceptible to intravitreal TTX than others. Within the neuropil of control puffs, 60% of the total mitochondrial population resided in dendrites, and the majority of dendritic mitochondria were highly reactive for CO. Axon terminals forming symmetrical synapses also contained darkly reactive mitochondria, whereas those forming asymmetrical synapses possessed very few and mainly lightly reactive mitochondria. Unmyelinated axon trunks, myelinated axons, and glia all exhibited low levels of CO activity. Synaptic count revealed a 3:1 ratio of asymmetrical to symmetrical synapses. Intravitreal TTX for 2-4 weeks adversely affects dendrites and symmetrical terminals much more so than other neuropil processes. There was a general decrease in darkly and moderately reactive mitochondria and an increase in lightly reactive mitochondria throughout the puffs, especially in dendrites. This indicates that afferent blockade is more detrimental to processes of higher metabolic activity. Changes also differed between central and peripheral regions of puffs, and indications of axonal and synaptic reorganization were more evident in the latter. Thus, stabilization of neuronal structure and synapses appears to be activity-dependent even in the adult. A working model of these metabolic and morphological responses to chronic TTX is proposed.

A quantitative study of morphological reorganization following chronic optic deafferentation in the adult cat dorsal lateral geniculate nucleus

Neuronal and synaptic reorganization in the lateral geniculate nucleus (dLGN) of adult cats following chronic visual deafferentation has been investigated with the aid of GABA immunocytochemistry and quantitative electron microscopy. The main purpose of this study was to establish the morphological counterpart of the functional plasticity of dLGN relay cells after total visual deafferentation (Eysel: Brain Res. 166:259-271, '79). The results provide evidence that the regained excitability of relay cells is not the result of disinhibition (caused hypothetically by the selective loss of GABAergic cells) since the proportion of GABA-positive and GABA-negative cells as well as the inhibitory synaptic density did not change. The alternative suggestion that the enhanced excitability could be the result of compensatory axonal sprouting by corticothalamic fibers had also to be dropped: the absolute number of corticothalamic axons to the deafferented dLGN remains unchanged. Because of shrinkage of the dendritic trees of dLGN neurons, however, the density of cortical synaptic input at dLGN cells becomes elevated by almost 60%. It is suggested that the regained excitability of relay neurons is the consequence of the combined effects of adaptive (structural) reduction in size ("atrophy") of retinally denervated nerve cells, and, as a consequence, increase of input resistance, reduced shunting effects, and relative increase in density of the excitatory cortical input per neuron.

The effects of partial retinal lesions on activity and size of cells in the dorsal lateral geniculate nucleus

The nasal parts of the adult cat retina were photocoagulated. In the dorsal lateral geniculate nucleus (dLGN) the projection of the remaining innervation was shown by anterograde transport of 3H-proline after eye injections. The neuronal activity was measured from single cells across the border region between innervated and deafferented parts of layer A of the dLGN contralateral to the lesions. A gradual decrease from normal light-excitability to total inexcitability was observed over a range of 300 micron. The perikaryal cell sizes measured in the same part of the dLGN displayed a concomitant decrease. Blockage of the afferent impulses by chronic application of tetrodotoxin did not change the results, suggesting that it is the loss of connections, not the loss of activity, that produces the transneuronal atrophy in the adult cat dLGN.

Recovery of function is not associated with proliferation of retinogeniculate synapses after chronic deafferentation in the dorsal lateral geniculate nucleus of the adult cat

A restricted recovery of visual excitation occurs in the partially deafferented dorsal lateral geniculate nucleus (dLGN) of the cat after retinal lesions. In the absence of axonal growth an increase of retinogeniculate synapses at the peripheral dendrites of deafferented cells could be a possible underlying mechanism. We labeled optic tract terminals with horseradish peroxidase and [3H]proline. The size and density of labeled boutons in the vicinity of regions deafferented by chronic retinal lesions were not different from those in normal parts of the dLGN. There was no indication for synaptic proliferation as a response to partial visual deafferentiation in the adult cat dLGN.