http://nova.newcastle.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7566 Circumscribed retinal lesions in adult cats result in a reorganization of circuitry in area 17 such that neurons in the lesion projection zone (LPZ) can now be activated, not from their original receptive fields (RFs) but from regions of normal retina adjacent to the lesion ('ectopic' RFs). We have studied this phenomenon further by making circumscribed monocular retinal lesions in 8-week-old kittens and recording responses to visual stimuli of neurons in the LPZ of area 17 when these cats reached adulthood. These responses have been compared with those in adult-lesioned cats either of relatively short postlesion survival (2–24 weeks) or long postlesion survival (3.5–4.5 years). In both kitten-lesioned and adult-lesioned animals most LPZ neurons recorded from the supragranular layers (II and III) not only exhibited new ectopic RFs when stimuli were presented via the lesioned eye but the RF properties (e.g. the sizes of excitatory RFs, orientation and direction selectivities, velocity preferences and upper cut-off velocities) were often indistinguishable from those seen when stimuli were presented via the nonlesioned eye. Similarly, in both kitten-lesioned and adult-lesioned animals, most LPZ neurons recorded from the granular and infragranular layers (IV, V, VI), like those recorded from the supragranular layers, were binocular. However, in adult-lesioned but not in kitten-lesioned animals, the responses and the upper cut-off velocities of LPZ cells recorded from the granular and infragranular layers to stimuli presented via ectopic RFs tended to be, respectively, substantially weaker and lower than those for stimuli presented via the nonlesioned eye. The age-related laminar differences in reorganizational plasticity of cat striate cortex correlate with the lamino-temporal pattern of distribution of N-methyl-d-aspartate glutamate receptors in striate cortex. 2011-04-12T06:00:01.960Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7564 We have previously reported to the Society that in cats in which monocular retinal lesions were made in adulthood or adolescence cells in the lesion projection zone (LPZ) of area 17 recovered sensitivity to photic stimuli but the receptive fields (RFs) were now ectopic, i.e. outside the lesion (Burke et al. 2000). When the lesions were made in adult cats (AL), stimuli presented via the lesioned eye gave lower peak discharge rates and lower cut-off velocities than those presented via the non-lesioned eye. By contrast, in kitten-lesioned cats (KL) the cut-off velocities and the peak discharge rates were similar for stimuli presented via the lesioned and the non-lesioned eye. There was, however, a difference in the time from lesion to experiment between the AL and KL groups, 2-24 weeks vs. 28-68 weeks. It was, therefore, important to see if this factor could be responsible for the neural effects. Ectopic RFs are believed to be mediated via axon collaterals of pyramidal cells interconnecting mainly in laminae 2 and 3. We, therefore, also investigated the location of the LPZ neurones (supragranular (SG) - laminae 1-3: granular/ infragranular (G/I) - laminae 4-6). In addition to the two groups already described (AL 4 cats; KL 5 cats) we prepared a third group in which the lesion was made in the adult cat but the cat then survived 3.5-4.5 years (AL/L 3 cats). Retinal lesions 8-12 deg in diameter were made in cats anaesthetized with xylazine (3 mg kg-1) and ketamine (30 mg kg-1). Single neurones in area 17 were studied in cats anaesthetized with 0.5-0.7 % halothane in 67/33 N2O/O2, given gallamine triethiodide 7.5 mg kg-1 h-1 I.V. and artificially respired. EEG, ECG, end-tidal CO2, lung pressure and deep body temperature were monitored and kept within normal limits. Animals were humanely killed at the end of the experiments. In the G/I laminae we found no difference between AL and AL/L cats but a significant difference between AL/L and KL cats and between AL and KL cats with regard to peak discharge rates and cut-off velocities (Wilcoxon, P < 0.05). Thus it seems that the interval between lesion and experiment is not a critical factor. By contrast, in the SG laminae there was no difference between AL and KL cats with respect to peak discharge rates and cut-off velocities. Thus following monocular retinal lesions there appears to be a critical period for the LPZ cells recorded from the G/I layers of area 17, after which the presumed cortical mechanisms underlying establishment of ectopic RFs are not capable of good compensation for the loss of the retinal input. 2011-04-12T05:50:03.507Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:7563 The 3α-hydroxy,5α-reduced pregnane steroids, allopregnanolone and allotetrahydrodeoxycorticosterone, are the most potent endogenous positive modulators of GABAᴀ receptor-mediated inhibition. This study presents the first immunohistochemical examination of the cellular distribution of 3α-hydroxy,5α-reduced pregnane steroids across the brain. We found a widespread distribution in the adult rat, with dense immunolabelling in the olfactory bulb, striatum and cerebral cortex, and lower density labelling in the brainstem reticular formation. In general terms, this distribution accords with the regional concentrations of 3α-hydroxy,5α-reduced steroids determined, in other laboratories, by brain region sampling and either gas chromatography-mass fragmentography or radioimmunoassay. However, immunohistochemistry allowed for a more detailed examination of regional distribution and cellular specificity. All immunoreactivity was confined to the cell bodies and thick dendrites of neurones; no identifiable glia were labelled. In most brain areas, the location and morphology of labelled cells identified them as excitatory neurones. In addition, cell populations known to be projecting GABAergic neurones (e.g. cerebellar Purkinje cells) were immunoreactive, whereas local inhibitory neurones generally were not. The cellular distribution of 3α-hydroxy,5α-reduced steroids suggests that sensory, motor, limbic and homeostatic systems can be influenced by neurosteroids at multiple stages of processing. 2011-04-12T05:50:02.464Z ]]> http://nova.newcastle.edu.au/vital/access/manager/Repository/uon:1388 Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems. 2010-04-27T06:53:05.847Z ]]>