Modified sensory processing in the barrel cortex of the adult mouse after chronic whisker stimulation.

Chronic stimulation of a mystacial whisker follicle for 24 h induces structural and functional changes in layer IV of the corresponding barrel, with an insertion of new inhibitory synapses on spines and a depression of neuronal responses to the stimulated whisker. Under urethane anesthesia, we analyzed how sensory responses of single units are affected in layer IV and layers II & III of the stimulated barrel column as well as in adjacent columns. In the stimulated column, spatiotemporal characteristics of the activation evoked by the stimulated whisker are not altered, although spontaneous activity and response magnitude to the stimulated whisker are decreased. The sensitivity of neurons for the deflection of this whisker is not altered but the dynamic range of the response is reduced as tested by varying the amplitude and repetition rate of the deflection. Responses to deflection of nonstimulated whiskers remain unaltered with the exception of in-row whisker responses that are depressed in the column corresponding to the stimulated whisker. In adjacent nonstimulated columns, neuronal activity remains unaltered except for a diminished response of units in layer II/III to deflection of the stimulated whisker. From these results we propose that an increased inhibition within the stimulated barrel reduced the magnitude of its excitatory output and accordingly the flow of excitation toward layers II & III and the subsequent spread into adjacent columns. In addition, the period of uncorrelated activity between pathways from the stimulated and nonstimulated whiskers weakens synaptic inputs from in-row whiskers in the stimulated barrel column.

Experience-dependent plasticity is impaired in adult rat barrel cortex after whiskers are unused in early postnatal life.

The capacity of adult barrel cortex to show experience-dependent plasticity after early restricted neonatal sensory deprivation was analyzed in barrel field cortex neurons. Selective sensory deprivation was induced by trimming two whiskers from postnatal day 0 (P0) to P21, namely, the principal D2 whisker plus one adjacent surround whisker (D3). At maturity (P90), responses of supragranular (layer II/III) and barrel (layer IV) neurons, all located in the D2 barrel column, were analyzed for modified responses to the deprived principal whisker (D2) and the nondeprived (D1) and deprived (D3) adjacent surround whiskers. For supragranular neurons, the responses to both principal and surround whiskers were reduced at maturity, whereas the barrel neurons showed mildly elevated responses to the principal whisker but a reduced response to the deprived surround whisker. In normal adult rats, trimming all but the principal D2 whisker and an adjacent D3 whisker for 3 d (whisker pairing) produced the expected bias: elevated responses from the intact D3 compared with the cut D1 whisker in both barrel and supragranular neurons. When the neonatally deprived D2 and D3 whiskers were paired at maturity, a similar D3/D1 bias was generated in barrel neurons, but no bias occurred in supragranular neuron responses. Pairing the maintained D1 and deprived D2 whiskers produced a much greater bias toward D1 compared with the deprived D3 whisker in barrel neurons than in supragranular neurons. There were minimal effects on response latencies in layer IV under any of the experimental conditions. These findings indicate that a restricted period of sensory deprivation in early postnatal life (1) impairs intracortical relay of deprived inputs from layer IV to layer II/III in barrel cortex at maturity and (2) degrades receptive field plasticity of the supragranular layer cells but not the thalamic-recipient barrel neurons.

Adult experience-dependent plasticity of S1 barrel cortex in the normal and monoamine oxidase-A knockout (Tg8) mouse.

By restricted use of D2 and D3 whiskers for 3-20 days at maturity (whisker pairing, WP), receptive field plasticity of adult D2 barrel cortex cells was compared in vivo for Tg8 mutant and normal (NOR) mice. Little plasticity was achieved until 20 days of WP in both mice. For Tg8, which lacks segregation of thalamocortical (TC) terminals into barrels, the first relay (TC) responses in layer IV to the principal whisker were potentiated more than in NOR mice by 20 days of WP. In parallel, secondary discharges were reduced more in Tg8 than NOR. It is suggested that both TC excitation and feed-forward inhibition in Tg8 are greater and potentiated more by WP than in NOR mice. Similar differences were reflected in supragranular (SG) cells. For Tg8 but not NOR mice, first latencies of one in five cells in layer IV to an adjacent surround whisker matched those of the principal whisker, increasing to one in three by 20 days of WP experience. Converse decreases occurred for the deprived surround whisker. Changes were similar but smaller for SG cells. Lack of TC segregation in Tg8, therefore, allows substantial overlapping TC terminals of immediate surround whiskers to activate neighbouring D2 column cells directly with potentiated relay to a whisker paired input and weakened relay to a deprived input. Although differing from NOR mice, experiential plasticity was not strongly compromised in Tg8 mice. Differences in WP plasticity from rat barrel cortex are discussed.