Differential effect of whisker trimming on excitatory and inhibitory transmission in primary somatosensory cortex of the adult rat in vivo

Neonatal sensory deprivation and the development of cortical function: unilateral and bilateral sensory deprivation result in different functional outcomes

The normal development of sensory perception in mammals depends on appropriate sensory experience between birth and maturity. Numerous reports have shown that trimming some or all of the large mystacial vibrissa (whiskers) on one side of the face after birth has a detrimental effect on the maturation of cortical function. The objective of the present study was to understand the differences that occur after unilateral whisker trimming compared with those that occur after bilateral deprivation. Physiological deficits produced by bilateral trimming (BD) of all whiskers for 2 mo after birth were compared with the deficits produced by unilateral trimming (UD) for the same period of time using extracellular recording under urethan anesthesia from single cells in rat barrel cortex. Fast spiking (FSUs) and regular spiking (RSUs) units were separated and their properties compared in four subregions identified by histological reconstructions of the electrode penetrations, namely: layer IV barrel and septum, and layers II/III above a barrel and above a septum. UD upregulated responses in layer IV septa and in layers II/III above septa and perturbed the timing of responses to whisker stimuli. After BD, nearly all responses were decreased, and poststimulus latencies were increased. Circuit changes are proposed as an argument for how inputs arising from the spared whiskers project to the undeprived cortex and, via commissural fibers, could upregulate septal responses after UD. Following BD, more global neural deficits create a signature difference in the outcome of UD and BD in rat barrel cortex.

Enhanced short-latency responses in the ventral posterior medial (VPM) thalamic nucleus following whisker trimming in the adult rat

This study examined the effects of whisker trimming on the functional organization of the adult somatosensory thalamus. In vivo extracellular unit recordings were made on ventral posterior medial (VPM) thalamic neurons in urethane anaesthetised adult rats. Neuronal response properties to controlled whisker deflection were recorded in untrimmed control animals and in animals where one row of whiskers had been trimmed for a median of 18 days. Trimming significantly increased short-latency responses to stimulation of the centre receptive field whisker (mean increase of 36%, p<.001). Longer latency responses to surround receptive field whiskers were unaffected. Spontaneous firing was significantly decreased in trimmed animals. A condition-test paradigm indicated that thalamic inhibition was reduced following whisker trimming, although this effect failed to reach statistical significance. These results demonstrate a capacity of the adult somatosensory thalamus to undergo functional reorganization in response to non-traumatic and innocuous whisker trimming.

Effects of sensory deprivation on columnar organization of neuronal circuits in the rat barrel cortex

We examined whether sensory deprivation during formation of the cortical circuitry influences the pattern of intracortical single-cell connections in rat barrel cortex. Excitatory postsynaptic potentials (EPSPs) from layer 2/3 (L2/3) pyramidal neurons were recorded in vitro using patch-clamp techniques. In order to evoke EPSPs, presynaptic neurons were stimulated by photolytically applied glutamate, thus generating action potentials. Synaptic connections between the stimulated and the recorded neuron were identified by the occurrence of PSPs following photostimulation. Sensory deprivation changed the pattern of projections from L4 and L2/3 neurons to L2/3 pyramidal cells. In slices of non-deprived rats 86% of the total presynaptic neurons were located in the first and only 10% in the second barrel column. Deprivation changed these values to 67% and 26%, respectively. Therefore, the probability of presynaptic cells projecting to L2/3 neurons was shifted from adjacent to more remote barrel columns. These results indicate that deprivation of sensory input influences the pattern of intracortical connections.

Switch time-point for rapid experience-dependent changes in zinc-containing circuits in the mouse barrel cortex

It has been previously demonstrated that in the mouse barrel cortex, synaptic zinc is regulated by sensory experience. In adult mice, cutting selected vibrissae produced a rapid but transient elevation of synaptic zinc in the corresponding barrels several hours later, whereas in 8 day-old animals this procedure did not affect synaptic zinc. In the present study, we wished to determine the postnatal age at which zinc-containing terminals gain the ability to respond rapidly to a restriction of sensory input. We therefore examined the effects of 1-day sensory deprivation starting at different postnatal ages. For this purpose we unilaterally trimmed all rows of vibrissae, except for row C, and we then visualized synaptic zinc in the barrel cortex 24h later. Up to postnatal day 15 such procedure had no effect on the level of synaptic zinc. However, beginning at postnatal day 16, 1-day sensory deprivation produced an increase in synaptic zinc within hollows of deprived rows of barrels as compared to non-deprived rows. These results show that during development there is a specific time-point after which zinc-containing circuits may respond rapidly to altered sensory inputs. A comparison of these findings with previous results obtained after chronic sensory deprivation suggests that a specific time window exists in development for persistent alterations in zinc-containing circuits.

Deprivation and denervation differentially affect zinc-containing circuitries in the barrel cortex of mice

In the neocortex, a population of glutamatergic synapses contains chelatable zinc that is released upon depolarization. The present study compares the effect of chronic tactile deprivation and vibrissectomy performed at different postnatal ages on the synaptic zinc distribution in the mouse barrel cortex. We found that a chronic unilateral tactile deprivation resulted in an increase of synaptic zinc in deprived barrels. Distribution and intensity of zinc staining in non-deprived barrels resembled the control situation. The increase of zinc staining was observed if chronic deprivation started in early postnatal life or in adolescent mice but not in 70-day-old animals. This suggests that a critical period exists for plasticity of zinc containing terminals in the barrel cortex. The alteration of zinc staining was localized to not only the thalamorecipient layers IV but also layer II/III, and upper layer V. Neonatal denervation of selected vibrissal rows resulted in rearrangement of synaptic zinc distribution following cytoarchitectonic alterations in the barrel field. However, no changes in the intensity of zinc staining were observed. Vibrissectomy performed after the critical period for barrel formation did not affect either the distribution or intensity of zinc staining. It appears that the integrity of vibrissa-barrel pathway is necessary to induce activity-dependent alterations in synaptic zinc.

Sleep deprivation but not a whisker trim increases nerve growth factor within barrel cortical neurons

Sleep is hypothesized to influence activity-driven changes in the brain microcircuitry. A change in the barrel cortex following the removal of the mystacial whiskers in rats is a model for synaptic plasticity. This model was combined with sleep deprivation and immunoreactivity for nerve growth factor (NGF) was determined. Sleep deprivation for 6 h after light onset significantly increased the number of NGF-immunoreactive pyramidal neurons in layer V of the barrel cortex. However, unilateral trimming of mystacial whiskers did not affect NGF immunoreactivity in the contralateral or ipsilateral barrel cortices when rats were allowed to sleep. If the rats received a unilateral whisker cut at light onset, and subsequently were deprived of sleep, increases in the NGF-immunoreactive neurons were only observed in the barrel cortex on the side that received input from the remaining intact whiskers. In contrast, NGF immunoreactivity on the side contralateral to the cut whiskers decreased in sleep-deprived animals to levels below those observed in the control animals that were allowed to sleep. These results suggest that NGF expression is influenced by the interaction of sleep, afferent input and the nature of ongoing synaptic reorganization. Further, results are consistent with the hypothesis that growth factors, such as NGF, form part of the mechanism responsible for sleep regulation and that they also form one facet of sleep-related synaptic plasticity.

Two directions of plasticity in the sensory-deprived adult cortex

Damage or deprivation of a localized region of the skin surface has been shown to induce a selective expansion of adjacent skin surface representations in the adult somatosensory cortex. Here, we use repeated optical imaging in conjunction with single unit recordings to assess the plasticity of a single whisker's functional representation in the adult rat. We observed a large-scale expansion of a single whisker's functional representation following innocuous removal of all neighboring whiskers. Surprisingly, the same manipulation can also induce a large-scale contraction of the representation if the animal is removed from its home cage and given a brief opportunity to use its whiskers for active exploration of a different environment. Both the expansion and contraction reverse upon regrowth of the deprived whiskers. Thus, allowing the animal to use its deprived receptor organ in active exploration can determine the direction of plasticity in the adult cortex.

Experience-dependent depression of vibrissae responses in adolescent rat barrel cortex

A short period of vibrissae deprivation in an adolescent (approximately 1 month old) rat can lead to depression of the cortical response to stimulation of the regrown vibrissae. In a barrel column representing the deprived vibrissa, depression is greater for neurons located close to the barrel column representing the spared vibrissa. One possible explanation is that the spared vibrissa produces heterosynaptic depression of the principal vibrissa response (Glazewski & Fox, 1996). To test this idea further, we compared the effect of depriving all vibrissae (no heterosynaptic influence at all) with depriving a single vibrissa (maximal heterosynaptic influence expected). In addition we tested the origin of the depression by recording from subcortical structures. After 7 days' deprivation and 6-8 days' regrowth, we tested the responses of barrel cortex cells, thalamic VPm neurons and trigeminal ganglion cells to stimulation of the regrown vibrissae. We found that depression was greater in cortex if a single vibrissa had been deprived than if all vibrissae had been deprived. (Average principal vibrissae responses in single vibrissae deprived animals were 36% of those in all vibrissae deprived animals for layer II/III and 41% for layer IV.) This implicates the spared vibrissae in actively down-regulating responses to the deprived vibrissae. However, some depression could also be produced in animals deprived of all vibrissae (layers II/III were 39% and layer IV 74% of control levels). These results indicate that simple withdrawal of activation has a depressive effect on responses but that depression is far greater if some active inputs remain. Neither form of deprivation had an effect on responses to principal vibrissa stimulation in the thalamus or trigeminal ganglion however, suggesting that depression originates in the cortex. Within the cortex, intracortical connections seem most affected as the greatest depression was found in layers II/III and in layer IV among cells responding at intermediate latencies (9-14 ms).

Differential effects of abnormal tactile experience on shaping representation patterns in developing and adult motor cortex

This study investigates the influence of early somatosensory experience on shaping movement representation patterns in motor cortex. Electrical microstimulation was used to map bilaterally the motor cortices of adult rats subjected to altered tactile experience by unilateral vibrissa trimming from birth (birth-trimmed group) or for comparable periods that began in adulthood (adult-trimmed group). Findings demonstrated that (1) vibrissa trimming from birth, but not when initiated in adulthood, led to a significantly smaller-sized primary motor cortex (M1) vibrissa representation in the hemisphere contralateral to the trimmed vibrissae, with no evidence for concomitant changes in size of the adjacent forelimb representation or the representation of the intact vibrissae in the opposite (ipsilateral) hemisphere; (2) in the contralateral hemispheres of the birth-trimmed group, an abnormal pattern of evoked vibrissa movement was evident in which bilateral or ipsilateral (intact) vibrissa movement predominated; (3) in both hemispheres of the birth-trimmed group, current thresholds for eliciting movement of the trimmed vibrissa were significantly lower than normal; and (4) in the adult-trimmed group, but not in the birth-trimmed group, there was a decrease bilaterally in the relative frequency of dual forelimb-vibrissa sites that form the common border between these representations. These results show that sensory experience early in life exerts a significant influence in sculpting motor representation patterns in M1. The mature motor cortex is more resistant to the type and magnitude of influence that tactile experience has on developing M1, which may indicate that such an influence is constrained by a developmentally regulated critical period.