Immediate and simultaneous sensory reorganization at cortical and subcortical levels of the somatosensory system

Anatomical correlates of representational map reorganization induced by partial vibrissectomy in the barrel cortex of adult mice

We examined the potential for changes in cortical connectivity to accompany long-term plastic changes in functional cortical representations of mystacial vibrissae. Plasticity in the barrel cortex of young adult mice was evoked by vibrissectomy that spared row C of whiskers. We found that 2-deoxyglucose brain mapping causes a progressive expansion of cortical representation of the spared vibrissae. Two months after vibrissectomy, when the width of the cortical map of the spared row of vibrissae doubled, living cortical slices of the barrel cortex were injected with fluorescent dextrans. The injections were centered on spared, deprived and control vibrissal columns. The injections labeled three intracortical projection systems: (i) local connections from one vibrissal column to neighboring columns; (ii) long-range projections running in the septa and walls of the barrels and spanning several barrels; and (iii) very-long-range fibers running horizontally in the lower part of layer V. The local, short-range projection system was analysed following small injections into the centers of columns in layers III and IV. We found that injections into spared barrels labeled axons extending for significantly greater distances in all layers (except layer V), and labeled cell bodies situated significantly further, than after injections into deprived or control barrels. Also, the total axonal density labeled by injections into the spared barrel was higher by 70% than for the deprived or control barrels. Alterations of topographical maps in adult somatosensory cortex may occur immediately after functional denervation, but may also increase with time, as in the case of our experimental situation. Our results indicate that persistent, long-term plastic change can remodel connectivity in the barrel cortex.

Deafferentation induced changes in GAD67 and GluR2 mRNA expression in mouse somatosensory cortex

Partial vibrissectomy in adult mice induces body map plasticity in SI barrel cortex. To examine if the disturbed balance of cortical activation affects the excitatory and inhibitory neurotransmitter systems, we studied glutamic acid decarboxylase (GAD 67) and AMPA receptor subunit GluR2 mRNA expression in the barrel cortex. At varying times post-vibrissectomy, sparing row C of whiskers on one side of the snout, the brains were processed for in situ hybridization using specific [(35)S]oligonucleotides to detect the laminar localization of GAD67 and GluR2 mRNAs. Three and seven days after vibrissectomy, the expression of GAD67 was decreased in the deafferented cortex, while 30 days post-lesion, no effects were observed. At 3 days post-lesion, an ipsilateral decrease in GAD67 mRNA expression was also observed. No decreases in GluR2 transcripts were found in the deafferented cortex, but an increased expression was observed in the representation of the spared row C of whiskers 3 days after vibrissectomy. Seven and 30 days post lesion no changes in GluR2 expression were found. These data indicate that in the barrel cortex, peripheral deafferentation transiently regulates GAD67 and GluR2 expression at the transcriptional level. We suggest that this may be a manifestation of adaptive processes.

Immediate thalamic sensory plasticity depends on corticothalamic feedback

Multiple neuron ensemble recordings were obtained simultaneously from both the primary somatosensory (SI) cortex and the ventroposterior medial thalamus (VPM) before and during the combined administration of reversible inactivation of the SI cortex and a reversible subcutaneous block of peripheral trigeminal nerve fibers. This procedure was performed to quantify the contribution of descending corticofugal projections on (i) the normal organization of thalamic somatosensory receptive fields and (ii) the thalamic somatosensory plastic reorganization that immediately follows a peripheral deafferentation. Reversible inactivation of SI cortex resulted in immediate changes in receptive field properties throughout the VPM. Cortical inactivation also significantly reduced but did not completely eliminate the occurrence of VPM receptive field reorganization resulting from the reversible peripheral deafferentation. This result suggests that the thalamic plasticity that is seen immediately after a peripheral deafferentation is dependent upon both descending corticofugal projections and ascending trigeminothalamic projections.

Source of inappropriate receptive fields in cortical somatotopic maps from rats that sustained neonatal forelimb removal

Previously this laboratory demonstrated that forelimb removal at birth in rats results in the invasion of the cuneate nucleus by sciatic nerve axons and the development of cuneothalamic cells with receptive fields that include both the forelimb-stump and the hindlimb. However, unit-cluster recordings from primary somatosensory cortex (SI) of these animals revealed few sites in the forelimb-stump representation where responses to hindlimb stimulation also could be recorded. Recently we reported that hindlimb inputs to the SI forelimb-stump representation are suppressed functionally in neonatally amputated rats and that GABAergic inhibition is involved in this process. The present study was undertaken to assess the role that intracortical projections from the SI hindlimb representation may play in the functional reorganization of the SI forelimb-stump field in these animals. The SI forelimb-stump representation was mapped during gamma-aminobutyric acid (GABA)-receptor blockade, both before and after electrolytic destruction of the SI hindlimb representation. Analysis of eight amputated rats showed that 75.8% of 264 stump recording sites possessed hindlimb receptive fields before destruction of the SI hindlimb. After the lesions, significantly fewer sites (13.2% of 197) were responsive to hindlimb stimulation (P < 0.0001). Electrolytic destruction of the SI lower-jaw representation in four additional control rats with neonatal forelimb amputation did not significantly reduce the percentage of hindlimb-responsive sites in the SI stump field during GABA-receptor blockade (P = 0.98). Similar results were obtained from three manipulated rats in which the SI hindlimb representation was silenced temporarily with a local cobalt chloride injection. Analysis of response latencies to sciatic nerve stimulation in the hindlimb and forelimb-stump representations suggested that the intracortical pathway(s) mediating the hindlimb responses in the forelimb-stump field may be polysynaptic. The mean latency to sciatic nerve stimulation at responsive sites in the GABA-receptor blocked SI stump representation of neonatally amputated rats was significantly longer than that for recording sites in the hindlimb representation [26.3 +/- 8.1 (SD) ms vs. 10.8 +/- 2.4 ms, respectively, P < 0.0001]. These results suggest that hindlimb input to the SI forelimb-stump representation detected in GABA-blocked cortices of neonatally forelimb amputated rats originates primarily from the SI hindlimb representation.

Spatio-temporal subthreshold receptive fields in the vibrissa representation of rat primary somatosensory cortex

Spatio-temporal subthreshold receptive fields in the vibrissa representation of rat primary somatosensory cortex. J. Neurophysiol. 80: 2882-2892, 1998. Whole cell recordings of synaptic responses evoked by deflection of individual vibrissa were obtained from neurons within adult rat primary somatosensory cortex. To define the spatial and temporal properties of subthreshold receptive fields, the spread, amplitude, latency to onset, rise time to half peak amplitude, and the balance of excitation and inhibition of subthreshold input were quantified. The convergence of information onto single neurons was found to be extensive: inputs were consistently evoked by vibrissa one- and two-away from the vibrissa that evoked the largest response (the "primary vibrissa"). Latency to onset, rise time, and the incidence and strength of inhibitory postsynaptic potentials (IPSPs) varied as a function of position within the receptive field and the strength of evoked excitatory input. Nonprimary vibrissae evoked smaller amplitude subthreshold responses [primary vibrissa, 9.1 +/- 0.84 (SE) mV, n = 14; 1-away, 5. 1 +/- 0.5 mV, n = 38; 2-away, 3.7 +/- 0.59 mV, n = 22; 3-away, 1.3 +/- 0.70 mV, n = 8] with longer latencies (primary vibrissa, 10.8 +/- 0.80 ms; 1-away, 15.0 +/- 1.2 ms; 2-away, 15.7 +/- 2.0 ms). Rise times were significantly faster for inputs that could evoke action potential responses (suprathreshold, 4.1 +/- 1.3 ms, n = 8; subthreshold, 12.4 +/- 1.5 ms, n = 61). In a subset of cells, sensory evoked IPSPs were examined by deflecting vibrissa during injection of hyperpolarizing and depolarizing current. The strongest IPSPs were evoked by the primary vibrissa (n = 5/5), but smaller IPSPs also were evoked by nonprimary vibrissae (n = 8/13). Inhibition peaked by 10-20 ms after the onset of the fastest excitatory input to the cortex. This pattern of inhibitory activity led to a functional reversal of the center of the receptive field and to suppression of later-arriving and slower-rising nonprimary inputs. Together, these data demonstrate that subthreshold receptive fields are on average large, and the spatio-temporal dynamics of these receptive fields vary as a function of position within the receptive field and strength of excitatory input. These findings constrain models of suprathreshold receptive field generation, multivibrissa interactions, and cortical plasticity.

Potential circuit mechanisms underlying concurrent thalamic and cortical plasticity

During the last two decades, plastic reorganization of both sensory and motor representations in the adult central nervous system has been demonstrated following a large variety of manipulations, ranging from partial lesions of the sensory receptor surface to modifications in sensory experience (see /14/ for review). Yet, little is known about the neural circuit mechanisms underlying such reorganization process. Despite the difficulty in addressing this issue, recent studies have provided some insights into this fundamental question. Altogether, these studies suggest that the process of plastic reorganization is a system-wide phenomenon, involving both cortical and subcortical representations. Contrary to classical beliefs, recent work also suggests that the final outcome of the reorganization process is not necessarily beneficial, since it can lead to abnormal perceptual experiences /31/, such as the phantom limb sensation, and even pain /31,32/. In this review, we focus on recent insights into the possible circuit mechanisms underlying sensory plasticity and discuss the potential implications of these findings. We then present physiological evidence supporting the view that the process of plasticity observed at the cortical level may reflect simultaneous changes in many subcortical structures.