Exp Brain Res (2001) 137:180–189 DOI 10.1007/s002210000652
R E S E A R C H A RT I C L E
Gianfranco Franchi
Persistence of vibrissal motor representation following vibrissal pad deafferentation in adult rats
Received: 21 April 2000 / Accepted: 6 November 2000 / Published online: 16 February 2001 © Springer-Verlag 2001
Abstract The effect of sensory vibrissal pad denervation on M1 organization was studied in adult rats 2 weeks after the infraorbital nerve was severed. Cortical motor output organization was assessed mapping the representation size and thresholds of vibrissa movements evoked by intracortical electrical microstimulation (ICMS). Motor cortex output patterns of control and sham groups of rats were compared with those of rats that had received unilateral or bilateral infraorbital nerve lesions. The mean size of the vibrissa representation in both unilateral and bilateral input-deprived hemispheres was not significantly different from those in control and sham hemispheres. The mean threshold required to evoke vibrissa movements was significantly higher in both groups of deafferented hemispheres than in control and sham groups of hemispheres. In contrast, the mean threshold required to evoke other types of movements from both groups of input-deprived hemispheres were similar to those found in the control and sham groups of hemispheres. These results indicate that input-deprived vibrissal motor representation reflects lower-than-normal excitability, although the size and topographic relationship with neighboring representations are normal. Keywords Motor cortex · Trigeminal deafferentation · Cortical reorganization · Plasticity
Introduction Neurophysiological studies show that, after peripheral nerve lesion, input-deprived sensory cortical areas and output-deprived motor cortical areas are occupied by neighboring representational fields (for review, see Sanes and Donoghue 1997; Kaas 1999). Sensory input to G. Franchi (✉) Dipartimento di Scienze Biomediche e Terapie Avanzate, Sezione di Fisiologia umana, Università di Ferrara, 44100 Ferrara, Italy e-mail:
[email protected] Tel.: +39-532-291236, Fax: +39-532-291242
the sensory cortex is essential for the maintenance of sensory maps as motor output is essential for the maintenance of motor maps. In the motor cortex of adult animals, the organization of movement representation can be largely modified by manipulations of peripheral motor nerves (Sanes et al. 1988; Donoghue et al. 1990). In adult rats, M1 output reorganization stabilizes during the second week following contralateral facial nerve lesion. After this period, forelimb and eye representations firmly occupy neighboring vibrissa representations (Sanes et al. 1990; Toldi et al. 1996). Following the reinnervation of the facial nerve, a re-representation of contralateral vibrissae movement reorganizes in a shrunken region localized in the medial part of the former vibrissa representation. The remaining portion of the vibrissa representation was firmly occupied by forelimb and eye representations, showing no signs of the reversal of the expansion displayed before facial nerve reinnervation (Franchi 2000). In contrast, the role of sensory input in maintaining representational maps in the motor cortex of adult animals is still unclear. In the rat, thalamic nuclei and somatosensory cortical areas relay peripheral sensory input to the M1 (Miyashita et al. 1994; Izraeli et al, 1995; Porter 1996), thus affecting the physiological properties of the MI neurons (Sievert et al. 1986; Kaneko et al. 1994). In adult rats, it has been shown that sensory feedback from the forelimb plays a role in short-term reshaping of motor cortical output. A sustained increase in the degree of input effectiveness to MI, caused by stretching forelimb muscles, expands the forelimb representation into the vibrissal representation (Sanes et al. 1992). In neonatal rats it has been shown that vibrissal sensory input plays a significant role in shaping representational maps in the motor cortex (Keller et al 1996; Huntley 1997b). Vibrissa trimming from birth induces shrinkage of the vibrissa movement representation that persists after trimming has ceased. In adult rats, the motor cortex was resistant to the effects of unilateral vibrissa trimming (Huntley 1997b) or the effects of bilateral vibrissa trimming reversed 5 days after clipping ceased (Keller et al 1996).
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The reversion of the shrinkage in the whisker movement representation presumably reflects the post-trimming resumption of normal whisking behavior. The present study was undertaken to define the role sensory input plays in maintaining representational maps in the motor cortex of adult animals. To this purpose, change in the size and excitability of motor representation was examined after persistent ablation of peripheral sensory input. Since most peripheral nerve manipulations involve damage to both sensory and motor axons, the decision was made to test the effect lesioning of a purely sensory nerve would have on MI output organization. In these experiments, the infraorbital nerve (ION) – the nerve supplying sensory innervation to vibrissal and muzzle skin – was used because it is anatomically separate from the vibrissal motor innervation and because the mystacial vibrissae have a relatively large representation in the rat MI cortex (Donoghue and Wise 1982; Neafsey et al. 1986). In order to study how tonic vibrissa input ablation affects MI organization, the effect of vibrissal pad deafferentation on MI excitability and representation patterns was investigated 2 weeks after infraorbital nerve lesion. Previous study evidenced that 2 weeks of recovery provides ample time for short-term modulatory influences on motor cortex reorganization (Sanes et al. 1988, 1990; Donoghue et al. 1990; Toldi et al. 1996) so that after this period detectable changes can be ascribed to long-term modulatory influences of somatic sensory input on motor cortical output. Since a large percentage of neurons in the somatosensory cortex receive bilateral vibrissal input (Armstrong-James and George 1988) the effect of unilateral and bilateral vibrissal deafferentation was tested in two groups of animals. These results were compared with those obtained in control and sham animals.
Materials and methods Experiments were carried out on 28 male Albino Wistar rats, weighing 300–350 g, divided into four groups of seven animals. In two groups of experimental rats, the infraorbital nerve (ION) was severed at the point where it exits from the infraorbital foramen: the surgery was unilateral in one group (Unilateral deafferented group), and bilateral in the other (Bilateral deafferented group). One of the two remaining groups was left untouched (Control group) and the last group underwent surgery on one side: the infraorbital nerve was isolated from surrounding tissues but left intact (Sham group). Infraorbital nerve surgery and follow-up All surgical procedures were performed under ketamine anesthesia (100 mg/kg IP and then supplemental dose IM as needed). Using aseptic procedures, under the operating microscope, the infraorbital nerve was exposed, separated from its adjacent tissues and legated; then it was cut distally to eliminate all remaining fine branches. The proximal stump was dried and covered with acrylic tissue adhesive (Histoacryl) to prevent the proximal axons from sprouting. The skin was closed with 6-0 sutures and the wound was cleansed with an antibiotic solution. In the post-operative period unilaterally and bilaterally injured animals, like the controls and shame groups, displayed bilateral rhythmical vibrissa move-
ments during natural whisking as they explored freely in their cages. During natural whisking the deafferented vibrissa did not suddenly retract when it hit against targets, as would normally be the case. After deafferentation the vibrissal pad proved unreactive to light pain-inducing sensory stimuli (i.e. light touch, squeeze or piercing). The loss of vibrissal pad sensitivity following deafferentation was clearly evidenced in all animals for the entire survival period. Intracortical stimulation mapping In each animal ICMS-evoked movements in the frontal agranular cortex were mapped. In the rats that underwent surgery, cortical mapping was carried out after the surgical wound had healed (14–18 days after surgery). Mapping was aimed at defining the extent of the vibrissa representation and the current threshold required to elicit vibrissa movement in the hemisphere contralateral to the side that underwent surgery. The mapping procedure was similar to the one described by Donoghue and Wise (1982) and Sanes et al. (1990), and detailed elsewhere (Franchi 2000). Briefly, the animals were placed in a Kopf stereotaxic apparatus and the frontal cortex of one side was exposed by a large craniotomy. The dura remained intact, and was kept moist with a 0.9% saline solution. The electrode penetrations were regularly spaced out over a 500 µm grid. Alteration in the coordinate grid, up to 200 µm, was sometimes necessary to prevent the electrode from penetrating the surface blood vessels. These adjustments in the coordinate grid were not reported in the reconstructing maps and were taken into account for quantitative map construction only when greater than 50 µm. Glass insulated tungsten electrodes (0.6–1 MΩ impedance at 1 kHz) were used for stimulation. The electrode was lowered vertically to 1.5 mm below the cortical surface and adjusted to ±200 µm so as to evoke movement at the lowest threshold. In a previous experiment this depth was found to correspond to layer V of the frontal agranular cortex (Franchi 2000). Cathodal monophasic pulses (30 ms train duration at 300 Hz, 200 µs pulse duration) of a maximum of 60 µA were passed through the electrode with a minimum interval of 2.5 s. Starting with a current of 60 µA, intensity was decreased in 5 µA steps until the movement was no longer evoked; then intensity was increased to a level at which nearly 50% of the stimulations elicited movement. This level defined the current threshold. If no movements or twitches were evoked with 60 µA, the site was recorded as negative. When movement was observed in two different body parts or bilateral whiskers, current thresholds were determined for each component. In general, at threshold current levels, only one movement was elicited from any given point. Body parts activated by microstimulation were identified by visual inspection and/or muscle palpation. During the experiment, when at rest, forelimbs and hindlimbs were approximately half-way between flexion and extension, and were alternately flexed and extended, particularly when defining representational borders. Electrophysiologic facial nerve testing At the conclusion of the mapping session, the compound muscle action potential (MAP) of vibrissa muscles was recorded (Fig. 1A). The recording and stimulating procedure was similar to the one described by Archibald et al. (1991) and detailed elsewhere (Franchi 2000). All MAPs were obtained by supramaximal nerve stimulation, and recorded by computer data acquisition for off-line analysis (Spike2 for Windows). MAP latency was calculated using cursors in 20 responses for each recording side. The mean MAP latency in control animals was similar to those found in animals that had undergone surgery (injured: 1.5±0.09 ms versus control: 1.6±0.23 ms; P>0.1; t-test, P0.2; t-test level of significance, P
