Neuronal plasticity in the nigrostriatal system of the rat after unilateral removal of vibrissae

Developmental milestones and behavior of infant rats: The role of sensory input from whiskers

Developmental milestones are behavioral and physical skills which are considered as markers of neurodevelopment. In rodents, sensory input from whiskers plays a crucial role in development of brain functions. Development of whisker system in rats includes the early period of passive whisker touch (PN1-8) before the onset of coordinated whisker movements which underlie active sensing. Inasmuch as transitioning from passive to active sensing requires a strong sensorimotor integration, we assume that the effect of whisker deprivation during the period of passive touch is unfavorable for neurodevelopment, but deprivation after the onset of active sensing might elicit less harmful effect due to compensatory neuroplalstic changes. Here we examined the effect of complete whisker trimming (WT) in WAG/Rij rats during PN1-8 and PN9-16 (active sensing) on achieving developmental milestones (e.g., eyelid opening, walking, self-grooming, rearing activity, physical maturation of forelimbs), locomotor activity and body weight. Control groups underwent sham trimming during the same periods. WT during PN1-8 caused a delay in achieving all investigated milestones, but WT during PN9-16 delayed only self-grooming. Both WT/sham trimming during PN9-16 caused a delay in explorative behavior, but accelerated self-grooming. These changes are likely to link with the effect of manipulations during PN9-16 in previously unhandled pups, but not specifically with WT. In general, developmental milestones appeared to be an informative tool to access neurodevelopment in rat pups and might have a translational value for studying developmental disorders during early life.

Abnormalities in whisking behaviour are associated with lesions in brain stem nuclei in a mouse model of amyotrophic lateral sclerosis

The transgenic SOD1(G93A) mouse is a model of human amyotrophic lateral sclerosis (ALS) and recapitulates many of the pathological hallmarks observed in humans, including motor neuron degeneration in the brain and the spinal cord. In mice, neurodegeneration particularly impacts on the facial nuclei in the brainstem. Motor neurons innervating the whisker pad muscles originate in the facial nucleus of the brain stem, with contractions of these muscles giving rise to "whisking" one of the fastest movements performed by mammals. A longitudinal study was conducted on SOD1(G93A) mice and wild-type litter mate controls, comparing: (i) whisker movements using high-speed video recordings and automated whisker tracking, and (ii) facial nucleus degeneration using MRI. Results indicate that while whisking still occurs in SOD1(G93A) mice and is relatively resistant to neurodegeneration, there are significant disruptions to certain whisking behaviours, which correlate with facial nuclei lesions, and may be as a result of specific facial muscle degeneration. We propose that measures of mouse whisker movement could potentially be used in tandem with measures of limb dysfunction as biomarkers of disease onset and progression in ALS mice and offers a novel method for testing the efficacy of novel therapeutic compounds.

Neonatal whisker clipping alters behavior, neuronal structure and neural activity in adult rats

Early experience plays critical roles during the development of sensory systems. For example, neonatal surgical manipulations of the whiskers in rodents lead to altered neural activity and behaviors later in life. However, while surgical procedures damage the sensory pathway; it is hard to examine the impact of whisker deprivation on adult animals. To address this issue, we performed a neonatal whisker clipping (WC0-3) paradigm, a non-invasive procedure, from the day of birth (P0) to postnatal day (P) 3, and examined behavioral performances in their adult age. With fully regrown whiskers, the WC0-3 rats exhibited shorter crossable distance than controls in a gap-crossing task, suggesting a defect in their whisker-specific tactile function. In their somatosensory cortex, the layer IV spiny stellate neurons had reduced dendritic complexity and spine density. After exploration in a novel environment, the expression of an activity-dependent immediate early gene, c-fos, increased dramatically in the somatosensory cortex. However, in WC0-3 rats, the number of c-Fos positive cells was less than those in control rats, indicating a fault in transducing sensory-related neural activity between cortical layers in WC0-3 rats. Together, our results demonstrate the roles of early tactile experience on the establishment of layer-specific excitatory connection in the barrel cortex. Early sensory insufficiency would leave long-lasting functional deficits in the sensory system.

Behavioural recovery after unilateral lesion of the dopaminergic mesotelencephalic pathway: effect of repeated testing

Functional recovery following a complete unilateral lesion of the nigrostriatal pathway in adult rats was studied. We examined the effect of training on the spontaneous or induced postural bias following the lesion. Two tasks measuring lateralization were used to assess the lesion-induced postural bias: spontaneous asymmetry was evaluated in the Y-maze, whereas induced body bias was measured by hanging the rat by its tail. Recovery was assessed at three different times following the lesion. The effects of lesion in adult rats in the short, medium and long term were evaluated and compared with the effects of dopaminergic transplants. In adult lesioned rats, destruction of dopaminergic innervation of the neostriatum induced initially an ipsilateral bias as measured in the "tail hang test" and the Y-maze. Recovery of function was observed in the tail hang test as ipsilateral bias declined on repeated testing. Apart from this effect, there was a post-lesion interval effect, since the postural bias disappeared more rapidly on repeated testing in the long-term lesioned rats. This spontaneous recovery was impaired by intrastriatal dopaminergic grafts. Furthermore, no spontaneous recovery was observed in the Y-maze test. These observations show that repeated testing can influence the long-term effects of damage to the nigrostriatal dopamine system.

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.

Peripheral and cerebral asymmetries in the rat

Rats learn a novel foraging pattern better with their right-side whiskers than with their left-side whiskers. They also learn better with the left cerebral hemisphere than with the right hemisphere. Rotating an already learned maze relative to the external environment most strongly reduces right-whisker performance; starting an already learned maze at a different location most strongly reduces left-whisker performance. These results suggest that the right-periphery-left-hemisphere system accesses a map-like representation of the foraging problem, whereas the left-periphery-right-hemisphere system accesses a rote path. Thus, as in humans, functional asymmetries in rats can be elicited by both peripheral and cortical manipulation, and each hemisphere makes qualitatively distinct contributions to a complex natural behavior.

Lateralized effect of unilateral somatosensory cortex contusion on behavior and cortical reorganization

Previous studies have shown that rats recover function after unilateral somatosensory cortex lesions, possibly by transfer of information processing to other brain areas not normally involved in those functions. In the present study, adult rats underwent unilateral contusions of the somatosensory cortex with ablation of the barrel receptor field. Behavioral testing with modified beam-walking and sensory neglect tasks demonstrated persistent somatosensory deficits in rats with left contusions but no apparent deficits in right injured animals. After 2 months, the [14C]2-deoxyglucose (2-DG) method was used to show the metabolic activity produced by unilateral stimulation of the facial vibrissae. In left injured animals, cortical metabolic activity rostral and caudal to the injury site was depressed both under basal conditions and during right vibrissal stimulation. On the other hand, comparison of the pattern of [14C]2-DG uptake in the intact, right cortex revealed changes in the pattern of glucose utilization associated with left injury combined with right vibrissal stimulation. Pattern changes were quantified by measuring the area in which glucose utilization was within the highest 25% of this range (high activity area; HAA). Right vibrissal stimulation in left injured rats caused an expansion of this HAA in the intact occipital/temporal cortex. Also, in the intact somatosensory cortex of left injured rats, there was an enlarged HAA whether or not vibrissal stimulation was performed. Thus, a combination of depressed peri-injury metabolic activity and aberrant activity in remote brain areas occurs following unilateral somatosensory cortex injury. It remains to be shown whether these factors ameliorate or contribute to persistent behavioral deficits.

Unilateral stimulation or removal of rat vibrissae: analysis of nerve growth factor and tyrosine hydroxylase mRNA in the brain

Previous work has shown that unilateral manipulation of vibrissae in the rat can lead to behavioral asymmetries and to neuronal changes in the basal ganglia: in brief, vibrissae stimulation led to increases in neostriatal dopamine release, whereas unilateral removal of vibrissae led to asymmetries in striatal afferents and to bilateral changes in mesencephalic dopamine mechanisms which were related to the occurrence of behavioral asymmetries and the later recovery therefrom. In the present study, the analysis of neuronal mechanisms possibly affected by vibrissae manipulation was extended to the nerve growth factor and the expression of tyrosine hydroxylase mRNA. Unilateral stimulation or removal of the vibrissae did not lead to significant changes in tissue levels of nerve growth factor in the neostriatum, parietal cortex (including the barrel cortex) or the hippocampus. In contrast, tyrosine hydroxylase mRNA in the substantia nigra and ventral tegmental area was affected by vibrissae removal but not by stimulation, as a bilateral increase in labeling was observed on the level of individual neurons. This effect was only observed in animals tested 4 h after vibrissae removal but not after 10 days. The results are discussed with respect to the interaction of vibrissae function with the basal ganglia, the neurotransmitter dopamine and mechanism of functional recovery.

Behavioral analysis of asymmetries induced by unilateral 6-OHDA injections into the substantia nigra

The intent of this study was to perform a detailed analysis of behavioral asymmetries (turning behavior) exhibited by animals which had sustained a unilateral lesion of the substantia nigra (SN). Rats were tested for behavior asymmetries over 16 days, once before, and 7 times after 6-OHDA had been injected into one SN. On the basis of the number of narrow diameter ipsiversive half turns produced during testing, they were then assigned to one of three groups: (a) those which showed an initial asymmetry from which they recovered, (b) those which were asymmetrical throughout testing, and (c) those in which the asymmetry only emerged during the testing period. We examined several different aspects of turning behavior. Recovery from asymmetry was associated with an increase in the number of contraversive, as well as a decrease in the number of ipsiversive narrow-angle turns. The average diameter of these ipsiversive turns also increased. Animals which recovered showed a contraversive asymmetry for wide-diameter turns which increased during testing. Narrow-diameter contraversive turns decreased during testing in the two nonrecovery groups. Both nonrecovery groups showed a contraversive asymmetry for wide-diameter turns. Asymmetry was attributable to a tendency to circle and move ipsiversively in the two nonrecovery groups, whereas it was due to circling behavior in animals which showed recovery. Additionally, more cells, labeled by HRP injected into the ipsilateral caudate putamen, were found in the damaged SN of animals which recovered.

Unilateral lesion in the tuberomammillary nucleus region: behavioral asymmetries and effects of histamine precursor

The subnuclei of tuberomammillary nucleus are located in the posterior part of the hypothalamus adjacent to the basolateral surface of the mammillary bodies. The neurons of this nucleus innervate extensive parts of the brain with several transmitters, particularly with histamine. In fact, they represent the only source of histaminergic projections in the brain. The present study deals with the effects of a lesion in this region on behavior. Unilateral electrolytic direct current (DC) lesions in the tuberomammillary nucleus led to an asymmetry in thigmotactic scanning; i.e., at 11 days, but not 1 day postlesion, the rats scanned the walls of an open field more with the vibrissae contralateral to the lesion than with those of the ipsilateral side. Furthermore, they emitted more ipsiversive than contraversive wide angle turns. The behavioral asymmetries are, in general, opposite in direction to those induced by lesion of the neighboring lateral hypothalamus and substantia nigra, indicating that they are specific to the tuberomammillary region destroyed. Application of the histamine precursor histidine led to a compensation of these asymmetries, suggesting that the tuberomammillary's histaminergic efferents are functionally related to the lesion-induced behavioral effects.

Time-dependent neuroplasticity in mesostriatal projections after unilateral removal of vibrissae in the adult rat: compartment-specific effects on horseradish peroxidase transport and cell size

In adult rats the mystacial vibrissae were clipped on one side of the snout, and the influence of this sensory deprivation on crossed and uncrossed striatal afferents from the substantia nigra, ventral tegmental area, and retrorubral area was examined with the horseradish peroxidase tract tracing technique. Unilateral removal of vibrissae was found to affect crossed and uncrossed nigrostriatal projections in a time-dependent manner. One to three days after hemivibrissotomy an apparent neuronal asymmetry was found in the crossed nigrostriatal projection arising in the rostral part of the substantia nigra, with more labeled neurons in the projection to the caudate-putamen on the side of vibrissae removal. This asymmetry resulted mainly from an asymmetry in the subset of nigrostriatal neurons reported to project to the striatal matrix ("dorsal cell type"). In contrast, 4-20 days after hemivibrissotomy reversed asymmetries were found in crossed and uncrossed nigrostriatal projections, with more labeled neurons in the projections to the caudate-putamen of the hemisphere opposite to vibrissae removal (the sensory deprived hemisphere). The asymmetry in the uncrossed projection was found throughout the substantia nigra, but was also most substantial in the projection from its rostral part. The asymmetry in the crossed projection was again restricted to the rostral substantia nigra; interestingly, however, it was limited to the subset of neurons reported to terminate in the striosomes ("ventral cell type"). Evidence was also found for time-dependent changes in size of neurons of the crossed nigrostriatal projections, as well as for changes in striatal afferents from the ventral tegmental area. The time course of these apparent changes in strength of mesostriatal projections is similar to the known time course of recovery from behavioral asymmetries induced by hemivibrissotomy, which is suggestive of a functional relationship between neuronal and behavioral changes. Moreover, the finding of a differential influence of hemivibrissotomy on nigrostriatal afferents to striosomes and matrix is indicative of a functional dissociation of these two systems.

Changes in the nigrostriatal projection associated with recovery from lesion-induced behavioral asymmetry

Possible neuronanatomical changes correlated with recovery from lesion-induced behavioral asymmetry were examined. Rats, with 6-OHDA injected into the substantia nigra (SN) on one side, were either assigned to a group with a 48-hour survival period, or one with a 15-day recovery period. Control groups, without a lesion, were also included. All animals were tested for behavioral asymmetry and, at the end of the survival period, had horseradish peroxidase (HRP) deposited in the caudate-putamen (CPU) ipsilateral to the lesion (right or left CPU in the controls). Both substantia nigrae of all animals were examined for HRP-labeled cells. Animals given a 15-day recovery period had more HRP-labeled cells in the SN ipsilateral to the HRP deposition site than those given a 1-day recovery period. Also animals which showed behavioral recovery had more HRP-labeled cells in their ipsilateral SN than either those which showed no recovery or those which were not given time to recover. Thus, our results suggested that behavioral recovery from lesion-induced asymmetry was associated with an increase in HRP uptake and retrograde transport by the remaining nigrostriatal terminals. Animals which recovered showed the expected decrease in ipsiversive, and increase in contraversive turning with time. Unexpectedly, animals which did not recover exhibited an increase in ipsiversive, and a decrease in contraversive turning with time.

Vibrissa-related behavior in mice: transient effect of ablation of the barrel cortex

Knowing that the mystacial vibrissae are an important part of the tactile sensory apparatus of rodents, we investigated the role of the barrel cortex - the endstation of the pathway between whiskerpad and cerebral cortex - in mouse behavior. We tested 15 female adult mice 2 and 10 weeks after both unilateral ablation of the barrel cortex and removal of the vibrissae on the same side in order to assess acute as well as transient effects of the cortical lesion. Two kinds of behavioral tests were performed on animals permanently provided with opaque lenses: one involved a passive stimulation of the vibrissae; the other was the 'gap-crossing' test which required the animal's active use of the vibrissae. Lesioned subjects did not show a deficit during passive stimulation of the vibrissae. On the contrary, there was a deficit during the gap-crossing test 2 weeks after the ablation of the barrel cortex. The deficit partly disappeared when the subjects were tested 10 weeks later. The results show that in mice, the barrel cortex is involved in the performance of complex behavioral tasks. The recovery of function could be due to changes in strategies to solve the gap-crossing test and/or to physical changes in neuronal circuitry. In either case, the results are relevant for the interpretation of cortical transplantation models using the whisker-to-barrel pathway.

Trigeminal-basal ganglia interaction: control of sensory-motor gating and positive reinforcement

Functional interactions between the basal ganglia and the perioral area were analyzed by means of electrical brain stimulation in the rat. The first experiment showed that unilateral stimulation of the substantia nigra sensitized the contralateral perioral area for a biting reflex upon its tactile stimulation. This biting reflex consists of lip withdrawal, orienting towards and biting into the stimulus source. The same sites in the substantia nigra also produced electrical self-stimulation using bar-pressing as the operant. A positive correlation was found between threshold currents for biting and for self-stimulation. However, the current levels necessary for reinforcement were considerably higher than those to facilitate the biting reflex. In the second experiment, it was found that manipulation of the perioral area by unilateral vibrissae removal reduced the rate of electrical self-stimulation in the substantia nigra. This effect was lateralized, depended on time after vibrissae removal, and could be reversed by systemic injections of the dopamine receptor agonist apomorphine. These results, which provide evidence for a reciprocal interaction between the basal ganglia and the perioral area, are discussed with respect to mechanisms of sensory-motor gating, motivation and reinforcement.

Asymmetries in thigmotactic scanning: evidence for a role of dopaminergic mechanisms

In two experiments, the influence of spontaneous asymmetries in thigmotactic scanning was analyzed on spontaneous and drug-induced behavior in the rat. The side of the face with which an animal performed more scanning in a baseline test was defined as the dominant vibrissae side. In experiment 1, repeated testing of either spontaneous thigmotactic scanning, or scanning after apomorphine or amphetamine, yielded no evidence that rats would preferably use one side of the face for scanning, when re-exposed to the same environment. However, an asymmetry in turning was observed both under apomorphine and amphetamine, that is, turning away from the dominant vibrissae side. In experiment 2, an influence of spontaneous asymmetries in scanning was found on behavioral asymmetries induced by unilateral vibrissae removal. Only animals, in which the vibrissae of the non-dominant side had been removed, showed more scanning with the intact vibrissae side, both undrugged and after apomorphine. Turning under apomorphine was more pronounced in animals in which the vibrissae on the dominant side had been removed. These animals showed an asymmetry in turning towards the intact vibrissae side. Furthermore, in both experiments we found evidence for left/right differences in turning or scanning. The results are discussed with respect to possible endogenous substrates of asymmetry, such as within the mesostriatal dopamine system.

The basal ganglia-orofacial system: studies on neurobehavioral plasticity and sensory-motor tuning

We have employed the unilateral removal of the vibrissae as a tool to examine ensuing behavioral changes in relation to concomitant changes in the central nervous system. In this paper we review a series of studies showing that unilateral removal of the vibrissae leads to behavioral asymmetries (e.g., in thigmotactic scanning) from which rats recover over time. Time-related to these behavioral changes we found neuronal alterations in striatal afferents, that is, in uncrossed and crossed projections from the substantia nigra and the tuberomammillary nucleus. The involvement of dopaminergic mechanisms was indicated by results showing that dopaminergic agonists can induce asymmetries in thigmotactic scanning and turning; the direction of these asymmetries was also dependent on time after vibrissae removal. Furthermore, it was shown that endogenous preferential use of one vibrissae side in thigmotactic scanning interacts with the expression of spontaneous and drug-induced behavioral asymmetries exhibited after unilateral vibrissae removal. Neurochemical studies indicated that both unilateral vibrissae removal and unilateral perioral stimulation can have lateralized effects on biogenic amines in the brain. Finally, using electrical stimulation of the substantia nigra, evidence was found for a lateralized and bidirectional interaction between basal ganglia and the orofacial systems, indicating an involvement in mechanisms of motivation and particular stimulation. These results are important from several perspectives. One, they indicate functional links between the orofacial systems and the basal ganglia. Two, they raise the possibility that unilateral removal of the vibrissae can serve as a model (a) to investigate the dynamics of recovery of function after CNS insults, in general, and specifically, (b) to study neuronal plasticity in the nigrostriatal and tuberomammillary-striatal pathways, and (c) to investigate the neuropharmacology of catecholamine systems in the brain.

Effects of hemivibrissotomy in the rat: time-dependent asymmetries in turning and biogenic amines induced by apomorphine

Behavioral and neurochemical changes accompanying unilateral removal of vibrissae were investigated in the rat. Rats were tested either 4 hours or 10 days after hemivibrissotomy. A systemic injection of apomorphine (0.5 mg/kg) induced turning behavior towards the intact vibrissae side in rats tested 4 hours after hemivibrissotomy. Compared to these animals, apomorphine induced more turning towards the side of vibrissae removal and less turning towards the intact side in animals tested 10 days after vibrissae removal. This reversal is suggestive of time-dependent changes in dopamine receptor sensitivity. Analysis of biogenic amines (dopamine, norepinephrine, serotonin) in the hemispheres ipsi- and contralateral to the side of vibrissae removal revealed evidence for neurochemical changes in apomorphine- and amphetamine-treated rats. Lateralized and bilateral differences were found in the neostriatum, septum and ventral mesencephalon, which were dependent on the side and duration of hemivibrissotomy. These results are discussed with respect to the behavioral and neural analogy between hemivibrissotomy and unilateral 6-hydroxydopamine lesions of the nigrostrial system.

Plasticity in crossed and uncrossed tuberomammillary-striatal projections in relation to recovery from behavioral asymmetries induced by hemivibrissotomy

The influence of unilateral removal of the adult rat's vibrissae on crossed and uncrossed tuberomammillary-striatal projections was examined with the horseradish peroxidase tract tracing technique. Striatal afferents from the caudal magnocellular and the postmammillary caudal magnocellular subnuclei of the tuberomammillary nucleus were investigated. Between four and 20 days after clipping the vibrissae on one side, more retrogradely labeled cells were found in the crossed and uncrossed projections from both subnuclei to the caudate-putamen of the hemisphere opposite to vibrissae removal (i.e. the sensory deprived hemisphere) than in the projections to the caudate-putamen on the vibrissae-clipped side. In contrast, between one and three days after clipping the vibrissae no asymmetries in cell-labeling in these projections were found. This time-course of changes in the tuberomammillary-striatal projections is similar to the time-course of behavioral recovery reported after unilateral removal of vibrissae. Thus, the neural changes in these projections appeared at the point in time when rats recovered from behavioral asymmetries induced by vibrissae removal.

Asymmetries in crossed and uncrossed nigrostriatal projections dependent on duration of unilateral removal of vibrissae in rats

The influence of unilateral removal of vibrissae on the crossed and uncrossed nigrostriatal projections was examined with the horseradish peroxidase tract tracing technique. Hemivibrissotomy mainly affected the projections arising from the rostral part of the substantia nigra. One to three days after clipping the vibrissae, rats were found to have more labeled neurons in the crossed projection to the caudate-putamen (CPU) on the same side as vibrissae removal than in the crossed projection to the CPU opposite to vibrissae removal. A reversed asymmetry was seen in rats examined 4-20 days after vibrissae removal. These animals had more labeled cells in the crossed and uncrossed projections terminating in the CPU opposite to the shaved side, i.e. in the hemisphere deprived of vibrissal sensory input. This time-course of neural alterations is similar to that of the recovery from behavioral asymmetries seen after hemivibrissotomy. Similar time-dependent alterations in the nigrostriatal projection had been found after unilateral injection of 6-OHDA into the substantia nigra.

Apomorphine reverses direction of asymmetry in facial scanning after 10 days of unilateral vibrissae removal in rat: vibrissotomy-induced denervation supersensitivity?

Sensorimotor asymmetries of hemivibrissotomized rats were investigated in an open field. To examine possible behavioral plasticity after unilateral removal of the vibrissae, we compared two groups of rats which had experienced the sensory imbalance for different durations. One group was shaved on one side of the face daily for 10 days before behavioral testing (group 10-Days). The other group was first shaved 4 h prior to the behavioral testing and was sham-shaved for the preceding 10 days (group 4-H). On day 11, the undrugged behavior was examined, and then, one hour later, the behavior induced by apomorphine (0.75 mg/kg, s.c.) was analyzed. In the undrugged state both groups tended to scan the walls of the enclosure with the vibrissae side of the face. After injection of apomorphine, the 4-H group also scanned the walls preferentially with the vibrissae side. In contrast, the 10-Days group reversed the scanning asymmetry under the influence of this drug. That is, animals in this latter group now preferred to scan the walls with the shaved side of the face. These findings are suggestive of a plasticity in dopamine transmission subsequent to unilateral removal of vibrissae. The reversal in direction of asymmetry under apomorphine of the 10-Days group is reminiscent of the apomorphine-induced reversal in sensorimotor asymmetries found after unilateral 6-hydroxydopamine lesion of the substantia nigra, which has been attributed to dopamine receptor supersensitivity.