A chronic unit study of the sensory properties of neurons in the forelimb areas of rat sensorimotor cortex

Synaptic and Genetic Bases of Impaired Motor Learning Associated with Modified Experience-Dependent Cortical Plasticity in Heterozygous Reeler Mutants

Patients with neurodevelopmental disorders show impaired motor skill learning. It is unclear how the effect of genetic variation on synaptic function and transcriptome profile may underlie experience-dependent cortical plasticity, which supports the development of fine motor skills. RELN (reelin) is one of the genes implicated in neurodevelopmental psychiatric vulnerability. Heterozygous reeler mutant (HRM) mice displayed impairments in reach-to-grasp learning, accompanied by less extensive cortical map reorganization compared with wild-type mice, examined after 10 days of training by intracortical microstimulation. Assessed by patch-clamp recordings after 3 days of training, the training induced synaptic potentiation and increased glutamatergic-transmission of cortical layer III pyramidal neurons in wild-type mice. In contrast, the basal excitatory and inhibitory synaptic functions were depressed, affected both by presynaptic and postsynaptic impairments in HRM mice; and thus, no further training-induced synaptic plasticity occurred. HRM exhibited downregulations of cortical synaptophysin, immediate-early gene expressions, and gene enrichment, in response to 3 days of training compared with trained wild-type mice, shown using quantitative reverse transcription polymerase chain reaction, immunohistochemisty, and RNA-sequencing. We demonstrated that motor learning impairments associated with modified experience-dependent cortical plasticity are at least partially attributed by the basal synaptic alternation as well as the aberrant early experience-induced gene enrichment in HRM.

Exercise modulates neuronal activation in the micturition circuit of chronically stressed rats: A multidisciplinary approach to the study of urologic chronic pelvic pain syndrome (MAPP) research network study

BACKGROUND

Rats exposed to water avoidance stress (WAS) show increased urinary frequency, increased somatosensory nociceptive reflex responses, as well as altered brain responses to bladder distension, analogous to similar observations made in patients with urologic chronic pelvic pain syndrome (UCPPS). Exercise has been proposed as a potential treatment option for patients with chronic urinary frequency and urgency. We examined the effects of exercise on urinary voiding parameters and functional brain activation during bladder distension in rats exposed to WAS.

METHODS

Adult, female Wistar Kyoto rats were exposed to 10 days of WAS and thereafter randomized to either voluntary exercise for 3 weeks or sedentary groups. Voiding parameters were assessed at baseline, post-WAS, and weekly for 3 weeks. Thereafter, cerebral blood flow (CBF) mapping was performed during isotonic bladder distension (20 cm H2O) after intravenous bolus injection of [14C]-iodoantipyrine. Regional CBF was quantified in autoradiographs of brain slices and analyzed in 3-D reconstructed brains by statistical parametric mapping. Functional connectivity was examined between regions of the micturition circuit through interregional correlation analysis.

RESULTS

WAS exposure in sedentary animals (WAS/no-EX) increased voiding frequency and decreased urinary volumes per void. Exercise exposure in WAS animals (WAS/EX) resulted in a progressive decline in voiding frequency back to the baseline, as well as increased urinary volumes per void. Within the micturition circuit, WAS/EX compared to WAS/no-EX demonstrated a significantly lower rCBF response to passive bladder distension in Barrington's nucleus that is part of the spinobulbospinal voiding reflex, as well as in the periaqueductal gray (PAG) which modulates this reflex. Greater rCBF was noted in WAS/EX animals broadly across corticolimbic structures, including the cingulate, medial prefrontal cortex (prelimbic, infralimbic areas), insula, amygdala, and hypothalamus, which provide a 'top-down' decision point where micturition could be inhibited or triggered. WAS/EX showed a significantly greater positive brain functional connectivities compared to WAS/no-EX animals within regions of the extended reflex loop (PAG, Barrington's nucleus, intermediodorsal thalamic nucleus, pons), as well as within regions of the corticolimbic decision-making loop of the micturition circuit, with a strikingly negative correlation between these pathways. Urinary frequency was positively correlated with rCBF in the pons, and negatively correlated with rCBF in the cingulate cortex.

CONCLUSION

Our results suggest that chronic voluntary exercise may decrease urinary frequency at two points of control in the micturition circuit. During the urine storage phase, it may diminish the influence of the reflex micturition circuit itself, and/or it may increase corticolimbic control of voiding. Exercise may be an effective adjunct therapeutic intervention for modifying the urinary symptoms in patients with UCPPS.

Chronic stability of single-channel neurophysiological correlates of gross and fine reaching movements in the rat

While substantial task-related neural activity has been observed during motor tasks in rodent primary motor cortex and premotor cortex, the long-term stability of these responses in healthy rats is uncertain, limiting the interpretability of longitudinal changes in the specific patterns of neural activity associated with learning or motor recovery following injury. This study examined the stability of task-related neural activity associated with execution of two distinct reaching tasks in healthy rodents. A novel automated rodent behavioral apparatus was constructed and rats were trained to perform a reaching task combining a ‘gross’ lever press and a ‘fine’ pellet retrieval. In each animal, two chronic microelectrode arrays were implanted in motor cortex spanning the caudal forelimb area (rodent primary motor cortex) and the rostral forelimb area (rodent premotor cortex). We recorded multiunit spiking and local field potential activity from 10 days to 7–10 weeks post-implantation to characterize the patterns of neural activity observed during each task component and analyzed the consistency of channel-specific task-related neural activity. Task-related changes in neural activity were observed on the majority of channels. While the task-related changes in multi-unit spiking and local field potential spectral power were consistent over several weeks, spectral power changes were more stable, despite the trade-off of decreased spatial and temporal resolution. These results show that neural activity in rodent primary and premotor cortex is associated with specific phases of reaching movements with stable patterns of task-related activity across time, establishing the relevance of the rodent for future studies designed to examine changes in task-related neural activity during recovery from focal cortical lesions.

Cortical Reorganization of Sensorimotor Systems and the Role of Intracortical Circuits After Spinal Cord Injury

The plasticity of sensorimotor systems in mammals underlies the capacity for motor learning as well as the ability to relearn following injury. Spinal cord injury, which both deprives afferent input and interrupts efferent output, results in a disruption of cortical somatotopy. While changes in corticospinal axons proximal to the lesion are proposed to support the reorganization of cortical motor maps after spinal cord injury, intracortical horizontal connections are also likely to be critical substrates for rehabilitation-mediated recovery. Intrinsic connections have been shown to dictate the reorganization of cortical maps that occurs in response to skilled motor learning as well as after peripheral injury. Cortical networks incorporate changes in motor and sensory circuits at subcortical or spinal levels to induce map remodeling in the neocortex. This review focuses on the reorganization of cortical networks observed after injury and posits a role of intracortical circuits in recovery.

Effects of water avoidance stress on peripheral and central responses during bladder filling in the rat: A multidisciplinary approach to the study of urologic chronic pelvic pain syndrome (MAPP) research network study

Stress plays a role in the exacerbation and possibly the development of functional lower urinary tract disorders. Chronic water avoidance stress (WAS) in rodents is a model with high construct and face validity to bladder hypersensitive syndromes, such as interstitial cystitis/bladder pain syndrome (IC/BPS), characterized by urinary frequency and bladder hyperalgesia and heightened stress responsiveness. Given the overlap of the brain circuits involved in stress, anxiety, and micturition, we evaluated the effects chronic stress has on bladder function, as well as its effects on regional brain activation during bladder filling. Female Wistar-Kyoto rats were exposed to WAS (10 days) or sham paradigms. One day thereafter, cystometrograms were obtained during titrated bladder dilation, with visceromotor responses (VMR) recorded simultaneously. Cerebral perfusion was assessed during passive bladder distension (20-cmH₂O) following intravenous administration of [¹⁴C]-iodoantipyrine. Regional cerebral blood flow was quantified by autoradiography and analyzed in 3-dimensionally reconstructed brains with statistical parametric mapping. WAS animals compared to controls demonstrated a decreased pressure threshold and visceromotor threshold triggering the voiding phase. At 20-cmH₂O, VMR was significantly greater in WAS animals compared to controls. WAS animals showed greater activation in cortical regions of the central micturition circuit, including the posterior cingulate, anterior retrosplenial, somatosensory, posterior insula, orbital, and anterior secondary (“supplementary”) motor cortices, as well as in the thalamus, anterior hypothalamus, parabrachial and Barrington nuclei, and striatum. Seed analysis showed increased functional connectivity of WAS compared to control animals of the posterior cingulate cortex to the pontine parabrachial nucleus; of the Barrington nucleus to the anterior dorsal midline and ventrobasilar thalamus and somatosensory and retrosplenial cortices; and of the posterior insula to anterior secondary motor cortex. Our findings show a visceral hypersensitivity during bladder filling in WAS animals, as well as increased engagement of portions of the micturition circuit responsive to urgency, viscerosensory perception and its relay to motor regions coordinating imminent bladder contraction. Results are consistent with recent findings in patients with interstitial cystitis, suggesting that WAS may serve as an animal model to elucidate the mechanisms leading to viscerosensitive brain phenotypes in humans with IC/BPS.

High-order motor cortex in rats receives somatosensory inputs from the primary motor cortex via cortico-cortical pathways

The motor cortex of rats contains two forelimb motor areas; the caudal forelimb area (CFA) and the rostral forelimb area (RFA). Although the RFA is thought to correspond to the premotor and/or supplementary motor cortices of primates, which are higher-order motor areas that receive somatosensory inputs, it is unknown whether the RFA of rats receives somatosensory inputs in the same manner. To investigate this issue, voltage-sensitive dye (VSD) imaging was used to assess the motor cortex in rats following a brief electrical stimulation of the forelimb. This procedure was followed by intracortical microstimulation (ICMS) mapping to identify the motor representations in the imaged cortex. The combined use of VSD imaging and ICMS revealed that both the CFA and RFA received excitatory synaptic inputs after forelimb stimulation. Further evaluation of the sensory input pathway to the RFA revealed that the forelimb-evoked RFA response was abolished either by the pharmacological inactivation of the CFA or a cortical transection between the CFA and RFA. These results suggest that forelimb-related sensory inputs would be transmitted to the RFA from the CFA via the cortico-cortical pathway. Thus, the present findings imply that sensory information processed in the RFA may be used for the generation of coordinated forelimb movements, which would be similar to the function of the higher-order motor cortex in primates.

Hypergravity within a critical period impacts on the maturation of somatosensory cortical maps and their potential for use-dependent plasticity in the adult

We investigated experience-dependent plasticity of somatosensory maps in rat S1 cortex during early development. We analyzed both short- and long-term effects of exposure to 2G hypergravity (HG) during the first 3 postnatal weeks on forepaw representations. We also examined the potential of adult somatosensory maps for experience-dependent plasticity after early HG rearing. At postnatal day 22, HG was found to induce an enlargement of cortical zones driven by nail displacements and a contraction of skin sectors of the forepaw map. In these remaining zones serving the skin, neurons displayed expanded glabrous skin receptive fields (RFs). HG also induced a bias in the directional sensitivity of neuronal responses to nail displacement. HG-induced map changes were still found after 16 wk of housing in normogravity (NG). However, the glabrous skin RFs recorded in HG rats decreased to values similar to that of NG rats, as early as the end of the first week of housing in NG. Moreover, the expansion of the glabrous skin area and decrease in RF size normally induced in adults by an enriched environment (EE) did not occur in the HG rats, even after 16 wk of EE housing in NG. Our findings reveal that early postnatal experience critically and durably shapes S1 forepaw maps and limits their potential to be modified by novel experience in adulthood.

Cortical reorganization after spinal cord injury: always for good?

Plasticity constitutes the basis of behavioral changes as a result of experience. It refers to neural network shaping and re-shaping at the global level and to synaptic contacts remodeling at the local level, either during learning or memory encoding, or as a result of acute or chronic pathological conditions. 'Plastic' brain reorganization after central nervous system lesions has a pivotal role in the recovery and rehabilitation of sensory and motor dysfunction, but can also be "maladaptive". Moreover, it is clear that brain reorganization is not a "static" phenomenon but rather a very dynamic process. Spinal cord injury immediately initiates a change in brain state and starts cortical reorganization. In the long term, the impact of injury - with or without accompanying therapy - on the brain is a complex balance between supraspinal reorganization and spinal recovery. The degree of cortical reorganization after spinal cord injury is highly variable, and can range from no reorganization (i.e. "silencing") to massive cortical remapping. This variability critically depends on the species, the age of the animal when the injury occurs, the time after the injury has occurred, and the behavioral activity and possible therapy regimes after the injury. We will briefly discuss these dependencies, trying to highlight their translational value. Overall, it is not only necessary to better understand how the brain can reorganize after injury with or without therapy, it is also necessary to clarify when and why brain reorganization can be either "good" or "bad" in terms of its clinical consequences. This information is critical in order to develop and optimize cost-effective therapies to maximize functional recovery while minimizing maladaptive states after spinal cord injury.

Different modulation of common motor information in rat primary and secondary motor cortices

Rodents have primary and secondary motor cortices that are involved in the execution of voluntary movements via their direct and parallel projections to the spinal cord. However, it is unclear whether the rodent secondary motor cortex has any motor function distinct from the primary motor cortex to properly control voluntary movements. In the present study, we quantitatively examined neuronal activity in the caudal forelimb area (CFA) of the primary motor cortex and rostral forelimb area (RFA) of the secondary motor cortex in head-fixed rats performing forelimb movements (pushing, holding, and pulling a lever). We found virtually no major differences between CFA and RFA neurons, regardless of neuron subtypes, not only in their basal spiking properties but also in the time-course, amplitude, and direction preference of their functional activation for simple forelimb movements. However, the RFA neurons, as compared with the CFA neurons, showed obviously a greater susceptibility of their functional activation to an alteration in a behavioral situation, a 'rewarding' response that leads to reward or a 'consummatory' response that follows reward water, which might be accompanied by some internal adaptations without affecting the motor outputs. Our results suggest that, although the CFA and RFA neurons commonly process fundamental motor information to properly control forelimb movements, the RFA neurons may be functionally differentiated to integrate motor information with internal state information for an adaptation to goal-directed behaviors.

Functional reorganization of motor and limbic circuits after exercise training in a rat model of bilateral parkinsonism

Exercise training is widely used for neurorehabilitation of Parkinson's disease (PD). However, little is known about the functional reorganization of the injured brain after long-term aerobic exercise. We examined the effects of 4 weeks of forced running wheel exercise in a rat model of dopaminergic deafferentation (bilateral, dorsal striatal 6-hydroxydopamine lesions). One week after training, cerebral perfusion was mapped during treadmill walking or at rest using [(14)C]-iodoantipyrine autoradiography. Regional cerebral blood flow-related tissue radioactivity (rCBF) was analyzed in three-dimensionally reconstructed brains by statistical parametric mapping. In non-exercised rats, lesions resulted in persistent motor deficits. Compared to sham-lesioned rats, lesioned rats showed altered functional brain activation during walking, including: 1. hypoactivation of the striatum and motor cortex; 2. hyperactivation of non-lesioned areas in the basal ganglia-thalamocortical circuit; 3. functional recruitment of the red nucleus, superior colliculus and somatosensory cortex; 4. hyperactivation of the ventrolateral thalamus, cerebellar vermis and deep nuclei, suggesting recruitment of the cerebellar-thalamocortical circuit; 5. hyperactivation of limbic areas (amygdala, hippocampus, ventral striatum, septum, raphe, insula). These findings show remarkable similarities to imaging findings reported in PD patients. Exercise progressively improved motor deficits in lesioned rats, while increasing activation in dorsal striatum and rostral secondary motor cortex, attenuating a hyperemia of the zona incerta and eliciting a functional reorganization of regions participating in the cerebellar-thalamocortical circuit. Both lesions and exercise increased activation in mesolimbic areas (amygdala, hippocampus, ventral striatum, laterodorsal tegmental n., ventral pallidum), as well as in related paralimbic regions (septum, raphe, insula). Exercise, but not lesioning, resulted in decreases in rCBF in the medial prefrontal cortex (cingulate, prelimbic, infralimbic). Our results in this PD rat model uniquely highlight the breadth of functional reorganizations in motor and limbic circuits following lesion and long-term, aerobic exercise, and provide a framework for understanding the neural substrates underlying exercise-based neurorehabilitation.

Serotonergic pharmacotherapy promotes cortical reorganization after spinal cord injury

Cortical reorganization plays a significant role in recovery of function after injury of the central nervous system. The neural mechanisms that underlie this reorganization may be the same as those normally responsible for skilled behaviors that accompany extended sensory experience and, if better understood, could provide a basis for further promoting recovery of function after injury. The work presented here extends studies of spontaneous cortical reorganization after spinal cord injury to the role of rehabilitative strategies on cortical reorganization. We use a complete spinal transection model to focus on cortical reorganization in response to serotonergic (5-HT) pharmacotherapy without any confounding effects from spared fibers left after partial lesions. 5-HT pharmacotherapy has previously been shown to improve behavioral outcome after SCI but the effect on cortical organization is unknown. After a complete spinal transection in the adult rat, 5-HT pharmacotherapy produced more reorganization in the sensorimotor cortex than would be expected by transection alone. This reorganization was dose dependent, extended into intact (forelimb) motor cortex, and, at least in the hindlimb sensorimotor cortex, followed a somatotopic arrangement. Animals with the greatest behavioral outcome showed the greatest extent of cortical reorganization suggesting that the reorganization is likely to be in response to both direct effects of 5-HT on cortical circuits and indirect effects in response to the behavioral improvement below the level of the lesion.

Reversible Inactivation of Rat Premotor Cortex Impairs Temporal Preparation, but not Inhibitory Control, During Simple Reaction-Time Performance

Previous studies by our lab and others have established a role for medial areas of the prefrontal cortex (mPFC) in the top-down control of action during simple reaction-time (RT) tasks. However, the neural circuits that allow mPFC to influence activity in the motor system have remained unclear. In the present study, we used a combination of tract-tracing and reversible inactivation methods to examine the role of a motor-related area in the rat frontal cortex, called the rostral forelimb area (RFA), in the top-down control of action. Neural tracing studies involved used electrical microstimulation to identify RFA and injections of biotinylated dextran amines (BDA) to map out connections of RFA with other parts of the frontal cortex. Connections were found between RFA and mPFC, the agranular insular cortex, and the primary motor cortex. Reversible inactivations using muscimol infusions into RFA increased response times and eliminated delay-dependent speeding, but did not increase premature responding. These results are markedly different from what is obtained when muscimol is infused into mPFC, which leads to excessive premature responding and a reduction of RTs to stimuli at short delays (Narayanan et al., 2006). We also tested animals during the RT task after inactivating the agranular insular cortex, which contains neurons that projects to and receives from RFA and mPFC, and found no effects on RT performance. Together, these studies suggest that RFA is a premotor region in the rat frontal cortex that competes with mPFC to control action selection. We suggest that RFA controls the threshold that is used to initiate responding and generates prepotent excitation over responding that is crucial for temporal preparation.

Corticofugal direct projections to primary afferent neurons in the trigeminal mesencephalic nucleus of rats

Little is known about projections from the cerebral cortex to the trigeminal mesencephalic nucleus (Vmes) which contains the cell bodies of primary sensory afferents innervating masticatory muscle spindles and periodontal ligaments of the teeth. To address this issue, we employed retrograde (Fluorogold, FG) and anterograde (biotinylated dextranamine, BDA) tracing techniques in the rat. After injections of FG into the Vmes, a large number of neurons were retrogradely labeled in the prefrontal cortex including the medial agranular cortex, anterior cingulate cortex, prelimbic cortex, infralimbic cortex, deep peduncular cortex and insular cortex; the labeling was bilateral, but with an ipsilateral predominance to the injection site. Almost no FG-labeled neurons were found in the somatic sensorimotor cortex. After BDA injections into the prefrontal cortex, anterogradely labeled axon fibers and boutons were distributed bilaterally in a topographic pattern within the Vmes, but with an ipsilateral predominance to the injection site. The rostral Vmes received more preferential projections from the medial agranular cortex, while the deep peduncular cortex and insular cortex projected more preferentially to the caudal Vmes. Several BDA-labeled axonal boutons made close associations (possible synaptic contacts) with the cell bodies of Vmes neurons. The present results have revealed the direct projections from the prefrontal cortex to the primary sensory neurons in the Vmes and their unique features, suggesting that deep sensory inputs conveyed by the Vmes neurons from masticatory muscle spindles and periodontal ligaments are regulated with specific biological significance in terms of the descending control by the cerebral cortex.

Rewiring of hindlimb corticospinal neurons after spinal cord injury

Little is known about the functional role of axotomized cortical neurons that survive spinal cord injury. Large thoracic spinal cord injuries in adult rats result in impairments of hindlimb function. Using retrograde tracers, we found that axotomized corticospinal axons from the hindlimb sensorimotor cortex sprouted in the cervical spinal cord. Mapping of these neurons revealed the emergence of a new forelimb corticospinal projection from the rostral part of the former hindlimb cortex. Voltage-sensitive dye (VSD) imaging and blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) revealed a stable expansion of the forelimb sensory map, covering in particular the former hindlimb cortex containing the rewired neurons. Therefore, axotomized hindlimb corticospinal neurons can be incorporated into the sensorimotor circuits of the unaffected forelimb.

Automated light-based mapping of motor cortex by photoactivation of channelrhodopsin-2 transgenic mice

Traditionally, mapping the motor cortex requires electrodes to stimulate the brain and define motor output pathways. Although effective, electrode-based methods are labor-intensive, potentially damaging to the cortex and can have off-target effects. As an alternative method of motor mapping, we photostimulated transgenic mice expressing the light-sensitive ion channel channelrhodopsin-2 in predominantly layer-5 output cortical neurons. We report that optical stimulation of these neurons in vivo using a stage scanning laser system resulted in muscle excitation within 10-20 ms, which can be recorded using implanted electromyogram electrodes or by a noninvasive motion sensor. This approach allowed us to make highly reproducible automated maps of the mouse forelimb and hindlimb motor cortex much faster than with previous methods. We anticipate that the approach will facilitate the study of changes in the location and properties of motor maps after skilled training or damage to the nervous system.

Suppression of activity in the forelimb motor cortex temporarily enlarges forelimb representation in the homotopic cortex in adult rats

After forelimb motor cortex (FMC) damage, the unaffected homotopic motor cortex showed plastic changes. The present experiments were designed to clarify the electrophysiological nature of these interhemispheric effects. To this end, the output reorganization of the FMC was investigated after homotopic area activity was suppressed in adult rats. FMC output was compared after lidocaine-induced inactivation (L-group) or quinolinic acid-induced lesion (Q-group) of the contralateral homotopic cortex. In the Q-group of animals, FMC mapping was performed, respectively, 3 days (Q3D group) and 2 weeks (Q2W group) after cortical lesion. In each animal, FMC output was assessed by mapping movements induced by intracortical microstimulation (ICMS) in both hemispheres (hemisphere ipsilateral and contralateral to injections). The findings demonstrated that in the L-group, the size of forelimb representation was 42.2% higher than in the control group (P < 0.0001). The percentage of dual forelimb-vibrissa movement sites significantly increased over the controls (P < 0.0005). The dual-movement sites occupied a strip of the map along the rostrocaudal border between the forelimb and vibrissa representations. This form of interhemispheric diaschisis had completely reversed, with the recovery of the baseline map, 3 days after the lesion in the contralateral FMC. This restored forelimb map showed no ICMS-induced changes 2 weeks after the lesion in the contralateral FMC. The present results suggest that the FMCs in the two hemispheres interact continuously through predominantly inhibitory influences that preserve the forelimb representation and the border vs. vibrissa representation.

Thin-film epidural microelectrode arrays for somatosensory and motor cortex mapping in rat

Assessments of somatosensory and motor cortical somatotopy in vivo can provide important information on sensorimotor physiology. Here, novel polyimide-based thin-film microelectrode arrays (72 contacts) implanted epidurally, were used for recording of somatosensory evoked potentials (SEPs) and somatosensory cortex somatotopic maps of the rat. The objective was to evaluate this method with respect to precision and reliability. SEPs and somatosensory maps were measured twice within one session and again after 8 days of rest. Additionally, motor cortex maps were acquired once to assess the spatial relationship between somatosensory and motor representations of fore- and hindlimb within one individual. Somatosensory maps were well reproduced within and between sessions. SEP amplitudes and latencies were highly reliable within one recording session (combined intraclass correlation 90.5%), but less so between sessions (21.0%). Somatosensory map geometry was stable within and between sessions. For the forelimb the somatosensory representation had a 30% overlap with the corresponding motor area. No significant overlap was found for the hindlimb. No evidence for cortical injury was found on histology (Nissl). Thin-film epidural electrode array technology enables a detailed assessment of sensorimotor cortex physiology in vivo and can be used in longitudinal designs enabling studies of learning and plasticity processes.

Short-term reorganization of input-deprived motor vibrissae representation following motor disconnection in adult rats

It has been proposed that abnormal vibrissae input to the motor cortex (M1) mediates short-term cortical reorganization after facial nerve lesion. To test this hypothesis, we cut first the infraorbital nerve (ION cut) and then the facial nerve (VII cut) in order to evaluate M1 reorganization without any aberrant, facial-nerve-lesion-induced sensory feedback. In each animal, M1 output was assessed in both hemispheres by mapping movements induced by intracortical microstimulation. M1 output was compared in different types of peripheral manipulations: (i) contralateral intact vibrissal pad (intact hemispheres), (ii) contralateral VII cut (VII hemispheres), (iii) contralateral ION cut (ION hemispheres), (iv) contralateral VII cut after contralateral ION cut (ION + VII hemispheres), (v) contralateral pad botulinum-toxin-injected after ION cut (ION + BTX hemispheres). Right and left hemispheres in untouched animals were the reference for normal M1 map (control hemispheres). Findings demonstrated that: (1) in ION hemispheres, the mean size of the vibrissae representation was not significantly different from those in intact and control hemispheres; (2) reorganization of the vibrissae movement representation clearly emerged only in hemispheres where the contralateral vibrissae pad had undergone motor output disconnection (VII cut hemispheres); (3) the persistent loss of vibrissae input did not change the M1 reorganization pattern during the first 48 h after motor paralysis (ION + VII cut and ION + BTX hemispheres). Thus, after motor paralysis, vibrissa input does not provide the gating signal necessary to trigger M1 reorganization.

Neonatal frontal lesion in unilateral hemisphere enhances the development of the intact higher motor cortex in the rat

The influence of the neonatal frontal lesion in unilateral cerebral hemisphere for the organization of intact forelimb motor cortex in the rat was investigated by intracortical microstimulation (ICMS). The relative size of the rostral forelimb area (RFL) compared to the caudal forelimb area (CFL) in the ipsilateral motor field of lesioned rat was significantly greater than those of contralateral in normal and lesioned rats. The optimal sites of the stimulation for ipsilateral responses in lesioned rats were located in the RFL, while the optimal sites for contralateral were located caudolaterally, as for those of normal rats. At the ipsilateral optimal sites within the RFL in the lesioned animals, the threshold for the ipsilateral responses was lower than that for the contralateral responses. That is, the intact hemisphere of the animal preferentially developed the RFL rather than the CFL, for the ipsilateral forelimb. This may suggest a critical role for the RFL in individual forelimb motor control within the normal hemisphere.

Long-Evans rats have a larger cortical topographic representation of movement than Fischer-344 rats: a microstimulation study of motor cortex in naïve and skilled reaching-trained rats

Intracortical microstimulation of the frontal cortex evokes movements in the contralateral limbs, paws, and digits of placental mammals including the laboratory rat. The topographic representation of movement in the rat consists of a rostral forelimb area (RFA), a caudal forelimb area (CFA), and a hind limb area (HLA). The size of these representations can vary between individual animals and the proportional representation of the body parts within regions can also change as a function of experience. To date, there have been no investigations of strain differences in the cortical map of rats, and this was the objective of the present investigation. The effect of cortical stimulation was compared in young male Long-Evans rats and Fischer-344 rats. The overall size of the motor cortex representation was greater in Long-Evans rats compared to Fischer-344 rats and the threshold required to elicit a movement was higher in the Fischer-344 rats. An additional set of animals were trained in a skilled reaching task to rule out the possibility that experiential differences in the groups could account for the result and to examine the relationship between the differences in topography of cortical movement representations and motor performance. The Long-Evans rats were quantitatively and qualitatively better in skilled reaching than the Fischer-344 rats. Also, Long-Evans rats exhibited a relatively larger area of the topographic representation and lower thresholds for eliciting movement in the contralateral forelimb. This is the first study to describe pronounced strain-related differences in the microstimulation-topographic map of the motor cortex. The results are discussed in relation to using strain differences as a way of examining the behavioral, the physiological, and the anatomical organization of the motor system.

Motor learning-dependent synaptogenesis is localized to functionally reorganized motor cortex

The regional specificity and functional significance of learning-dependent synaptogenesis within physiologically defined regions of the adult motor cortex are described. In comparison to rats in a motor activity control group, rats trained on a skilled reaching task exhibited an areal expansion of wrist and digit movement representations within the motor cortex. No expansion of hindlimb representations was seen. This functional reorganization was restricted to the caudal forelimb area, as no differences in the topography of movement representations were observed within the rostral forelimb area. Paralleling the physiological changes, trained animals also had significantly more synapses per neuron than controls within layer V of the caudal forelimb area. No differences in the number of synapses per neuron were found in either the rostral forelimb or hindlimb areas. This is the first demonstration of the co-occurrence of functional and structural plasticity within the same cortical regions and provides strong evidence that synapse formation may play a role in supporting learning-dependent changes in cortical function.

Sensorimotor experience modulates age-dependent alterations of the forepaw representation in the rat primary somatosensory cortex

In a previous study, we found that the forepaw representation in the primary somatosensory cortex of rats housed in standard laboratory conditions was drastically altered during the aging process. In other studies we reported that exposure to an enriched environment improved the topographical organization and increased the spatial resolution of the forepaw cutaneous map in young adult rats, whereas housing in impoverished environment resulted in a loss of somatotopic details in the forepaw map. The main purpose of the present study was to investigate the influence of differential sensorimotor experience promoted by exposure to enriched or impoverished environments on the mutability of the cortical forepaw representation during aging. Two groups of Long-Evans rats were reared in enriched and impoverished environments from weaning to the age of 3.5-5 months (young adults), 6.5-8 months (mature rats), and 23-28 months (senescent rats). The electrophysiological maps of the forepaw representation were based on the somatosensory 'submodality' (cutaneous vs. non-cutaneous), size, and location of the receptive fields of small clusters of layer IV neurons. Moreover, the mechanical thresholds of neuronal response to cutaneous stimulation were assessed with calibrated von Frey filaments in mature and senescent animals. Age-related alterations of the topographic features of the forepaw map were characterized by a decrease in and a fragmentation of the cortical zones serving the glabrous skin of the forepaw. These changes were less pronounced in the enriched rats than in the impoverished rats. Glabrous skin receptive fields were smaller in young adult and mature enriched rats than in their impoverished counterparts. However, during aging glabrous receptive fields increased in the enriched rats, but decreased in the senescent impoverished rats so that old rats of either groups displayed receptive fields of similar sizes; in contrast, the size of hairy skin receptive fields was not affected by housing conditions or aging. Measurement of the neuronal responses to calibrated forces applied to the skin indicated that cortical excitability to near-threshold cutaneous input was lower in senescent rats than in mature rats, regardless of environmental conditions. The present study demonstrates that use-dependent remodeling of somatosensory maps occurs throughout life and that environmental and social interactions can partially offset the age-related breakdown of somatosensory cortical maps.

Transient changes in cortical distribution of S100 proteins during reorganization of somatotopy in the primary motor cortex induced by facial nerve transection in adult rats

In adult rats, the primary motor cortex (MI) comprises a somatotopic map of muscle representations. This somatotopy is modified after transection of the facial nerve (N7x). Mapping with cortical stimulation revealed that the underlying cortical reorganization is biphasic. Primary changes cause a transient disinhibition of long cortico-cortical connections in both hemispheres. While the first reaction vanishes within a few hours, short intra-areal connections are disinhibited within MI contralateral to N7x. The resulting co-operation between adjacent parts of MI persists as long as peripheral reinnervation is prevented. Cellular mechanisms underlying this cortical reorganization are largely unknown. Here, we utilized changes in immunoreactivity of S100 proteins (S100-IR) known as a sensitive indicator of astroglial reactions during plastic reactions in the central nervous system. Within 1 h of N7x, zones with enhanced S100-IR appeared in both hemispheres. Between 3. 5 and 18 h, reaction patterns with changing topography were transiently prominent in many cortical areas including parts of MI which surrounded the facial muscle representation fields. After 24 h, the facial muscle representation contralateral to N7x became labelled while S100-IR enhancement disappeared in most of the cortex. S100-IR-enhancement vanished completely during the next day of survival. Data presented suggest that (i) enhancement of S100-IR labels cortical tissue during the functional reorganization that is induced by N7x, (ii) large parts of the cerebral cortex participate in the reorganization, before it is finally focused on the representation field of MI that corresponds with contralateral N7x, and (iii) temporo-spatial patterns of astrocytic reactions apparently play a role in the underlying plasticity reaction.

Functional reorganization of the rat motor cortex following motor skill learning

Functional reorganization of the rat motor cortex following motor skill learning. J. Neurophysiol. 80: 3321-3325, 1998. Adult rats were allocated to either a skilled or unskilled reaching condition (SRC and URC, respectively). SRC animals were trained for 10 days on a skilled reaching task while URC animals were trained on a simple bar pressing task. After training, microelectrode stimulation was used to derive high resolution maps of the forelimb and hindlimb representations within the motor cortex. In comparison with URC animals, SRC animals exhibited a significant increase in mean area of the wrist and digit representations but a decrease in elbow/shoulder representation within the caudal forelimb area. No between-group differences in areal representation were found in either the hindlimb or rostral forelimb areas. These results demonstrate that motor skill learning is associated with a reorganization of movement representations within the rodent motor cortex.

Neural activity related to reaching and grasping in rostral and caudal regions of rat motor cortex

The objective of this study was to assess the relation of motor cortical neural activity in the rat to self-paced reach-to-grasp movements. Overall, around 40% of excitatory and 60% of inhibitory modulations in neuronal activity began prior to reach onset. These data are consistent with a role for rat motor cortex in the initiation and control of the reaching movement. In addition, although the reach only lasted a short time, 30% of excitations and inhibitions began while it was in progress. The existence of such modulations occurring during the reach is consistent with previous data showing activity of cortical neurons late in the reach, and suggests a heavy involvement of cortical neurons in controlling the recently described, complex movements associated with grasping that are seen in the rat. These features were broadly similar in neurones from both the caudal and rostral subdivisions of rat motor cortex.

Organization of somatosensory cortex in monotremes: in search of the prototypical plan

The present investigation was designed to determine the number and internal organization of somatosensory fields in monotremes. Microelectrode mapping methods were used in conjunction with cytochrome oxidase and myelin staining to reveal subdivisions and topography of somatosensory cortex in the platypus and the short-billed echidna. The neocortices of both monotremes were found to contain four representations of the body surface. A large area that contained neurons predominantly responsive to cutaneous stimulation of the contralateral body surface was identified as the primary somatosensory area (SI). Although the overall organization of SI was similar in both mammals, the platypus had a relatively larger representation of the bill. Furthermore, some of the neurons in the bill representation of SI were also responsive to low amplitude electrical stimulation. These neurons were spatially segregated from neurons responsive to pure mechanosensory stimulation. Another somatosensory field (R) was identified immediately rostral to SI. The topographic organization of R was similar to that found in SI; however, neurons in R responded most often to light pressure and taps to peripheral body parts. Neurons in cortex rostral to R were responsive to manipulation of joints and hard taps to the body. We termed this field the manipulation field (M). The mediolateral sequence of representation in M was similar to that of both SI and R, but was topographically less precise. Another somatosensory field, caudal to SI, was adjacent to SI laterally at the representation of the face, but medially was separated from SI by auditory cortex. Its position relative to SI and auditory cortex, and its topographic organization led us to hypothesize that this caudal field may be homologous to the parietal ventral area (PV) as described in other mammals. The evidence for the existence of four separate representations in somatosensory cortex in the two species of monotremes indicates that cortical organization is more complex in these mammals than was previously thought. Because the two monotreme families have been separate for at least 55 million years (Richardson, B.J. [1987] Aust. Mammal. 11:71-73), the present results suggest either that the original differentiation of fields occurred very early in mammalian evolution or that the potential for differentiation of somatosensory cortex into multiple fields is highly constrained in evolution, so that both species arrived at the same solution independently.

Comparison of the connectional properties of the two forelimb areas of the rat sensorimotor cortex: support for the presence of a premotor or supplementary motor cortical area

The existence of multiple motor cortical areas that differ in some of their properties is well known in primates, but is less clear in the rat. The present study addressed this question from the point of view of connectional properties by comparing the afferent and efferent projections of the caudal forelimb area (CFA), considered to be the equivalent of the forelimb area of the primary motor cortex (MI), and a second forelimb motor representation, the rostral forelimb area (RFA). As a result of various tracing experiments (including double labeling), it was observed that CFA and RFA had reciprocal corticocortical connections characterized by preferential, asymmetrical, laminar distribution, indicating that RFA may occupy a different hierarchical level than CFA, according to criteria previously discussed in the visual cortex of primates. Furthermore, it was found that RFA, but not CFA, exhibited dense reciprocal connections with the insular cortex. With respect to their efferent projection to the basal ganglia, it was observed that CFA projected very densely to the lateral portion of the ipsilateral caudate putamen, whereas the contralateral projection was sparse and more restricted. The ipsilateral projection originating from RFA was slightly less dense than that from CFA, but it covered a larger portion of the caudate putamen (in the medial direction); the contralateral projection from RFA to the caudate putamen was of the same density and extent as the ipsilateral projection. The reciprocal thalamocortical and corticothalamic connections of RFA and CFA differed from each other in the sense that CFA was mainly interconnected with the ventrolateral thalamic nucleus, while RFA was mainly connected with the ventromedial thalamic nucleus. Altogether, these connectional differences, compared with the pattern of organization of the motor cortical areas in primates, suggest that RFA in the rat may well be an equivalent of the premotor or supplementary motor area. In contrast to the corticocortical, corticostriatal, and thalamocortical connections, RFA and CFA showed similar efferent projections to the subthalamic nucleus, substantia nigra, red nucleus, tectum, pontine nuclei, inferior olive, and spinal cord.

Modulation of sustained electromyographic activity by single intracortical microstimuli: comparison of two forelimb motor cortical areas of the rat

In rats, a rostral and a caudal forelimb motor area (RFA and CFA, respectively) have been distinguished on the basis of intracortical microstimulation effects (see Neafsey et al., 1986, for a review). The goal of the present study was to assess and compare their relative connectional strength with target motor units of the forelimb. This was achieved by averaging modulation responses of sustained electromyographic (EMG) activity triggered by single intracortical microstimuli (S-ICMS) of relatively low intensity (mostly below 35 microA) to minimize both direct and transsynaptic current spread. In chronically prepared and ketamine-sedated rats, this method produced prominent peaks and troughs in the averaged EMG at short latencies with S-ICMS currents as low as 5 microA. S-ICMS at 30-50 microA in CFA sometimes even elicited visible twitches and an EMG burst of the contralateral wrist or digits following each stimulation pulse. Increasing S-ICMS currents to about 1.5 mA revealed a sudden shortening of EMG response latencies, which was most likely induced by current spread to brainstem motor centers. S-ICMS at near-threshold intensity in the majority of effective sites in both CFA and RFA produced modulation responses in more than one group of forelimb muscles, frequently also including muscles of the ipsilateral forelimb. Usually the ipsilateral responses were weaker, as were the suppression effects. Comparison of CFA and RFA revealed similar effects in terms of the number of modulated muscle groups and the response latencies. In contralateral wrist/digit muscles, facilitation responses were elicited at latencies of 9.7 +/- 1.8 msec (CFA) and 9.6 +/- 1.9 msec (RFA), with the shortest latencies around 6 msec. However, modulations by S-ICMS in RFA had significantly smaller amplitudes, had slower rates of buildup, and required higher thresholds than those obtained from S-ICMS in CFA. It is concluded, on the basis of the S-ICMS method, that both the CFA and the RFA exert a prominent and relatively direct influence on forelimb motoneurons. The present findings, together with calculations of conduction time, suggest that a contingent of corticospinal axons of the rat has oligosynaptic and possibly even monosynaptic connections with forelimb motoneurons. The recruitment of a relatively large number of muscles, including those of the ipsilateral forelimb, by S-ICMS in both areas may be explained by the prominent divergence of corticospinal axons. Further investigations are required to understand the relative positions and roles of the two areas in motor control and their possible homology with primary and nonprimary motor areas of primates.

Spontaneous forelimb grasping in free feeding by rats: motor cortex aids limb and digit positioning

Forelimb use in grasping food during free feeding was studied in control and motor cortex damaged rats using videoanalysis and Eshkol-Wachmann Movement Notation (EWMN). Rats detected food using olfaction, grasped it by mouth, and then sat and reached for it with their paws. Once held in the paws, the food was eaten. A reach consists of: (1) lifting the forelimbs from the ground, (2) positioning them elbows-in, so that the paws were adjacent to the mouth, and (3) clasping the food in the digits. These movements were executed mainly with the upper arm. Limb movements were usually bilaterally symmetrical but when asymmetrical movements occurred, the forelimb least involved in weight support initiated the movement. As the limb was positioned for grasping, the aperture of the digits was adjusted to anticipate the size of the food and the food was grasped and manipulated with the tips of the digits. Following unilateral motor cortex lesions to the forelimb area: (1) the ipsilateral limb (good limb) initiated lifting, positioning, and grasping movements, (2) appropriate adjustment of the digits of the contralateral limb (bad limb) and grasping were impaired, and (3) when contact with food was lost, the bad limb adopted an extended, closed-fist spastic posture and could not be repositioned independently. The gross impairments cleared within 2 weeks, and after a few months impairments were infrequently observed. These findings show that: (1) spontaneous food grasping uses both proximal movements of the limb and distal movements of the digits, (2) digit aperture anticipates food size in reaching, and (3) motor cortex damage impairs both proximal and distal movements more profoundly when the limb is used independently than when it is used in conjunction with the good limb. The results are discussed in relation to kinematic studies on primates and humans.

Skilled reaching in rats and humans: evidence for parallel development or homology

Forelimb reaching by the rat is used as a paradigm for the experimental study of neural control, plasticity, and recovery of function after injury, in the expectation that results are generalizable to humans. The present study was done to compare rat to human reaching movements. The movements of both species were videorecorded and subjected to frame-by-frame analysis using Cartesian (spatial and velocity) and Eshkol-Wachman Movement Notation (EWMN) systems. The component movements of reaching, their sequence and velocity profiles, and their topography were similar in the two species. Both species also displayed more supination and lengthened grasping times when reaching for small as opposed to large objects. Both rats and humans moved the limb medially using the upper arm to aim it when they were required to reach through an aperture but in a free reaching test only rats continued to aim the limb. Human movements were characterized by greater blending of movement components, more variability, and independent digit use. Arguments are presented that the similarities and differences in rat and human reaching are not trivially accounted for by limb and task similarities. The many similarities in the movements of the two species provide evidence for at least parallel development or perhaps even homology.

Neocortical localization of tactile/proprioceptive limb placing reactions in the rat

The present study was aimed at delineating the neocortical substrate of tactile/proprioceptive limb placing reactions in rats by means of behavioral tests that excluded the participation of facial stimuli in limb function. Using a photochemical technique, we made unilateral focal lesions in the frontal and parietal neocortex. Fore- and/or hindlimb placing deficits resulted from damage to a fronto-parietal region lying between the medial agranular cortex and the primary somatosensory (whisker barrel field) cortex. When the antero-posterior coordinate was varied from 4 mm anterior to 1 mm posterior to bregma, tactile/proprioceptive forelimb dysfunction was more pronounced after damage to the parietal forelimb area, but lesions confined to the frontal lateral agranular cortex also yielded clear-cut forelimb placing deficits. Damage to either area alone allowed for partial recovery of forelimb function. However, following combined, total destruction of both frontal and parietal forelimb areas, forelimb deficits did not recover. This resembled the irreversible hindlimb deficits after near-total destruction of the parietal hindlimb area. Damage to the medial agranular cortex left limb placing intact. Likewise, for as long as the medial edge of lesions to the whisker barrel field did not come closer than 3 mm to the midline, thus remaining outside the parietal hindlimb area, limb placing remained normal. This sharp medial and lateral delineation of the cortical substrate subserving tactile/proprioceptive limb placing coincides with the borders of a thick, dense subfield of large pyramidal neurons in the deeper parts of layer V. Limb placing remained intact when medial agranular cortex lesions damaged only 30% of that subfield, whereas 70% destruction of that layer following more laterally placed lesions in the parietal hindlimb area produced irreversible hindlimb dysfunction. The severity of hindlimb placing deficits was related to the amount of incursion by whisker barrel field lesions into the subfield of deep layer V large pyramidal neurons. Finally, very large lesions of the occipital cortex did not affect tactile/proprioceptive limb placing. We discuss the neocortical areal and laminar specificity of tactile/proprioceptive limb function in the context of recent neuroanatomical and electrophysiological findings, and their relevance to normal cortical function, recovery from neocortical stroke (including diaschisis), and age-related cortical dysfunction.

Cortical modulation of thalamo-cortical neurons relaying exteroceptive information: a microstimulation study in the guinea pig

The nature and organization of cortical influences on somatosensory thalamic neurons were investigated in the guinea pig in order to ascertain if mechanisms subserving sensory-motor integration in the thalamus are as precise as has previously been demonstrated in the agranular frontal cortex (AGr) and granular parietal cortex (Gr). The study was carried out on 14 chronically-implanted awake animals. In each experiment one or two motor foci within AGr and Gr were identified according to the region of the movement evoked by intracortical microstimulation at the lowest threshold stimulation (usually 5-15 microA). Spontaneous activity of 182 thalamo-cortical single neurons was recorded in the nucleus ventralis thalami (VT). The neurons were also identified by their response to activation of cutaneous receptive fields (RFs) located in regions of vibrissae or limbs, and then tested for cortical stimulation with a pulse intensity equal to the threshold for evoking motor effects. During the cortico-thalamic tests, the duration of stimulating trains was reduced in order to avoid the appearance of limb or vibrissa movements which could activate somatosensory ascending pathways forwarding peripheral messages to VT. The cortical control on VT neurons appears to be organized in a very precise manner. It was seen that: 1) The influences on these neurons relaying exteroceptive signals specifically emanated from AGr and Gr areas which in turn received exteroceptive input. 2) The vibrissa units responded to stimulation of foci in either AGr or Gr but the reactivity was greater upon stimulation of Gr than AGr. The incidence of responses was very high when the vibrissa RF was overlapping or adjacent to the region of the cortically-evoked vibrissa movement. The response pattern was mostly excitatory. Responses were rarely observed when vibrissa RF lay distant from the vibrissa moved by cortical stimulation. 3) Neurons with limb RFs responded constantly to stimulation of Gr foci only when the RF was overlapping or adjacent to the region of the cortical motor target; in these two conditions the response pattern was excitatory and inhibitory, respectively. Inhibitions only concerned neurons with forelimb RFs. Responses to stimulation of AGr were rarely obtained. From a functional point of view, the excitatory nature of the cortical control on thalamo-cortical VT neurons suggests that a cortical signal inducing movement of a given body part is able to enhance the afferent transmission of somatosensory messages arising in the same body part.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of changes in the periphery on development of the corticospinal motor system in the rat

Effects of changes in the periphery on development of the corticospinal (CS) motor system were studied in the rat. Unilateral forelimb restraint between ages 5 and 30 days resulted in an increase in the number of CS neurons which persisted in the adult. The effect was most marked ipsilateral to limb restraint where both crossed and uncrossed CS connections were increased, but it also occurred to a lesser extent on the contralateral side. Animals with limb restraint had enlargement of the areas of cerebral cortex in which CS neurons occurred. The enlargement of motor cortex regions and increase in CS neurons closely resembled the changes found in the remaining cerebral hemisphere after neonatal hemispherectomy. The findings in animals with forelimb restraint differed markedly from those after forelimb amputation, where little change occurred in either number or location of CS neurons. Limb restraint initiated at the time of postnatal hemispherectomy had no effects on location or number of CS neurons beyond those of hemispherectomy alone. It is proposed that transient CS axons that occur normally in the postnatal rat may be recruited for formation of permanent connections under very diverse conditions, i.e. hemispherectomy and limb restraint. Failure to observe an additional effect of limb restraint in hemispherectomized animals may be due to the fact that after hemispherectomy all available transient fibers in the remaining hemisphere are recruited for innervation of the side of the spinal cord that has lost its cortical input.

The impairments in reaching and the movements of compensation in rats with motor cortex lesions: an endpoint, videorecording, and movement notation analysis

Reaching for food by rats, with the limb contralateral to limb area motor cortex damage, was analyzed using end-point scores, videoanalysis, and Eshkol-Wachmann Movement Notation (EWMN). End point results from groups of rats with small, medium, and large lesions showed reaching success and amount of food grasped per reach decreased with increases in lesion size. Videoanalysis and EWMN showed that the impairments were attributable to: (1) an inability to pronate the paw over the food by abduction of the upper arm, and (2) an inability to supinate the paw at the wrist to orient the food to the mouth. There were no obvious impairments in locating food using olfaction, in positioning the body in order to initiate a reach, or in clasping the digits to grasp food. There were only mild impairments in lifting, aiming, and advancing the limb. In rats with medium and large lesions, loss of pronation and supination were compensated for by a variety of whole body movements. These findings are discussed in reference to neural and behavioral mechanisms underlying recovery of function and the contribution of the motor cortex to skilled movements in the rat and other species.

Functional subdivisions of the rat somatic sensorimotor cortex

The behavioural impairments and subsequent recovery were studied in rats with circumscribed unilateral lesions in the somatic sensorimotor cortex (SMC). Lesions were made in the caudal forelimb region (CFL), the rostral forelimb region (RFL), the anteromedial cortex (AMC) or the hindlimb area. Rats with damage in the CFL produced a deficit in placing the forelimb contralateral to the lesion during exploratory locomotion on a grid surface. Rats with AMC damage circled in the direction ipsilateral to the lesion. Lesions in the CFL or AMC produced an ipsilateral somatosensorimotor asymmetry on the bilateral-stimulation test (responding to adhesive patches placed on the contralateral forelimb was slower) that recovered in 7 days following AMC lesions or 28 days following CFL lesions. Finally, RFL lesions produced an ipsilateral asymmetry on the bilateral-stimulation task that was more severe and enduring (recovery in 60 days). After behavioral recovery, the effects of an additional lesion placed in the homotopic contralateral cortex were examined (two-stage bilateral lesion). Rats receiving two-stage bilateral lesions in the RFL or CFL responded slower to tactile stimulation of the forelimb contralateral to the second lesion. In the case of CFL-damaged rats, placing deficits also appeared contralateral to the most recent injury. In contrast, rats receiving two-stage bilateral AMC lesions did not exhibit behavioral asymmetries following the second lesion. These results provide evidence to suggest that subdivisions of the rat SMC can be distinguished with lesion/behavioral experiments. Moreover, a comparison of the effects of unilateral and two-stage bilateral lesions may help in the parcellation of the rat SMC into functionally distinct subareas and provide a basis for studying the processes of recovery and maintenance of function following brain damage.

Multiple representations of the body and input-output relationships in the agranular and granular cortex of the chronic awake guinea pig

The organization of somatosensory input and the input-output relationships in regions of the agranular frontal cortex (AGr) and granular parietal cortex (Gr) were examined in the chronic awake guinea pig, using the combined technique of single-unit recording and intracortical microstimulation (ICMS). AGr, which was cytoarchitectonically subdivided into medial (AGrm) and lateral (AGrl) parts, also can be characterized on a functional basis. AGrl contains the head, forelimb, and most hindlimb representations; only a small number of hindlimb neurons are confined in AGrm. Different distributions of submodalities exist in AGr and Gr: AGr receives predominantly deep input (with the exception of the vibrissa region, which receives cutaneous input), whereas neurons of Gr respond almost exclusively to cutaneous input. The cutaneous or deep receptive field (RF) of each neuron was determined by natural peripheral stimulation. All studied neurons were activated by small RFs, with the exception of lip, nose, pinna, and limb units of lateral Gr (Grl), for which the RFs were larger. Microelectrode mapping experiments revealed the existence of three spatially separate, incomplete body maps in which somatosensory and motor representations overlap. One body map, with limbs medially and head rostrolaterally, is contained in AGr. A second map, comparable to the first somatosensory cortex (SI) of other mammals, is found in Gr, with hindlimb, trunk, forelimb, and head representations in an orderly mediolateral sequence. An unresponsive zone separates the head area from the forelimb region. A third map, with the forelimb rostrally and the hindlimb caudally, lies adjacent and lateral to the SI head area. This limb representation, which is characterized by an upright and small size compared to that found in SI, can be considered to be part of the second somatosensory cortex (SII). A distinct head representation was not recognized as properly belonging to SII, but the evidence that neurons of the SI head region respond to stimulation of large RFs located in lips, nose, and pinna leads us to hypothesize that the SII face area overlaps that of SI to some extent, or, alternatively, that the two areas strictly contiguous and the limits are ambiguous, making them difficult to distinguish. The input-output relationships were based on the results of RF mapping and ICMS in the same electrode penetration. The intrinsic specific interconnections of cortical neurons whose afferent input and motor output is related to identical body regions show a considerable degree of refinement.(ABSTRACT TRUNCATED AT 400 WORDS)

Areal distributions of cortical neurons projecting to different levels of the caudal brain stem and spinal cord in rats

Distributions of corticospinal and corticobulbar neurons were revealed by tetramethylbenzidine (TMB) processing after injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) into the cervical or lumbar enlargements of the spinal cord, or medullary or pontine levels of the brain stem. Sections reacted for cytochrome oxidase (CO) allowed patterns of labeled neurons to be related to the details of the body surface map in the first somatosensory cortical area (SI). The results indicate that a number of cortical areas project to these subcortical levels: (1) Projection neurons in granular SI formed a clear somatotopic pattern. The hindpaw region projected to the lumbar enlargement, the forepaw region to the cervical enlargement, the whisker pad field to the lower medulla, and the more rostral face region to more rostral brain stem levels. (2) Each zone of labeled neurons in SI extended into adjacent dysgranular somatosensory cortex, forming a second somatotopic pattern of projection neurons. (3) A somatotopic pattern of projection neurons in primary motor cortex (MI) paralleled SI in mediolateral sequence corresponding to the hindlimb, forelimb, and face. (4) A weak somatotopic pattern of projection neurons was suggested in medial agranular cortex (Agm), indicating a premotor field with a rostromedial-to-caudolateral representation of hindlimb, forelimb, and face. (5) A somatotopic pattern of projection neurons representing the foot to face in a mediolateral sequence was observed in medial parietal cortex (PM) located between SI and area 17. (6) In the second somatosensory cortical area (SII), neurons projecting to the brain stem were immediately adjacent caudolaterally to the barrel field of SI, whereas neurons projecting to the upper spinal cord were more lateral. No projection neurons in this region were labeled by the injections in the lower spinal cord. (7) Other foci of projection neurons for the face and forelimb were located rostral to SII, providing evidence for a parietal ventral area (PV) in perirhinal cortex (PR) lateral to SI, and in cortex between SII and PM. None of these regions, which may be higher-order somatosensory areas, contained labeled neurons after injections in the lower spinal cord. Thus, more cortical fields directly influence brain stem and spinal cord levels related to sensory and motor functions of the face and forepaw than the hindlimb. The termination patterns of corticospinal and corticobulbar projections were studied in other rats with injections of WGA:HRP in SI.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographical organization of the efferent projections of the medial prefrontal cortex in the rat: an anterograde tract-tracing study with Phaseolus vulgaris leucoagglutinin

The purpose of the present investigation was to examine the topographical organization of efferent projections from the cytoarchitectonic divisions of the mPFC (the medial precentral, dorsal anterior cingulate and prelimbic cortices). We also sought to determine whether the efferents from different regions within the prelimbic division were organized topographically. Anterograde transport of Phaseolus vulgaris leucoagglutinin was used to examine the efferent projections from restricted injection sites within the mPFC. Major targets of the prelimbic area were found to include prefrontal, cingulate, and perirhinal cortical structures, the dorsomedial and ventral striatum, basal forebrain nuclei, basolateral amygdala, lateral hypothalamus, mediodorsal, midline and intralaminar thalamic nuclei, periaqueductal gray region, ventral midbrain tegmentum, laterodorsal tegmental nucleus, and raphe nuclei. Previously unreported projections of the prelimbic region were also observed, including efferents to the anterior olfactory nucleus, the piriform cortex, and the pedunculopontine tegmental-cuneiform region. A topographical organization governed the efferent projections from the prelimbic area, such that the position of terminal fields within target structures was determined by the rostrocaudal, dorsoventral, and mediolateral placement of the injection sites. Efferent projections from the medial precentral and dorsal anterior cingulate divisions (dorsomedial PFC) were organized in a similar topographical fashion and produced a pattern of anterograde labeling different from that seen with prelimbic injection sites. Target structures innervated primarily by the dorsomedial PFC included certain neocortical fields (the motor, somatosensory, and visual cortices), the dorsolateral striatum, superior colliculus, deep mesencephalic nucleus, and the pontine and medullary reticular formation. Previously unreported projections to the paraoculomotor central gray area and the mesencephalic trigeminal nucleus were observed following dorsomedial PFC injections. These results indicate that the efferent projections of the mPFC are topographically organized within and across the cytoarchitectonic divisions of the medial wall cortex. The significance of topographically organized and restricted projections of the rat mPFC is discussed in light of behavioral and physiological studies indicating functional heterogeneity of this region.

Kinesthetic cortical area anterior to primary somatic sensory cortex in the raccoon (Procyon lotor)

Extracellular microelectrode recording of cortical unit activity, with subsequent histological examination, was used to determine the extent, organization, and cytoarchitecture of the zone of muscle afferent projections (kinesthetic cortex) anterior to the primary somatic sensory cortex in anesthetized raccoons. Activity was evoked in response to mechanical stimulation of muscles from which the overlying skin had been dissected away. Most kinesthetic responses were elicited in a contiguous cortical area, which included: the anterior bank of the lateral arm, and the fundus and posterior bank of the medial arm of the medial central sulcus; and the anterior two-thirds of the interfundic rise within the interbrachial sulcus. Some responses were recorded in a separate small area of the anterior bank at the medial end of the lateral central sulcus. Somatotopy was evident with forelimb represented lateral to hindlimb. Proximal limb muscles were represented in the center of the medial central sulcus; distal muscle projections were medial (hindlimb) or lateral (forelimb) in the same sulcus. Most representations were of flexor and extensor muscles of the contralateral carpus and forepaw digits. Activity at a given recording locus in the kinesthetic area could be elicited by both flexor and extensor muscles, which acted about a common joint. Low amplitude units evoked by cutaneous stimulation of the dissected skin were recorded in the kinesthetic area; these were from receptive fields of skin that normally overlay the muscles whose higher-amplitude evoked kinesthetic units were represented in that same recording locus. The kinesthetic zone was anterior to primary somatic sensory cortex, where the outer stripe of Baillarger and granular layer IV become attenuated. In the hindlimb muscle representation area, the additional criterion of area 3a (large pyramidal cells in layer V) was seen. However, no cytoarchitecture could be identified that was consistently associated with the kinesthetic cortex.