Posture-independent sensorimotor analysis of inter-hemispheric receptor asymmetries in neostriatum

Somatosensorimotor function of the superior colliculus, somatosensory cortex, and lateral hypothalamus in the rat

The role of the superior colliculus in multimodal sensory function is unsettled, in large part because a clear distinction between the somatosensory effects and the postural/motor effects of damage to the deep layers of the superior colliculus has not been obtained. Unilateral lesions of the entire superior colliculus impair orienting of the head and eyes to tactile, visual, and auditory stimuli presented on the side of the body contralateral to the lesion; however, even in the absence of sensory stimulation animals with such a lesion tend to circle ipsiversively and fail to make contralateral head movements. To determine whether or not unilateral damage to the superior colliculus produces a somatosensory asymmetry independently of head movement/circling biases, a neurological test was used in which lateral head or trunk movements were not required. Small pieces of adhesive-backed paper were attached to each forelimb and the latencies to contact and remove the stimuli were recorded. A battery of standard neurological tests was administered as well. The entire superior colliculus was removed unilaterally, and for comparison, the sensorimotor cortex or lateral hypothalamus were damaged in additional groups. Lesions of the superior colliculus produced the expected deficit in contralateral orienting and ipsilateral circling/postural biases, but failed to produce a somatosensory asymmetry in the head movement-independent sensorimotor test. In contrast, both sensorimotor cortex and lateral hypothalamus lesions produced a severe asymmetry in the head-movement-independent sensorimotor test. We conclude that the superior colliculus is involved in the control of lateral head movements and that its role in somatosensory function is fundamentally different from that of the sensorimotor cortex or lateral hypothalamus regions.

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.

Recovery of function after brain damage: severe and chronic disruption by diazepam

Following unilateral damage to the anterior-medial region of the neocortex (AMC) in rats a sensory asymmetry appeared, but recovered within a week. In a separate group of rats with AMC lesions daily 3-week exposure to diazepam (Valium, 5 mg/kg) beginning 12 h after surgery caused recovery to be delayed indefinitely. The efficiency and speed (as opposed to symmetry) of behavior was not impaired. More than 9 weeks after discontinuation of diazepam (12 weeks postsurgery), recovery was still not apparent. Postmortem analysis ruled out lesion size as a contributing factor. In a second experiment undrugged animals with AMC lesions were allowed to recover for at least 3 weeks before being exposed to diazepam. These animals showed only a transient (2-day) reinstatement of asymmetry despite continuous drug treatment. We conclude that important mechanisms serving recovery of function may be vulnerable during a short period soon after brain damage.

Lateralized and compulsive exteroceptive orientation in rats treated with apomorphine

A new behavioral test afforded the original demonstration of a compulsive and extremely lateralized taxis for edges in a minority (16%) of rats treated with the dopamine (DA) receptor agonist apomorphine (APO) (1.25 mg/kg, s.c.). APO also induced a reliable lateralization of locomotor direction in a rotometer in 29% of rats. The lateralizing effects of the drug on taxis for edges and rotational direction were not reliably associated, however, suggesting that these effects of APO are independent of each other. Interhemispheric asymmetries of DA receptors in functionally distinctive brain regions may underlie the diverse lateralizing effects of APO in subpopulations of rats.

Control of sensorimotor function by dopaminergic nigrostriatal neurons: influence on eating and drinking

The literature on the effects of lesions of the lateral hypothalamic area (LHA) on eating and drinking is reviewed in an effort to understand the function of the neural substrate destroyed. The data suggest that damage to the dopaminergic nigrostriatal neurons that course through the LHA results in a decrease in sensorimotor facilitation; that is, an increase in the threshold for responding to stimuli that elicit orientation, approach and consumption. This increase results in decreased consumption of food and water. Evidence is also reviewed suggesting the possibility that striatal dopaminergic activity may mediate a negative feedback signal related to blood glucose level that influences responsiveness to food, and therefore eating. There is no evidence that the nigrostriatal system mediates a similar signal related to water balance and drinking. A second deficit associated with LHA lesions, caused by damage to the pallidofugal neurons that descend through this area, is a dysfunction of motor control of the head and mouth. This results in an increase in the effort required to consume food and water, also leading to decreased consumption. These two behavioral factors: an increased threshold for responding to the sensory properties of food and water and an increase in the effort required to eat and drink are used to explain the symptoms making up the lateral hypothalamic syndrome without postulating changes in physiological regulatory (set point) mechanisms.

Neonatal hemidecortication and bilateral cutaneous stimulation in rats

In humans, a dominant somatosensory consequence of extensive unilateral neocortex damage is "simultaneous extinction," which is an interhemispheric perceptual interaction that is operationally distinguishable from neglect. A tactile stimulus presented on the contralateral side of the body is detected when presented singly, but is completely masked during bilateral stimulation. Analogous tests designed to calibrate somatosensory asymmetries in rats were used to determine the long-term effects of hemidecortication sustained on postnatal Day 1. These data were compared with that observed in adult operated rats at a comparable postoperative period. In one respect the neonatal brain was more vulnerable than the adult brain. That is, unlike adult operated rats which were tested at 3 postoperative months, a sensory asymmetry appeared to be permanent in the neonatally operated rats, at least for the duration of testing (3-9 months). Further analysis suggested that in another way the neonates were more resistant to the effects of hemidecortication than were the adults. Neonatally operated rats appeared to be capable of processing input from both sides of the body simultaneously, even during markedly asymmetrical input. In other words, the early occurrence of brain damage may have spared them from a condition reminiscent of "simultaneous extinction." Finally, the adult operated and neonatally operated rats both displayed a subtle motor abnormality. Thus, depending on the test used, the neonatal operation yielded more severe, less severe, or comparable behavior deficits.

Contralateral circling induced by tegmental morphine: anatomical localization, pharmacological specificity, and phenomenology

Unilateral morphine application to sites in the ventral tegmentum caused contralateral circling. Positive sites were restricted to the region of the dopamine-containing cells in the ventral tegmental area and substantia nigra; rates were highest with ventral tegmental application. The opiate receptor antagonists naltrexone and naloxone blocked morphine-induced circling whether given before or during the sessions. Pimozide pretreatment also blocked morphine-induced circling at doses that spared muscimol-induced circling. Morphine induced little postural asymmetry and induced forward locomotion in all 4 limbs. The radius of morphine-induced circling was large or small depending on the size of the test environment; thus the behavior was controlled by environmental (sensory) and not central (motor) constraints. These studies suggest that morphine-induced circling results from activation of dopaminergic circuitry involved in sensory-motor integration, particularly that involved in forward locomotion.

Speed of movement initiation performance predicts differences in [3H]spiroperidol receptor binding in normal rats

Speed of movement initiation is altered in normal aging and in neurological disorders such as Parkinson's disease. This study was undertaken to extend our previous results, which suggested a relationship between nigrostriatal dopamine function and an animal model of movement initiation speed (reactive capacity). Fisher 344 rats exhibiting exceptionally fast or slow reactive capacity but otherwise normal were examined for differences in the striatal binding of the dopaminergic ligand, [3H]spiroperidol. Rats with fast reactive capacity (Fast rats) exhibited significantly higher binding than did rats with slow reactive capacity (Slow rats). Also, Fast rats responded nearly maximally on the reactive capacity task regardless of the duration of time provided in which to respond, whereas Slow rats reacted more slowly when more response time was provided. The differences in [3H]spiroperidol binding and the differential influence of time provided to respond on the response latency of these two normal groups of rats was similar to that observed in old or model Parkinson's disease rats having nigrostriatal dopamine deficits. These results strengthen the relationship between an animal model of reactive capacity and nigrostriatal dopamine function.