Initiation and execution of locomotion elicited by diencephalic stimulation: regional differences in response to nembutal

Locomotion and head scanning initiated by hypothalamic stimulation are inversely related

Stimulation in the hypothalamus elicits locomotor stepping. Before stepping is initiated, head scanning movements occur. We determined the relationships between the latency of locomotor initiation and the number, extent and direction of the head scanning movements. Chronic stimulation electrodes were stereotaxically implanted in and around the hypothalamus of 29 rats. Under awake conditions, 38 locomotor sites were tested in a runway apparatus. Behaviors occurring between the onset of stimulation and the first step were recorded on videotape. Points on the rat were digitized at sampling rate of 6 Hz to produce measures of head angles in the vertical, horizontal, and sagittal planes. The priming paradigm was used with a current selected for each site that was minimally sufficient to produce reliable stepping. In trials at approximately 1-min intervals, a 5-s train of stimulation (the control) was followed by a second train (the test) delivered 5-20 s later. Initiation latency on control trains was strongly correlated with head movement measures. Vertical and lateral head movements were independent of one another. Together, their frequency and extent accounted for 85% of the variance in locomotor initiation latencies. In effective priming trials, when locomotor initiation latencies were reduced on the test train, the frequency and extent of vertical and lateral head movements were also reduced. In non-effective priming trials, when latencies were not reduced, head movements were not reduced. Head scanning and locomotor initiation reflect reciprocal processes.

Coordination of neck and hindlimb extension during the initiation of locomotion elicited by hypothalamic stimulation

We tested the hypothesis that during the initiation of stepping elicited by hypothalamic stimulation, hindlimb extension was coordinated with head extension in the sagittal plane. Chronic stimulation electrodes (monopolar stainless-steel, 125 microm diameter) were implanted bilaterally into the perifornical hypothalamus of anaesthetized rats (N = 15) under stereotaxic control. Under freely moving and awake conditions, 18 sites which reliably elicited forward locomotion at a latency of approximately 3 s were tested in a videotaping session. The locomotor stimulation was a constant current train of 5 s duration composed of biphasic pulses at 40-50 Hz. The videotape records were digitized at a sampling rate of 6 Hz for seven points on the rat: Nose, pinnae, midpoint of inter-pinnae line, right forepaw, right hindpaw and base of tail. A characteristic pattern of coordinated movements preceded, by approximately 0.5 s, the execution of the first locomotor step. The pattern included a movement of the pelvis in the anterior or superior direction that was produced by hindlimb extension and an extension of the neck forward along the sagittal plane. There was considerable flexibility in this pattern, but it was invariant to the extent that it occurred at a variety of latencies and after several types of head movements. Associated with the coordinated extensions of the neck and hindlimbs was a lowering of the head angle which had a more variable time course. These data indicate that there is significant coupling between the systems that produce hindlimb extension and control head position when the rat prepares to step.

Priming of locomotor initiation by electrical stimulation in the hypothalamus and preoptic region in the anesthetized rat

Electrical stimulation at a locomotor site can prime (i.e., shorten the latency to initiate) stepping elicited by subsequent stimulation of the same or a different site. We tested for the priming effect in representative sites along the medial forebrain bundle, and determined if its magnitude showed regional differences. Rats (n = 20) were anesthetized with Nembutal and held in a stereotaxic apparatus over a wheel. Stepping was detected by accelerometers attached to the hindlimbs. Priming and test trains of stimulation (0.5-ms cathodal pulses, 50 Hz, 25-75 microA, 7-9-s train duration) separated by 20 s were delivered every 90 s. When the priming and test stimulations were applied to the same site, the priming effects were similar along the entire extent of the medial forebrain bundle. When the priming and test sites were different, the priming effect depended on their relative positions. Anterior stimulation primed posterior sites at magnitude comparable to those produced by stimulating the same posterior site. Posterior stimulation primed anterior sites at a level half of that produced by stimulation of the same anterior site. This pattern was found for priming and test sites that were ipsilateral and contralateral. Priming is a general and robust phenomenon with properties that may be useful for studying locomotor initiation pathways.

Preoptic and hypothalamic neurons and the initiation of locomotion in the anesthetized rat

Despite its insensate condition and apparent motoric depression, the anesthetized rat can provide useful information about the systems involved in locomotor initiation. The preparation appears to be particularly appropriate for the study of the appetitive locomotor systems and may be more limited for the study of the circuits involved in exploratory and defensive locomotion. In the anesthetized rat, pharmacological evidence indicates that the preoptic basal forebrain contains neurons which initiate locomotor stepping. Mapping with low levels of electrical stimulation indicates, but does not prove, that a region centered in the lateral preoptic area might be the location of these neurons. Several lines of evidence indicate that locomotor stepping elicited by electrical stimulation of the hypothalamus is mediated by neurons in the perifornical and lateral hypothalamus. Locomotor effects of hypothalamic stimulation persist in the absence of descending fibers of passage from the ipsilateral preoptic locomotor regions but are severely impaired by kainic acid lesions in the area of stimulation. Injections of glutamate into the perifornical and lateral hypothalamus elicit locomotor stepping at short latencies. Anatomical evidence suggests that the two regions are components of a network for appetitive locomotion. The recognition that multiple systems initiate locomotion both clarifies and complicates the study of locomotion. It provides a framework that incorporates disparate findings but it also underscores the need for increased attention to behavioral issues in studies of locomotor circuitry.

Microstimulation mapping of the basal forebrain in the anesthetized rat: the "preoptic locomotor region"

Previous studies have indicated that the basal forebrain at the level of the preoptic area contains neurons which participate in the initiation of locomotion. This study attempted to localize those neurons by mapping sites at which 25- and 50-microA stimulation (50 Hz, 0.5 ms cathodal pulses, 10-s trains) initiated hindlimb stepping. Anesthetized rats were held in a stereotaxic apparatus supported by a sling so that stepping movements rotated a wheel. Anesthesia was maintained by periodic injections of Nembutal (7 mg/kg) supplemented by lidocaine injections. Stimulation was applied through 50-70-microns diameter pipettes filled with 2 M NaCl at approximately 1600 sites in the basal forebrain, adjacent thalamus, and striatum. A circumscribed grouping of 25-microA locomotor sites, centered in the lateral preoptic area, defined the preoptic locomotor region. It extended into the ventral bed nucleus of the stria terminalis, the lateral part of the medial preoptic area, the anterior hypothalamic area, the medial and rostral parts of the ventral pallidum, medial substantia innominata, and the horizontal limb of the diagonal band. This general region is known to project to the midbrain locomotor region and the ventral tegmental area; it is proposed to initiate locomotion in service of primary motivational systems. Among the structures generally negative for locomotor sites were the dorsal and ventral striata, septal complex, bed nucleus of stria terminalis, and lateral ventral pallidum and substantia innominata. These findings indicate that low current stimulation applied to a circumscribed area centered in the lateral preoptic area produces locomotor stepping in the anesthetized rat. Whether the activated elements in this preoptic locomotor region are cells or fibers is not yet known. The degree of localization afforded by these findings indicates that the areas that are most likely to contain the mediating elements are quite limited in extent.

Locomotor sites mapped with low current stimulation in intact and kainic acid damaged hypothalamus of anesthetized rats

To determine whether local neurons mediated the locomotor effects of electrical stimulation of the lateral hypothalamus, kainic acid injections (0.5-1.25 micrograms), intended to destroy neural somata as opposed to fibers of passage, were made unilaterally in the tuberal-posterior hypothalamus of 22 rats. The area of lesion and its contralateral homolog were mapped for locomotor stepping sites in Nembutal-anesthetized rats mounted in a stereotaxic apparatus such that locomotor stepping rotated a wheel. Stimulation (25 and 50 microA, 50 Hz, 0.5-ms cathodal pulses, 10-s trains) was delivered through 50-80 microns glass pipettes filled with 2 M saline. Contralateral to the lesion, locomotor stepping sites were common in the perifornical lateral and medial hypothalamus and less dense in the zona incerta. On the side of the kainic-acid lesion, locomotor sites were generally absent in the central part of the damaged area. If they did appear within the area of lesion, they tended to be near the border with intact tissue. In a few cases, locomotor stepping sites were found centrally located in the lesion amidst widespread loss of somata. In four rats, additional maps of anterior locomotor regions in the preoptic area ipsilateral to the lesion suggested that their descending fibers were largely spared by the kainic lesions. Local neurons appear to be major contributors to the locomotion elicited by electrical stimulation of the lateral hypothalamus, but fibers of passage may also participate.

Latency to initiate locomotion elicited by stimulation of the diencephalon positively correlates in awake and anesthetized rats

Locomotor stepping can be elicited by brain stimulation at various diencephalic sites under moderate levels of Nembutal. This study determined if locomotor initiation measured under anesthesia provides a valid measure of the intersite factors which determine initiation in the awake condition. We compared the latencies to initiate locomotor stepping elicited by electrical stimulation (50 microA, 0.5-msec pulses, 10 to 160 Hz) by rats tested while awake and unrestrained in a rotary runway or anesthetized and held in a stereotaxic apparatus. In the latter tests, initial anesthesia was provided by Nembutal (25 mg/kg) and 2% halothane and maintenance anesthesia was provided by 7 mg/kg as needed and local injections of lidocaine. For 30 sites in 16 rats, average locomotor initiation latency in the awake condition and the shortest latencies in the anesthetized condition were positively correlated (r = .78). Locomotion at sites with long latencies in the awake condition was frequently blocked in the anesthetized condition, but sites with short latencies were rarely blocked. The results indicate that the shortest locomotor latencies in the anesthetized condition approximate the latencies measured in the awake condition. It is concluded that the anesthetized condition can provide valid initiation measures, but sites with long latencies in the awake condition are prone to depression under anesthesia.