Midbrain areas required for locomotion initiated by electrical stimulation of the lateral hypothalamus in the anesthetized rat

The priming effect of rewarding brain stimulation in rats depends on both the cost and strength of reward but survives blockade of D2-like dopamine receptors

Receipt of an intense reward boosts motivation to work for more of that reward. This phenomenon is called the priming effect of rewards. Using a novel measurement method, we show that the priming effect of rewarding electrical brain stimulation depends on the cost, as well as on the strength, of the anticipated reward. Previous research on the priming effect of electrical brain stimulation utilized a runway paradigm in which running speed serves as the measure of motivation. In the present study, the measure of motivation was the vigour with which rats executed a two-lever response chain, in a standard operant-conditioning chamber, to earn rewarding electrical stimulation of the lateral hypothalamus. In a second experiment, we introduced a modification that entails self-administered priming stimulation and alternating blocks of primed and unprimed trials. Reliable, consistent priming effects of substantial magnitude were obtained in the modified paradigm, which is closely analogous to the runway paradigm. In a third experiment, the modified paradigm served to assess the dependence of the priming effect on dopamine D2-like receptors. The priming effect proved resilient to the effect of eticlopride, a selective D2-like receptor antagonist. These results are discussed within the framework of a new model of brain reward circuitry in which non-dopaminergic medial forebrain bundle fibers and dopamine axons provide parallel inputs to the final common paths for reward and incentive motivation.

siRNA silencing of estrogen receptor-α expression specifically in medial preoptic area neurons abolishes maternal care in female mice

The medial preoptic area has been shown to be intricately involved in many behaviors, including locomotion, sexual behavior, maternal care, and aggression. The gene encoding estrogen receptor-α (ERα) protein is expressed in preoptic area neurons, and a very dense immunoreactive field of ERα is found in the preoptic region. ERα knockout animals show deficits in maternal care and sexual behavior and fail to exhibit increases in these behaviors in response to systemic estradiol treatment. In the present study, we used viral-vector mediated RNA interference to silence ERα expression specifically in the preoptic area of female mice and measured a variety of behaviors, including social and sexual aggression, maternal care, and arousal activity. Suppression of ERα in the preoptic area almost completely abolished maternal care, significantly increasing the latency to pup retrieval and significantly reducing the time the moms spent nursing and licking the pups. Strikingly, maternal aggression toward a male intruder was not different between control and preoptic ERα-silenced mice, demonstrating the remarkably specific role of ERα in these neurons. Reduction of ERα expression in preoptic neurons significantly decreased sexual behavior in female mice and increased aggression toward both sexual partners and male intruders in a seminatural environment. Estrogen-dependent increases in arousal, measured by home cage activity, were not mediated by ERα expression in the preoptic neurons we targeted, as ERα-suppressed mice had increases similar to control mice. Thus, we have established that a specific gene in a specific group of neurons is required for a crucially important natural behavior.

Relationship of arousal to circadian anticipatory behavior: ventromedial hypothalamus: one node in a hunger-arousal network

The mechanisms by which animals adapt to an ever-changing environment have long fascinated scientists. Different forces, conveying information regarding various aspects of the internal and external environment, interact with each other to modulate behavioral arousal. These forces can act in concert or, at times, in opposite directions. These signals eventually converge and are integrated to influence a common arousal pathway which, depending on all the information received from the environment, supports the activation of the most appropriate behavioral response. In this review we propose that the ventromedial hypothalamic nucleus (VMN) is part of the circuitry that controls food anticipation. It is the first nucleus activated when there is a change in the time of food availability, silencing of VMN ghrelin receptors decreases food-anticipatory activity (FAA) and, although lesions of the VMN do not abolish FAA, parts of the response are often altered. In proposing this model it is not our intention to exclude parallel, redundant and possibly interacting pathways that may ultimately communicate with, or work in concert with, the proposed network, but rather to describe the neuroanatomical requirements for this circuit and to illustrate how the VMN is strategically placed and connected to mediate this complex behavioral adaptation.

Electrical stimulation in the lateral hypothalamus in rats in the activity-based anorexia model

Object

One quarter of patients with anorexia nervosa have a poor outcome and continue to suffer chronically or die. Electrical brain stimulation may be of therapeutic benefit in some of these patients; however, the brain target for inducing symptom relief is unknown. In this study, the authors evaluated the effects of acute and chronic electrical stimulation in the lateral hypothalamus on food intake, locomotor activity, and survival time in rats in an activity-based anorexia model.

Methods

In an acute experiment, the authors electrically stimulated at 100 Hz and 0, 25, 50 and 75% of the maximal stimulation amplitude (that is, the amplitude leading to severe side effects) in the lateral hypothalamus on consecutive days during 4 test sessions in 10 rats and evaluated food intake and locomotor activity. In a chronic experiment, they compared food intake, wheel revolutions, and survival time between 6 rats that underwent electrical stimulation in the lateral hypothalamus (50% of maximal stimulation amplitude) and 8 rats that did not undergo stimulation.

Results

In the acute experiment, overall electrical stimulation (25, 50, and 75% combined) and stimulation at 75% of the maximal stimulation amplitude significantly decreased the locomotor activity. However, if the authors omitted results of 1 rat, in which the electrode tip was not located in the lateral hypothalamus on one side but rather in the supraoptic chiasm, the remaining results did not yield significance. No other differences were observed.

Conclusions

When the findings of the current study are extrapolated to patients with anorexia nervosa, the authors do not expect major effects on symptoms with electrical stimulation at high frequency in the lateral hypothalamus.

Movements and muscle activity initiated by brain locomotor areas in semi-intact preparations from larval lamprey

In in vitro brain/spinal cord preparations from larval lamprey, locomotor-like ventral root burst activity can be initiated by pharmacological (i.e., "chemical") microstimulation in several brain areas: rostrolateral rhombencephalon (RLR); dorsolateral mesencephalon (DLM); ventromedial diencephalon (VMD); and reticular nuclei. However, the quality and symmetry of rhythmic movements that would result from this in vitro burst activity have not been investigated in detail. In the present study, pharmacological microstimulation was applied to the above brain locomotor areas in semi-intact preparations from larval lamprey. First, bilateral pharmacological microstimulation in the VMD, DLM, or RLR initiated symmetrical swimming movements and coordinated muscle burst activity that were virtually identical to those during free swimming in whole animals. Unilateral microstimulation in these brain areas usually elicited asymmetrical undulatory movements. Second, with synaptic transmission blocked in the brain, bilateral pharmacological microstimulation in parts of the anterior (ARRN), middle (MRRN), or posterior (PRRN) rhombencephalic reticular nucleus also initiated symmetrical swimming movements and muscle burst activity. Stimulation in effective sites in the ARRN or PRRN initiated higher-frequency locomotor movements than stimulation in effective sites in the MRRN. Unilateral stimulation in reticular nuclei elicited asymmetrical rhythmic undulations or uncoordinated movements. The present study is the first to demonstrate in the lamprey that stimulation in higher-order locomotor areas (RLR, VMD, DLM) or reticular nuclei initiates and sustains symmetrical, well-coordinated locomotor movements and muscle activity. Finally, bilateral stimulation was a more physiologically realistic test of the function of these brain areas than unilateral stimulation.

MCH-containing neurons in the hypothalamus of the cat: searching for a role in the control of sleep and wakefulness

Neurons that utilize melanin-concentrating hormone (MCH) and others that employ hypocretin as neurotransmitter are located in the hypothalamus and project diffusely throughout the CNS, including areas that participate in the generation and maintenance of the states of sleep and wakefulness. In the present report, immunohistochemical methods were employed to examine the distribution of MCHergic and hypocretinergic neurons. In order to test the hypothesis that the MCHergic system is capable of influencing specific behavioral states, we studied Fos immunoreactivity in MCH-containing neurons during (1) quiet wakefulness, (2) active wakefulness with motor activity, (3) active wakefulness without motor activity, (4) quiet sleep and (5) active sleep induced by carbachol (AS-carbachol). We determined that MCHergic neuronal somata in the cat are intermingled with hypocretinergic neurons in the dorsal and lateral hypothalamus, principally in the tuberal and tuberomammillary regions; however, hypocretinergic neurons extended more in the anterior-posterior axis than MCHergic neurons. Axosomatic and axodendritic contacts were common between these neurons. In contrast to hypocretinergic neurons, which are known to be active during motor activity and AS-carbachol, Fos immunoreactivity was not observed in MCH-containing neurons in conjunction with any of the preceding behavioral conditions. Non-MCHergic, non-hypocretinergic neurons that expressed c-fos during active wakefulness with motor activity were intermingled with MCH and hypocretin-containing neurons, suggesting that these neurons are related to some aspect of motor function. Further studies are required to elucidate the functional sequela of the interactions between MCHergic and hypocretinergic neurons and the phenotype of the other neurons that were active during motor activity.

Modulatory effect of substance P to the brain stem locomotor command in lampreys

Substance P initiates locomotion when injected in the brain stem of mammals. This study examined the possible role of this peptide on the supraspinal locomotor command system in lampreys. Substance P was bath applied or locally injected into an in vitro isolated brain stem, and the effects of the drug were examined on reticulospinal cells and on the occurrence of swimming in a semi-intact preparation. Bath applications of substance P induced sustained depolarizations occurring rhythmically in intracellularly recorded reticulospinal cells. Spiking activity was superimposed on the depolarizations and swimming was induced. The sustained depolarizations were abolished by tetrodotoxin, and substance P did not affect the membrane resistance of reticulospinal cells nor their firing properties, suggesting that it did not directly effect reticulospinal cells. To establish where the effects were exerted, successive lesions of the brain stem were made as well as local applications of the drug in the brain stem. Removing the mesencephalon abolished the sustained depolarizations, whereas large ejections of the drug in the mesencephalon excited reticulospinal cells and elicited bouts of swimming. More local injections into the mesencephalic locomotor region (MLR) also elicited swimming. After an injection of substance P, the current threshold needed to induce locomotion by MLR stimulation was decreased, and the size of the postsynaptic responses of reticulospinal cells to MLR stimulation was increased. Substance P also reduced the frequency of miniature spontaneous postsynaptic currents in reticulospinal cells. Taken together, these results suggest that substance P plays a neuromodulatory role on the brain stem locomotor networks of lampreys.

Distribution of melanin-concentrating hormone neurons projecting to the medial mammillary nucleus

The melanin-concentrating hormone and neuropeptide glutamic acid-isoleucine are expressed in neurons located mainly in the hypothalamus that project widely throughout the CNS. One of the melanin-concentrating hormone main targets is the medial mammillary nucleus, but the exact origin of these fibers is unknown. We observed melanin-concentrating hormone and neuropeptide glutamic acid-isoleucine immunoreactive fibers coursing throughout the mammillary complex, showing higher density in the pars lateralis of the medial mammillary nucleus, while the lateral mammillary nucleus showed sparse melanin-concentrating hormone innervation. The origins of these afferents were determined by using implant of the retrograde tracer True Blue in the medial mammillary nucleus. Double-labeled neurons were observed in the lateral hypothalamic area, rostromedial zona incerta and dorsal tuberomammillary nucleus. A considerable population of retrogradely labeled melanin-concentrating hormone perikaryal profiles was also immunoreactive to neuropeptide glutamic acid-isoleucine (74+/-15% to 85+/-15%). The afferents from the lateral hypothalamic area, rostromedial zona incerta and dorsal tuberomammillary nucleus to the medial mammillary nucleus were confirmed using implant of the anterograde tracer Phaseolus vulgaris leucoagglutinin. In addition, using double-labeled immunohistochemistry, we found no co-localization between neurons expressing melanin-concentrating hormone and adenosine deaminase (histaminergic marker) in the dorsal tuberomammillary nucleus. We hypothesize that these melanin-concentrating hormone projections participate in spatial memory process mediated by the medial mammillary nucleus. These pathways would enable the animal to look for food during the initial moments of appetite stimulation.

Muscle tone facilitation and inhibition after orexin-a (hypocretin-1) microinjections into the medial medulla

Orexins/hypocretins are synthesized in neurons of the perifornical, dorsomedial, lateral, and posterior hypothalamus. A loss of hypocretin neurons has been found in human narcolepsy, which is characterized by sudden loss of muscle tone, called cataplexy, and sleepiness. The normal functional role of these neurons, however, is unclear. The medioventral medullary region, including gigantocellular reticular nucleus, alpha (GiA) and ventral (GiV) parts, participates in the induction of locomotion and muscle tone facilitation in decerebrate animals and receives moderate orexinergic innervation. In the present study, we have examined the role of orexin-A (OX-A) in muscle tone control using microinjections (50 microM, 0.3 microl) into the GiA and GiV sites in decerebrate rats. OX-A microinjections into GiA sites, previously identified by electrical stimulation as facilitating hindlimb muscle tone bilaterally, produced a bilateral increase of muscle tone in the same muscles. Bilateral lidocaine microinjections (4%, 0.3 microl) into the dorsolateral mesopontine reticular formation decreased muscle rigidity and blocked muscle tone facilitation produced by OX-A microinjections into the GiA sites. The activity of cells related to muscle rigidity, located in the pedunculopontine tegmental nucleus and adjacent reticular formation, was correlated positively with the extent of hindlimb muscle tone facilitation after medullary OX-A microinjections. OX-A microinjections into GiV sites were less effective in muscle tone facilitation, although these sites produced a muscle tone increase during electrical stimulation. In contrast, OX-A microinjections into the gigantocellular nucleus (Gi) sites and dorsal paragigantocellular nucleus (DPGi) sites, previously identified by electrical stimulation as inhibitory points, produced bilateral hindlimb muscle atonia. We propose that the medioventral medullary region is one of the brain stem target for OX-A modulation of muscle tone. Facilitation of muscle tone after OX-A microinjections into this region is linked to activation of intrinsic reticular cells, causing excitation of midbrain and pontine neurons participating in muscle tone facilitation through an ascending pathway. Moreover, our results suggest that OX-A may also regulate the activity of medullary neurons participating in muscle tone suppression. Loss of OX function may, therefore, disturb both muscle tone facilitatory and inhibitory processes at the medullary level.

Brainstem regions with neuronal activity patterns correlated with priming of locomotor stepping in the anesthetized rat

Locomotor stimulation in the perifornical hypothalamus produces a transient facilitation of subsequent locomotion, a priming effect, such that stepping to a second train of stimulation occurs with a shorter latency of onset and increased amplitude. Neurons responsible for the initiation of this facilitated stepping presumably respond to locomotor stimulation with a similar priming effect, i.e. either a shorter latency or a larger change in activity rate. This study used anesthetized rats (urethane, 800mg/kg) to compare brainstem regions in terms of the relative rates of occurrence of single neurons that showed both specific responses to locomotor stimulation and also priming effects. Specific responses were characterized by a progressive increase in activity prior to the first step (a Type I pattern). In that they co-varied in time with the increased probability of stepping onset, Type I responses were more specific than Type II responses, which peaked early in the stimulation train several seconds before the onset of stepping. Regions with high proportions of neurons showing Type I responses and priming effects included the anterior dorsal tegmentum lateral to the central gray, the oral pontine reticular nucleus and the medial gigantocellular nucleus. Few Type I neurons showed a modulation of activity related to the step cycle. Type I primed neurons were uncommon in the cuneiform and the pedunculopontine regions, but neurons showing other patterns (decreases and antidromic responses) were relatively prevalent there. The ventral tegmental area was generally unresponsive. The results indicate that stepping elicited by perifornical stimulation in the anesthetized rat is mediated by circuits that differ at midbrain levels from the circuits implicated in other types of locomotion. Two regions, the anterior dorsal tegmentum and the oral pontine reticular nucleus, warrant further attention to determine their possible roles in the initiation of locomotion.

Hippocampal theta activity and facilitated locomotor stepping produced by GABA injections in the midbrain raphe region

Inactivation of neurons in the midbrain raphe region produces increases in locomotor activity, and it appears that they function to suppress locomotion. Inactivation of neurons there also produces hippocampal slow wave (theta) activity and it appears that they also function to inhibit rhythmic activity in the hippocampus. We determined whether the degree of association between the two effects was consistent with the operation of a single mechanism. Stimulation electrodes were implanted into locomotor sites of the hypothalamus of 34 urethane-anesthetized rats. Hindlimb stepping was elicited by 5.12-s trains of perifornical electrical stimulation presented once per minute. Hippocampal theta activity was recorded across the CA1 layer of the dorsal hippocampus. GABA injections were used to locate raphe sites at which neuronal inactivation influenced stepping and hippocampal activity. A glass pipette (80-microm tip) was inserted into the midbrain, and injections of GABA (50-100 mg/0.1-0.2 microl) were made in 70 sites in the midbrain. Injections at 34 sites facilitated stimulation-elicited stepping, and at 17 sites, they also produced intertrial stepping. Facilitating injections, but not ineffective or suppressive injections, increased the mean peak frequency of hippocampal activity, and increased power in the 4-5 Hz band during the period that preceded the stimulation trains, but did not change the 5-6 Hz activity produced during the stimulation trains. Priming locomotor stimulation which also facilitated stepping produced generally similar increases in pre-stimulation peak frequency and 4-Hz power. The magnitudes of the increases in stepping and 4-Hz power were uncorrelated. The increase in 4-Hz power appeared earlier than the increase in stepping in 18 of 34 cases, and later in 11 cases; no increases in 4-Hz power were apparent in five cases. The results indicate that pre-locomotor 4-Hz hippocampal activity in the urethane-anesthetized rat is loosely coupled with facilitated locomotor initiation. Neurons in the midbrain raphe region appear to suppress both processes, but the low degree of association between the magnitudes and onset times of increases in stepping and hippocampal 4-Hz power indicate the operation of multiple mechanisms.

GABA injected into the anterior dorsal tegmentum (ADT) of the midbrain blocks stepping initiated by stimulation of the hypothalamus

Previous work showed that the activity rates of certain neurons in the anterior dorsal tegmentum (ADT) of the midbrain correlated with the onset of stepping elicited by hypothalamic stimulation. This study determined if reversible inactivation of the ADT would block locomotion elicited by hypothalamic stimulation of anesthetized rats (urethane, 800 mg/kg). GABA (concentrations 0.25-1.0 mg/microl in saline) were injected in 52 sites in 21 rats. GABA at volumes of 0.1 or 0.2 microl blocked hindlimb stepping in 18 cases. Locomotor blocks occurred within 5 min of the injection, and typically recovered within 10-20 min. The effective blocking sites were clustered around the interstitial nucleus of the medial longitudinal fasciculus. Sites more dorsal and more anterior were not as effective as sites in and ventral to this nucleus. The data are consistent with a role for the ADT of the midbrain in locomotor initiation.

Study of the origins of melanin-concentrating hormone and neuropeptide EI immunoreactive projections to the periaqueductal gray matter

Previous studies have described the distribution of melanin-concentrating hormone (MCH) and neuropeptide EI (NEI) in the rat central nervous system (CNS), and revealed this peptidergic system to be primarily localized in neurons within the lateral hypothalamic area (LHA) and zona incerta (ZI). Moreover, an extensive MCH- and NEI-immunoreactive (ir) fiber distribution has been described throughout the CNS, including a dense innervation within the periaqueductal gray matter (PAG). MCH and NEI have become important markers for the LHA, which harbors a variety of neuronal types as well as the medial forebrain bundle, a complex system of fibers which extends rostrocaudally throughout this area. In the present study, the projection patterns of MCH- and NEI-ir fibers within the PAG were characterized using a diamino benzidine immunoperoxidase procedure to localize each of these peptides in normal rat brain sections. MCH- and NEI-ir fibers were seen coursing through all of its subdivisions the entire length of the PAG, with a more condensed number of fibers in the periaqueductal medial zone. The primary origin(s) of these PAG afferents were determined in combined retrograde tracing immunofluorescent studies in which true blue (TB) was injected into various subdivisions of the PAG. TB-filled MCH-ir neurons were identified mainly in the rostral portion of the medial ZI (ZIm) and in the tuberal LHA (LHAt). Studies confirming this MCH-ir projection in which anterograde tracer (Phaseolus vulgaris leucoagglutinin) was injected into various regions in and around the LHA and ZI revealed a distinction in the PAG projections arising from these nuclei. ZIm injections resulted in labeled fibers mainly within the rostral dorsomedial and dorsolateral regions of the PAG, whereas injections in the LHAt revealed an innervation at intermediate and caudal levels in the ventrolateral region. Since the MCH and NEI fiber distribution patterns in the PAG are identical, this would suggest that these peptides are colocalized within the hypothalamus. Sequential immunofluorescent staining for MCH and NEI on tissue from rats who had received TB injections into the PAG confirmed this, and revealed that approximately 15% of all tracer-filled neurons in the LHA and ZI were both MCH- and NEI-ir. In fact, the vast majority of MCH-ir neurons within these regions also colocalize with NEI. Therefore, the MCH/NEI projection patterns within the PAG arise from two major sources: the ZIm which supplies afferents via a medial pathway that enters the PAG dorsally at rostral levels, and a pathway originating in the LHA that enters the PAG ventrally at more caudal levels. The ZIm and LHA are believed to be the primary, if not the only, sources of MCH and NEI projections to the PAG.

Organisation of the reticular thalamic projection to the intralaminar and midline nuclei in rats

This study examines the projection of the reticular thalamic nucleus to the classic "nonspecific" dorsal thalamic nuclei of rats. Individual nuclei of the intralaminar (central-lateral, paracentral, central-medial, parafascicular) and the midline (reuniens/rhomboid, parataenial) nuclear groups, together with the reticular nucleus itself, were injected with the neuronal tracers biotinylated dextran or fluorescent latex microspheres (red or green). Reticular cells projecting to the intralaminar and midline nuclei are limited largely to the rostral pole of the nucleus. Within the rostral pole, most reticular cells projecting to the intralaminar and midline nuclear groups are found in largely distinct sectors; cells that project to the intralaminar nuclei tend to lie more laterally, whereas those projecting to the midline nuclei lie more medially within the pole. Among the individual nuclei of both the intralaminar and midline nuclear groups, however, the segregation is far less distinct. For instance, the reticular cells that project to the intralaminar central-lateral, central-medial, paracentral, and parafascicular nuclei are intermixed completely on the lateral edge of the rostral pole. After separate injections of different colored latex microspheres into individual intralaminar nuclei, the incidence of double-labelled reticular cells is about 37%, a percentage much higher than among the "specific" dorsal thalamic nuclei (< 1%). All the above-mentioned results refer to the reticular labelling seen on the side ipsilateral to the injection. After separate injections into the intralaminar central-medial nucleus, the midline nuclei, and the reticular nucleus itself, we also see a very small group of reticular cells labelled on the contralateral side. In general, our results indicate that the reticular projection to the intralaminar and midline nuclei is far more diffuse than the reticular projection to the specific dorsal thalamic nuclei.

Descending projections of the posterior nucleus of the hypothalamus: Phaseolus vulgaris leucoagglutinin analysis in the rat

No previous report in any species has systematically examined the descending projections of the posterior nucleus of the hypothalamus (PH). The present report describes the descending projections of the PH in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin. PH fibers mainly descend to the brainstem through two routes: dorsally, within the central tegmental tract, and ventromedially, within the mammillo-tegmental tract and its caudal extension, ventral reticulo-tegmental tracts. PH fibers were found to distribute densely to several nuclei of the brainstem. They are (from rostral to caudal) 1) lateral/ ventrolateral regions of the diencephalo-mesopontine periaqueductal gray (PAG); 2) the peripeduncular nucleus; 3) discrete nuclei of pontomesencephalic central gray (dorsal raphe nucleus, laterodorsal tegmental nucleus, and Barrington's nucleus); 4) the longitudinal extent of the central core of the mesencephalic through meduallary reticular formation (RF); 5) the ventromedial medulla (nucleus gigantocellularis pars alpha, nucleus raphe magnus, and nucleus raphe pallidus); 6) the ventrolateral medulla (nucleus reticularis parvocellularis and the rostral ventrolateral medullary region); and 7) the inferior olivary nucleus. PH fibers originating from the caudal PH distribute much more heavily than those from the rostral PH to the lower brainstem. The PH has been linked to the control of several important functions, including respiration, cardiovascular activity, locomotion, antinociception, and arousal/wakefulness. It is likely that descending PH projections, particularly those to the PAG, the pontomesencephalic RF, Barrington's nucleus, and parts of the ventromedial and ventrolateral medulla, serve a role in a PH modulation of complex behaviors involving integration of respiratory, visceromotor, and somatomotor activity.

Neural activity in the midbrain correlated with hindlimb extension initiated by locomotor stimulation of the hypothalamus of the anesthetized rat

Midbrain neuronal activity that correlated with the initiation of locomotion produced by hypothalamic stimulation was studied. Locomotion was elicited by electrical stimulation in the perifornical hypothalamus of 59 rats anesthetized with Nembutal. The first hindlimb extension indexed stepping onset. Single and multiple neurons were recorded ipsilateral to the stimulation site at 2230 sites in the anterior and posterior midbrain. To classify responses, activity patterns averaged around stimulation onset and around the extension onset were examined. Responses with specific correlations to extension onset were Type I; responses not specifically related to the extension onset were Type II. In the anterior midbrain, 6% of sites were Type I and 8% were Type II. The larger Type I responses were frequent in the anterior tegmentum near the central gray. The relative frequency of Type I patterns in the posterior ventrolateral tegmentum was similar. Other regions showed relatively more Type II responses; they included the ventral tegmental area, and the regions near the superior cerebellar peduncle and the posterior central gray. Regional population profiles showed that during the initiation of locomotion, neurons in the posterior peribrachial region responded early and neurons in the anterior dorsal and the posterior ventrolateral tegmentum responded later. The initiation-related activity of Type I neurons in the anterior and posterior midbrain tegmentum suggest that they warrant further study for a role in locomotor initiation.

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.

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

At moderate levels of Nembutal, within the anesthetic range, locomotor stepping can be elicited by brain stimulation. We determined if Nembutal (7, 14 and 28 mg/kg) had different effects on locomotion elicited by stimulation at different brain regions. Two regions were compared: the medial forebrain bundle (MFB, 13 sites) and the areas medial and dorsal to it (MED/DORSAL, 20 sites). Locomotion was produced by electrical stimulation (50 microA, 0.5 msec pulses, 10 to 160 Hz) of unrestrained rats in a rotary runway. The latency to initiate locomotion and the time to complete 1 revolution of the rotary were measured. With no drug, MFB locomotion was initiated sooner but took longer to complete than MED/DORSAL locomotion. Nembutal at 7 mg/kg did not affect initiation of MFB or MED/DORSAL locomotion. Nembutal at 14 mg/kg shortened MFB initiations, but this dose prolonged MED/DORSAL initiations. Initiations with both types of sites were blocked with 28 mg/kg. The 7 and 14 mg/kg doses prolonged the locomotor completion times of the MFB sites but not of the MED/DORSAL sites. The results indicate that the response to Nembutal differs qualitatively for locomotion elicited by stimulation of the MFB and locomotion elicited by stimulation of the medial and dorsal hypothalamus. The mechanisms underlying the difference remain to be elucidated; they may relate to nonlocomotor behaviors also elicited by stimulation or to the motivational states reflected in those behaviors.