Microinfusion Of Muscimol Research Articles

A Possible Neural Basis for Attentional Capture of Faces Revealed by Functional Magnetic Resonance Imaging and Causal Pharmacological Inactivation in Macaques

Abstract In primates, the presence of a face in a visual scene captures attention and rapidly directs the observer's gaze to the face, even when the face is not relevant to the task at hand. Here, we explored a neural circuit that might potentially play a causal role in this powerful behavior. In our previous research, two monkeys received microinfusions of muscimol, a γ-aminobutyric acid type A (GABAA)-receptor agonist, or saline (as a control condition) in separate sessions into individual or pairs of four inferotemporal face patches (middle and anterior lateral and fundal), as identified by an initial localizer experiment. Then, using fMRI, we measured the impact of each inactivation condition on responses in the other face patches relative to the control condition. In this study, we used the same method and measured the impact of each inactivation condition on responses in the FEF and the lateral intraparietal area, two regions associated with attentional processing, while face and nonface object stimuli were viewed. Our results revealed potential relationships between inferotemporal face patches and these two attention-related regions: The inactivation of the middle lateral and anterior fundal face patches had a pronounced impact on FEF, whereas the inactivation of the middle and anterior lateral face patches had a noticeable influence on LIP. Together, these initial exploratory findings document a circuit that potentially underlies the attentional capture of faces. Confirmation of the role of this circuit remains to be accomplished in the context of a paradigm that explicitly tests the attentional capture of faces.

Inactivation of the dorsal CA1 hippocampus impairs the consolidation of discriminative avoidance memory by modulating the intrinsic and extrinsic hippocampal circuitry

Despite progress in understanding the role of the dorsal hippocampus in the acquisition, consolidation and retrieval of episodic-like memory, plastic changes within the intra- and extrahippocampal circuits for aversive memory formation and anxiety-like behaviours must still be identified since both processes contribute to multiple aspects of flexible decision-making. Here, we investigated the effect of reversible inactivation induced by a muscimol microinfusion into the dorsal CA1 subfield (dCA1) either prior to acquisition or to retrieval testing of a discriminative avoidance task performed in a plus-maze apparatus (PM-DAT). Differential cAMP-response-element-binding protein 1 (CREB-1) expression in the dorsal and ventral CA1 and CA3 of the hippocampus (dCA1, dCA3, vCA1, and vCA3), dorsal dentate gyrus (dDG), and infralimbic (IL) and prelimbic (PrL) regions of the medial prefrontal cortex was also assessed to investigate the molecular changes associated with the consolidation or retrieval of episodic-like memory and anxiety. Adult male Wistar rats were assigned to two control groups, learning (no surgery/no microinfusion, n = 7) and sham-operated (sham surgery/no microinfusion, n = 6) groups, or four experimental groups, in which the vehicle (0.5 µl per side, n = 8/per group) or a GABAA receptor agonist (0.5 µg/0.5 µl muscimol/per side) was bilaterally microinfused in the dCA1 30 min prior to training (n = 9) or prior to testing sessions (n = 6) with a 24 h intertrial interval. Memory was evaluated using the percentage of time spent in the nonaversive enclosed arms, whereas anxiety was measured by calculating the percentages of time spent and entries into open arms and the percentage of time spent self-grooming. Our findings corroborated previous data showing that the dCA1 is required for discriminative avoidance consolidation. Furthermore, additional information indicated that impaired long-term memory was associated with downregulated CREB-1 expression in the dDG and vCA3. Moreover, memory retrieval was not impaired by dCA1 inactivation prior to the testing session, which was associated with the upregulation of CREB-1 in the dCA3 and vCA1 and downregulation in the dCA1 and vCA3. Differential expression of CREB was not identified in the IL or PrL areas. These results improve our understanding of how the hippocampal circuitry mediates the acquisition and retrieval of aversive memory and anxiety.

GABA-A receptor signaling in the anterior cingulate cortex modulates aggression and anxiety-related behaviors in socially isolated mice

Dysfunctional modulation of brain circuits that regulate the emotional response to potentially threatening stimuli is associated to an inappropriate representation of the emotional salience. Reduced top-down control by cortical areas is assumed to underlie several behavioral abnormalities including aggression and anxiety related behaviors. Previous studies have identified disrupted GABA signaling in the anterior cingulate cortex (ACC) as a possible mechanism underlying the top-down regulation of aggression and anxiety. In this study, we investigate a role for GABA-A receptor in the ACC in the regulation of aggression and anxiety related behaviors in socially isolated mice. We evaluated the effects of site directed injections of the GABA-A receptor agonist, muscimol or the GABA-A receptor antagonist, bicuculline into the ACC on these behaviors.Results showed that hyper-aggressive behavior, the anxiety and avoidance behavior in socially isolated mice were increased by muscimol microinfusion into ACC, while the sociability was not affected. In contrast, hyper-aggressive behavior in socially isolated mice was inhibited following bicuculline microinfusion without affecting anxiety. Furthermore, microinfusion of bicuculline into ACC decreased avoidance intensity and significantly reinforced social behavior, suggesting that GABA-A receptor inhibition in ACC specifically regulated aggression and sociability. Together, our results confirm a role for GABA-A receptor signaling in the ACC in the regulation of aggressive, social and anxiety related behaviors in socially isolated mice.

Substantia nigra pars reticulata-mediated sleep and motor activity regulation.

The substantia nigra pars reticulata (SNR) is a major output nucleus of the basal ganglia. Animal studies have shown that lesions of the SNR cause hyposomnia and motor hyperactivity, indicating that the SNR may play a role in the control of sleep and motor activity. Eight 8- to 10-week-old adult male Sprague-Dawley rats were used. After 3 days of baseline polysomnographic recording, dialysates were collected from the lateral SNR across natural sleep-wake states. Muscimol and bicuculline were microinfused into the lateral SNR. We found that GABA release in the lateral SNR is negatively correlated with slow wave sleep (SWS; R = -0.266, p < 0.01, n = 240) and positively correlated with waking (R = 0.265, p < 0.01, n = 240) in rats. Microinfusion of muscimol into the lateral SNR decreased sleep time and sleep quality, as well as eliciting motor hyperactivity in wake and increased periodic leg movement in SWS, while bicuculline infused into the lateral SNR increased sleep and decreased motor activity in SWS in rats. Muscimol infusion skewed the distribution of inter-movement intervals, with most between 10 and 20 s, while a flat distribution of intervals between 10 and 90 s was seen in baseline conditions. Activation of the lateral SNR is important for inducing sleep and inhibiting motor activity prior to and during sleep, and thus to the maintenance of sleep. Abnormal function of the lateral SNR may cause hyposomnia and motor hyperactivity in quiet wake and in sleep.

Assessment of the effect of continuous theta burst stimulation of the motor cortex on manual dexterity in non-human primates in a direct comparison with invasive intracortical pharmacological inactivation.

Non-invasive reversible perturbation techniques of brain output such as continuous theta burst stimulation (cTBS), commonly used to modulate cortical excitability in humans, allow investigation of possible roles in functional recovery played by distinct intact cortical areas following stroke. To evaluate the potential of cTBS, the behavioural effects of this non-invasive transient perturbation of the hand representation of the primary motor cortex (M1) in non-human primates (two adult macaques) were compared with an invasive focal transient inactivation based on intracortical microinfusion of GABA-A agonist muscimol. The effects on the contralateral arm produced by cTBS or muscimol were directly compared based on a manual dexterity task performed by the monkeys, the "reach and grasp" drawer task, allowing quantitative assessment of the grip force produced between the thumb and index finger and exerted on the drawer's knob. cTBS only induced modest to moderate behavioural effects, with substantial variability on manual dexterity whereas the intracortical muscimol microinfusion completely impaired manual dexterity, producing a strong and clear cortical inhibition of the M1 hand area. In contrast, cTBS induced mixed inhibitory and facilitatory/excitatory perturbations of M1, though with predominant inhibition. Although cTBS impacted on manual dexterity, its effects appear too limited and variable in order to use it as a reliable proof of cortical vicariation mechanism (cortical area replacing another one) underlying functional recovery following a cortical lesion in the motor control domain, in contrast to potent pharmacological block generated by muscimol infusion, whose application is though limited to an animal model such as non-human primate.

Neuronal activity and microglial activation support corticospinal tract and proprioceptive afferent sprouting in spinal circuits after a corticospinal system lesion

Spared corticospinal tract (CST) and proprioceptive afferent (PA) axons sprout after injury and contribute to rewiring spinal circuits, affecting motor recovery. Loss of CST connections post-injury results in corticospinal signal loss and associated reduction in spinal activity. We investigated the role of activity loss and injury on CST and PA sprouting. To understand activity-dependence after injury, we compared CST and PA sprouting after motor cortex (MCX) inactivation, produced by chronic MCX muscimol microinfusion, with sprouting after a CST lesion produced by pyramidal tract section (PTx). Activity suppression, which does not produce a lesion, is sufficient to trigger CST axon outgrowth from the active side to cross the midline and to enter the inactivated side of the spinal cord, to the same extent as PTx. Activity loss was insufficient to drive significant CST gray matter axon elongation, an effect of PTx. Activity suppression triggered presynaptic site formation, but less than PTx. Activity loss triggered PA sprouting, as PTx. To understand injury-dependent sprouting further, we blocked microglial activation and associated inflammation after PTX by chronic minocycline administration after PTx. Minocycline inhibited myelin debris phagocytosis contralateral to PTx and abolished CST axon elongation, formation of presynaptic sites, and PA sprouting, but not CST axon outgrowth from the active side to cross the midline. Our findings suggest sprouting after injury has a strong activity dependence and that microglial activation after injury supports axonal elongation and presynaptic site formation. Combining spinal activity support and inflammation control is potentially more effective in promoting functional restoration than either alone.

Temporary inactivation reveals that the CA1 region of the mouse dorsal hippocampus plays an equivalent role in the retrieval of long-term object memory and spatial memory

Recognition of a previously experienced item or object depends upon the successful retrieval of memory for the object. The neural mechanisms that support object recognition memory in the mammalian brain are not well understood. The rodent hippocampus plays a well-established role in spatial memory, and we previously demonstrated that temporary inactivation of the mouse hippocampus impairs object memory, as assessed with a novel object preference (NOP) test. The present studies were designed to test some remaining issues regarding the contribution of the CA1 sub-region of the mouse dorsal hippocampus to long-term object memory. Specifically, we examined whether the retrieval of spatial memory (as assessed by the Morris water maze; MWM) and object recognition memory are differentially sensitive to inactivation of the CA1 region. The current study used pre-test local microinfusion of muscimol directly into the CA1 region of dorsal hippocampus to temporarily interrupt its function during the respective retrieval phases of both behavioral tasks, in order to compare the contribution of the CA1 to object memory and spatial memory. Histological analyses revealed that local intra-CA1 injection of muscimol diffused within, and not beyond, the CA1 region of dorsal hippocampus. The degree of memory retrieval impairment induced by muscimol was comparable in the two tasks, supporting the view that object memory and spatial memory depend similarly on the CA1 region of rodent hippocampus. Further, we confirmed that the muscimol-induced impairment of CA1 function is temporary. First, mice that exhibited impaired object memory retrieval immediately after intra-CA1 muscimol, subsequently exhibited unimpaired retrieval of object memory when tested 24h later. Secondly, a cohort of mice that exhibited impaired object memory retrieval after intra-CA1 muscimol later acquired spatial memory in the MWM comparable to that of control mice. Together, these results offer further support for the involvement of the CA1 region of mouse hippocampus in object recognition memory, and provide evidence to suggest that the NOP task is as much a test of hippocampal function as the classic MWM test.

Motor Cortex Activity Organizes the Developing Rubrospinal System.

The corticospinal and rubrospinal systems function in skilled movement control. A key question is how do these systems develop the capacity to coordinate their motor functions and, in turn, if the red nucleus/rubrospinal tract (RN/RST) compensates for developmental corticospinal injury? We used the cat to investigate whether the developing rubrospinal system is shaped by activity-dependent interactions with the developing corticospinal system. We unilaterally inactivated M1 by muscimol microinfusion between postnatal weeks 5 and 7 to examine activity-dependent interactions and whether the RN/RST compensates for corticospinal tract (CST) developmental motor impairments and CST misprojections after M1 inactivation. We examined the RN motor map and RST cervical projections at 7 weeks of age, while the corticospinal system was inactivated, and at 14 weeks, after activity returned. During M1 inactivation, the RN on the same side showed normal RST projections and reduced motor thresholds, suggestive of precocious development. By contrast, the RN on the untreated/active M1 side showed sparse RST projections and an immature motor map. After M1 activity returned later in adolescent cat development, RN on the active M1/CST side continued to show a substantial loss of spinal terminations and an impaired motor map. RN/RST on the inactivated side regressed to a smaller map and fewer axons. Our findings suggest that the developing rubrospinal system is under activity-dependent regulation by the corticospinal system for establishing mature RST connections and RN motor map. The lack of RS compensation on the non-inactivated side can be explained by development of ipsilateral misprojections from the active M1 that outcompete the RST. Significance statement: Skilled movements reflect the activity of multiple descending motor systems and their interactions with spinal motor circuits. Currently, there is little insight into whether motor systems interact during development to coordinate their emerging functions and, if so, the mechanisms underlying this process. This study examined activity-dependent interactions between the developing corticospinal and rubrospinal systems, two key systems for skilled limb movements. We show that the developing rubrospinal system competes with the corticospinal system in establishing the red nucleus motor map and rubrospinal tract connections. This is the first demonstration of one motor system steering development, and ultimately function, of another. Knowledge of activity-dependent competition between these two systems helps predict the response of the rubrospinal system following corticospinal system developmental injury.

Transient inactivation of the medial prefrontal cortex impairs performance on a working memory-dependent conditional discrimination task.

The rodent medial prefrontal cortex (mPFC) has been implicated in working memory function; lesions and inactivation of this region have been shown to result in impairments in spatial working memory (WM) tasks. Our laboratory has developed a tactile-visual conditional discrimination (CD) task, which uses floor insert cues to signal the correct goal-arm choice in a T maze. This task can be manipulated by altering the floor insert cues to be present throughout the trial (CDSTANDARD) or to be present only at the beginning of the trial (CDWM), thus making the task either WM-independent or WM-dependent, respectively. This ability to manipulate the working memory demand of the task while holding all other task features constant allows us to rule out the possibility that confounding performance variables contribute to the observed impairment. A previous study from our lab showed that mPFC inactivation did not impair performance on CDSTANDARD, confirming that mPFC inactivation does not induce sensorimotor or motivational deficits that could impact task performance. To examine whether mPFC inactivation impairs CDWM, the current study transiently inactivated the mPFC with bilateral microinfusions of muscimol immediately prior to testing on the CDWM task. As predicted, CDWM task performance was significantly impaired during the muscimol-infusion session compared with the control saline-infusion sessions. Together with our previous demonstration that the mPFC in not required for CDSTANDARD, these results not only confirm that the mPFC is crucial for working memory, but also set the stage for using the task-comparison approach to investigate corticolimbic interactions during working memory.

The zona incerta modulates spontaneous spike-wave discharges in the rat

The contribution of the zona incerta (ZI) of the thalamus on spike-wave discharges (SWDs) was investigated. Chronic recordings of bilateral cortices, bilateral vibrissa muscle, and unilateral ZI were performed in Long-Evans rats to examine the functional role of SWDs. Rhythmic ZI activity appeared at the beginning of SWD and was accompanied by higher-oscillation frequencies and larger spike magnitudes. Bilateral lidocaine injections into the mystacial pads led to a decreased oscillation frequency of SWDs, but the phenomenon of ZI-related spike magnitude enhancement was preserved. Moreover, 800-Hz ZI microstimulation terminates most of the SWDs and whisker twitching (WT; >80%). In contrast, 200-Hz ZI microstimulation selectively stops WTs but not SWDs. Stimulation of the thalamic ventroposteriomedial nucleus showed no obvious effect on terminating SWDs. A unilateral ZI lesion resulted in a significant reduction of 7- to 12-Hz power of both the ipsilateral cortical and contralateral vibrissae muscle activities during SWDs. Intraincertal microinfusion of muscimol showed a significant inhibition on SWDs. Our present data suggest that the ZI actively modulates the SWD magnitude and WT behavior.

Directional Responding of C57BL/6J Mice in the Morris Water Maze Is Influenced by Visual and Vestibular Cues and Is Dependent on the Anterior Thalamic Nuclei

Recent findings indicate that rats navigate in spatial tasks such as the Morris water maze (MWM) using a local cue-based reference frame rather than a distal cue-based reference frame. Specifically, rats swim in a particular direction to a location relative to pool-based cues, rather than to an absolute location defined by room-based cues. Neural mechanisms supporting this bias in rodents for relative responding in spatial tasks are not yet understood. Anterior thalamic neurons discharge according to the current directional heading of the animal. The contribution of head direction (HD) cell activity to navigation has been difficult to elucidate. We found that male C57BL/6J mice trained for 4 or 7 d in the MWM exhibited an overwhelming preference for swimming in a direction relative to pool-based cues over absolute responding during a platform-less probe test. Rotation of extramaze cues caused a corresponding rotation of the direction mice swam during the probe test, suggesting that both pool- and room-based reference frames guide platform search. However, disorienting the mice before the probe test disturbed relative responding. Therefore, relative responding is guided by both internal and external cue sources. Selective inactivation of anterior thalamic nuclei (ATN) by microinfusion of muscimol or fluorophore-conjugated muscimol caused a near complete shift in preference from relative to absolute responding. Interestingly, inactivation of the dorsal CA1 region of the hippocampus did not affect relative responding. These data suggest that ATN, and HD cells therein, may guide relative responding in the MWM, a task considered by most to reflect hippocampal processing.

Medial prefrontal cortex activity can disrupt the expression of stress response habituation

Recent findings suggest that the expression of hypothalamic–pituitary–adrenal (HPA) axis stress response adaptation in rats depends on top–down neural control. We therefore examined whether the medial prefrontal cortex (mPFC) modulates expression of stress response habituation. We transiently suppressed (muscimol microinfusion) or stimulated (picrotoxin microinfusion) mPFC neural activity in rats and studied the consequence on the first time response to psychological stress (restraint) or separately on the development and expression of habituation to repeated restraint. We monitored both the hormonal (corticosterone) and neural (forebrain c-fos mRNA) response to stress. Inactivation of the mPFC had no effect on the HPA-axis response to first time restraint, however increased mPFC activity attenuated stress-induced HPA-axis activity. In a three day repeated restraint stress regimen, inactivation of the mPFC on days 1 and 2, but not day 3, prevented the expression of HPA-axis hormone response habituation. In these same rats, the mPFC activity on day 3 interfered with the expression of c-fos mRNA habituation selectively within the mPFC, lateral septum and hypothalamic paraventricular nucleus. In contrast, inactivation of the mPFC only on day 3, or on all 3 days did not interfere with the expression of habituation. We conclude that the mPFC can permit or disrupt expression of HPA-axis stress response habituation, and this control depends on alteration of neural activity within select brain regions. A possible implication of these findings is that the dysregulation of PFC activity associated with depression and post-traumatic stress disorder may contribute to impaired expression of stress-response adaptation and consequently exacerbation of those disorders.

The basolateral amygdala is necessary for the encoding and the expression of odor memory

Conditioned odor avoidance (COA) results from the association between a novel odor and a delayed visceral illness. The present experiments investigated the role of the basolateral amygdala (BLA) in acquisition and retrieval of COA memory. To address this, we used the GABA(A) agonist muscimol to temporarily inactivate the BLA during COA acquisition or expression. BLA inactivation before odor-malaise pairing greatly impaired COA tested 3 d later. In contrast, muscimol microinfusion between odor and malaise spared retention. Moreover, inactivation of the BLA before pre-exposure to the odor prevented latent inhibition of COA. This suggests that neural activity in the BLA is essential for the formation of odor representation. BLA inactivation before the retrieval test also blocked COA memory expression when performed either 3 d (recent memory) or 28 d (remote memory) after acquisition. This effect was transitory as muscimol-treated animals were not different from controls during the subsequent extinction tests. Moreover, muscimol infusion in the BLA neither affected olfactory perception nor avoidance behavior, and it did not induce a state-dependent learning. Altogether, these findings suggest that neural activity in the BLA is required for the encoding and the retrieval of odor memory. Moreover, the BLA seems to play a permanent role in the expression of COA.

Dorsal hippocampal N-methyl-d-aspartate receptors underlie spatial working memory performance during non-matching to place testing on the T-maze

Previous lesion studies have suggested a functional dissociation along the septotemporal axis of the hippocampus. Whereas the dorsal hippocampus has been implicated in spatial memory processes, the ventral hippocampus may play a role in anxiety. However, these lesion studies are potentially confounded by demyelination of fibres passing through the lesion site, and the possibility of secondary, downstream changes in associated brain structures as a consequence of their chronic denervation following the lesion. In the present study, we have used the microinfusion of muscimol to temporarily inactivate either the dorsal or ventral hippocampus in order to re-examine the contribution of the hippocampal sub-regions to spatial memory. Microinfusion studies spare fibres of passage and offer fewer opportunities for compensatory changes because the effects are transient and short-lasting. Rats were infused prior to spatial working memory testing on a non-matching to place T-maze alternation task. Spatial working memory was impaired by dorsal but not ventral hippocampal inactivation. In a second experiment, infusion of the NMDAR antagonist, D-AP5, into dorsal hippocampus also impaired spatial working memory performance, suggesting that NMDAR function within the dorsal hippocampus makes an essential contribution to this aspect of hippocampal information processing.

Differential involvement of the core and shell subregions of the nucleus accumbens in conditioned cue-induced reinstatement of cocaine seeking in rats.

The nucleus accumbens (NAC) is theorized to be a critical element of the neural circuitry that mediates relapse to cocaine seeking. Evidence suggests that the NAC is a functionally heterogeneous structure, and the core (NACc) and shell (NACs) regions of the NAC may play a differential role in stimulus-induced motivated behavior. Thus, determination of the involvement of NAC subregions in conditioned cue-induced reinstatement of cocaine seeking is warranted. The present study compared the effects of GABA agonist-induced inactivation of the NACc versus NACs on conditioned cue-induced reinstatement of cocaine seeking behavior. Rats were trained to lever press for cocaine infusions (0.20 mg/infusion, IV) paired with presentations of a light-tone stimulus complex. Responding was then allowed to extinguish prior to reinstatement testing. Reinstatement of cocaine seeking (i.e. responses on the previously cocaine-paired lever) was measured in the presence of response-contingent presentation of the light-tone stimulus complex following microinfusion of muscimol+baclofen (Mus+Bac, 0.1/1.0 mM, respectively, 0.3 microl/side) or vehicle into the NACc or NACs. The effects of these manipulations on locomotor activity were also examined. Mus+Bac-induced inactivation of the NACc abolished, whereas inactivation of the NACs failed to alter, conditioned cue-induced reinstatement of operant responding relative to vehicle pretreatment. Time course analyses of the effects of these manipulations on locomotion versus operant responding confirmed that the effects of Mus+Bac on reinstatement were not due to suppression of general activity. The functional integrity of the NACc, but not the NACs, is necessary for conditioned cue-induced reinstatement of cocaine seeking behavior.

Sex differences in GABAAergic system in rat substantia nigra pars reticulata

The substantia nigra pars reticulata (SNR) is involved in the control of movement disorders including seizures through its GABAergic neurons. Microinfusions of muscimol (a GABAA receptor agonist) produce specific effects on seizures depending on sex, infusion site (SNRanterior or SNRposterior) and age. To assess whether these effects are due to sex differences in GABAergic indices within the SNR we analyzed the expression of α1 subunit mRNA of the GABAA receptor and the levels of GABA immunoreactivity (IR) of male and female rats at postnatal day 15 (PN15) and PN30. In each age, within the same SNR region, expression of α1 subunit mRNA and intensity of GABA IR per neuron was higher in females compared to males. At PN15, in both sexes, there were no regional differences in expression of α1 subunit mRNA and intensity of GABA IR. However, at PN30 in both sexes, expression of α1 subunit mRNA and intensity of GABA IR per cell was higher in SNRanterior than in SNRposterior. These results demonstrate that expression of α1 subunit mRNA for GABAA receptor and levels of GABA IR in the SNR are sex- and site-specific, which may contribute to sex-, regional- and age-related differences in the expression of movement disorders and seizures.

Relative involvement of globus pallidus and subthalamic nucleus in the regulation of somatodendritic dopamine release in substantia nigra is dopamine-dependent

Previously, we have shown that GABA A receptors and glutamate receptors in substantia nigra play distinct roles in the regulation of somatodendritic dopamine release. GABAergic input to substantia nigra was found to be the primary determinant of the level of spontaneous somatodendritic dopamine release. In contrast, acute blockade of dopamine receptors by systemic haloperidol administration produced an increase in somatodendritic dopamine release in substantia nigra that was found to be dependent exclusively upon activation of nigral glutamate receptors. The focus of the present study was to identify anatomical structures that may participate in the differential regulation of somatodendritic dopamine release by GABA and glutamate under these two conditions. To this end, we pharmacologically inhibited the activity of either globus pallidus or subthalamic nucleus using microinfusion of the GABA A receptor agonist muscimol. The effects of these manipulations on spontaneous efflux of somatodendritic dopamine and on increases in this measure produced by systemic haloperidol administration were determined in ipsilateral substantia nigra using in vivo microdialysis. As observed previously, administration of haloperidol (0.5 mg/kg, i.p.) significantly increased extracellular dopamine in substantia nigra. Microinfusion of muscimol (400 ng/200 nl) into globus pallidus also produced a significant increase in somatodendritic dopamine efflux. When haloperidol was administered systemically in conjunction with microinfusion of muscimol into globus pallidus, an increase in nigral dopamine efflux was observed that was significantly greater than that which was produced singly by muscimol microinfusion into globus pallidus or by systemic haloperidol administration. The additive nature of the increases in somatodendritic dopamine release produced by these two manipulations indicates that independent neural circuitries may be involved. Inactivation of subthalamic nucleus by microinfusion of muscimol (200 ng/100 nl) had no effect on spontaneous somatodendritic dopamine efflux. Muscimol application into subthalamic nucleus, however, completely abolished the stimulatory effect of systemic haloperidol on dendritic dopamine efflux in substantia nigra. The present data extend our previous findings by demonstrating: 1) an important involvement of globus pallidus efferents in the GABAergic regulation of somatodendritic dopamine efflux in substantia nigra under normal conditions and, 2) an emergent predominant role of subthalamic nucleus efferents in the glutamate-dependent increase in somatodendritic dopamine efflux observed after systemic haloperidol administration. Thus, the relative influence of globus pallidus and subthalamic nucleus in the determination of the level of somatodendritic dopamine release in substantia nigra qualitatively varies as a function of dopamine receptor blockade. These findings are relevant to current models of basal ganglia function under both normal and pathological conditions, e.g. Parkinson's disease.

Lack of robust anticonvulsant effects of muscimol microinfusions in the anterior substantia nigra of kindled rats

The substantia nigra pars reticulata is thought to control the spread of seizures in various seizure models. Potentiation of γ-aminobutyrate (GABA)-mediated transmission in this region by intranigral administration of drugs such as muscimol has been shown to inhibit seizure propagation in such models, including the kindling model of epilepsy. More recent studies have shown that the effects on seizures are site-specific within the substantia nigra pars reticulata. Using flurothyl to induce clonic seizures, it was reported that bilateral microinfusions of muscimol into the anterior substantia nigra pars reticulata were anticonvulsant, while similar infusions into the posterior pars reticulata were proconvulsant. This prompted us to reevaluate the effects of intranigral muscimol in the kindling model with particular emphasis on the anterior substantia nigra pars reticulata. In amygdala kindled rats, muscimol was bilaterally infused into the anterior pars reticulata at doses of either 60 or 120 ng. Thirty minutes later, the threshold for induction of afterdischarges in the amygdala and the threshold for generalized seizures were determined in each rat. Furthermore, severity and duration of seizures at threshold currents were recorded. Unexpectedly, muscimol failed to increase seizure thresholds or to significantly reduce seizure severity or duration of motor seizures, although there was a moderate reduction in motor seizure duration in several rats. The data indicate that, in contrast to flurothyl seizures, in kindled rats the anterior pars reticulata of the substantia nigra is not a site at which muscimol causes robust anticonvulsant effects.

Sexual dimorphism and developmental regulation of substantia nigra function.

The substantia nigra is an important brain nucleus involved in the expression of movement disorders and seizures. The two most common movement disorders affecting the substantia nigra, Parkinson's disease and Tourette syndrome, show gender differences and age-related onset. To assess the substrates for the gender and age specificity of substantia nigra-related disorders, we determined the functional properties of the substantia nigra gamma-aminobutyric acid (GABAA) system along its anterior-posterior axis, using localized microinfusions of muscimol (a GABAA agonist) and susceptibility to motor seizures in rats. In the substantia nigra, there are sex-specific differences in the topographic segregation and functionality of GABAA systems. In mature male rats there are two distinct regions mediating opposite effects on seizures; in female rats there is only one region that can affect seizures. In the neonatal period, the presence of circulating testosterone is essential for the development of a substantia nigra region that exerts proconvulsant effects throughout the rat's life, a unique feature of the male substantia nigra. The final maturation of the substantia nigra occurs in the peripubertal period, and is in part regulated by testosterone as well. The recognition of the existence of distinct sex- and age-specific substantia nigra features can be translated into new cures of disorders affecting the substantia nigra.

Effects of reversible inactivation of the supplementary motor area (SMA) on unimanual grasp and bimanual pull and grasp performance in monkeys

The supplementary motor area (SMA) was reversibly inactivated by muscimol microinfusion in two monkeys while they were performing two motor tasks: (1) a delayed conditional bimanual drawer pulling and grasping sequence which was initiated on a self-paced basis; (2) a unimanual reach and grasp task (modified Kluver board task). Unilateral or bilateral inactivation of the SMA induced a prominent deficit in trial initiation of bimanual sequential movements, affecting the hand contralateral to the inactivated side or both hands, respectively. The deficit was a long lasting (10-15 min or more) inability of the monkey to place its hand (s) in the ready position on start touch-sensitive pads, a condition required to initiate the drawer task. However, if after such a deficit period, the experimenter put his hand on the start touch-sensitive pad to initiate the trial, then the monkey executed the drawer task without obvious motor deficit. SMA inactivation did not affect unimanual reaching and grasping movements in the board task. In contrast to the SMA, inactivation of other motor areas (primary, premotor dorsal, anterior intraparietal area) did not affect the initiation of movement sequences in the drawer task. These data thus indicate that the SMA plays a crucial and specific role in initiation of self-paced movement sequences. However, SMA inactivation did not prevent the monkeys to perform coordinated movements of the two forelimbs and hands, indicating that SMA is not necessary for bimanual coordination.