Role of the ventromedial nucleus of the thalamus in motor behaviour--I. Effects of focal injections of drugs

The nucleus reuniens of the thalamus sits at the nexus of a hippocampus and medial prefrontal cortex circuit enabling memory and behavior

The nucleus reuniens of the thalamus (RE) is a key component of an extensive network of hippocampal and cortical structures and is a fundamental substrate for cognition. A common misconception is that RE is a simple relay structure. Instead, a better conceptualization is that RE is a critical component of a canonical higher-order cortico-thalamo-cortical circuit that supports communication between the medial prefrontal cortex (mPFC) and the hippocampus (HC). RE dysfunction is implicated in several clinical disorders including, but not limited to Alzheimer's disease, schizophrenia, and epilepsy. Here, we review key anatomical and physiological features of the RE based primarily on studies in rodents. We present a conceptual model of RE circuitry within the mPFC-RE-HC system and speculate on the computations RE enables. We review the rapidly growing literature demonstrating that RE is critical to, and its neurons represent, aspects of behavioral tasks that place demands on memory focusing on its role in navigation, spatial working memory, the temporal organization of memory, and executive functions.

Motor thalamus supports striatum-driven reinforcement

Reinforcement has long been thought to require striatal synaptic plasticity. Indeed, direct striatal manipulations such as self-stimulation of direct-pathway projection neurons (dMSNs) are sufficient to induce reinforcement within minutes. However, it’s unclear what role, if any, is played by downstream circuitry. Here, we used dMSN self-stimulation in mice as a model for striatum-driven reinforcement and mapped the underlying circuitry across multiple basal ganglia nuclei and output targets. We found that mimicking the effects of dMSN activation on downstream circuitry, through optogenetic suppression of basal ganglia output nucleus substantia nigra reticulata (SNr) or activation of SNr targets in the brainstem or thalamus, was also sufficient to drive rapid reinforcement. Remarkably, silencing motor thalamus—but not other selected targets of SNr—was the only manipulation that reduced dMSN-driven reinforcement. Together, these results point to an unexpected role for basal ganglia output to motor thalamus in striatum-driven reinforcement.

Regulation of cortical activity and arousal by the matrix cells of the ventromedial thalamic nucleus

The “non-specific” ventromedial thalamic nucleus (VM) has long been considered a candidate for mediating cortical arousal due to its diffuse, superficial projections, but direct evidence was lacking. Here, we show in mice that the activity of VM calbindin1-positive matrix cells is high in wake and REM sleep and low in NREM sleep, and increases before cortical activity at the sleep-to-wake transition. Optogenetic stimulation of VM cells rapidly awoke all mice from NREM sleep and consistently caused EEG activation during slow wave anesthesia, while arousal did not occur from REM sleep. Conversely, chemogenetic inhibition of VM decreased wake duration. Optogenetic activation of the “specific” ventral posteromedial nucleus (VPM) did not cause arousal from either NREM or REM sleep. Thus, matrix cells in VM produce arousal and broad cortical activation during NREM sleep and slow wave anesthesia in a way that accounts for the effects classically attributed to “non-specific” thalamic nuclei.

The ventromedial thalamus (VM) is thought to control cortical arousal through its diffuse projections to cortex. Here the authors record and manipulate the activity of calbindin1-positive matrix cells in VM and show that they bidirectionally regulate the sleep-wake transition.

Ventral Medial Thalamic Nucleus Promotes Synchronization of Increased High Beta Oscillatory Activity in the Basal Ganglia-Thalamocortical Network of the Hemiparkinsonian Rat

Loss of dopamine is associated with increased synchronization and oscillatory activity in the subthalamic nucleus and basal ganglia (BG) output nuclei in both Parkinson's disease (PD) patients and animal models of PD. We have previously observed substantial increases in spectral power in the 25-40 Hz range in LFPs recorded in the substantia nigra pars reticulata (SNpr) and motor cortex (MCx) in the hemiparkinsonian rat during treadmill walking. The current study explores the hypothesis that SNpr output entrains activity in the ventral medial thalamus (VM) in this frequency range after loss of dopamine, which in turn contributes to entrainment of the MCx and BG. Electrode bundles were implanted in MCx, SNpr, and VM of rats with unilateral dopamine cell lesions. Spiking and LFP activity were recorded during epochs of rest and walking on a circular treadmill. After dopamine cell lesion, 30-36 Hz LFP activity in the VM became more robust during treadmill walking and more coherent with LFP activity in the same range in MCx and SNpr. Infusion of the GABAA antagonist picrotoxin into the VM reduced both high beta power in MCx and SNpr and coherence between MCx and SNpr while temporarily restoring walking ability. Infusion of the GABAA agonist muscimol into the VM also reduced MCx-SNpr coherence and beta power but failed to improve walking. These results support the view that synchronized neuronal activity in the VM contributes to the emergence of high beta oscillations throughout the BG-thalamocortical network in the behaving parkinsonian rat.

SIGNIFICANCE STATEMENT

Parkinson's disease symptoms are associated with dramatic increases in synchronized beta range (15-35 Hz) oscillatory local field activity in several brain areas involved in motor control, but the mechanisms promoting this activity and its functional significance remain unresolved. This oscillatory activity can be recorded in awake behaving rats with unilateral dopamine cell lesions using chronically implanted electrodes. Although these rats have motor deficits, they can walk on a circular treadmill in the direction ipsilateral to their lesion. This study establishes a critical role for the ventral medial thalamus in the propagation of this exaggerated beta range oscillatory activity and the sequential entrainment of structures throughout the basal ganglia-thalamocortical loop in the lesioned hemisphere of hemiparkinsonian rats during treadmill walking.

Motor thalamus integration of cortical, cerebellar and basal ganglia information: implications for normal and parkinsonian conditions

Motor thalamus (Mthal) is implicated in the control of movement because it is strategically located between motor areas of the cerebral cortex and motor-related subcortical structures, such as the cerebellum and basal ganglia (BG). The role of BG and cerebellum in motor control has been extensively studied but how Mthal processes inputs from these two networks is unclear. Specifically, there is considerable debate about the role of BG inputs on Mthal activity. This review summarizes anatomical and physiological knowledge of the Mthal and its afferents and reviews current theories of Mthal function by discussing the impact of cortical, BG and cerebellar inputs on Mthal activity. One view is that Mthal activity in BG and cerebellar-receiving territories is primarily "driven" by glutamatergic inputs from the cortex or cerebellum, respectively, whereas BG inputs are modulatory and do not strongly determine Mthal activity. This theory is steeped in the assumption that the Mthal processes information in the same way as sensory thalamus, through interactions of modulatory inputs with a single driver input. Another view, from BG models, is that BG exert primary control on the BG-receiving Mthal so it effectively relays information from BG to cortex. We propose a new "super-integrator" theory where each Mthal territory processes multiple driver or driver-like inputs (cortex and BG, cortex and cerebellum), which are the result of considerable integrative processing. Thus, BG and cerebellar Mthal territories assimilate motivational and proprioceptive motor information previously integrated in cortico-BG and cortico-cerebellar networks, respectively, to develop sophisticated motor signals that are transmitted in parallel pathways to cortical areas for optimal generation of motor programmes. Finally, we briefly review the pathophysiological changes that occur in the BG in parkinsonism and generate testable hypotheses about how these may affect processing of inputs in the Mthal.

Decreased GABA receptor in the striatum and spatial recognition memory deficit in epileptic rats: effect of Bacopa monnieri and bacoside-A

AIM OF THE STUDY

Gamma-aminobutyric acid A receptors are the principal mediators of synaptic inhibition in striatal neurons and play an important role in preventing the spreading of seizures through the striatum. In the present study, effect of Bacopa monnieri (L.) Pennel and its active component bacoside-A on spatial recognition memory deficit and alterations of GABA receptor in the striatum of epileptic rats were investigated.

MATERIALS AND METHODS

Total GABA and GABA(A) receptor numbers in the control and epileptic rats were evaluated using [(3)H]GABA and [(3)H]bicuculline binding. GABA(Aalpha1,) GABA(Aalpha5,) GABA(Agamma3) and GABA(Adelta) gene expressions were studied. Behavioral performance was assed using Y-maze.

RESULTS

Scatchard analysis of [(3)H]GABA and [(3)H]bicuculline in the striatum of epileptic rats showed significant decrease in B(max) compared to control. Real-Time PCR amplification of GABA(A) receptor subunits such as GABA(Aalpha1,) GABA(Aalpha5) and GABA(Adelta), were down regulated (p<0.001) in the striatum of epileptic rats compared to control. Epileptic rats have deficit in Y-maze performance. Bacopa monnieri and bacoside-A treatment reversed these changes to near control.

CONCLUSION

Our results suggest that decreased GABA receptors in the striatum have an important role in epilepsy associated motor learning deficits and Bacopa monnieri and bacoside-A has a beneficial effect in the management of epilepsy.

Behavioral arrest: in search of the neural control system

Scientists have spent hundreds of years trying to understand how the brain controls movement. Why has there been so little interest in knowing how the brain STOPS movement? This review calls attention to behavioral phenomena in which an animal or human undergoes temporary total-body arrest of movement, that is, behavioral arrest (BA). These states can be actively induced by visual stimuli, by body and limb manipulations, and by drugs. Historically, these states have been considered as unrelated, and their literature does not cross-connect. What is known about the causal mechanisms is scant, limited mostly to implication of the brainstem in manipulation-induced BA and dopaminergic blockade in the striatum in the case of drug-induced BA. The possibility has not been experimentally tested that all of these states share with each other not only an active global immobility in which awkward postures are maintained, but also underlying neural mechanisms. This review identifies key brainstem, diencephalic, and basal forebrain areas that seem to be involved in causing BA. We review the evidence that suggest a possible role in BA for the following brain structures: entopeduncular nucleus, medullary and pontine reticular zones, parabrachial region, pedunculopontine nucleus and nearby areas, substantia nigra, subthalamic nucleus, ventromedial thalamic nucleus, and zona incerta. Such areas may operate as a BA control system. Confirmation of which brain areas operate collectively in BA would require testing of several kinds of BA in the same animals with the same kinds of experimental tests. Areas and mechanisms might be elucidated through a strategic combination of the following research approaches: imaging (fMRI, c-fos), lesions (of areas, of afferent and efferent pathways), chemical microstimulation, and electrical recording (of multiple units and field potentials, with an emphasis on testing coherence among areas). We suggest the working hypothesis that BA is created and sustained by coherent, perhaps oscillatory, activity among a group of basal forebrain and brainstem areas that collectively disrupt the normal spinal and supraspinal sequencing controls of reciprocal actions on the extensors and flexors that otherwise produce movement.

Electrophysiological and behavioral output of the rat basal ganglia after intrastriatal infusion of d-amphetamine: lack of support for the basal ganglia model

Dopamine, by acting upon D1 and D2 dopamine receptors located on striatonigral and striatopallidal neurons, respectively, has been postulated to inhibit output from the substantia nigra pars reticulata (SNpr) and internal pallidal segment (GPi). The inhibition of the SNpr/GPi should, in turn, disinhibit the thalamus to facilitate movement. The present study tests this prediction in intact (unlesioned) rats by attempting to correlate changes in the single unit activities of SNpr neurons with motor (i.e. behavioral) responses in the 20-30 min after infusions of d-amphetamine into the striatum. Unilateral injections of amphetamine (20 microg/microl) into either the dorsal-rostral, central, or ventral-lateral striatum failed to appreciably alter behavior and, in parallel electrophysiological studies, failed to consistently or significantly alter the activities of SNpr neurons in either chloral hydrate-anesthetized rats or awake locally anesthetized rats. However, when amphetamine was infused bilaterally into the ventral-lateral striatum (VLS; 20 microg/microl per side), a robust behavioral activation ensued (increased locomotor activity, oral movements, and sniffing) with an onset ranging from immediate to 20 min post-infusion and persisting for at least 40 min. In parallel studies, bilateral amphetamine infusions into VLS also caused changes in the firing frequency of a majority of SNpr neurons. However, the changes in firing were extremely variable and, contrary to expectation, the net population response of SNpr neurons was an increase in firing which corresponded in time with the period of peak behavioral activation. These results show that (i) bilateral but not unilateral activation of striatal dopamine receptors is needed to elicit behavioral and electrophysiological output from the basal ganglia, and (ii) motor activation is apparently not signaled by a generalized inhibition of SNpr firing, as is predicted by the basal ganglia model.

Kir6.2 oligoantisense administered into the globus pallidus reduces apomorphine-induced turning in 6-OHDA hemiparkinsonian rats

ATP-sensitive inwardly rectifying potassium channels (KATPs) couple cell metabolism with its membrane potential. The best characterized KATP is the pancreatic KATP which is an heteromultimer of Kir6.2 and SUR1 protein subunits. KATPs are found in a variety of excitable cells, including neurons of the central nervous system. Basal ganglia (BG), especially in the substantia nigra (SN) reticulata and the globus pallidus (GP), have a high density of KATPs. Pharmacological modulation of the KATPs within the BG alters GABAergic activity and produces behavioural changes. However, the relatively high concentrations of drugs used might not have been entirely selective for the KATPs and may have acted at presynaptic nerve terminals as well as on the post-synaptic neurons. As an alternative means of examining the role of KATPs in regulating motor behavior, we used oligoantisense technology to diminish selectively Kir6.2 formation in the GP neurons. We then examined the effect of reduction in Kir6.2 expression on apomorphine-induced turning behavior in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the SN. Two weeks after injection of 6-OHDA, contralateral circling in response to apomorphine (0.25 mg/kg sc) was recorded. Kir6.2 antisense oligodeoxyribonucleotide (ODN) was then administered daily for 6 days into the GP ipsilateral to the 6-OHDA injection. Responses to apomorphine were then tested again and the animals killed to determine the effect of the antisense ODN on Kir6. 2 mRNA. Administration of Kir6.2 antisense ODN significantly attenuated apomorphine-induced contralateral turning and specifically reduced Kir6.2 mRNA in the injected GP. These results are consistent with pharmacological experiments which suggest that KATP channels in the GP are involved in motor responses to apomorphine in 6-OHDA lesioned rats, localizing the effects to the GP neurons, probably through modulation of the GABAergic system.

Differential expression and regulation of leptin receptor isoforms in the rat brain: effects of fasting and oestrogen

Leptin affects body weight and reproduction mainly via receptors in the central nervous system. Different isoforms of the leptin receptor (leptin-R) exist, including a long isoform (leptin-RL) with signalling capacity and short isoforms (leptin-RS) with unknown function. The aim of this study was to examine leptin-R gene expression in different regions of the brain under conditions with altered body weight, in the female rat, including ovariectomy (OVX), oestradiol (E2) treatment, fasting and a genetic model of obesity (Zucker fa/fa). Leptin-R gene expression was analysed by in situ hybridization using probes recognizing all receptor isoforms (leptin-R) or specifically leptin-RL. Transcripts recognized by the leptin-R probe were abundant in the choroid plexus (CP), arcuate nucleus (ARC), ventromedial nucleus (VMN), thalamus (TH) and piriform cortex (PC). Leptin-RL transcripts were detected in the ARC, VMN, TH and PC but not in the CP. Although no sex difference was observed, leptin-R gene expression was reduced by E2 administration and increased by OVX. Administration of E2 reduced leptin-RL gene expression in the ARC and VMN but did not alter the expression in the TH or PC. OVX had no effect on the expression of leptin-RL mRNA. Fasting also caused a differential regulation of leptin-R mRNAs, with an increase in abundance of leptin-RL transcripts in the TH despite a decrease in leptin-R in this area. Obese Zucker rats had a similar pattern of expression with an increased expression of leptin-RL transcripts in all brain areas analysed and a decrease in leptin-R gene expression. These results demonstrate a differential regulation of leptin-RL and leptin-RS which could provide a mechanism for regulating access to, and sensitivity of, discrete regions of the brain for circulating leptin. We suggest that fasting and E2 alter the balance between leptin-RL and leptin-RS and that this could increase tissue sensitivity to leptin.

MK 801 reverses haloperidol-induced catalepsy from both striatal and extrastriatal sites in the rat brain

The present study investigated whether the anticataleptic effect of (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)-cyclohepten-5,10-imine (MK 801) is due to a blockade of N-methyl-D-aspartate (NMDA) receptors in striatal output pathways as well as in the striatum. Catalepsy induced by haloperidol (1 mg/kg i.p.) was more effectively reversed by MK 801 (0.2 mg/kg i.p.) given 10 min prior to rather than 45 min after the neuroleptic. Catalepsy evoked by intrastriatal haloperidol (7 micrograms/side) was also strongly attenuated by systemic MK 801 (0.2 mg/kg i.p.). We also found that the cataleptic rigidity induced by systemic haloperidol (1 mg/kg i.p.) could be prevented by prior injection of MK 801 into the striatum (10 micrograms), subthalamic nucleus (5 micrograms), entopeduncular nucleus (5 micrograms) or substantia nigra pars reticulata (1 microgram). These results suggest that the anticataleptic action of systemic MK 801 versus haloperidol, is due to the blockade of NMDA receptors in the striatum as well as in striatal output circuits through the subthalamus. However, systemic MK 801 (0.2 mg/kg i.p.) was without effect on the catalepsy elicited by injecting muscimol into the globus pallidus (25 ng) or ventromedial thalamus (50 ng). These findings suggest that MK 801 has little influence over thalamic excitatory feedback to the cortex, and that hypoactivity of the pallidum may not be a prerequisite for hyperactivity in the subthalamus.

Intrathalamic striatal grafts survive and affect circling behaviour in adult rats with excitotoxically lesioned striatum

Current models of basal ganglia disorders suggest that choreoathetosis is the end result of reduced GABAergic inhibition of the motor thalamus. Graft-derived release of GABA from intrastriatal striatal grafts has also been reported. In the present work, cell suspension grafts from embryonic day 14-15 rat striatal primordia were implanted close to the ventromedial thalamic nucleus to investigate whether they can develop and survive in this ectopic location, and whether they induce changes in the circling behaviour of the host. The grafts were implanted either in normal rats or in rats whose striatum had been lesioned with ibotenic acid. These grafts were implanted either ipsilateral or contralateral to the lesioned striatum. Additionally, some rats received intrastriatal grafts, and lesioned but non-grafted rats and lesioned rats that had received injections of saline or of cell suspensions from fetal spinal cord in the thalamus were used as control. Four to eight months after transplantation, circling behaviour after amphetamine or apomorphine injection was evaluated. Serial sections were stained with Cresyl Violet and studied immunohistochemically with antibodies against DARPP-32 (dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein, as striatal marker), Fos protein, glutamate decarboxylase (67,000 mol. wt), glutamate decarboxylase (65,000 mol. wt) and GABA. Cresyl Violet sections showed that the intrathalamic striatal grafts developed into tissue masses resembling those observed in intrastriatal striatal grafts. DARPP-32 immunohistochemistry revealed that the grafts were composed of DARPP-32 immunoreactive (striatum-like) and DARPP-32-negative patches. The intrathalamic grafts of rats which had received a low dose of apomorphine (0.25 mg/kg) 2 h before perfusion showed clusters of intensely Fos-immunoreactive nuclei throughout the transplant, indicating that these cells had developed dopamine receptors and supersensitivity to dopamine agonists. Double Fos and DARPP-32 immunohistochemistry revealed that the Fos-positive nuclei were located in the striatum-like areas. Finally, the intrathalamic grafts also contained neurons immunoreactive to GABA and glutamate decarboxylase (65,000 and 67,000 mol. wt). Rats that had received intrathalamic grafts contralateral to the lesioned striatum (i.e. contralateral to the lesion-induced turning direction) showed a significant reduction of circling both after amphetamine (78% reduction) or apomorphine (77% reduction) injection. Rats that had received grafts ipsilateral to the lesioned striatum showed a 75% decrease in amphetamine-induced circling, but no significant change in apomorphine-induced circling. No significant drug-induced circling was observed in non-lesioned and grafted rats. Sham grafting (saline) or grafting of weakly GABAergic tissue (fetal spinal cord) had no significant effects on lesion-induced circling behaviour.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of the nigrotectal and nigrothalamic pathways in kappa opioid-induced circling

The relationship between kappa opioid-induced movement and output stations of the substantia nigra pars reticulata (SNpr) was examined using the rodent circling model. Contralateral rotation produced by intranigral microinjection of the kappa opiate U50,488 was lower in animals with ibotenic acid lesions of either the ipsilateral ventromedial thalamus or superior colliculus than in control animals without lesions. These results suggest that endogenous kappa opioids in the SNpr may influence movement through actions on the nigrothalamic and nigrotectal pathways. In contrast, animals with ipsilateral lesions of the striatum showed an increase in circling relative to controls, possibly due to kappa receptor supersensitivity in the SNpr.

Behavioral demonstration of a reciprocal interaction between dopamine receptor subtypes in the mouse striatum: possible involvement of the striato-nigral pathway

It is well known that stimulation of the D-2 dopamine receptor in vitro inhibits the increased efflux of cyclic adenosine monophosphate caused by D-1 receptor agonists. Furthermore, behavioral data suggest that the striato-nigral pathway is more involved with the dopamine agonist-induced expression of oral behaviors, which are, in turn, mediated by stimulation of the D-1 receptor. We examined an in vivo model to determine whether this D-1/D-2 reciprocal interaction is detectable at a behavioral level. First, mice were pretreated with wide range of doses of the D-2 antagonist, spiperone, and then injected with a behaviorally active dose of apomorphine (a nonspecific direct dopamine agonist) and were observed for incidence of oral behavior and rated for stereotypic behavior. A biphasic effect of spiperone pretreatment was observed, at some low doses both stereotypy and oral behavior were enhanced, while at high doses, both agonist-induced behaviors were progressively inhibited. To test the specificity of this effect for the striato-nigral pathway, mice were administered discrete electrolytic lesions in the ventral portion of the internal capsule in one hemisphere. The animals that responded to apomorphine by rotating ipsilaterally to the lesion were used in two, five-point apomorphine dose-response curves, one with, and one without, pretreatment with the dose of spiperone which most enhanced stereotypic behavior and incidence of oral behavior. The spiperone pretreatment caused a clear increase in the maximum rotational response to apomorphine without affecting the ED50. These data suggest that behavior associated with the striato-nigral efferent from striatum is marked by the opposition of D-1 and D-2 receptors.

Bromocriptine-induced rotation: characterization using a striatal efferent lesion in the mouse

Two lesion techniques were used to elucidate the mode of action of bromocriptine (BRC)-induced behavior in mice. With the first lesion, a unilateral 6-hydroxydopamine (6-OHDA) preparation, BRC administration resulted in contralateral rotation which was blocked by alpha-methyl-para-tyrosine (AMPT), comparable to previous reports using rats. After striatonigral/entopeduncular lesion, mice did not rotate in response to doses of BRC up to 30 mg/kg but did show general activation which was also inhibited by AMPT pretreatment. It is concluded that BRC does not elicit rotation when there is no dopaminergic asymmetry such as that caused by a unilateral 6-OHDA lesion or no asymmetry in the striatonigral or striatoentopeduncular efferents. Since BRC-induced behaviors are dependent on intact presynaptic dopamine and BRC is predominantly a D-2 agonist, behaviors elicited in response to BRC must be the result of coactivation of D-1 receptors by endogenous dopamine. Thus, the behavioral effects of BRC, and perhaps D-2 agonists in general, must be mediated by efferents other than the striatonigral and striatoentopeduncular pathways.

Lateral hypothalamus-zona incerta region as an output station for the catalepsy induced by the blockade of striatal D1 and D2 dopamine receptors

Our previous study reported that the blockade of GABAA receptors of the lateral hypothalamus-zona incerta region (LH-ZI) by local injections of bicuculline methiodide inhibited the haloperidol-induced catalepsy. The aim of the present study was to determine (1) whether the blockade of GABAA receptors of the LH-ZI may counteract the catalepsy evoked by SCH 23390 and by sulpiride, and (2) whether the GABAA receptors of the LH-ZI affect the function of the striatal dopaminergic system. Bicuculline methiodide (2.5 and 5 ng/side) injected bilaterally into the LH-ZI inhibited in a dose-dependent manner the catalepsy induced by SCH 23390 administered peripherally (0.2 mg/kg s.c.). SCH 23390 (2 micrograms/side) and sulpiride (1 microgram/side) injected bilaterally into the rostroventral part of the striatum induced potent catalepsy. The catalepsy induced by injection of SCH 23390 (2 micrograms) and sulpiride (1 microgram) into the striatum was inhibited by bicuculline methiodide (2.5 ng and 5 ng) injected into the LH-ZI. Neither bicuculline (5 ng/side) nor muscimol (50 ng/side) injected bilaterally into the LH-ZI changed the levels of dopamine and its intraneuronal metabolite, 3,4-dihydroxyphenyl-acetic acid, or the concentration of noradrenaline and 5-hydroxyindole-acetic acid measured in the striatum and nucleus accumbens by HPLC with an electrochemical detection. It is concluded that GABAA receptors of the LH-ZI are an output station for the catalepsy induced by the blockade of the striatal D2 and D1 dopamine receptors.

Circling behavior induced by intranigral administration of ruthenium red and 4-aminopyridine in the rat

We have studied the effects of the unilateral intranigral microinjection of Ruthenium Red and 4-aminopyridine in the rat, as compared with that of muscimol. The three drugs produced contralateral turning when injected into the central nigra reticulata. Muscimol was the most effective but its effect disappeared in 3-4 h, whereas that of Ruthenium Red lasted for up to 3 days. When injected into the caudoventromedial nigra, Ruthenium Red produced intense ipsiversive turning, 4-aminopyridine weak ipsiversive turning and muscimol intense contraversive turning. Pretreatment with haloperidol (i.p.) abolished the effect of Ruthenium Red after injection into the caudoventromedial nigra but only partially reduced it after administration into the central nigra. The effect of muscimol, when injected into either of the nigral regions studied, was only slightly diminished by haloperidol. The release of [3H]GABA in slices of the Ruthenium Red-injected substantia nigra was not altered. Histological examination showed that the microinjected Ruthenium Red was located mainly inside the soma of nigral neurons. It is concluded that alterations of transmitter release are probably responsible for the circling behavior induced by 4-aminopyridine, but the effects of Ruthenium Red seem to be secondary to its penetration into the neuronal somas. Dopaminergic neurons seem to play an important role in the ipsilateral turning induced by Ruthenium Red when injected into the caudoventromedial nigra.

Output pathway for turning behavior from the neostriatum and substantia nigra in cats

The goal of the present work was to study the output pathway of the information for turning behavior originating in the striatum and coursing through the substantia nigra pars reticulata (SNR). In 45 adult cats distributed in 3 groups, Ni-Cr electrodes were implanted in the caudate nucleus and substantia nigra pars reticulata and depending on the animal group in the superior colliculus (SC), nucleus ventralis lateralis/nucleus ventralis medialis (VL/VM) complex or nucleus tegmenti pedunculopontinus (TPP) of one cerebral hemisphere. The threshold current required to evoke turning behavior was determined in each animal for each implantation site. An electrolytic lesion of the superior colliculus, the VL/VM complex or the nucleus tegmenti pedunculopontinus was carried out in each group of cats. The effects of the lesions on behavior and on the electrical threshold currents were determined and compared with the prelesion values. Finally the extent of the lesions and electrode positions were analyzed. The results show that the substantia nigra pars reticulata is the structure with the lowest thresholds for turning and that the superior colliculus appears to be more relevant for carrying the information for turning than either the VL/VM complex or the nucleus tegmenti pedunculopontinus.

The role of the ventromedial thalamic nucleus in the catalepsy evoked from the substantia nigra pars reticulata in rats

Picrotoxin (25, 50 and 100 ng), injected unilaterally into the posterior part of the substantia nigra pars reticulata (SNR) of rats, evoked a dose-dependent catalepsy. The catalepsy evoked by 100 ng of picrotoxin injected into the SNR was abolished by a subsequent bilateral injection of the same drug (200 ng) into the ventromedial thalamic nuclei. It is suggested that impulses pertinent to the catalepsy evoked from the SNR are transmitted via a GABAergic pathway to the ventromedial thalamic nucleus, wherefrom they reach the striatum, as had been shown previously.

The neostriatal inhibition of catalepsy, but not of muscle rigidity, evoked from the substantia nigra pars reticulata

The effects of a bilateral blockade of neo- and palleostriatal GABAergic mechanisms on catalepsy and muscle rigidity resulting from picrotoxin injection into the substantia nigra pars reticulata (SNR) were studied. The catalepsy and rigidity were induced by a unilateral injection of 100 ng/0.5 microliter of picrotoxin. Bilateral injections of 250 ng/l microliter of picrotoxin into the intermediate-ventral parts of the caudato-putamen (CP) abolished the catalepsy but had no effect on the muscle rigidity induced by an intranigral injection of the drug. Bilateral injections of 250 ng/l microliter of picrotoxin into the globus pallidus (GP) did not influence the catalepsy and rigidity induced by the intranigral injection of the drug. The results indicate that the impulses, connected with the catalepsy evoked from the SNR seem to be transmitted back to the CP and blocked therein by inhibition of GABAergic synapses in its intermediate-ventral part. The impulses, connected with the muscle rigidity evoked from the SNR, presumably do not return to the striatum.

Cerebral glucose utilization following striatal lesions: the effects of the GABA agonist, muscimol, and the dopaminergic agonist, apomorphine

Local cerebral glucose utilization was measured in 21 discrete regions of the rat CNS following unilateral kainic acid lesions of the caudate nucleus, and subsequent pharmacological challenge with GABAergic or dopaminergic agonists. The most pronounced increases in glucose use were observed in those ipsilateral areas of the brain to which the lesioned striatum normally projects (globus pallidus, entopeduncular nucleus and substantia nigra pars reticulata), although several other brain regions with known anatomical connections to the striato-pallido-nigral system were also affected, most notably in the thalamus and epithalamus. These effects were similar to those reported previously from a different group of animals in which the injection protocol was slightly different. The consequences of striatal lesion, in terms of alterations in local rates of glucose use, were attenuated by i.v. administration of the putative GABA agonist, muscimol. In one area, the ventromedial thalamus, glucose use was more markedly affected by muscimol treatment bilaterally in lesioned animals than in intact animals. The consequences of striatal lesions upon the response to the putative dopaminergic agonist apomorphine, were both complex and profound. In some regions (e.g. globus pallidus), striatal lesion eliminated, or masked the normal response to apomorphine. Elsewhere, the apomorphine response, although in evidence, was significantly attenuated by striatal lesion (e.g. entopeduncular nucleus), but in only two brain areas, substantia nigra pars reticulata and ventrolateral thalamus, the apomorphine response was significantly potentiated by striatal lesion. These studies add further weight to the concept of disinhibition, mediated via striatal GABA fibres, as an organizing principle in striatonigral function and indicate a complex interaction of intrinsic GABAergic pathways with dopaminergic systems in the integrated response to stimulation of dopaminergic receptors in the extrapyramidal motor system.

Behavioral actions of baclofen in the rat ventromedial thalamic nucleus: antagonism by delta-aminovalerate

Baclofen, an agonist at GABAB receptors injected locally into the rat ventromedial thalamic nucleus (VM) induced catalepsy in a dose-dependent, stereospecific and site-specific way. Baclofen-induced catalepsy was accompanied by tonic activity in the electromyogram (EMG) recorded from the gastrocnemius muscle, which is considered to be a measure of limb rigidity. delta-Aminovalerate (delta-AVA) coadministered with baclofen into the VM prevented the development of both catalepsy and limb rigidity. delta-AVA injected alone into the VM was devoid of depressant or stimulating behavioral effects. In contrast to delta-AVA, bicuculline, a GABAA antagonist, failed to antagonize baclofen-induced catalepsy. On the other hand, catalepsy due to local injection of a GABAA agonist, muscimol into the VM was antagonized by bicuculline, but not by delta-AVA. The present results suggest that delta-AVA may be useful as an antagonist of central pharmacological actions of baclofen.

Motor actions of excitatory amino acids and their antagonists within the rat ventromedial thalamic nucleus

The present study investigates the role of excitatory amino acid receptors within the rat ventromedial thalamic nucleus (VM) for the mediation of motor behaviour. For this purpose changes of motility were monitored after microinjections of excitatory amino acids and of various excitatory amino acid antagonists into the VM. N-Methyl-D-aspartate (NMDA) and kainate (KA), but not quisqualate (QA), led to a dose-dependent increase of exploratory activity. A specific NMDA antagonist (-)-2-amino-7-phosphonoheptanoate (-)-AP7), preferential non-NMDA antagonists, 1-(p-chlorobenzoyl)-piperazine-2,3-dicarboxylate (pCB-PzDA) and gamma-D-glutamylaminomethylsulphonate (GAMS), and a broad spectrum antagonist, kynurenate (KYN), induced catalepsy in a dose-dependent manner. The catalepsy induced by (-)-AP7 was antagonized by NMDA, but not by KA, the pCB-PzDA-induced catalepsy was blocked by KA, but not by NMDA and the KYN-induced catalepsy was reversed by either NMDA or KA. These data point to a role of both NMDA and KA receptors within the VM for the regulation of motility.

A neuropharmacological study of the periventricular neural substrate involved in flight

This paper reviews results obtained in experiments concerning the neurochemical characteristics of the substrate involved in the control of flight reactions and the induction of aversive effects in the rat. These experiments investigated the behavioural effects produced by microinjecting into the periaqueductal grey matter (PAG) or the medial hypothalamus (MH) compounds known to interfere with the functioning of some neurotransmitter systems known to exist in these structures. The data obtained show that: the activity of the substrate involved in the production of flight reactions is tonically inhibited by the release of GABA (gamma-aminobutyric acid); the behavioural reactions produced by microinjecting GABA antagonists can be clearly distinguished, depending on whether such drugs were injected into the PAG or the MH, despite the fact that jumps were produced from either level; behavioural effects, comparable to some extent to those produced by microinjections of GABA antagonists, can be obtained by injecting drugs which act on non-GABAergic neurochemical substrates, namely opioidergic or cholinergic systems; and behavioural effects, comparable to those produced by injecting GABA antagonists into the PAG, can be obtained by injecting such drugs into various sites located in other parts of the tectum such as the inferior colliculus or adjacent structures.

GABA mechanisms of ventromedial thalamic nucleus in morphine-induced muscle rigidity

The aim of the study was to investigate the role of the ventromedial thalamic nucleus in the rigidity induced by morphine. Muscle rigidity was assessed using an electromyographic method (EMG) in non-anaesthesized rats with electrodes implanted unilaterally in the gastrocnemius soleus muscle. Subcutaneous (s.c.) injections of morphine in doses of 5, 10 or 20 mg/kg evoked tonic EMG activity in the gastrocnemius soleus muscle; this was estimated as muscle rigidity. Picrotoxin was injected bilaterally into the ventromedial nucleus in doses of 50-400 ng/0.5 microliter 30 min after morphine administration. Picrotoxin in doses of 200 and 400 ng attenuated the tonic EMG activity induced by morphine, 10 mg/kg s.c. Picrotoxin in a dose of 400 ng reduced the tonic activity induced by morphine, 20 mg/kg s.c. The results suggest that the thalamic ventromedial nucleus mediates the morphine-induced rigidity.

Thalamus as a relay station for catalepsy and rigidity

The aim of the study was to determine to what extent catalepsy and tonic rigidity of muscles induced by muscimol administration into the ventral thalamic nuclei disturb the motor activity of rats. This study also aimed to test whether the ventromedial thalamic nucleus (Vm) was involved in transmitting effects evoked by the systemic injection of neuroleptics or opioids. For this purpose muscimol and/or picrotoxin was injected into the ventral thalamic nuclei and the behaviour of the animals was assessed in a series of test situations. It was found that muscimol administration to the Vm disturbs not only the initiation and performance of voluntary movements but also the occurrence of avoidance when the animal's life is endangered. Postural reflexes remained, however, undisturbed. Those effects seemed to be GABA- and site-specific to Vm. The haloperidol catalepsy was strongly inhibited by administration of picrotoxin to the Vm while the morphine catalepsy remained unchanged after picrotoxin. The Vm plays a crucial role in the motor behaviour and transmission of cataleptogenic effects of haloperidol, whereas similar effects produced by morphine appear to by-pass the investigated thalamic region.

Regulation of cerebello-cortical transmission in the rat ventromedial thalamic nucleus

On the basis of antidromic stimulation we have identified two distinct neuronal populations in the rat ventromedial thalamic nucleus. The largest population (96%) are thalamo-cortical relay cells which project via the internal capsule to the cerebral cortex. The smaller population of cells (4%) project caudally to the reticular formation and superior colliculus. These two cell types could be distinguished further on the basis of their patterns of spontaneous discharge. Relay cells fluctuate between two activity patterns (i) a rhythmic pattern characterized by periods of high-frequency bursting, and (ii) a more tonic discharge pattern of single spikes. The caudally projecting cells had a characteristic fast, regular type of spontaneous firing. Brachium conjunctivum stimulation evokes two distinct responses in thalamic relay cells. (i) a short-latency single spike, (ii) a longer latency, rhythmic response of 2-3 spikes. Both excitatory responses are followed by a period of cell quiescence. The type of response is dependent upon the cell's firing pattern. The short-latency response occurs during tonic, single-spike activity whilst the longer latency response occurs during high-frequency bursting activity. The short-latency response can be altered to the long latency response by increasing the level of anaesthesia or by applying a conditioning shock to known inhibitory pathways. Conversely the long latency response can be altered to the short-latency response by decreasing anaesthesia or by stimulation of the reticular formation. It is argued that both response types are evoked monosynaptically by activation of the same cerebello-thalamic fibres but that different ionic conductances which are active at different levels of membrane polarization are responsible for the two response patterns. Efficient time-locked cerebello-thalamo-cortical transmission occurs only during tonic single-spike activity, when cerebellar stimulation evokes a short-latency response. Such transmission is allowed or disallowed by the fine balance between converging excitatory and inhibitory afferents. In addition to a monosynaptic excitatory input from the cerebellar nuclei, relay cells received converging synaptic inputs from the substantia nigra, cerebral cortex, reticular formation and superior colliculus. Due to the anatomical arrangement in the rat it proved impossible to assess the role of the pallidum. The population of caudally projecting cells also received several converging synaptic inputs, but unlike those influencing relay cells, these inputs were all excitatory.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of the ventromedial nucleus of the thalamus in motor behaviour--II. Effects of lesions

Rotational behaviour was initiated in naive rats by injecting muscimol into one substantia nigra pars reticulata, or in unilaterally 6-hydroxydopamine-treated rats with systemic or intracaudate apomorphine. Electrolytic or kainic acid lesions were made in one or both ventromedial nuclei of the thalamus and their effects on the components of circling studied. A unilateral ventromedial electrolesion imposed a weak ipsilateral posture and occasionally elicited weak ipsiversive circling acutely, but not chronically. Challenging these rats with a large subcutaneous dose of apomorphine invariably provoked ipsiversive circling, however old was the lesion. Bilateral electrolesions caused slight hypoactivity. Kainic acid treatments of one or both ventromedial thalami produced uncontrolled hypermotility initially, with subsequent loss of ventromedial neurones and recovery of normal motor behaviour. No form of ventromedial lesion affected the incidence of stereotypy. Acute (but not chronic) contralateral or ipsilateral ventromedial electrolesions, or both, slowed muscimol and apomorphine-induced circling (often in different ways) through complex changes in posture and/or locomotor drive. Animals lesioned during the course of a circling episode often showed the biggest changes in circling to begin with, only to recover minutes later. Rapidly circling rats were sometimes more readily inhibited than slowly circling rats. Toxin injury of the ventromedial nucleus appeared to suppress muscimol and not apomorphine circling. Any ventromedial lesion (electrical or chemical, acute or chronic), if positioned opposite a contraversive circling stimulus, intensified the associated posture. Ipsilateral lesions tended to abolish posture altogether or, like bilateral treatments, to suppress locomotion. Sham operations had none of these effects. Acute electrical lesions and drug-induced inhibition of one or both ventromedial thalami were more or less identical in their effects on rat circling behaviour, save that bilateral muscimol injection caused profound catalepsy while lesions did not. It is suggested that the ventromedial thalamus is more concerned with the registration of striatal dopamine-mediated behaviours in drug-stimulated than in spontaneously behaving rats, and that other output pathways may rapidly compensate for any impairment of function in the ventromedial nuclei.