Effects of substantia nigra and pallidum stimulation on hippocampal interictal activity in the cat

Response Flexibility: The Role of the Lateral Habenula

The ability to make appropriate decisions that result in an optimal outcome is critical for survival. This process involves assessing the environment as well as integrating prior knowledge about the environment with information about one’s current internal state. There are many neural structures that play critical roles in mediating these processes, but it is not yet known how such information coalesces to influence behavioral output. The lateral habenula (LHb) has often been cited as a structure critical for adaptive and flexible responding when environmental contexts and internal state changes. A challenge, however, has been understanding how LHb promotes response flexibility. In this review, we hypothesize that the LHb enables flexible responding following the integration of context memory and internal state information by signaling downstream brainstem structures known to drive hippocampal theta. In this way, animals respond more flexibly in a task situation not because the LHb selects a particular action, but rather because LHb enhances a hippocampal neural state that is often associated with greater attention, arousal, and exploration. In freely navigating animals, these are essential conditions that are needed to discover and implement appropriate alternative choices and behaviors. As a corollary to our hypothesis, we describe short- and intermediate-term functions of the LHb. Finally, we discuss the effects on the behavior of LHb dysfunction in short- and intermediate-timescales, and then suggest that new therapies may act on the LHb to alleviate the behavioral impairments following long-term LHb disruption.

A supramodal role of the basal ganglia in memory and motor inhibition: Meta-analytic evidence

The ability to stop actions and thoughts is essential for goal-directed behaviour. Neuroimaging research has revealed that stopping actions and thoughts engage similar cortical mechanisms, including the ventro- and dorso-lateral prefrontal cortex. However, whether and how these abilities require similar subcortical mechanisms remains unexplored. Specifically of interest are the basal ganglia, subcortical structures long-known for their motor functions, but less so for their role in cognition. To investigate the potential common mechanisms in the basal ganglia underlying action and thought stopping, we conducted meta-analyses using fMRI data from the Go/No-Go, Stop-signal, and Think/No-Think tasks. All three tasks require active stopping of prepotent actions or thoughts. To localise basal ganglia activations, we performed high-resolution manual segmentations of striatal subregions. We found that all three tasks recovered clusters in the basal ganglia, although the specific localisation of these clusters differed. Although the Go/No-Go and Stop-signal tasks are often interchangeably used for measuring action stopping, their cluster locations in the basal ganglia did not significantly overlap. These different localised clusters suggest that the Go/No-Go and Stop-signal tasks may recruit distinct basal ganglia stopping processes, and therefore should not be treated equivalently. More importantly, the basal ganglia cluster recovered from the Think/No-Think task largely co-localised with that from the Stop-signal task, but not the Go/No-Go task, possibly indicating that the Think/No-Think and Stop-signal tasks share a common striatal circuitry involved in the cancellation of unwanted thoughts and actions. The greater similarity of the Think/No-Think task to the Stop-Signal rather than Go/No-Go task also was echoed at the cortical level, which revealed highly overlapping and largely right lateralized set of regions including the anterior DLPFC, VLPFC, Pre-SMA and ACC. Overall, we provide novel evidence suggesting not only that the basal ganglia are critical for thought stopping, but also that they are involved in specific stopping subprocesses that can be engaged by tasks in different domains. These findings raise the possibility that the basal ganglia may be part of a supramodal network responsible for stopping unwanted processes more broadly.

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Stopping actions and thoughts both consistently activate the basal ganglia.

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Action prevention and action cancellation engage distinct basal ganglia processes.

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Thought stopping co-localises with action cancellation, but not prevention.

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Basal ganglia may support a supramodal process cancellation mechanism.

Basal ganglia–hippocampal interactions support the role of the hippocampal formation in sensorimotor integration

Experiments were carried out to evaluate whether neural activity in the basal ganglia is functionally related to the neural activity underlying mechanisms of theta band oscillation and synchrony in the hippocampal formation. Experiment 1 demonstrated that electrical stimulation administered to the substantia nigra, globus pallidus (GP) and caudate-putamen (CPu) in urethane anesthetized rats elicited theta field activity in the hippocampal formation. Subsequent microinfusion of the local anesthetic procaine hydrochloride into the medial septum reversibly abolished this effect. In Experiment 2, single cell discharge profiles established for 152 cells recorded in nuclei of the basal ganglia resulted in 101 (66%) being classified as theta-related and 51 (34%) classified as nonrelated. Theta-related cells were further subclassified as tonic theta-ON cells (n = 79) and tonic theta-OFF (n = 22). Tonic theta-ON and tonic theta-OFF cells displayed irregular or regular (tonic) discharge patterns. Rhythmic discharge patterns did not occur in any theta-related cells in the nuclei of the basal ganglia. However, analyses using Kaneoke and Vitek's [J. Neurosci. Methods 68, (1996) 211] algorithms revealed that 51/101 (50%) theta-related cells displayed periodicity in their discharge patterns whereas 27/51 (53%) of the nonrelated cells displayed periodicity in their discharge patterns. The periodicities in the majority of cells were in frequency ranges above that of theta band oscillation and synchrony. The results support the following conclusions: (1) the cellular activity of the basal ganglia, composed of nuclei traditionally associated with motor functions, is functionally connected with the neural circuitry involved in the generation of theta band oscillation and synchrony in the hippocampal formation; (2) the observed functional connectivity provides support for the role of the hippocampal formation in sensorimotor integration.

Cognitive outcome after unilateral pallidal stimulation in Parkinson's disease

OBJECTIVES

Chronic high frequency electrostimulation of the globus pallidus internus mimics pallidotomy and improves clinical symptoms in Parkinson's disease. The aim of this study was to investigate the cognitive consequences of unilateral deep brain stimulation.

METHODS

Twenty non-demented patients with Parkinson's disease (age range 38-70 years) were neuropsychologically assessed 2 months before and 3 months after unilateral pallidal stimulation. The cognitive assessment included measures of memory, spatial behaviour, and executive and psychomotor function. In addition to group analysis of cognitive change, a cognitive impairment index (CII) was calculated for each individual patient representing the percentage of cognitive measures that fell more than 1 SD below the mean of a corresponding normative sample.

RESULTS

Neurological assessment with the Hoehn and Yahr scale and the unified Parkinson's disease rating scale disclosed a significant postoperative reduction in average clinical Parkinson's disease symptomatology (p<0.001). Repeated measures multivariate analysis of variance (using right/left side of stimulation as a between subjects factor) showed no significant postoperative change in cognitive performance for the total patient group (main effect of operation). The side of stimulation did not show a significant differential effect on cognitive performance (main effect of lateralisation). There was no significant operation by lateralisation interaction effect. Although the patients experienced significant motor symptom relief after pallidal stimulation, they remained mildly depressed after surgery. Analysis of the individual CII changes showed a postoperative cognitive decline in 30% of the patients. These patients were significantly older and took higher preoperative doses of levodopa than patients showing no change or a postoperative cognitive improvement.

CONCLUSIONS

Left or right pallidal stimulation for the relief of motor symptoms in Parkinson's disease seems relatively safe, although older patients and patients needing high preoperative doses of levodopa seem to be more vulnerable for cognitive decline after deep brain stimulation.

Action of GP 47779, the active metabolite of oxcarbazepine, on the corticostriatal system. I. Modulation of corticostriatal synaptic transmission

Oxcarbazepine (OCBZ) is the keto-analogue of carbamazepine (CBZ). In humans, OCBZ is rapidly and almost completely metabolized to 10, 11-dihydro-10-hydroxy-CBZ (GP 47779), the main metabolite responsible for the drug's antiepileptic activity. The corticostriatal pathway is involved in the propagation of epileptic discharges. We characterized the electrophysiological effects of GP 47779 on striatal neurons by making intracellular recordings from corticostriatal slices. GP 47779 (3-100 microM) produced a dose-dependent inhibition of glutamatergic excitatory postsynaptic potentials (EPSPs). This effect was not coupled either with changes of the membrane potential of these cells or with alterations of their postsynaptic sensitivity to excitatory amino acids (EAA) suggesting a presynaptic site of action. GP 47779 reduced the current-evoked firing discharge only at concentrations > 100 microM. GP 47779 did not affect the presynaptic inhibitory action of adenosine, showing that presynaptic adenosine receptors were not implicated in the GP 47779-mediated reduction of corticostriatal EPSPs. Our data indicate that GP 47779 apparently acts directly on corticostriatal terminals to reduce the release of EAA, probably by inhibiting high-voltage-activated (HVA) calcium (Ca2+) currents (described in the accompanying article). The inhibitory action of GP 47779 on corticostriatal transmission may contribute to the antiepileptic effects of this drug.

Excitatory neurotransmitters in the lateral habenula and pedunculopontine nucleus of rat modulate limbic seizures induced by pilocarpine

The involvement of the excitatory neurotransmitter system in the lateral habenula and pedunculopontine nucleus in the initiation and propagation of limbic seizures induced by pilocarpine has been investigated in the rat. Limbic seizures occur in animals following bilateral microinjection into the lateral habenula of N-methyl-D-aspartate (NMDA) (5 and 12.5 nmol) or kainate (100 and 200 pmol), 15 min prior to a subconvulsant dose of pilocarpine (150 mg/kg, i.p.). In the absence of pilocarpine NMDA (5 and 12.5 nmol) or kainate (100 and 200 pmol), injected focally into the lateral habenula or pedunculopontine nucleus, produced sniffing, grooming and tremor but no electrographic or behavioural seizures. Limbic seizures also occur after a subconvulsant dose of pilocarpine when it is preceded by injection of NMDA (5 and 12.5 nmol) or kainate (50, 100 and 200 pmol) into the pedunculopontine nucleus. Behavioural and electrographic signs of limbic seizures following pilocarpine (380 mg/kg, i.p.) were attenuated or completely antagonized by focal injection into the lateral habenula of the NMDA antagonist, 2-amino-7-phosphonoheptanoate (AP7) (10 and 50 pmol) or kainate antagonist, gamma-D-glutamylaminomethylsulphonate (GAMS) (20 nmol). In addition, AP7 (0.05, 0.1 and 1.0 nmol) or GAMS (40 nmol) injected into the pedunculopontine nucleus suppressed limbic seizures induced by i.p. administration of pilocarpine (380 mg/kg). The relative efficacy of NMDA and non-NMDA receptor antagonists revealed that the selective NMDA antagonist, AP7, was more potent in its anticonvulsant activity in comparison to GAMS, a non-NMDA receptor antagonist.

Relay stations and neurotransmitters between the pallidal region and the hippocampus

The effects of internal pallidum and lateral habenula stimulation on epileptiform activity of cat's hippocampus were studied. A steady interictal activity was induced by locally applied sodium penicillin (PCN) solution. Both pallidal and habenular electrical stimulation caused an increase in spike frequency and amplitude. Intraperitoneally injected atropine sulphate failed to modify pallidal and habenular influences. Intraperitoneal methysergide bimaleate (5-HT antagonist) suppressed the effects of habenular stimulation. In contrast to the effects of pallidal and habenular stimulation, raphe electrical stimulation inhibited hippocampal spiking and intra-raphal muscimol (a GABA receptor agonist) enhanced hippocampal-based epilepsy. After muscimol, raphe stimulation at the same threshold parameters failed to affect hippocampal activity. In cats with habenular lesions hippocampal spike frequency and amplitude were reduced and intra-raphal muscimol did not affect the hippocampus. The results are discussed in the light of a complex interrelationship between basal ganglia and hippocampus. The role of the lateral habenula and of the medial raphe as relay stations between the two regions is emphasized.

Nigral influence on focal epilepsy

The substantia nigra (SN) has been proposed as a structure involved in epileptiform phenomena. Previous investigations demonstrated that SN is able to elicit hippocampal rhythmic slow activity (RSA) as well as to inhibit hippocampal interictal spikes induced by parenteral administration of penicillin. The present series of experiments was carried out in order to characterize the action of SN on a focal model of hippocampal epilepsy. Experiments were performed on encéphale isolé cats in which steady epileptiform activity was induced by locally applied penicillin. Electrical stimulation of SN pars reticulata (pr) caused a statistically significant decrease of hippocampal spike frequency and amplitude in 30% of the total number of stimulation sessions. Stimulation of SN pars compacta (pc) was even more effective. It induced inhibitory effects on hippocampal spikes in 91% of the cases. In 30% of the cats, RSA was noted on hippocampal recordings in correspondence to nigral activation. Experimental data support the hypothesis that the SNpc influences hippocampal excitability: a differential role may be played by SNpc and SNpr in the control of seizure processes.

Regulation of seizure threshold by excitatory amino acids in the striatum and entopeduncular nucleus of rats

The participation of excitatory neurotransmitter systems in the basal ganglia in the initiation and propagation of limbic seizures induced by pilocarpine has been investigated in the rat. Limbic seizures (electrographic and motor) occur in rats receiving bilateral microinjections into the entopeduncular nucleus of 12.5 nmol N-methyl-D-aspartate or 0.1 nmol kainate, 15 min prior to a subconvulsant dose of pilocarpine (150 mg/kg, intraperitoneally). N-Methyl-D-aspartate (12.5 nmol) or kainate (0.5 nmol), injected alone bilaterally into the entopeduncular nucleus, induce sniffing and grooming but no electrographic or behavioural seizures. Limbic seizures also occur after a subconvulsant dose of pilocarpine when it is preceded by injection of N-methyl-D-aspartate (12.5 nmol) or kainate (0.5 or 2 nmol) into the dorsal striatum. Behavioural and electrographic signs of limbic seizures following pilocarpine (380 mg/kg) are suppressed by the focal microinjection into the entopeduncular nucleus of the N-methyl-D-aspartate antagonist, 2-amino-7-phosphonoheptanoate (0.02 nmol) or the kainate antagonist, gamma-D-glutamylamino-methylsulphonate (40 nmol). Seizure threshold within the limbic system is modulated by excitatory systems controlling basal ganglia outputs. The relative importance of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor systems varies between different components of the basal ganglia.

Anti-epileptic effects of focal micro-injection of excitatory amino acid antagonists

The role of excitatory synaptic activity at various brain regions in the development and spread of seizure activity has been investigated by the focal microinjection of 2-amino-7-phosphono-heptanoate (2-APH), a selective antagonist at the N-methyl-D-aspartate preferring receptor, or gamma-D-glutamyl-aminomethyl sulphonate (GAMS), a partially selective antagonist at the kainate receptor. In genetically epilepsy prone rats the seizure response to a loud sound in most effectively suppressed by focal injections of 2-APH, 0.1-1.0 nmol, in the inferior colliculus. Protection is also seen after injections of 2-APH, 25 nmoles, in the substantia nigra (pars reticulata) or the midbrain reticular formation. Motor limbic seizures induced by pilocarpine, 380 mg/kg intraperitoneally, are prevented by prior injection into the substantia nigra, pars reticulata, or the entopeduncular nucleus, of 2-APH, 10 nmol or 10 pmol, respectively. Similar protection follows the injection of 2-APH, 1-5 pmol in the piriform cortex. The convulsant effects of pilocarpine are also blocked by the focal injection of GAMS, 10 nmol in the entopeduncular nucleus. This experimental approach can indicate critical sites at which seizure activity is initiated in particular models (e.g., inferior colliculus in sound-induced seizures, and piriform cortex in limbic seizures) and the pathways controlling seizure expression, such as the basal ganglia outputs. It also identifies specific receptors at which anticonvulsant drugs may operate.

An electrophysiological study of habenular influence on hippocampus

The action of lateral habenula (LH) stimulation on focal epileptiform activity in the hippocampus was studied. Local microinjection of sodium penicillin induced a steady interictal activity in the dorsal hippocampus. Low frequency electrical stimulation of the habenula caused a marked enhancement of spike activity in both frequency and amplitude. The effect was blocked by intraperitoneally injected methysergide. The facilitatory influence of the habenula on hippocampal activity might be due to a disinhibitory mechanism. The results are regarded as suggesting that the habenula may be a relay station between the basal ganglia and the hippocampal formation. LH as well as basal ganglia might modulate hippocampal excitability, exerting a control on the genesis and diffusion of abnormal activities.

Hippocampal seizures and striatal regulation: a possible functional pathway

Experimental findings have suggested the possibility of a functional relationship between the basal ganglia and the hippocampus. Previous research has revealed a predominantly inhibitory action of the caudate nucleus (CN) and an excitatory effect of the globus pallidus (GP) on electrically induced hippocampal afterdischarges (HAD). The effects of electrolytic destruction of the CN on the threshold and duration of HAD has been studied in the 'encéphale isolé' cat. The threshold and duration of HAD was also studied following conditioning stimulation of the CN in animals in which the inner segment of the globus pallidus (GPi) and medial septal nucleus (MSN) had been destroyed. Following CN lesions, the hippocampal excitability threshold underwent a significant reduction, while the duration of HAD appeared to be increased. Following destruction of the GPi and MSN, the threshold and duration of HAD exhibited no change following conditioning stimulation of the CN. The results reveal a tonic inhibitory effect of the CN on the hippocampus and suggest that a strio-pallido-septal pathway is the anatomical substrate for the effect.

Negative occurrence between hippocampal rhythmic slow activity and epileptiform spikes: influence of the striatum

The effects of caudate and septal stimulation on hippocampal activity were studied. Sodium penicillin was intravenously injected in order to induce a steady rate of interictal epileptic activity. Penicillin injection caused significant reduction of the rate of occurrence and duration of hippocampal rhythmic slow activity (RSA). Spontaneous RSA as well as RSA-eliciting caudate and septal stimulation induced a marked inhibition on frequency of epileptiform complexes. Lesions of the medial septal nucleus made it impossible to record RSA or to observe any sort of influence on hippocampal epileptiform activity by caudate stimulation. This result suggests that the septum is important for RSA genesis in the striato-hippocampal pathway or in the reciprocal septo-hippocampal connections.