Substantia Nigra Reticulata Research Articles

Stimulation of delta opioid receptors located in substantia nigra reticulata but not globus pallidus or striatum restores motor activity in 6‐hydroxydopamine lesioned rats: new insights into the role of delta receptors in parkinsonism

The delta opioid peptide (DOP) receptor has been proposed as a target in the symptomatic therapy of Parkinson's disease. However, the circuitry underlying the antiparkinsonian action of DOP receptor agonists and their site of action have never been adequately investigated. Systemic administration of the DOP receptor agonist (+)-4-[(alphaR)-alpha-(2S,5R)-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N-N-diethylbenzamide (SNC-80) attenuated akinesia/bradykinesia and improved motor activity in 6-hydroxydopamine hemilesioned rats. Opposite effects were produced by the selective DOP receptor antagonist naltrindole (NTD), suggesting that endogenous enkephalins tonically sustain movement under parkinsonian conditions. Microdialysis revealed that SNC-80 reduced GABA release in globus pallidus (GP) while NTD elevated it. Moreover, SNC-80 reduced GABA and glutamate release in substantia nigra reticulata (SNr) whereas NTD reduced GABA without affecting glutamate release. The bar test coupled to microdialysis showed that perfusion with NTD in SNr but not GP or striatum prevented the antiakinetic effect of systemic SNC-80 and its neurochemical correlates. Consistently, microinjections of SNC-80 into SNr or bicuculline in GP attenuated parkinsonian-like symptoms while SNC-80 microinjections in GP or striatum were ineffective. This study demonstrates that nigral DOP receptors mediate antiparkinsonian actions of SNC-80 and challenges the common view that DOP receptor agonists solely attenuate parkinsonism via pallidal mechanisms.

The novel nociceptin/orphanin FQ receptor antagonist Trap‐101 alleviates experimental parkinsonism through inhibition of the nigro‐thalamic pathway: positive interaction with L‐DOPA

In this study we investigated whether the recently discovered antagonist of the nociceptin/orphanin FQ (N/OFQ) opioid peptide (NOP) receptor, 1-[1-(cyclooctylmethyl)-1,2,3,6-tetrahydro-5-(hydroxymethyl)-4-pyridinyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (Trap-101) changed motor activity in naïve rats and mice, and alleviated parkinsonism in 6-hydroxydopamine hemilesioned rats. In naïve rats, Trap-101 stimulated motor activity at 10 mg/Kg and inhibited it at 30 mg/Kg. Such dual action was also observed in wild-type but not NOP receptor knockout mice suggesting specific involvement of NOP receptors. Trap-101 alleviated akinesia/bradykinesia and improved overall gait ability in hemiparkinsonian rats, being effective starting at 1 mg/Kg and without worsening motor deficit at 30 mg/Kg. To investigate the circuitry involved in the Trap-101 action, behavioral tests were performed in rats undergoing microdialysis. The anti-akinetic/anti-bradykinetic effects of Trap-101, given systemically (10 mg/Kg) or perfused in substantia nigra reticulata (10 microM), were associated with reduced glutamate and enhanced GABA release in substantia nigra, and reduced GABA release in ipsilateral ventro-medial thalamus. When combined with ineffective doses of l-DOPA (0.1 mg/Kg), Trap-101 evoked larger neurochemical and behavioral responses. These data show that Trap-101 is an effective NOP receptor antagonist in vivo and confirm that NOP receptor antagonists alleviate parkinsonism through blockade of nigral NOP receptors and impairment of nigro-thalamic transmission.

Metabolic environment in substantia nigra reticulata is critical for the expression and control of hypoglycemia-induced seizures.

Seizures represent a common and serious complication of hypoglycemia. Here we studied mechanisms of control of hypoglycemic seizures induced by insulin injection in fasted and nonfasted rats. We demonstrate that fasting predisposes rats to more rapid and consistent development of hypoglycemic seizures. However, the fasting-induced decrease in baseline blood glucose concentration cannot account for the earlier onset of seizures in fasted versus nonfasted rats. Data obtained with c-Fos immunohistochemistry and [14C]2-deoxyglucose uptake implicate a prominent involvement of the substantia nigra reticulata (SNR) among other structures in the hypoglycemic seizure control. This is supported by data showing that fasting decreases the SNR expression of K(ATP) channels, which link metabolism with activity, and is further confirmed with microinfusions of K(ATP) channel agonist and antagonist. Data obtained with whole-cell and perforated patch recordings from SNR neurons in slices in vitro demonstrate that both presynaptic and postsynaptic K(ATP) channels participate in the failure of the SNR to control hypoglycemic seizures. The results suggest that fasting and insulin-induced hypoglycemia can lead to impairment in the function of the SNR, leading thus to hypoglycemic seizures.

Altered aminergic neurotransmission in the brain of organic cation transporter 3‐deficient mice

Organic cation transporters (OCTs) are carrier-type polyspecific permeases known to participate in low-affinity extraneuronal catecholamine uptake in peripheral tissues. OCT3 is the OCT subtype most represented in the brain, yet its implication in central aminergic neurotransmission in vivo had not been directly demonstrated. In a detailed immunohistochemistry study, we show that OCT3 is expressed in aminergic pathways in the mouse brain, particularly in dopaminergic neurons of the substantia nigra compacta, non-aminergic neurons of the ventral tegmental area, substantia nigra reticulata (SNr), locus coeruleus, hippocampus and cortex. Although OCT3 was found mainly in neurons, it was also occasionally detected in astrocytes in the SNr, hippocampus and several hypothalamic nuclei. In agreement with this distribution, OCT3/Slc22a3-deficient mice show evidence of altered monoamine neurotransmission in the brain, with decreased intracellular content and increased turnover of aminergic transmitters. The behavioral characterization of these mutants reveal subtle behavioral alterations such as increased sensitivity to psychostimulants and increased levels of anxiety and stress. Altogether our data support a role of OCT3 in the homeostatic regulation of aminergic neurotransmission in the brain.

Differential effect of NR2A and NR2B subunit selective NMDA receptor antagonists on striato‐pallidal neurons: relationship to motor response in the 6‐hydroxydopamine model of parkinsonism

We previously demonstrated that NMDA receptors containing the NR2A or NR2B subunits differentially regulate striatal output pathways. We now investigate whether such a differential control is altered under parkinsonian conditions and whether subunit selective antagonists have different abilities to attenuate parkinsonian-like motor deficits. Three microdialysis probes were simultaneously implanted in the dopamine-depleted striatum, globus pallidus and substantia nigra reticulata of 6-hydroxydopamine hemilesioned rats. The NR2A antagonist NVP-AAM077 perfused in the striatum reduced pallidal GABA, but not glutamate, levels whereas the NR2B antagonist Ro 25-6981 was ineffective. Neither antagonist affected striatal or nigral amino acid levels. To investigate whether these neurochemical responses were predictive of different antiparkinsonian activities, antagonists were administered systemically and motor activity evaluated in different motor tasks. Neither antagonist attenuated akinesia/bradykinesia in the bar and drag test. However, NVP-AAM077 dually modulated rotarod performance (low doses being facilitatory and higher ones inhibitory) while Ro 25-6981 monotonically improved it. Microdialysis revealed that motor facilitating doses reduced pallidal GABA levels while motor inhibiting doses increased them. We conclude that, under parkinsonian conditions, the striato-pallidal pathway is driven by striatal NR2A subunits. Motor improvement induced by NVP-AAM077 and Ro 25-6981 is accomplished by blockade of striatal NR2A and extrastriatal NR2B subunits, respectively.

6-OHDA-induced hemiparkinsonism and chronic l-DOPA treatment increase dopamine D1-stimulated [ 3H]-GABA release and [ 3H]-cAMP production in substantia nigra pars reticulata of the rat

It has been proposed that striatonigral GABAergic transmission in the substantia nigra reticulata (SNr) is enhanced during Parkinson's disease and subsequent l-DOPA treatment. To evaluate this proposal we determined the effects of activating dopamine D1 receptors on depolarization induced [ 3H]-GABA release and on [ 3H]-cAMP accumulation in slices of SNr of rats with unilateral 6-OHDA lesions with and without l-DOPA treatment. Denervation increased depolarization induced D1-stimulated [ 3H]-GABA release, while repeated l-DOPA treatment further enhanced this response. Both also enhanced the effects of forskolin on [ 3H]-cAMP production and [ 3H]-GABA release, while neither modified the stimulating effects of 8-Br-cAMP on the release. These results shown that, after 6-OHDA lesions and l-DOPA treatment, cAMP signaling is enhanced. Furthermore, the results suggest that activation of sites in the signaling cascade downstream of cAMP synthesis is not required to increase release.

Topographical Fos induction within the ventral midbrain and projection sites following self-stimulation of the posterior mesencephalon

Rats will readily perform an operant response to self-administer electrical stimulation to the posterior mesencephalon (PM). Previous results show that axons that support self-stimulation travel between the PM and the ventral tegmental area (VTA) and that their activation increases firing of VTA neurons. The present work sought to extend these findings by describing the distribution of ventral midbrain neurons affected by PM self-stimulation. In Experiment 1, ventral midbrain Fos-immunoreactivity (IR) was assessed in three groups of rats implanted with a monopolar electrode; two groups were trained to self-administer stimulation, but only one was allowed to self-stimulate on the test day, whereas the third was never trained or tested. Self-stimulation induced prominent Fos-IR that was differentially distributed within the VTA and substantia nigra (SN). Control rats showed only sparse labeling. In Experiment 2, ventral midbrain Fos-IR was assessed with three additional groups trained to self-administer PM stimulation and tested as follows: Group-1 was allowed to self-stimulate, Group-2 received stimulation at parameters that failed to support self-stimulation (deemed non-rewarding) “yoked” to the rate of responding of Group-1, and Group-3 received no stimulation. PM self-stimulation induced Fos-IR throughout the rostral–caudal VTA and within the SN reticulata. Non-rewarding stimulation induced sparse Fos-IR, comparable to no stimulation. Fos-IR specific to PM self-stimulation was also observed within the bed nucleus of the stria terminalis (BNST) and nucleus accumbens (NAS)-shell, but not within NAS-core, caudate putamen, medial prefrontal or orbital cortices. These findings are consistent with evidence that reward or positive reinforcement can be triggered by chemical and electrical stimulation over a large rostral–caudal extent of the VTA. They suggest that among ventral midbrain projection sites, the BNST and NAS-shell constitute important components of the circuitry implicated in reward. They provide additional support for the functional link between neurons that support PM and VTA self-stimulation, and offer topographical guidance to future attempts at their identification.

Nociceptin/Orphanin FQ Modulates Motor Behavior and Primary Motor Cortex Output Through Receptors Located in Substantia Nigra Reticulata

This study was set to investigate whether motor effects of nociceptin/orphanin FQ (N/OFQ) can be related to changes in primary motor cortex output. N/OFQ injected i.c.v. biphasically modulated motor performance, low doses being facilitating and higher ones inhibitory. These effects were counteracted by the N/OFQ receptor antagonist [Nphe(1) Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101) confirming the specificity of N/OFQ action. However, UFP-101 alone facilitated motor performance, suggesting that endogenous N/OFQ inhibits motor function. N/OFQ and UFP-101 injected into the substantia nigra reticulata but not motor cortex replicated these effects, suggesting motor responses were mediated by subcortical circuits involving the basal ganglia. Intracortical microstimulation technique showed that i.c.v. N/OFQ also biphasically modulated motor cortex excitability and movement representation. Low N/OFQ doses caused a leftward shift of threshold distribution curve in the forelimb area without affecting the number of effective sites. Conversely, high N/OFQ doses increased unresponsive and reduced excitable (movement) sites in vibrissa but not forelimb area. However, increased threshold currents and rightward shift of threshold distribution curve were observed in both areas, suggesting an overall inhibitory effect on cortical motor output. UFP-101 alone evoked effects similar to low N/OFQ doses, suggesting tonic inhibitory control over forelimb movement by endogenous N/OFQ. As shown in behavioral experiments, these effects were replicated by intranigral, but not intracortical, N/OFQ or UFP-101 injections. We conclude that N/OFQ receptors located in the substantia nigra reticulata mediate N/OFQ biphasic control over motor behavior, possibly through changes of primary motor cortex output.

Complex EPSCs evoked in substantia nigra reticulata neurons are disrupted by repetitive stimulation of the subthalamic nucleus

Although substantia nigra reticulata (SNR) neurons fire bursts of action potentials during normal movement, excessive burst firing correlates with symptoms of Parkinson's disease. A major excitatory output from the subthalamic nucleus (STN) to the SNR is thought to provide the synaptic impetus for burst firing in SNR neurons. Using patch pipettes to record from SNR neurons in rat brain slices, we found that a single electrical stimulus delivered to the STN evokes a burst of action potentials. Under voltage-clamp conditions, STN stimulation evokes a complex EPSC that is comprised of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a long-lasting inward current. Using varied stimulation frequencies, we found that the initial EPSC was significantly reduced or abolished after 2 s of 50-100 Hz STN stimulation. However, only 4 s of 1 Hz stimulation was required to abolish the late component of the complex EPSC. We suggest that differential effects of repetitive STN stimulation on early and late components of complex EPSCs may help explain the frequency-dependent effects of deep brain stimulation of the STN that is used in the treatment of Parkinson's disease.

The pedunculopontine nucleus in Parkinson's disease: primate studies

Gait freezing and poor balance are two of the most disabling symptoms of advanced Parkinson's disease (PD), and also of other untreatable progressive neurological disorders, such as multi-system atrophy (MSA) and progressive supranuclear palsy (PSP). In PD, these symptoms are currently inadequately managed by drugs and also the present surgical treatment of deep brain stimulation (DBS) of the sub-thalamic nucleus (STN) and the globus pallidus internus (GPi). The pedunculopontine nucleus (PPN) has been implicated in these symptoms. The PPN is in the upper brain stem. The major inhibitory input is from the GPi and substantia nigra reticulata (SNr), and bilateral output is to the substantia nigra compacta (SNc), thalamus and spinal cord. Stimulation of the PPN in the decerebrate rat, cat and dog induced gait-like movements. In autopsy studies in PD, MSA, PSP and the DYT-1 dystonic brain, the PPN is degenerate. Autoradiography of the MPTP-Parkinsonian primate shows excessive inhibition in the PPN. Lesions of the PPN in the normal primate induced PD-type bradykinesia, which was persistent with bilateral lesions. In the MPTP-primate model, microinjections of the gamma aminobutyric acid A (GABA) antagonist bicuculine into the PPN reversed Parkinsonian akinesia implying that stimulation of this region might have a therapeutic role in drug resistant PD. Low frequency (5–10Hz) stimulation of the PPN in the same model reversed akinesia independently of l-dopa; moreover, l-dopa and stimulation effects were additive, implying the involvement of non-dopaminergic pathways.

Normalization of GABAA receptor specific binding in the substantia nigra reticulata and the prevention of L‐dopa‐induced dyskinesias in MPTP parkinsonian monkeys

L-Dopa therapy in Parkinson's disease (PD) is counfounded by the development of involuntary movements such as L-Dopa-induced dyskinesias (LIDs). In this study GABA(A) receptor autoradiography was assessed using [(3)H]flunitrazepam binding to the benzodiazepine site of the GABA(A) receptor and [(35)S]t-butylbicyclophosphorothionate (TBPS) binding to the chloride channel of GABA(A) receptors in the substantia nigra reticulata (SNr) and subthalamic nucleus (STN). L-Dopa-treated parkinsonian monkeys experiencing LIDs were compared to animals in which LIDs was prevented by adjunct treatments with CI-1041, a selective antagonist of the NR1A/2B subtype of NMDA receptor, or low doses of the dopamine D2 receptor agonist, cabergoline. Our results demonstrated a decrease of GABA(A) receptor specific binding in the posterior part of the SNr in dyskinetic monkeys compared to nondyskinetic animals, while no modulation has been observed in the STN. These results provide evidence for the first time that pharmacological treatments preventing LIDs in nonhuman primate model of PD are associated with normalization of GABA(A) receptor-mediated signalling in the SNr.

NR2A and NR2B subunit containing NMDA receptors differentially regulate striatal output pathways

Triple probe microdialysis was employed to investigate whether striatal NR2A and NR2B subunit containing NMDA receptors regulate the activity of striato-pallidal and striato-nigral projection neurons. Probes were implanted in the striatum, ipsilateral globus pallidus and substantia nigra reticulata. Intrastriatal perfusion with the NR2A subunit selective antagonist (R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077) reduced pallidal GABA and increased nigral glutamate (GLU) release whereas perfusion with the NR2B subunit selective antagonist (R-(R*,S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidinepropanol (Ro 25-6981) reduced nigral GABA and elevated striatal and pallidal GLU release. To confirm that changes in GABA levels were because of blockade of (GLUergic-driven) tonic activity of striatofugal neurons, tetrodotoxin was perfused in the striatum. Tetrodotoxin reduced both pallidal and nigral GABA release without changing GLU levels. To investigate whether striatal NR2A and NR2B subunits were also involved in phasic activation of striatofugal neurons, NVP-AAM077 and Ro 25-6981 were challenged against a NMDA concentration able to evoke GABA release in the three areas. Both antagonists prevented the NMDA-induced striatal GABA release. NVP-AAM077 also prevented the NMDA-induced surge in GABA release in the globus pallidus, whereas Ro 25-6981 attenuated it in the substantia nigra. We conclude that striatal NMDA receptors containing NR2A and NR2B subunits preferentially regulate the striato-pallidal and striato-nigral projection neurons, respectively.

Mapping P2X and P2Y receptor proteins in striatum and substantia nigra: An immunohistological study

Our work aimed to provide a topographical analysis of all known ionotropic P2X1–7 and metabotropic P2Y1,2,4,6,11–14 receptors that are present in vivo at the protein level in the basal ganglia nuclei and particularly in rat brain slices from striatum and substantia nigra. By immunohistochemistry-confocal and Western blotting techniques, we show that, with the exception of P2Y11,13 receptors, all other subtypes are specifically expressed in these areas in different amounts, with ratings of low (P2X5,6 and P2Y1,6,14 in striatum), medium (P2X3 in striatum and substantia nigra, P2X6,7 and P2Y1 in substantia nigra) and high. Moreover, we describe that P2 receptors are localized on neurons (colocalizing with neurofilament light, medium and heavy chains) with features that are either dopaminergic (colocalizing with tyrosine hydroxylase) or GABAergic (colocalizing with parvalbumin and calbindin), and they are also present on astrocytes (P2Y2,4, colocalizing with glial fibrillary acidic protein). In addition, we aimed to investigate the expression of P2 receptors after dopamine denervation, obtained by using unilateral injection of 6-hydroxydopamine as an animal model of Parkinson’s disease. This generates a rearrangement of P2 proteins: most P2X and P2Y receptors are decreased on GABAergic and dopaminergic neurons, in the lesioned striatum and substantia nigra, respectively, as a consequence of dopaminergic denervation and/or neuronal degeneration. Conversely, P2X1,3,4,6 on GABAergic neurons and P2Y4 on astrocytes augment their expression exclusively in the lesioned substantia nigra reticulata, probably as a compensatory reaction to dopamine shortage. These results disclose the presence of P2 receptors in the normal and lesioned nigro-striatal circuit, and suggest their potential participation in the mechanisms of Parkinson’s disease.

Bursting in Substantia Nigra Pars Reticulata Neurons In Vitro: Possible Relevance for Parkinson Disease

Projection neurons of the substantia nigra reticulata (SNr) convey basal ganglia (BG) processing to thalamocortical and brain stem circuits responsible for movement. Two models try to explain pathological BG performance during Parkinson disease (PD): the rate model, which posits an overexcitation of SNr neurons due to hyperactivity in the indirect pathway and hypoactivity of the direct pathway, and the oscillatory model, which explains PD as the product of pathological pattern generators disclosed by dopamine reduction. These models are, apparently, incompatible. We tested the predictions of the rate model by increasing the excitatory drive and reducing the inhibition on SNr neurons in vitro. This was done pharmacologically with bath application of glutamate agonist N-methyl-d-aspartate and GABA(A) receptor blockers, respectively. Both maneuvers induced bursting behavior in SNr neurons. Therefore synaptic changes forecasted by the rate model induce the electrical behavior predicted by the oscillatory model. In addition, we found evidence that Ca(V)3.2 Ca(2+) channels are a critical step in generating the bursting firing pattern in SNr neurons. Other ion channels involved are: hyperpolarization-activated cation channels, high-voltage-activated Ca(2+) channels, and Ca(2+)-activated K(+) channels. However, although these channels shape the temporal structure of bursting, only Ca(V)3.2 Ca(2+) channels are indispensable for the initiation of the bursting pattern.

Lack of Spike-Count and Spike-Time Correlations in the Substantia Nigra Reticulata Despite Overlap of Neural Responses

Previous studies of single neurons in the substantia nigra reticulata (SNr) have shown that many of them respond to similar events. These results, as well as anatomical studies, suggest that SNr neurons share inputs and thus may have correlated activity. Different types of correlation can exist between pairs of neurons. These are traditionally classified as either spike-count ("signal" and "noise") or spike-timing (spike-to-spike and joint peristimulus time histograms) correlations. These measures of neuronal correlation are partially independent and have different implications. Our purpose was to probe the computational characteristics of the basal ganglia output nuclei through an analysis of these different types of correlation in the SNr. We carried out simultaneous multiple-electrode single-unit recordings in the SNr of two monkeys performing a probabilistic delayed visuomotor response task. A total of 113 neurons (yielding 355 simultaneously recorded pairs) were studied. Most SNr neurons responded to one or more task-related events, with instruction cue (69%) and reward (63%) predominating. Response-match analysis, comparing peristimulus time histograms, revealed a significant overlap between response vectors. However, no measure of average correlation differed significantly from zero. The lack of significant SNr spike-count population correlations appears to be an exceptional phenomenon in the brain, perhaps indicating unique event-related processing by basal ganglia output neurons to achieve better information transfer. The lack of spike-timing correlations suggests that the basal high-frequency discharge of SNr neurons is not driven by the common inputs and is probably intrinsic.

Behavioral Electrophysiology of Psychostimulants

The motor-activating effects of amphetamine and other psychostimulants such as cocaine depend on an increase in dopamine (DA) transmission in the striatum, a key component of the basal ganglia and the forebrain motive circuit. This review focuses on research aimed at using electrophysiological techniques--including extracellular unit recording and iontophoresis--in alert, fully functioning animals to understand how these drugs alter striatal neuronal processing under behaviorally relevant conditions. The data indicate that DA works in conjunction with glutamate (GLU), an excitatory amino acid, to enhance the signal-to-noise ratio of afferent information. This DA-GLU interaction appears to play a critical role in the amphetamine-induced activation of striatal neurons. The pattern of striatal activation, moreover, changes as the behavioral response changes from unfocused locomotion to highly focused, stereotyped behavior, but interestingly, the striatal response pattern is not reflected in substantia nigra reticulata, a primary target of striatal efferents. Although cocaine also activates striatal neurons during behavior, the underlying mechanisms appear to be complicated by factors unique to this drug and deserve further evaluation. Collectively, these findings provide unique insight into the neuronal processes by which the striatum participates in psychostimulant-induced motor behavior.

Histamine H3 receptor agonists reduce L‐dopa–induced chorea, but not dystonia, in the MPTP‐lesioned nonhuman primate model of Parkinson's disease

L-dopa-induced dyskinesia (LID) remains a major complication of the treatment of Parkinson's disease. The neural mechanisms underlying LID are thought to involve overactivity of striatal glutamatergic neurotransmission, with resultant underactivation of the output regions of the basal ganglia. Histamine H3 heteroreceptors can reduce glutamate and gamma-aminobutyric acid (GABA) transmission in the striatum and substantia nigra reticulata, respectively. Thus, we tested whether the histamine H3 receptor agonists immepip and imetit can alleviate LID in the MPTP-lesioned marmoset model of Parkinson's disease. Coadministration of immepip (1 mg/kg) with L-dopa (15 mg/kg) was associated with significantly less total dyskinesia than L-dopa alone. When dyskinesia was separately rated as chorea and dystonia, coadministration of L-dopa with either immepip or imetit (both 10 mg/kg) significantly reduced chorea but had no effect on dystonia. The antidyskinetic actions of the H3 agonists were not accompanied by alteration of the antiparkinsonian actions of L-dopa. However, immepip (10 mg/kg), when administered as monotherapy, significantly increased parkinsonian disability compared to vehicle. Overall, the results obtained in this study suggest that histamine H3 receptors may be involved in the neural mechanisms underlying L-dopa-induced dyskinesia in Parkinson's disease.

GABAergic mechanisms in regulating the activity state of substantia nigra pars reticulata neurons

Substantia nigra reticulata is the major output structure of the basal ganglia involved in somatosensory integration and organization of movement. While previous work in vitro and in anesthetized animal preparations suggests that these neurons are autoactive and points to GABA as a primary input regulating their activity, single-unit recording coupled with iontophoresis was used in awake, unrestrained rats to further clarify the role of tonic and phasic GABA input in maintenance and fluctuations of substantia nigra reticulata neuronal activity under physiologically relevant conditions. In contrast to glutamate, which was virtually ineffective at stimulating substantia nigra reticulata neurons in awake rats, all substantia nigra reticulata neurons tested were inhibited by iontophoretic GABA and strongly excited by bicuculline, a GABA-A receptor blocker. The GABA-induced inhibition had short onset and offset latencies, a fading response pattern (a rapid decrease in rate followed by its relative restoration), and was independent of basal discharge rate. The bicuculline-induced excitation was inversely related to discharge rate and current (dose)-dependent in individual units. However, the average discharge rate during bicuculline applications at different currents increased to a similar plateau (∼60 impulses/s), which was about twice the mean basal rates. The excitatory effects of bicuculline were phasically inhibited or completely blocked by brief GABA applications and generally mimicked by gabazine, another selective GABA antagonist. These data as well as neuronal inhibitions induced by nipecotic acid, a selective GABA uptake inhibitor, suggest that substantia nigra reticulata neurons in awake, quietly resting conditions are under tonic, GABA-mediated inhibition. Therefore, because of inherent autoactivity and specifics of afferent inputs, substantia nigra reticulata neurons are very sensitive to phasic alterations in GABA input, which appears to be the primary factor determining fluctuations in their activity states under physiological conditions. While these cells are relatively insensitive to direct activation by glutamate, and resistant to a continuous increase in GABA input, they appear to be very sensitive to a diminished GABA input, which may release them from tonic inhibition and determine their functional hyperactivity.

Tau‐predominant–associated pathology in a sporadic late‐onset Hallervorden–Spatz syndrome

Hallervorden-Spatz syndrome (HSS) is a heterogeneous clinicopathological disorder currently included within the broader title of neurodegeneration with brain iron accumulation (NBIA). The classic histological hallmarks of HSS are axonal spheroids and excessive iron-containing granules accompanied by neuronal loss and gliosis in the globus pallidus and substantia nigra reticulata. In the modern literature, attention has been drawn to the co-occurrence of two other histological markers: Lewy bodies mainly composed of abnormal alpha-synuclein, and neurofibrillary tangles due to hyperphosphorilated tau aggregation. Discrepancies exist regarding the importance of these molecular changes and its relevance for the nosology of HSS. Most authors have emphasized the importance of the Lewy body-like pathology, favoring the inclusion of HSS within the alpha-synucleinopathies. We report on a case of late-onset HSS, with the typical histological findings restricted to the basal ganglia and cerebellum in which tau pathology was exceedingly more abundant than alpha-synuclein pathology. This case contributes to the increasing evidence about the heterogeneity of HSS. We favor the view that the molecular changes and the protein misfolding underlying the Lewy body and tangle formation in HSS/NBIA are secondary to the main pathological process and should not be taken as the basis for its nosological classification.

GABAA Receptors as Broadcasters of Sexually Differentiating Signals in the Brain

Epileptic seizures are more common in males than in females. One of the areas that has recently been implicated in the higher susceptibility of males to seizures is the substantia nigra reticulata (SNR). Several studies support the existence of phenotypic differences between male and female infantile SNR neurons, and particularly in several aspects of the GABAergic system, including its ability to control seizures. We have recently found that at postnatal day 14-17 (PN14-17) rats, which are equivalent to infants, activation of GABA(A) receptors has different physiological effects in male and female SNR neurons. This is likely due to the differences in the expression of the neuronal-specific potassium-chloride co-transporter KCC2, which regulates the intracellular chloride concentration. In male PN14-17 SNR neurons, GABA(A)-receptor activation with muscimol causes depolarization and increments in intracellular calcium concentration and the expression of calcium regulated genes, such as KCC2. Blockade of L-type voltage-sensitive calcium channels (L-VSCC) by nifedipine decreases KCC2 mRNA expression. However, in PN14-17 females, muscimol hyperpolarizes the SNR neurons, does not increase intracellular calcium, and decreases KCC2 mRNA expression. In PN15 females, nifedipine has no effect on KCC2 mRNA expression in the SNR. This sexually dimorphic function of GABA(A) receptors also creates divergent patterns of estradiol signaling. In male PN15 rats, estradiol decreases KCC2 mRNA expression in SNR neurons. Pretreatment with the GABA(A)-receptor antagonist bicuculline or with nifedipine, prevents the appearance of estradiol-mediated downregulation of KCC2 mRNA expression. In contrast, in PN15 females, estradiol does not influence KCC2 expression. These findings show that, in infantile rats, drugs or conditions that modulate the activity of GABA(A) receptors or L-VSCCs have different effects on the differentiation of the SNR. As a result, they have the potency of causing long-term changes in the function of the SNR in the control of seizures, movement, and the susceptibility to and course of epilepsy and movement disorders.