Modulation of GABA release by dopamine in the substantia nigra

5-HT1B receptors mediate dopaminergic inhibition of vesicular fusion and GABA release from striatonigral synapses

The substantia nigra pars reticulata (SNr), a crucial basal ganglia output nucleus, contains a dense expression of dopamine D1 receptors (D1Rs), along with dendrites belonging to dopaminergic neurons of substantia nigra pars compacta. These D1Rs are primarily located on the terminals of striatonigral medium spiny neurons, suggesting their involvement in the regulation of neurotransmitter release from the direct pathway in response to somatodendritic dopamine release. To explore the hypothesis that D1Rs modulate GABA release from striatonigral synapses, we conducted optical recordings of striatonigral activity and postsynaptic patch-clamp recordings from SNr neurons in the presence of dopamine and D1R agonists. We found that dopamine inhibits optogenetically triggered striatonigral GABA release by modulating vesicle fusion and Ca 2+ influx in striatonigral boutons. Notably, the effect of DA was independent of D1R activity but required activation of 5-HT1B receptors. Our results suggest a serotonergic mechanism involved in the therapeutic actions of dopaminergic medications for Parkinson's disease and psychostimulant-related disorders.

Combined and sequential effects of alcohol and methamphetamine in animal models

Comorbid drug use, often alcohol with other drugs, poses significant health and societal concerns. Methamphetamine is among the illicit drugs most often co-used with alcohol. The current review examines the animal literature for impacts of comorbid alcohol and methamphetamine exposure. We found evidence for additive or synergistic effects of combined or sequential exposure on behavior and physiology. Dopaminergic, serotonergic, and glutamatergic systems are all impacted by combined exposure to alcohol and methamphetamine and cyclooxygenase-2 activity plays an important role in their combined neurotoxic effects. Adverse consequences of comorbid exposure include altered brain development with prenatal exposure, impaired learning and memory, motor deficits, gastrotoxicity, hepatotoxicity, and augmented intake under some conditions. Given high susceptibility to drug experimentation in adolescence, studies of co-exposure during the adolescent period and of how adolescent exposure to one drug impacts later use or sensitivity to the other drug should be a priority. Further, to gain traction on prevention and treatment, additional research to identify motivational and neurobiological drivers and consequences of comorbid use is needed.

Ethanol-induced alterations of amino acids measured by in vivo microdialysis in rats: a meta-analysis

PURPOSE

In recent years in vivo microdialysis has become an important method in research studies investigating the alterations of neurotransmitters in the extracellular fluid of the brain. Based on the major involvement of glutamate and γ-aminobutyric acid (GABA) in mediating a variety of alcohol effects in the mammalian brain, numerous microdialysis studies have focused on the dynamical behavior of these systems in response to alcohol.

METHODS

Here we performed multiple meta-analyses on published datasets from the rat brain: (i) we studied basal extracellular concentrations of glutamate and GABA in brain regions that belong to a neurocircuitry involved in neuropsychiatric diseases, especially in alcoholism (Noori et al., Addict Biol 17:827-864, 2012); (ii) we examined the effect of acute ethanol administration on glutamate and GABA levels within this network and (iii) we studied alcohol withdrawal-induced alterations in glutamate and GABA levels within this neurocircuitry.

RESULTS

For extraction of basal concentrations of these neurotransmitters, datasets of 6932 rats were analyzed and the absolute basal glutamate and GABA levels were estimated for 18 different brain sites. In response to different doses of acute ethanol administration, datasets of 529 rats were analyzed and a non-linear dose response (glutamate and GABA release) relationship was observed in several brain sites. Specifically, glutamate in the nucleus accumbens shows a decreasing logarithmic dose response curve. Finally, regression analysis of 11 published reports employing brain microdialysis experiments in 104 alcohol-dependent rats reveals very consistent augmented extracellular glutamate and GABA levels in various brain sites that correlate with the intensity of the withdrawal response were identified.

CONCLUSIONS

In summary, our results provide standardized basal values for future experimental and in silico studies on neurotransmitter release in the rat brain and may be helpful to understand the effect of ethanol on neurotransmitter release. Furthermore, this study illustrates the benefit of meta-analyses using the generalization of a wide range of preclinical data.

Dopamine-nociceptin/orphanin FQ interactions in the substantia nigra reticulata of hemiparkinsonian rats: involvement of D2/D3 receptors and impact on nigro-thalamic neurons and motor activity

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor antagonists proved to be effective in alleviating experimental parkinsonism. Nonetheless, loss of effectiveness or even worsening of parkinsonian symptoms have been observed at high doses. With the aim of clarifying the circuitry underlying the dual action of NOP receptor antagonists and the role of endogenous dopamine, the NOP receptor antagonist 1-benzyl-N-[3-[spiroisobenzofuran-1(3H),4'-piperidin-1-yl]propyl]pyrrolidine-2-carboxamide (Compound 24) and the D(2)/D(3) receptor antagonist raclopride were used in 6-hydroxydopamine hemilesioned rats. Systemically administered Compound 24 improved motor activity in the 0.1-10mg/kg dose range being ineffective at 30 mg/kg. To confirm NOP selectivity, Compound 24 improved motor performance in wild-type mice at 1 and 10mg/kg and inhibited it at 60 mg/kg, being ineffective in NOP receptor knockout mice. To prove that the bell-shaped profile was mediated by nigral NOP receptors, reverse dialysis of Compound 24 (0.03 μM) in substantia nigra reticulata ameliorated akinesia whereas Compound 24 (3 μM) was ineffective. To demonstrate that motor responses were mediated by tuning inhibitory and excitatory inputs to nigro-thalamic neurons, the low concentration elevated GABA and reduced glutamate in substantia nigra, simultaneously reducing GABA levels in ventro-medial thalamus. Conversely, the higher concentration reduced nigral and elevated thalamic GABA, without affecting nigral glutamate levels. Co-perfusion with raclopride (1 μM) abolished the antiakinetic action of Compound 24 (0.03 μM) and turned the ineffectiveness of Compound 24 (3 μM) into an antiakinetic effect. The low concentration reduced nigral but did not affect thalamic GABA whereas the higher concentration elevated nigral and reduced thalamic GABA. Neither concentration affected nigral glutamate. We conclude that dual motor effects of Compound 24 in hemiparkinsonian rats are accomplished through blockade of nigral NOP receptors resulting in opposite modulation of nigro-thalamic neurons. Endogenous dopamine contributes to these responses affecting the level of GABAergic inhibition of the nigral output via D(2)/D(3) receptors.

Dopaminergic presynaptic modulation of nigral afferents: its role in the generation of recurrent bursting in substantia nigra pars reticulata neurons

Previous work has shown the functions associated with activation of dopamine presynaptic receptors in some substantia nigra pars reticulata (SNr) afferents: (i) striatonigral terminals (direct pathway) posses presynaptic dopamine D₁-class receptors whose action is to enhance inhibitory postsynaptic currents (IPSCs) and GABA transmission. (ii) Subthalamonigral terminals posses D₁- and D₂-class receptors where D₁-class receptor activation enhances and D₂-class receptor activation decreases excitatory postsynaptic currents. Here we report that pallidonigral afferents posses D₂-class receptors (D₃ and D₄ types) that decrease inhibitory synaptic transmission via presynaptic modulation. No action of D₁-class agonists was found on pallidonigral synapses. In contrast, administration of D₁-receptor antagonists greatly decreased striatonigral IPSCs in the same preparation, suggesting that tonic dopamine levels help in maintaining the function of the striatonigral (direct) pathway. When both D₃ and D₄ type receptors were blocked, pallidonigral IPSCs increased in amplitude while striatonigral connections had no significant change, suggesting that tonic dopamine levels are repressing a powerful inhibition conveyed by pallidonigral synapses (a branch of the indirect pathway). We then blocked both D₁- and D₂-class receptors to acutely decrease direct pathway (striatonigral) and enhance indirect pathways (subthalamonigral and pallidonigral) synaptic force. The result was that most SNr projection neurons entered a recurrent bursting firing mode similar to that observed during Parkinsonism in both patients and animal models. These results raise the question as to whether the lack of dopamine in basal ganglia output nuclei is enough to generate some pathological signs of Parkinsonism.

Extrastriatal dopaminergic circuits of the Basal Ganglia

The basal ganglia are comprised of the striatum, the external and internal segment of the globus pallidus (GPe and GPi, respectively), the subthalamic nucleus (STN), and the substantia nigra pars compacta and reticulata (SNc and SNr, respectively). Dopamine has long been identified as an important modulator of basal ganglia function in the striatum, and disturbances of striatal dopaminergic transmission have been implicated in diseases such as Parkinson's disease (PD), addiction and attention deficit hyperactivity disorder. However, recent evidence suggests that dopamine may also modulate basal ganglia function at sites outside of the striatum, and that changes in dopaminergic transmission at these sites may contribute to the symptoms of PD and other neuropsychiatric disorders. This review summarizes the current knowledge of the anatomy, functional effects and behavioral consequences of the dopaminergic innervation to the GPe, GPi, STN, and SNr. Further insights into the dopaminergic modulation of basal ganglia function at extrastriatal sites may provide us with opportunities to develop new and more specific strategies for treating disorders of basal ganglia dysfunction.

Ultrastructural localization and function of dopamine D1-like receptors in the substantia nigra pars reticulata and the internal segment of the globus pallidus of parkinsonian monkeys

The motor symptoms of Parkinson's disease (PD) are commonly attributed to striatal dopamine loss, but reduced dopamine innervation of basal ganglia output nuclei, the internal globus pallidus (GPi) and the substantia nigra pars reticulata (SNr) may also contribute to symptoms and signs of PD. Both structures express dopamine D1 and D5 receptors under normal conditions, and we have recently demonstrated that their local activation reduces neuronal discharge rates and enhances bursts and oscillatory activity in both nuclei of normal monkeys [M.A. Kliem et al. (2007)J. Neurophysiol., 89, 1489-1500]. Here, we determined the ultrastructural localization and function of D1-like receptors in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys. In both normal and MPTP-treated monkeys, most of the D1 and D5 receptor immunoreactivity was associated with unmyelinated axons, but we also found significant postsynaptic D5 receptor immunostaining in dendrites of GPi and SNr neurons. A significant proportion of axonal D1 immunostaining was bound to the plasma membrane in both normal and MPTP-treated monkeys. Local microinjections of the D1/D5 receptor agonist SKF82958 significantly reduced discharge rates in GPi and SNr neurons, while they increased burst firing and oscillatory activity in the 3-15-Hz band in SNr, but not in GPi, of parkinsonian monkeys. Together with our recent findings from normal monkeys, these data provide evidence that functional D1/D5 receptors are expressed in GPi and SNr in both normal and parkinsonian states, and that their activation by endogenous dopamine (under normal conditions) or dopamine receptor agonists (in parkinsonism) may regulate basal ganglia outflow.

Dendritic transmitter release: a comparison of two model systems

Information flow through neurones was historically considered to be linear, with dendrites receiving information from incoming synaptic terminals, the soma processing the information and the axon carrying it to the terminal that synapses upon another cell or end organ. However, recent studies have shown that dendrites can release transmitters themselves, and thereby communicate with neighbouring structures, whether these are adjacent neurones or incoming synapses. Due to their anatomical features, the magnocellular vasopressin and oxytocin containing neurones of the hypothalamic supraoptic and paraventricular nuclei and the dopamine neurones of the substantia nigra have revealed important aspects of dendritic function including mechanisms of dendritic transmitter release.

Activation of nigral and pallidal dopamine D1-like receptors modulates basal ganglia outflow in monkeys

Studies of the effects of dopamine in the basal ganglia have focused on the striatum, whereas the functions of dopamine released in the internal pallidal segment (GPi) or in the substantia nigra pars reticulata (SNr) have received less attention. Anatomic and biochemical investigations have demonstrated the presence of dopamine D1-like receptors (D1LRs) in GPi and SNr, which are primarily located on axons and axon terminals of the GABAergic striatopallidal and striatonigral afferents. Our experiments assessed the effects of D1LR ligands in GPi and SNr on local gamma-aminobutyric acid (GABA) levels and neuronal activity in these nuclei in rhesus monkeys. Microinjections of the D1LR receptor agonist SKF82958 into GPi and SNr significantly reduced discharge rates in GPi and SNr, whereas injections of the D1LR antagonist SCH23390 increased firing in the majority of GPi neurons. D1LR activation also increased bursting and oscillations in neuronal discharge in the 3- to 15-Hz band in both structures, whereas D1LR blockade had the opposite effects in GPi. Microdialysis measurements of GABA concentrations in GPi and SNr showed that the D1LR agonist increased the level of the transmitter. Both findings are compatible with the hypothesis that D1LR activation leads to GABA release from striatopallidal or striatonigral afferents, which may secondarily reduce firing of basal ganglia output neurons. The antagonist experiments suggest that a dopaminergic "tone" exists in GPi. Our results support the finding that D1LR activation may have powerful effects on GPi and SNr neurons and may mediate some of the effects of dopamine replacement therapies in Parkinson's disease.

Dynamic changes in vesicular glutamate transporter 1 function and expression related to methamphetamine-induced glutamate release

The vesicular glutamate (GLU) transporter (VGLUT1) is a critical component of glutamatergic neurons that regulates GLU release. Despite the likely role of GLU release in drug abuse pathology, there is no information that links VGLUT1 with drugs of abuse. This study provides the first evidence that methamphetamine (METH) alters the dynamic regulation of striatal VGLUT1 function and expression through a polysynaptic pathway. METH increases cortical VGLUT1 mRNA, striatal VGLUT1 protein in subcellular fractions, and the Vmax of striatal vesicular GLU uptake. METH also increases glyceraldehyde-3-phosphate dehydrogenase (GAPDH) protein in the crude vesicle fraction. METH-induced increases in cortical VGLUT1 mRNA, as well as striatal VGLUT1 and GAPDH, are GABA(A) receptor-dependent because they are blocked by GABA(A) receptor antagonism in the substantia nigra. These results show that VGLUT1 can be dynamically regulated via a polysynaptic pathway to facilitate vesicular accumulation of GLU for subsequent release after METH.

Dopamine D(1) receptor facilitation of depolarization-induced release of gamma-amino-butyric acid in rat striatum is mediated by the cAMP/PKA pathway and involves P/Q-type calcium channels

Transmission in the "direct" pathway through the basal ganglia, which has an important role in the control of motor movement, is markedly facilitated by the concurrent activation of dopamine D(1) receptors. Consistent with this, Ca(2+)-dependent, depolarization-induced release of [(3)H]-GABA from striatal slices from rats pretreated with reserpine was greatly increased in the presence of 1 microM SKF 38393, a dopamine D(1)-like receptor agonist. The effect of SKF 38393 was mimicked by 1 mM 8-bromo-cyclic AMP (Br-cAMP) and inhibited by the protein kinase A (PKA) inhibitor H-89, mean inhibition 92% +/- 4% with 10 microM H-89 (n = 3). The effects of SKF 38393 and Br-cAMP were not additive. The stimulatory effects of SKF 38393 and Br-cAMP were practically abolished in the presence of the histamine H(3) receptor agonist immepip (1 microM). The depolarization-induced release of [(3)H]-GABA in the presence of SKF 38393 was not significantly inhibited by 5 microM nimodipine, an L-type Ca(2+) channel blocker, or by 0.3 microM omega-conotoxin MVIIA, a selective blocker of N-type channels. However, preincubation of the slices with 0.95 microM omega-agatoxin TK, a P/Q-type channel blocker, followed by washing before changing to a depolarizing medium containing SKF 38393, resulted in a marked inhibition of the stimulated release of [(3)H]-GABA, mean 68% +/- 4% (n = 3). These observations provide evidence that dopamine D(1) agonist facilitation of the depolarization-induced release of GABA from striatal terminals is mediated by the cAMP/PKA pathway and involves mainly P/Q-type Ca(2+) channels.

Interactions between methamphetamine and environmental stress: role of oxidative stress, glutamate and mitochondrial dysfunction

AIMS

Methamphetamine is an amphetamine derivative that is abused increasingly world-wide at an alarming rate over the last decade. Pre-clinical and human studies have shown that methamphetamine is neurotoxic to brain dopamine and serotonin. Other lines of study indicate that stress enhances the vulnerability to drug abuse. The purpose of this review is to shed light on the biochemical similarities between methamphetamine and stress in an effort to highlight the possibility that prior exposure to stress may interact with methamphetamine to exacerbate neurotoxicity.

METHODS

A review of the literature on methamphetamine and stress was conducted that focused on the common neurotoxic and biochemical consequences of methamphetamine administration and stress exposure.

RESULTS

Experimental findings of a large number of studies suggest that there are parallels between stress and methamphetamine with regard to their ability to increase glutamate release, produce a metabolic compromise and cause oxidative damage.

CONCLUSION

A combination of methamphetamine administration and stress can act synergistically and/or additively to cause or augment toxicity in brain regions such as striatum and hippocampus.

Subchronic administration of l-DOPA to adult rats with a unilateral 6-hydroxydopamine lesion of dopamine neurons results in a sensitization of enhanced GABA release in the substantia nigra, pars reticulata

L-DOPA is the most effective pharmacological agent used for the symptomatic treatment of Parkinson's disease but long-term L-DOPA treatment induces involuntary abnormal movements such as dyskinesias. The present study, using in vivo microdialysis, investigated the effects of a single or subchronic administration of L-DOPA to adult rats with a unilateral 6-OHDA lesion of dopamine neurons on GABA release in the substantia nigra, pars reticulata (SNr). The results indicate that a challenge injection of L-DOPA (50 mg/kg, i.p.) significantly increases GABA levels in the SNr of rats treated with a daily repeated administration of L-DOPA (50 mg/kg, i.p.). Further statistical analysis between groups also showed that extracellular GABA levels were significantly higher in the subchronic L-DOPA group than in the group receiving only one injection of L-DOPA. These results show that the subchronic administration of L-DOPA results in a sensitization of enhanced extracellular GABA levels in the SNr.

High-dose methamphetamine acutely activates the striatonigral pathway to increase striatal glutamate and mediate long-term dopamine toxicity

Methamphetamine (METH) has been shown to increase the extracellular concentrations of both dopamine (DA) and glutamate (GLU) in the striatum. Dopamine, glutamate, or their combined effects have been hypothesized to mediate striatal DA nerve terminal damage. Although it is known that METH releases DA via reverse transport, it is not known how METH increases the release of GLU. We hypothesized that METH increases GLU indirectly via activation of the basal ganglia output pathways. METH increased striatonigral GABAergic transmission, as evidenced by increased striatal GAD65 mRNA expression and extracellular GABA concentrations in substantia nigra pars reticulata (SNr). The METH-induced increase in nigral extracellular GABA concentrations was D1 receptor-dependent because intranigral perfusion of the D1 DA antagonist SCH23390 (10 microm) attenuated the METH-induced increase in GABA release in the SNr. Additionally, METH decreased extracellular GABA concentrations in the ventromedial thalamus (VM). Intranigral perfusion of the GABA-A receptor antagonist, bicuculline (10 microm), blocked the METH-induced decrease in extracellular GABA in the VM and the METH-induced increase in striatal GLU. Intranigral perfusion of either a DA D1 or GABA-A receptor antagonist during the systemic administrations of METH attenuated the striatal DA depletions when measured 1 week later. These results show that METH enhances D1-mediated striatonigral GABAergic transmission (1), which in turn activates GABA-A receptors in the SNr (2), leading to a decrease in GABAergic nigrothalamic activity (3), an increase in corticostriatal GLU release (4), and a consequent long-term depletion of striatal DA content (5).

The GABA uptake inhibitor beta-alanine reduces pilocarpine-induced tremor and increases extracellular GABA in substantia nigra pars reticulata as measured by microdialysis

Substantia nigra pars reticulata (SNr) is a major output nucleus of the basal ganglia that receives GABAergic projections from neostriatum and globus pallidus. Previous research has shown that local pharmacological manipulations of GABA in SNr can influence tremulous jaw movements in rats. Tremulous jaw movements are defined as rapid vertical deflections of the lower jaw that resemble chewing but are not directed at a particular stimulus, and evidence indicates that these movements share many characteristics with parkinsonian tremor in humans. In order to investigate the role of GABA in motor functions related to tremor, the present study tested the GABA uptake blocker beta-alanine for its ability to reduce pilocarpine-induced tremulous jaw movements. In a parallel experiment, the effect of an active dose of beta-alanine on dialysate levels of GABA in SNr was assessed using microdialysis methods. GABA levels in dialysis samples were measured using high performance liquid chromatography with electrochemical detection. beta-Alanine (250-500 mg/kg) significantly reduced tremulous jaw movements induced by pilocarpine (4.0 mg/kg). Moreover, systemic administration of beta-alanine at a dose that reduced tremulous jaw movements (500 mg/kg) resulted in a substantial increase in extracellular levels of GABA in SNr compared to the pre-injection baseline. Thus, the present results are consistent with the hypothesis that GABAergic tone in SNr plays a role in the regulation of tremulous jaw movements. This research may lead to a better understanding of how parkinsonian symptoms are modulated by SNr GABA mechanisms.

Sampling glutamate and GABA with microdialysis: suggestions on how to get the dialysis membrane closer to the synapse

Microdialysis is currently optimized to sample the extrasynaptic pool. As such, the technique has facilitated discovery of ischemia-induced excitotoxic glutamate overflow (Benveniste H, Drejer J, Schousboe A, Diemer NH, 1987, Regional cerebral glucose phosphorylation and blood flow after insertion of a microdialysis fiber through the dorsal hippocampus in the rat. J. Neurochem., 49, 729-734) and adenosinergic sleep drive (Porkka-Heiskanen T, Strecker RE, Thakkar M, Bjorkum AA, Greene RW, McCarley RW, 1997, Adenosine: a mediator of the sleep-inducing effects of prolonged wakefulness. Science, 276 (5316), 1265-1268); and is proving essential for clinical monitoring of glutamate and cellular metabolites in stroke and head trauma (Sarrafzadeh AS, Sakowitz OW, Kiening KL, Benndorf G, Lanksch WR, Unterberg AW. Bedside microdialysis: a tool to monitor cerebral metabolism in subarachnoid hemorrhage patients? Crit. Care Med. 2002, 30 (5): 1062-1070). Study of the origin of extrasynaptic glutamate sampled with microdialysis has advanced understanding of extrasynaptic signal processing (Baker DA, Xi ZX, Shen H, Swanson CJ, Kalivas PW. The origin and neuronal function of in vivo nonsynaptic glutamate. J. Neurosci. 2002, 22 (20): 9134-9141; Baker DA, McFarland K, Lake RW, Shen H, Tang XC, Toda S, Kalivas PW, 2003, Neuroadaptations in cystine-glutamate exchange underlie cocaine relapse. Nat. Neurosci., 6, 743-749) in the CNS. Microdialysis studies furthermore demonstrate that synaptic pools of some neurotransmitters spill into the extrasynaptic space. For this reason, microdialysis has provided a window into the synaptic pool that has significantly advanced understanding of neurotransmitter control of behavior (Tanda G, Pontieri FE, Di Chiara G, 1997, Cannabinoid and heroin activation of mesolimbic dopamine transmission by a common mu1 opioid receptor mechanism. Science, 276, 2048-2050). Nonetheless, ability to sample synaptic pools of neurotransmitters is limited. Here we summarize evidence that microdialysis often fails to sample synaptic pools of neurotransmitters, such as glutamate and GABA because of rapid clearance and limited diffusion of these neurotransmitters from the synapse. Moreover, we consider means to move the dialysis membrane closer to the synapse to facilitate sampling of the synaptic pool of these neurotransmitters by minimizing tissue trauma, decreasing probe size and increasing temporal resolution.

Substantia nigra pars reticulata GABA is involved in the regulation of operant lever pressing: pharmacological and microdialysis studies

Substantia nigra pars reticulata (SNr) is an important mesencephalic nucleus that functions as a relay area for basal ganglia output. SNr receives GABAergic inputs from the neostriatum and globus pallidus, and in turn sends projections to a variety of motor areas. Although a large number of studies have focused upon the behavioral functions of basal ganglia dopamine, much less is known about the behavioral functions of SNr GABA. The present studies were undertaken to investigate the role of SNr GABA in lever pressing behavior. In the first experiment, the GABA(A) antagonist bicuculline was infused locally into SNr to determine if blockade of GABA receptors interfered with the performance of lever pressing on a fixed ratio 5 schedule. SNr injections of bicuculline produced a dose-related suppression of operant responding. Analysis of interresponse time bins showed that SNr bicuculline produced a response slowing characterized by a relative reduction in the number of fast interresponse times, and an increase in the relative number of pauses. In an additional experiment, microdialysis methods were used to determine if extracellular GABA is elevated during the performance of fixed ratio five lever pressing. During the 30 min lever pressing session, extracellular GABA showed a significant and substantial increase relative to baseline levels. These data support the hypothesis that SNr GABA is involved in the regulation of motor output, and indicate that GABA release in this structure is increased during behavioral stimulation.

Inhibition of calpains prevents neuronal and behavioral deficits in an MPTP mouse model of Parkinson's disease

The molecular mechanisms mediating degeneration of midbrain dopamine neurons in Parkinson's disease (PD) are poorly understood. Here, we provide evidence to support a role for the involvement of the calcium-dependent proteases, calpains, in the loss of dopamine neurons in a mouse model of PD. We show that administration of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) evokes an increase in calpain-mediated proteolysis in nigral dopamine neurons in vivo. Inhibition of calpain proteolysis using either a calpain inhibitor (MDL-28170) or adenovirus-mediated overexpression of the endogenous calpain inhibitor protein, calpastatin, significantly attenuated MPTP-induced loss of nigral dopamine neurons. Commensurate with this neuroprotection, MPTP-induced locomotor deficits were abolished, and markers of striatal postsynaptic activity were normalized in calpain inhibitor-treated mice. However, behavioral improvements in MPTP-treated, calpain inhibited mice did not correlate with restored levels of striatal dopamine. These results suggest that protection against nigral neuron degeneration in PD may be sufficient to facilitate normalized locomotor activity without necessitating striatal reinnervation. Immunohistochemical analyses of postmortem midbrain tissues from human PD cases also displayed evidence of increased calpain-related proteolytic activity that was not evident in age-matched control subjects. Taken together, our findings provide a potentially novel correlation between calpain proteolytic activity in an MPTP model of PD and the etiology of neuronal loss in PD in humans.

Somatodendritic dopamine release in rat substantia nigra influences motor performance on the accelerating rod

The physiological role of somatodendritic dopamine release in the rat substantia nigra was evaluated with a combination of dual probe microdialysis and simultaneous motor performance tests on an accelerating rod. Three main findings support a modulating influence of somatodendritic dopamine release on motor coordination. (1) The rod performance tests were associated with an increase in extracellular dopamine but not 5-hydroxytryptamine concentrations in substantia nigra and with increases in both dopamine and 5-hydroxytryptamine concentrations in the striatum. (2) Nigral application of dopamine antagonists without intrinsic activity resulted in changed performances on the accelerating rod. The response to nigral perfusion with low concentrations (0.1, 1.0 microM) of the D(2)/D(3)-antagonist raclopride consisted of an impairment in rod performance to 63% of the pre-perfusion performance. Higher concentrations (10, 100 microM), however, were not associated with impaired rod performance, but with increased striatal dopamine concentrations. Perfusion of the substantia nigra with 1, 10 and 100 microM of the D(1)/D(5)-antagonist SCH 23390 dose-dependently impaired rod performance. SCH 23390 consistently increased dopamine and 5-hydroxytryptamine concentrations in substantia nigra but did not change the dialysate in the striatum. (3) In unilaterally 6-hydroxydopamine-lesioned rats, a dose-dependent improvement in rod performance was observed during perfusion of the substantia nigra with the non-selective dopamine agonist apomorphine.

Evidence for different exocytosis pathways in dendritic and terminal dopamine release in vivo

Although dendritic release was first proposed in the 1970s, the mechanism of release is still subject to debate. We have used in vivo microdialysis to study the acute effects of botulinum toxin A, B and tetanus toxin injected in the substantia nigra or striatum of freely moving rats. Spontaneous and evoked dopamine release decreased in both regions after treatment with the SNAP-25 (synaptosome-associated protein of 25 kDa) cleaving protease botulinum toxin A (1000 mouse lethal doses, MLD). Tetanus toxin (4000 MLD) did not significantly change spontaneous or evoked dopamine release in striatum or in the substantia nigra. Another synaptobrevin cleaving protease, botulinum toxin B, inhibited release in the striatum by 55% but did not affect dopamine release when injected in the substantia nigra. The results indicate that both terminal and somatodendritic dopamine release need intact SNAP-25 to occur, but somatodendritic dopamine release in contrast to terminal release depends on a botulinum toxin B resistant pathway.

A possible mechanism for the dopamine-evoked synergistic disinhibition of thalamic neurons via the "direct" and "indirect" pathways in the basal ganglia

The mechanism of synaptic plasticity which we have previously proposed for striatal spiny neurons, along with published data on the predominance of dopamine-sensitive D1/D2 receptors on strionigral/striopallidal neurons, was used as the basis to propose the hypothesis that the induction of long-term potentiation/depression of the efficiency of the cortical inputs to these cells may result from the excitatory/inhibitory actions of dopamine on the activity of the neurons originating the "direct" and "indirect" pathways through the basal ganglia. Thus, the action of dopamine increases disinhibition of thalamic neurons via the "direct" pathway and decreases their inhibition via the "indirect" pathway. Both effects lead to increases in the activity of thalamic cells and in the activity of the efferent neocortical neurons which they excite. The actions of dopamine on striosomal neurons, which mainly have D1 receptors, may also be to induce long-term potentiation of cortical inputs. This effect should lead to increased inhibition of dopaminergic cells and decreases in their dopamine release, which may promote the maintenance of a stable dopamine concentration in the cortex-basal ganglia-thalamus-cortex neural network.

Dopamine selectively reduces GABA(B) transmission onto dopaminergic neurones by an unconventional presynaptic action

The functioning of midbrain dopaminergic neurones is closely involved in mental processes and movement. In particular the modulation of the inhibitory inputs on these cells might be crucial in controlling firing activity and dopamine (DA) release in the brain. Here, we report a concentration-dependent depressant action of dopamine on the GABA(B) IPSPs intracellularly recorded from dopaminergic neurones. Such effect was observed in spite of the presence of D(1)/D(2) dopamine receptor antagonists. A reduction of the GABA(B) IPSPs was also caused by noradrenaline (norepinephrine) and by L-beta-3,4-dihydroxyphenylalanine (L-DOPA), which is metabolically transformed into DA. The DA-induced depression of the IPSPs was partially antagonised by the alpha2 antagonists yohimbine and phentolamine. DA did not change the postsynaptic effects of the GABA(B) agonist baclofen, suggesting a presynaptic site of action. Furthermore, DA did not modulate the GABA(A)-mediated IPSP. The DA-induced depression of the GABA(B) IPSP occluded the depression produced by serotonin and was not antagonized by serotonin antagonists. The DA- and 5-HT-induced depression of the GABA(B) IPSP persisted when calcium and potassium currents were reduced in to the presynaptic terminals. These results describe an unconventional presynaptic, D(1) and D(2) independent action of DA on the GABA(B) IPSP. This might have a principal role in determining therapeutic/side effects of L-DOPA and antipsychotics and could be also involved in drug abuse.

The cortico-basal ganglia-thalamocortical circuit with synaptic plasticity. II. Mechanism of synergistic modulation of thalamic activity via the direct and indirect pathways through the basal ganglia

A possible mechanism underlying the modulatory role of dopamine, adenosine and acetylcholine in the modification of corticostriatal synapses, subsequent changes in signal transduction through the "direct" and "indirect" pathways in the basal ganglia and variations in thalamic and neocortical cell activity is proposed. According to this mechanism, simultaneous activation of dopamine D1/D2 receptors as well as inactivation of adenosine A1/A(2A) receptors or muscarinic M4/M1 receptors on striatonigral/striatopallidal inhibitory cells can promote the induction of long-term potentiation/depression in the efficacy of excitatory cortical inputs to these cells. Subsequently augmented inhibition of the activity of inhibitory neurons of the output nuclei of the basal ganglia through the "direct" pathway together with reduced disinhibition of these nuclei through the "indirect" pathway synergistically increase thalamic and neocortical cell firing. The proposed mechanism can underlie such well known effects as "excitatory" and "inhibitory" influence of dopamine on striatonigral and striatopallidal cells, respectively; the opposite action of dopamine and adenosine on these cells; antiparkinsonic effects of dopamine receptor agonists and adenosine or acetylcholine muscarinic receptor antagonists.

Effects of methamphetamine-induced neurotoxicity on the development of neural circuitry: a hypothesis

Exposure of the developing brain to methamphetamine has well-studied biochemical and behavioral consequences. We review: (1) the effects of methamphetamine on mature serotonergic and dopaminergic pathways; (2) the mechanisms of methamphetamine neurotoxicity and (3) the role of serotonergic and dopaminergic signaling in sculpting developing neural circuitry. Consideration of these data suggest the types of neural circuit alterations that may result from exposure of the developing brain to methamphetamine and that may underlie functional defects.

GABA in the deep layers of the superior Colliculus/Mesencephalic reticular formation mediates the enhancement of startle by the dopamine D1 receptor agonist SKF 82958 in rats

GABA transmission in the deep layers of the superior colliculus/deep mesencephalic reticular formation (deep SC/Me) mediates several motor responses, including those expressed after systemic administration of dopamine agonists. In the present study we examined the role of the deep SC/Me in the modulation of the acoustic startle reflex and its enhancement by the dopamine D(1) agonist SKF 82958. Rats were implanted with bilateral cannulas into the deep SC/Me or superficial layers of the SC (super SC) and 1 week later were infused with various compounds. The GABA(A) antagonist bicuculline (0, 5, and 10 ng) produced a dose- and time-dependent enhancement of startle after infusion into the deep SC/Me, but not the super SC. Infusion of the GABA(A) agonist muscimol (0.1 microg) into the deep SC/Me, but not the super SC, blocked the enhancement of startle by systemic SKF 82958 (1 mg/kg) but had no effect on baseline startle by itself. This effect was not produced by infusion of the D(1) antagonist SCH 23390(1 microg) or the glutamate antagonist NBQX (0.1 microg). Deposits of FluoroGold into the deep SC/Me, combined with immunohistochemistry for glutamic acid decarboxylase (GAD), confirmed a direct GABAergic input from the substantia nigra pars reticulata (SNr) to the deep SC/Me. These results suggest that GABA tone in the deep SC/Me modulates the expression of startle as well as the enhancement of startle by dopamine D(1) agonists. On the basis of these data and previous work, we have proposed a striatonigral-tectal-reticular neural pathway mediating the effects of dopamine D(1) agonists on startle.