Dopamine D1 heteroreceptors on striatonigral axons are not stimulated by endogeneous dopamine either tonically or after amphetamine: evidence from terminal excitability

Abundance and location of DARPP-32 in striato-tegmental circuits of domestic chicks

The striatum is reciprocally connected to the brainstem dopaminergic nuclei and receives a strong dopaminergic input. In the present study the spatial relation between the dopaminergic and dopaminoceptive structures of the avian medial striatum (formerly: lobus parolfactorius) was observed by confocal laser scanning microscope in the domestic chick (Gallus domesticus). We also analysed the connections in the area ventralis tegmentalis and the substantia nigra. To label the dopaminergic structures, anti-tyrosine hydroxylase was used and DARPP-32 (dopamine and cAMP regulated phosphoprotein) was a marker of dopaminoceptive elements. The tyrosine hydroxylase positive fibres formed baskets of juxtapositions around the DARPP-32 containing cells of the medial striatum. However, such baskets were also observed to juxtapose DARPP-32 immunonegative cells. In the tegmentum, DARPP-32 was observed in axons descending from the telencephalon via the ansa lenticularis. These varicose fibers innervated the ventral tegmental area and substantia nigra and were often juxtaposed to dopaminergic neurons and dendrites. Approximately 40% of the striatal projection neurons targeting the ventral tegmentum, and 60% of striatal projection neurons targeting the nigra were immunoreactive to DARPP-32, as revealed by retrograde pathway tracing with Fast Blue. Endogenous dopamine may exert a retrograde synaptic effect on the afferent striato-tegmental fibers, apart from the reported extrasynaptic action. The abundance of juxtapositions observed in the avian brainstem and medial striatum corroborates the possibility of reciprocal striato-tegmental circuits, relevant to the reinforcement of behaviour.

Modulation of GABA release by dopamine in the substantia nigra

The role of specific dopamine receptor subtypes in the regulation of GABA release in the substantia nigra was investigated using microdialysis in the awake rat. Both basal and potassium-stimulated changes in the extracellular concentrations of GABA were examined in response to the local perfusion of tetrodotoxin (TTX), the D1 agonist SKF 38393, or the D2 agonist LY 171555 through the microdialysis probe in the substantia nigra. Although TTX (1 microM) did not alter the basal extracellular concentrations of GABA in the substantia nigra, it attenuated the potassium-stimulated (80 mM K+) release of GABA. SKF 38393 had no effect on basal extracellular concentrations of GABA, but did potentiate K+ -stimulated release of GABA in a concentration-dependent manner. The potentiated response at the highest concentration of SKF 38393 (100 microM) was blocked by the D1 antagonist SCH 23390. In contrast to the effect of the D1 agonist, the D2 agonist LY 171555 attenuated the stimulated release of GABA. These data indicate that although basal extracellular concentrations of GABA in the substantia nigra may not be derived from neuronal pools, K+ -stimulated release of GABA is impulse-mediated and is modulated by the D1 and the D2 receptors. Local interactions between dopamine and GABA in the substantia nigra may have important implications for the direct regulation of basal ganglia efferent activity and motor behavior.

Glutamate-dependent long-term presynaptic changes in corticostriatal excitability

We have previously shown that brief high frequency stimulation of the anteromedial prefrontal cortex induces a long-term decrease in excitability of the glutamatergic corticostriatal terminal field. In contrast, a long-term increase in presynaptic corticostriatal excitability may be induced by presenting two brief cortical tetanizing stimuli separated by 2-3 min such that the second tetanus coincides with a period of increased excitability elicited by the first. In the present study, we examined the glutamate receptor subtypes involved in these long-term changes in presynaptic excitability. A specific glutamate receptor antagonist was infused into the rat striatum 10-25 min prior to either a single or double cortical tetanic stimulation. To eliminate the participation of intrinsic striatal cells, a subset of animals received a striatal kainic acid lesion eight to 20 days before the recording experiment. Antagonists of the N-methyl-D-aspartate and metabotropic glutamate receptor subtypes were effective in blocking the decrease in excitability induced by single cortical tetanic stimulation whereas an antagonist of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptor did not prevent the induction of a long-term reduction in excitability. In contrast, each of these antagonists prevented the induction of a long-term increase in excitability. These long-term modifications in excitability of the presynaptic glutamate axon terminals appear to be induced by similar mechanisms to those postulated to operate in long-term potentiation and depression. These enduring changes in presynaptic excitability are likely to represent important mechanisms for the selective modification of information processing in the striatum.

Amphetamine-induced changes in nigrostriatal terminal excitability are modified following repeated amphetamine pretreatment

To investigate neural mechanisms associated with behavioral sensitization to amphetamine, we studied the effect of an intrastriatal infusion of amphetamine on nigrostriatal axon terminal electrical excitability in rats following withdrawal from repeated systemic treatment. Rats were injected with amphetamine 2.5 mg/kg s.c. or saline daily for 4 days. Either 24 h or 14 days after the last injection, extracellular recordings were obtained from dopaminergic neurons of the substantia nigra, in a blind design in which the experimenter did not know the pretreatment regime. In order to assess the electrical excitability of the nigrostriatal axonal field, neurons were activated antidromically by stimulating their terminal fields in the striatum. As previously reported, striatal infusion of amphetamine (1 microM/0.3 microliter) in control animals resulted in a significant reduction in excitability as indicated by an increase in striatal stimulus current necessary to evoke antidromic activity. In contrast, intrastriatal amphetamine administration to amphetamine-pretreated animals did not decrease excitability. Spontaneous firing rates and patterns of cell discharge did not differ between saline- and amphetamine-treated animals. The chronic amphetamine-induced change in the effect of an acute intrastriatal amphetamine infusion on nigrostriatal terminal excitability may be due to enduring alterations in the amphetamine-induced release of dopamine and other striatal neurotransmitters or to changes in the sensitivity of presynaptic hetero- and/or autoreceptors on the dopaminergic axons.

Interaction of the dopaminergic and serotonergic systems in the rat striatum: effects of selective antagonists and uptake inhibitors

The effects of some selective dopamine (DA) and 5-hydroxytryptamine (5-HT) antagonists and uptake inhibitors on the outflow of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) in the striatum were studied using microdialysis in conscious rats. Eticlopride (D2 antagonist, 2.5 mg/kg) increased HVA but decreased 5-HIAA, while nomifensine (DA uptake inhibitor, 10 mg/kg) decreased both. Mianserin (5-HT2 antagonist, 2 mg/kg) and citalopram (5-HT uptake inhibitor, 11 mg/kg) also decreased both HVA and 5-HIAA. The selectivity of these drugs and the time course of their effects on HVA and 5-HIAA suggested that the dopaminergic drugs may affect the serotonergic systems primarily via changes in extraneuronal DA and conversely, serotonergic drugs affect the dopaminergic systems via changes in extraneuronal 5-HT. The results obtained are consistent with current evidence that DA and 5-HT release from striatal terminals are regulated by D2 and 5-HT1B autoreceptors, respectively. They also support the idea that, in the rat striatum, presynaptic inhibitory heteroreceptors are a major mechanism that contributes to a reciprocal interaction of the dopaminergic and serotonergic systems.

Dopamine D1 receptors in the sub-commissural part of the globus pallidus and their role in oro-facial dyskinesia in cats

The possible role of dopamine D1 receptors in the sub-commissural part of the globus pallidus in the induction of oro-facial dyskinesia was studied in cats. The present study reveals two findings. Firstly, bilateral injections of the D1 agonist (+/-)-SK& F38393 into the ventral pallidal area elicited oro-facial dyskinesia, which was quantified in terms of numbers of tongue protrusions. The results show that the dose-effect curve was bell-shaped (1.0, 1.75, 2.5, 5.0 micrograms/0.5 microliters (+/-)-SK&F38393). The oro-facial dyskinesia elicited by (+/-)-SK&F38393 was highly comparable to the oro-facial dyskinesia elicited by injections of the GABA antagonist picrotoxin or the acetylcholine agonist carbachol into the sub-commissural part of the globus pallidus. Secondly, the inactive enantiomer of SK&F38393, i.e. S(-)-SK&F38393, was found to be ineffective in eliciting oro-facial dyskinesia when injected in a dose equivalent to 50% of the most effective dose of the racemic mixture of (+/-)-SK&F38393. Furthermore, the effect elicited by 2.5 micrograms/0.5 microliters (+/-)-SK&F38393 was significantly attenuated by local injection of the D1 antagonist R(+)-SCH23390 in a dose which had no effect itself (1.0 micrograms/0.5 microliters). These findings indicate that the effects elicited by (+/-)-SK&F38393 are D1-specific. The present results thus clearly indicate that dopamine D1 receptors within the sub-commissural part of the globus pallidus are involved in mediating oro-facial dyskinesia in cats.

Ultrastructural single- and double-label immunohistochemical studies of substance P-containing terminals and dopaminergic neurons in the substantia nigra in pigeons

The vast majority of striatonigral projection neurons in pigeons contain substance P (SP), and the vast majority of SP-containing fibers terminating in the substantia nigra arise from neurons in the striatum. To help clarify the role of striatonigral projection neurons, we conducted electron microscopic single- and double-label immunohistochemical studies of SP+ terminals and/or dopaminergic neurons (labeled with either anti-dopamine, DA, or anti-tyrosine hydroxylase, TH) in pigeons to determine: (1) the synaptic organization of SP+ terminals, (2) the synaptic organization of TH+ perikarya and/or dendrites, and (3) the synaptic relationship between SP+ terminals and TH+ neurons in the substantia nigra. Tissue single-labeled for SP revealed numerous SP+ terminals contacting thin unlabeled dendrites in the substantia nigra, but few SP+ terminals were observed contacting perikarya or large-diameter dendrites. SP+ terminals contained round, densely packed, clear vesicles, and often contained one or more dense-core vesicles. Synaptic junctions between SP+ terminals and their targets were more often symmetric (86%) than asymmetric. In tissue single-labeled for DA, we observed few terminals contacting DA+ perikarya, whereas terminals contacting DA+ dendrites were more abundant. Terminals contacting DA+ structures comprised at least four different morphologically distinct types based on the morphology of the clear synaptic vesicles and the type of synaptic junction. One type of terminal contained round clear vesicles and made symmetric synapses, and thus resembled the predominant type of SP+ terminal. The second type contained round clear vesicles and made asymmetric synapses, the third type contained medium-size pleomorphic clear vesicles and made symmetric synapses, and the fourth type contained small pleomorphic clear vesicles and made symmetric synapses. The presence of contacts between SP+ terminals and dopaminergic dendrites in the substantia nigra was directly demonstrated in tissue double-labeled for SP (by the peroxidase-antiperoxidase procedure, or PAP, with diaminobenzidine) and TH (by either the silver-intensified immunogold procedure or the PAP procedure with benzidine dihydrochloride). SP+ terminals commonly contacted thin TH+ dendrites in the substantia nigra, but few SP+ terminals contacted large-diameter TH+ dendrites or perikarya. Synapses between SP+ terminals and TH+ neurons were always symmetric. TH+ dendrites also were contacted by terminals not labeled for SP, which were more abundant than were SP+ terminals. Non-TH+ neurons were also contacted by both SP+ terminals and non-SP+ terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of dopaminergic terminal excitability by D1 selective agents: further characterization

We have previously shown that stimulation of striatal D1 receptors affects dopaminergic nigrostriatal terminal excitability, which is thought to be an index of biophysical events resulting from the activation of receptors on the presynaptic membrane. The experiments presented here further examine the locus and bases of these D1 effects in the rat. We now report that striatal administration of the D1 receptor selective antagonist R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazapine+ ++-7-ol-HCl (SCH 23390) produces a paradoxical agonist-like decrease in dopaminergic terminal excitability. This effect is blocked by pretreatment with the dopamine synthesis inhibitor, alpha-methyl-paratyrosine, suggesting that the action of SCH 23390 is dependent upon endogenous dopamine. Further, haloperidol pretreatment also prevents the SCH 23390-induced decrease in terminal excitability, confirming that dopamine, acting through a dopamine receptor, is responsible for this agonist-like action. Striatal application of the active R-(+) enantiomer of the dopaminergic D1-selective agonist 1-phenyl-2,3,4,5-tetrahydrol-(1H)-3-benzazepine-7,8-diol-HCl (R-SKF 38393) decreases terminal excitability in the alpha-methyl-paratyrosine pretreated animal, indicating that dopamine is not required for the agonist action. In an effort to ascertain the presynaptic or postsynaptic location of these actions, an extensive destruction of postsynaptic neurons in the neostriatum was produced by local administration of the neurotoxin, kainic acid. It was observed that the neurotoxin-induced neostriatal neuronal loss did not disrupt the action of R-SKF 38393 nor its reversal by SCH 23390.(ABSTRACT TRUNCATED AT 250 WORDS)