Enhancing effect of dopamine blockers on evoked acetylcholine release in rat striatal slices: a classical D-2 antagonist response?

Chlorpromazine-induced inhibition of catecholamine secretion by a differential blockade of nicotinic receptors and L-type Ca2+ channels in rat pheochromocytoma cells

We investigated the effect of chlorpromazine (CPZ), a phenothiazine neuroleptic, on catecholamine secretion in rat pheochromocytoma (PC12) cells. CPZ inhibited [3H]norepinephrine ([3H]NE) secretion induced by 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), an agonist of nicotinic acetylcholine receptors (nAChRs) with an IC50 value of 1.0 +/- 0.2 microM. The DMPP-induced rise in cytosolic free Ca2+ concentration [Ca2+]i was inhibited by CPZ with an IC50 of 1.9 +/- 0.1 microM. The DMPP-induced increase in cytosolic free Na+ concentration [Na+]i was also inhibited by CPZ with a similar potency. Furthermore, the binding of [3H]nicotine to PC12 cells was inhibited by CPZ with an IC50 value of 2.7 +/- 0.6 microM, suggesting that the nAChRs themselves are inhibited by CPZ. In addition, both 70 mM K+-induced [3H]NE secretion and [Ca2+]i increase were inhibited by CPZ with IC50 of 7.9 +/- 1.1 and 6.2 +/- 0.3 microM, respectively. Experiments with Ca2+ channel antagonists suggest that L-type Ca2+ channels are mainly responsible for the inhibition. We conclude that CPZ inhibits catecholamine secretion by blocking nAChRs and L-type Ca2+ channels, with the former being more sensitive to CPZ.

Dual character, asynaptic and synaptic, of the dopamine innervation in adult rat neostriatum: a quantitative autoradiographic and immunocytochemical analysis

Dopamine (DA) axon terminals (varicosities) in the neostriatum of adult rats were examined for shape, size, content, synaptic incidence, type of junction, synaptic targets, and microenvironment after electron microscopic identification either by [3H]DA uptake autoradiography or by immunocytochemistry with monoclonal antibodies against DA-glutaraldehyde-protein conjugate. Both approaches yielded comparable results. Whether they were from the paraventricular or the mediodorsal neostriatum, respectively, the [3H]DA-labeled and DA-immunostained varicosities were generally oblong and relatively small; more than 60% contained one or more mitochondria. Sixty to seventy percent were asynaptic, and 30-40% were endowed with a synaptic membrane differentiation (junctional complex), as inferred by stereological extrapolation from single thin sections (both approaches) or observed directly in long, uninterrupted series of thin sections (immunocytochemistry). The synaptic DA varicosities always displayed symmetrical junctions: 67% with dendritic branches, 30% with dendritic spines, and 2-3% with neuronal cell bodies. DA varicosities juxtaposed to one another were frequent. Other axonal varicosities were more numerous in the immediate vicinity of DA varicosities than around randomly selected, unlabeled terminals. The respective microenvironments of DA and unlabeled varicosities also showed enrichment in the preferred synaptic targets of both groups of varicosities, with dendritic branches for DA and dendritic spines for the unlabeled ones. These data suggest a dual mode of operation that is diffuse as well as synaptic for the nigrostriatal DA system. In such a densely DA-innervated brain region, they also lead to the hypothesis that a basal level of extracellular DA might be maintained permanently around every tissue constituent and, thus, contribute to the mechanisms of action, properties, and functions (or dysfunctions) of DA within the neostriatum itself and as part of the basal ganglia circuitry.

Effects of haloperidol, lithium, and valproate on phosphoinositide turnover in rat brain

The effects of acute, subacute, and chronic treatment with haloperidol, lithium, and valproate on inositol phosphate (IP) formation were examined. Acute treatment with haloperidol or the combination of haloperidol and lithium significantly reduced IP basal cortical levels. Subacute (three days) treatment with lithium decreased the IP basal level in the frontal cortex. Chronic treatment with haloperidol (14 and 28 days) caused a significant attenuation of carbachol-sensitive IP accumulation in the frontal cortex and striatum and a significant decrease in norepinephrine (NE)-induced IP formation in the frontal cortex (14 and 28 days) and striatum (28 days). Lithium treatment for 14 days produced a significant reduction in the IP basal cortical value, and a significant reduction in cortical carbachol- and NE-induced IP formation was found after 28 days of lithium treatment. The combination of haloperidol and lithium for 28 days decreased the striatal carbachol- and cortical NE-induced IP accumulation and caused a significant increase in NE-sensitive IP formation in the striatum at 14 days. Valproate treatment for 28 days was associated with a significant attenuation in striatal agonist-stimulated IP formation. Therefore, three drugs with different specificities for primary neurotransmitters may have common effects on second-messenger systems.

SCH 39166, a potential antipsychotic drug, does not evoke movement disorders in cebus monkeys

Subchronic administration of a neuroleptic in cebus monkeys can reliably mimic the abnormal movements produced by these drugs in humans. SCH 39166 is the best candidate to test in this model to determine if selective antagonism at the dopamine D1 receptor is devoid of these side effects. It has superior selectivity for the dopamine D1 site versus several other sites and a significantly longer duration in primates than the prototypical D1 antagonist, SCH 23390. In contrast to haloperidol, weekly administration of SCH 39166 for 14 weeks did not produce abnormal movements but did produce equivalent sedative effects. Thus dopamine D1 antagonists are uniquely different from D2 antagonists with regards to the production of abnormal movements.

Chronic haloperidol treatment attenuates receptor-mediated phosphoinositide turnover in rat brain slices

The long-term effects of haloperidol on phosphoinositide turnover in rat brain slices were investigated. Continuous treatment with haloperidol decanoate (21 mg/kg I.M. biweekly for 6 weeks) significantly attenuated carbachol- and norepinephrine (NE)-induced inositol phosphate accumulation in rat frontal cortex and hippocampus. In the striatum, the haloperidol treatment also significantly decreased carbachol-stimulated inositol phosphate level but did not significantly affect NE-sensitive phosphoinositide turnover. These effects were not observed in rats treated with a single dose of haloperidol (1.5 mg/kg). Basel levels of inositol phosphate in these 3 brain regions did not change following continuous or single haloperidol doses.

Muscarinic and dopaminergic receptor subtypes on striatal cholinergic interneurons

Unilateral stereotaxic injection of small amounts of the cholinotoxin, AF64A, caused minimal nonselective tissue damage and resulted in a significant loss of the presynaptic cholinergic markers [3H]hemicholinium-3 (45% reduction) and choline acetyltransferase (27% reduction). No significant change from control was observed in tyrosine hydroxylase or tryptophan hydroxylase activity; presynaptic neuronal markers for dopamine- and serotonin-containing neurons, respectively. The AF64A lesion resulted in a significant reduction of dopamine D2 receptors as evidenced by a decrease in [3H]sulpiride binding (42% reduction) and decrease of muscarinic non-M1 receptors as shown by a reduction in [3H]QNB binding in the presence of 100 nM pirenzepine (36% reduction). Saturation studies revealed that the change in [3H]sulpiride and [3H]QNB binding was due to a change in Bmax not Kd. Intrastriatal injection of AF64A failed to alter dopamine D1 or muscarinic M1 receptors labeled with [3H]SCH23390 and [3H]pirenzepine, respectively. In addition, no change in [3H]forskolin-labeled adenylate cyclase was observed. These results demonstrate that a subpopulation of muscarinic receptors (non-M1) are presynaptic on cholinergic interneurons (hence, autoreceptors), and a subpopulation of dopamine D2 receptors are postsynaptic on cholinergic interneurons. Furthermore, dopamine D1, muscarinic M1 and [3H]forskolin-labeled adenylate cyclase are not localized to striatal cholinergic interneurons.

Decreased striatal release of acetylcholine following withdrawal from long-term treatment with haloperidol: modulation by cholinergic, dopamine-D1 and -D2 mechanisms

The effect of chronic treatment with haloperidol (2.7-5.3 mumol/kg/day) on K(+)-evoked release of [3H]acetylcholine (ACh) from superfused slices of the striatum was assessed. Acute injections of haloperidol (0.7-13.3 mumol/kg) produced 5-54% increases in the release of [3H]ACh in the striatum. Chronic treatment with haloperidol for 2.5 and 5 months also resulted in enhanced release of [3H]ACh in the striatum (28-35%). However, withdrawal from 2.5 and 5 months of treatment produced 34 and 38% decreases in K(+)-evoked release of [3H]ACh in the striatum, respectively. The drug SKF 38393 (D1-agonist), produced concentration-dependent (0.1-10 microM) increases (24-59%) in the release of [3H]ACh in the striatum which were blocked by the selective D1-antagonist, SCH 23390. The effect of stimulation of D1-receptors was significantly reduced after 2.5 or 5 months of chronic treatment with haloperidol. Both LY171555 (D2-agonist) and carbachol (muscarinic agonist) produced concentration-dependent (0.1-10 microM) inhibitions of the release of [3H]ACh in the striatum (LY171555: 28-62%; carbachol: 23-63%). Long-term treatment with haloperidol (2.5 and 5 months) elicited increases in sensitivity to the effect of LY171555, while the effect of carbachol was diminished only after the 5-month treatment period. These findings demonstrate that withdrawal from chronic exposure to haloperidol in the rat results in a reduction in the release of acetylcholine in the striatum. This effect is accompanied by (1) attenuated dopaminergic D1 mechanisms which ordinarily facilitate evoked release of ACh, (2) enhanced D2 mechanism which elicits inhibition of the release of ACh in the striatum, and (3) diminished muscarinic inhibitory influence which regulates the release of ACh.

Spiny neurons lacking choline acetyltransferase immunoreactivity are major targets of cholinergic and catecholaminergic terminals in rat striatum

The ultrastructural substrate for functional interactions between intrinsic cholinergic neurons and catecholaminergic afferents to the caudate-putamen nucleus and nucleus accumbens septi (NAS) was investigated immunocytochemically. Single sections of glutaraldehyde-fixed rat brain were processed 1) for the immunoperoxidase labeling of a rat monoclonal antibody against the acetylcholine-synthesizing enzyme choline acetyltransferase (CAT) and 2) for the immunoautoradiographic localization of a rabbit polyclonal antiserum against the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). The ultrastructural morphology and cellular associations did not significantly differ in the caudate-putamen versus NAS. Immunoperoxidase reaction for CAT versus NAS. Immunoperoxidase reaction for CAT was seen in perikarya, dendrites, and terminals, whereas immunoautoradiography for TH was in terminals. The perikarya and dendrites immunolabeled for CAT were large, sparsely spiny, and postsynaptic mainly to unlabeled axon terminals. Only 2-3% of the CAT-labeled terminals (n = 136) and less than 1% of the TH-labeled terminals (n = 86) were apposed to, or formed synapses with, perikarya or dendrites immunoreactive for CAT. Most unlabeled and all labeled terminals formed symmetric synapses. In the same sample, 18% of the CAT and 16% of the TH-labeled terminals were directly apposed to each other. Unlabeled dendritic shafts received the major (40% for CAT versus 23% for TH) synaptic input from cholinergic terminals, while unlabeled spines received the major (47% for TH versus 23% for CAT) synaptic input from catecholaminergic terminals. Neither the unlabeled dendrites or spines received detectable convergent input from CAT and TH-labeled terminals. Thirteen percent of the CAT-labeled and 14% of TH-labeled terminals were in apposition to unlabeled terminals forming asymmetric, presumably excitatory, synapses with unlabeled dendritic spines. We conclude that in both the caudate-putamen and NAS cholinergic and catecholaminergic terminals 1) form symmetric, most likely inhibitory, synapses primarily with non-cholinergic neurons, 2) differentially synapse on shafts or spines of separate dendrites, and 3) have axonal appositions suggesting the possibility of presynaptic physiological interactions. These results support the hypothesis that the cholinergic-dopaminergic balance in striatal function may be mediated through inhibition of separate sets of spiny projection neurons with opposing excitatory and inhibitory functions.

D1 and D2 dopamine receptor interactions with pilocarpine-induced oral activity in rats

To investigate the relationship between dopamine (DA) and acetylcholine (ACh) systems in the control of oral movement, we studied the effects of specific D1 and D2 drugs on vacuous chewing movements induced by the muscarinic ACh agonist, pilocarpine. In previous experiments we found that when given alone, the D1 agonist SKF 38393 increased vacuous chewing and the D1 antagonist SCH 23390 decreased it, while both the D2 agonist LY 171555 (quinpirole) and the D2 antagonist sulpiride decreased vacuous chewing. In the present experiment, the effects of the D1 drugs had similar effects in rats concurrently given pilocarpine. In contrast, the effects of both of the D2 drugs were altered by pilocarpine. Surprisingly, the actions of D2 agonist and antagonist were affected in opposite ways. The effect of sulpiride in reducing oral movement activity was eliminated by pilocarpine, while the effect of LY 171555 in reducing oral movement was enhanced by pilocarpine.

Localization of nigrostriatal dopamine receptor subtypes and adenylate cyclase

Quantitative autoradiography using [3H]-SCH 23390, [3H]-sulpiride and [3H]-forskolin was used to assess the effects of single and combined neurotoxin lesions of the nigrostriatal pathway in the rat brain on dopamine (DA) receptor subtypes and adenylate cyclase (AC), respectively. Ibotenic acid (IA) lesions of the caudate-putamen (CPu) resulted in near total loss of both [3H]-SCH 23390 and of [3H]-forskolin binding in the ipsilateral CPu and substantia nigra reticulata (SNR). [3H]-sulpiride binding in the CPu was only partially removed by this same lesion, and nigral [3H]-sulpiride binding was virtually unchanged. 6-Hydroxydopamine (6-OHDA) and IA lesions of the substantia nigra compacta (SNC) did not affect [3H]-SCH 23390 or [3H]-forskolin binding, but largely removed [3H]-sulpiride binding in the SNC. A 6-OHDA lesion of the nigrostriatal pathway followed by an ipsilateral IA injection of the CPu failed to further reduce [3H]-sulpiride binding in the CPu. These results demonstrate that postsynaptic DA receptors in the CPu are of both the D1 and D2 variety; however, a portion of D2 receptors in the CPu may be presynaptic on afferent nerve terminals to this structure. D1 receptors in the SNR are presynaptic on striatonigral terminals, whereas the D2 receptors of the SNC are autoreceptors on nigral DA neurons. The existence of presynaptic D2 receptors on nigrostriatal DA-ergic terminals could not be confirmed by this study. Co-localization of D1 receptors and AC occurs in both the CPu and SNR.

D1 dopamine receptor--the search for a function: a critical evaluation of the D1/D2 dopamine receptor classification and its functional implications

The present review focuses on the hypothesized D1/D2 dopamine (DA) receptor classification, originally based on the form of receptor coupling to adenylate cyclase activity. The pharmacological effects of compounds exhibiting putative selective agonist or antagonist profiles at those DA receptors positively coupled to adenylate cyclase activity (D1 DA receptors) are extensively reviewed. Comparisons are made with the effects of putative selective D2 DA receptor agonists and antagonists, and on the basis of this work, the DA receptor classification is critically evaluated. A variety of biochemical, behavioral, and electrophysiological evidence is presented which supports the view that D1 and D2 DA receptors can interact in both an opposing and synergistic fashion. Particular attention is focused on the possibility that D1 receptor stimulation is required to enable the expression of certain D2 receptor-mediated effects, and the functional consequences of this form of interaction are considered. A hypothetical model is presented which considers how both the opposing and enabling forms of interaction between D1 and D2 DA receptors can control behavioral expression. Finally, the clinical relevance of this work is discussed and the potential use of selective D1 receptor agonists and antagonists in the treatment of psychotic states and Parkinson's disease is considered.