Muscarinic receptors on dopamine terminals in the cat caudate nucleus: neuromodulation of [3H]dopamine release in vitro by endogenous acetylcholine

Omega 3 fatty acid improves sexual and erectile function in BPF-treated rats by upregulating NO/cGMP signaling and steroidogenic enzymes activities

Bisphenol F (BPF) is an environmental pollutant that has been implicated in sexual dysfunction. Omega 3 fatty acid (O3FA), on the other hand, is an antioxidant with the ability to improve fertility indices. However, no study has explored the possible ameliorative effect of O3FA on BPF-induced sexual dysfunction. Thus, the effect of BPF and/or O3FA on male sexual performance was investigated. Male Wistar rats were randomized into 6 groups, corn oil-treated, O3FA low and high dose (100 and 300 mg/kg), BPF-treated, BPF + O3FA low and BPF + O3FA high dose. BPF significantly impaired male sexual competence, evidenced by a reduction in motivation to mate, prolonged mount, intromission and ejaculation latency, and post-ejaculatory index. Furthermore, a reduction in mount, intromission, and ejaculation frequency were observed. Also, BPF caused a decrease in gonadotropin releasing hormone, follicle stimulating hormone, luteinizing hormone, testosterone, nitric oxide (NO) cyclic guanosine monophosphate (cGMP), 3beta-hydroxysteroid dehydrogenase (3β-HSD), 17beta-hydroxysteroid dehydrogenase (17β-HSD), dopamine, and acetylcholine esterase. Furthermore, it was accompanied by a significant increase in prolactin and estrogen and poor pregnancy outcomes. These observed BPF-led alterations were abolished by O3FA administration. This study showed that O3FA ameliorates BPF-induced sexual dysfunction by upregulating NO/cGMP signaling and steroidogenic enzymes activities.

Contribution of cholinergic interneurons to striatal pathophysiology in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder caused by the loss of nigral dopaminergic neurons innervating the striatum, the main input structure of the basal ganglia. This creates an imbalance between dopaminergic inputs and cholinergic interneurons (ChIs) within the striatum. The efficacy of anticholinergic drugs, one of the earliest therapy for PD before the discovery of L-3,4-dihydroxyphenylalanine (L-DOPA) suggests an increased cholinergic tone in this disease. The dopamine (DA)-acetylcholine (ACh) balance hypothesis is now revisited with the use of novel cutting-edge techniques (optogenetics, pharmacogenetics, new electrophysiological recordings). This review will provide the background of the specific contribution of ChIs to striatal microcircuit organization in physiological and pathological conditions. The second goal of this review is to delve into the respective contributions of nicotinic and muscarinic receptor cholinergic subunits to the control of striatal afferent and efferent neuronal systems. Special attention will be given to the role played by muscarinic acetylcholine receptors (mAChRs) in the regulation of striatal network which may have important implications in the development of novel therapeutic strategies for motor and cognitive impairment in PD.

Muscarinic acetylcholine M4 receptors play a critical role in oxotremorine-induced DARPP-32 phosphorylation at threonine 75 in isolated medium spiny neurons

Dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) play essential roles in dopamine (DA) transmission in the striatum. It is suggested that a link exists between muscarinic acetylcholine receptors (mAChRs) and DA/DARPP-32 signaling, but the molecular mechanisms mediating this relationship have not been elucidated. The predominant mAChRs subtypes in the striatum are M₁ and M₄. In this study, we investigated the functions of these two receptors, particularly M₄, in regulating cAMP production and DARPP-32 phosphorylation in rat striatal medium spiny neurons (MSNs). We used time-resolved fluorescence resonance energy transfer, immunofluorescence confocal microscopy, and western blot assays. In cultured intact MSNs, we confirmed that muscarinic M₁ and M₄ receptors were highly expressed. Notably, M₄ receptors were co-expressed with D₁ receptors in only a portion of the cultured MSNs. The nonselective muscarinic agonist oxotremorine M (OX) slightly enhanced cAMP production, but this effect was independent of M₁ or M₄ receptors. However, OX directly participated in DARPP-32 phosphorylation, phosphorylating DARPP-32 at Thr75 (the CDK5 site) and concomitantly de-phosphorylating DARPP-32 at Thr34 (the PKA site) in virtually cultured MSNs, whereas APO phosphorylated DARPP-32 at both Thr34 and Thr75. The OX-induced time-dependent increase in DARPP-32 phosphorylation at Thr75 was accompanied by increased p35 and CDK5 activity. Specifically, elevated immunoreactivity for phospho-DARPP-32-Thr75 and p35 was detected in M₄ receptor-expressing MSNs. Both genetic knockdown and pharmacologic inhibition of M₄ receptors with MT3, an M₄ receptor-selective antagonist, decreased the OX-induced DARPP-32-Thr75 phosphorylation in MSNs. These results indicate that the M₄ muscarinic receptor plays a critical role in modulating phosphorylation of DARPP-32-Thr75 in MSNs. The results suggest that M₄ receptor activation acts antagonistically with dopamine D₁-like receptors within the striatum, and indicate that M₄ receptors may be a potential target for the treatment of Parkinson's disease and other relevant central nervous system disorders.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

Muscarinic regulation of dopamine and glutamate transmission in the nucleus accumbens

Cholinergic transmission in the striatum functions as a key modulator of dopamine (DA) transmission and synaptic plasticity, both of which are required for reward and motor learning. Acetylcholine (ACh) can elicit striatal DA release through activation of nicotinic ACh receptors (nAChRs) on DA axonal projections. However, it remains controversial how muscarinic ACh receptors (mAChRs) modulate striatal DA release, with studies reporting both potentiation and depression of striatal DA transmission by mAChR agonists. This study investigates the mAChR-mediated regulation of release from three types of midbrain neurons that project to striatum: DA, DA/glutamate, and glutamate neurons. We found that M5 mAChRs potentiate DA and glutamate release only from DA and DA/glutamate projections from the midbrain. We also show that M2/M4 mAChRs depress the nAChR-dependent mechanism of DA release in the striatum. These results suggest that M5 receptors on DA neuron terminals enhance DA release, whereas M2/M4 autoreceptors on cholinergic terminals inhibit ACh release and subsequent nAChR-dependent DA release. Our findings clarify the mechanisms of mAChR-dependent modulation of DA and glutamate transmission in the striatum.

α2-Adrenoceptors in the treatment of major neuropsychiatric disorders

Presynaptic autoreceptors mediate a retrograde transfer of information by a negative feedback mechanism mediated by the transmitter of the neuron, and fulfill an autoregulatory function in neurotransmission in the peripheral and central nervous system (CNS). Starting with norepinephrine (NE), it was later reported that an autoreceptor-mediated negative feedback mechanism exists for other neurotransmitters, including dopamine (DA), serotonin, acetylcholine, histamine, GABA, and glutamate. This feedback mechanism regulates calcium-dependent transmitter release and synthesis through terminal presynaptic autoreceptors, while the firing rate of the neuron is regulated through somatodendritic autoreceptors.

Striatal cholinergic dysfunction as a unifying theme in the pathophysiology of dystonia

Dystonia is a movement disorder of both genetic and non-genetic causes, which typically results in twisted posturing due to abnormal muscle contraction. Evidence from dystonia patients and animal models of dystonia indicate a crucial role for the striatal cholinergic system in the pathophysiology of dystonia. In this review, we focus on striatal circuitry and the centrality of the acetylcholine system in the function of the basal ganglia in the control of voluntary movement and ultimately clinical manifestation of movement disorders. We consider the impact of cholinergic interneurons (ChIs) on dopamine-acetylcholine interactions and examine new evidence for impairment of ChIs in dysfunction of the motor systems producing dystonic movements, particularly in animal models. We have observed paradoxical excitation of ChIs in the presence of dopamine D2 receptor agonists and impairment of striatal synaptic plasticity in a mouse model of DYT1 dystonia, which are improved by administration of recently developed M1 receptor antagonists. These findings have been confirmed across multiple animal models of DYT1 dystonia and may represent a common endophenotype by which to investigate dystonia induced by other types of genetic and non-genetic causes and to investigate the potential effectiveness of pharmacotherapeutics and other strategies to improve dystonia.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Regulation of striatal dopamine release by presynaptic auto- and heteroreceptors

Striatal dopamine neurotransmission is critical for normal voluntary movement, affect and cognition. Dysfunctions of its regulation are implicated in a broad range of behaviors and disorders including Parkinson's disease, schizophrenia and drug abuse. Extracellular dopamine levels result from a dynamic equilibrium between release and reuptake by dopaminergic terminals. Both processes are regulated by multiple mechanisms. Here we review data characterizing how dopamine levels are regulated by presynaptic autoreceptors and heteroreceptors, an area intensively investigated due to advances in real time electrochemical detection of extracellular dopamine, i.e., fast-scan cyclic voltammetry and amperometry, and the development of mutant mouse lines with deletions for specific receptors.

Muscarinic mechanisms in psychotic disorders

Schizophrenia is a devastating disease with several broad symptom clusters and the current monoamine-based treatments do not adequately treat the disease, especially negative and cognitive symptoms. A proposed alternative approach for treating schizophrenia is through the use of compounds that activate certain muscarinic receptor subtypes, the so-called muscarinic cholinergic hypothesis theory. This theory has been revitalized with a number of recent and provocative findings including postmortem reports in schizophrenia patients showing decreased numbers of muscarinic M(1) and M(4) receptors in brain regions associated with schizophrenia as well as decreased muscarinic receptors in an in vivo imaging study. Studies with M(4) knockout mice have shown that there is a reciprocal relationship between M(4) and dopamine receptor function, and a number of muscarinic agonists have shown antidopaminergic activity in a variety of preclinical assays predictive of antipsychotic efficacy in the clinic. Furthermore, the M(1)/M(4) preferring partial agonist xanomeline has been shown to have antipsychotic-like and pro-cognitive activity in preclinical models and in clinical trials to decrease psychotic-like behaviors in Alzheimer's patients and positive, negative, and cognitive symptoms in patients with schizophrenia. Therefore, we propose that an agonist with M(1) and M(4) interactions would effectively treat core symptom clusters associated with schizophrenia. Currently, research is focused on developing subtype-selective muscarinic agonists and positive allosteric modulators that have reduced propensity for parasympathetic side-effects, but retain the therapeutic benefit observed with their less selective predecessors.

Dopamine signaling in dorsal versus ventral striatum: the dynamic role of cholinergic interneurons

Mesostriatal dopaminergic neurons and striatal cholinergic interneurons participate in signaling the motivational significance of environmental stimuli and regulate striatal plasticity. Dopamine (DA) and acetylcholine (ACh) have potent interactions within the striatum at multiple levels that include presynaptic regulation of neurotransmitter release and postsynaptic effects in target cells (including ACh neurons). These interactions may be highly variable given the dynamic changes in the firing activities of parent DA and ACh neurons. Here, we consider how striatal ACh released from cholinergic interneurons acting at both nicotinic and muscarinic ACh receptors powerfully modulates DA transmission. This ACh–DA interaction varies in a manner that depends on the frequency of presynaptic activation, and will thus strongly influence how DA synapses convey discrete changes in DA neuron activity that are known to signal events of motivational salience. Furthermore, this ACh modulation of DA transmission within striatum occurs via different profiles of nicotinic and muscarinic receptors in caudate–putamen compared to nucleus accumbens, which may ultimately enable region-specific targeting of striatal function.

The cholinergic system and neostriatal memory functions

The striatum is one of the major forebrain regions that strongly expresses muscarinic and nicotinic cholinergic receptors. This article reviews the current knowledge and our new findings about the striatal cholinoceptive organization and its role in a variety of cognitive functions. Pharmacological and genetic manipulations have indicated that the cholinergic and dopaminergic system in the striatum modulate each other's function. In addition to modulating the dopaminergic system, nicotinic cholinergic receptors facilitate GABA release, whereas muscarinic receptors attenuate GABA release. The striatal cholinergic system has also been implicated in various cognitive functions including procedural learning and intradimensional set shifting. Together, these data indicate that the cholinergic system in the striatum is involved in a diverse set of cognitive functions through interactions with other neurotransmitter systems including the dopaminergic and GABAergic systems.

Release-enhancing pre-synaptic muscarinic and nicotinic receptors co-exist and interact on dopaminergic nerve endings of rat nucleus accumbens

Dopaminergic nerve endings in the corpus striatum possess nicotinic (nAChRs) and muscarinic cholinergic receptors (mAChRs) mediating release of dopamine (DA). Whether nAChRs and mAChRs co-exist and interact on the same nerve endings is unknown. We here investigate on these possibilities using rat nucleus accumbens synaptosomes pre-labeled with [(3)H]DA and exposed in superfusion to cholinergic receptor ligands. The mixed nAChR-mAChR agonists acetylcholine (ACh) and carbachol provoked [(3)H]DA release partially sensitive to the mAChR antagonist atropine but totally blocked by the nAChR antagonist mecamylamine. Addition of the mAChR agonist oxotremorine at the minimally effective concentration of 30 micromol/L, together with 3, 10, or 100 micromol/L (-)nicotine provoked synergistic effect on [(3)H]DA overflow. The [(3)H]DA overflow elicited by 100 micromol/L (-)nicotine plus 30 micromol/L oxotremorine was reduced by atropine down to the release produced by (-)nicotine alone and it was abolished by mecamylamine. The ryanodine receptor blockers dantrolene or 8-bromo-cADP-ribose, but not the inositol 1,4,5-trisphosphate receptor blocker xestospongin C inhibited the (-)nicotine/oxotremorine evoked [(3)H]DA overflow similarly to atropine. This overflow was partly sensitive to 100 nmol/L methyllycaconitine which did not prevent the synergistic effect of (-)nicotine/oxotremorine. Similarly to (-)nicotine, the selective alpha4beta2 nAChR agonist RJR2403 exhibited synergism when added together with oxotremorine. To conclude, in rat nucleus accumbens, alpha4beta2 nAChRs exert a permissive role on the releasing function of reportedly M(5) mAChRs co-existing on the same dopaminergic nerve endings.

Acetylcholine-dopamine interactions in the pathophysiology and treatment of CNS disorders

Dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area of the midbrain form the nigrostriatal and mesocorticolimbic dopaminergic pathways that, respectively, project to dorsal and ventral striatum (including prefrontal cortex). These midbrain dopaminergic nuclei and their respective forebrain and cortical target areas are well established as serving a critical role in mediating voluntary motor control, as evidenced in Parkinson's disease, and incentive-motivated behaviors and cognitive functions, as exhibited in drug addiction and schizophrenia, respectively. Although it cannot be disputed that excitatory and inhibitory amino acid-based neurotransmitters, such as glutamate and GABA, play a vital role in modulating activity of midbrain dopaminergic neurons, recent evidence suggests that acetylcholine may be as important in regulating dopaminergic transmission. Midbrain dopaminergic cell tonic and phasic activity is closely dependent upon projections from hindbrain pedunculopontine and the laterodorsal tegmental nuclei, which comprises the only known cholinergic inputs to these neurons. In close coordination with glutamatergic and GABAergic activity, these excitatory cholinergic projections activate nicotinic and muscarinic acetylcholine receptors within the substantia nigra and ventral tegmental area to modulate dopamine transmission in the dorsal/ventral striatum and prefrontal cortex. Additionally, acetylcholine-containing interneurons in the striatum also constitute an important neural substrate to provide further cholinergic modulation of forebrain striatal dopaminergic transmission. In this review, we examine neurological and psychopathological conditions associated with dysfunctions in the interaction of acetylcholine and dopamine and conventional and new pharmacological approaches to treat these disorders.

Striatal muscarinic receptors promote activity dependence of dopamine transmission via distinct receptor subtypes on cholinergic interneurons in ventral versus dorsal striatum

Striatal dopamine (DA) and acetylcholine (ACh) regulate motivated behaviors and striatal plasticity. Interactions between these neurotransmitters may be important, through synchronous changes in parent neuron activities and reciprocal presynaptic regulation of release. How DA signaling is regulated by striatal muscarinic receptors (mAChRs) is unresolved; contradictory reports indicate suppression or facilitation, implicating several mAChR subtypes on various neurons. We investigated whether mAChR regulation of DA signaling varies with presynaptic activity and identified the mAChRs responsible in sensorimotor- versus limbic-associated striatum. We detected DA in real time at carbon fiber microelectrodes in mouse striatal slices. Broad-spectrum mAChR agonists [oxotremorine-M, APET (arecaidine propargyl ester tosylate)] decreased DA release evoked by low-frequency stimuli (1-10 Hz, four pulses) but increased the sensitivity of DA release to presynaptic activity, even enhancing release by high frequencies (e.g., >25 Hz for four pulses). These bidirectional effects depended on ACh input to striatal nicotinic receptors (nAChRs) on DA axons but not GABA or glutamate input. In caudate-putamen (CPu), knock-out of M(2)- or M(4)-mAChRs (not M(5)) prevented mAChR control of DA, indicating that M(2)- and M(4)-mAChRs are required. In nucleus accumbens (NAc) core or shell, mAChR function was prevented in M(4)-knock-outs, but not M(2)- or M(5)-knock-outs. These data indicate that striatal mAChRs, by inhibiting ACh release from cholinergic interneurons and thus modifying nAChR activity, offer variable control of DA release probability that promotes how DA release reflects activation of dopaminergic axons. Furthermore, different coupling of striatal M(2)/M(4)-mAChRs to the control of DA release in CPu versus NAc suggests targets to influence DA/ACh function differentially between striatal domains.

Specific inhibitory effect of amyloid-beta on presynaptic muscarinic receptor subtypes modulating neurotransmitter release in the rat nucleus accumbens

In this study we investigate on the effect of amyloid-beta1-40 (A beta 1-40) on the oxotremorine (OXO)-induced release of [(3)H] dopamine (DA), [(3)H]GABA and [(3)H]acetylcholine (ACh) from synaptosomes in the rat nucleus accumbens (NAc). OXO in presence of himbacine (HIMBA) was able to increase the basal release of [(3)H]GABA. The OXO-elicited [(3)H]GABA overflow was significantly antagonized by atropine (A; 94%), by the M3 antagonists DAU5884 (96%) and 4-DAMP (70%), and by A beta 1-40 (65%). Exposure of NAc synaptosomes to OXO produced a dose-dependent increase of [(3)H]DA overflow which was antagonized by A, partially inhibited by A beta 1-40 (100 nM) but unaffected by DAU5884 and 4-DAMP. The K(+)-evoked [(3)H]ACh overflow was inhibited by OXO. This effect was counteracted by the M2 antagonist AFDX-116 but not by the selective M4 antagonist mamba toxin 3 (MT3). The K(+)-evoked [(3)H]GABA overflow was also inhibited by OXO but conversely, this effect was counteracted by MT3 and not by AFDX-116. A beta 1-40 (100 nM) did not modify the inhibitory effect of OXO both on the K(+)-evoked [(3)H]ACh and [(3)H]GABA overflow. The results show that in the rat NAc, A beta 1-40 selectively inhibits the function of the muscarinic subtypes which stimulate neurotransmitter release and not those which modulate negatively the stimulated release.

Involvement of KCNQ2 subunits in [3H]dopamine release triggered by depolarization and pre-synaptic muscarinic receptor activation from rat striatal synaptosomes

KCNQ2 and KCNQ3 subunits encode for the muscarinic-regulated current (I(KM)), a sub-threshold voltage-dependent K+ current regulating neuronal excitability. In this study, we have investigated the involvement of I(KM) in dopamine (DA) release from rat striatal synaptosomes evoked by elevated extracellular K+ concentrations ([K+]e) and by muscarinic receptor activation. [3H]dopamine ([3H]DA) release triggered by 9 mmol/L [K+]e was inhibited by the I(KM) activator retigabine (0.01-30 micromol/L; Emax = 54.80 +/- 3.85%; IC50 = 0.50 +/- 0.36 micromol/L). The I(KM) blockers tetraethylammonium (0.1-3 mmol/L) and XE-991 (0.1-30 micromol/L) enhanced K+-evoked [3H]DA release and prevented retigabine-induced inhibition of depolarization-evoked [3H]DA release. Retigabine-induced inhibition of K+-evoked [3H]DA release was also abolished by synaptosomal entrapment of blocking anti-KCNQ2 polyclonal antibodies, an effect prevented by antibody pre-absorption with the KCNQ2 immunizing peptide. Furthermore, the cholinergic agonist oxotremorine (OXO) (1-300 micromol/L) potentiated 9 mmol/L [K+]e-evoked [3H]DA release (Emax = 155 +/- 9.50%; EC50 = 25 +/- 1.80 micromol/L). OXO (100 micromol/L)-induced [3H]DA release enhancement was competitively inhibited by pirenzepine (1-10 nmol/L) and abolished by the M3-preferring antagonist 4-diphenylacetoxy N-methylpiperidine methiodide (1 micromol/L), but was unaffected by the M1-selective antagonist MT-7 (10-100 nmol/L) or by Pertussis toxin (1.5-3 microg/mL), which uncouples M2- and M4-mediated responses. Finally, OXO-induced potentiation of depolarization-induced [3H]DA release was not additive to that produced by XE-991 (10 micromol/L), was unaffected by retigabine (10 micromol/L), and was abolished by synaptosomal entrapment of anti-KCNQ2 antibodies. Collectively, these findings indicate that, in rat striatal nerve endings, I(KM) channels containing KCNQ2 subunits regulate depolarization-induced DA release and that I(KM) suppression is involved in the reinforcement of depolarization-induced DA release triggered by the activation of pre-synaptic muscarinic heteroreceptors.

Towards a muscarinic hypothesis of schizophrenia

Although the neurotransmitter dopamine plays a prominent role in the pathogenesis and treatment of schizophrenia, the dopamine hypothesis of schizophrenia fails to explain all aspects of this disorder. It is increasingly evident that the pathology of schizophrenia also involves other neurotransmitter systems. Data from many streams of research including pre-clinical and clinical pharmacology, treatment studies, post-mortem studies and neuroimaging suggest an important role for the muscarinic cholinergic system in the pathophysiology of schizophrenia. This review will focus on evidence that supports the hypothesis that the muscarinic system is involved in the pathogenesis of schizophrenia and that muscarinic receptors may represent promising novel targets for the treatment of this disorder.

Muscarinic acetylcholine receptor knockout mice: novel phenotypes and clinical implications

Muscarinic acetylcholine receptors (mAChRs; M1-M5) play key roles in regulating the activity of many important functions of the central and peripheral nervous system. Because of the lack of ligands endowed with a high degree of receptor subtype selectivity and the fact that most tissues or cell types express two or more mAChR subtypes, identification of the physiological and pathophysiological roles of the individual mAChR subtypes has proven a difficult task. To circumvent these difficulties, several laboratories recently employed gene-targeting techniques to generate mutant mouse strains deficient in each of the five mAChR subtypes. Phenotyping studies showed that each mutant mouse line displayed characteristic physiological, pharmacological, behavioral, biochemical, or neurochemical deficits. The novel insights gained from these studies should prove instrumental for the development of novel classes of muscarinic drugs.

Novel insights into M5 muscarinic acetylcholine receptor function by the use of gene targeting technology

Until recently, little was known about the possible physiological functions of the M(5) muscarinic acetylcholine receptor subtype, the last member of the muscarinic receptor family (M(1)-M(5)) to be cloned. To learn more about the potential physiological roles of this receptor subtype, we generated and analyzed M(5) receptor-deficient mice (M5 -/- mice). Strikingly, acetylcholine, a potent dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5 -/- mice, suggesting that endothelial M(5) receptors mediate this activity in wild-type mice. This effect was specific for cerebral blood vessels, since acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5 -/- mice. In addition, in vitro neurotransmitter release experiments indicated that M(5) receptors located on dopaminergic nerve terminals play a role in facilitating muscarinic agonist-induced dopamine release in the striatum, consistent with the observation that the dopaminergic neurons innervating the striatum almost exclusively express the M(5) receptor subtype. We also found that the rewarding effects of morphine, the prototypical opiate analgesic, were substantially reduced in M5 -/- mice, as measured in the conditioned place preference paradigm. Furthermore, both the somatic and affective components of naloxone-induced morphine withdrawal symptoms were significantly attenuated in M5 -/- mice. It is likely that these behavioral deficits are caused by the lack of mesolimbic M(5) receptors, activation of which is known to stimulate dopamine release in the nucleus accumbens. These results convincingly demonstrate that the M(5) muscarinic receptor is involved in modulating several important pharmacological and behavioral functions. These findings may lead to novel therapeutic strategies for the treatment of drug addiction and certain cerebrovascular disorders.

Muscarinic and nicotinic cholinergic mechanisms in the mesostriatal dopamine systems

The striatum and its dense dopaminergic innervation originating in the midbrain, primarily from the substantia nigra pars compacta and the ventral tegmental area, compose the mesostriatal dopamine (DA) systems. The nigrostriatal system is involved mainly in motor coordination and in disorders such as Tourette's syndrome, Huntington's disease, and Parkinson's disease. The dopaminergic projections from the ventral tegmental area to the striatum participate more in the processes that shape behaviors leading to reward, and addictive drugs act upon this mesolimbic system. The midbrain DA areas receive cholinergic innervation from the pedunculopontine tegmentum and the laterodorsal pontine tegmentum, whereas the striatum receives dense cholinergic innervation from local interneurons. The various neurons of the mesostriatal systems express multiple types of muscarinic and nicotinic acetylcholine receptors as well as DA receptors. Especially in the striatum, the dense mingling of dopaminergic and cholinergic constituents enables potent interactions. Evidence indicates that cholinergic and dopaminergic systems work together to produce the coordinated functioning of the striatum. Loss of that cooperative activity contributes to the dysfunction underlying Parkinson's disease.

Cholinergic modulation of basal and amphetamine-induced dopamine release in rat medial prefrontal cortex and nucleus accumbens

Behavioral evidence suggests that muscarinic/cholinergic inhibition of brain dopaminergic activity may be a useful principle for developing novel antipsychotic drugs (APDs). Thus, oxotremorine, a muscarinic agonist, attenuates amphetamine-induced locomotor activity in rodents, an effect also produced by a wide variety of proven APDs, whereas scopolamine, a muscarinic antagonist, has the opposite effect. Since atypical APDs such as clozapine, olanzapine, risperidone, ziprasidone and quetiapine, increase brain acetylcholine as well as dopamine (DA) release in a region-specific manner, their effects on cholinergic and dopaminergic neurotransmission may also contribute to various actions of these drugs. Oxotremorine (0.5-1.5 mg/kg) dose-dependently and preferentially increased DA release in rat medial prefrontal cortex (mPFC), compared to the nucleus accumbens (NAC). However, S-(-)-scopolamine (0.5-1.5 mg/kg) produced similar increases in DA release in the mPFC, but the effect was much less than that of oxotremorine. Whereas a dose of S-(-)-scopolamine of 0.5 mg/kg comparably increased DA release in the mPFC and NAC, 1.5 mg/kg had no effect on DA release in the NAC. Oxotremorine-M (0.5 mg/kg), a M(1/4)-preferring agonist, also increased DA release in the mPFC, but not the NAC, an effect completely abolished by telenzepine (3 mg/kg), a M(1/4)-preferring antagonist, which by itself had no effect on DA release in either region. Oxotremorine (0.5, but not 1.5, mg/kg) attenuated amphetamine (1 mg/kg)-induced DA release in the NAC, whereas S-(-)-scopolamine did not. Oxotremorine (1.5 mg/kg) and S-(-)-scopolamine (0.5 mg/kg) modestly but significantly potentiated amphetamine (1 mg/kg)-induced DA release in the mPFC. These results suggest that stimulation of muscarinic receptors, in particular M(1/4), as indicated by the effect of oxotremorine-M and telenzepine, may preferentially increase cortical DA release and inhibit amphetamine-induced DA release in the NAC.

Multiple muscarinic acetylcholine receptor subtypes modulate striatal dopamine release, as studied with M1-M5 muscarinic receptor knock-out mice

A proper balance between striatal muscarinic cholinergic and dopaminergic neurotransmission is required for coordinated locomotor control. Activation of striatal muscarinic acetylcholine receptors (mAChRs) is known to modulate striatal dopamine release. To identify the mAChR subtype(s) involved in this activity, we used genetically altered mice that lacked functional M1-M5 mAChRs [knock-out (KO) mice]. In superfused striatal slices from wild-type mice, the non-subtype-selective muscarinic agonist oxotremorine led to concentration-dependent increases in potassium-stimulated [3H]dopamine release (by up to 60%). The lack of M1 or M2 receptors had no significant effect on the magnitude of these responses. Strikingly, oxotremorine-mediated potentiation of stimulated striatal [3H]dopamine release was abolished in M4 receptor KO mice, significantly increased in M3 receptor-deficient mice, and significantly reduced (but not abolished) in M5 receptor KO mice. Additional release studies performed in the presence of tetrodotoxin suggested that the dopamine release-stimulating M4 receptors are probably located on neuronal cell bodies, but that the release-facilitating M5 and the release-inhibiting M3 receptors are likely to be located on nerve terminals. Studies with the GABA(A) receptor blocker bicuculline methochloride suggested that M3 and M4 receptors mediate their dopamine release-modulatory effects via facilitation or inhibition, respectively, of striatal GABA release. These results provide unambiguous evidence that multiple mAChR subtypes are involved in the regulation of striatal dopamine release. These findings should contribute to a better understanding of the important functional roles that the muscarinic cholinergic system plays in striatal function.

Hyperactivity, elevated dopaminergic transmission, and response to amphetamine in M1 muscarinic acetylcholine receptor-deficient mice

Acetylcholine serves an important modulatory role in the central nervous system. Pharmacological evidence has suggested that cholinergic activity can modulate central dopaminergic transmission; however, the nature of this interaction and the receptors involved remain undefined. In this study we have generated mice lacking the M1 muscarinic acetylcholine receptor and examined the effects of M1 deletion on dopaminergic transmission and locomotor behavior. We report that M1 deficiency leads to elevated dopaminergic transmission in the striatum and significantly increased locomotor activity. M1-deficient mice also have an increased response to the stimulatory effects of amphetamine. Our results provide direct evidence for regulation of dopaminergic transmission by the M1 receptor and are consistent with the idea that M1 dysfunction could be a contributing factor in psychiatric disorders in which altered dopaminergic transmission has been implicated.

Cholinergic dilation of cerebral blood vessels is abolished in M(5) muscarinic acetylcholine receptor knockout mice

The M(5) muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M(1)-M(5)) to be cloned. At present, the physiological relevance of this receptor subtype remains unknown, primarily because of its low expression levels and the lack of M(5) receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M(5) receptor-deficient mice (M5R(-/-) mice). M5R(-/-) mice did not differ from their wild-type littermates in various behavioral and pharmacologic tests. However, in vitro neurotransmitter release experiments showed that M(5) receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M(5) receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M(5) receptors led to changes in vascular tone by using several in vivo and in vitro vascular preparations. Strikingly, acetylcholine, a powerful dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5R(-/-) mice. This effect was specific for cerebral blood vessels, because acetylcholine-mediated dilation of extra-cerebral arteries remained fully intact in M5R(-/-) mice. Our findings provide direct evidence that M(5) muscarinic receptors are physiologically relevant. Because it has been suggested that impaired cholinergic dilation of cerebral blood vessels may play a role in the pathophysiology of Alzheimer's disease and focal cerebral ischemia, cerebrovascular M(5) receptors may represent an attractive therapeutic target.

Involvement of intrinsic cholinergic and GABAergic innervation in the effect of NMDA on striatal dopamine efflux and metabolism as assessed by microdialysis studies in freely moving rats

Microdialysis perfusion was used to study the participation of striatal cholinergic and gamma-aminobutyric acid-ergic (GABAergic) neurotransmission in basal and N-methyl-D-aspartate (NMDA) receptor-modulated dopamine release and metabolism in the striatum of the freely moving rat. Reverse dialysis of atropine (1-50 microM) induced a concentration-related increase in dopamine efflux and decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) efflux. (+)-Bicuculline (10-100 microM) similarly increased dopamine efflux, but was without consistent effect on metabolite efflux. Reverse dialysis of NMDA (1 mM) evoked an approximately twofold increase in dopamine efflux and decreased DOPAC and HVA efflux to 30-40% of basal levels. The effect of NMDA on dopamine efflux was completely abolished by coadministration of tetrodotoxin (TTX; 1 microM) or atropine (10 microM), and markedly potentiated (approximately fourfold) by coadministration of (+)-bicuculline (50 microM). The NMDA-induced decrease in dopamine metabolite efflux was inhibited by coadministration of TTX or (+)-bicuculline, but was unaffected by atropine. Our data suggest that dopamine release in the striatum is subject to both cholinergic and GABAergic tonic inhibitory mechanisms mediated through muscarinic and GABAA receptors, respectively. Furthermore, NMDA-stimulated dopamine release also involves obligatory cholinergic facilitation and an inhibitory GABAergic component mediated through these respective receptors.

Postsynaptic integration of cholinergic and dopaminergic signals on medium-sized GABAergic projection neurons in the neostriatum

The effects of cholinergic drugs and the interaction between cholinergic and dopaminergic compounds were studied on electrically evoked [3H]gamma-aminobutyric acid (GABA) overflow in slices of the rat neostriatum. Slices were prepared and loaded with [3H]GABA in the presence of beta-alanine and then superfused with Krebs-bicarbonate buffer containing aminooxyacetic acid and nipecotic acid to inhibit GABA uptake and metabolism, respectively. The nonselective muscarinic agonist oxotremorine (0.1-10 microM) increased the release of [3H]GABA and the selective M1 receptor agonist McN-A-343 (0.1-10 microM) exerted similar effect. The stimulatory effect of oxotremorine (10 microM) on [3H][GABA overflow was antagonized by the nonselective muscarinic antagonist atropine (1 microM) and the selective M1 receptor antagonist pirenzepine (0.1-1.0 microM). The M2 receptor antagonist methoctramine (1.0 microM) did not alter the stimulatory effect of oxotremorine. Of the muscarinic receptor antagonists atropine, pirenzepine, and methoctramine (1.0 microM) failed to affect [3H]GABA overflow. The M3 receptor antagonist p-F-HHSiD (1 microM) increased [3H]GABA overflow and p-F-HHSiD and oxotremorine were found to be additive in increasing this effect. The D2 dopamine receptor antagonist sulpiride (10 microM) increased the electrical stimulation-induced [3H]GABA overflow, and this stimulation was counteracted by concomitant administration of atropine (1 microM). McN-A-343 and sulpiride also increased the KCl-induced [3H]GABA overflow from superfused neostriatal slices and tetrodotoxin (1 microM) did not affect these stimulations. These data indicate that the release of GABA in the neostriatum is under the control of M1 stimulatory and M3 inhibitory muscarinic receptors. Dopamine, which exerts inhibition on GABA release via D2 receptors, may counteract the M1 facilitation, and M1 and D2 receptors involved in the cholinergic-dopaminergic interaction may be located postsynaptically on medium-sized spiny GABAergic projection neurons.

Intermittent morphine treatment causes a protracted increase in cholinergic striatal neurotransmission measured ex vivo

Considering the long-lasting neuroadaptations that occur in the brain after exposure to drugs of abuse, we found that the facilitatory effect of an EC50 concentration (0.1 microM) of the acetylcholinesterase inhibitor physostigmine, unlike that of the muscarinic receptor agonist oxotremorine, on K(+)-induced [3H]dopamine release from rat striatal slices was enhanced about 2-fold 1 month after cessation of intermittent morphine treatment. Similarly, the inhibitory effect of physostigmine on K(+)-induced [14C]acetylcholine release from the slices was enhanced subsequent to morphine treatment, whereas that of oxotremorine appeared to be unchanged. Therefore, intermittent morphine administration may cause a very long-lasting increase of muscarinic receptor activation by released endogenous acetylcholine in rat striatum, which may play a pivotal role in the enduring character of stimulus hyperresponsiveness after exposure to drugs of abuse.

Effects of SUN 8399, a potent and selective 5-HT1A agonist, on conflict behavior and ambulatory activity in mice: comparison with those of buspirone, tandospirone and diazepam

Behavioral effects of p.o. administration of SUN 8399, a selective 5-HT1A agonist, on the operant behavior under a MULT VI 1.5 min/FR 5-punishment schedule of food reinforcement and on the ambulatory activity were evaluated in mice, and the characteristics were compared with those of other 5-HT1A agonists, buspirone and tandospirone, and the benzodiazepine diazepam. Diazepam (3 and 10 mg/kg) significantly increased the punished response without eliciting any significant change in the non-punished response; i.e., showing anticonflict action. SUN 8399 (3-30 mg/kg) and buspirone (1-10 mg/kg) did not significantly change either the punished or non-punished responses. Tandospirone significantly increased the non-punished response at 10 mg/kg, but significantly decreased both the punished and non-punished responses at 30 mg/kg. The single administration of SUN 8399 (10 mg/kg), buspirone (3 and 30 mg/kg) and tandospirone (10 and 30 mg/kg) significantly increased the ambulatory activity, while diazepam tended to decrease it. The ambulation-increasing effect of methamphetamine (2 mg/kg, s.c.) was reduced by buspirone (10 and 30 mg/kg) and tandospirone (10 and 30 mg/kg), but enhanced by diazepam (3 and 10 mg/kg). Buspirone (30 mg/kg), tandospirone (10 and 30 mg/kg) and diazepam (3 and 10 mg/kg) significantly reduced the ambulation-increasing effect of scopolamine (0.5 mg/kg, s.c.). SUN 8399 (3-100 mg/kg) did not modify the effects of either methamphetamine or scopolamine. The present results suggest that 5-HT1A agonists scarcely show anticonflict action on the Geller-type conflict behavior in mice. However, SUN 8399 possesses different behavioral characteristics from those of the other two 5-HT1A agonists in terms of interactions with methamphetamine and scopolamine.

Do nerve terminals and cell bodies of nigrostriatal dopaminergic neurons of the rat contain similar receptors?

The question was investigated whether dopamine release-controlling receptors are evenly distributed over somatodendritic sites and nerve terminals of nigrostriatal dopaminergic neurons of the rat. Prototypical drugs of 5 different (sub)types of receptors (D2, cholinergic, GABAB, NMDA and non-NMDA) were infused via a microdialysis probe into the striatum, and effects on dopamine released from nerve terminals were determined by microdialysis. In separate experiments the same drugs were infused into the substantia nigra and effects on dendritic release of dopamine were recorded. In addition, the effect of calcium depletion and tetrodotoxin infusion (1 mumol/l) was studied in both areas. Infusion of (-)-N0437 (1 mumol/l), (-)-sulpiride (1 mumol/l), NMDA (300 mumol/l), AMPA (100 mumol/l), kainic acid (30 mumol), tetrodotoxin and depletion of calcium induced comparable changes in the release of dopamine when applied into the striatum as well as into the nigra. Carbachol (100 mumol/l) and baclofen (5 mumol/l) inhibited dendritic dopamine release when administered into the nigra; however, the latter drugs were not effective when infused into the striatum. It is concluded that the release-controlling receptors are not evenly distributed over somata and nerve terminals of dopaminergic neurons.

Muscarinic potentiation of excitatory amino acid-evoked dopamine release in mesencephalic cells: specificity for the NMDA response and role of intracellular messengers

Of the five cloned muscarinic receptor subtypes, dopamine (DA) neurons in the substantia nigra and ventral tegmental areas have been shown to be selectively enriched with the mRNA for the m5 subtype, suggesting that muscarinic modulation of DA neurons may have a distinct pharmacology. In the present study we have used dissociated cell cultures of fetal rat ventral mesencephalon to characterize muscarinic modulation of DA neurons. [3H]DA release stimulated by activation of N-methyl-D-aspartate (NMDA) receptors was potentiated by carbachol, a mixed muscarinic-nicotinic agonist, and by oxotremorine-M, a muscarinic agonist. Neither carbachol nor oxotremorine-M had an effect on [3H]DA release evoked by the non-NMDA agonists, kainate or quisqualate. A nicotinic agonist, DMPP, had no effect on NMDA-stimulated release. Potentiation of NMDA-stimulated [3H]DA release by oxotremorine-M was inhibited by the broad spectrum muscarinic antagonist, QNB, and by low concentrations of a putative M1 antagonist, pirenzepine, while much higher concentrations of a purported M2 antagonist, AF-DX 384, were required to reverse the oxotremorine-M effect. The muscarinic antagonist, 4-DAMP, was active in a concentration range between that required for pirenzepine and AF-DX 384. Further experiments examined intracellular messenger mechanisms coupled to the muscarinic receptors modulating NMDA-stimulated [3H]DA release. In contrast to oxotremorine-M, two muscarinic agents with only weak partial agonism with respect to phosphoinositide turnover, pilocarpine and arecoline, had no effect on NMDA-stimulated [3H]DA release.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of acetyl-L-carnitine on extracellular amino acid levels in vivo in rat brain regions

Acetyl-L-carnitine (ALCAR) was found to have beneficial effects in senile patients. In recent years many of its effects on the nervous system have been examined, but its mechanism(s) of action remains to be elucidated. We previously reported that it causes release of dopamine in the striatum. In the present paper we report that ALCAR, when administered at intracerebral sites via microdialysis, stimulates the release of amino acids in a concentration-dependent and regionally heterogeneous manner. The effect was strong in the striatum and cerebellum, less so in the frontal cortex, and weak in the thalamus. Seven amino acids were measured: the increase in the level of aspartate, glutamate, and taurine was substantial, and the increase in the level of glycine, serine, threonine, alanine, and glutamine in the microdialysate was minor. The stimulatory effect of ALCAR on the release of amino acids in the striatum was inhibited by the muscarinic antagonist atropine, but was not inhibited by the nicotinic antagonist mecamylamine. The effect of ALCAR on the levels of most of the amino acids tested was independent of the presence of Ca2+ in the perfusate. These results indicate that ALCAR, when administered intracerebrally at fairly high concentrations, can affect the level and the release not only of such neurotransmitters as acetylcholine and dopamine, but also of amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-competitive antagonism by hirsuteine of nicotinic receptor-mediated dopamine release from rat pheochromocytoma cells

Effects of hirsuteine, an indole alkaloid extracted from Uncaria genus, on nicotine- and high K-induced responses were investigated in rat pheochromocytoma PC12 cells. Hirsuteine (300 nM-10 microM) inhibited dopamine release evoked by 100 microM nicotine in a concentration-dependent manner. Hirsuteine did not produce a parallel shift of the concentration-response relationship curve for nicotine, but reduced maximal dopamine release. Dopamine release evoked by 60 and 155 mM KCl was also inhibited by hirsuteine, but the concentration necessary for significant inhibition was higher (more than 10 microM). Under whole cell voltage-clamp, hirsuteine reversibly inhibited inward currents activated by 100 microM nicotine. The current inhibition was slightly accelerated by hyperpolarization. The results suggest that hirsuteine non-competitively antagonizes nicotine-evoked dopamine release by blocking ion permeation through nicotinic receptor channel complexes. The blockade of Ca channels, which are activated during nicotine-evoked depolarization, may not play a major role in the antagonism.

NMDA receptor mediates dopamine release in the striatum of unanesthetized rats as measured by brain microdialysis

We have studied the characteristics associated with the activation of the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor on the release of dopamine (DA) in the striatum of awake rats as measured by brain microdialysis technique. NMDA dose-dependently stimulated the striatal DA release in Mg(2+)-free Ringer's solution. The stimulation was significant at 90 microM and the maximum observed effect was at the highest concentration tested (800 microM). The selective NMDA receptor antagonist, 2-amino-5-phosphonovalerate (AP5; 300 microM), blocked the stimulatory effect of NMDA. The NMDA-induced release of DA was reduced by 1.2 mM Mg2+ and totally blocked by 2.5 mM of the cation. Glycine (200 microM) potentiated the response evoked by 300 microM NMDA while 7-chloro-kynurenate (100 microM), an antagonist of the glycine site, reduced markedly this response. Neither atropine (100 microM) nor tetrodotoxin (TTX) (5 microM) prevented the stimulatory effect of NMDA. These results suggest that glutamate released from corticostriatal terminals presynaptically stimulates the release of DA via an NMDA receptor.

Acetyl-L-carnitine releases dopamine in rat corpus striatum: an in vivo microdialysis study

The effect of acetyl-L-carnitine, a compound reported to be beneficial for senile patients, on the release of dopamine (DA) from the striatum was studied by using in vivo brain dialysis in anesthetized rats coupled with HPLC-electrochemical detection. Striatal infusion of acetyl-L-carnitine increased the efflux of DA with no apparent changes in efflux of DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (HVA). The DA-releasing effect of acetyl-L-carnitine was concentration- and Ca(2+)-dependent, and was abolished by omega-conotoxin fraction GVIA and tetrodotoxin, inhibitors of the voltage-dependent Ca2+ and Na+ channels, respectively. Nomifensine, an inhibitor of DA reuptake did not alter the DA-releasing property of acetyl-L-carnitine. DA released from the striatum by acetyl-L-carnitine was decreased by reserpine pretreatment whereas the d-amphetamine-evoked DA outflow was not affected. In contrast to acetyl-L-carnitine, d-amphetamine reduced the extracellular concentrations of DOPAC and HVA. We conclude from the present data that acetyl-L-carnitine evokes DA release from the vesicular pools of the nigrostriatal dopaminergic neurons by a Ca(2+)-dependent, exocytotic process.

Decreased (3H) quinuclidinyl benzilate binding to lymphocytes in Gilles de la Tourette syndrome

In spite of an unknown pathophysiology, it has been suggested that central dopaminergic hyperactivity exists in Gilles de la Tourette syndrome (TS). Cholinergic influences have also been postulated as a dopaminergic-cholinergic balance seems to be important in other movement disorders. If TS is due to alterations of cholinergic activity, this may also be expressed at postsynaptic levels. Recently, we showed that circulating lymphocytes may serve as useful peripheral markers reflecting induced alterations or inherent changes in muscarinic receptors in the central nervous system (CNS). In the present study, we compared the muscarinic binding characteristics in peripheral lymphocytes as measured by (3H) quinuclidinyl benzilate [(3H)-QNB] in 27 unmedicated TS patients, against 22 healthy (age and gender-matched) controls. B(max) and Kd values were determined using Lineweaver-Burke plots. The mean B(max) values in nontreated TS patients was markedly and significantly lower than in controls (10.59 +/- 8.4 versus 40.16 +/- 9.2 fmole/10(6) cells, p less than 10(-6), while Kd values were similar in both groups. Our findings suggest that changes in cholinergic receptors may play a role in the pathophysiology of Tourette syndrome.

Muscarinic antagonists attenuate neurotensin-stimulated accumbens and striatal dopamine metabolism

The effect of scopolamine and atropine upon the increase in extracellular 3,4-dihydroxyphenylacetic acid induced by central injection of neurotensin was examined in the nucleus accumbens and the striatum of anaesthetized rats using in vivo differential pulse voltammetry with carbon fibre electrodes. Scopolamine (1 and 3 mg/kg, i.p.) and atropine (20 micrograms, i.c.v.) did not alter the 3,4-dihydroxyphenylacetic acid level in the nucleus accumbens or the striatum, measured for 60 min after administration. Neurotensin (10 micrograms, i.c.v.) increased the 3,4-dihydroxyphenylacetic acid peak height in both regions. Pretreatment with scopolamine (1 mg/kg) 15 min before neurotensin injection blocked the increase in extracellular 3,4-dihydroxyphenylacetic acid in the striatum but not in the nucleus accumbens whilst scopolamine (3 mg/kg) partially attenuated the effect of neurotensin in the nucleus accumbens and blocked the increase in 3,4-dihydroxyphenylacetic acid in the striatum. Atropine partially attenuated the effect produced by neurotensin in the nucleus accumbens and blocked the increase in 3,4-dihydroxyphenylacetic acid induced by the peptide in the striatum. However, the increase in extracellular 3,4-dihydroxyphenylacetic acid induced by haloperidol (1 mg/kg, s.c.) was not altered by scopolamine (1 mg/kg) or atropine. Also, the increase in dopamine metabolism in the nucleus accumbens and the striatum after centrally injected haloperidol (10 micrograms, i.c.v.) was not altered by atropine (20 micrograms, i.c.v.). Together, the results demonstrate a functional interaction between muscarinic antagonists and neurotensin on in vivo dopamine metabolism in the nucleus accumbens and the striatum but with a greater effect in the latter region.

Glutamate stimulation of tyrosine hydroxylase is mediated by NMDA receptors in the rat striatum

We have studied the action of glutamate on striatal tyrosine hydroxylase activity and determined which type of glutamate receptors are involved. Glutamate stimulated (EC50 = 4 +/- 2 microM) the activity of tyrosine hydroxylase in slices of rat neostriatum. The selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (10 microM) blocked the stimulation; however, both the non-NMDA receptor antagonist glutamate diethyl ester (10 microM) and the general excitatory amino acid antagonist kynurenate (10 microM) had no effect. NMDA was even more potent than glutamate in stimulating tyrosine hydroxylase activity. Quisqualate (100 microM) only slightly stimulated the enzyme, and kainate had practically no effect. Omission of Mg2+ from the incubation medium potentiated the glutamate stimulation. Neither tetrodotoxin nor atropine prevented the stimulation. These results suggest that glutamate stimulates striatal tyrosine hydroxylase activity via NMDA receptors. The lack of effect of tetrodotoxin and atropine suggests that glutamate acts on NMDA receptors located on the dopaminergic nigrostriatal terminal. The stimulation may involve the entry of Ca2+ into the terminal through the NMDA receptor ionophore, since a Ca(2+)-free medium or cadmium totally blocked the stimulation of the enzyme by glutamate.

Reduced D2 dopamine and muscarinic cholinergic receptor densities in caudate specimens from fluctuating parkinsonian patients

Binding of spiperone and 3-quinuclidinyl benzilate (QNB), both labeled with hydrogen 3 (3H), were measured in caudate tissue obtained from 8 living parkinsonian patients at the time of cerebral transplantation. This was clinically homogeneous group of patients. All remained predominantly responsive to levodopa, although with marked disability secondary to clinical fluctuations (short-duration responses) and medication-induced dyskinesias; all were receiving substantial doses of levodopa and 6 of the 8 patients were additionally receiving bromocriptine or pergolide. Binding densities of dopamine D2 receptors, as measured by [3H]spiperone binding, were reduced in this group of patients, compared to caudate specimens from autopsy control subjects. This findings may reflect medication-induced receptor downregulation. Parallel changes occurred with muscarinic cholinergic receptors; [3H]QNB binding was significantly reduced, compared to autopsy control values. This reduction of muscarinic receptors might be due to loss of nigrostriatal terminals that are known to contain muscarinic receptors. Alternatively, muscarinic receptors may have been downregulated by increased corticostriatal glutamatergic input to cholinergic cells, inferred to be present based on the prominent levodopa-induced dyskinesias. Finally, receptor deficits could have been a reflection of more widespread degenerative cerebral disease, although levodopa-refractory symptoms were generally not pronounced in these patients.

Effects of various experimental manipulations on neostriatal acetylcholine and dopamine release

The release of endogenous acetylcholine and dopamine and the appearance of their metabolites, choline and dihydroxyphenylacetic acid (DOPAC), from neostriatal slices prepared from Fischer 344 rats was examined under various experimental conditions. There was a dose-dependent increase in the amount of neurotransmitter or metabolite as the medium potassium concentration was increased from 5 to 50 mM. Over an eight minute period in Krebs Ringer bicarbonate buffer containing 25 mM potassium, the rate of release of acetylcholine was 6 to 13 times greater than that of dopamine. The dopamine endogenous to the slice preparation appeared to have little effect on the release of endogenous acetylcholine since manipulations that significantly altered dopamine release (depletion with 6-hydroxydopamine or uptake inhibition with nomifensine) had minimal effects on the cholinergic neurons. In contrast, increasing the endogenous acetylcholine in the preparation by inhibiting acetylcholinesterase resulted in a 1.2 to 12 fold increase in dopamine release depending upon the incubation time and the potassium concentration. These studies indicate that within the neostriatal slices there is minimal influence of the endogenous dopamine on the cholinergic neurons, whereas the extracellular acetylcholine can influence dopamine release when its concentration is increased by inhibition of acetylcholinesterase.

Pharmacologically defined M1 and M2 muscarinic cholinergic binding sites in the cat's substantia nigra: development and maturity

Muscarinic cholinergic binding in the substantia nigra of the cat was documented during development and at maturity with autoradiographic methods by labeling the pharmacologically defined M1 and M2 subtypes of muscarinic binding sites. In cats from age embryonic day 40 to postnatal day 6 and at adulthood, M1 sites were labeled with [3H]pirenzepine and M2 sites were labeled with [3H]N-methylscopolamine in competition with pirenzepine. Comparisons were made among binding site distributions, acetylcholinesterase staining and tyrosine hydroxylase-like immunoreactivity in serial or neighboring nigral tissue sections. M1 and M2 binding sites were present in the substantia nigra at all ages studied. Qualitative comparisons showed that M1 binding delineated the substantia nigra more distinctly than did M2 binding. For M1 binding sites in particular, the embryonic pars reticulata of the substantia nigra was more prominently labeled than the pars compacta. At adulthood both nigral subdivisions clearly exhibited M1 and M2 binding, with the pars compacta demonstrating some internal heterogeneity of binding density. These findings provide further evidence that the substantia nigra is a site of cholinergic transmission and suggest that the functional balance between acetylcholine and dopamine in the basal ganglia acts here as well as in the striatum.

Effect of cocaine and 5-HT3 receptor antagonists on 5-HT-induced [3H]dopamine release from rat striatal synaptosomes

The effect of serotonin (5-HT) on the release of tritium from striatal synaptosomes previously loaded with [3H]dopamine ([3H]DA) was studied. 5-HT stimulated both the spontaneous and Ca(2+)-evoked efflux of tritium in a concentration-dependent manner. This effect was not mimicked by the non-selective 5-HT agonist, d-lysergic acid diethylamide. Further, the stimulatory effects of 5 muM 5-HT were unaffected by the selective 5-HT3 receptor antagonists, MDL-72222 and GR-38032F. On the other hand, cocaine and the selective DA uptake inhibitor, nomifensine completely antagonized the effect of 5 muM 5-HT on spontaneous tritium efflux with IC50 values of 0.2 and 0.09 muM, respectively. The effect of 5-HT on Ca(2+)-evoked tritium efflux was also blocked by these DA uptake inhibitors, albeit at somewhat higher concentrations. These data support the hypothesis that 5-HT induces the release of DA from striatal nerve terminals via a mechanism involving the transport of 5-HT into the dopaminergic terminal, rather than by activating 5-HT3 receptors as has been proposed to account for the effect of 5-HT observed in striatal slices.