Muscarinic antagonists in substantia nigra influence the decarboxylation of L-dopa in striatum

Intractable hiccups (singultus) abolished by risperidone, but not by haloperidol

Hiccups or singulata are rhythmic involuntary movements of the diaphragm, caused by a variety of conditions that interfere with the functions of the nerve nuclei in the medulla and supra-spinal hiccup center. Although neurotransmitters and receptors involved in the pathophysiology of hiccups are not defined well, dopamine has been considered to play an important role. In some cases, chlorpromazine or other antipsychotics are used for the treatment of intractable hiccups but their efficacy is often limited. This report involves an 18-year-old patient who experienced two episodes of intractable hiccups triggered by stress, which lasted for weeks or even months. In both episodes, haloperidol was initially used, but there was no significant effect. In contrast, risperidone, the second-generation antipsychotic that possesses a dopamine-serotonin antagonist property, completely abolished the hiccups 6 hours after administration. This is one of few case reports in which two antipsychotics were challenged for a single patient with hiccups, and the effects of the drugs were obviously different. Our finding suggests that, in addition to dopaminergic system, the serotonergic systems may be involved in the pathophysiology of some hiccup cases and that the serotonin-acting antipsychotics such as risperidone should be considered as a choice in the drug treatment of intractable hiccups.

Motor activity-induced dopamine release in the substantia nigra is regulated by muscarinic receptors

Nigro-striatal neurons release dopamine not only from their axon terminals in the striatum, but also from somata and dendrites in the substantia nigra. Somatodendritic dopamine release in the substantia nigra can facilitate motor function by mechanisms that may act independently of axon terminal dopamine release in the striatum. The dopamine neurons in the substantia nigra receive a cholinergic input from the pedunculopontine nucleus. Despite recent efforts to introduce this nucleus as a potential target for deep brain stimulation to treat motor symptoms in Parkinson's disease; and the well-known antiparkinsonian effects of anticholinergic drugs; the cholinergic influence on somatodendritic dopamine release is not well understood. The aim of this study was to investigate the possible regulation of locomotor-induced dopamine release in the substantia nigra by endogenous acetylcholine release. In intact and 6-OHDA hemi-lesioned animals alike, the muscarinic antagonist scopolamine, when perfused in the substantia nigra, amplified the locomotor-induced somatodendritic dopamine release to approximately 200% of baseline, compared to 120-130% of baseline in vehicle-treated animals. A functional importance of nigral muscarinic receptor activation was demonstrated in hemi-lesioned animals, where motor performance was significantly improved by scopolamine to 82% of pre-lesion performance, as compared to 56% in vehicle-treated controls. The results indicate that muscarinic activity in the substantia nigra is of functional importance in an animal Parkinson's disease model, and strengthen the notion that nigral dopaminergic regulation of motor activity/performance is independent of striatal dopamine release.

Anatomy, function and regulation of neuropeptide EI (NEI)

This review is focused on the anatomy, role and behavior of neuropeptide-glutamic acid-isoleucine (NEI), providing a general report on the neuropeptide. In addition to hormone release, this peptide also takes part in the regulation of grooming behavior and locomotor activity. NEI is produced by cleavage of prepro-MCH that probably takes place at the Lys(129)-Arg(130) and Arg(145)-Arg(146) sites (the glycine residue on the C-terminus of NEI strongly suggests that this peptide is amidated). This same prohormone is also the precursor of MCH, widely studied in relation to food and water intake, and NGE, of which little is known. NEI and MCH are extensively colocalized throughout the central nervous system (CNS), and NEI is also present in peripheral tissues. The latter is also effective in stimulating luteinizing hormone (LH) release and, to a lesser extent, FSH from primary pituitary cell cultures. In addition to releasing LH from the medial eminence, NEI also acts directly on gonadotropes. Lastly, this neuropeptide also acts at the CNS level on gonadotropin-releasing hormone (GnRH) neurons.

Midbrain muscarinic receptor mechanisms underlying regulation of mesoaccumbens and nigrostriatal dopaminergic transmission in the rat

Laterodorsal (LDT) and pedunculopontine (PPT) tegmental nuclei in the mesopontine project cholinergic inputs to the midbrain ventral tegmental area (VTA) and substantia nigra pars compacta (SNc), respectively, to directly and indirectly influence the activity of dopamine neuronal cells via actions on muscarinic and nicotinic receptors. The present study investigated the role of midbrain muscarinic receptors in the functional modulation of VTA and SNc dopamine cell activity as reflected by alterations in, respectively, nucleus accumbens (NAc) and striataldopamine efflux. In vivo chronoamperometry was used to measure changes in basal dopamine efflux via stearate-graphite paste electrodes implanted unilaterally in the NAc or striatum of urethane-anaesthetized rats, following blockade or activation of, respectively, VTA or SNc muscarinic receptors. Intra-VTA or -SNc infusion of the muscarinic antagonist scopolamine (200 microg/microL) reduced, respectively, NAc and striatal dopamine efflux while infusion of the muscarinic and nicotinic agonist carbachol (0.5 microg/microL) or the prototypical muscarinic agonist muscarine (0.5 microg/microL) increased NAc and striatal dopamine efflux. Transient decreases in dopamine efflux preceded these increases selectively in the striatum, suggesting a reduction in excitatory or increase in inhibitory drive to the SNc by preferential activation of M3 muscarinic receptors on GABA interneurons and glutamatergic inputs. This was confirmed by showing that selective blockade of M3 receptors with p-F-HHSiD (0.5 microg/microL) increased striatal, but not NAc, dopamine efflux. Together, these findings suggest that midbrain muscarinic receptors, probably M5 subtypes on VTA and SNc dopamine neurons, contribute to the tonic excitatory regulation of forebrain basal dopamine transmission whereas presynaptic M3 receptors serve to counter excessive excitation of nigral dopamine cell activity.

Muscarinic M2 and M4 receptors in anterior cingulate cortex: relation to neuropsychiatric symptoms in dementia with Lewy bodies

Alterations in cholinergic functions have been reported to be associated with neuropsychiatric symptoms in dementia. Increased M1 muscarinic receptor binding in temporal cortex is associated with delusions in dementia with Lewy bodies (DLB) patients and increased M2/M4 receptor binding with psychosis in Alzheimer's disease. However, the relation between M2 and M4 muscarinic receptor and psychotic symptoms in DLB is unknown. The aim of this study was to measure M2 and M4 receptors in the anterior cingulate cortex in DLB and to correlate the neurochemical findings with neuropsychiatric symptoms. Muscarinic M2 and M4 receptor levels in the anterior cingulate cortex and adjacent cortex (Brodmann's area [BA] 32) were measured separately by using a radioligand binding protocol based on binding of [(3)H]AF-DX 384 in the presence and absence of dicyclomine, a potent M4 receptor antagonist. M2 receptor binding was significantly increased, while M4 receptor binding was unchanged in the cingulate cortex and BA32 of DLB patients compared with age-matched controls. Impaired consciousness was significantly associated with increased M4 binding and delusions were significantly associated with increased M2 binding. Increased M2 and M4 receptor binding in DLB was also associated with visual hallucinations. Upregulation of M2 and M4 muscarinic receptors in cingulate and adjacent cortex may thus contribute to the development of psychosis in DLB, with potential implications for treatments with drugs acting on these receptors.

M4 muscarinic receptors regulate the dynamics of cholinergic and dopaminergic neurotransmission: relevance to the pathophysiology and treatment of related CNS pathologies

Dopaminergic dysfunction is an important pathogenetic factor for brain pathologies such as Parkinson's disease, ADHD, schizophrenia, and addiction as well as for metabolic disorders and anorexia. Dopaminergic neurons projecting from the midbrain to forebrain regions, such as the nucleus accumbens and the prefrontal cortex, regulate motor and cognitive functions and coordinate the patterned response of the organism to sensory, affective, and rewarding stimuli. In this study, we showed that dopaminergic neurotransmission is highly dependent on M4 cholinergic muscarinic receptor function. Using in vivo microdialysis, we found elevated dopamine (DA) basal values and enhanced DA response to psychostimulants in the nucleus accumbens of M4 knockout mice. We also demonstrated impaired homeostatic control of cholinergic activity that leads to increased basal acetylcholine efflux in the midbrain of these animals. Thus, loss of M4 muscarinic receptor control of cholinergic function effectuates a state of dopaminergic hyperexcitability. This may be responsible for pathological conditions, in which appetitive motivation as well as affective and cognitive processing is impaired. We propose that M4 receptor agonists could represent an innovative strategy for the treatment of pathologies associated with hyperdopaminergia.

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.

Synthesis and pharmacology of benzoxazines as highly selective antagonists at M(4) muscarinic receptors

Previously, we reported on PD 102807 (41) as being the most selective synthetic M(4) muscarinic antagonist identified to date. Synthesized analogues of 41 showed no improvement in affinity and selectivity at that time. However, several newly synthesized compounds exhibit a 7-fold higher affinity at M(4) receptors and demonstrate a selectivity of at least 100-fold over all other muscarinic receptor subtypes. For example, compound 28 showed an affinity of pK(i) = 9.00 at M(4) receptors and a selectivity of M(1)/M(4) = 13 183-fold, M(2)/M(4) = 339-fold, M(3)/M(4) = 151-fold, and M(5)/M(4) = 11 220-fold. This high selectivity along with high affinity has not been reported for any synthetic muscarinic antagonist, nor for natural occurring M(4) antagonists such as the M(4) selective Eastern Green Mamba venom MT3 (M(4) pK(b) = 8.7, M(1)/M(4) = 40-fold, M(2)/M(4) > or = 500-fold, M(3)/M(4) > or = 500-fold, and M(5)/M(4) > or = 500-fold). Derivative 24, a compound with a high selectivity pattern as well, has been tested for in vivo efficacy. It was able to block the L-3,4-dihydroxyphenylalanine accumulation produced by CI-1017, an M(1)/M(4) selective muscarinic agonist, in the mesolimbic region and striatum, which suggests that 24 is capable of crossing the blood-brain barrier and confirms the pharmacokinetic data obtained on this compound. This is evidence that suggests that agonist-induced increase in catecholamine synthesis observed in these regions is mediated by M(4) receptors.

Muscarinic modulation of hypoxia-induced release of catecholamines from the cat carotid body

Chemotransduction of arterial hypoxemia by the cat carotid body is generally thought to begin with a hypoxia-induced depolarization of the glomus cells (GCs) of the carotid body (CB). This depolarization activates voltage-gated calcium channels with the subsequent entry of calcium, movement of transmitter-containing vesicles to the synaptic-like juncture between the GC and apposed sensory afferent neuron. The vesicles exocytotically release their transmitters which then proceed to the receptors on both the postsynaptic neuron and on the GCs themselves (autoreceptors). Action potentials and their modulation in the sensory fibers are the result, along with the modulation of further neurotransmitter release from the GCs. The purpose of the present study was to: (1) determine the parameters of an incubated cat CB preparation capable of releasing measurable amounts of catecholamines (CAs) in response to hypoxia; (2) determine the impact of muscarinic activities on CA release during the hypoxic challenge; (3) determine if the muscarinic activity preferentially modified the release of one CA more than another; (4) determine if there were any differences in the pattern of hypoxia-induced release of dopamine (DA) vs. norepinephrine (NE). CBs were harvested from deeply anesthetized cats. Cleaned of fat and connective tissue, they were incubated in Krebs Ringer bicarbonate solution at 37 degrees C, and bubbled with a hyperoxic mixture of gases (95% O(2)-5% CO(2)) for 30 min. The first series of experiments to address the CB's hypoxia-induced release of CAs explored the effects of incubating CBs for 2 h with hyperoxia vs. normoxia (21% O(2)-6% CO(2)) followed by a 30 min hypoxic challenge, with or without L-dihydroxyphenylalanine (L-DOPA). In the second series of experiments the CBs, after the first 30 min of hyperoxia, were next challenged with hypoxia (4% O(2)-5% CO(2)) for intervals of 3-20 min with intervening recovery periods of hyperoxia to determine the effect of the duration of the hypoxic exposure on CA release. In the third series of experiments the CBs, after the first 30 min of hyperoxia, were challenged with hypoxia for intervals of 10-40 min in the presence or absence of an M1 or M2 muscarinic receptor antagonist. CAs released into the incubation medium were analyzed by means of high performance liquid chromatography-electrochemical detection using standard procedures. Incubated cat CBs challenged for 2 h with hyperoxia followed by 30 min of hypoxia, released much more measurable amounts of CAs in the presence of 40 microM L-DOPA than without it. Moving from hyperoxia to hypoxia produced a better yield than moving from normoxia to hypoxia, and at least 10-20 min exposures were needed for measurable amounts of CAs. The M1 muscarinic receptor antagonist, pirenzepine, reduced the hypoxia-induced release of CAs during each exposure. Further, the reduction appeared to be dose-related. The M2 muscarinic receptor antagonist, methoctramine, enhanced the hypoxia-induced release of CAs during each exposure. These data support a role for acetylcholine (ACh) in the hypoxia-induced release of CAs, and suggest a significant, if modest, muscarinic dimension to it. And although hypoxia induced a greater release of DA than of NE, the muscarinic modulation of the release (both decreasing it and increasing it) may have had a greater impact on NE release than on DA release. Finally, the patterns of hypoxia-induced release of DA and NE from incubated cat carotid bodies are significantly different.