Facilitation of striatal acetylcholine release by dopamine D1 receptor stimulation: involvement of enhanced nitric oxide production via neurokinin-2 receptor activation

Cholinergic Receptor Modulation as a Target for Preventing Dementia in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative condition characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) in the midbrain resulting in progressive impairment in cognitive and motor abilities. The physiological and molecular mechanisms triggering dopaminergic neuronal loss are not entirely defined. PD occurrence is associated with various genetic and environmental factors causing inflammation and mitochondrial dysfunction in the brain, leading to oxidative stress, proteinopathy, and reduced viability of dopaminergic neurons. Oxidative stress affects the conformation and function of ions, proteins, and lipids, provoking mitochondrial DNA (mtDNA) mutation and dysfunction. The disruption of protein homeostasis induces the aggregation of alpha-synuclein (α-SYN) and parkin and a deficit in proteasome degradation. Also, oxidative stress affects dopamine release by activating ATP-sensitive potassium channels. The cholinergic system is essential in modulating the striatal cells regulating cognitive and motor functions. Several muscarinic acetylcholine receptors (mAChR) and nicotinic acetylcholine receptors (nAChRs) are expressed in the striatum. The nAChRs signaling reduces neuroinflammation and facilitates neuronal survival, neurotransmitter release, and synaptic plasticity. Since there is a deficit in the nAChRs in PD, inhibiting nAChRs loss in the striatum may help prevent dopaminergic neurons loss in the striatum and its pathological consequences. The nAChRs can also stimulate other brain cells supporting cognitive and motor functions. This review discusses the cholinergic system as a therapeutic target of cotinine to prevent cognitive symptoms and transition to dementia in PD.

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.

Access to the CNS: Biomarker Strategies for Dopaminergic Treatments

Despite substantial research carried out over the last decades, it remains difficult to understand the wide range of pharmacological effects of dopaminergic agents. The dopaminergic system is involved in several neurological disorders, such as Parkinson's disease and schizophrenia. This complex system features multiple pathways implicated in emotion and cognition, psychomotor functions and endocrine control through activation of G protein-coupled dopamine receptors. This review focuses on the system-wide effects of dopaminergic agents on the multiple biochemical and endocrine pathways, in particular the biomarkers (i.e., indicators of a pharmacological process) that reflect these effects. Dopaminergic treatments developed over the last decades were found to be associated with numerous biochemical pathways in the brain, including the norepinephrine and the kynurenine pathway. Additionally, they have shown to affect peripheral systems, for example the hypothalamus-pituitary-adrenal (HPA) axis. Dopaminergic agents thus have a complex and broad pharmacological profile, rendering drug development challenging. Considering the complex system-wide pharmacological profile of dopaminergic agents, this review underlines the needs for systems pharmacology studies that include: i) proteomics and metabolomics analysis; ii) longitudinal data evaluation and mathematical modeling; iii) pharmacokinetics-based interpretation of drug effects; iv) simultaneous biomarker evaluation in the brain, the cerebrospinal fluid (CSF) and plasma; and v) specific attention to condition-dependent (e.g., disease) pharmacology. Such approach is considered essential to increase our understanding of central nervous system (CNS) drug effects and substantially improve CNS drug development.

Impact of 6-hydroxydopamine lesions and cocaine exposure on mu-opioid receptor expression and regulation of cholinergic transmission in the limbic-prefrontal territory of the rat dorsal striatum

Information processing within the striatum is regulated by local circuits involving dopamine, cholinergic interneurons and neuropeptides released by recurrent collaterals of striatal output neurons. In the limbic-prefrontal territory of the dorsal striatum, enkephalin inhibits the NMDA-evoked release of acetylcholine directly through micro-opioid receptors (MORs) located on cholinergic interneurons and indirectly through MORs of output neurons of striosomes. In this territory, we investigated the consequence of changes in dopamine transmission, bilateral 6-hydroxydopamine-induced degeneration of striatal dopaminergic innervation or cocaine (acute and chronic) exposure on (i) MOR expression in both cholinergic interneurons and output neurons of striosomes, and (ii) the direct and indirect enkephalin-MOR regulations of the NMDA-evoked release of acetylcholine. Expression of MORs in cholinergic interneurons was preserved after 6-hydroxydopamine and down-regulated after cocaine treatments. Accordingly, the direct enkephalin-MOR control of acetylcholine release was preserved after 6-hydroxydopamine treatment and lost after cocaine exposure. Expression of MORs in output neurons of striosomes was down-regulated in the 6-hydroxydopamine situation and either preserved or up-regulated after acute or chronic cocaine exposure, respectively. Accordingly, the indirect enkephalin-MOR control of acetylcholine release disappeared in the 6-hydroxydopamine situation but surprisingly, despite preservation of MORs in striosomes, disappeared after cocaine treatment. Showing that MORs of striosomes are still functional in this situation, the MOR agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin inhibited the NMDA-evoked release of acetylcholine after cocaine exposure. Therefore, alteration in the regulation of cholinergic transmission by the enkephalin-MOR system might play a major role in the motivational and cognitive disorders associated with dopamine dysfunctions in fronto-cortico-basal ganglia circuits.

M1 muscarinic receptors contribute to, whereas M4 receptors inhibit, dopamine D1 receptor-induced [3H]-cyclic AMP accumulation in rat striatal slices

In rat striatal slices labelled with [(3)H]-adenine and in the presence of 1 mM 3-isobutyl-1-methylxantine (IBMX), cyclic [(3)H]-AMP ([(3)H]-cAMP) accumulation induced by the dopamine D(1) receptor agonist SKF-81297 (1 microM; 177 +/- 13% of basal) was inhibited by the general muscarinic agonist carbachol (maximum inhibition 72 +/- 3%, IC(50) 0.30 +/- 0.06 microM). The muscarinic toxin 7 (MT-7), a selective antagonist at muscarinic M(1) receptors, reduced the effect of SKF-81297 by 40+/-7% (IC(50) 251+/- 57 pM) and enhanced the inhibitory action of a submaximal (1 microM) concentration of carbachol (69 +/- 4% vs. 40 +/- 7% inhibition, IC(50) 386 +/- 105 pM). The toxin MT-1, agonist at M(1) receptors, stimulated [(3)H]-cAMP accumulation in a modest but significant manner (137 +/- 11% of basal at 400 nM), an action additive to that of D(1) receptor activation and blocked by MT-7 (10 nM). The effects of MT-7 on D(1) receptor-induced [(3)H]-cAMP accumulation and the carbachol inhibition were mimicked by the PKC inhibitors Ro-318220 (200 nM) and Gö-6976 (200 nM). Taken together our results indicate that in addition to the inhibitory role of M(4) receptors, in rat striatum acetylcholine stimulates cAMP formation through the activation of M(1 )receptors and PKC stimulation.

Phasic dopaminergic transmission increases NO efflux in the rat dorsal striatum via a neuronal NOS and a dopamine D(1/5) receptor-dependent mechanism

Dysfunctional neurotransmission within striatal networks is believed to underlie the pathophysiology of several neurological and psychiatric disorders. Nitric oxide (NO)-producing interneurons have been shown to play a critical role in modulating striatal synaptic transmission. These interneurons receive synaptic contacts from midbrain dopamine (DA) neurons and may be regulated by DA receptor activation. In the current study, striatal NO efflux was measured in anesthetized male rats using an NO-selective electrochemical microsensor and the role of DA in modulating NO synthase (NOS) activity was assessed during electrical or chemical (bicuculline) stimulation of the substantia nigra (SN). Electrical stimuli were patterned to approximate the natural single spike or burst firing activity of midbrain DA neurons. Electrical stimulation of the SN at low frequencies induced modest increases in striatal NO efflux. In contrast, train stimulation of the SN robustly increased NO efflux in a stimulus intensity-dependent manner. NO efflux evoked by SN stimulation was similar in chloral hydrate- and urethane-anesthetized rats. The facilitatory effect of train stimulation on striatal NO efflux was transient and attenuated by systemic administration of the neuronal NOS inhibitor 7-nitroindazole and the nonselective NOS inhibitor methylene blue. Moreover, the increase in NO efflux observed during chemical and train stimulation of the SN was attenuated following systemic administration of the DA D(1/5) receptor antagonist SCH 23390. SCH 23390 also blocked NO efflux induced by systemic administration of the D(1/5) agonist SKF 81297. These results indicate that neuronal NOS is activated in vivo by nigrostriatal DA cell burst firing via a DA D(1/5)-like receptor-dependent mechanism.

Striatal glutamate release evoked in vivo by NMDA is dependent upon ongoing neuronal activity in the substantia nigra, endogenous striatal substance P and dopamine

The aim of the present microdialysis study was to investigate whether the increase in striatal glutamate levels induced by intrastriatal perfusion with NMDA was dependent on the activation of extrastriatal loops and/or endogenous striatal substance P and dopamine. The NMDA-evoked striatal glutamate release was mediated by selective activation of the NMDA receptor-channel complex and action potential propagation, as it was prevented by local perfusion with dizocilpine and tetrodotoxin, respectively. Tetrodotoxin and bicuculline, perfused distally in the substantia nigra reticulata, prevented the NMDA-evoked striatal glutamate release, suggesting its dependence on ongoing neuronal activity and GABA(A) receptor activation, respectively, in the substantia nigra. The NMDA-evoked glutamate release was also dependent on striatal substance P and dopamine, as it was antagonized by intrastriatal perfusion with selective NK(1) (SR140333), D(1)-like (SCH23390) and D(2)-like (raclopride) receptor antagonists, as well as by striatal dopamine depletion. Furthermore, impairment of dopaminergic transmission unmasked a glutamatergic stimulation by submicromolar NMDA concentrations. We conclude that in vivo the NMDA-evoked striatal glutamate release is mediated by activation of striatofugal GABAergic neurons and requires activation of striatal NK(1) and dopamine receptors. Endogenous striatal dopamine inhibits or potentiates the NMDA action depending on the strength of the excitatory stimulus (i.e. the NMDA concentration).

cGMP: a second messenger for acetylcholine in the brain?

Already 30 years ago, it became apparent that there exists a relationship between acetylcholine and cGMP in the brain. Acetylcholine plays a role in a great number of processes in the brain, however, the role of cGMP in these processes is not known. A review of the data shows that, although the connection between NO-mediated cGMP synthesis and acetylcholine is firmly established, the complexities of the heterosynaptic pathways and the oligosynaptic structures involved preclude a clear definition of the role of cGMP in the functioning of acetylcholine presently.

A 25 year adventure in the field of tachykinins

Several aspects of our 25 year adventure in the field of tachykinins will be successively described. They concern: substance P (SP) synthesis and release in the basal ganglia, the identification and pharmacological characterization of central tachykinin NK(1), NK(2) and NK(3) binding sites and their topographical distribution, the description of some new biological tests for corresponding receptors, the identification of tachykinin NK(1) receptor subtypes or conformers sensitive to all endogenous tachykinins (substance P, neurokinin A (NKA), neurokinin B (NKB), neuropeptide gamma (NP gamma) and neuropeptide K (NPK)) and finally, the functional involvement of these receptors and their subtypes in tachykinin-induced regulations of dopamine and acetylcholine release in the striatum.

The new neurokinin 1-sensitive receptor mediates the facilitation by endogenous tachykinins of the NMDA-evoked release of acetylcholine after suppression of dopaminergic transmission in the matrix of the rat striatum

Using an in vitro microsuperfusion procedure, the NMDA-evoked release of [3H]ACh was studied after suppression of dopamine (DA) transmission (alpha-methyl-p-tyrosine) in striatal compartments of the rat. The effects of tachykinin neurokinin 1 (NK1) receptor antagonists and the ability of appropriate agonists to counteract the antagonist responses were investigated to determine whether tachykinin NK1 classic, septide-sensitive and/or new NK1-sensitive receptors mediate these regulations. The NK1 antagonists, SR140333, SSR240600, GR205171 but not GR82334 and RP67580 (0.1 and 1 microM) markedly reduced the NMDA (1 mm + D-serine 10 microM)-evoked release of [3H]ACh only in the matrix. These responses unchanged by coapplication with NMDA of NK2 or NK3 agonists, [Lys5,MeLeu9,Nle10]NKA(4-10) or senktide, respectively, were completely counteracted by the selective NK1 agonist, [Pro9]substance P but also by neurokinin A and neuropeptide K (1 nM each). According to the rank order of potency of agonists for counteracting the antagonist responses ([Pro9]substance P, 0.013 nM > neurokinin A, 0.15 nM >> substance P(6-11) 7.7 nM = septide 8.7 nM), the new NK1-sensitive receptors mediate the facilitation by endogenous tachykinins of the NMDA-evoked release of ACh in the matrix, after suppression of DA transmission. Solely the NK1 antagonists having a high affinity for these receptors could be used as indirect anti-cholinergic agents.

Autoradiographic distribution of tachykinin NK2 binding sites in the rat brain: comparison with NK1 and NK3 binding sites

The autoradiographic distribution of tachykinin NK(2) binding sites was determined in the adult rat brain using [(125)I]neurokinin A in the presence of either senktide (NK(3) agonist) and [Pro(9)]substance P (NK(1) agonist) or senktide and SR 140333 (NK(1) antagonist). Indeed, this radioligand labels two subtypes of NK(1) binding sites (which present a high affinity not only for SP but also for neurokinin A, neuropeptide K and neuropeptide gamma) as well as NK(3) binding sites. The distribution of NK(2) binding sites was also compared with those of NK(1) and NK(3) binding sites, these sites being labeled with [(125)I]Bolton and Hunter substance P and [(125)I]Bolton and Hunter eledoisin, respectively. In agreement with our results obtained with membranes from various brain structures, NK(2)-sensitive [(125)I]neurokinin A labeling was mainly observed in few structures including the dorsal and ventral hippocampus, the septum, the thalamus and the prefrontal cortex. The density of NK(2) binding sites was weak when compared with those of NK(1) and NK(3) binding sites. Marked differences were observed in the distributions of NK(1), NK(2) and NK(3) binding sites. These results are discussed taking into consideration differences or similarities between the distributions of NK(2)-sensitive [(125)I]neurokinin A binding sites and of their endogenous ligands (neurokinin A, neuropeptide K and neuropeptide gamma) but also local NK(2) agonist responses blocked by NK(2) antagonists. Insights on the roles of endogenous tachykinins in several brain functions are also discussed on the basis of the respective distributions of different neurokinin binding sites.

Selective stimulation of striatal dopamine receptors of the D1- or D2-class causes opposite changes of fos expression in the rat cerebral cortex

It has been suggested that activation of striatal neurons expressing D1 or D2 dopamine receptors elicits opposite changes in the net output of the basal ganglia circuitry and, consequently, in the functional interactions of the circuit with the cerebral cortex. In particular, it has been recently reported that striatal D1 receptors may regulate cortex function. To further address this issue, we mapped cerebral expression of Fos protein following intrastriatal stimulation of D1- or D2-class receptors in freely moving animals. Using permanent cannulas implanted in the right striatum, Sprague-Dawley rats received intrastriatal microinfusions of SKF 38393 (D1 agonist) or quinpirole (D2 agonist) or saline (controls), combined with systemic administration of D1 antagonist SCH 23390 or D2 antagonist eticlopride or saline. Animals treated with SKF 38393 showed dose-dependent, massive Fos increases in the motor, somatosensory, auditory, visual and limbic regions of the cerebral cortex, ipsilaterally to the injected striatum. Consistent Fos expression was also found in the injected striatum and, bilaterally, in the nucleus accumbens shell. These increases were effectively counteracted by systemic SCH 23390. Conversely, quinpirole did not induce significant cortical or striatal expression of Fos, which was instead observed after the systemic administration of eticlopride. Fos was not detected in any of the other basal ganglia nuclei, regardless of the dopamine agonists or antagonists used. Our results confirm that striatal D1 dopamine receptors play a central role in the modulation of cortical activity, thus providing additional information on the functional interaction between basal ganglia circuitry and cerebral cortex.

Facilitation by endogenous tachykinins of the NMDA-evoked release of acetylcholine after acute and chronic suppression of dopaminergic transmission in the matrix of the rat striatum

Using a microsuperfusion method in vitro, the effects of the NK1, NK2, and NK3 tachykinin receptor antagonists SR140333, SR48968, and SR142801, respectively, on the NMDA-evoked release of [3H]-acetylcholine were investigated after both acute and chronic suppression of dopamine transmission in striosomes and matrix of the rat striatum. NMDA (1 mm) alone or with D-serine (10 microm) in the presence of alpha-methyl-p-tyrosine (100 microm) markedly enhanced the release of [3H]-acetylcholine through a dopamine-independent inhibitory process. In both conditions, as well as after chronic 6-OHDA-induced denervation of striatal dopaminergic fibers, SR140333, SR48968, or SR142801 (0.1 microm each) reduced the NMDA-evoked release of [3H]-acetylcholine in the matrix but not in striosome-enriched areas. These responses were selectively abolished by coapplication with NMDA of the respective tachykinin agonists, septide, [Lys5,MeLeu9,Nle10]NKA(4-10), or senktide. Distinct mechanisms are involved in the effects of the tachykinin antagonists because the inhibitory response of SR140333 was additive with that of either SR48968 or SR142801. In addition, the SR140333-evoked response remained unchanged, whereas those of SR48968 and SR142801 were abolished in the presence of N(G)-monomethyl-l-arginine (nitric oxide synthase inhibitor). Therefore, in the matrix but not in striosomes, the acute or chronic suppression of dopamine transmission unmasked the facilitatory effects of endogenously released substance P, neurokinin A, and neurokinin B on the NMDA-evoked release of [3H]-acetylcholine. Whereas substance P and neurokinin A are colocalized in same efferent neurons, their responses involve distinct circuits because the substance P response seems to be mediated by NK1 receptors located on cholinergic interneurons, while those of neurokinin A and neurokinin B are nitric oxide-dependent.

Group III metabotropic glutamate receptors and D1-like and D2-like dopamine receptors interact in the rat nucleus accumbens to influence locomotor activity

Evidence for functional interactions between metabotropic glutamate (mGlu) receptors and dopamine (DA) neurotransmission is now clearly established. In the present study, we investigated interactions between group III mGlu receptors and D1- and D2-like receptors in the nucleus accumbens (NAcc). Administration, into the NAcc, of the selective group III mGlu receptor agonist, AP4, resulted in an increase in locomotor activity, which was blocked by pretreatment with the group III mGlu receptor antagonist, MPPG. In addition, pretreatment with AP4 further blocked the increase in motor activity induced by the D1-like receptor agonist, SKF 38393, but potentiated the locomotor responses induced by either the D2-like receptor agonist, quinpirole, or coinfusion of SKF 38393 and quinpirole. MPPG reversed the effects of AP4 on the motor responses induced by D1-like and/or D2-like receptor activation. These results confirm that glutamate transmission may control DA-dependent locomotor function through mGlu receptors and further indicate that group III mGlu receptors oppose the behavioural response produced by D1-like receptor activation and favour those produced by D2-like receptor activation.

Permissive role of neurokinin NK(3) receptors in NK(1) receptor-mediated activation of the locus coeruleus revealed by SR 142801

The present experiments investigated the role of neurokinin-1 (NK(1)) and neurokinin-3 (NK(3)) receptors on the activity of the locus coeruleus (LC)-noradrenergic system by using a dual probe microdialysis technique in anesthetized guinea pigs. The local application in the LC of the selective NK(1) receptor agonists [SAR(9),Met(O(2))(11)]-SP (10 microM) and septide (1 microM) as well as the selective NK(3) receptor agonist senktide (1 microM), enhanced the extracellular norepinephrine (NE) levels in the prefrontal cortex. The enhancing effect of [SAR(9),Met(O(2))(11)]-SP was completely blocked by the peripheral administration of the selective non peptide NK(1) and NK(3) receptor antagonists, GR 205171 (1 mg/kg, i.p.) and SR 142801 (0.1 mg/kg, i.p.), respectively, whereas SR 142806 (0.1 mg/kg, i.p.) the inactive enantiomer of SR 142801 had no effect. Moreover, the [SAR(9),Met(O(2))(11)]-SP-induced increase in LC DOPAC concentrations, is only antagonized by GR 205171. In contrast, only SR 142801 (0.3 mg/kg, i.p.) could block stereoselectively the senktide-evoked increase in NE levels. Both [SAR(9),Met(O(2))(11)]-SP and senktide effects were blocked by local infusion into the LC of SR 142801 (10(-9) M). These results demonstrate that stimulation of NK(1) and NK(3) receptors located in the LC area modulates the activity of the LC-NE system, and that the excitatory effects of NK(1) receptor agonists require NKB/NK(3) receptor activation in the LC.

Presence of NK2 binding sites in the rat brain

Attempts were made to label tachykinin NK2 binding sites in the adult rat brain using [125I]neurokinin A (NKA) as ligand in the presence of NK1 and NK3 agonist or antagonist to avoid labelling of NK1 and NK3 binding sites, respectively. A high-affinity, specifically NK2-sensitive, [125I]NKA-binding, temperature-dependent, reversible, sensitive to GTPgammaS and correspondence to a single population of binding sites (K(D) and B(max) values: 2.2 nM and 7.3 fmol/mg protein) was demonstrated on hippocampal membranes. Competition studies performed with tachykinins and tachykinin-related compounds indicated that the pharmacological properties of these NK2-sensitive [125I]NKA binding sites were identical to those identified in the rat urinary bladder and duodenum. NKA, neuropeptide K, and neuropeptide gamma, as well as the potent and selective NK2 antagonists SR 144190, SR 48968 and MEN 10627, presented a nanomolar affinity for these sites. The regional distribution of these NK2-sensitive [125I]NKA binding sites differs markedly from those of NK1 and NK3 binding sites, with the largest labeling being found in the hippocampus, the thalamus and the septum. Binding in other brain structures was low or negligible. A preliminary autoradiographic analysis confirmed [125I]NKA selective binding in hippocampal CA1 and CA3 areas, particularly, and in several thalamic nuclei.

Tachykinins as modulators of the micturition reflex in the central and peripheral nervous system

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex. The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.

Nitric oxide and peroxynitrite as factors to stimulate neurotransmitter release in the CNS

This review summarizes the stimulatory potentials of NO and peroxynitrite (OONO-) on neurotransmitter release in the central nervous system. Exogenous and endogenous NO stimulates to release neurotransmitter. NO synthesized intracellularly diffuses out through neuronal membrane and acts on the outer side of membrane to depolarize neuronal membrane, which triggers neurotransmitter release. NO-induced release of neurotransmitters is mediated by Ca2+-dependent and -independent processes. The latter process is operated by reverse process of the Na+-dependent carrier-mediated neurotransmitter uptake system or by unknown mechanisms. Ca2+-dependent release of neurotransmitter occurs in part subsequent to increase in Ca2+ influx via VDCCs, although N-type VDCCs may not involve in this action of NO because of suppression of Ca2+ influx through N-type VDCCs by NO. Participation of cGMP formation by NO on neurotransmitter release is controversial. A superoxide scavenger, Ca2+, Zn(2+)-superoxide dismutase, abolishes NO-induced neurotransmitter release and synthesized OONO- induces neurotransmitter release, indicating that OONO- participates in NO-evoked neurotransmitter release.

Expression of tachykinin NK2 receptor mRNA in human brain

Tachykinin NK2 receptors have been suggested to play an important role in the central nervous system. This study, using reverse transcription-polymerase chain reaction revealed a detectable expression of NK2 receptor mRNA in various human brain regions, including the caudate nucleus, the putamen, the hippocampus, the substantia nigra and the cerebral cortex. The distribution of NK2 receptor expression in the cortex revealed a major expression in frontal and temporal cortex compared to occipital and parietal areas. These results provide a molecular basis for considering a role of NK2 receptors in human pathophysiology.

Neurotensin increases endogenous glutamate release in rat cortical slices

In the present study, the effects of the tridecapeptide neurotensin [NT(1-13)] and its fragments, NT(1-7) and NT(8-13), on endogenous glutamate release from rat cortical slices, were evaluated. NT(1-13) (100-1000 nM) slightly increased spontaneous glutamate release, while it was ineffective at 1 and 10 nM concentrations. Neither the biologically active NT fragment NT(8-13) nor the inactive one NT(1-7) affected basal glutamate release. NT(1-13) (1-1000 nM) enhanced potassium (35 mM)-evoked glutamate release displaying a bell-shaped concentration response curve. In addition NT(8-13) (10 nM) increased K+-evoked-glutamate release similarly to the parent peptide (10 nM), while the biologically inactive fragment NT(1-7) (10-100 nM) was ineffective. The effects of NT(1-13) and NT(8-13) were fully counteracted by the selective neurotensin receptor antagonist SR48692 (100 nM). These findings suggest that NT plays a role in regulating cortical glutamate transmission.

Caffeine-mediated induction of c-fos, zif-268 and arc expression through A1 receptors in the striatum: different interactions with the dopaminergic system

Adenosine and the adenosine receptor antagonist, caffeine, modulate locomotor activity and striatal neuropeptide expression through interactions with the dopaminergic system by mechanisms which remain partially undetermined. We addressed this question by using quantitative immunocytochemistry and in situ hybridization, combined with retrograde tracing of striatal neurons, to characterize the mechanism(s) leading to the striatal increase in the immediate early genes (IEG), c-fos, zif-268 and arc, following a single injection of caffeine or the A1 antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). Caffeine and DPCPX induced c-fos, zif-268 and arc expression, both at mRNA and protein levels, in large proportions of striatonigral and striatopallidal neurons. The involvement of dopamine systems was evaluated by manipulations of the dopaminergic transmission. Quinpirole, a D2 agonist, almost completely blocked the caffeine-induced IEG increase in both striatopallidal and striatonigral neurons. Conversely, the lesion of the nigrostriatal pathway and the D1 antagonist SCH23390 abolished the caffeine effects in striatonigral neurons but had no or slight effect, respectively, on its action in striatopallidal neurons. These observations demonstrate that caffeine- and DPCPX-mediated IEG inductions involved different mechanisms in striatonigral and striatopallidal neurons through blockade of A1 receptors. Immediate early gene inductions result from a stimulation of dopamine release in striatonigral neurons and from activation of glutamate release and probably also acetylcholine release in striatopallidal neurons. These results also support the idea that, besides A2A receptors, adenosine acting at the A1 receptor plays pivotal functions in the basal ganglia physiology and that blockade of these receptors by specific or nonspecific antagonists, DPCPX and caffeine, may influence a broad range of neuronal functions in the striatum.

Opposite modulation of cortical N-methyl-D-aspartate receptor-mediated responses by low and high concentrations of dopamine

To examine whether dopamine modulates cortical N-methyl-D-aspartate receptor-mediated glutamate transmission, whole-cell recordings were made from identified pyramidal cells located in layers V and VI of the medial prefrontal cortex of the rat using a slice preparation. In the presence of tetrodotoxin and the absence of Mg2+, a brief local application of N-methyl-D-aspartate evoked an inward current which was blocked by the N-methyl-D-aspartate antagonist dizocilpine maleate but not affected by the non-N-methyl-D-aspartate antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline, suggesting that the observed current is mediated by N-methyl-D-aspartate receptors located on recorded cells. Bath application of dopamine produced opposite effects on the N-methyl-D-aspartate current depending on the concentrations of dopamine applied. At low concentrations (<50 microM), dopamine enhanced the N-methyl-D-aspartate current, whereas at higher concentrations, dopamine suppressed the current. The same concentrations of dopamine did not significantly affect the inward current induced by the non-N-methyl-D-aspartate agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. The enhancing effect of dopamine on the N-methyl-D-aspartate response was mimicked by the D1 agonist SKF38393 and blocked by the D1 antagonist SCH31966, whereas the suppressing effect was mimicked by the D2 agonist quinpirole and blocked by the D2 antagonist eticlopride. The above results suggest that dopamine at low concentrations acts preferentially on D1-like receptors to promote N-methyl-D-aspartate receptor-mediated transmission, while at high concentrations dopamine also activates D2-like receptors, leading to a suppression of the N-methyl-D-aspartate function. This differential modulation of N-methyl-D-aspartate function may have significant implications for understanding behaviors and disorders involving both cortical dopamine- and glutamate-mediated neurotransmission.

Activation of dopaminergic and cholinergic neurotransmission by tachykinin NK3 receptor stimulation: an in vivo microdialysis approach in guinea pig

The regulation of dopaminergic and cholinergic function by neurokinin-3 (NK3) receptor activation was examined in vivo in urethane-anaesthetized guinea pigs with microdialysis probes. The local application of the NK3 tachykinin receptor agonist senktide in the region of dopamine cell bodies (pars compacta of the substantia nigra and ventral tegmental area) and in the area of cholinergic cell bodies (septal area) markedly enhanced the extracellular dopamine (DA) and acetylcholine (ACh) concentration throughout their respective target areas, i.e. striatum, nucleus accumbens, prefrontal cortex for dopaminergic systems and hippocampus for cholinergic neurons. The enhancing effect of senktide on neurotransmitter release was dose dependently blocked by the selective non-peptide NK3 receptor antagonist SR142801 (0.1-1 mg/kg, i.p.), whereas its inactive S-enantiomer SR142806 (0.3-1 mg/kg, i.p.) did not exert any antagonistic activity on the effect of intranigral or intraseptal application of senktide. These results demonstrate that NK3 receptors can modulate the activity of central DA and ACh systems.

Expression and presence of septal neurokinin-2 receptors controlling hippocampal acetylcholine release during sensory stimulation in rat

We examined the expression and presence of NK2 receptors in the septal area of rat brain, and investigated their functional role in the regulation of the septohippocampal cholinergic system. Using reverse transcription-polymerase chain reaction (RT-PCR) analysis, we showed the presence of NK2 receptor mRNA expression in the septal area, and detected septal NK2 binding sites by using a fluorescent-tagged neurokinin A (NKA) derivative. In vivo microdialysis was employed to explore the functional role of NK2 receptors in the release of hippocampal acetylcholine evoked by tactile stimulation in freely moving rats. Two sessions of stroking of the neck and back of the rat for 30 min, at 90 min intervals, produced a marked and reproducible increase in hippocampal acetylcholine release. This effect was dose-dependently prevented by intraperitoneal administration of the two selective non-peptide tachykinin NK2 receptor antagonists SR144190 (0.03-0.3 mg/kg, i.p.) and SR48968 (0.3 and 1 mg/kg, i.p.), but not by the inactive enantiomer of SR48968 (SR48965, 1 mg/kg) nor by the two non-peptide NK1 receptor antagonists SR140333 (3 mg/kg, i.p.) and GR205171 (1 mg/kg, i.p.). Furthermore, the intraseptal application of SR144190 (10(-8) M) reduced the sensory response. Finally, intraseptal perfusion of neurokinin A (0.01-10 microM) in anaesthetized rats produced a concentration-dependent increase in hippocampal acetylcholine release. The response to neurokinin A (0.1 microM) was prevented by SR144190 (0.03-0.3 mg/kg, i.p.) and SR48968 (0.3-1 mg/kg, i.p.). In conclusion, this study provides direct evidence for the role of endogenous NKA/substance P, through the activation of NK2 receptors, in regulating the septohippocampal cholinergic function.