Localization of striatal and nigral tachykinin receptors in the rat

A Second Wave for the Neurokinin Tac2 Pathway in Brain Research

Despite promising advances in basic research of the neurokinin B/Tac2 pathway in both animals and humans, clinical applications are yet to be implemented. This is likely because of our limited understanding of the action of the pathway in the brain. While this system controls neuronal activity in multiple regions, the precise impact of Tac2-induced cellular responses on behavior remains unclear. Recently, elegant studies revealed a key contribution to stress-related behaviors and memory. Here, we discuss the crucial importance of bridging the gap between the Tac2 pathway's involvement in cell physiology and cognition to comprehend its role in health and disease. We propose that a better understanding of the Tac2 pathway in the brain could provide an essential perspective for basic investigations, which in turn will feed clinical research.

Mapping the binding pocket of a novel, high-affinity, slow dissociating tachykinin NK3 receptor antagonist: biochemical and electrophysiological characterization

The NK3 receptor is a GPCR that is prominently expressed in limbic areas of the brain, many of which have been implicated in schizophrenia. Phase II clinical trials in schizophrenia with two selective NK3 antagonists (osanetant and talnetant) have demonstrated significant improvement in positive symptoms. The objective of this study was to characterize the properties of a novel dual NK2/NK3 antagonist, RO5328673. [(3)H]RO5328673 bound to a single saturable site on hNK2, hNK3 and gpNK3 with high-affinity. RO5328673 acted as an insurmountable antagonist at both human and guinea-pig NK3 receptors in the [(3)H]IP accumulation assay. In binding kinetic analyses, [(3)H]RO5328673 had fast association and dissociation rates at hNK2 while it had a fast association rate and a remarkably slow dissociation rate at gp and hNK3. In electrophysiological recordings of gp SNpc, RO5328673 inhibited the senktide-induced potentiation of spontaneous activity of dopaminergic neurons with an insurmountable mechanism of action. RO5328673 exhibited in-vivo activity in gerbils, robustly reversing the senktide-induced locomotor activity. The TM2 residue gpNK3-A114(2.58) (threonine in all other species) was identified as the critical residue for the RO5328673's slower dissociation kinetics and stronger insurmountable mode of antagonism in the guinea-pig as compared to hNK3-T139(2.58). Using site-directed mutagenesis, [(3)H]RO5328673 binding and rhodopsin-based modeling, the important molecular determinants of the RO5328673-binding pocket of hNK3 were determined. A comparison of the RO5328673-binding pocket with that of osanetant showed that two antagonists have similar contact sides on hNK3 binding crevice except for three mutations V95L(1.42), Y247W(5.38), V255I(5.46), which behaved differently between interacting modes of two antagonists in hNK3.

3D-Quantitative structure-activity relationship and docking studies of the tachykinin NK3 receptor

The tachykinin NK(3) receptor (NK(3)R) is a novel drug target for schizophrenia and drug abuse. Since few non-peptide antagonists of this G protein-coupled receptor are available, we have initiated this study to gain a better understanding of the structure-activity relationships of NK(3) antagonist compounds. We developed a 3D comparative molecular similarity index analysis (CoMSIA) model that gave cross-validated PLS values with q(2) >0.5 which were validated using a test set. We also describe the development of a homology model of the NK(3)R. The model was then used to develop a pharmacophore for docked ligands. This pharmacophore showed two aromatic, two hydrogen donor and one acceptor/aromatic points. These data will be useful for future structure-based drug discovery of ligands for the NK(3)R.

Characterization of nuclear neurokinin 3 receptor expression in rat brain

Ligand-induced translocation of the G-protein-coupled receptor, neurokinin 3 (NK3-R), to the nucleus of hypothalamic neurons was reported using antibodies (ABs) raised against the C-terminal region of NK3-R. The current work was undertaken to substantiate the ability of NK3-R to enter the nucleus and identify which portion of the NK3-R molecule enters the nucleus. ABs directed at epitopes in the N-terminal and second extracellular loop of the rat NK3-R molecule were used to evaluate western blots of whole tissue homogenates and nuclear fractions from multiple brain areas. Specificity of the protein bands recognized by these ABs was demonstrated using Chinese hamster ovary (CHO) cells transfected with rat or human NK3-R. Both ABs prominently recognized a diffuse protein band of ∼56-65 kDa (56 kDa=predicted size) and distinct ∼70-kDa and 95-kDa proteins in homogenates of multiple brain areas. The ∼95-kDa protein recognized by the extracellular loop AB was enriched in nuclear fractions. Recognition of these proteins by ABs directed at different regions of the NK3-R supports their identification as NK3-R. The size differences reflect variable glycosylation and possibly linkage to different cytosolic and nuclear proteins. Recognition of protein bands by both ABs in nuclear fractions is consistent with the full-length NK3-R entering the nucleus. Hypotension increased the density of the ∼95-kDa band in nuclear fractions from the supraoptic nucleus indicating activity-induced nuclear translocation. Since NK3-R is widely distributed in the CNS, the presence of NK3-R in nuclei from multiple brain regions suggests that it may broadly influence CNS gene expression in a ligand-dependent manner.

Characterization of RO4583298 as a novel potent, dual antagonist with in vivo activity at tachykinin NK₁ and NK₃ receptors

BACKGROUND AND PURPOSE

Clinical results of osanetant and talnetant (selective-NK₃ antagonists) indicate that blocking the NK₃ receptor could be beneficial for the treatment of schizophrenia. The objective of this study was to characterize the in vitro and in vivo properties of a novel dual NK₁/NK₃ antagonist, RO4583298 (2-phenyl-N-(pyridin-3-yl)-N-methylisobutyramide derivative).

EXPERIMENTAL APPROACH

RO4583298 in vitro pharmacology was investigated using radioligand binding ([³H]-SP, [³H]-osanetant, [³H]-senktide), [³H]-inositol-phosphate accumulation Schild analysis (SP- or [MePhe⁷]-NKB-induced) and electrophysiological studies in guinea-pig substantia nigra pars compacta (SNpc). The in vivo activity of RO4583298 was assessed using reversal of GR73632-induced foot tapping in gerbils (GFT; NK₁) and senktide-induced tail whips in mice (MTW; NK₃).

KEY RESULTS

RO4583298 has a high-affinity for NK₁ (human and gerbil) and NK₃ (human, cynomolgus monkey, gerbil and guinea-pig) receptors and behaves as a pseudo-irreversible antagonist. Unusually it binds with high-affinity to mouse and rat NK₃, yet with a partial non-competitive mode of antagonism. In guinea-pig SNpc, RO4583298 inhibited the senktide-induced potentiation of spontaneous activity of dopaminergic neurones with an apparent non-competitive mechanism of action. RO4583298 (p.o.) robustly blocked the GFT response, and inhibited the MTW.

CONCLUSIONS AND IMPLICATIONS

RO4583298 is a high-affinity, non-competitive, long-acting in vivo NK₁/NK₃ antagonist; hence providing a useful in vitro and in vivo pharmacological tool to investigate the roles of NK₁ and NK₃ receptors in psychiatric disorders.

Tachykinin neurokinin 3 receptor antagonists: a patent review (2005 - 2010)

INTRODUCTION

The neurokinin 3 (NK(3)) receptor is a GPCR that has been shown to modulate monoaminergic systems within regions of the brain implicated in schizophrenia. Preclinical and Phase II clinical results of osanetant and talnetant in schizophrenic patients have indicated that NK(3) antagonists may provide significant improvement of the positive symptoms and cognitive impairment associated with this disorder. Recent findings have also indicated that neurokinin B (NKB)-NK(3) signaling plays a key role in the hypothalamic regulation of reproduction in humans.

AREAS COVERED

This review article discusses the latest medicinal chemistry strategies used to derive novel NK(3) receptor antagonists which have been patented during the period 2005 - 2010.

EXPERT OPINION

Since the report of a beneficial effect of osanetant in schizophrenic patients, major pharmaceutical companies have been involved in this field, leading to a very large number of patent applications disclosed. Nevertheless, only three NK(3) selective antagonists entered into Phase II, but were then terminated for various reasons. Currently, the main challenge to move forward a selective NK(3) antagonist into the clinic would be to define a safety margin between the desired therapeutic effect and the effect on testosterone levels. The involvement of NKB-NK(3) signaling in reproduction in humans may also lead to new exciting indications, such as treatment for sex steroid-sensitive cancers of breast and prostate.

Psychomotor and cognitive effects of a single oral dose of talnetant (SB223412) in healthy volunteers compared with placebo or haloperidol

Central Nervous System (CNS) effects of talnetant, an NK-3 antagonist in development for schizophrenia, were compared to those of haloperidol and placebo. The study was randomised, double-blind, three-way crossover of talnetant 200 mg, haloperidol 3 mg or placebo. Twelve healthy males participated and EEG, saccadic and smooth pursuit eye movements, adaptive tracking, body sway, finger tapping, hormones, visual analogue scales (VAS) for alertness, mood and calmness and psychedelic effects, left/right distraction task, Tower of London and Visual and Verbal Learning Task were assessed. Haloperidol showed (difference to placebo; 95% CI; p-value) decreases in EEG alpha power (-0.87microV; -1.51/-0.22; p = 0.0110), saccadic inaccuracy (2.0%; 0.5/3.6; p = 0.0133), smooth pursuit eye movements (-7.5%; -12.0/-3.0; p = 0.0026), adaptive tracking (-3.5%; -5.4/-1.7; p = 0.0009), alertness (-6.8 mm; -11.1/-2.4; p = 0.0039), negative mood (-4.6 mm; -8.6/-0.6; p = 0.0266), the ability to control thoughts (1.2 mm; 0.2/2.3; p = 0.0214), and an increase of serum prolactin (ratio 4.1; 3.0/5.6; p < 0.0001). Talnetant showed decreased alpha power (-0.69 muV; -1.34/-0.04; p = 0.0390), improved adaptive tracking (1.9%; 0.1/3.7; p = 0.0370) and reduced calmness on VAS Bond and Lader (-4.5 mm; -8.0/-1.0; p = 0.0151). Haloperidol effects were predominantly CNS-depressant, while those of talnetant were slightly stimulatory. The results suggest that talnetant penetrates the brain, but it remains to be established whether this dose is sufficient and whether the observed effect profile is class-specific for NK3-antagonists.

In vitro and in vivo characterization of the non-peptide NK3 receptor antagonist SB-223412 (talnetant): potential therapeutic utility in the treatment of schizophrenia

Neurokinin-3 (NK3) receptors are concentrated in forebrain and basal ganglia structures within the mammalian CNS. This distribution, together with the modulatory influence of NK3 receptors on monoaminergic neurotransmission, has led to the hypothesis that NK3 receptor antagonists may have therapeutic efficacy in the treatment of psychiatric disorders. Here we describe the in vitro and in vivo characterization of the highly selective NK3 receptor antagonist talnetant (SB-223412). Talnetant has high affinity for recombinant human NK3 receptors (pKi 8.7) and demonstrates selectivity over other neurokinin receptors (pKi NK2 = 6.6 and NK1<4). In native tissue-binding studies, talnetant displayed high affinity for the guinea pig NK3 receptor (pKi 8.5). Functionally, talnetant competitively antagonized neurokinin B (NKB)-induced responses at the human recombinant receptor in both calcium and phosphoinositol second messenger assay systems (pA2 of 8.1 and 7.7, respectively). In guinea pig brain slices, talnetant antagonized NKB-induced increases in neuronal firing in the medial habenula (pKB = 7.9) and senktide-induced increases in neuronal firing in the substantia nigra pars compacta (pKB = 7.7) with no diminution of maximal agonist efficacy, suggesting competitive antagonism at native NK3 receptors. Talnetant (3-30 mg/kg i.p.) significantly attenuated senktide-induced 'wet dog shake' behaviors in the guinea pig in a dose-dependent manner. Microdialysis studies demonstrated that acute administration of talnetant (30 mg/kg i.p.) produced significant increases in extracellular dopamine and norepinephrine in the medial prefrontal cortex and attenuated haloperidol-induced increases in nucleus accumbens dopamine levels in the freely moving guinea pigs. Taken together, these data demonstrate that talnetant is a selective, competitive, brain-penetrant NK3 receptor antagonist with the ability to modulate mesolimbic and mesocortical dopaminergic neurotransmission and hence support its potential therapeutic utility in the treatment of schizophrenia.

Substance P depolarizes striatal projection neurons and facilitates their glutamatergic inputs

The striatum is the main basal ganglia input nucleus, receiving extensive glutamatergic inputs from cortex and thalamus. Medium spiny striatal projection neurons (MSNs) are GABAergic, and their axon collaterals synapse on other MSNs. Approximately 50% of MSNs corelease substance P (SP), but how this neurotransmitter controls MSN activity is poorly understood. We used whole-cell recordings to investigate how SP affects MSNs and their glutamatergic inputs. SP elicited slow depolarizations in 47/90 MSNs, which persisted in the presence of tetrodotoxin (TTX). SP responses were mimicked by the NK1 receptor agonist [Sar9,Met(O(2))11]-substance P (SSP), and fully blocked by the NK1 receptor antagonists L-732,138, or extracellular zinc. When intracellular chloride was altered, the polarity of SP responses depended on the sign of the chloride driving force. In voltage-clamp, SP-induced currents reversed around -68 mV and displayed marked inward rectification. These data indicate that SP increased a ClC-2-type chloride conductance in MSNs, acting through NK1 receptors. SP also strongly increased glutamatergic responses in 49/89 MSNs. Facilitation of glutamatergic responses (which was observed both in MSNs directly affected by SP and in non-affected ones) was reduced by application of L-732,138, and fully blocked by coapplication of L-732,138 and SB222200 (an NK3 receptor antagonists), showing that both NK1 and NK3 receptors were involved. SP-induced facilitation of glutamatergic responses was accompanied by a marked decrease in paired-pulse ratio, indicating a presynaptic mechanism of action. These data provide an electrophysiological correlate for the anatomically known connections between SP-positive MSN terminals and the dendrites and somata of other MSNs.

Disruption of the neurokinin-3 receptor (NK3) in mice leads to cognitive deficits

RATIONALE

The structurally related neuropeptides, substance P, neurokinin A, and neurokinin B, belong to a family of molecules termed tachykinins and are widely distributed in the central and peripheral nervous systems. These peptides mediate their effects through three G protein coupled receptor subtypes, the neurokinin-1, neurokinin-2 and neurokinin-3 receptors, respectively.

OBJECTIVE

To study the physiological functions of NK3, a line of NK3 knockout mice were generated and characterized in a broad spectrum of well-established behavioral tests.

RESULTS

In several tests, including spontaneous locomotor activity, elevated plus maze, forced swim, and hot plate, wild-type and knockout mice performed similarly. However, in several cognition tests, including passive avoidance, acquisition of conditioned avoidance responding (CAR), and the Morris water maze, NK3 knockout mice displayed deficits compared to wild-type mice. Although NK3 wild-type and knockout mice performed similarly in the training phase of the passive avoidance test, knockout mice had shorter latencies to enter the dark compartment on days 3 and 4, suggesting impaired retention. In the acquisition phase of the conditioned avoidance responding assay, NK3 knockout mice acquired the CAR task at a slower rate than wild-type mice. Once the CAR test was acquired, both NK3 wild-type and knockout mice responded similarly to clozapine and risperidone, drugs which suppress responding in this test. In the Morris water maze, NK3 knockout mice showed increased latencies to find the escape platform on day 3 of training, suggesting a modest, but significant delay in acquisition compared to wild-type mice.

CONCLUSION

These studies suggest a role for NK3 in learning and memory in mice.

Neurokinin receptor antagonism attenuates cocaine's behavioural activating effects yet potentiates its dopamine-enhancing action in the nucleus accumbens core

Several lines of evidence indicate a role for neurokinin3 receptors (NK3-Rs) in behavioural activation and mechanisms governing reinforcement processes. In this study we investigated the effect of pretreatment with the NK3-R antagonist, SR142801, (0.2 and 2.0 mg/kg) on the cocaine-induced (10.0 mg/kg i.p.) increase in extracellular dopaminergic activity in the nucleus accumbens (NAc). In vivo microdialysis in the NAc of freely moving rats showed that cocaine increased concentrations of dopamine (DA) to approximately 350% in the core and approximately 450% in the shell. Pre-treatment with SR142801 significantly potentiated this effect in the core (to approximately 550%), whereas this effect was not found in the shell. We also investigated the effects of NK3-Rs antagonism on cocaine-induced hyperactivity and conditioned place preference. SR142801 blocked the hyperactivity, but neither the conditioned place preference nor the conditioned locomotor activity induced by cocaine, although there was a slight tendency towards a reduced place preference. When given alone, SR142801 had no effects on behaviour or extracellular dopamine concentrations in any of the structures investigated. These data provide evidence for a contribution of NK3-Rs in the acute behavioural and neurochemical effects of cocaine, involving dopaminergic activity in the core of the nucleus accumbens.

Predominant surface distribution of neurokinin-3 receptors in non-dopaminergic dendrites in the rat substantia nigra and ventral tegmental area

Neurokinin-3 (NK(3)) receptors are prevalent within the substantia nigra (SN) and ventral tegmental area (VTA), where their activation can affect motor and motivational behaviors as well as cardiovascular function and stress responses. These actions are mediated, in part, by dopaminergic neurons in each region. To determine the relevant sites for activation of these receptors, we examined the electron microscopic localization of NK(3) receptors and tyrosine hydroxylase (TH), the catecholamine synthesizing enzyme in dopaminergic neurons in the SN and VTA of rat brain. In each region, immunogold-silver labeling for NK(3) receptors was detected in many somatodendritic profiles, some of which contained TH-immunoreactivity. NK(3)-immunogold particles were largely associated with endomembranes resembling smooth endoplasmic reticulum, and only occasionally located on the plasma membrane in TH-labeled dendrites. In comparison with these dendrites, non-TH immunoreactive dendrites contained significantly more total (VTA) and more plasmalemmal (VTA and SN) NK(3)-immunogold particles. In each region, NK(3) gold particles also were seen in axonal as well as glial profiles, some of which contacted TH-immunoreactive dendrites. The NK(3)-labeled axon terminals formed either symmetric or asymmetric, excitatory-type synapses, the latter of which were significantly more prevalent in the VTA, compared with SN. These results provide the first ultrastructural evidence indicating that NK(3) receptors are available in cytoplasmic reserve in dopaminergic neurons, but more immediately accessible at the plasmalemmal surface of non-dopaminergic dendrites in both the SN and VTA. The activation of these receptors, together with the NK(3) receptors in either the presynaptic axon terminals or glia may contribute to the diverse physiological effects of tachykinins in each region, and most prominently involving excitatory inputs to the VTA.

Opinion: NK3 receptor antagonists: the next generation of antipsychotics?

Although current antipsychotic drugs are effective at treating the psychotic (positive) symptoms of schizophrenia, they have one or more serious side effects, including extrapyramidal symptoms, weight gain, cardiovascular liabilities and type II diabetes. However, recent data from clinical trials of selective neurokinin 3 (NK(3)) receptor antagonists in schizophrenia - osanetant and talnetant - have shown significant improvement in positive symptoms, with no major side-effects reported as yet. Here we discuss the preclinical and clinical evidence that indicates that NK(3) receptor antagonists might represent a new approach to the treatment of schizophrenia and possibly other neuropsychiatric disorders.

Intranigral antagonism of neurokinin 1 and 3 receptors reduces intrastriatal dopamine D1 receptor-stimulated locomotion in the rat

Stimulation of striatal dopamine (DA) D1 receptors increases the activity of the direct striatonigral pathway resulting in movement. While GABA has long been considered the primary effector of this pathway, co-released tachykinin peptides and their respective nigral tachykinin receptors are also in position to influence movement. Therefore, the present studies determined to what extent nigral tachykinin receptor subtypes contribute to striatal D1-mediated locomotion. Adult male Sprague-Dawley rats bearing chronic cannulae in the dorsal striatum and/or substantia nigra (SN) were tested for locomotor responses to various drug infusions. Unilateral intranigral infusions of the neurokinin-1 (NK1) antagonist LY306740 (0 and 50 nmol) but not the neurokinin-3 (NK3) antagonist SR142801 (0 and 50 nmol) led to ipsilateral rotations. Bilateral intrastriatal infusions of the full D1 agonist SKF 82958 (0, 1.2 and 12.0 nmol) dose-dependently increased locomotion. Prior bilateral intranigral infusions of LY306740 or SR142801 (0, 5.0 and 50 nmol) dose-dependently attenuated locomotor activity induced by intrastriatal SKF 82958 (12.0 nmol). These findings indicate that NK1, but not NK3, receptors within the SN may be tonically stimulated. However, activation of both nigral NK1 and NK3 receptors appears to be required for increased locomotion in response to striatal D1 receptor stimulation.

Implication of nigral tachykinin NK3 receptors in the maintenance of hypertension in spontaneously hypertensive rats: a pharmacologic and autoradiographic study

1. The role of nigral tachykinin NK(1), NK(2) and NK(3) receptors in central cardiovascular regulation was studied by measuring the effects of selective agonists and antagonists on mean arterial pressure (MAP) and heart rate (HR) after bilateral microinjection into the substantia nigra of spontaneously hypertensive rats (SHR). Quantitative in vitro autoradiography was also performed in the midbrain of SHR and Wistar-Kyoto (WKY) with the NK(3) receptor ligand [(125)I]-HPP-Senktide. 2. Tachycardia was elicited by the NK(1) ([Sar(9),Met(O(2))(11)]SP) and NK(2) ([betaAla(8)]NKA(4-10)) agonists at 25 and 100 pmol while the NK(3) agonist (senktide, 50 and 100 pmol) had no significant effect. The three agonists had no effect on behaviour, and increases in MAP were elicited by the NK(1) agonist only. 3. Whereas antagonists at NK(1) (RP 67580, 500 pmol) and NK(2) (SR 48968, 500 pmol) receptors had no significant effect on MAP and HR, the NK(3) antagonist (R-820, 500 pmol) reduced MAP for over 3 h in SHR. That anti-hypertensive effect did not occur after intracerebroventricular or intravenous injection of R-820. Also, R-820 had no cardiovascular effect in WKY. 4. The affinity (K(D): 0.7 nM) and densities of specific NK(3) receptor binding sites measured in the substantia nigra, ventral tegmental area, hippocampus and amygdala were not significantly different in SHR and WKY. 5. It is concluded that endogenous tachykinins exert a tonic activity on NK(3) receptors in the substantia nigra of SHR to maintain high blood pressure. Hence, nigral tachykinin NK(3) receptors may represent a promising therapeutic target in the treatment of arterial hypertension.

Regional decreases in NK-3, but not NK-1 tachykinin receptor binding in dystonic hamster (dt(sz)) brains

Although the pathophysiology of primary dystonias is currently unknown, it is thought to involve changes in the basal ganglia-thalamus-cortex circuit, particularly activity imbalances between direct and indirect striatal pathways. Substance P, a member of the tachykinin family of neuropeptides, is a major component in the direct pathway from striatum to basal ganglia output nuclei. In the present study quantitative autoradiography was used to examine changes in neurokinin-1 (NK-1) and neurokinin-3 (NK-3) receptors in mutant dystonic hamsters (dt(sz)), a well characterized model of paroxysmal dystonia. NK-1 receptors were labeled in 10 dystonic brains and 10 age-matched controls with 3 nM [(3)H]-[Sar(9), Met(O(2))(11)]-SP. NK-3 binding sites were labeled in adjacent sections with 2.5 nM [(3)H]senktide. NK-1 binding was found to be unaltered in 27 brain areas examined. In contrast, NK-3 binding was significantly reduced in layers 4 and 5 of the prefrontal (-46%), anterior cingulate (-42%) and parietal (-45%) cortices, ventromedial thalamus (-42%) and substantia nigra pars compacta (-36%) in dystonic brains compared to controls. The latter effects may be particularly relevant in view of evidence that activation of NK-3 receptors on dopaminergic neurons in the substantia nigra pars compacta can increase nigrostriatal dopaminergic activity. Since previous studies indicated that a reduced basal ganglia output in mutant hamsters is based on an overactivity of the direct pathway which also innervates substantia nigra pars compacta neurons, the decreased NK-3 binding could be related to a receptor down-regulation. The present finding of decreased NK-3 receptor density in the substantia nigra pars compacta, thalamic and cortical areas substantiates the hypothesis that disturbances of the basal ganglia-thalamus-cortex circuit play a critical role in the pathogenesis of paroxysmal dystonia.

Neurokinin receptors modulate the neurochemical actions of cocaine

The psychostimulant cocaine is an indirect agonist that increases synaptic dopamine (DA) by binding with high affinity to the DA transporter (DAT) and blocking the active transport of synaptic DA back into the terminal. The resulting increase in extracellular DA alters postsynaptic activity in the circuitry of the basal ganglia. This study examines the role of neurokinin receptors on cocaine-evoked DA overflow in the striatum. Male Sprague-Dawley rats (n = 8) were treated with cocaine (10 mg/kg body weight i.p.) acutely or chronically. The pattern of DA release was assessed using in vivo microdialysis. In separate experiments two different neurokinin-1 (NK-1) receptor antagonists (D-Arg(1), D-Pro(2), D-Trp(7,9), Leu(11), or L-733,060) were perfused via the microdialysis probe for one hour in awake and freely moving animals that were subsequently injected i.p. with cocaine. Throughout the procedure, DA release was monitored at 1/2-hour intervals at a flow rate of 1 microl/min. In the groups of rats preperfused with NK-1 antagonists into the striatum, the cocaine-evoked release of DA was significantly reduced. This result suggests a significant role for the NK-1 receptor in the striatal response to acute and chronic cocaine administration.

Modulation of cardiac activity by tachykinins in the rat substantia nigra

1. The effects of tachykinin NK(1), NK(2) and NK(3) receptor agonists and antagonists were measured on blood pressure (MAP) and heart rate (HR) after bilateral microinjection into the substantia nigra (SN) of awake, unrestrained rats. 2. Increasing doses (25 pmol - 1 nmol) of selective agonists at NK(1) ([Sar(9),Met(O(2))(11)]SP), NK(2) ([beta-Ala(8)]NKA(4 - 10)) and NK(3) (senktide) receptors into the SN produced tachycardia which was selectively and reversibly blocked by the prior injection of tachykinin antagonists at NK(1) (RP67580, 250 pmol), NK(2) (SR48968, 250 pmol) and NK(3) (R-820, 500 pmol) receptor. A rapid fall in MAP followed by a pressor response was seen with 1 nmol of [Sar(9),Met(O(2))(11)]SP. Behavioural activity was elicited by 1 nmol of [Sar(9),Met(O(2)(11)]SP (sniffing > face washing = grooming) and senktide (sniffing > wet dog shake > rearing = locomotion). Tachykinin antagonists had no direct cardiovascular or behavioural effects. 3. The tachycardia produced by 100 pmol of [beta-Ala(8)]NKA(4 - 10) or senktide was abolished by an i.v. treatment with atenolol (beta(1)-adrenoceptor antagonist, 5 mg kg(-1)) while that evoked by [Sar(9),Met(O(2))(11)]SP was reduced. A combination of atenolol (5 mg kg(-1)) and atropine (muscarinic antagonist, 1 mg kg(-1)) blocked the response evoked by [Sar(9),Met(O(2))(11)]SP. 4. These data suggest that the SN is a potential site of modulation of cardiac activity by tachykinins. In addition to the withdrawal of the cardiovagal activity by NK(1) receptor, the three tachykinin receptors appear to increase the sympatho/adrenal drive to the heart. This occurs independently of changes in MAP and behaviour. Hence, this study highlights a new central regulatory mechanism of cardiac autonomic activity.

Substance P modulates cocaine-evoked dopamine overflow in the striatum of the rat brain

To study the role of the neuropeptide substance P in modulating some of the effects of cocaine in the striatum, we administered cocaine to rats and measured preprotachykinin-A (PPT-A) messenger RNA and substance P peptide in the nigrostriatal pathway. We also measured the effect of a neurokinin-1 (NK-1) receptor antagonist on striatal cocaine-evoked dopamine overflow by in vivo microdialysis in freely moving animals. Acute administration of cocaine to naive rats (15 mg/kg of body weight) increased preprotachykinin-A mRNA levels in the dorsal and ventral aspects of the caudate putamen 4 hours after the intraperitoneal injection of cocaine. Concomitantly, in a separate group of animals, substance P peptide levels were decreased in the ventral caudate putamen and substantia nigra (38% below controls). In a separate experiment, infusion through the microdialysis probe of the neurokinin-1 receptor antagonist L-733,060 significantly decreased cocaine-evoked striatal dopamine overflow (approximately 50% inhibition at 30 minutes after cocaine administration). Taken together, these results suggest a direct role for substance P in the modulation of some of the actions of cocaine in the striatum of the rat brain.

Neurokinin-1 receptor antagonists block acute cocaine-induced horizontal locomotion

Systemic exposure to neurokinin-1 receptor antagonists CP099994 or LY306740 prior to cocaine administration (10 mg/kg i.p.) blocks acute, cocaine-induced horizontal locomotion. CP099994 (30 mg/kg) was delivered i.p. 30 minutes before cocaine exposure, and LY306740 (5 mg/kg) was delivered continuously by osmotic minipump for 12-14 hours before cocaine administration. These results suggest that endogenous substance P acting via neurokinin-1 receptors is necessary for the expression of acute cocaine-induced hyperactivity.

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.

NK-3 receptors are expressed on mouse striatal gamma-aminobutyric acid-ergic interneurones and evoke [(3)H] gamma-aminobutyric acid release

In the present study the ability of tachykinin agonists and antagonists to modulate gamma-aminobutyric acid (GABA) release has been correlated with tachykinin receptor expression in the mouse striatum. Significant GABA release was observed when striatal slices were challenged with the NK-3 receptor agonist senktide, the selectivity of which was confirmed using the NK-3 receptor antagonist SR142801. In situ hybridisation revealed co-expression of NK-3 receptors with nitric oxide synthase (NOS)/preprosomatostatin containing GABAergic interneurones. These findings suggest that tachykinins modulate GABA release within the striatum via interaction with NK-3 receptors on somatostatin/NOS interneurones.

Tachykinins increase [3H]acetylcholine release in mouse striatum through multiple receptor subtypes

Tachykinins have been suggested to play a significant role in the mammalian striatum, at least in part by the control of acetylcholine release from cholinergic interneurons. In the present study, we have examined the ability of known tachykinin agonists and antagonists to modulate the activity of these interneurons in mouse striatal slices. Using whole-cell patch-clamp recordings, the selective neurokinin-1, neurokinin-2 and neurokinin-3 receptor agonists [sar9,Met(O2)11]substance P, [beta-ala8]neurokinin A(4-10) and senktide each produced a dose-dependent depolarization of visually identified cholinergic interneurons that was retained under conditions designed to interrupt synaptic transmission. The nature of these neurons and the expression of multiple tachykinin receptors was confirmed using single-cell reverse transcriptase-polymerase chain reaction analysis. Using in vitro superfusion techniques, the selective neurokinin-1, neurokinin-2 and neurokinin-3 receptor agonists [sar9,Met(O2)11]substance P, [beta-ala8]neurokinin A(4-10) and senktide, respectively, each produced a dose-dependent increase in acetylcholine release, the selectivity of which was confirmed using the neurokinin-1, neurokinin-2 and neurokinin-3 receptor antagonists SR140333, GR94800 and SR142801 (100 nM). U73122 (10 microM), a phospholipase C inhibitor, blocked [sar9,Met(O2)11]substance P- and senktide-induced acetylcholine release, but had no effect on [beta-ala8]neurokinin A(4-10)-induced release. The protein kinase C inhibitors chelerythrine and Ro-31-8220 (both 1 microM) significantly inhibited responses induced by all three agonists. These findings indicate that tachykinins modulate the activity of mouse striatal cholinergic interneurons. Furthermore, neurokinin-2 receptors are shown to perform a role in mouse that has not been identified previously in other species.

Intracerebroventricular administration of substance P increases dopamine content in the brain of 6-hydroxydopamine-lesioned rats

The interactions existing between substance P- and dopamine-positive neurons, notably in the basal ganglia, suggest that substance P may have therapeutic use in treatment of Parkinson's disease characterized by impaired dopaminergic transmission. The effects of intracerebroventricularly administered substance P were tested on the levels of dopamine and its metabolites in the striatum, nucleus accumbens and frontal cortex of 6-hydroxydopamine-lesioned rats. Intracerebroventricular injection of 6-hydroxydopamine decreased the levels of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the brain structures under investigation. Administration of substance P in low dose (0.35 nmol/kg) had no effect on the 6-hydroxydopamine-induced reduction of the dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid contents in the brain. However, treatment with substance P in higher dose (3.5 nmol/kg) increased the concentrations of dopamine and its metabolites in the striatum, nucleus accumbens and frontal cortex relative to saline-treated group. Additionally, 6-hydroxydopamine lesions significantly increased 3,4-dihydroxyphenylacetic acid/dopamine and homovanillic acid/dopamine ratios in the striatum and nucleus accumbens. Substance P (3.5 nmol/kg) partially reversed lesion-induced increases in 3,4-dihydroxyphenylacetic acid/dopamine and homovanillic acid/dopamine ratios in the striatum, but did not alter these ratios in nucleus accumbens. To test whether substance P fragmentation is responsible for this phenomenon, substance P(5-11), which is one of the main substance P fragments in rat CNS, was administered in equimolar dose. Substance P(5-11) was found to have no effect on the content of dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid in the striatum and nucleus accumbens. In the frontal cortex, substance P(5-11) produced decreases in dopamine levels and increases in homovanillic acid/dopamine ratio. The results of this study suggest that substance P helps to restore dopamine deficit in the brain in an animal model of Parkinson's disease, with the positive effects being more prominent on the nigrostriatal than on the mesocorticolimbic dopaminergic system, but substance P(5-11) is not responsible for this effect.

Blockade of neurokinin3 receptors antagonizes drug-induced population response and depolarization block of midbrain dopamine neurons in guinea pigs

In vivo extracellular recording techniques were used to investigate the effects of neurokinin3 (NK3) receptor blockade on the pharmacological activation of midbrain dopamine (DA) neurons in the guinea pig substantia nigra (A9) and ventral tegmental area (A10). The number of spontaneously active DA cells (population response) was largely increased in A10 and A9 by acute administration of haloperidol (1 and 0.5 mg/kg i.p., respectively) and this effect was dose-dependently prevented in both areas by the selective NK3 receptor antagonist SR142801 (0.3, 1, 3, and 1, 3, 10 mg/kg i.p., respectively). This compound, which was totally inactive by itself, also antagonized the increase of population response induced in A10 cells by the neurotensin receptor antagonist SR142948 (1 mg/kg i.p.) and in A9 cells by the NK2 receptor antagonist SR144190 (1 mg/kg i.p.). None of the effects of SR142801 were reproduced by SR142806, its (R)-enantiomer with 240-fold lower affinity for NK3 receptors. In addition, neither SR144190 (0.3 mg/kg i.p.) nor the NK1 receptor antagonist GR205171 (1 mg/kg i.p.) affected the haloperidol-induced response. The antagonistic effects of SR142801 (3 mg/kg i.p.) were also observed on the depolarization block-related decrease of A10 cell population response evoked by repeated administration (22 days) of haloperidol. Finally, SR142801 (3 mg/kg i.p.) prevented depolarization block induced in A10 cells by acute co-administration of SR142948 and haloperidol, both on population response and on single cell firing. These results on pharmacologically induced activation and depolarization block of dopamine neurons suggest that NK3 receptors play a key role in the midbrain DA function, presumably through activation by neurokinin B.

Neurokinin-3 receptor distribution in rat and human brain: an immunohistochemical study

Autoradiographic and immunohistochemical studies have shown that the neurokinin-3 receptor is widely distributed in the rodent CNS. Expression of the neurokinin-3 receptor in human brain, however, has been debated. These conflicting findings, as well as the poor resolution of autoradiographic images, prompted us to develop a polyclonal antibody against an oligopeptide derived from the carboxy-terminus consensus sequence of both the rat and human neurokinin-3 receptor ([C]ASTTSSFISSPYTSVDEYS, amino acids 434-452 of the rat neurokinin-3 receptor). Western blot analysis of both human and rat brain tissue revealed a major band in the molecular weight range 65,000-67,000, the proposed molecular weight of the neurokinin-3 receptor based on its amino acid sequence and presumed glycosylation state. The distribution of selective high affinity neurokinin-3 receptor agonist [3H]senktide binding and neurokinin-3 receptor immunoreactivity were virtually identical in the brains of male Fischer 344 rats. The highest concentrations of neurokinin-3 receptors were observed in cortical layers IV-V; the basolateral amygdaloid nucleus; the hypothalamic paraventricular, perifornical and supraoptic nuclei; the zona incerta; and the entopeduncular and interpeduncular nuclei. [3H]senktide binding and neurokinin-3 receptor immunoreactivity were compared in homologous cortical areas of the human and rat brain. In contrast to the rat, autoradiographic analysis of normal control human brains (35-75 years) revealed a distinct and predominant superficial cortical labeling in the glia limitans and the cortical layer I. However, neurokinin-3 receptor immunoreactivity could be found not only in the superficial cortical layers, but also on pyramidal neurons and astrocytes in the neuropil and white matter. These findings suggest species differences in both the cellular and anatomical distribution of the neurokinin-3 receptor.

Characterization of the mechanism of action of tachykinins in rat striatal cholinergic interneurons

The mechanism by which substance P depolarizes cholinergic interneurons in the rat striatum was studied using whole-cell recording techniques. In all cases the effects of substance P were mimicked by the neurokinin1 receptor agonist [Sar9, Met(O2)11] substance P and were antagonized by the neurokinin1 receptor antagonist SR140333. [Sar9, Met(O2)11] substance P was found to depolarize cholinergic interneurons by the induction of a calcium-activated inward current at -60 mV. This inward current was irreversibly potentiated by photolysis of caged GTPgammaS within neurons implicating the involvement of a G-protein. The [Sar9, Met(O2)11] substance P-induced inward current was inhibited by the phospholipase C inhibitor U-73122, and by the inclusion of the inositol-1,4,5-triphosphate receptor antagonist heparin in the electrode solution. These findings suggest that neurokinin1 receptors depolarize cholinergic interneurons in the rat striatum primarily through a phosphoinositide signalling pathway.

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.

Tachykinin NK-1 and NK-3 selective agonists induce analgesia in the formalin test for tonic pain following intra-VTA or intra-accumbens microinfusions

Experiments were designed to examine the analgesic effects induced by selective tachykinin receptor agonists microinfused into either the ventral tegmental area (VTA) or nucleus accumbens septi (NAS). Rats were tested in the formalin test for tonic pain following an injection of 0.05 ml of 2.5% formalin into one hind paw immediately after bilateral intra-VTA infusions of either the NK-1 agonist, GR-73632 (0.005, 0.05 or 0.5 nmol/side), the NK-3 agonist, senktide (0.005, 0.5 or 1.5 nmol/side), or saline. Two weeks later, the saline-treated rats were assessed in the tail-flick test for phasic pain after infusions of the tachykinin agonists. Tail-flick latencies were recorded following immersion of the tail in 55 degrees C hot water at 10 min intervals for 1 h immediately after intra-VTA infusions of either GR-73632 (0.5 nmol/side), senktide (1.5 nmol/side) or saline. In a second group of rats, the same effects were studied after infusions into the nucleus accumbens (NAS) of GR-73632 (0.005, 0.5 or 1.5 nmol/side), senktide (0.005, 0.5 or 1.5 nmol/side), or saline. In both the VTA and NAS, the NK-1 and the NK-3 agonists caused significant analgesia in the formalin test, although the NK-1 agonist appeared to be more effective. Naltrexone (2.0 mg/kg) pretreatment failed to reverse the analgesic effects in the formalin test induced by intra-VTA infusions of the substance P (SP) analog, DiMe-C7 (3.0 microg/side), GR-73632 (0.5 nmol/side), or senktide (1.5 nmol/side). Neither compound given at either site was effective in the tail-flick test. These findings suggest that SP-dopamine (DA) interactions within the mesolimbic DA system play an important role in the inhibition of tonic pain. Furthermore, they support our earlier ideas that activation of midbrain DA systems by SP might play a role in stress- and/or pain-induced analgesia.

Neuropeptides in the immunomodulation: substance P-induced stimulation of immune response in mice

The effects of substance P (SP) and its new structural analog EC-1 administered systemically to CBA mice on the immune responsiveness have been examined. Three main findings are presented in this study. First, the principal effect of SP and EC-1 on the immunity is a stimulatory one. It is shown that in the doses of 1, 10 and 100 mkg/kg of the body weight both neuropeptides significantly increased the number of plaque- and rosette-forming cells in the spleen of animals at the peak of the immune reactions. Second, the destruction of the pituitary stalk prevented the immune response stimulation caused by neuropeptides. Third, SP-induced immunostimulation was not observed after combination with the antagonist of the postsynaptic dopamine (DA) D-2 receptors haloperidol (2 mg/kg) suggesting the involvement of the DAergic system in the realization of this effect. Thus, these results demonstrate neurochemical DAergic mechanisms underlying the immunostimulating influence of tachykinins.

Tachykinin neurokinin-1 and neurokinin-3 receptor-mediated responses in guinea-pig substantia nigra: an in vitro electrophysiological study

The effects of tachykinin receptor agonists and antagonists were investigated using intra- and extracellular recordings on spontaneously firing nigral neurons in guinea-pig brain slices. In 70 of 76 electrophysiologically identified dopaminergic neurons, a concentration-dependent increase in firing rate was induced by the selective neurokinin-3 tachykinin agonist senktide and by the natural tachykinin agonists neurokinin B and substance P, with EC50 values of 14.7, 31.2 and 12200 nM respectively. These responses were inhibited in a concentration- and time-dependent manner by the selective non-peptide neurokinin-3 receptor antagonist SR 142801 (1-100 nM; n=23), but neither by its S-enantiomer SR 142806 (100 nM; n=4) nor by selective antagonists of neurokinin-1 (SR 140333) or neurokinin-2 (SR 48968) receptors (both at 100 nM; n=3). The selective neurokinin-1 agonist [Sar9,Met(O2)11]substance P (30-100 nM; n=23) and the selective neurokinin-2 agonist [Nle10]neurokinin A(4-10)(30-100 nM; n=13) were without any effect on dopaminergic cells. In 13 of 21 electrophysiologically identified, presumably GABAergic neurons located in the pars compacta of the substantia nigra, excitatory responses were evoked concentration dependently by substance P and [Sar9,Met(O2)11]substance P, with EC50 values of 18.6 and 41.9 nM respectively. These responses were inhibited by SR 140333 (100 nM; n=3), but neither by its R-enantiomer SR 140603 nor by SR 142801 (both at 100 nM; n=3). Senktide and [Nle10]neurokinin A(4-10) (both at 30-100 nM; n=10) were without effect on these presumed GABAergic neurons. A small population (12%) of pars compacta neurons was insensitive to any of the three selective tachykinin agonists. In the nigral pars reticulata, 12 neurons were recorded which had an electrophysiological profile similar to that of presumed GABAergic neurons in the pars compacta. Of these 12 cells, seven did not respond to any of the selective tachykinin agonists tested, while five were excited by senktide in a concentration-dependent manner (EC50=98.5 nM). Although this value was significantly higher than that found for dopaminergic neurons in the pars compacta, senktide-evoked responses were inhibited by SR 142801 (100 nM; n=3). We conclude that, in the guinea-pig substantia nigra, tachykinins evoke excitatory responses in both dopaminergic and non-dopaminergic neurons; however, the sensitivity to tachykinin agonists (neurokinin-1 versus neurokinin-3) depends on both neuronal type and localization.

Spatial distribution of kainate receptor subunit mRNA in the mouse basal ganglia and ventral mesencephalon

In an attempt to gain knowledge of the possible functions of kainate receptors, we have used in situ hybridization to examine the regional and cellular expression patterns of glutamate receptor subunits GluR5-7, KA1 and KA2 in the adult mouse basal ganglia, known to play a pivotal role in the translation of motivation into actions. Kainate receptor subunits were found to be differentially expressed in the circuitry forming the basal ganglia. They differ from each other in expression levels and their spatial localization. GluR6 appeared as the key subunit for the descending gamma-aminobutyric acid (GABA)ergic-glutamatergic pathways, with highest message levels in the caudate putamen, globus pallidus and subthalamic nucleus as well as in the nucleus accumbens and olfactory tubercle. GluR7 exhibited highest expression in the ascending nigrostriatal and mesolimbic dopaminergic neurons. GluR5 had a restricted distribution pattern, with high expression in the ventral pallidum, the islands of Calleja and pars compacta of the substantia nigra. KA2 was usually coexpressed with GluR6, although with a generally lower level of expression. Finally, KA1 mRNA was barely detectable in these neuronal circuits. These data suggest that kainate receptors in general may be involved in the functions associated with the basal ganglia, with a key role in the control of the central dopaminergic transmission. Thus, they might be implicated in the neurodegenerative and psychic disorders associated with an impairment of the basal ganglia.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Evidence for modulation of dopamine-neuronal function by tachykinin NK3 receptor stimulation in gerbil mesencephalic cell cultures

Primary cultures of gerbil mesencephalon were used for studying the modulation exerted by tachykinin NK(3) receptor activation on the activity of dopamine (DA) neurons. Dopamine neurons were identified by their ability to take up [(3)H]DA in a nomifensine-dependent manner. Moreover, tyrosine hydroxylase immunohistochemistry revealed that these neurons accounted for 5-7% of the total cell population. The NK(3) receptor agonists, senktide (EC(50) = 0.58 nM) and [MePhe(7)]neurokinin B (EC(50) = 3 nM), increased spontaneous [(3)H]DA release in a concentration-dependent manner. In contrast, tested at a supramaximal concentration (IC(50) = 0.89 nM), neither septide nor substance P were found to affect [(3)H]DA release. The senktide-evoked [(3)H]DA release was not observed when extracellular Ca(2+) was chelated, but was unaffected by nomifensine. This indicates that this increase in [(3)H]DA outflow resulted more from an exocytotic process than from reversal of carrier-mediated DA uptake. Moreover, the senktide effect was unaffected by the Na+ channel blocker tetrodotoxin, a result suggesting a direct action of senktide on DA neurons. The non-peptide NK(3) receptor antagonist, SR 142801, shifted or blocked (IC(50) = 0.89 nM) the senktide-evoked [(3)H]DA release, while its (-)-antipode, SR 142806, was 80-fold less potent, in agreement with binding data. Selective antagonists for Nk1 (SR 140333) or Nk2 (SR 48968) receptors failed to reduce the senktide effect. Light scanning microscopic analysis of mesencephalic cells loaded with the Ca(2+) sensitive dye, fluo-3, showed that senktide induced a rise in cytosolic Ca(2+) in 8-10% of the cell population. The senktide-induced elevation in intracellular Ca(2+) was rapid in onset and transient (at 10-8 M) or more sustained with no further increase in fluorescence intensity (at 10(-7) M). The proportion of senktide-responsive cells was not significantly modified when extracellular Ca(2+) was chelated, but was reduced by 87% in the presence of SR 142801 and by 75% in cultures that were pre-treated with the DA neurotoxin 1-methyl-4-phenylpyridinium. The present study shows that enhancement of spontaneous [(3)H]DA release and intracellular Ca(2+) mobilization may be observed after NK(3) receptor stimulation and that both biochemical events are likely to occur in DA neurons.

Pharmacological characterization of grooming induced by a selective NK-1 tachykinin receptor agonist

Bilateral intranigral administration of the selective NK-1 tachykinin receptor agonist [AcArg6, Sar9, Met(O2)11]SP6-11 (0-1 nmol total bilateral dose) selectively induced grooming in rats. This response was blocked by concurrent intranigral administration of the NK-1 tachykinin receptor antagonist RP 67580 (2 nmol), but not by NK-2 (L-659,877) or NK-3 ([Trp7, beta-Ala8]NKA4-10) antagonists. Pretreatment with systemic opioid (naloxone 1.5 mg/kg) and D1 dopamine (SCH 23390 100 micrograms/kg) receptor antagonists also attenuated tachykinin-induced grooming, which was unaffected by D2 dopamine (sulpiride 30 mg/kg) or 5-HT2A+C (ritanserin 2 mg/kg) antagonists. Grooming induced by intranigral [AcArg6, Sar9, Met(O2)11]SP6-11 was also attenuated by bilateral 6-hydroxydopamine lesions of the substantia nigra. These findings indicate that grooming induced by intranigral tachykinins reflects activation of NK-1 receptors and is dependent upon endogenous dopamine and consequent selective stimulation of D1 dopamine receptors.

Pharmacology of tachykinin receptors on neurones in the ventral tegmental area of rat brain slices

The pharmacology of tachykinin receptors within the ventral tegmental area of rat brain slices was studied using in vitro electrophysiological techniques. The selective tachykinin NK3 receptor agonist senktide (100 nM) increased the action potential firing rate from 1.9 to 3.9 Hz in 70% of spontaneously active cells tested (n = 27). Senktide was the most potent agonist tested with an EC50 of 4 nM. In contrast the NK1 receptor agonists substance P-O-methyl ester (100-300 nM) or GR 73632 (1 microM) were inactive at the concentrations tested. Responses to neurokinin B (EC50 = 32 nM) were not blocked by the tachykinin NK1 receptor antagonist CP 99,994 (1 microM) nor by the tachykinin NK2 receptor antagonist SR 48968 (300 nM). Similarly responses to the tachykinin NK2 receptor agonist beta-[Ala8]neurokinin A-(4-10) (EC50 = 427 nM) were not antagonised by the tachykinin NK2 receptor antagonist SR 48968 (300 nM) and thus were likely to be due to the activation of tachykinin NK3 receptors. These data demonstrate that NK3, and not NK1 or NK2 receptors, mediate the principal excitatory effects of exogenously applied tachykinin receptor agonists on dopamine neurones within the rat ventral tegmental area.