The neurokinin-1 receptor antagonist WIN51,708 attenuates the anxiolytic-like effects of ventralpallidal substance P injection

Involvement of substance P and the NK-1 receptor in human pathology

The peptide substance P (SP) shows a widespread distribution in both the central and peripheral nervous systems, but it is also present in cells not belonging to the nervous system (immune cells, liver, lung, placenta, etc.). SP is located in all body fluids, such as blood, cerebrospinal fluid, breast milk, etc. i.e. it is ubiquitous in human body. After binding to the neurokinin-1 (NK-1) receptor, SP regulates many pathophysiological functions in the central nervous system, such as emotional behavior, stress, depression, anxiety, emesis, vomiting, migraine, alcohol addiction, seizures and neurodegeneration. SP has been also implicated in pain, inflammation, hepatitis, hepatotoxicity, cholestasis, pruritus, myocarditis, bronchiolitis, abortus, bacteria and viral infection (e.g., HIV infection) and it plays an important role in cancer (e.g., tumor cell proliferation, antiapoptotic effects in tumor cells, angiogenesis, migration of tumor cells for invasion, infiltration and metastasis). This means that the SP/NK-1 receptor system is involved in the molecular bases of many human pathologies. Thus, knowledge of this system is the key for a better understanding and hence a better management of many human diseases. In this review, we update the involvement of the SP/NK-1 receptor system in the physiopathology of the above-mentioned pathologies and we suggest valuable future therapeutic interventions involving the use of NK-1 receptor antagonists, particularly in the treatment of emesis, depression, cancer, neural degeneration, inflammatory bowel disease, viral infection and pruritus, in which that system is upregulated.

Cocaine sensitization does not alter SP effects on locomotion or excitatory synaptic transmission in the NAc of rats

Substance P (SP) and cocaine employ similar mechanisms to modify excitatory synaptic transmission in the nucleus accumbens (NAc), a region implicated in substance abuse. Here we explored, using NAc slices, whether SP effects on these synaptic responses were altered in rats that have been sensitized to cocaine and whether SP could mimic cocaine in triggering increased locomotion in sensitized rats. Intraperitoneal (IP) injection of naïve rats with cocaine (15 mg/kg) caused increased locomotion by 408.5 ± 85.9% (n = 5) which further increased by 733.1 ± 157.8% (n = 5) following a week of cocaine sensitization. A similar challenge with 10 mg/kg of SP after cocaine sensitization did not produce significant changes in locomotion (170.6 ± 61.0%; n = 4). In contrast to cocaine, IP injection of rats with SP or SP(5-11) (10-100 mg/kg) with or without phosphoramidon did not elicit changes in locomotion. In electrophysiological studies, both cocaine and SP depressed evoked NMDA and non-NMDA receptor-mediated excitatory synaptic currents (EPSCs) in slices obtained from naïve rats. In slices derived from cocaine-sensitized rats, cocaine but not SP produced a more profound decrease in non-NMDA compared to NMDA responses. Similar to that in naïve rats, cocaine's effect on the EPSCs in these sensitized rats occluded those of SP. Thus, although SP and cocaine may employ similar mechanisms to depress EPSCs in the NAc, IP injection of SP does not mimic cocaine-induced hyperlocomotion indicating that not all of cocaine's effects are mimicked by SP. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Cortical GABA, striatal dopamine and midbrain serotonin as the key players in compulsive and anxiety disorders--results from in vivo imaging studies

Various factors are discussed in the pathophysiology of anxiety disorders, including dysfunctions of the (DA)ergic, serotonin (5-HT)ergic and GABAergic system. We assessed the contribution of the individual synaptic constituents by subjecting all available in vivo imaging studies on patients with anxiety disorders to a retrospective analysis. On a total of 504 patients with obsessive-compulsive disorder (OCD), generalized anxiety disorder (GAD), panic disorder (PD), phobia, or posttraumatic stress-disorder (PTSD) and 593 controls, investigations of VMAT2, DAT, SERT, D1, D2, 5-HTIA, 5-HT2A, GABA(A), and NK1 receptor binding in neostriatum, ventral striatum, thalamus, neocortex, limbic system, cingulate, midbrain/ pons or cerebellum were performed using either PET or SPECT. Separate analyses of the individual disorders showed significant decreases of striatal D2 receptors in OCD (-18%), mesencephalic SERT in OCD (-13%), frontocortical GABAA receptors in PD (-13%) and temporocortical GABAA receptors in GAD (-16%). Pooling of all disorders yielded a significant reduction of mesencephalic SERT (-13%), mesencephalic (-27%) as well as cingulate 5-HT1A receptors (-18%), striatal D2 receptors (-21%) and frontal (-14%), temporal (-14%), occipital (-13%) and cingulate GABAA receptors (-15%). The results show that DA, 5-HT, and GABA play a major role in all subtypes of anxiety disorders. In particular, the findings imply that the regulation state of DA as modulated by GABA and 5-HT may be crucial for the development of anxiety- and compulsion-related disorders. As GABA and 5-HT inhibit DAergic neurotransmission, the reductions of GABAA, 5-HT1A and SERT can be assumed to result in an enhanced activity of the mesolimbic DAergic system. This notion is also reflected by the decrease of striatal D2 receptor binding, which is indicative of an increased availability of synaptic DA.

Deficits in substance P mRNA levels in the CeA are inversely associated with alcohol-motivated responding

In the present study, in vitro and in vivo studies were conducted to determine the relationship between innate substance P (SP) levels and alcohol-motivated behavior in alcohol-preferring (P) and nonpreferring (NP) rat lines. In Experiment 1, in situ hybridization and quantitative autoradiography were used to detect and measure SP mRNA levels in discrete brain loci of the P and NP rats. The results indicated significantly lower SP mRNA levels in the central nucleus of the amygdala (CeA) of P compared with those of NP rats. Experiment 2 evaluated the effects of SP, microinfused into the CeA, on alcohol (10%, v/v) and sucrose (2%, w/v) motivated responding in the P rat. The results revealed that, when infused into the CeA (1-8 microg), SP reduced alcohol responding by 48-85% of control levels, with no effects on sucrose responding. Neuroanatomical control infusions (1-8 microg) into the caudate putamen (CPu) also failed to significantly alter alcohol- or sucrose-motivated behaviors. Given the selective reductions on alcohol (compared to sucrose) responding by direct intracranial infusion of SP, the data suggest that deficits in SP signaling within the CeA (an anxiety regulating locus) are inversely associated with alcohol-motivated behaviors. Activation of SP receptors in the CeA may reduce anxiety-like behavior in the P rat and contribute to reductions on alcohol responding. The SP system may be a suitable target for the development of drugs to reduce alcohol-drinking behavior in humans.

Modulation of basal and stress-induced amygdaloid substance P release by the potent and selective NK1 receptor antagonist L-822429

It has been shown that anxiety and stress responses are modulated by substance P (SP) released within the amygdala. However, there is an important gap in our knowledge concerning the mechanisms regulating extracellular SP in this brain region. To study a possible self-regulating role of SP, we used a selective neurokinin-1 (NK1) receptor antagonist to investigate whether blockade of NK1 receptors results in altered basal and/or stress-evoked SP release in the medial amygdala (MeA), a critical brain area for a functional involvement of SP transmission in enhanced anxiety responses induced by stressor exposure. In vitro binding and functional receptor assays revealed that L-822429 represents a potent and selective rat NK1 receptor antagonist. Intra-amygdaloid administration of L-822429 via inverse microdialysis enhanced basal, but attenuated swim stress-induced SP release, while the low-affinity enantiomer of L-822429 had no effect. Using light and electron microscopy, synaptic contacts between SP-containing fibres and dendrites expressing NK1 receptors was demonstrated in the medial amygdala. Our findings suggest self-regulatory capacity of SP-mediated neurotransmission that differs in the effect on basal and stress-induced release of SP. Under basal conditions endogenous SP can serve as a signal that tonically inhibits its own release via a NK1 receptor-mediated negative feedback action, while under stress conditions SP release is further facilitated by activation of NK1 receptors, likely leading to high local levels of SP and activation of receptors to which SP binds with lower affinity.

Anxiolytic-like effects of substance P administration into the dorsal, but not ventral, hippocampus and its influence on serotonin

Substance P (SP) is known to be involved in processes related to learning and memory, fear, anxiety and stress. SP and NK1 receptors are localized in the hippocampus, a brain structure involved in learning and memory as well as emotional processes. As there is evidence for differential functions of the ventral (VH) and dorsal (DH) hippocampus in a variety of behaviors, we here evaluated the effects of injections of SP into the VH and DH in rats submitted to the elevated plus-maze (EPM) and open field (OF) tests. The results obtained showed that infusions of 100 and 1000 ng of SP into the DH, but not VH, increased open arm activity in the EPM and in the central zone of the OF, indicative of anxiolytic-like action. These effects were observed in the absence of significant changes in general motor activity. In an additional experiment to examine whether these effects of SP are mediated by local serotoninergic mechanisms, extracellular concentrations of this monoamine were assessed by use of in vivo microdialysis. Infusions of SP into the DH did not influence the extracellular concentration of serotonin. These data indicate that neurokinins in the DH, but not VH, are involved in mechanisms associated with anxiety and that the mediation of SP in anxiety-related behaviors is independent of local serotonergic mechanisms.

Morphine upregulates functional expression of neurokinin-1 receptor in neurons

Neuronkinin-1 receptor (NK-1R), the neuropeptide substance P (SP) preferring receptor, is highly expressed in areas of the central nervous system (CNS) that are especially implicated in depression, anxiety, and stress. Repeated exposure to opioids may sensitize neuronal systems involved in stress response. We examined the effects of morphine, the principal metabolite of heroin, on the functional expression of NK-1R in the cortical neurons. NK-1R and mu-opioid receptor (MOR) are co-expressed in the cortical neurons. Morphine enhanced NK-1R expression in the cortical neurons at both the mRNA and protein levels. The upregulated NK-1R by morphine had functional activity, because morphine-treated cortical neurons had greater SP-induced Ca(2+) mobilization than untreated neurons. Blocking opioid receptors on the cortical neurons by naltrexone or CTAP (a mu-opioid receptor antagonist) abolished the morphine action. Investigation of the mechanism(s) responsible for the morphine action showed that morphine activated NK-1R promoter and induced the phosphorylation of p38 MAPK protein in the cortical neurons. These in vitro data provide a plausible cellular mechanism for opioid-mediated neurological disorders.

The role of substance P in stress and anxiety responses

Substance P (SP) is one of the most abundant peptides in the central nervous system and has been implicated in a variety of physiological and pathophysiological processes including stress regulation, as well as affective and anxiety-related behaviour. Consistent with these functions, SP and its preferred neurokinin 1 (NK1) receptor has been found within brain areas known to be involved in the regulation of stress and anxiety responses. Aversive and stressful stimuli have been shown repeatedly to change SP brain tissue content, as well as NK1 receptor binding. More recently it has been demonstrated that emotional stressors increase SP efflux in specific limbic structures such as amygdala and septum and that the magnitude of this effect depends on the severity of the stressor. Depending on the brain area, an increase in intracerebral SP concentration (mimicked by SP microinjection) produces mainly anxiogenic-like responses in various behavioural tasks. Based on findings that SP transmission is stimulated under stressful or anxiety-provoking situations it was hypothesised that blockade of NK1 receptors may attenuate stress responses and exert anxiolytic-like effects. Preclinical and clinical studies have found evidence in favour of such an assumption. The status of this research is reviewed here.

The NK1 receptor antagonist NKP608 lacks anxiolytic-like activity in Swiss-Webster mice exposed to the elevated plus-maze

The selective non-peptide NK(1) receptor antagonist NKP608 has been shown to exert potent anxiolytic-like effects in the rat and gerbil social interaction tests. In vitro binding of NKP608 in cortical, striatal and rest-of-brain tissue samples from mice, rats and gerbils indicated comparable pIC(50) values for rats and mice (in all three tissues) and only slightly higher values for gerbils. It would therefore be expected that doses previously found to produce anxiolytic-like effects in rats and gerbils would also be active in mice. The present study evaluated NKP608 in one of the most widely-used animal models of anxiety, the mouse elevated plus-maze. Two consecutive experiments were conducted in which the effects of NKP608 (0.0003-10.0 mg/kg, p.o.) were compared to those produced by the prototypical benzodiazepine anxiolytic, chlordiazepoxide (CDP, 15 mg/kg, p.o.). Ethological scoring methods were used to provide comprehensive behavioural profiles for each compound. In both experiments, acute CDP treatment resulted in significant anxioselective effects, i.e., reductions in measures of open arm avoidance without any alteration in general activity levels (closed arm entries and rearing). Although the results of Experiment 1 (0.001-10.0 mg/kg NKP608) suggested a weak anxiolytic-like action of NKP608 at 0.001 mg/kg (significant increase in percent open arm entries), Experiment 2 failed both to replicate this effect or to find any behavioural activity at lower (0.0003 mg/kg) or higher (0.03 mg/kg) doses. Present findings suggest that the anxiolytic efficacy of this NK(1) receptor antagonist may be test-specific and thus limited to particular subtypes of anxiety. These new data are also discussed in relation to the general difficulty of relating the behavioural profiles of NK(1) receptor antagonists to their potency at NK(1) receptors.

Pretraining or previous non-spatial experience improves spatial learning in the Morris water maze of nucleus basalis lesioned rats

Previous experiments have shown that infusions of ibotenic acid in the nucleus basalis magnocellularis (NBM) induce a strong impairment in spatial navigation for a hidden platform in the Morris water maze. This effect was initially attributed to a cholinergic deficit, but later studies showed that performance level did not correlate with the degree of cholinergic denervation. Therefore, this impairment is due to a combined cholinergic and non-cholinergic deficit. However, it is not clear in which particular processes the NBM is involved. In this study we have evaluated the origin of behavioural impairment in spatial navigation in the water maze after an ibotenic acid-induced lesion of NBM. In the first experiment, Wistar rats were trained preoperatively in an allocentric navigation task. Postoperatively, they were tested in the same task. All lesioned animals showed a performance level similar to controls. Lesions did not impede the acquisition of new positions in the water maze, nor did affect the ability of animals to remember new platform positions after an intertrial interval of 20s, even if animals had received only allocentric experience with the platform position, or allocentric and path integration information concurrently. Lesions also failed to affect the ability to locate a hidden platform in a new environment. However, hippocampal infusions of scopolamine (5 microg) produced a severe impairment in NBM-damaged animals, without impairing performance of controls. In the second experiment Wistar rats with the same lesion were first trained in a visual-guided task in the water maze, and subsequently evaluated in the spatial task. In both tasks lesioned animals were not different from controls. These results suggest that the NBM played an important role during acquisition phases but not in the execution of spatial navigation. Moreover, the excessive emotional response displayed by lesioned animals is postulated as a relevant cause for the impairment observed in spatial navigation after NBM damage.

Anxiolytic-like effects of substance P fragment (SP(1-7)) in non-human primates (Callithrix penicillata)

The behavioral effects of the amino (N)-terminal fragment of substance P (SP(1-7)) on the marmoset (Callithrix penicillata) predator confrontation test of fear/anxiety were investigated. The test apparatus consisted of a figure-eight maze with three parallel arms interconnected at each extremity to a perpendicular arm. A taxidermized oncilla cat (Felis tigrina) was placed outside the maze facing one of its corners. Subjects were submitted to seven 30 min maze habituation trials (HTs), in the absence of the 'predator', and then to six 30 min treatment trials (TTs), in the presence of the 'predator', consisting of four doses of SP(1-7) (5, 50, 250 and 500 microg/kg; IP), saline and sham injection. SP(1-7) treatment reversed, in a dose-dependent way, the fear-induced avoidance behavior due to the predator's presence and increased the frequency of exploratory behaviors. Locomotor activity decreased during successive HTs, yet increased after all SP(1-7) treatments. These results indicate that systemic administration of SP(1-7) produces anxiolytic-like effects in marmosets tested in the predator confrontation model of fear/anxiety.

Sustained blockade of neurokinin-1 receptors enhances serotonin neurotransmission

BACKGROUND

Antagonists of neurokinin-1 (NK(1)) receptors, through which substance P acts, have been proposed to belong to a new class of antidepressants with a unique mode of action. It was postulated that they exert this putative therapeutic effect independently of the serotonin (5-HT) neurons.

METHODS

The aim of the present study was to assess, using in vivo electrophysiological paradigms, the effects of sustained administration of the nonpeptidic NK(1) antagonist CP-96,345 on the firing activity of rat dorsal raphe 5-HT neurons, the responsiveness of pre- and postsynaptic 5-HT(1A) receptors, and overall 5-HT neurotransmission in the hippocampus.

RESULTS

Both short- and long-term treatments with CP-96,345 significantly increased the spontaneous firing activity of dorsal raphe 5-HT neurons, and this increase was associated with an attenuation of somatodendritic 5-HT(1A) autoreceptor responsiveness. In contrast, the inactive enantiomer of CP-96,345 at NK(1) receptors, CP-96,344, did not alter these parameters after short-term administration. Because 5-HT(1A) receptor activation inhibits the firing activity of dorsal hippocampus CA(3) pyramidal neurons, the degree of disinhibition produced by the selective 5-HT(1A) receptor antagonist WAY 100635 was determined to assess the net change in 5-HT neurotransmission. Intravenous injection of WAY 100635 did not disinhibit CA(3) pyramidal neuron firing in rats given saline, CP-96,345 for 2 days, or CP-96,344 for 14 days, but produced a significant enhancement of firing in rats treated with CP-96,345 for 2 weeks. Therefore, only long-term treatment with CP-96,345 enhanced the tonic activation of postsynaptic 5-HT(1A) receptors.

CONCLUSIONS

Similar to all other major types of antidepressant treatments, these data indicate that substance P antagonists might alleviate anxiety and major depression, at least in part, by enhancing the degree of activation of some 5-HT receptors in the forebrain.

Molecular pathways of anxiety revealed by knockout mice

Anxiety is a normal reaction to threatening situations, and serves a physiological protective function. Pathological anxiety is characterized by a bias to interpret ambiguous situations as threatening, by avoidance of situations that are perceived to be harmful, and/or by exaggerated reactions to threat. Although much evidence indicates the involvement of the gamma-aminobutyric acid, serotonin, norepinephrine, dopamine, and neuropeptide transmitter systems in the pathophysiology of anxiety, little is known about how anxiety develops and what genetic/environmental factors underlie susceptibility to anxiety. Recently, inactivation of several genes, associated with either chemical communication between neurons or signaling within neurons, has been shown to give rise to anxiety-related behavior in knockout mice. Apart from confirming the involvement of serotonin, gamma-aminobutyric acid, and corticotrophin-releasing hormone as major mediators of anxiety and stress related behaviors, two novel groups of anxiety-relevant molecules have been revealed. The first group consists of neurotrophic-type molecules, such as interferon gamma, neural cell adhesion molecule, and midkine, which play important roles in neuronal development and cell-to-cell communication. The second group comprises regulators of intracellular signaling and gene expression, which emphasizes the importance of gene regulation in anxiety-related behaviors. Defects in these molecules are likely to contribute to the abnormal development and/or function of neuronal networks, which leads to the manifestation of anxiety disorders.

Substance P and its role in neural mechanisms governing learning, anxiety and functional recovery

The neurokinin Substance P (SP) is widely distributed in the central nervous system and has been extensively studied in various functional aspects. This review focuses on the behavioral relevance of SP. Here we show that SP can have memory-promoting, reinforcing and anxiolytic-like effects when administered systemically or into the nucleus basalis of the ventral pallidum. These effects seem to be mediated via the SP-preferring NK(1)receptor and differentially related to N- versus C-terminal fragments of the undecapeptide. Secondly, SP injection into the ventral pallidum can lead to increases of acetylcholine in frontal cortex and dopamine in nucleus accumbens, suggesting that the hypermnestic, positively reinforcing and anxiolytic effects observed upon basal forebrain injection of SP are mediated by activation of the nucleus accumbens-ventral pallidum circuitry. Furthermore, SP and certain SP-fragments may not only be considered to have beneficial behavioral effects in normal animals, but can also prevent lesion-induced functional deficits and improve the speed of recovery. This indicates that SP agonists might also have a neuroprotective capacity in parallel with recovery-promoting actions.

Anxiolytic-like effects in rats produced by ventral pallidal injection of both N- and C-terminal fragments of substance P

Prior studies have shown that the neurokinin substance P (SP) has anxiolytic-like effects when administered into the nucleus basalis (NB) area of the rat ventral pallidum. The present work was performed to examine whether the anxiolytic effects of SP in the nucleus basalis can be assigned its amino (N)- or carboxy (C)-terminal moiety. Using the elevated plus-maze model of anxiety in combination with unilateral injection of N-terminal SP(1-7) or C-terminal SP(7-11) into the NB region, we found that the treatment with either SP-fragment increased the number of entries into and time spent on the open arms as well as excursions into the end of the open arms, indicative of an anxiolytic-like profile. Furthermore, the effective doses of SP(1-7) (0.67 ng) and SP(7-11) (0.45 ng) were equimolar to the dosage of the whole SP molecule (1 ng), which was effective to reduce anxiety. Thus, the results support earlier findings that ventral pallidal injection of SP has anxiolytic-like effects and provide new evidence that fragments of SP, representing the N- and C-terminal domain of the peptide can reduce fear-parameters at a concentration similar to that of the parent peptide.