Effect of GR205171 on autonomic dysreflexia induced by colorectal distension in spinal cord injured rats.

STUDY DESIGN: Preclinical pharmacology. OBJECTIVES: To determine whether blocking substance P signaling attenuates the hypertension and bradycardia evoked by colorectal distension (CRD) in spinal cord injured (SCI) rats. SETTING: University laboratory in Pennsylvania, U.S.A. METHODS: Tachykinin NK1 receptor antagonists were administered 30 min prior to CRD three weeks after complete spinal cord transection at the 4th thoracic (T4) level. The dose range, route of administration, and pretreatment time was based on published data demonstrating occupancy of brain NK1 receptors in rodents. RESULTS: Subcutaneous (SC) administration of 10-30 mg/kg GR205171 ((2S,3S)-N-[[2-methoxy-5-[5-(trifluoromethyl)tetrazol-1-yl]phenyl]methyl]-2-phenylpiperidin-3-amine dihydrochloride) reduced CRD-induced hypertension and bradycardia by 55 and 49%, respectively, compared with pretreatment values. There was no effect of GR205171 on resting blood pressure or heart rate. In contrast, the same dose range of CP-99,994 ((2S,3S)-N-[(2-methoxyphenyl)methyl]-2-phenyl-3-piperidinamine dihydrochloride) had no effect on CRD-induced cardiovascular responses. CONCLUSIONS: The effective dose range of GR205171 to alleviate autonomic dysreflexia is consistent with the blockade of NK1 receptors on pelvic sensory afferents in the lumbosacral spinal cord, which may in turn prevent the over-excitation of sympathetic preganglionic neurons (SPNs) that regulate blood pressure and heart rate. The findings provide preclinical support for the utility of NK1 receptor antagonists to treat autonomic dysreflexia in people with SCI. The difference in the effects of the two NK1 receptor antagonists may reflect the ~200-fold lower affinity of CP-99,994 than GR205171 for the rat NK1 receptor.

Colorectal and bladder prokinetic activity of [Lys5, MeLeu9, Nle10]-NKA(4-10) after intranasal or sublingual delivery in dogs.

The feasibility of eliciting defecation and urination after intranasal (IN) or sublingual (SL) delivery of a small peptide NK2 receptor agonist, [Lys5, MeLeu9, Nle10]-NKA(4-10), was examined using prototype formulations in dogs. In anesthetized animals, administration of 100 or 300 µg/kg IN or 2.0-6.7 mg/kg SL increased colorectal peak pressure and area under the curve. Peak bladder pressure was also increased at the same doses, and this was accompanied by highly efficient voiding at normal physiological bladder pressure. The onset of these effects was rapid (≤2.5 min), and the primary contractions lasted ∼25 min, returning to baseline in <60 min. Slight hypotension lasting a few minutes and causing <10% change from baseline was detected after higher doses and was statistically significant after only 100 µg/kg IN. In conscious dogs, there was a dose-related increase in voiding responses and reduction in the latency to urinate and defecate after 300 and 1000 µg/kg IN; emesis was also observed at these doses. SL administration of 6.7 mg/kg induced urination within 10 min, but not defecation or emesis. These findings support the feasibility of developing a convenient dosage form of small peptide NK2 receptor agonists as on-demand defecation or urination therapies.

Potency, efficacy, and selectivity of GR64349 at human recombinant neurokinin NK2 and NK1 receptors.

The affinity, potency, efficacy, and selectivity of the NK2 receptor agonist GR64349 ([Lys3,Gly8,-R-γ-lactam-Leu9]NKA(3-10)) at human recombinant NK2 and NK1 receptors was examined. In radioligand binding studies, GR64349 displaced [125I]-NKA binding to NK2 receptors with high affinity (pKi 7.77 + 0.10) but only weakly displaced [3H]-septide binding to NK1 receptors (pKi <5). In functional studies examining increases in intracellular inositol-1 phosphate (IP-1) accumulation, calcium levels, and cyclic AMP synthesis, GR64349 was a full agonist by reference to the endogenous agonists NKA (NK2 receptors) and substance P (NK1 receptors). GR64349 increased IP-1 accumulation with 1,400-fold greater potency in cells expressing NK2 receptors (pEC50 9.10 + 0.16) than cells expressing NK1 receptors (pEC50 5.95 + 0.80). For calcium responses, GR64349 was 500-fold more potent in the assay using NK2 receptors (pEC50 9.27 + 0.26) than NK1 receptors (pEC50 6.55 + 0.16). GR64349 also stimulated cyclic AMP synthesis in both cell lines, and was almost 900-fold more potent at NK2 receptors (pEC50 10.66 + 0.27) than NK1 receptors (pEC50 7.71 + 0.41). These findings confirm that GR64349 is the most selective NK2 receptor agonist described to date.

Prokinetic effects of the neurokinin NK2 receptor agonist [Lys5,MeLeu9,Nle10]-NKA(4-10) on bladder and colorectal activity in minipigs.

The effects of the neurokinin NK2 receptor agonist [Lys5,MeLeu9,Nle10]-NKA(4-10) (LMN-NKA) on bladder and colorectal function were examined in minipigs. In anesthetized animals, subcutaneous (SC) administration of 30-100 µg/kg increased peak bladder and colorectal pressures. Increases in bladder and colorectal pressure were inhibited by a 15 min pretreatment with the NK2 receptor antagonist GR 159897 (1 mg/kg intravenously (IV)). Bladder and colorectal pressures were also increased after IV (0.3 µg/kg), intranasal (IN; 100 µg/kg) and sublingual administration (SL; 5 mg/kg). There was a nonsignificant trend for hypotension (16 or 12% decrease in mean arterial pressure) after 100 µg/kg SC and 0.3 µg/kg IV, respectively, but not after 100 µg/kg IN or 5 mg/kg SL. In conscious minipigs, 30-300 µg/kg SC caused a dose-related increase in defecation that was accompanied by emesis in 38% of subjects receiving 300 µg/kg. Urination was increased after 100 µg/kg SC but not lower or higher doses. The peak plasma exposure (Cmax) after 100 µg/kg SC was 123 ng/mL, and area under the curve (AUC) was 1790 min * ng/mL. Defecation response rates (~82%) were maintained after SC administration of LMN-NKA (30 µg/kg) given 3 times daily over 5 consecutive days. Defecation rates were higher after a single dose of 100 µg/kg IN compared with vehicle, but this did not reach significance. After 7-10 mg/kg SL, 83% of animals urinated and defecated, and none had emesis. The data support the feasibility of developing a convenient and well-tolerated route of administration of LMN-NKA for human use. Minipigs may be a suitable species for toxicology studies with LMN-NKA due to the relatively low rate of emesis in this species.

Affinity, potency, efficacy, and selectivity of neurokinin A analogs at human recombinant NK2 and NK1 receptors.

A series of peptide NK2 receptor agonists was evaluated for affinity, potency, efficacy, and selectivity at human recombinant NK2 and NK1 receptors expressed in CHO cells to identify compounds with the greatest separation between NK2 and NK1 receptor agonist activity. Binding studies were performed using displacement of [125I]-NKA binding to NK2 receptors and displacement of [3H]-Septide binding to NK1 receptors expressed in CHO cells. Functional studies examining the increase in intracellular calcium levels and cyclic AMP stimulation were performed using the same cell lines. A correlation was demonstrated between binding affinities (Ki) and potency to increase intracellular calcium (EC50) for NK2 and NK1 receptors. Ranking compounds by their relative affinity (Ki) or potency (EC50) at NK2 or NK1 receptors indicated that the most selective NK2 agonists tested were [Lys5,MeLeu9,Nle10]-NKA(4-10) (NK1/NK2 Ki ratio = 674; NK1/NK2 EC50 ratio = 105) and [Arg5,MeLeu9,Nle10]-NKA(4-10) (NK1/NK2 Ki ratio = 561; NK1/NK2 EC50 ratio = 70). The endogenous peptide, NKA, lacked selectivity with an NK1/NK2 Ki ratio = 20 and NK1/NK2 EC50 ratio = 1. Of the compounds selected for evaluation in cyclic AMP stimulation assays, [ß-Ala8]-NKA(4-10) had the greatest selectivity for activation of NK2 over NK1 receptors (NK1/NK2 EC50 ratio = 244), followed by [Lys5,MeLeu9,Nle10]-NKA(4-10) (ratio = 74), and NKA exhibited marginal selectivity (ratio = 2.8).

The Hypothalamic-Pituitary-Adrenal Axis and Serotonin Metabolism in Individual Brain Nuclei of Mice with Genetic Disruption of the NK1 Receptor Exposed to Acute Stress.

Mice lacking the substance P (SP) neurokinin-1 (NK1) receptor (NK1R-/-mice) were used to investigate whether SP affects serotonin (5-HT) function in the brain and to assess the effects of acute immobilisation stress on the hypothalamic-pituitary-adrenocortical (HPA) axis and 5-HT turnover in individual brain nuclei. Basal HPA activity and the expression of hypothalamic corticotropin-releasing hormone (CRH) in wild-type (WT)- and NK1R-/- mice were identical. Stress-induced increases in plasma ACTH concentration were considerably higher in NK1R-/- mice than in WT mice while corticosterone concentrations were equally elevated in both mouse lines. Acute stress did not alter the expression of CRH. In the dorsal raphe nucleus (DRN), basal 5-HT turnover was increased in NK1R-/- mice and a 15 min stress further magnified 5-HT utilisation in this region. In the frontoparietal cortex, medial prefrontal cortex, central nucleus of amygdala, and the hippocampal CA1 region, stress increased 5-HT and/or 5-hydroxyindoleacetic acid (5-HIAA) concentrations to a similar extent in WT and NK1R-/- mice. 5-HT turnover in the hypothalamic paraventricular nucleus was not affected by stress, but stress induced similar increases in 5-HT and 5-HIAA in the ventromedial and dorsomedial hypothalamic nuclei in WT and NK1R-/- mice. Our findings indicate that NK1 receptor activation suppresses ACTH release during acute stress but does not exert sustained inhibition of the HPA axis. Genetic deletion of the NK1 receptor accelerates 5-HT turnover in DRN under basal and stress conditions. No differences between the responses of serotonergic system to acute stress in WT and NK1R-/- mice occur in forebrain nuclei linked to the regulation of anxiety and neuroendocrine stress responses.

Colorectal and cardiovascular effects of [Lys5,MeLeu9,Nle10]-NKA(4-10) in anesthetized macaques.

The effects of the tachykinin NK2 receptor agonist LMN-NKA ([Lys5,MeLeu9,Nle10]-NKA(4-10)) on colorectal and arterial blood pressure were examined in anesthetized macaques. Intravenous (IV) administration of 1-100 µg/kg caused dose-related increases in colorectal pressure up to 120 mmHg above baseline, and area under the curve (AUC) up to 24,987 mmHg*s. This was accompanied at all doses by transient hypotension, with up to 26% reduction in mean arterial pressure (MAP) from baseline. Hypotension, but not the increase in colorectal pressure, was inhibited by a 10-min pretreatment with the NK1 receptor antagonist CP-99,994. In a pilot experiment using subcutaneous (SC) injection, a similar dose range of LMN-NKA (3-100 µg/kg) again appeared to increase colorectal pressure with a similar AUC (up to 18,546 mmHg*s) to that seen after IV injection, but lower peak amplitude (up to 49 mmHg). Unlike the effects of IV injection, hypotension was only present after the highest SC dose (100 µg/kg) in one of two animals. Pharmacokinetic analysis revealed markedly lower plasma exposures after SC compared with IV administration. Cmax was 39.6 versus 1070 ng/mL, and AUCinf was 627 versus 2090 ng/mL*min, respectively. These findings are consistent with previous observations in anesthetized dogs and indicate that the prokinetic effects of LMN-NKA may be achieved without hypotension using a route of administration that avoids unnecessarily high plasma exposures.

[Lys5,MeLeu9,Nle10]-NKA(4-10) Elicits NK2 Receptor-Mediated Micturition and Defecation, and NK1 Receptor-Mediated Emesis and Hypotension, in Conscious Dogs.

Tachykinin neurokinin 2 (NK2) receptor agonists may have potential to alleviate clinical conditions associated with bladder and gastrointestinal underactivity by stimulating contraction of visceral smooth muscle. The ability of [Lys5,MeLeu9,Nle10]-neurokinin A(4-10) (LMN-NKA) to elicit micturition and defecation was examined after repeated administration in groups of 2-10 conscious dogs. Administration of 10-100 µg/kg, i.v., four times daily for six consecutive days, reliably elicited micturition after ≥90% of doses and defecation after ≥50% of doses. Voiding occurred <4 minutes after dosing and was short lasting (<10 minutes). LMN-NKA was well tolerated, with emesis after ∼25% of doses at 100 µg/kg, i.v. Hypotension was induced by 100 µg/kg, i.v., of LMN-NKA but not by lower doses. Administration of 30-300 µg/kg, s.c., twice daily for seven consecutive days, reliably elicited both urination and defecation after 88%-100% of doses, and was accompanied by a high rate of emesis (50%-100%). The onset of voiding was rapid (<7 minutes) but was more prolonged than after intravenous administration (30-60 minutes). Emesis induced by 30 or 300 µg/kg, s.c., of LMN-NKA was significantly reduced (from 58% to 8% and from 96% to 54%, respectively) by a 30-minute pretreatment with the neurokinin 1 (NK1) receptor antagonist, (2S,3S)-N-(2-methoxybenzyl)-2-phenylpiperidin-3-amine (CP-99,994; 1 mg/kg, s.c.). The ability of selective NK2 receptor agonists to elicit on-demand voiding could potentially address a major unmet need in people lacking voluntary control of micturition and/or defecation. LMN-NKA unexpectedly activated NK1 receptors at doses that stimulated voiding, causing emesis and hypotension that may limit the clinical utility of nonselective NK2 receptor agonists.

NK2 and NK1 receptor-mediated effects of NKA and analogs on colon, bladder, and arterial pressure in anesthetized dogs.

Tachykinin NK2 receptor (NK2R) agonists have potential to alleviate clinical conditions associated with bladder and gastrointestinal under activity. The effects of agonists with differing selectivity for NK2R over NK1Rs on colorectal, bladder, and cardiovascular function were examined in anesthetized dogs. Intravenous (IV) administration of NKA, LMN-NKA ([Lys5,MeLeu9,Nle10]-NKA(4-10)), and [ß-Ala8]-NKA(4-10) caused a dose-related increase in colorectal pressure (up to 98 mmHg) that was blocked by pretreatment with the NK2R antagonist GR 159897 (1 mg/kg), and hypotension (decrease in mean arterial pressure of ~40 mmHg) that was blocked by the NK1R antagonist CP-99,994 (1 mg/kg). Despite the greater in vitro selectivity of LMN-NKA and [ß-Ala8]-NKA(4-10) for NK2R over NK1Rs compared with NKA, all 3 agonists increased colorectal pressure and caused hypotension within a similar dose range when administered as a bolus (0.1-300 µg/kg IV), or even as a slow IV infusion over 5 min (NKA; 0.02-0.6 µg/kg/min). In contrast, subcutaneous (SC) administration of LMN-NKA (3-10 µg/kg) increased colorectal pressure (up to 50 mmHg) and elicited micturition (≧ 85% voiding efficiency) without causing hypotension. NK2R agonists can produce rapid-onset, short-duration, colorectal contractions, and efficient voiding of urine without hypotension after SC administration, indicating that routes of administration that avoid the high plasma concentrations associated with IV dosing improve the separation between desired and unwanted pharmacodynamic effects. The potent hypotensive effect of NKA in dogs was unexpected based on published studies in humans in which IV infusion of NKA did not affect blood pressure at doses that increased gastrointestinal motility.

Prokinetic effects of neurokinin-2 receptor agonists on the bladder and rectum of rats with acute spinal cord transection.

The suitability of various neurokinin-2 (NK2) receptor agonists and routes of administration to elicit on-demand voiding of the bladder and bowel, as future therapy for individuals with spinal cord injury, was examined using a rat model. The current study examined the feasibility of alternative routes of administration, which are more practical for clinical use than intravenous (IV) administration. Voiding and isovolumetric cystometry were recorded in anesthetized, acutely spinalized, female rats after IV, subcutaneous (SC), intramuscular (IM), intranasal (IN), or sublingual (SL) administration of [Lys5,MeLeu9,Nle10]-NKA(4-10) (LMN-NKA). Administration of LMN-NKA (1-10µg/kg IV; 10-300µg/kg SC or IM; 15-1000µg/kg IN or 300-1500µg/kg SL) elicited rapid-onset, short-duration, dose-related increases in bladder pressure and voiding with the rank order for time of both onset and duration being IV < IN < SC = IM < SL. The incidence of voiding was dependent on the dose and route, with all routes resulting in a high voiding efficiency (~ 70%). Like LMN-NKA, neurokinin A (NKA 1-100µg/kg IV) and GR 64349 (0.1-30µg/kg IV or 1-300µg/kg SC) produced rapid-onset, short-duration increases in bladder pressure, as well as colorectal pressure. Administration of vehicle never produced bladder or rectal contractions or voiding. Transient hypotension was observed after IV injection of LMN-NKA, which was less pronounced after SC injection. Hypotension was not apparent with GR 64349. In conclusion, selective NK2 receptor agonists, administered through various non-IV routes of administration, may provide a safe, convenient, and efficacious method for inducing voiding.

NK1 receptor antagonists for depression: Why a validated concept was abandoned.

BACKGROUND: NK1 receptor antagonists were abandoned despite antidepressant efficacy in five randomized clinical trials. The loss of confidence may be attributed to the failure of a Phase III clinical program with the NK1 receptor antagonist aprepitant in Major Depression. This review examines how PET receptor occupancy was used to select doses for aprepitant and that these may not have achieved adequate exposure. METHODS: PubMed, Google Scholar, and FDA databases were searched for articles concerning NK1 receptor antagonists, human PET receptor occupancy and clinical trials in Major Depression. RESULTS: Antidepressant efficacy was initially demonstrated with three NK1 receptor antagonists, including aprepitant. A nanoparticle formulation of aprepitant was then developed to improve oral bioavailability. In PET studies, doses of 80 and 160mg achieved a high level (~ 90%) of occupancy of NK1 receptors in the human brain and were selected for Phase III. The efficacy of these doses of the nanoparticle formulation may not have been established in depressed patients prior to Phase III, and previous formulations required a dose of 300mg of aprepitant for efficacy. No antidepressant effect of 80 or 160mg of aprepitant was found, and it was concluded that the NK1 antagonist concept was flawed. However, subsequent studies with other compounds showed that a higher level of NK1 receptor occupancy (100%) was required for antidepressant efficacy. LIMITATIONS: Key data concerning the bioequivalence of different formulations of aprepitant have not been published. The importance of NK1 antagonists for pharmacotherapy of depression and other psychiatric disorders has not been established in clinical practice. CONCLUSION: Aprepitant may have failed in Phase III because of an inadequate understanding of the relationship between brain NK1 receptor occupancy and clinical response. A validated and novel mechanistic approach to treat depression has been misperceived as ineffective and abandoned. Caution should be exercised in the appropriate use of PET occupancy data to select doses for drug development programs in neuropsychiatry. The relationship between exposure, receptor occupancy and clinical response should be established. A crisis of confidence has followed the failure of this and other programs in neuropsychiatry, with a far reaching and detrimental impact on pharmaceutical research.

Cyclopentane-based human NK1 antagonists. Part 2: development of potent, orally active, water-soluble derivatives.

The synthesis and optimization of a cyclopentane-based hNK1 antagonist scaffold 3, having four chiral centers, will be discussed in the context of its enhanced water solubility properties relative to the marketed anti-emetic hNK1 antagonist EMEND (Aprepitant). Sub-nanomolar hNK1 binding was achieved and oral activity comparable to Aprepitant in two in vivo models will be described.

Species differences in tachykinin receptor distribution: further evidence that the substance P (NK1) receptor predominates in human brain.

Marked species differences in the distribution of central tachykinin receptors are reported but uncertainty remains about the ability of available ligands to detect NK2 and NK3 receptors in human brain. We compared the distribution of NK1, NK2, and NK3 receptors in sections from rodent, primate, and human brain using the 125I-labeled ligands substance P (SP) for the NK1 receptor, neurokinin A (NKA) for the NK2 receptor, and neurokinin B (NKB) and eledoisin for NK3 receptors. Duration of exposure to autoradiographic film was from 7 days for [125I]SP up to 90 days for the other ligands. High levels of specific [125I]SP binding were seen throughout the brains of all species studied. Specific [125I]NKA binding was detected in brains from neonatal rat, and to a lesser level in adult rat, gerbil, and guinea pig; it was not detected in monkey or human brain, but was present in circular muscle of human duodenum, confirming that this ligand binds to human NK2 receptors under our experimental conditions. Specific [125I]NKB and [125I]eledoisin binding was widespread in brain sections from rats, gerbils, and guinea pigs, and very low levels were also detected in marmoset, squirrel monkey, and rhesus monkey brain after prolonged (up to 90 days) exposure. We failed to identify specific eledoisin binding in human brain, even after prolonged exposures. These findings demonstrate that the NK1 receptor is the predominant tachykinin receptor expressed in primate and human brain, but that low levels of NK3 receptor are present in nonhuman, primate brain.

Chronic psychosocial stress in tree shrews: effect of the substance P (NK1 receptor) antagonist L-760735 and clomipramine on endocrine and behavioral parameters.

RATIONALE: Substance P and its preferred receptor, the neurokinin 1 receptor (NK(1)R), have been proposed as possible targets for new antidepressant therapies, although results of a recently completed phase III trial failed to demonstrate that the NK(1)R antagonist MK-869 is more effective than placebo in the treatment of depression. METHODS: In the present study, we compared the effects of the NK(1)R antagonist L-760735 with the tricyclic antidepressant clomipramine on endocrine and behavioral parameters in chronically stressed tree shrews. Animals were subjected to a 7-day period of psychosocial stress before receiving daily oral administration of L-760735 (10 mg/kg/day) or clomipramine (50 mg/kg/day). The psychosocial stress continued throughout the treatment period of 21 days. Daily morning urine was collected to measure cortisol and norepinephrine levels. All animals were videotaped daily and three types of behavior were analyzed. RESULTS: Chronic psychosocial stress resulted in a significant increase of urinary cortisol and norepinephrine concentrations. Moreover, stressed animals displayed decreased marking behavior and locomotor activity, while grooming remained unaffected. Neither treatment with clomipramine nor L-760735 was able to normalize the stress-induced elevation of cortisol or norepinephrine. On the behavioral parameters, L-760735 had a time-dependent restorative influence on marking behavior close to normal levels, without affecting locomotor activity. Grooming behavior was significantly increased by the 3 weeks of drug treatment. CONCLUSIONS: These results suggest that L-760735 was able to counteract certain stress-induced behavioral alterations in an animal model of depression.

Increased neurogenesis and brain-derived neurotrophic factor in neurokinin-1 receptor gene knockout mice.

It has previously been shown that chronic treatment with antidepressant drugs increases neurogenesis and levels of brain-derived neurotrophic factor in the hippocampus. These changes have been correlated with changes in learning and long-term potentiation and may contribute to the therapeutic efficacy of antidepressant drug treatment. Recently, antagonists at the neurokinin-1 receptor, the preferred receptor for the neuropeptide substance P, have been shown to have antidepressant activity. Mice with disruption of the neurokinin-1 receptor gene are remarkably similar both behaviourally and neurochemically to mice maintained chronically on antidepressant drugs. We demonstrate here that there is a significant elevation of neurogenesis but not cell survival in the hippocampus of neurokinin-1 receptor knockout mice. Neurogenesis can be increased in wild-type but not neurokinin-1 receptor knockout mice by chronic treatment with antidepressant drugs which preferentially target noradrenergic and serotonergic pathways. Hippocampal levels of brain-derived neurotrophic factor are also two-fold higher in neurokinin-1 receptor knockout mice, whereas cortical levels are similar. Finally, we examined hippocampus-dependent learning and memory but found no clear enhancement in neurokinin-1 receptor knockout mice. These data argue against a simple correlation between increased levels of neurogenesis or brain-derived neurotrophic factor and mnemonic processes in the absence of increased cell survival. They support the hypothesis that increased neurogenesis, perhaps accompanied by higher levels of brain-derived neurotrophic factor, may contribute to the efficacy of antidepressant drug therapy.

Neuropeptide systems as novel therapeutic targets for depression and anxiety disorders.

The health burden of stress-related diseases, including depression and anxiety disorders, is rapidly increasing, whereas the range of available pharmacotherapies to treat these disorders is limited and suboptimal with regard to efficacy and tolerability. Recent findings support a major role for neuropeptides in mediating the response to stress and thereby identify neuropeptide systems as potential novel therapeutic targets for the treatment of depression and anxiety disorders. In preclinical models, pharmacological and/or genetic manipulation of substance P, corticotropin-releasing factor (CRF), vasopressin, neuropeptide Y and galanin function alters anxiety- and depression-related responses. Recently, specific and highly potent small-molecule neuropeptide receptor agonists and antagonists have been developed that can readily cross the blood-brain barrier. Clinical assessment of several compounds is currently underway, with antidepressant efficacy confirmed in double-blind, placebo-controlled trials of tachykinin NK(1) (substance P) receptor antagonists, and preliminary evidence of antidepressant activity in an open-label trial of a CRF(1) receptor antagonist.

Comparison of the functional blockade of rat substance P (NK1) receptors by GR205171, RP67580, SR140333 and NKP-608.

Extensive screening of compound libraries was undertaken to identify compounds with high affinity for the rat NK(1) receptor based on inhibition of [(125)I]-substance P binding. RP67580, SR140333, NKP-608 and GR205171 were selected as compounds of interest, with cloned rat NK(1) receptor binding K(i) values of 0.15-1.9 nM. Despite their high binding affinity, NKP-608 and GR205171 exhibited only a moderate functional antagonism of substance P-induced inositol-1-phosphate accumulation and acidification rate at 1 microM using cloned or native rat NK(1) receptors in vitro. The ability of the compounds to penetrate the CNS was determined by inhibition of NK(1) agonist-induced behaviours in gerbils and rats. GR205171 and NKP-608 potently inhibited GR73632-induced foot drumming in gerbils (ID(50) 0.04 and 0.2 mg/kg i.v., respectively). In contrast, RP67580 and SR140333 were poorly brain penetrant in gerbils (no inhibition at 10 mg/kg i.v.) and were not examined further in vivo. In rats, only high doses of GR205171 (10 or 30 mg/kg s.c.) inhibited NK(1) agonist-induced sniffing and hypertension, whilst NKP-608 (1 or 10 mg/kg i.p.) was without effect. GR205171 (3-30 mg/kg s.c.) caused only partial inhibition of separation-induced vocalisations in rat pups, a response that is known to be NK(1) receptor mediated in other species. These observations demonstrate the shortcomings of currently available NK(1) receptor antagonists for rat psychopharmacology assays.

Spirocyclic NK(1) antagonists II: [4.5]-spiroethers.

A series of novel spiroether-based neurokinin-1 (NK(1)) antagonists is described. The effect of modifications to the spiroether ring and aromatic substituents are discussed, leading to the identification of compounds with high affinity and excellent CNS penetration.

Substance P (neurokinin 1) receptor antagonists enhance dorsal raphe neuronal activity.

Substance P receptor [neurokinin 1 (NK1] antagonists (SPAs) represent a novel mechanistic approach to antidepressant therapy with comparable clinical efficacy to selective serotonin reuptake inhibitors (SSRIs). Because SSRIs are thought to exert their therapeutic effects by enhancing central serotonergic function, we have examined whether SPAs regulate neuronal activity in the dorsal raphe nucleus (DRN), the main source of serotonergic projections to the forebrain. Using in vivo electrophysiological techniques in the guinea pig, we found that administration of the highly selective NK1 receptor antagonist 1-(5-[[(2R,3S)-2-([(1R)-1-[3,5-bis(trifluoromethyl)phenyl]ethyl]oxy)-3-(4-phenyl)morpholin-4-yl]methyl]-2H-1,2,3-triazol-4-yl)-N,N-dimethylmethanamine (L-760735) caused an increase in DRN neuronal firing rate. However, unlike chronic treatment with fluoxetine, there was no detectable 5-HT1A autoreceptor desensitization. In vitro electrophysiological investigation showed that these effects were not mediated by a direct action in the DRN, an observation supported by immunocytochemical analysis that identified the lateral habenula (LHb) as a more likely site of action. Subsequently, we found that local application of L-760735 into the LHb increased firing in the DRN, which, together with our data showing that L-760735 increased metabolic activity in the cingulate cortex, amygdala, LHb, and DRN, indicates that the effects of L-760735 may be mediated by disinhibition of forebrain structures acting via a habenulo raphe projection. These findings support other evidence for an antidepressant profile of SPAs and suggest that regulation of DRN neuronal activity may contribute to their antidepressant mechanism of action but in a manner that is distinct from monoamine reuptake inhibitors.

New insights into the antidepressant actions of substance P (NK1 receptor) antagonists.

Considerable progress has been made in understanding the neural circuits involved the antidepressant and anxiolytic efficacy of substance P (NK, receptor) antagonists (SPAs). Progress has been hampered by species differences in the pharmacology of the NK1 receptor, and the availability of NK1R-/- mice has been a particularly useful resource in overcoming this difficulty. Using neuroanatomical, behavioural, and electrophysiological techniques, studies have now established that pharmacological blockade or deletion of the NK1 receptor produces an antidepressant and anxiolytic-like profile in a range of behavioural assays that is distinct from that of established drugs. There is evidence from focal injection studies that some of these effects may be mediated directly by blockade of NK, receptors in the amygdala and its projections to the hypothalamus, periaqueductal gray, and reticulopontine nucleus. Substance P and NK1 receptors are also intimately associated with ascending 5-HT and norepinephrine projections to the forebrain, and alterations in the function of these systems are also likely to be related to the antidepressant efficacy of SPAs. Unlike some established drugs, SPAs are generally well tolerated and do not induce sedation or motor impairment in preclinical species. These findings are consistent with a novel antidepressant mechanism of action of SPAs.