5-hydroxytryptamine (5-HT)1A autoreceptor adaptive changes in substance P (neurokinin 1) receptor knock-out mice mimic antidepressant-induced desensitization

Antagonists at substance P receptors of the neurokinin 1 (NK1) type have been shown to represent a novel class of antidepressant drugs, with comparable clinical efficacy to the selective serotonin (5-HT) reuptake inhibitors (SSRIs). Because 5-HT 1A receptors may be critically involved in the mechanisms of action of SSRIs, we examined whether these receptors could also be affected in a model of whole-life blockade of NK1 receptors, i.e. knock-out mice lacking the latter receptors (NK1Ϫ/Ϫ). 5-HT 1A receptor labeling by the selective antagonist radioligand receptor agonist ipsapirone to inhibit the discharge of serotoninergic neurons in the dorsal raphe nucleus within brainstem slices, and reduced hypothermic response to 8-OH-DPAT, were noted in NK1Ϫ/Ϫ versus NK1ϩ/ϩ mice. On the other hand, cortical 5-HT overflow caused by systemic injection of the SSRI paroxetine was four-to sixfold higher in freely moving NK1Ϫ/Ϫ mutants than in wild-type NK1ϩ/ϩ mice. Accordingly, the constitutive lack of NK1 receptors appears to be associated with a downregulation/functional desensitization of 5-HT 1A autoreceptors resembling that induced by chronic treatment with SSRI antidepressants. Double immunocytochemical labeling experiments suggest that such a heteroregulation of 5-HT 1A autoreceptors in NK1Ϫ/Ϫ mutants does not reflect the existence of direct NK1-5-HT 1A receptor interactions in normal mice

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.This research was supported by the Wellcome Trust, a European Community Marie Curie fellowship to S.M. and a Boehringer Ingelheim Fonds Predoctoral Fellowship to M.P. C.A.G. is on the Wellcome Trust Four-year PhD in Neuroscience at UCL.Peer reviewe

Pharmacological blockade or genetic deletion of substance P (NK1) receptors attenuates neonatal vocalisation in guinea-pigs and mice

The regulation of stress-induced vocalisations by central NK1 receptors was investigated using pharmacological antagonists in guinea-pigs, a species with human-like NK1 receptors, and transgenic NK1R−/− mice. In guinea-pigs, i.c.v. infusion of the selective substance P agonist GR73632 (0.1 nmol) elicited a pronounced vocalisation response that was blocked enantioselectively by the NK1 receptor antagonists CP-99,994 and L-733,060 (0.1–10 mg/kg). GR73632-induced vocalisations were also markedly attenuated by the antidepressant drugs imipramine and fluoxetine (30 mg/kg), but not by the benzodiazepine anxiolytic diazepam (3 mg/kg) or the 5-HT1A agonist buspirone (10 mg/kg). Similarly, vocalisations in guinea-pig pups separated from their mothers were blocked enantioselectively by the highly brain-penetrant NK1 receptor antagonists L-733,060 and GR205171 (ID50 3 mg/kg), but not by the poorly brain-penetrant compounds LY303870 and CGP49823 (30 mg/kg). Separation-induced vocalisations were also blocked by the anxiolytic drugs diazepam, chlordiazepoxide and buspirone (ID50 0.5–1 mg/kg), and by the antidepressant drugs phenelzine, imipramine, fluoxetine and venlafaxine (ID50 3–8 mg/kg). In normal mouse pups, GR205171 attenuated neonatal vocalisations when administered at a high dose (30 mg/kg) only, consistent with its lower affinity for the rat than the guinea-pig NK1 receptor. Ultrasound calls in NK1R−/− mouse pups were markedly reduced compared with those in WT pups, confirming the specific involvement of NK1 receptors in the regulation of vocalisation. These observations suggest that centrally-acting NK1 receptor antagonists may have clinical utility in the treatment of a range of anxiety and mood disorders.Peer reviewe

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, [β-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)

5-Hydroxytryptamine (5-HT)1A autoreceptor adaptive changes in substance P (neurokinin 1) receptor knock-out mice mimic antidepressant-induced desensitization

Antagonists at substance P receptors of the neurokinin 1 (NK1) type have been shown to represent a novel class of antidepressant drugs, with comparable clinical efficacy to the selective serotonin (5-HT) reuptake inhibitors (SSRIs). Because 5-HT1A receptors may be critically involved in the mechanisms of action of SSRIs, we examined whether these receptors could also be affected in a model of whole-life blockade of NK1 receptors, i.e. knock-out mice lacking the latter receptors (NK1 -/-). 5-HT1A receptor labeling by the selective antagonist radioligand [3H]N-[2-[4-(2-methoxyphenyl)1-piperazinyl]-ethyl]-N- (2-pyridinyl)-cyclohexanecarboxamide (WAY 100635) and 5-HT1A-dependent [35S] GTP-γ-S binding at the level of the dorsal raphe nucleus (DRN) in brain sections, as well as the concentration of 5-HT1A mRNA in the anterior raphe area were significantly reduced (-19 to -46%) in NK1-/- compared with NK1+/+ mice. Furthermore, a ∼10-fold decrease in the potency of the 5-HT1A receptor agonist ipsapirone to inhibit the discharge of serotoninergic neurons in the dorsal raphe nucleus within brainstem slices, and reduced hypothermic response to 8-OH-DPAT, were noted in NK1 -/- versus NK1 +/+ mice. On the other hand, cortical 5-HT overflow caused by systemic injection of the SSRI paroxetine was four- to sixfold higher in freely moving NK1 -/- mutants than in wild-type NK1 +/+ mice. Accordingly, the constitutive lack of NK1 receptors appears to be associated with a downregulation/functional desensitization of 5-HT1A autoreceptors resembling that induced by chronic treatment with SSRI antidepressants. Double immunocytochemical labeling experiments suggest that such a heteroregulation of 5-HT1A autoreceptors in NK1 -/- mutants does not reflect the existence of direct NK1-5-HT1A receptor interactions in normal mice.Peer Reviewe