1-Phenyl-8-azabicyclo[3.2.1]octane ethers are NK(1) receptor antagonists. Substitution at the 6-exo-position led to high affinity NK(1) antagonists with a prolonged duration of action in vivo. Incorporation of an alpha-methyl substituent in the pendent benzyl ether side chain gave compounds with increased selectivity over the hERG channel.

A series of 8-azabicyclo[3.2.1]octane amine hNK1 antagonists has been investigated and structure-activity relationships of the benzylamine and 6-exo substituents described. Acidic substituents at C6 give a series of high affinity compounds for hNK1 with selectivity over the hERG channel.

A new class of high affinity hNK1R antagonists based on seven-membered ring cores has been identified. This series, with relatively simple, compact structures, includes compounds with high affinity, good selectivity, and promising in vivo properties.

The mechanism of action of conventional antidepressants (e.g. imipramine) has been linked to modulation of central monoamine systems. Substance P (NK1) receptor antagonists may have antidepressant and anxiolytic effects in patients with major depressive disorder and high anxiety but, unlike conventional antidepressants, are independent of activity at monoamine reuptake sites, transporters, receptors, or monoamine oxidase. To investigate the possibility that substance P receptor antagonists influence central monoamine systems indirectly, we have compared the effects of chronic administration of imipramine with that of the substance P receptor antagonist L-760735 on the spontaneous firing activity of locus coeruleus neurones. Electrophysiological recordings were made from brain slices prepared from guinea-pigs that had been dosed orally every day for 4 weeks with either L-760735 (3 mg/kg), imipramine (10 mg/kg), or vehicle (water), or naive animals. Chronic, but not acute, treatment with the Substance P receptor antagonist L-760735. induced burst firing of neurones in the locus coeruleus. This effect resembles that of the conventional antidepressant imipramine. However, their effects are dissociable since. in contrast to chronic imipramine treatment, chronic L-760735 treatment does not cause functional desensitisation of somatic alpha(2) adrenoceptors.The mechanism by which chronic substance P receptor antagonist or conventional antidepressant treatment influences the pattern of firing activity of norepinephrine neurotics remains to be elucidated. However, an indirect action in the periphery or distant brain nuclei has been excluded by the use of the in vitro slice preparation, suggesting a local site of action in the locus coeruleus. (C) 2002 IBRO. Published by Elsevier Science Ltd. All rights reserved.

The phenotype of NK1R-/- mice was compared with that of acute pharmacological blockade of the tachykinin NK1 receptor on sensorimotor function and in assays relevant to depressive illness and anxiety. The dose range for L-760735 and GR205171 that was associated with functional blockade of central NK1 receptors in the target species was established by antagonism of the behavioural effects of intracerebroventricular NK1 agonist challenge in gerbils, mice and rats. The caudal grooming and scratching response to GR73632 was absent in NK1R-/- mice, confirming that the receptor had been genetically ablated. There was no evidence of sedation or motor impairment in NK1R-/- mice or following administration of L-760735 to gerbils, even at doses in excess of those required for central NK1 receptor occupancy. In the resident-intruder and forced swim test, the behaviour of NK1R-/- mice, or animals treated acutely with L-760735 or GR205171, resembled that seen with the clinically used antidepressant drug fluoxetine. However, the effects of GR205171 were not clearly enantioselective in mice. In contrast, although NK1R-/- mice also exhibited an increase in the duration of struggle behaviour in the tail suspension test, this was not observed following pharmacological blockade with L-760735 in gerbils or GR205171 in mice, suggesting that this may reflect a developmental alteration in the knockout mouse. There was no effect of NK1 receptor blockade with L-760735 in guinea-pigs or GR205171 in rats, or deletion of the NK1 receptor in mice, on behaviour in the elevated plus-maze test for anxiolytic activity. These findings extend previous observations on the phenotype of the NK1R-/- mouse and establish a broadly similar profile following acute pharmacological blockade of the receptor. These studies also serve to underscore the limitations of currently available antagonists that are suitable for use in rat and mouse behavioural assays.

The regulation of stress-induced vocalisations by central NK(1) receptors was investigated using pharmacological antagonists in guinea-pigs, a species with human-like NK(1) 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 NK(1) 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-HT(1A) agonist buspirone (10 mg/kg). Similarly, vocalisations in guinea-pig pups separated from their mothers were blocked enantioselectively by the highly brain-penetrant NK(1) receptor antagonists L-733,060 and GR205171 (ID(50) 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 (ID(50) 0.5-1 mg/kg), and by the antidepressant drugs phenelzine, imipramine, fluoxetine and venlafaxine (ID(50) 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 NK(1) receptor. Ultrasound calls in NK1R-/- mouse pups were markedly reduced compared with those in WT pups, confirming the specific involvement of NK(1) receptors in the regulation of vocalisation. These observations suggest that centrally-acting NK(1) receptor antagonists may have clinical utility in the treatment of a range of anxiety and mood disorders.

The distribution of NK(1) receptor immunoreactivity in the amygdaloid complex, induction of NK(1) receptor endocytosis in the amygdala following immobilisation stress, and the ability of pretreatment with the substance P antagonist L-760,735 or imipramine to block this response were examined in gerbils, a species with human-like NK(1) receptor pharmacology. Highest levels of immunolabelling were observed in the anterior, amygdalo-hippocampal and medial nuclei. Less dense labelling was observed in the basolateral nucleus, where it was possible to clearly visualise the distal dendrites of NK(1) immunoreactive neurones and quantify the effect of immobilisation stress on NK(1) receptor endocytosis morphology, a marker of local substance P release. Immobilisation for 1 h caused an approximately 60% increase in the number of dendritic processes undergoing NK(1) receptor endocytosis in the basolateral amygdala that was inhibited by acute pretreatment of animals with L-760,735 (3 mg/kg), but not by imipramine (10 mg/kg). These findings are consistent with other evidence that the amygdala represents a possible site of action for the antidepressant and anxiolytic efficacy of substance P antagonists.

The distribution of NK1 receptor immunoreactivity in the amygdaloid complex, induction of NK1 receptor endocytosis in the amygdala following immobilisation stress, and the ability of pretreatment with the substance P antagonist L-760,735 or imipramine to block this response were examined in gerbils, a species with human-like NK1 receptor pharmacology. Highest levels of immunolabelling were observed in the anterior, amygdalo-hippocampal and medial nuclei. Less dense labelling was observed in the basolateral nucleus, where it was possible to clearly visualise the distal dendrites of NK1 immunoreactive neurones and quantify the effect of immobilisation stress on NK1 receptor endocytosis morphology, a marker of local substance P release. Immobilisation for 1 h caused an approximately 60% increase in the number of dendritic processes undergoing NK1 receptor endocytosis in the basolateral amygdala that was inhibited by acute pretreatment of animals with L-760,735 (3 mg/kg), but not by imipramine (10 mg/kg). These findings are consistent with other evidence that the amygdala represents a possible site of action for the antidepressant and anxiolytic efficacy of substance P antagonists. (C) 1999 Elsevier Science B.V. All rights reserved.

Chronic inflammatory conditions produce a state of hyperalgesia which is evident from a few hours to days after administration of an inflammatory stimulus. The molecular mechanisms involved in the inflation of hyperalgesia are not well understood and in this study we have investigated the role of prostaglandins in this process in the rat. Unilateral intraplantar injection of Freund's complete adjuvant produces an immediate localized swelling (oedema) with the development of altered pain responses in the ipsilateral paw such as a reduced threshold to noxious stimuli (hyperalgesia) and lowered thresholds such that normally innocuous stimuli produce a pain response (allodynia). We have monitored levels of cyclooxygenase messenger RNA and prostaglandins in lumbar spinal cord in parallel with these behavioural responses (oedema, hyperalgesia and allodynia) and identified a marked increase in cyclooxygenase-2 messenger RNA (3-fold), maximal at 2–4h after Freund's complete adjuvant, followed by a significant increase in 6-keto prostaglandin F1α and prostaglandin E2 which is maximal by 8h. Pretreatment of animals with the unselective cyclooxygenase inhibitor indomethacin attenuated oedema (approximately 40%) and allodynia (80–100%), but had no effect on the development of mechanical hyperalgesia. Pretreatment with the cyclooxygenase-2 selective inhibitors DuP 697, flosulide and SC58125 also attenuated allodynia (by 80–100%) but had no effect on the development of oedema or mechanical hyperalgesia.The marked increase in cyclooxygenase-2 messenger RNA in the lumbar spinal cord following intraplantar Freund's complete adjuvant suggests that the cyclooxygenase enzyme and its product may have a role in the adaptive response that occurs in the lumbar spinal cord during a peripheral inflammatory reaction. Pharmacological analysis reveals that prostaglandins are directly involved in the development of allodynia. However, these studies show that the development of mechanical hyperalgesia does not require the production of prostaglandins indicating that more than one pathway mediates the altered pain responses associated with a peripheral inflammatory lesion.