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Antidepressants are widely prescribed in the treatment of depression, although the mechanism of how they exert their therapeutic effects is poorly understood. To shed further light on their mode of action, we have attempted to identify a common proteomic signature in guinea pig brains after chronic treatment with two different antidepressants. Both fluoxetine and the substance P receptor (NK1R) antagonist (SPA) L-000760735 altered cortical expression of multiple heat shock protein 60 forms along with neurofilaments and related proteins that are critical determinants of synaptic structure and function. Analysis of NK1R−/− mice showed similar alterations of neurofilaments confirming the specificity of the effects observed with chronic NK1R antagonist treatment. To determine if these changes were associated with structural modification of synapses, we carried out electron microscopic analysis of cerebral cortices from fluoxetine-treated guinea pigs. This showed an increase in the percentage of synapses with split postsynaptic densities (PSDs), a phenomenon that is characteristic of activity-dependent synaptic rearrangement. These findings suggest that cortical alterations of the neurofilament pathway and increased synaptic remodeling are associated with the mechanism of these two antidepressant drug treatments and may contribute to their psychotherapeutic actions.

We have generated embryonic stem (ES) cells and transgenic mice with green fluorescent protein (GFP) inserted into the Pitx3 locus via homologous recombination. In the central nervous system, Pitx3-directed GFP was visualized in dopaminergic (DA) neurons in the substantia nigra and ventral tegmental area. Live primary DA neurons can be isolated by fluorescence-activated cell sorting from these transgenic mouse embryos. In culture, Pitx3–GFP is coexpressed in a proportion of ES-derived DA neurons. Furthermore, ES cell-derived Pitx3–GFP expressing DA neurons responded to neurotrophic factors and were sensitive to DA-specific neurotoxin N-4-methyl-1, 2, 3, 6-tetrahydropyridine. We anticipate that the Pitx3–GFP ES cells could be used as a powerful model system for functional identification of molecules governing mDA neuron differentiation and for preclinical research including pharmaceutical drug screening and transplantation. The Pitx3 knock-in mice, on the other hand, could be used for purifying primary neurons for molecular studies associated with the midbrain-specific DA phenotype at a level not previously feasible. These mice would also provide a useful tool to study DA fate determination from embryo- or adult-derived neural stem cells.

We used a molecular screen to identify genes upregulated in regenerating adult rat dorsal root ganglion cells. FLRT3 mRNA and protein characterized by a fibronectin type III domain and a leucine-rich repeat motif was upregulated in damaged sensory neurons. The protein was then transported into their peripheral and central processes where the FLRT3 protein was localized to presynaptic axon terminals. In vitro, the FLRT3 protein was expressed at the cell surface, regulated neurite outgrowth in sensory neurons, but did not exhibit homophilic binding. FLRT3 was widely expressed in the developing embryo, particularly in the central nervous system and somites. However, in the adult, we found no evidence for accumulation or reexpression of the FLRT3 protein in damaged axons of the central nervous system. We conclude that FLRT3 codes for a putative cell surface receptor implicated in both the development of the nervous system and in the regeneration of the peripheral nervous system (PNS).

Human African trypanosomiasis, also known as sleeping sickness, affects the CNS at the late stage of the disease. Untreated the disease is invariably fatal, and melarsoprol, the only available and effective treatment for CNS disease, is associated in up to 10% of cases with a severe post‐treatment reactive encephalopathy (PTRE), which can itself cause death. We used a reproducible mouse model of the PTRE to investigate the pathogenesis and treatment of this condition. Mice infected with Trypanosoma brucei brucei and treated subcuratively with diminazene aceturate develop a severe meningoencephalitis that closely resembles PTRE. We previously reported that substance P plays an important role in PTRE. We investigated the effect of disrupting the gene encoding for the NK1 receptor in mice on the clinical and neuroinflammatory response in this model. After induction of PTRE, NK1–/– mice showed a significant reduction in clinical impairment compared with NK1+/+ mice, but the severity of the neuroinflammatory response was significantly greater in NK1–/– mice. To explore the mechanisms of this dissociated phenotype, we treated infected NK1–/– mice with antagonists to NK2 and NK3 receptors, either singly or in combination. While none of these antagonist treatments altered the clinical score, combined treatment with the NK2 and NK3 antagonists significantly reduced the neuroinflammatory grading score in the NK1–/– mice. Thus, the clinical and neuroinflammatory responses to parasite invasion can be mediated by different pathways, and, importantly, the neuroinflammatory response is altered by alternative tachykinin receptor usage. These findings could be exploited to develop novel anti‐inflammatory therapies in Human African trypanosomiasis by modulating the NK1 receptor as well as the parasite.

Mice lacking the neurokinin-1 (NK1) receptor, the preferred receptor for the neuropeptide substance P (SP), do not show many of the behaviors associated with morphine reward. To identify the areas of the brain that might contribute to this effect, we assessed the behavioral effects of ablation of neurons expressing the NK1 receptor in specific regions of the mouse brain using the neurotoxin substance P-saporin. In a preliminary investigation, bilateral ablation of these neurons from the amygdala, but not the nucleus accumbens and dorsomedial caudate putamen, brought about reductions in morphine reward behavior. Subsequently, the effect of ablation of these neurons in the amygdala on anxiety behavior was assessed using the elevated plus maze (EPM), before conditioned place preference (CPP), and locomotor responses to morphine were measured. Loss of NK1 receptor-expressing neurons in the amygdala caused an increase in anxiety-like behavior on the EPM. It also brought about a reduction in morphine CPP scores and the stimulant effect of acute morphine administration relative to saline controls, without affecting CPP to cocaine. NK1 receptor-expressing neurons in the mouse amygdala therefore modulate morphine reward behaviors. These observations mirror those observed in NK1 receptor knock-out (NK1-/-) mice and suggest that the amygdala is an important area for the effects of SP and the NK1 receptor in the motivational properties of opiates, as well as the control of behaviors related to anxiety.

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.

Substance P and neurokinin-1 receptors (NK1) modulate the respiratory activity and are expressed early during development. We tested the hypothesis that NK1 receptors are involved in prenatal development of the respiratory network by comparing the resting respiratory activity and the respiratory response to hypoxia of control mice and mutant mice lacking the NK1 receptor (NK1-/-). In vitro and in vivo experiments were conducted on neonatal, young and adult mice from wild-type and NK1-/- strains. In the wild strain, immunohistological, pharmacological and electrophysiological studies showed that NK1 receptors were expressed within medullary respiratory areas prior to birth and that their activation at birth modulated central respiratory activity and the membrane properties of phrenic motoneurons. Both the membrane properties of phrenic motoneurons and the respiratory activity generated in vitro by brainstem-spinal cord preparation from NK1-/- neonate mice were similar to that from the wild strain. In addition, in vivo ventilation recordings by plethysmography did not reveal interstrain differences in resting breathing parameters. The facilitation of ventilation by short-lasting hypoxia was similar in wild and NK1-/- neonates but was significantly weaker in adult NK1-/- mice. Results demonstrate that NK1 receptors do appear to be necessary for a normal respiratory response to short-lasting hypoxia in the adult. However, NK1 receptors are not obligatory for the prenatal development of the respiratory network, for the production of the rhythm, or for the regulation of breathing by short-lasting hypoxia in neonates.

More than half of all chronic cancer pain arises from metastases to bone, and bone cancer pain is one of the most difficult of all persistent pain states to fully control. Several tumor types including sarcomas and breast, prostate, and lung carcinomas grow in or preferentially metastasize to the skeleton where they proliferate, and induce significant bone remodeling, bone destruction, and cancer pain. Many of these tumors express the isoenzyme cycloxygenase-2 (COX-2), which is involved in the synthesis of prostaglandins. To begin to define the role COX-2 plays in driving bone cancer pain, we used an in vivo model where murine osteolytic 2472 sarcoma cells were injected and confined to the intramedullary space of the femur in male C3HHeJ mice. After tumor implantation, mice develop ongoing and movement-evoked bone cancer pain-related behaviors, extensive tumor-induced bone resorption, infiltration of the marrow space by tumor cells, and stereotypic neurochemical alterations in the spinal cord reflective of a persistent pain state. Thus, after injection of tumor cells, bone destruction is first evident at day 6, and pain-related behaviors are maximal at day 14. A selective COX-2 inhibitor was administered either acutely [NS398; 100 mg/kg, i.p.] on day 14 or chronically in chow {MF. tricyclic; 0.015%, p.o.} from day 6 to day 14 after tumor implantation. Acute administration of a selective COX-2 inhibitor attenuated both ongoing and movement-evoked bone cancer pain, whereas chronic inhibition of COX-2 significantly reduced ongoing and movement-evoked pain behaviors, and reduced tumor burden, osteoclastogenesis, and bone destruction by >50%. The present results suggest that chronic administration of a COX-2 inhibitor blocks prostaglandin synthesis at multiple sites, and may have significant clinical utility in the management of bone cancer and bone cancer pain.

Mice lacking the NK1 receptor, the preferred receptor for substance P, demonstrate normal analgesic responses to morphine on the hot plate assay, but have been predicted, on the basis of conditioned place preference studies, to be insensitive to the rewarding properties of opiates. In this study, self-administration and the development and maintenance of locomotor sensitisation of both morphine and cocaine were investigated in mice that lacked the NK1 gene (NK1 knockout mice, NK1−/−). Both wildtype and NK1−/− mice learned an operant lever-press response to obtain food. When intravenous infusions of morphine (0.2 mg/kg/infusion) were substituted for the food reward, the wildtype mice initially reduced rates of lever pressing, but then increased them on the rewarded lever to obtain approx. 10 infusions per 90 min session; in contrast, NK1−/− mice continued to operate both the rewarded, and non-rewarded levers at low rates. Additionally, NK1−/− mice failed, following repeated administration, to sensitise to the locomotor stimulant effects of morphine (15 mg/kg, i.p.). These deficits were specific to opiates, since NK1−/− mice responding for food or cocaine self-administration (0.65 mg/kg/infusion) did not differ from wildtypes, and they showed normal behavioural sensitisation to repeated cocaine administration (10 mg/kg, i.p.). These results demonstrate that NK1 receptors are critical for the reinforcing properties of morphine, and for adaptive responses elicited by repeated opiate administration. It is postulated that substance P and the NK1 receptor may be necessary for the development of opiate, but not cocaine addiction.

Behavioral analysis of the NK1 receptor gene knock-out (NK1−/−) mouse indicated that substance P was closely involved in orchestrating the physiological and behavioral response of the animal to major environmental stressors. In particular, endogenous pain control mechanisms, such as stress-induced analgesia were substantially impaired in mutant mice, suggesting a reduction in descending inhibitory controls to the spinal cord from the brainstem. To directly test the integrity of descending controls in NK1−/− mice, we have analyzed c-Fos expression in laminae I-II of the lumbar and cervical cord and in the rostral ventromedial medulla in an experimental paradigm known to require recruitment of descending inhibitory controls. Anesthetized mice were stimulated with water at 50°C either on their forepaw, hindpaw, or on both the hindpaw plus forepaw concurrently. Wild-type mice, naïve or treated with an NK1 antagonist (RP67580) or its inactive isomer (RP68651), were compared with NK1−/− mice. C-Fos expression at the lumbar laminae I-II level was significantly reduced, whereas it was significantly greater in the raphe magnus and pallidus nuclei in the double stimulation situation in wild-type compared with NK1−/− mice. Blocking the NK1 receptor pharmacologically reproduced, in an enantiomere-selective manner, the data from NK1−/− mice, with no evidence for recruitment of descending inhibition at the lumbar cord level after forepaw stimulation. The present study demonstrates that the NK1 receptor is essential for the full development of noxiously evoked descending inhibition.

The phenotype of NK1R−/− mice was compared with that of acute pharmacological blockade of the tachykinin NK 1 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 NK 1 receptors in the target species was established by antagonism of the behavioural effects of intracerebroventricular NK 1 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 NK 1 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 NK 1 receptor blockade with L-760735 in guinea-pigs or GR205171 in rats, or deletion of the NK 1 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 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.

The number and immunocytochemical characteristics of trigeminal ganglion neurons providing sensory innervation to the cornea were studied in the mouse. Corneal neurons were retrogradely labelled with fluorogold placed on the cornea after removal of the epithelium with n-heptanol. Corneal neurons were counted, sized and characterized immunocytochemically with antisera against substance P (SP), calcitonin gene-related peptide (CGRP), calbindin, calretinin, and with a monoclonal antibody (RT97) against neurofilament proteins. A total of 258 corneal neurons were counted, most of them located in the ophthalmic division of the trigeminal ganglion. They represent only a small fraction (1.3%) of the population of trigeminal ganglion neurons. More than 70% of corneal neurons were classified as 'small dark' according to their cell body area and the absence of immunoreactivity to RT97. A low percentage of corneal neurons, usually large in size, contained calcium binding proteins. Fifty-eight percent of the corneal neurons were immunoreactive to CGRP, and 20% to SP. Corneal wounding with NaOH, which affects stromal nerve trunk, did not modify the total number of corneal neurons or their neuropeptide content. However, this increased the total number of calbindin-positive and decreased the RT97-positive neurons. Thus, unlike in other nociceptive neurons, peripheral axotomy did not modify the SP/CGRP content of corneal neurons.Trigeminal ganglion neurons projecting to the cornea are similar in size and neuropeptide content to nociceptive neurons of other territories. Their number is high in relation to the corneal surface, thus confirming that the cornea has a large nociceptive representation in the trigeminal ganglion.

The proto-oncogene c-Jun has been implicated in the control of neuronal responses to injury and in axonal growth during regenerative processes. We have investigated the expression of c-Jun during normal terminal remodelling in trigeminal ganglion neurons innervating the cornea and after acute injury of epithelial nerve terminals or parent axons. Remodelling and rearrangement, or damage limited to corneal epithelium endings, was not a trigger for activation of c-Jun expression. However, injury of parent axons in the stroma or in the orbital ciliary nerves induced c-Jun expression in 50% of the population of corneal neurons, which included all of the large myelinated and 20% of the small neuropeptide-containing corneal neurons. This suggests that c-Jun expression in trigeminal ganglion neurons is not associated with normal remodelling or regeneration of peripheral nerve terminals, and that it takes place only when parent axons are injured. A substantial number of damaged neurons do not express c-Jun, indicating that in primary sensory neurons, injury and regeneration may not always be coupled to the expression of this proto-oncogene.

Neurokinin-1 receptors (NK1) are present within the nucleus of the solitary tract, a nucleus which plays a vital role in cardiovascular and respiratory homeostasis. We compared the efficacy of the baroreceptor and pulmonary chemoreflexes between NK1 knockout, heterozygous and wild-type urethane-anaesthetised mice. The magnitude of the baroreceptor reflex mediated bradycardia, induced by a phenylephrine induced pressor response, was significantly greater in NK1 knockout mice (P<0.001) compared to heterozygous and wild-type animals. In comparison, administration of an NK1 antagonist, CP-99,994 (1.5 mg/kg i.v.) to wild-type animals, had no significant effect on baroreceptor reflex performance. In contrast to the baroreceptor reflex, there were no significant differences in the magnitude of the reflex evoked falls in heart rate, arterial pressure, or respiratory depression between the three groups of mice when the pulmonary chemoreflex was evoked with right atrial injections of phenylbiguanide. It is concluded that the baroreceptor reflex pathway over-compensates for the lack of NK1 receptors in knockout mice. Plausible mechanisms accounting for the enhanced baroreceptor reflex responsiveness in NK1 knockout animals are discussed.

The peptide neurotransmitter substance P modulates sensitivity to pain by activating the neurokinin-1 (NK-1) receptor, which is expressed by discrete populations of neurons throughout the central nervous system. Substance P is synthesized by small-diameter sensory 'pain' fibres, and release of the peptide into the dorsal horn of the spinal cord following intense peripheral stimulation promotes central hyperexcitability and increased sensitivity to pain. However, despite the availability of specific NK-1 antagonists, the function of substance P in the perception of pain remains unclear. Here we investigate the effect of disrupting the gene encoding the NK-1 receptor in mice. We found that the mutant mice were healthy and fertile, but the characteristic amplification ('wind up') and intensity coding of nociceptive reflexes was absent. Although substance P did not mediate the signalling of acute pain or hyperalgesia, it was essential for the full development of stress-induced analgesia and for an aggressive response to territorial challenge, demonstrating that the peptide plays an unexpected role in the adaptive response to stress.