The mechanism of central pressor actions of tachykinin NK-3 receptor in the paraventricular nucleus of the hypothalamus in rats

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

Function and pharmacology of spinally-projecting sympathetic pre-autonomic neurones in the paraventricular nucleus of the hypothalamus

The paraventricular nucleus (PVN) of the hypothalamus has been described as the "autonomic master controller". It co-ordinates critical physiological responses through control of the hypothalamic-pituitary-adrenal (HPA)-axis, and by modulation of the sympathetic and parasympathetic branches of the central nervous system. The PVN comprises several anatomical subdivisions, including the parvocellular/ mediocellular subdivision, which contains neurones projecting to the medulla and spinal cord. Consensus indicates that output from spinally-projecting sympathetic pre-autonomic neurones (SPANs) increases blood pressure and heart rate, and dysfunction of these neurones has been directly linked to elevated sympathetic activity during heart failure. The influence of spinally-projecting SPANs on cardiovascular function high-lights their potential as targets for future therapeutic drug development. Recent studies have demonstrated pharmacological control of these spinally-projecting SPANs with glutamate, GABA, nitric oxide, neuroactive steroids and a number of neuropeptides (including angiotensin, substance P, and corticotrophin-releasing factor). The underlying mechanism of control appears to be a state of tonic inhibition by GABA, which is then strengthened or relieved by the action of other modulators. The physiological function of spinally-projecting SPANs has been subject to some debate, and they may be involved in physiological stress responses, blood volume regulation, glucose regulation, thermoregulation and/or circadian rhythms. This review describes the pharmacology of PVN spinally-projecting SPANs and discusses their likely roles in cardiovascular control.

Compensatory mechanisms to maintain blood pressure in paraplegic rats: implication of central tachykinin NK-1 and NK-3 receptors?

People with high level spinal cord injury (SCI) suffer from both hypotension and spontaneous hypertension due to loss of supraspinal control of spinal sympathetic outflow. Few reports have addressed whether any changes occur in central regulation of blood pressure (BP) and heart rat (HR) at the supraspinal level. Central tachykinin NK-1 and NK-3 receptors are located in many cardiovascular areas in the brain and are known to modulate BP and HR. This study examined the intracerebroventricular (i.c.v.) effects of the selective NK-1 receptor agonist [Sar(9), Met(O(2))(11)]SP (65pmol, n=6) and NK-3 receptor agonist senktide (650pmol, n=6) on mean arterial pressure (MAP) and HR before and after complete spinal cord transection at thoracic level 4 (T4). [Sar(9), Met(O(2))(11)]SP evoked increases in MAP and HR which were still present 4days after the T4 SCI. Further analysis using the beta(1)-adrenoceptor antagonist atenolol (10mgkg(-1)) revealed an increased contribution of HR in the MAP increase after SCI. For senktide, 2 and 5weeks after T4 SCI, the rise in MAP induced by senktide was significantly increased in magnitude and was similar to a normal response at 8weeks. These effects were accompanied by a bradycardia, which was still present and amplified at 8weeks. Our results reveal a transient potentiation of the senktide-mediated MAP effect and a greater contribution of the HR in MAP increase by [Sar(9), Met(O(2))(11)]SP in T4 transected rats. Although the significance of these changes remains to be established. This suggest a reorganization of supraspinal mechanisms regulating BP and HR after a high level SCI. Central NK-1 and NK-3 receptors might therefore contribute to the maintenance of MAP following high thoracic SCI.

Substance P targets sympathetic control neurons in the paraventricular nucleus

The paraventricular nucleus (PVN) contains spinally-projecting neurons implicated in fine-tuning the cardiovascular system. In vivo activity of "presympathetic" parvocellular neurons is suppressed by tonic inhibition from GABA-ergic inputs, inhibition of which increases sympathetic pressor activity and heart rate. Targeting of this specific neuronal population could potentially limit elevations of heart rate and blood pressure associated with disease. Here we show, for the first time, that "presympathetic" PVN neurons are disinhibited by the neuropeptide substance P (SP) acting via tachykinin NK1 receptor inhibition of GABA(A) currents. Application of SP to the paraventricular nucleus of rats increases heart rate and blood pressure. In in vitro brain slice experiments, in the presence of GABA, 1 micromol/L SP increased action current frequency by a factor of 2.7+/-0.6 (n=5, P< or =0.05, ANOVA). Furthermore, 1 micromol/L SP inhibited GABA(A) currents by 70+/-8% (n=8, P< or =0.005 paired t test). These effects were abolished by NK1 antagonists, but not NK2 and NK3 antagonists. GABA(A) inhibition was not reproduced by NK2 or NK3 agonists. The inhibition of parvocellular GABA(A) currents by SP was also abolished by a protein kinase C (PKC) inhibitor peptide and mimicked by application of phorbol-12-myristate-13-acetate (PMA), implicating a PKC-dependent mechanism. Single-channel analysis indicates that SP acts through reduction of channel mean open-time (cmot): GABA(A) cmot being reduced by approximately 60% by SP (P< or =0.05 ANOVA, Bonferroni). These data suggest that tachykinins mediate their pressor activity by increasing the excitability of spinally-projecting neurons and identifies NK1 receptors as potential targets for therapeutic modulation of the cardiovascular system.

Activation of paraventricular nucleus neurones by the dorsomedial hypothalamus via a tachykinin pathway in rats

The dorsomedial hypothalamus (DMH) innervates the paraventricular nucleus (PVN) with substance P (SP) immunoreactive neurones. The PVN itself powerfully influences both the neuroendocrine and the cardiovascular systems. In this in vitro study, we examine the DMH-to-PVN pathway electrophysiologically. Glutamate application to the DMH increased action current frequency in the PVN. This effect was prevented by the glutamate antagonist kynurenic acid or by synaptic block with a high-Mg(2)(+) low-Ca(2)(+) buffer solution. Crucially, the selective tachykinin NK1 receptor antagonist L-703606 also inhibited DMH-to-PVN neurotransmission. Thus we show, for the first time, an excitatory connection between the DMH and PVN that uses tachykinin NK1 receptors. This pathway may be important for the hypothalamic control of neuroendocrine and/or cardiovascular function.

Coexpression of dynorphin and neurokinin B immunoreactivity in the rat hypothalamus: Morphologic evidence of interrelated function within the arcuate nucleus

Considerable evidence suggests that dynorphin and neurokinin B (NKB) neurons in the hypothalamic arcuate nucleus participate in the sex-steroid regulation of reproduction. In the present study, we used dual-label immunofluorescence to explore the distribution of prodynorphin and proNKB immunoreactivity in the rat hypothalamus. Additionally, we investigated whether arcuate prodynorphin-ir (immunoreactive) neurons expressed the neurokinin 3 receptor (NK3R) or nuclear estrogen receptor-alpha (ERalpha). We found that the majority of prodynorphin-ir neurons in the rat arcuate nucleus expressed proNKB, whereas nearly all (99%) of the proNKB neurons were immunoreactive for prodynorphin. The arcuate nucleus was the only site in the hypothalamus where neuronal somata coexpressing prodynorphin and proNKB-immunoreactivity were identified. A dense plexus of double-labeled prodynorphin/proNKB-ir fibers was found within the arcuate nucleus extending to the median eminence and throughout the periventricular zone of the hypothalamus. Prodynorphin/proNKB fibers were also identified in the paraventricular nucleus, anterior hypothalamic area, medial preoptic area, median preoptic nucleus, anteroventral periventricular nucleus, and bed nucleus of the stria terminalis in a distribution consistent with previously described arcuate nucleus projections. Interestingly, the majority of prodynorphin-ir neurons in the arcuate nucleus expressed NK3R, and nearly 100% of the prodynorphin-ir neurons contained nuclear ERalpha. Our results suggest that there is a close functional relationship between dynorphin and NKB peptides within the arcuate nucleus of the rat, which may include an autofeedback loop mediated through NK3R. The diverse hypothalamic projections of fibers expressing both prodynorphin and proNKB provide evidence that these neurons may participate in a variety of homeostatic and neuroendocrine processes.

Central injection of senktide, an NK3 receptor agonist, or neuropeptide Y inhibits LH secretion and induces different patterns of Fos expression in the rat hypothalamus

Arcuate neurokinin B (NKB) neurons express estrogen receptor-alpha and are strongly modulated by gonadal steroids. Although numerous studies suggest that NKB neurons participate in the reproductive axis, there is no information on the regulation of luteinizing hormone (LH) secretion by NKB or its receptor, NK3. In the present study, we determined if central injection of senktide, a selective NK3 receptor agonist, would alter serum LH in ovariectomized, estrogen-primed rats. The effects of senktide were compared to neuropeptide Y (NPY), a well-characterized modulator of LH secretion. Saline, senktide, or NPY was injected into the lateral ventricle of unanesthetized rats and serial blood samples were collected for LH radioimmunoassay. The rats were sacrificed 90 min after injection and the brains were removed and processed for Fos immunocytochemistry. A significant inhibition of serum LH was observed from 30 to 90 min after injection of senktide relative to saline controls. In the senktide-injected rats, the inhibition of serum LH was accompanied by increased Fos expression in the medial preoptic area and arcuate nucleus--two reproductive control centers. Senktide also induced Fos in the paraventricular nuclei (PVN) and supraoptic nuclei (SON). Injection of NPY also inhibited serum LH but increased Fos expression only in the PVN and SON. This study provides the first demonstration of alterations in LH secretion by an NK3 receptor agonist. These data, combined with the induction of Fos in medial preoptic and arcuate neurons, strongly support the hypothesis that NKB neurons play a role in the regulation of gonadotropin secretion.

Central neurokinin 3 receptors increase systemic oxytocin release: interaction with norepinephrine

Stimulation of central tachykinin receptors contributes to neuroendocrine functions of the hypothalamo-neurohypophyseal system. However, the specific role of each tachykinin receptor subtype has not been completely characterized. Specifically, while neurokinin 3 (NK3) receptor stimulation increases systemic vasopressin, the effects on oxytocin (OT) are not known. Therefore, the present studies investigated the effect of central NK3 receptor stimulation with senktide on release of systemic and central OT. Furthermore, since central NK3 receptors activate noradrenergic systems, which contribute to OT release, the effects of alpha-adrenergic receptor blockade on senktide-induced changes in OT release were evaluated. Female rats were implanted with a cannula in the third cerebral ventricle, and changes in plasma OT concentration determined before and following central administration of senktide in vehicle-treated rats, and animals following central administration of the alpha-adrenergic antagonist phentolamine. Other rats were implanted with microdialysis probes adjacent to the paraventricular nucleus (PVN), and dialysate and plasma OT concentrations were determined before and during administration of senktide through the dialysis probe. Central senktide increased systemic OT release, which was prevented by pretreatment with phentolamine. Furthermore, there was no detectable change in extracellular OT concentration in the PVN during dialysis administration of senktide. These data demonstrate that activation of central NK3 receptors stimulates systemic release of OT by activation of central noradrenergic systems, apparently without increasing intranuclear OT release in the PVN.

Distribution of Fos-like immunoreactivity within the rat brain following intraventricular injection of the selective NK(3) receptor agonist senktide

Neurokinin B (NKB) is one member of an evolutionarily conserved family of neuropeptides, the tachykinins. Preferential binding of NKB to endogenous NK(3) receptors affects a variety of biological and physiological processes, including endocrine secretions, sensory transmission, and fluid and electrolyte homeostasis. In light of its widespread biological actions, immunohistochemical detection of the c-Fos protein product was used to study the distribution of neuronal activation in the rat brain caused by intraventricular (icv) injections of the selective NK(3) receptor agonist (succinyl-[Asp(6), N-Me-Phe(8)] substance P [6-11]), senktide. Quantitative analysis revealed that treatment with isotonic saline or 200 ng senktide resulted in the differential expression of Fos-like immunoreactivity (FLI) throughout the brain. Senktide induced the highest number of FLI neurons in the lateral septum, bed nucleus of the stria terminalis, amygdala, paraventricular nucleus of the hypothalamus, median preoptic nucleus, organum vasculosum of the lamina terminalis, supraoptic nucleus, periaqueductal gray, and medial nucleus of the solitary tract compared to isotonic saline controls. Additional regions that contained elevated FLI following icv injection of senktide, relative to saline injection, included the cerebral cortex, lateral hypothalamic nucleus, suprachiasmatic nucleus, ventral tegmental area, substantia nigra, inferior colliculus, locus coeruleus, zona incerta, and arcuate nucleus. Our data indicate that activation of NK(3) receptors induces the expression of FLI within circumscribed regions of the rat brain. This pattern of neuronal activation overlaps with nuclei known to regulate homeostatic processes, such as endocrine secretion, cardiovascular function, salt intake, and nociception.

Neurokinin-3 receptor distribution in rat and human brain: an immunohistochemical study

Autoradiographic and immunohistochemical studies have shown that the neurokinin-3 receptor is widely distributed in the rodent CNS. Expression of the neurokinin-3 receptor in human brain, however, has been debated. These conflicting findings, as well as the poor resolution of autoradiographic images, prompted us to develop a polyclonal antibody against an oligopeptide derived from the carboxy-terminus consensus sequence of both the rat and human neurokinin-3 receptor ([C]ASTTSSFISSPYTSVDEYS, amino acids 434-452 of the rat neurokinin-3 receptor). Western blot analysis of both human and rat brain tissue revealed a major band in the molecular weight range 65,000-67,000, the proposed molecular weight of the neurokinin-3 receptor based on its amino acid sequence and presumed glycosylation state. The distribution of selective high affinity neurokinin-3 receptor agonist [3H]senktide binding and neurokinin-3 receptor immunoreactivity were virtually identical in the brains of male Fischer 344 rats. The highest concentrations of neurokinin-3 receptors were observed in cortical layers IV-V; the basolateral amygdaloid nucleus; the hypothalamic paraventricular, perifornical and supraoptic nuclei; the zona incerta; and the entopeduncular and interpeduncular nuclei. [3H]senktide binding and neurokinin-3 receptor immunoreactivity were compared in homologous cortical areas of the human and rat brain. In contrast to the rat, autoradiographic analysis of normal control human brains (35-75 years) revealed a distinct and predominant superficial cortical labeling in the glia limitans and the cortical layer I. However, neurokinin-3 receptor immunoreactivity could be found not only in the superficial cortical layers, but also on pyramidal neurons and astrocytes in the neuropil and white matter. These findings suggest species differences in both the cellular and anatomical distribution of the neurokinin-3 receptor.

Age and species-dependent differences in the neurokinin B system in rat and human brain

Neurokinin B and its cognate neurokinin-3 receptor are expressed more in the forebrain than in brain stem structures but little is known about the primary function of this peptide system in the central processing of information. In general, few studies have specifically addressed age-related changes of tachykinins, notably the changes in number and/or distribution of the neurokinin B-expressing and neurokinin-3 receptor-bearing neurons. Data on functions and changes of neurokinins in physiological aging are limited and apply mainly to the substance P/neurokinin-1 receptor system. In the present study, we analyzed neurokinin B/neurokinin-3 receptor system in young (5 months) versus middle aged (15 months) and old rats (23-25 months) and also in aging human brains. For the majority of the immunohistochemically examined regions of the rat brain, there was no statistically significant change in neuronal number and size of the neurokinin B and neurokinin-3 receptor staining. In the adult human brain, there was no age-associated change of the number or size of neurokinin-B-positive neurons. However, we found a major decline in number of neurokinin-3 receptor-expressing neurons between young/middle aged (30 years to 69 years) versus old (70 years and older) adults. Interestingly, numbers of neurokinin-3 receptor-positive microglia increased whereas the neurokinin-3 receptor-positive astrocytes remained unchanged in both aging rat and human brains. Finally, in addition to assessing the morphological and quantitative changes of the neurokinin B/neurokinin-3 receptor system in the rat and human brain, we discuss functional implications of the observed interspecies differences.

Renal effects of intracerebroventricularly injected tachykinins in the conscious saline-loaded rat: receptor characterization

1. The effects of intracerebroventricularly (i.c.v.) injected substance P (SP), neurokinin A (NKA) and [MePhe7]neurokinin B (NKB) were investigated on renal excretion of water, sodium and potassium in the conscious saline-loaded rat. The central effects of [MePhe7]NKB were characterized with selective tachykinin antagonists for NK1 (RP 67580), NK2 (SR 48968) and NK3 (R 820) receptors. 2. Whereas SP or NKA (65 or 650 pmol) failed to modify the renal responses, [MePhe7]NKB (65-6500 pmol) produced dose-dependent and long-lasting (30-45 min) decreases in renal excretion of water (maximal reduction at 65 pmol: from 66.14 +/- 7.62 to 21.07 +/- 3.79 microliters min-1), sodium (maximal reduction at 65 pmol: from 10.19 +/- 2.0 to 1.75 +/- 0.48 mumol min-1) and potassium (maximal reduction at 65 pmol: from 4.31 +/- 1.38 to 0.71 +/- 0.27 mumol min-1). While 650 pmol [MePhe7]NKB elevated urinary osmolality, neither 65 pmol nor 6.5 nmol [MePhe7]NKB altered this parameter. 3. Both the antidiuresis and antinatriuresis induced by [MePhe7]NKB (65 pmol) were significantly blocked by the prior i.c.v. injection of R 820 (1.3 nmol, 5 min earlier), although the potassium excretion was only partially reduced. However, R 820 did not affect the antidiuresis and antinatriuresis elicited by endothelin-1 (1 pmol, i.c.v.). On its own, R 820 decreased renal potassium excretion with no effect on urinary osmolality and renal excretion of water and sodium. The i.c.v. co-injection of RP 67580 and SR 48968 (6.5 nmol each, 5 min earlier) failed to modify the renal responses to [MePhe7]NKB in a similar study. 4. The central effects of [MePhe7]NKB (65 pmol) on renal excretion were blocked by the prior i.v. administration of a linear peptide vasopressin V2 receptor antagonist (50 micrograms kg-1, 5 min earlier). 5. These results suggest that the central NK3 receptor, probably located in the hypothalamus, is implicated in the renal control of water and electrolyte homeostasis through the release of vasopressin in the conscious saline-loaded rat.

Intracerebroventricular responses to neuropeptide gamma in the conscious rat: characterization of its receptor with selective antagonists

1. The cardiovascular and behavioural effects elicited by the intracerebroventricular (i.c.v.) administration of neuropeptide gamma (NP gamma) in the conscious rat were assessed before and 5 min after i.c.v. pretreatment with antagonists selective for NK1 (RP 67,580), NK2 (SR 48,968) and NK3 (R 820) receptors. In addition, the central effects of NP gamma before and after desensitization of the NK1 and NK2 receptors with high doses of substance P (SP) and neurokinin A (NKA) were compared. 2. Intracerebroventricular injection of NP gamma (10-780 pmol) evoked dose- and time-dependent increases in mean arterial blood pressure (MAP), heart rate (HR), face washing, head scratching, grooming and wet-dog shake behaviours. Similar injection of vehicle or 1 pmol NP gamma had no significant effect on those parameters. 3. The cardiovascular and behavioural responses elicited by NP gamma (25 pmol) were significantly and dose-dependently reduced by pretreatment with 650 pmol and 6.5 nmol of SR 48,968. No inhibition of NP gamma responses was observed when 6.5 nmol of RP 67,580 was used in a similar study. Moreover, the prior co-administration of SR 48,968 (6.5 nmol) and RP 67,580 (6.5 nmol) with or without R 820 (6.5 nmol) did not reduce further the central effects of NP gamma and significant residual responses (30-50%) remained. 4. No tachyphylaxis to NP gamma-induced cardiovascular and behavioural changes was observed when two consecutive injections of 25 pmol NP gamma were given 24 h apart. 5. Simultaneous NK1 and NK2 receptor desensitization reduced significantly central effects mediated by 25 pmol NP gamma. However, significant residual responses persisted as seen after pretreatment with SR 48,968. 6. The results suggest that the central effects of NP gamma are mediated partly by NK2 receptors and by another putative tachykinin receptor subtype (NP gamma receptor?) that appears to be different from NK1 and NK3 receptors.

Characterization of the tachykinin receptors involved in spinal and supraspinal cardiovascular regulation

The pharmacological characterization of the tachykinin receptors involved in spinal and supraspinal cardiovascular regulation is reviewed in this report. In conscious rats, substance P (SP), neurokinin A (NKA), neurokinin B (NKB), neuropeptide K (NPK), and neuropeptide gamma (NP gamma) were injected either intrathecally (i.t.) or intracerebroventricularly (i.c.v.), and their effects were assessed on mean arterial blood pressure (MAP) and heart rate (HR). Moreover, selective antagonists for NK1 ((+/-)-CP-96045 and RP-67580), NK2 (SR-48968), and NK3 (R-486) receptors were tested against the agonists. I.t. tachykinins elicited dose-dependent increases in MAP and HR (NPK > NP gamma > SP > NKA > NKB). The cardiovascular response to i.t. SP, NPK, and NP gamma was significantly attenuated by the prior i.t. administration of (+/-)-CP-96345 and RP-67580 but not by SR-48968 and R-486. By the i.c.v. route, tachykinins also elicited pressor and tachycardiac responses dose dependently (NPK > NP gamma > SP > NKA > NKB). Senktide and [MePhe7]NKB, two NK3-selective agonists, were slightly more potent than NKB on both parameters. Whereas the cardiovascular response to NPK was largely blocked by (+/-)-CP-96345 and RP-67580, that to SP was reduced by 40-50%. This treatment had no effect on the cardiovascular response to NKA and [MePhe7]NKB. Conversely, SR-48968 reduced by 40-50% the NKA-induced cardiovascular changes without affecting the central mediated effects of NPK, SP, and [MePhe7]NKB. However, when coadministered, RP-67580 and SR-48968 abolished the effects to SP and NKA while leaving untouched those induced by [MePhe7]NKB. Finally, the central effects mediated by [MePhe7]NKB, senktide, and NKB were blocked by R-486. These findings suggest that the i.t. action of tachykinins on the rat cardiovascular system is mediated by a NK1 receptor in the spinal cord, while NK1, NK2, and NK3 receptors are likely involved in the supraspinal (hypothalamus) effects of these neuropeptides. It is also concluded that NPK is a pure and powerful NK1 agonist, in contrast to SP and NKA, which are not selective for NK1 and NK2 receptors, respectively.

Cardiovascular and behavioural effects of centrally administered tachykinins in the rat: characterization of receptors with selective antagonists

1. The effects of intracerebroventricular (i.c.v.) injection of selective and potent NK1 (RP 67580), NK2 (SR 48968) and NK3 (R 486, [Trp7, beta-Ala8]NKA(4-10)) receptor antagonists were assessed on the cardiovascular and behavioural responses elicited by the i.c.v. injection of substance P (SP), neurokinin A (NKA) or [MePhe7]neurokinin B ([MePhe7]NKB) in the conscious freely moving rat. 2. SP, NKA and [MePhe7]NKB (5-650 pmol) evoked dose-dependent increases in mean arterial blood pressure (MAP) and heart rate (HR) with the rank order of potency SP > NKA > [MePhe7]NKB. The cardiovascular responses were accompanied by excessive face washing, grooming and wet dog shakes. 3. The cardiovascular effects and face washing behaviour induced by SP (25 pmol) were significantly reduced by the pre-injection (i.c.v., 5 min earlier) of RP 67580 (6.5 nmol). However, this antagonist failed to affect the central effects of 25 pmol NKA or [MePhe7]NKB. 4. The cardiovascular and behavioural responses (except for wet dog shakes) elicited by NKA (25 pmol) were significantly reduced by 6.5 nmol SR 48968. However, the latter antagonist had no effect on the SP or [MePhe7]NKB-mediated responses. 5. Both cardiovascular and behavioural effects produced by either SP or NKA (25 pmol) were completely abolished when rats were pretreated with a combination of RP 67580 (6.5 nmol) and SR 48968 (6.5 nmol), yet this combination of antagonists failed to modify the central effects of [MePhe7]NKB. 6. R 486 (6.5 nmol) inhibited the cardiovascular effects as well as wet dog shakes produced by [MePhe7]NKB, but it was inactive against the responses induced by either SP or NKA. 7. None of the tachykinin receptor antagonists or agonists caused motor impairment or respiratory distress. All antagonists blocked in a reversible manner and were devoid of intrinsic activity except R486 (6.5 nmol) which produced a transient increase of MAP and HR.8. These results suggest that the central effects of SP, NKA and [MePhe7]NKB are primarily mediated by central NK1, NK2 and NK3 receptors, respectively. However, a minor activation of NK2 receptors bySP and NK1 receptors by NKA was seen during blockade of both receptors. This study therefore supports the existence of functional NK1, NK2 and NK3 receptors in the adult rat brain.