SR142801 behaves as a tachykinin NK-3 receptor agonist on a spinal nociceptive reflex in the rat

Tacr3/NK3R: Beyond Their Roles in Reproduction

The Tacr3 gene encodes tachykinin receptor 3 (NK3R), which belongs to the tachykinin receptor family. This family of proteins includes typical G protein-coupled receptors and belongs to the rhodopsin subfamily. NK3R functions by binding to its high-affinity ligand, neurokinin B(NKB). The role of Tacr3/NK3R in growth and reproduction has been extensively studied, but Tacr3/NK3R is also widely expressed in the nervous system from the spinal cord to the brain and is involved in both physiological and pathological processes in the nervous system, including mood disorders, chronic pain, learning and memory deficiencies, Alzheimer's disease, Parkinson's disease, addiction-related processes, hypoxic-ischemic encephalopathy, body fluid management, neural development, and schizophrenia. Here, we summarize the structure of NK3R/NKB and its cellular signaling as well as the expression of Tacr3/NK3R in the nervous system, and we provide a comprehensive summary of the role of Tacr3/NK3R in neurological diseases, including reproduction-related disorders and other neurological diseases. At the end of this review, we propose the hypothesis that Tacr3/NK3R mediates a variety of brain functions by affecting the excitability of different neurons with specific functions. On the basis of this "excited or not" hypothesis, more studies related to Tacr3 should be carried out in other nervous system diseases in order to better understand the biological roles of Tacr3.

Ultrastructural transneuronal degeneration study of axonal elements within the paratrigeminal nucleus in sinoaortic deafferented rats

OBJECTIVE

Morphological study that searched to authenticate the presence of sinoaortic baroreceptor inputs within the dorsolateral medullary nucleus under electron microscopy analysis.

METHODS

After a 5-day survival period, 9 baroreceptor-denervated rats deeply anaesthetized with equithesin were transcardially perfused and their brains were histologically processed.

RESULTS

The neuronal cytoarchitecture of the paratrigeminal nucleus comprehends afferent projections from other nuclei that have a distributive character regarding visceral and nociceptive functions in the cardiovascular reflex integration response.

CONCLUSION

The medial portion of the nucleus receives afferent projections of the rostral ventrolateral medulla, as shown by retrograde neurotracing studies. The present results show that the medial extent of the paratrigeminal nucleus contains degenerated axoplasmic cellular components in sinoaortic deafferented rats. The number of degenerated axonal fibers was also larger in this area of the nucleus.

Characterization of neurons that express preprotachykinin B in the dorsal horn of the rat spinal cord

Although it is established that neurokinin B is expressed by some neurons in laminae I-III of the rat spinal dorsal horn, little is known about the proportions of cells in these laminae that express neurokinin B, or whether these are excitatory or inhibitory neurons. Neurokinin B is derived from preprotachykinin B, and we have used an antibody against preprotachykinin B to address these issues. We found that preprotachykinin B-immunoreactive neurons were present throughout laminae I-III, constituting 10-11% of the neuronal population in laminae I-II, and 4% of that in lamina III. They formed a prominent band in the ventral half of lamina II (where they made up 16% of the population) and the dorsalmost part of lamina III. The great majority (99%) of preprotachykinin B-immunoreactive axonal boutons contained the vesicular glutamate transporter 2, while none contained glutamic acid decarboxylase. Since most of these boutons are likely to be derived from local preprotachykinin B-expressing cells, these observations suggest that most of the latter are excitatory interneurons. Although 9% of preprotachykinin B-labeled axonal varicosities were substance P-immunoreactive, none contained calcitonin gene-related peptide, which is consistent with reports that neurokinin B is not expressed by primary afferent axons. Many of the preprotachykinin B-immunoreactive cells contained compounds that are present in putative excitatory neurons in laminae I-III: calbindin (84%), protein kinase Cgamma (76%) or somatostatin (31%). However, there was little or no overlap between preprotachykinin B and three other markers associated with excitatory neurons in these laminae: the mu opioid receptor MOR-1, the neurokinin 1 receptor and neurotensin. These results suggest that neurokinin B is expressed by specific populations of excitatory neurons in the superficial dorsal horn. By examining expression of Fos protein in response to intraplantar injection of formaldehyde we provide evidence that many of the preprotachykinin B cells in lamina I and the outer part of lamina II respond to noxious stimulation.

New challenges in the study of the mammalian tachykinins

There is an expanding repertoire of mammalian tachykinins produced by a variety of tachykinin genes, gene splicing events and peptide processing. Novel tachykinin-binding molecules/receptors are proposed, but only, three tachykinin receptors are identified with certainty. The question remains - do more tachykinin receptors exist or is there just the need to reappraise our understanding of the known receptors? The tachykinin NK1 receptor, the preferred receptor for both substance P and the peripheral SP-like endokinins, exists in several tissue-specific conformations and isoforms and may provide some clues. This review addresses recent advances in this exciting field and raises challenging new concepts.

DIFFERENTIAL EFFECTS OF ACTIVATION OF PERIPHERAL AND SPINAL TACHYKININ NEUROKININ 3 RECEPTORS ON THE MICTURITION REFLEX IN RATS

PURPOSE

We clarified the roles of tachykinin neurokinin (NK)3 receptors in the bladder or spinal cord for control of the micturition reflex in rats.

MATERIALS AND METHODS

In female rats under urethane anesthesia repetitive bladder contractions were elicited by saline infusion into the bladder through intravesical bladder catheters. The effects of peripheral receptor activation were first examined by topical application of the tachykinin NK3 receptor agonist [MePhe]-NKB (Calbiochem, Darmstadt, Germany) in normal rats and rats pretreated with capsaicin (Sigma Chemical Co., St. Louis, Missouri) 4 days before the experiments. Subsequently the effects of spinal NK3 receptor activation were examined by intrathecal administration of [MePhe]-NKB via implanted intrathecal catheters. The effects of the tachykinin NK3 receptor antagonist SB235375 and the opioid receptor antagonist naloxone on changes in bladder activity induced by [MePhe]-NKB were also investigated.

RESULTS

Topical application of [MePhe]-NKB onto the bladder surface decreased intercontraction intervals and bladder capacity, and increased baseline bladder pressure in dose dependent fashion. [MePhe]-NKB induced bladder overactivity was inhibited by simultaneous topical administration of SB235375 or by capsaicin pretreatment. In contrast, intrathecal injection of [MePhe]-NKB increased intercontraction intervals in dose dependent fashion and at a high dose it induced overflow incontinence or inefficient voiding. These inhibitory effects of [MePhe]-NKB in the spinal cord were antagonized by the intrathecal injection of SB235375 or naloxone.

CONCLUSIONS

These results indicate that the tachykinin NK3 receptor mediated neural control of the micturition reflex has dual actions depending on the location of receptor activation. Activation of tachykinin NK3 receptors located in the bladder can induce bladder overactivity at least in part via the activation of capsaicin sensitive C-fiber afferents, while tachykinin NK3 receptor activation in the spinal cord can inhibit the micturition reflex through an opioid mechanism.

Cardiovascular and behavioural effects induced by naloxone-precipitated morphine withdrawal in rat: characterization with tachykinin antagonists

This study examined the intracerebroventricular (i.c.v.) effects of three selective tachykinin receptor antagonists on the cardiovascular and behavioural responses induced by naloxone-precipitated morphine withdrawal in rats. I.c.v. injection of naloxone (10 microg) to morphine pre-treated rats (i.c.v. for 5 days) induced an immediate increase in blood pressure ( approximately 10 mmHg) and behavioural activity (sniffing > rearing > face washing approximately grooming approximately wet dog shake) without causing significant heart rate changes. The prior i.c.v. injection of the NK(1) receptor antagonist (6.5 nmol LY306740) reduced face washing and grooming during morphine withdrawal. NK(2) and NK(3) receptor antagonists (6.5 nmol SR48968 and R820) did not affect behavioural effects, yet the co-injection of the three tachykinin antagonists reduced all behavioural activity. The pressor response was not affected by the selective inhibition of NK(1) and NK(3) receptors while both blood pressure and heart rate were markedly enhanced by SR48968 during morphine withdrawal. The potentiating effect of SR48968 was prevented following simultaneous blockade of the three tachykinin receptors. In addition to confirming the involvement of central tachykinins in behavioural manifestations to morphine withdrawal, data suggest a modulatory function for tachykinins, especially the NK(2) receptor, in brain autonomic control of blood pressure and heart rate in supraspinal noloxone-precipitated withdrawal.

Diencephalic neurons producing melanin-concentrating hormone are influenced by local and multiple extra-hypothalamic tachykininergic projections through the neurokinin 3 receptor

As melanin-concentrating hormone (MCH) neurons express the neurokinin 3 receptor (NK3) in the rat diencephalon, their innervation by tachykininergic fibers, the origin of this innervation and the effect of a NK3 agonist on MCH mRNA expression were researched. The obtained results show that the tachykininergic system develops complex relationships with MCH neurons. Overall, MCH cell bodies appeared targeted by both NKB- and SP-inputs. These afferents have multiple hypothalamic and extra-hypothalamic origins, but a local (intra-lateral hypothalamic area) origin from small interneurons was suspected as well. MCH cell bodies do not express NK1, but around 2.7% of the MCH neurons contained SP after colchicine injection. Senktide, a NK3 agonist, produced an increase of the MCH mRNA expression in cultured hypothalamic slices. This effect was reversed by two NK3 antagonists. Tachykinins enhance MCH mRNA expression, and, thus, may modulate the effect of MCH in functions such as feeding and reproductive behaviors in which this peptide has been experimentally involved.

Intestinal anti-nociceptive behaviour of NK3 receptor antagonism in conscious rats: evidence to support a peripheral mechanism of action

The involvement of neurokinin receptors in visceral nociception is well documented. However, the role and localization of NK3 receptors is not clearly established. This study was designed to determine whether NK3 receptor antagonists crossing (talnetant) or not (SB-235375) the blood-brain barrier reduce the nociceptive response to colo-rectal distension (CRD) and whether NK3 antagonism reduces inflammation- or stress-induced hypersensitivity to rectal distension. Isobaric CRD and isovolumic rectal distensions were performed in rats equipped with intramuscular electrodes to record abdominal muscle contractions. In controls, CRD induced a pressure-related (15-60 mmHg) increase in the number of abdominal contractions. Both talnetant and SB-235375 [50 mg x kg-1, per oral (p.o.)], which had no effect on colo-rectal tone, reduced the number of contractions associated with CRDs from 30 to 60 mmHg. Three days after rectal instillation of TNBS, abdominal contractions were increased for rectal distension volume of 0.4 mL. This effect was not modified by talnetant (30 mg x kg-1, p.o.). Partial restraint stress increased abdominal contractions at all distension volumes (0-1.2 mL). Talnetant (10 mg kg-1, p.o.) abolished the increase observed for 0.8 and 1.2 mL. These results indicate that peripheral NK3 receptor antagonism reduced nociception associated with CRD and hypersensitivity induced by stress but not inflammation.

Implication of nigral tachykinin NK3 receptors in the maintenance of hypertension in spontaneously hypertensive rats: a pharmacologic and autoradiographic study

1. The role of nigral tachykinin NK(1), NK(2) and NK(3) receptors in central cardiovascular regulation was studied by measuring the effects of selective agonists and antagonists on mean arterial pressure (MAP) and heart rate (HR) after bilateral microinjection into the substantia nigra of spontaneously hypertensive rats (SHR). Quantitative in vitro autoradiography was also performed in the midbrain of SHR and Wistar-Kyoto (WKY) with the NK(3) receptor ligand [(125)I]-HPP-Senktide. 2. Tachycardia was elicited by the NK(1) ([Sar(9),Met(O(2))(11)]SP) and NK(2) ([betaAla(8)]NKA(4-10)) agonists at 25 and 100 pmol while the NK(3) agonist (senktide, 50 and 100 pmol) had no significant effect. The three agonists had no effect on behaviour, and increases in MAP were elicited by the NK(1) agonist only. 3. Whereas antagonists at NK(1) (RP 67580, 500 pmol) and NK(2) (SR 48968, 500 pmol) receptors had no significant effect on MAP and HR, the NK(3) antagonist (R-820, 500 pmol) reduced MAP for over 3 h in SHR. That anti-hypertensive effect did not occur after intracerebroventricular or intravenous injection of R-820. Also, R-820 had no cardiovascular effect in WKY. 4. The affinity (K(D): 0.7 nM) and densities of specific NK(3) receptor binding sites measured in the substantia nigra, ventral tegmental area, hippocampus and amygdala were not significantly different in SHR and WKY. 5. It is concluded that endogenous tachykinins exert a tonic activity on NK(3) receptors in the substantia nigra of SHR to maintain high blood pressure. Hence, nigral tachykinin NK(3) receptors may represent a promising therapeutic target in the treatment of arterial hypertension.

Two major distinct subpopulations of neurokinin-3 receptor-expressing neurons in the superficial dorsal horn of the rat spinal cord

This study was designed to provide evidence for elucidating the mechanisms of neurokinin-3 receptor (NK3) in spinal pain modulation. First, colocalization of NK3 with the micro -opioid receptor (MOR1) was studied in the spinal dorsal horn of the rat. Confocal microscopy showed that about 44% of NK3-expressing neurons in laminae I and II were immunoreactive for MOR1, which corresponded to about 93% of the total population of MOR1-containing neurons in these laminae. Second, the relationship between NK3/MOR1-coexpressing neurons and those that express nitric oxide synthase (NOS) was examined by using a triple immunofluorescent staining method. About 37% of NK3-immunoreactive neurons were also NOS-immunoreactive, which constituted about 82% of NOS-immunoreacitve neurons in the superficial laminae. However, no triple-labelled neurons were detected. The present results indicate that there are two major distinct subpopulations of NK3-expressing neurons in the superficial dorsal horn, which suggests that the involvement of NK3 receptor in spinal nociception could be mediated by two distinct mechanisms, i.e. opioid and nitric oxide.

Sensitization of enteric reflexes in the rat colon in vitro

We have investigated sensitization of reflexes in the isolated rat colon in order to develop a model that might prove useful for investigating how the sensitivity of enteric reflexes can be altered by prior stimulation. Records were taken of circular muscle tension, 7-10 mm oral and anal to radial distension exerted by a hook passed through the wall of the colon. A test stimulus of 1.5 g produced consistent contractions both oral and anal to the distension. A conditioning protocol, consisting of repeated application of 3 g for 30 s with 30 s between the stimuli for 30 min, doubled the amplitudes of reflex contractions that were evoked by the test stimuli but did not change the sensitivity of the muscle to the direct action of carbachol. The enhanced responses persisted for at least 40 min. The enhancement of reflexes was not reduced by antagonists of tachykinin NK3 receptors or of 5-HT3 receptors, but the reflex oral to stimulation was reduced by NK1 and NK3 antagonists added together. Sensitization was abolished by the cyclo-oxygenase and thromboxane synthase inhibitor, indomethacin. We conclude that sensitization can be reliably induced in vitro and that the model described in the present work can be used to investigate drugs that interfere with the sensitization process.

Modulation of cardiac activity by tachykinins in the rat substantia nigra

1. The effects of tachykinin NK(1), NK(2) and NK(3) receptor agonists and antagonists were measured on blood pressure (MAP) and heart rate (HR) after bilateral microinjection into the substantia nigra (SN) of awake, unrestrained rats. 2. Increasing doses (25 pmol - 1 nmol) of selective agonists at NK(1) ([Sar(9),Met(O(2))(11)]SP), NK(2) ([beta-Ala(8)]NKA(4 - 10)) and NK(3) (senktide) receptors into the SN produced tachycardia which was selectively and reversibly blocked by the prior injection of tachykinin antagonists at NK(1) (RP67580, 250 pmol), NK(2) (SR48968, 250 pmol) and NK(3) (R-820, 500 pmol) receptor. A rapid fall in MAP followed by a pressor response was seen with 1 nmol of [Sar(9),Met(O(2))(11)]SP. Behavioural activity was elicited by 1 nmol of [Sar(9),Met(O(2)(11)]SP (sniffing > face washing = grooming) and senktide (sniffing > wet dog shake > rearing = locomotion). Tachykinin antagonists had no direct cardiovascular or behavioural effects. 3. The tachycardia produced by 100 pmol of [beta-Ala(8)]NKA(4 - 10) or senktide was abolished by an i.v. treatment with atenolol (beta(1)-adrenoceptor antagonist, 5 mg kg(-1)) while that evoked by [Sar(9),Met(O(2))(11)]SP was reduced. A combination of atenolol (5 mg kg(-1)) and atropine (muscarinic antagonist, 1 mg kg(-1)) blocked the response evoked by [Sar(9),Met(O(2))(11)]SP. 4. These data suggest that the SN is a potential site of modulation of cardiac activity by tachykinins. In addition to the withdrawal of the cardiovagal activity by NK(1) receptor, the three tachykinin receptors appear to increase the sympatho/adrenal drive to the heart. This occurs independently of changes in MAP and behaviour. Hence, this study highlights a new central regulatory mechanism of cardiac autonomic activity.

Kinin receptors in pain and inflammation

Kinins are among the most potent autacoids involved in inflammatory, vascular and pain processes. These short-lived peptides, including bradykinin, kallidin and T-kinin, are generated during tissue injury and noxious stimulation. However, emerging evidence also suggests that kinins are stored in neuronal elements of the central nervous system (CNS) where they are thought to play a role as neuromediators in various cerebral functions, particularly in the control of nociceptive information. Kinins exert their biological effects through the activation of two transmembrane G-protein-coupled receptors, denoted bradykinin B(1) and B(2). Whereas the B(2) receptor is constitutive and activated by the parent molecules, the B(1) receptor is generally underexpressed in normal tissues and is activated by kinins deprived of the C-terminal Arg (des-Arg(9)-kinins). The induction and increased expression of B(1) receptor occur following tissue injury or after treatment with bacterial endotoxins or cytokines such as interleukin-1 beta and tumor necrosis factor-alpha. This review summarizes the most recent data from various animal models which convey support for a role of B(2) receptors in the acute phase of the inflammatory and pain response, and for a role of B(1) receptors in the chronic phase of the response. The B(1) receptor may exert a strategic role in inflammatory diseases with an immune component (diabetes, asthma, rheumatoid arthritis and multiple sclerosis). New information is provided regarding the role of sensory mechanisms subserving spinal hyperalgesia and intrapleural neutrophil migration that occur upon B(1) receptor activation in streptozotocin-treated rats, a model of insulin-dependent diabetes mellitus in which the B(1) receptor seems to be rapidly overexpressed. Although it is widely accepted that the blockade of kinin receptors with specific antagonists could be of benefit in the treatment of somatic and visceral inflammation and pain, recent molecular and functional evidence suggests that the activation of B(1) receptors with an agonist may afford a novel therapeutic approach in the CNS inflammatory demyelinating disorder encountered in multiple sclerosis by reducing immune cell infiltration (T-lymphocytes) into the brain. Hence, the B(1) receptor may exert either a protective or detrimental effect depending on the inflammatory disease. This dual function of the B(1) receptor deserves to be investigated further.

Proteinase-activated receptor-2 and hyperalgesia: A novel pain pathway

Using a combined pharmacological and gene-deletion approach, we have delineated a novel mechanism of neurokinin-1 (NK-1) receptor-dependent hyperalgesia induced by proteinase-activated receptor-2 (PAR2), a G-protein-coupled receptor expressed on nociceptive primary afferent neurons. Injections into the paw of sub-inflammatory doses of PAR2 agonists in rats and mice induced a prolonged thermal and mechanical hyperalgesia and elevated spinal Fos protein expression. This hyperalgesia was markedly diminished or absent in mice lacking the NK-1 receptor, preprotachykinin-A or PAR2 genes, or in rats treated with a centrally acting cyclooxygenase inhibitor or treated by spinal cord injection of NK-1 antagonists. Here we identify a previously unrecognized nociceptive pathway with important therapeutic implications, and our results point to a direct role for proteinases and their receptors in pain transmission.

The psychopharmacology of tachykinin NK-3 receptors in laboratory animals

The present article reviews the studies so far published on the psychopharmacological effects mediated by tachykinin NK-3 receptors in laboratory animals. Central administration of NK-3 receptor agonists has been reported to attenuate alcohol intake in alcohol-preferring rats and to evoke conditioned place preference. These findings suggest that NK-3 receptors may affect reward processes to drugs of abuse. Anxiolytic-like and antidepressant-like effects have been previously reported for NK-1 receptor antagonists, and anxiolytic-like effects for NK-2 receptor antagonists. More recently, it has been shown that NK-3 receptor agonists have anxiolytic-like and antidepressant-like effects in mice and rats, while an NK-3 receptor antagonist was reported to be anxiogenic in mice. These findings indicate that different TK receptor subtypes may be involved in anxiolytic-like and antidepressant-like effects in laboratory animals and raise interest for the possible role of NK-3 receptors in the control of anxiety and depression in man.

Effects of supraspinal administration of PG-SPI and PG-KII, two amphibian tachykinin peptides, on nociception in the rat

We investigated and compared the effects of two amphibian tachykinins, the NK1 receptor agonist PG-SPI and the NK3 receptor agonist PG-KII, and the mammalian tachykinins substance P, neurokinin A and neurokinin B on the reaction time to a painful radiant heat stimulus (tail-flick test in rats) after intracerebroventricular injection. PG-SPI (1, 10 and 20 microg) and PG-KII (1, 5 and 10 microg) significantly increased the reaction time. Substance P (10 microg) injected intracerebroventricularly induced antinociception, whereas neurokinin A and neurokinin B did not. Like analgesia evoked by exogenous substance P, PG-SPI-evoked analgesia was blocked by pretreatment with naloxone. Naloxone left PG-KII antinociception unchanged, but the NK3 receptor selective antagonist markedly reduced it. These findings suggest NK1 and NK3 tachykinin receptor system involvement in supraspinal analgesia in rats.