Expression and presence of septal neurokinin-2 receptors controlling hippocampal acetylcholine release during sensory stimulation in rat

Neurokinin1 - cholinergic receptor mechanisms in the medial Septum-Dorsal hippocampus axis mediates experimental neuropathic pain

The neurokinin-1 receptors (NK1Rs) in the forebrain medial septum (MS) region are localized exclusively on cholinergic neurons that partly project to the hippocampus and the cingulate cortex (Cg), regions implicated in nociception. In the present study, we explored the hypothesis that neurotransmission at septal NK1R and hippocampal cholinergic mechanisms mediate experimental neuropathic pain in the rodent chronic constriction injury model (CCI). Our investigations showed that intraseptal microinjection of substance P (SP) in rat evoked a peripheral hypersensitivity (PH)-like response in uninjured animals that was attenuated by systemic atropine sulphate, a muscarinic-cholinergic receptor antagonist. Conversely, pre-emptive destruction of septal cholinergic neurons attenuated the development of PH in the CCI model that also prevented the expression of cellular markers of nociception in the spinal cord and the forebrain. Likewise, anti-nociception was evoked on intraseptal microinjection of L-733,060, an antagonist at NK1Rs, and on bilateral or unilateral microinjection of the cholinergic receptor antagonists, atropine or mecamylamine, into the different regions of the dorsal hippocampus (dH) or on bilateral microinjection into the Cg. Interestingly, the effect of L-733,060 was accompanied with a widespread decreased in levels of CCI-induced nociceptive cellular markers in forebrain that was not secondary to behaviour, suggesting an active modulation of nociceptive processing by transmission at NK1R in the medial septum. The preceding suggest that the development and maintenance of neuropathic nociception is facilitated by septal NK1R-dH cholinergic mechanisms which co-ordinately affect nociceptive processing in the dH and the Cg. Additionally, the data points to a potential strategy for pain modulation that combines anticholinergics and anti-NKRs.

Neurokinin receptor mechanisms in forebrain medial septum modulate nociception in the formalin model of inflammatory pain

The present study has explored the hypothesis that neurokinin1 receptors (NK1Rs) in medial septum (MS) modulate nociception evoked on hind paw injection of formalin. Indeed, the NK1Rs in MS are localized on cholinergic neurons which have been implicated in nociception. In anaesthetized rat, microinjection of L-733,060, an antagonist at NK1Rs, into MS antagonized the suppression of CA1 population spike (PS) evoked on peripheral injection of formalin or on intraseptal microinjection of substance P (SP), an agonist at NK1Rs. The CA1 PS reflects the synaptic excitability of pyramidal cells in the region. Furthermore, microinjection of L-733,060 into MS, but not LS, attenuated formalin-induced theta activation in both anaesthetized and awake rat, where theta reflects an oscillatory information processing by hippocampal neurons. The effects of L-733,060 on microinjection into MS were nociceptive selective as the antagonist did not block septo-hippocampal response to direct MS stimulation by the cholinergic receptor agonist, carbachol, in anaesthetized animal or on exploration in awake animal. Interestingly, microinjection of L-733,060 into both MS and LS attenuated formalin-induced nociceptive flinches. Collectively, the foregoing novel findings highlight that transmission at NK1R provide an affective valence to septo-hippocampal information processing and that peptidergic transmission in the septum modulates nociceptive behaviours.

A Second Wave for the Neurokinin Tac2 Pathway in Brain Research

Despite promising advances in basic research of the neurokinin B/Tac2 pathway in both animals and humans, clinical applications are yet to be implemented. This is likely because of our limited understanding of the action of the pathway in the brain. While this system controls neuronal activity in multiple regions, the precise impact of Tac2-induced cellular responses on behavior remains unclear. Recently, elegant studies revealed a key contribution to stress-related behaviors and memory. Here, we discuss the crucial importance of bridging the gap between the Tac2 pathway's involvement in cell physiology and cognition to comprehend its role in health and disease. We propose that a better understanding of the Tac2 pathway in the brain could provide an essential perspective for basic investigations, which in turn will feed clinical research.

Neuropeptides in learning and memory

Dementia conditions and memory deficits of different origins (vascular, metabolic and primary neurodegenerative such as Alzheimer's and Parkinson's diseases) are getting more common and greater clinical problems recently in the aging population. Since the presently available cognitive enhancers have very limited therapeutical applications, there is an emerging need to elucidate the complex pathophysiological mechanisms, identify key mediators and novel targets for future drug development. Neuropeptides are widely distributed in brain regions responsible for learning and memory processes with special emphasis on the hippocampus, amygdala and the basal forebrain. They form networks with each other, and also have complex interactions with the cholinergic, glutamatergic, dopaminergic and GABA-ergic pathways. This review summarizes the extensive experimental data in the well-established rat and mouse models, as well as the few clinical results regarding the expression and the roles of the tachykinin system, somatostatin and the closely related cortistatin, vasoactive intestinal polypeptide (VIP) and pituitary adenylate-cyclase activating polypeptide (PACAP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), opioid peptides and galanin. Furthermore, the main receptorial targets, mechanisms and interactions are described in order to highlight the possible therapeutical potentials. Agents not only symptomatically improving the functional impairments, but also inhibiting the progression of the neurodegenerative processes would be breakthroughs in this area. The most promising mechanisms determined at the level of exploratory investigations in animal models of cognitive disfunctions are somatostatin sst4, NPY Y2, PACAP-VIP VPAC1, tachykinin NK3 and galanin GALR2 receptor agonisms, as well as delta opioid receptor antagonism. Potent and selective non-peptide ligands with good CNS penetration are needed for further characterization of these molecular pathways to complete the preclinical studies and decide if any of the above described targets could be appropriate for clinical investigations.

Neurokinin3 receptor as a target to predict and improve learning and memory in the aged organism

Impaired learning and memory performance is often found in aging as an early sign of dementia. It is associated with neuronal loss and reduced functioning of cholinergic networks. Here we present evidence that the neurokinin3 receptors (NK3-R) and their influence on acetylcholine (ACh) release may represent a crucial mechanism that underlies age-related deficits in learning and memory. Repeated pharmacological stimulation of NK3-R in aged rats was found to improve learning in the water maze and in object-place recognition. This treatment also enhanced in vivo acetylcholinergic activity in the frontal cortex, hippocampus, and amygdala but reduced NK3-R mRNA expression in the hippocampus. Furthermore, NK3-R agonism incurred a significantly higher increase in ACh levels in aged animals that showed superior learning than in those that were most deficient in learning. Our findings suggest that the induced activation of ACh, rather than basal ACh activity, is associated with superior learning in the aged. To test whether natural variation in NK3-R function also determines learning and memory performance in aged humans, we investigated 209 elderly patients with cognitive impairments. We found that of the 15 analyzed single single-nucleotide ploymorphism (SNPs) of the NK3-R-coding gene, TACR3, the rs2765 SNP predicted the degree of impairment of learning and memory in these patients. This relationship could be partially explained by a reduced right hippocampus volume in a subsample of 111 tested dementia patients. These data indicate the NK3-R as an important target to predict and improve learning and memory performance in the aged organism.

Study on the distribution sites and the molecular mechanism of analgesia after intracerebroventricular injection of rat/mouse hemokinin-1 in mice

Hemokinin-1 is a peptide encoded by Pptc, which belongs to the family of mammalian tachykinins. Our previous results showed that rat/mouse hemokinin-1 (r/m HK-1) produced striking analgesia after intracerebroventricular (i.c.v.) injection in mice, and the analgesia could be blocked by the NK1 receptor antagonist and the opioid receptor antagonist, respectively. However, the precise distribution sites and the molecular mechanism involved in the analgesic effect after i.c.v. administration of r/m HK-1 are needed to be further investigated deeply. Using the fluorescence labeling method, our present results directly showed that r/m HK-1 peptides were mainly distributed at the ventricular walls and several juxta-ventricular structures for the first time. Our results showed that the mRNA expressions of NK1 receptor, PPT-A, PPT-C, KOR, PDYN, DOR and PENK were not changed markedly, as well as the protein expression of NK1 receptor was hardly changed. However, both the transcripts and proteins of MOR and POMC were up-regulated significantly, indicating that the analgesic effect induced by i.c.v. administration of r/m HK-1 is related to the activation of NK1 receptor first, then it is related to the release of endogenous proopiomelanocortin, as well as the increased expression level of μ opioid receptor. These results should facilitate further the analysis of the analgesia of r/m HK-1 in the central nerval system in acute pain and may open novel pharmacological interventions.

Are biological actions of neurokinin A in the adult brain mediated by a cross-talk between the NK1 and NK2 receptors?

Mice lacking the NK(1) receptor (NK(1)R-/- mice) and selective, high-affinity, non-peptide, NK(1), NK(2) and NK(3) receptor antagonists were used to identify the tachykinin receptor subtype(s) mediating the central responses induced by neurokinin A (NKA). The peptides, substance P (SP), NKA and senktide and the antagonists were injected intracerebroventricularly (ICV) through an implanted cannula. NKA (50 pmol) was as potent as SP (50 pmol) in inducing grooming behaviour (face washing and hind limb grooming) in wild-type mice, but both peptides failed to induce behavioural responses in NK(1)R-/- mice. In wild-type mice, the NK(1) receptor antagonist, RP 67580 (2 nmol), effectively inhibited grooming behaviour elicited by SP, but was inactive against grooming induced by NKA, which in turn was abolished after pre-treatment with the selective NK(2) receptor agonist, SR 48968 (2 nmol). Unlike NKA, the selective NK(2) receptor agonists, (β Ala(8)) NKA 4-10 and (NLeu(10)) NKA 4-10, injected ICV at doses of 50 or 100 pmol did not elicit any behavioural response in wild-type mice. The NK(3) receptor antagonist, SR 142801, inhibited behaviours induced by the NK(3) receptor agonist, senktide, but did not alter behavioural responses to either SP or NKA in wild-type mice. The present findings demonstrate that central biological actions of SP and senktide are mediated by activation of NK(1) and NK(3) receptors, respectively. Our results also indicate that NK(1) receptors are essential for generating central actions induced by NKA, which are most probably mediated by a cross-talk between the NK(1) and NK(2) receptors.

Neurokinin-2 receptor antagonism in medial septum influences temporal-order memory for objects and forebrain cholinergic activity

In the mammalian brain the neurokinin NK(2) receptors are predominantly located in the hippocampus, thalamus, septum and frontal cortex. It has been shown that administration of the NK(2) receptor agonist, neurokinin A (NKA), into the medial septum of rats increases extracellular levels of acetylcholine (ACh) in the hippocampus and that NK(2) receptor antagonism blocks this increase. Therefore, given the prominent role of hippocampal ACh in information processing, we hypothesized that NK(2) receptor antagonism in the medial septum would negatively affect learning and memory via its influence on the cholinergic neurons of the basal forebrain. We investigated the action of local application of the peptidic NK(2) receptor antagonist, Bz-Ala-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH (1, 10 and 100pmol), into the medial septum on object memory for temporal order and spatial location using an object novelty paradigm. By means of in vivo microdialysis and HPLC analyses, we also examined the influence of NK(2) receptor antagonism in the medial septum on ACh in major cholinergic projection areas of the basal forebrain, namely, hippocampus, frontal cortex and amygdala.

RESULTS

Injection of vehicle alone into the medial septum impaired memory for temporal order and spatial location of objects. Application of 1pmol of the NK(2) receptor antagonist partially reversed this deficit by reinstating memory for temporal order. Injection of 10pmol of the NK(2) receptor antagonist into the medial septum decreased levels of ACh in the hippocampus (at 30min post-injection), and frontal cortex (at 30 and 80min post-injection) in comparison to vehicle. However, this apparent decrease was the result of the blockade of a saline-induced increase in ACh levels.

Neuropeptide and sigma receptors as novel therapeutic targets for the pharmacotherapy of depression

Among the most prevalent of mental illnesses, depression is increasing in incidence in the Western world. It presents with a wide variety of symptoms that involve both the CNS and the periphery. Multiple pharmacological observations led to the development of the monoamine theory as a biological basis for depression, according to which diminished neurotransmission within the CNS, including that of the dopamine, noradrenaline (norepinephrine) and serotonin systems, is the leading cause of the disorder. Current conventional pharmacological antidepressant therapies, using selective monoamine reuptake inhibitors, tricyclic antidepressants and monoamine oxidase inhibitors, aim to enhance monoaminergic neurotransmission. However, the use of these agents presents severe disadvantages, including a delay in the alleviation of depressive symptoms, significant adverse effects and high frequencies of non-responding patients. Neuroendocrinological data of recent decades reveal that depression and anxiety disorders may occur simultaneously due to hypothalamus-pituitary-adrenal (HPA) axis hyperactivity. As a result, the stress-diathesis model was developed, which attempts to associate genetic and environmental influences in the aetiology of depression. The amygdala and the hippocampus control the activity of the HPA axis in a counter-balancing way, and a plethora of regulatory neuropeptide signalling pathways are involved. Intervention at these molecular targets may lead to alternative antidepressant therapeutic solutions that are expected to overcome the limitations of existing antidepressants. This prospect is based on preclinical evidence from pharmacological and genetic modifications of the action of neuropeptides such as corticotropin-releasing factor, substance P, galanin, vasopressin and neuropeptide Y. The recent synthesis of orally potent non-peptide micromolecules that can selectively bind to various neuropeptide receptors permits the onset of clinical trials to evaluate their efficacy against depression.

Neurokinins robustly activate the majority of septohippocampal cholinergic neurons

In the brain, tachykinins acting via the three cloned neurokinin (NK) receptors are implicated in stress-related affective disorders. Hemokinin-1 is a novel tachykinin that reportedly prefers NK1 to NK2 or NK3 receptors. Although NK1 and NK3 receptors are abundantly expressed in the brain, NK2-receptor-mediated electrophysiological effects have rarely been described as NK2 receptors are expressed only in a few brain regions such as the nucleus of the medial septum/diagonal band. Medial septal/diagonal band neurons that control hippocampal mnemonic functions also colocalize NK1 and NK3 receptors. Functionally, intraseptal activation of all three NK receptors increases hippocampal acetylcholine release and NK2 receptors have specifically been implicated in stress-induced hippocampal acetylcholine release. Electrophysiological studies on the effects of NKs on septohippocampal cholinergic neurons are lacking and electrophysiological effects of hemokinin-1 have thus far not been reported in brain neurons. In the present study we examined the electrophysiological and pharmacological effects of multiple NKs on fluorescently tagged septohippocampal cholinergic neurons using whole-cell patch-clamp recordings in a rat brain slice preparation. We demonstrate that a vast majority of septohippocampal cholinergic cells are activated by NK1, NK2 and NK3 receptor agonists as well as by hemokinin-1 via direct post-synaptic mechanisms. Pharmacologically, hemokinin-1 recruits not only NK1 but also NK2 and NK3 receptors to activate septohippocampal cholinergic neurons that are the primary source of acetylcholine for the hippocampus.

Effects of the vasopressin (V1b) receptor antagonist, SSR149415, and the corticotropin-releasing factor 1 receptor antagonist, SSR125543, on FG 7142-induced increase in acetylcholine and norepinephrine release in the rat

Arginine vasopressin and corticotropin-releasing factor are two neuroactive peptides that regulate hypothalamic-pituitary-axis and associated stress response. While the potential antidepressant and anxiolytic profiles of corticotropin-releasing factor 1 antagonists have been well studied, the concept of blockade of vasopressin system as another approach for the treatment of emotional processes has only been made available recently by the synthesis of the first non-peptide antagonist at the V1b receptor, SSR149415. In the present study SSR149415 has been compared with the corticotropin-releasing factor 1 antagonist SSR125543 and with anxiolytic and antidepressant drugs on the response of hippocampal cholinergic and cortical noradrenergic systems to the anxiogenic benzodiazepine receptor inverse agonist FG 7142. Acute (0.3-10 mg/kg, i.p.) and long-term administration (10 mg/kg, i.p., 21 days) of SSR149415 and SSR125543 reduced the FG 7142-induced increase in extracellular concentrations of acetylcholine in the hippocampus of anesthetized rats measured by microdialysis. By contrast acute and long-term administration of SSR149415 failed to reduce the FG 7142-induced increase in the release of norepinephrine in the cortex of freely moving rats. The present results demonstrate that the two compounds have similar profiles in a model of activation by an anxiogenic drug of the hippocampal cholinergic system and they suggest that SSR149415 and SSR125543 may have anti-stress anxiolytic and antidepressant effects via a mechanism of action different from classical benzodiazepine ligands and noradrenergic antidepressants.

A 25 year adventure in the field of tachykinins

Several aspects of our 25 year adventure in the field of tachykinins will be successively described. They concern: substance P (SP) synthesis and release in the basal ganglia, the identification and pharmacological characterization of central tachykinin NK(1), NK(2) and NK(3) binding sites and their topographical distribution, the description of some new biological tests for corresponding receptors, the identification of tachykinin NK(1) receptor subtypes or conformers sensitive to all endogenous tachykinins (substance P, neurokinin A (NKA), neurokinin B (NKB), neuropeptide gamma (NP gamma) and neuropeptide K (NPK)) and finally, the functional involvement of these receptors and their subtypes in tachykinin-induced regulations of dopamine and acetylcholine release in the striatum.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Autoradiographic distribution of tachykinin NK2 binding sites in the rat brain: comparison with NK1 and NK3 binding sites

The autoradiographic distribution of tachykinin NK(2) binding sites was determined in the adult rat brain using [(125)I]neurokinin A in the presence of either senktide (NK(3) agonist) and [Pro(9)]substance P (NK(1) agonist) or senktide and SR 140333 (NK(1) antagonist). Indeed, this radioligand labels two subtypes of NK(1) binding sites (which present a high affinity not only for SP but also for neurokinin A, neuropeptide K and neuropeptide gamma) as well as NK(3) binding sites. The distribution of NK(2) binding sites was also compared with those of NK(1) and NK(3) binding sites, these sites being labeled with [(125)I]Bolton and Hunter substance P and [(125)I]Bolton and Hunter eledoisin, respectively. In agreement with our results obtained with membranes from various brain structures, NK(2)-sensitive [(125)I]neurokinin A labeling was mainly observed in few structures including the dorsal and ventral hippocampus, the septum, the thalamus and the prefrontal cortex. The density of NK(2) binding sites was weak when compared with those of NK(1) and NK(3) binding sites. Marked differences were observed in the distributions of NK(1), NK(2) and NK(3) binding sites. These results are discussed taking into consideration differences or similarities between the distributions of NK(2)-sensitive [(125)I]neurokinin A binding sites and of their endogenous ligands (neurokinin A, neuropeptide K and neuropeptide gamma) but also local NK(2) agonist responses blocked by NK(2) antagonists. Insights on the roles of endogenous tachykinins in several brain functions are also discussed on the basis of the respective distributions of different neurokinin binding sites.

Control by tachykinin NK(2) receptors of CRF(1) receptor-mediated activation of hippocampal acetylcholine release in the rat and guinea-pig

In vivo microdialysis was employed to explore the effects of different selective non-peptides NK(1),NK(2) and NK(3) receptor antagonists on the corticotropin releasing factor (CRF)-induced release of acetylcholine (ACh) in the hippocampus of rats and guinea-pigs. In both species, the intracerebroventricular (i.c.v.) administration of CRF produced a time- and dose-dependent increase in hippocampal ACh release that was totally suppressed by an intraperitoneally (i.p.) pretreatment with the selective non-peptide CRF(1) receptor antagonist antalarmin (30 mg/kg). Pretreatment with the selective NK(2) receptor antagonist SR48968 (1mg/kg, i.p.) significantly reduced the increase of ACh induced by CRF. In contrast, its low-affinity enantiomer SR48965 (1mg/kg, i.p.) or the NK(1) receptor antagonist, GR205171 (1mg/kg, i.p.) did not exert any antagonist effect. Moreover, administration of the selective NK(3) receptor antagonist SR142801 (1mg/kg, i.p.) did not significantly reduce the CRF-induced hippocampal ACh release in guinea-pigs (the only species studied). The selective activity of SR48968 versus GR205171 or SR142801 indicates that NK(2) receptors play a major role in the control of CRF-induced hippocampal ACh release. Moreover, in freely moving rats, two sessions of stroking of the neck and back of the rat for 30 min, at 90 min intervals, known to be a stressful stimulus, produced a marked and reproducible increase in hippocampal ACh release. This effect was prevented by the administration of the two selective non-peptide CRF1 and NK(2) receptor antagonists antalarmin (30 mg/kg, i.p.) and SR48968 (1mg/kg, i.p.), respectively. This suggests that stress-induced activation of the hippocampal ACh system may be under the control of both endogenously released CRF and NKA, and opens the possibility of the existence of a functional interplay between the pathways containing these peptides as we observed in our experiments on anaesthetized animals.

Facilitation by endogenous tachykinins of the NMDA-evoked release of acetylcholine after acute and chronic suppression of dopaminergic transmission in the matrix of the rat striatum

Using a microsuperfusion method in vitro, the effects of the NK1, NK2, and NK3 tachykinin receptor antagonists SR140333, SR48968, and SR142801, respectively, on the NMDA-evoked release of [3H]-acetylcholine were investigated after both acute and chronic suppression of dopamine transmission in striosomes and matrix of the rat striatum. NMDA (1 mm) alone or with D-serine (10 microm) in the presence of alpha-methyl-p-tyrosine (100 microm) markedly enhanced the release of [3H]-acetylcholine through a dopamine-independent inhibitory process. In both conditions, as well as after chronic 6-OHDA-induced denervation of striatal dopaminergic fibers, SR140333, SR48968, or SR142801 (0.1 microm each) reduced the NMDA-evoked release of [3H]-acetylcholine in the matrix but not in striosome-enriched areas. These responses were selectively abolished by coapplication with NMDA of the respective tachykinin agonists, septide, [Lys5,MeLeu9,Nle10]NKA(4-10), or senktide. Distinct mechanisms are involved in the effects of the tachykinin antagonists because the inhibitory response of SR140333 was additive with that of either SR48968 or SR142801. In addition, the SR140333-evoked response remained unchanged, whereas those of SR48968 and SR142801 were abolished in the presence of N(G)-monomethyl-l-arginine (nitric oxide synthase inhibitor). Therefore, in the matrix but not in striosomes, the acute or chronic suppression of dopamine transmission unmasked the facilitatory effects of endogenously released substance P, neurokinin A, and neurokinin B on the NMDA-evoked release of [3H]-acetylcholine. Whereas substance P and neurokinin A are colocalized in same efferent neurons, their responses involve distinct circuits because the substance P response seems to be mediated by NK1 receptors located on cholinergic interneurons, while those of neurokinin A and neurokinin B are nitric oxide-dependent.

Characterization of the profile of neurokinin-2 and neurotensin receptor antagonists in the mouse defense test battery

Defensive behaviors of lower mammals confronted with a predatory stimulus provide an appropriate laboratory model for investigating behavior relevant to human emotional disorders. The mouse defense test battery (MDTB) has been developed because it combines many of the aspects of defense. Briefly, it consists of five tests either associated with potential threat (contextual defense) or the actual presence of an approaching threat (a rat). These latter focus on changes in flight, risk assessment and defensive threat and attack behaviors. Investigations with anxiolytic compounds have shown that these defense reactions may be used to differentiate between several classes of anxiolytic drugs. Here we used the MDTB to compare the behavioral profile of the benzodiazepine diazepam with that of neuropeptide receptor antagonists which have been shown to be involved in the modulation of stress response, namely the NK(2) receptor antagonists, SR48968 (0.01-1mg/kg) and SR144190 (1-10mg/kg), and the NT(1) receptor antagonist, SR48692 (1-30mg/kg). Results showed that all compounds decreased defensive threat/attack, but only diazepam and, to a lesser extent, SR48692 significantly modified risk assessment or flight. Further, none of the neuropeptide receptor antagonists modified contextual defense. Overall, the behavioral profile displayed by diazepam and these latter compounds in the MDTB are consistent with an anxiolytic-like action. However, our results suggest that, while NK(2) and NT(1) receptor antagonists may have limited efficacy on anxiety-related responses including cognitive aspects (i.e. risk assessment), they may have a potential against some forms of anxiety disorders which involve adaptative responses to extreme stress stimuli (e.g. direct confrontation with the threat stimulus).

Presence of NK2 binding sites in the rat brain

Attempts were made to label tachykinin NK2 binding sites in the adult rat brain using [125I]neurokinin A (NKA) as ligand in the presence of NK1 and NK3 agonist or antagonist to avoid labelling of NK1 and NK3 binding sites, respectively. A high-affinity, specifically NK2-sensitive, [125I]NKA-binding, temperature-dependent, reversible, sensitive to GTPgammaS and correspondence to a single population of binding sites (K(D) and B(max) values: 2.2 nM and 7.3 fmol/mg protein) was demonstrated on hippocampal membranes. Competition studies performed with tachykinins and tachykinin-related compounds indicated that the pharmacological properties of these NK2-sensitive [125I]NKA binding sites were identical to those identified in the rat urinary bladder and duodenum. NKA, neuropeptide K, and neuropeptide gamma, as well as the potent and selective NK2 antagonists SR 144190, SR 48968 and MEN 10627, presented a nanomolar affinity for these sites. The regional distribution of these NK2-sensitive [125I]NKA binding sites differs markedly from those of NK1 and NK3 binding sites, with the largest labeling being found in the hippocampus, the thalamus and the septum. Binding in other brain structures was low or negligible. A preliminary autoradiographic analysis confirmed [125I]NKA selective binding in hippocampal CA1 and CA3 areas, particularly, and in several thalamic nuclei.

Cholinergic neurons expressing neuromedin K receptor (NK3) in the basal forebrain of the rat: a double immunofluorescence study

By using a double immunofluorescence method we have examined the distribution of cholinergic neurons expressing neuromedin K receptor (NK3) in the rat brain and spinal cord. The distribution of neuromedin K receptor-like immunoreactive neurons completely overlapped with that of choline acetyltransferase-positive neurons in certain regions of the basal forebrain, e.g. the medial septal nucleus, nucleus of the diagonal band of Broca, magnocellular preoptic nucleus and substantia innominata. Partially overlapping distributions of neuromedin K receptor-like immunoreactive and choline acetyltransferase-positive neurons were found in the basal nucleus of Meynert, globus pallidus, ventral pallidum of the forebrain, tegmental nuclei of the pons and dorsal motor nucleus of the vagus. Neurons showing both neuromedin K receptor-like and choline acetyltransferase immunoreactivities, however, were found predominantly in the medial septal nucleus, nucleus of the diagonal band of Broca and magnocellular preoptic nucleus of the basal forebrain: 66-80% of these choline acetyltransferase-positive neurons displayed neuromedin K receptor-like immunoreactivity. Neurons showing both neuromedin K receptor-like and choline acetyltransferase immunoreactivities were hardly detected in other aforementioned regions of the forebrain, brainstem and spinal cord. The present study has provided morphological evidence for direct physiological modulation or regulation of cholinergic neurons by tachykinins through the neuromedin K receptor in the basal forebrain of rats.

Cholinergic neurons expressing substance P receptor (NK(1)) in the basal forebrain of the rat: a double immunocytochemical study

Cholinergic neurons expressing substance P receptor (SPR, NK(1)) were examined in the rat brain using double immunofluorescence. The distribution of SPR-like immunoreactive (SPR-LI) neurons completely overlapped with that of choline acetyltransferase (ChAT)-LI neurons in the medial septal nucleus, the nucleus of diagonal band of Broca, the magnocellular preoptic nucleus, the substantia innominata of basal forebrain, the caudate-putamen, and the ventral pallidum of the basal ganglia. In the mesopontine tegmentum and the cranial motor nuclei of the brainstem, the distribution of SPR-LI and ChAT-LI neurons was partially overlapping. Neurons showing both SPR-like and ChAT-like immunoreactivities, however, were predominantly found above basal forebrain regions and 82-90% of these ChAT-LI neurons displayed SPR-like immunoreactivity, in addition to the confirmatory observation that 100% of the ChAT-LI neurons exhibit SPR-like immunoreactivity in the basal ganglia. In contrast, neurons double-labeled for SPR-like and ChAT-like immunoreactivities were hardly detected in aforementioned regions of the brainstem. The present study has provided morphological evidence for direct physiological modulation of cholinergic neurons by tachykinins through substance P receptor in the basal forebrain of the rat.

Expression of tachykinin NK2 receptor mRNA in human brain

Tachykinin NK2 receptors have been suggested to play an important role in the central nervous system. This study, using reverse transcription-polymerase chain reaction revealed a detectable expression of NK2 receptor mRNA in various human brain regions, including the caudate nucleus, the putamen, the hippocampus, the substantia nigra and the cerebral cortex. The distribution of NK2 receptor expression in the cortex revealed a major expression in frontal and temporal cortex compared to occipital and parietal areas. These results provide a molecular basis for considering a role of NK2 receptors in human pathophysiology.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

Characterization and regulation of tachykinin receptors in the nucleus tractus solitarius

1. The characteristics, localization and regulation of tachykinin receptors in the rat nucleus tractus solitarius (NTS) involved in respiratory control were investigated using a combination of in vivo microinjection and in vitro autoradiographic techniques. 2. Microinjection of receptor-selective tachykinin agonists and antagonists into the NTS of urethane-anaesthetized rats suggests that stimulation of NK1 and NK3 receptors increases tidal volume, whereas NK2 and NK3 receptor activation produces a bradypnoea. 3. Depletion of NK1 receptors in the NTS due to either ageing or acute hypoxia correlates with a markedly reduced respiratory response to substance P. In contrast, chemical ablation of sensory neurons by neonatal capsaicin administration dramatically increases the respiratory response to a variety of NK1, NK2 and NK3 agonists. 4. These studies suggest that all three tachykinin receptors are present in the rat NTS and that these receptors are subject to both acute and chronic regulation.

Receptor-mediated internalization of somatostatin in rat cortical and hippocampal neurons

Binding of neuropeptides to their receptors usually results in internalization of receptor-ligand complexes. This process serves a crucial role in receptor downregulation, resensitization, and transmembrane signaling. It has mainly been investigated in cells ectopically expressing recombinant receptors. In the present study, we investigated whether rat central neurons and astrocytes naturally expressing somatostatin (SRIF) receptors internalized this neuropeptide. We demonstrated that 29% of cortical and 45% of hippocampal neurons in culture expressed the SRIF receptor sst(2A) and that 40-50% of the neurons internalized fluorescent SRIF. Similarly, an important proportion of astrocytes expressed sst(2A) (up to 60% in cortical cultures) and internalized fluo-SRIF. Competition experiments using the sst(2)/sst(5)-preferring agonist SMS 201-995 (octreotide) showed that a subpopulation of neurons internalized fluo-SRIF via sst(2) and/or sst(5) receptors, but that others also did so via other subtypes. Fluo-SRIF labeling was barely competed for by the sst(1)-selective agonist CH-275, indicating that sst(1) was unlikely to be mediating SRIF internalization in hippocampal and cortical neurons. Given the paucity of sst(5) receptors in cerebral cortex and hippocampus and the poor yield of sst(4) internalization in transfected cells, we conclude that sst(2) and sst(3) subtypes are the most likely to be responsible for SRIF internalization in our culture systems.

Different subtypes of tachykinin NK(1) receptor binding sites are present in the rat brain

(2-[(125)I]iodohistidyl(1))Neurokinin A ([(125)I]NKA), which labels "septide-sensitive" but not classic NK(1) binding sites in peripheral tissues, was used to determine whether septide-sensitive binding sites are also present in the rat brain. Binding studies were performed in the presence of SR 48968 (NK(2) antagonist) and senktide (NK(3) agonist) because [(125)I]NKA also labels peripheral NK(2) binding sites and, as shown in this study, central NK(3) binding sites. [(125)I]NKA was found to label not only septide-sensitive binding sites but also a new subtype of NK(1) binding site distinct from classic NK(1) binding sites. Both subtypes of [(125)I]NKA binding sites were sensitive to tachykinin NK(1) antagonists and agonists but also to the endogenous tachykinins NKA, neuropeptide K (NPK), and neuropeptide gamma (NPgamma). However, compounds of the septide family such as substance P(6-11) [SP(6-11)] and propionyl-[Met(O(2))(11)]SP(7-11) and some NK(1) antagonists, GR 82334, RP 67580, and CP 96345, had a much lower affinity for the new NK(1)-sensitive sites than for the septide-sensitive sites. The hypothalamus and colliculi possess only this new subtype of NK(1) site, whereas both types of [(125)I]NKA binding sites were found in the amygdala and some other brain structures. These results not only explain the central effects of septide or SP(6-11), but also those of NKA, NPK, and NPgamma, which can be selectively blocked by NK(1) receptor antagonists.

Respiratory actions of tachykinins in the nucleus of the solitary tract: characterization of receptors using selective agonists and antagonists

1. The respiratory response to microinjection of tachykinins and analogues into the commissural nucleus of the solitary tract (cNTS) of urethane-anaesthetized rats was investigated in the presence and absence of selective tachykinin NK(1), NK(2) and NK(3) antagonists (RP 67580, SR 48968 and SR 142801, respectively). 2. All tachykinins, except for the selective NK(2) agonist, [Nle(10)]-NKA(4-10), increased tidal volume (VT). The rank potency order of naturally-occurring tachykinins was neurokinin A (NKA)> or =substance P (SP)>>NKB, whereas the rank order for selective analogues was senktide> or = septide>> [Sar(9),Met(O(2))(11)]-SP>>[Nle(10)]-NKA(4-10). Septide (NK(1)-selective) and senktide (NK(3)-selective) were 22 fold more potent (pD(2) approximately 12) at stimulating VT than SP (pD(2) approximately 10.5). 3. Tachykinin agonists produced varying degrees of respiratory slowing, independent of changes in VT. At doses producing maximum stimulation of VT, agonists induced either a mild (<10 breaths min(-1) decrease; SP and septide), moderate (10 - 25 breaths min(-1) decrease; NKA, NKB and [Sar(9),Met(O(2)]-SP) or severe ( approximately 40 breaths min(-1) decrease; senktide) bradypnoea. [Nle(10)]-NKA(4-10) produced a dose-dependent bradypnoea without affecting VT. 4. RP 67580 significantly attenuated the VT response to SP (33 pmol) and NKA (10 pmol) but not NKB (100 pmol). In the presence of RP 67580, the mild bradypnoeic response to NKB was significantly enhanced whereas SP and NKA induced a bradyapnea which was not observed in the absence of RP 67580. SR 48968 had no effect on the VT response to SP or NKB, markedly enhanced the VT response to NKA and completely blocked the bradypnoeic response to [Nle(10)]-NKA(4-10). Only SR142801 attenuated the VT response to NKB. 5. The present data suggest that all three tachykinin receptors (NK(1), NK(2) and NK(3)) are present in the cNTS and are involved in the central control of respiration.

Facilitation of cholinergic transmission by substance P methyl ester in the mouse hippocampal slice preparation

Using sharp microelectrode recording from CA1 pyramidal neurons of the adult mouse hippocampal slice preparation, we studied the modulatory action of the selective neurokinin 1 (NK1) receptor agonist substance P methyl ester (SPME), a peptidase-resistant analogue of the peptide substance P (SP), on cholinergic responses. While SPME (0.1-1 microM) had only slight effects on membrane potential and input resistance of CA1 neurons, it largely and reversibly enhanced the membrane depolarization and oscillatory activity induced by the cholinergic agonist carbachol (CCh; 0.1-100 microM). This effect of SPME was prevented by the selective NK1 receptor antagonist SR 140333 (4 microM). In about half of the tested neurons the action of SPME was preserved in tetrodotoxin (TTX) solution, suggesting that it partly occurred at the level of pyramidal cells. Cholinergic slow excitatory postsynaptic potentials (sEPSPs) were reversibly enhanced by SPME which increased their amplitude and prolonged any associated bursting activity. This action was also blocked by SR 140333. The present results suggest that SPME largely enhances cholinergic activity in the mouse hippocampus, an effect which can help to explain, in this brain area, the recently reported facilitation of seizures by SP.

Blockade of neurokinin3 receptors antagonizes drug-induced population response and depolarization block of midbrain dopamine neurons in guinea pigs

In vivo extracellular recording techniques were used to investigate the effects of neurokinin3 (NK3) receptor blockade on the pharmacological activation of midbrain dopamine (DA) neurons in the guinea pig substantia nigra (A9) and ventral tegmental area (A10). The number of spontaneously active DA cells (population response) was largely increased in A10 and A9 by acute administration of haloperidol (1 and 0.5 mg/kg i.p., respectively) and this effect was dose-dependently prevented in both areas by the selective NK3 receptor antagonist SR142801 (0.3, 1, 3, and 1, 3, 10 mg/kg i.p., respectively). This compound, which was totally inactive by itself, also antagonized the increase of population response induced in A10 cells by the neurotensin receptor antagonist SR142948 (1 mg/kg i.p.) and in A9 cells by the NK2 receptor antagonist SR144190 (1 mg/kg i.p.). None of the effects of SR142801 were reproduced by SR142806, its (R)-enantiomer with 240-fold lower affinity for NK3 receptors. In addition, neither SR144190 (0.3 mg/kg i.p.) nor the NK1 receptor antagonist GR205171 (1 mg/kg i.p.) affected the haloperidol-induced response. The antagonistic effects of SR142801 (3 mg/kg i.p.) were also observed on the depolarization block-related decrease of A10 cell population response evoked by repeated administration (22 days) of haloperidol. Finally, SR142801 (3 mg/kg i.p.) prevented depolarization block induced in A10 cells by acute co-administration of SR142948 and haloperidol, both on population response and on single cell firing. These results on pharmacologically induced activation and depolarization block of dopamine neurons suggest that NK3 receptors play a key role in the midbrain DA function, presumably through activation by neurokinin B.

Blockade of cannabinoid receptors by SR141716 selectively increases Fos expression in rat mesocorticolimbic areas via reduced dopamine D2 function

The present study investigated, in rats, whether blockade of cannabinoid CB1 receptors may alter Fos protein expression in a manner comparable to that observed with antipsychotic drugs. Intraperitoneal administration of the selective CB1 receptor antagonist, SR141716, dose-dependently (1.0, 3.0 and 10 mg/kg) increased Fos-like immunoreactivity in mesocorticolimbic areas (prefrontal cortex, ventrolateral septum, shell of the nucleus accumbens and dorsomedial caudate-putamen), while motor-related structures such as the core of the nucleus accumbens and the dorsolateral caudate-putamen were unaffected. In the ventrolateral septum, taken as a representative structure, the Fos-inducing effect of SR141716 (10 mg/kg) was maximal 2 h after injection and returned to near control levels by 4 h. Within the prefrontal cortex, SR141716 increased the number of Fos-positive cells predominantly in the infralimbic and prelimbic cortices, presumptive pyramidal cells being the major cell types in which Fos was induced. The D1-like receptor antagonist, SCH23390 (0.1 mg/kg), did not prevent the Fos-inducing effect of SR141716 in any brain region examined (prefrontal cortex, nucleus accumbens, ventrolateral septum and dorsomedial caudate-putamen), although SCH23390 significantly reduced Fos expression induced by cocaine (20 mg/kg) in all these regions. By contrast, the dopamine D2-like agonist, quinpirole (0.25 mg/ kg), counteracted SR141716-induced Fos-like immunoreactivity in the ventrolateral septum, the nucleus accumbens and the dorsomedial caudate-putamen, while no antagonism was observed in the prefrontal cortex. Microdialysis experiments in awake rats indicated that SR141716, at doses which increased Fos expression (3 and 10 mg/kg), did not alter dopamine release in the shell of the nucleus accumbens. Finally, SR141716 increased the levels of neurotensin-like immunoreactivity in the nucleus accumbens, but not in the caudate-putamen. Collectively, the present results show that blockade of cannabinoid receptors increases Fos- and neurotensin-like immunoreactivity with characteristics comparable to those reported for atypical neuroleptic drugs.

Activation of dopaminergic and cholinergic neurotransmission by tachykinin NK3 receptor stimulation: an in vivo microdialysis approach in guinea pig

The regulation of dopaminergic and cholinergic function by neurokinin-3 (NK3) receptor activation was examined in vivo in urethane-anaesthetized guinea pigs with microdialysis probes. The local application of the NK3 tachykinin receptor agonist senktide in the region of dopamine cell bodies (pars compacta of the substantia nigra and ventral tegmental area) and in the area of cholinergic cell bodies (septal area) markedly enhanced the extracellular dopamine (DA) and acetylcholine (ACh) concentration throughout their respective target areas, i.e. striatum, nucleus accumbens, prefrontal cortex for dopaminergic systems and hippocampus for cholinergic neurons. The enhancing effect of senktide on neurotransmitter release was dose dependently blocked by the selective non-peptide NK3 receptor antagonist SR142801 (0.1-1 mg/kg, i.p.), whereas its inactive S-enantiomer SR142806 (0.3-1 mg/kg, i.p.) did not exert any antagonistic activity on the effect of intranigral or intraseptal application of senktide. These results demonstrate that NK3 receptors can modulate the activity of central DA and ACh systems.