Substance P enhances NMDA channel function in hippocampal dentate gyrus granule cells

Orexin receptors in the hippocampal dentate gyrus modulated the restraint stress-induced analgesia in the animal model of chronic pain

Previous studies have shown that stressful stimuli induced an adaptive response of reduced nociception, known as stress-induced analgesia (SIA). Since orexin neuropeptides are involved in pain modulation, and orexin neurons, primarily located in the lateral hypothalamus (LH), project to various hippocampal regions, such as the dentate gyrus (DG), the current study aimed to examine the role of orexin receptors within the DG region in the restraint SIA in the animal model of chronic pain. One hundred-thirty adult male Wistar rats (230-250 g) were unilaterally implanted with a cannula above the DG region. Animals were given SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol, 0.5 µl/rat) into the DG region as orexin-1 receptor (OX1r) and orexin-2 receptor (OX2r) antagonists, respectively, five min before exposure to a 3-hour restraint stress (RS) period. Animals were then undergone the formalin test to assess pain-related behaviors as the animal model of chronic pain. The results showed that RS produces an analgesic response during the early and late phases of the formalin test. However, intra-DG microinjection of OX1r and OX2r antagonists attenuated the restraint SIA. OX2r antagonist was more potent than OX1r antagonist in the early phase of the formalin test, while OX1r antagonist was little more effective in the late phase. Predominantly, it could be concluded that the orexinergic system in the DG region might act as a potential endogenous pain control system and a novel target for treating stress-related disorders.

The integrative role of orexin-1 and orexin-2 receptors within the hippocampal dentate gyrus in the modulation of the stress-induced antinociception in the formalin pain test in the rat

The stressful experiences, by triggering a cascade of hormonal and neural changes, can produce antinociception commonly referred to as stress-induced antinociception (SIA). Orexin neuropeptides have an essential role in stress responses and pain modulation. The dentate gyrus receives orexinergic projections and has been shown to be involved in pain processing. The current study investigated the possible role of orexin-1 and orexin-2 receptors (OX1r and OX2r, respectively) within the dentate gyrus in SIA in a rat model of formalin-induced pain behavior in one hind paw. Male Wistar rats weighing 230-250 g underwent stereotaxic surgery and a cannula was implanted in their brains, above the dentate gyrus region. Either SB334867 or TCS OX2 29 (OX1r and OX2r antagonists, respectively) was microinjected into the dentate gyrus region at a range of doses at 1, 3, 10, and 30 nmol (control group received DMSO 12% as vehicle), 5 min before the forced swim stress (FSS) exposure. The formalin test was performed to assess pain-related behaviors. The results indicated that FSS exposure relieves pain-related behavior in the early and late phases of the formalin test. Blockade of intra-dentate gyrus OX1 or OX2 receptors reduced the antinociceptive responses induced by FSS in the formalin test, with more impact during the late phase. Our findings support the potential role of intra-dentate gyrus orexin receptors as target sites of orexin neurons in painful and stressful situations. Therefore, understanding the exact mechanisms of SIA and the role of the orexinergic system in this phenomenon can lead to identifying the strategies to guide future research and offer a new approach to discovering new pain therapeutic agents.

Bringing to light the physiological and pathological firing patterns of human induced pluripotent stem cell-derived neurons using optical recordings

Human induced pluripotent stem cells (hiPSCs) are a promising approach to study neurological and neuropsychiatric diseases. Most methods to record the activity of these cells have major drawbacks as they are invasive or they do not allow single cell resolution. Genetically encoded voltage indicators (GEVIs) open the path to high throughput visualization of undisturbed neuronal activity. However, conventional GEVIs perturb membrane integrity through inserting multiple copies of transmembrane domains into the plasma membrane. To circumvent large add-ons to the plasma membrane, we used a minimally invasive novel hybrid dark quencher GEVI to record the physiological and pathological firing patterns of hiPSCs-derived sensory neurons from patients with inherited erythromelalgia, a chronic pain condition associated with recurrent attacks of redness and swelling in the distal extremities. We observed considerable differences in action potential firing patterns between patient and control neurons that were previously overlooked with other recording methods. Our system also performed well in hiPSC-derived forebrain neurons where it detected spontaneous synchronous bursting behavior, thus opening the path to future applications in other cell types and disease models including Parkinson's disease, Alzheimer's disease, epilepsy, and schizophrenia, conditions associated with disturbances of neuronal activity and synchrony.

Role of orexin receptors within the dentate gyrus in antinociception induced by chemical stimulation of the lateral hypothalamus in an animal model of inflammatory pain

Pain is a complex experience consisting of sensory, affective-motivational, and cognitive dimensions. Hence, identifying the multiple neural pathways subserving these functional aspects is a valuable task. The role of dentate gyrus (DG) as a relay station of neocortical afferents in the hippocampal formation (HF) in persistent pain is still controversial. The lateral hypothalamus (LH)-HF neural circuits are involved in numerous situations such as anxiety-like behavior, reward processing, feeding, orofacial as well as acute pain. Nonetheless, to our knowledge, the involvement of the LH-DG neural circuit in persistent pain has already remained unexplored. Adult male Wistar rats weighing 220-250 g were undergone stereotaxic surgery for unilateral implantation of two separate cannulae into the LH and DG. Intra-DG administration of the orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists, SB334867 and TCS OX2 29, respectively, was performed 5 min before intra-LH microinjection of carbachol. Animals were then undergone the formalin test using 50 μl formalin injection (2.5%) into the plantar surface of the hind paw. Microinjection of SB334867 or TCS OX2 29 into the DG region attenuated the antinociceptive effect produced by carbachol microinjection into the LH. The preventive effect of SB334867 and TCS OX2 29 on intra-LH carbachol-induced antinociception was approximately equal in both early and late phases of formalin nociception. The results suggest a neural pathway from the LH to the DG, which contributes to the modulation of formalin-induced inflammatory pain through the recruitment of OX1 and OX2 receptors within the DG.

NK1 tachykinin receptor treatment is superior to capsaicin pre-treatment in improving functional outcome following acute ischemic stroke

Previous results from our laboratory have shown that blockade of the substance P (SP) pathway with an NK1 tachykinin receptor antagonist significantly reduces blood brain barrier breakdown, cerebral edema and functional deficits following ischemic stroke. However, it is unclear whether removal of all neuropeptides is more efficacious than blocking SP alone. As such, the aim of the present study was to determine the effect of neuropeptide depletion with capsaicin pre-treatment on functional outcome following acute ischemic stroke in rats. Animals received 125 mg/kg of capsaicin or equal volume of saline vehicle, administered subcutaneously over a 3-day period. At 14 days following treatment animals were subject to 2h of middle cerebral artery occlusion followed by reperfusion. A subset of animals was treated with an NK1 tachykinin receptor antagonist (NAT) or vehicle at 4h after the onset of stroke only. The functional outcome of animals was assessed for a 7-day period following stroke using a rotarod device, the bilateral asymmetry test, modified neurological severity score, open field and angleboard. Although capsaicin pre-treatment improved outcome, treatment with an NK1 tachykinin receptor antagonist was superior in improving post-stroke functional outcome. This data suggests that some neuropeptides may play a beneficial role following stroke, whilst others such as SP are deleterious.

Requirement of phospholipase C and protein kinase C in cholecystokinin-mediated facilitation of NMDA channel function and anxiety-like behavior

Although cholecystokinin (CCK) has long been known to exert anxiogenic effects in both animal anxiety models and humans, the underlying cellular and molecular mechanisms are ill-defined. CCK interacts with CCK-1 and CCK-2 receptors resulting in up-regulation of phospholipase C (PLC) and protein kinase C (PKC). However, the roles of PLC and PKC in CCK-mediated anxiogenic effects have not been determined. We have shown previously that CCK facilitates glutamate release in the hippocampus especially at the synapses formed by the perforant path and dentate gyrus granule cells via activations of PLC and PKC. Here we further demonstrated that CCK enhanced NMDA receptor function in dentate gyrus granule cells via activation of PLC and PKC pathway. At the single-channel level, CCK increased NMDA single-channel open probability and mean open time, reduced the mean close time, and had no effects on the conductance of NMDA channels. Because elevation of glutamatergic functions results in anxiety, we explored the roles of PLC and PKC in CCK-induced anxiogenic actions using the Vogel Conflict Test (VCT). Our results from both pharmacological approach and knockout mice demonstrated that microinjection of CCK into the dentate gyrus concentration-dependently increased anxiety-like behavior via activation of PLC and PKC. Our results provide a novel unidentified signaling mechanism whereby CCK increases anxiety.

Regional changes in gene expression after limbic kindling

Repeated electrical stimulation results in development of seizures and a permanent increase in seizure susceptibility (kindling). The permanence of kindling suggests that chronic changes in gene expression are involved. Kindling at different sites produces specific effects on interictal behaviors such as spatial cognition and anxiety, suggesting that causal changes in gene expression might be restricted to the stimulated site. We employed focused microarray analysis to characterize changes in gene expression associated with amygdaloid and hippocampal kindling. Male Long-Evans rats received 1 s trains of electrical stimulation to either the amygdala or hippocampus once daily until five generalized seizures had been kindled. Yoked control rats carried electrodes but were not stimulated. Rats were euthanized 14 days after the last seizures, both amygdala and hippocampus dissected, and transcriptome profiles compared. Of the 1,200 rat brain-associated genes evaluated, 39 genes exhibited statistically significant expression differences between the kindled and non-kindled amygdala and 106 genes exhibited statistically significant differences between the kindled and non-kindled hippocampus. In the amygdala, subsequent ontological analyses using the GOMiner algorithm demonstrated significant enrichment in categories related to cytoskeletal reorganization and cation transport, as well as in gene families related to synaptic transmission and neurogenesis. In the hippocampus, significant enrichment in gene expression within categories related to cytoskeletal reorganization and cation transport was similarly observed. Furthermore, unique to the hippocampus, enrichment in transcription factor activity and GTPase-mediated signal transduction was identified. Overall, these data identify specific and unique neurochemical pathways chronically altered following kindling in the two sites, and provide a platform for defining the molecular basis for the differential behaviors observed in the interictal period.

An in vivo mouse model of long-term potentiation at synapses between primary afferent C-fibers and spinal dorsal horn neurons: essential role of EphB1 receptor

Background

Long-term potentiation (LTP), a much studied cellular model of synaptic plasticity, has not been demonstrated at synapses between primary afferent C-fibers and spinal dorsal horn (DH) neurons in mice in vivo. EphrinB-EphB receptor signaling plays important roles in synaptic connection and plasticity in the nervous system, but its role in spinal synaptic plasticity remains unclear.

Results

This study characterizes properties of LTP at synapses of C-fibers onto neurons in the superficial DH following high-frequency stimulation (HFS) of a peripheral nerve at an intensity that activates C-fibers and examines associated activation of Ca²⁺/calmodulin-activated protein kinase II (p-CaMKII), extracellular signal-regulated kinase (p-ERK) and the cyclic AMP response element binding protein (p-CREB) and expression of c-Fos, and it investigates further roles for the EphB1 receptor in LTP. HFS induced LTP within 5 min and lasts for 3–8 h during the period of recording and resulted in upregulation of p-CaMKII, p-ERK and p-CREB protein levels in the spinal cord and expression of c-Fos in DH. Intrathecal pretreatment of MK-801 or EphB2-Fc prevented LTP and significantly reduced upregulation of p-CaMKII, p-ERK, p-CREB and c-Fos. Further, targeted mutation of EphB1 receptor prevented induction of LTP and associated increases in phosphorylation of CaMKII, ERK, and CREB.

Conclusion

This study provides an in vivo mouse model of LTP at synapses of C-fibers onto the superficial DH neurons that will be valuable for studying the DH neuron excitability and their synaptic plasticity and hyperalgesia. It further takes advantage of examining functional implications of a specific gene targeted mice and demonstrates that the EphB1 receptor is essential for development of LTP.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Understanding LTP in pain pathways

Long-term potentiation (LTP) at synapses of nociceptive nerve fibres is a proposed cellular mechanism underlying some forms of hyperalgesia. In this review fundamental properties of LTP in nociceptive pathways are described. The following topics are specifically addressed: A concise definition of LTP is given and a differentiation is made between LTP and "central sensitisation". How to (and how not to) measure and how to induce LTP in pain pathways is specified. The signal transduction pathways leading to LTP at C-fibre synapses are highlighted and means of how to pre-empt and how to reverse LTP are delineated. The potential functional roles of LTP are evaluated at the cellular level and at the behavioural level in experimental animals. Finally, the impact of LTP on the perception of pain in human subjects is discussed.

The impact of chronic nandrolone decanoate administration on the NK1 receptor density in rat brain as determined by autoradiography

Adult male Sprague-Dawley rats were treated with the anabolic androgenic steroid nandrolone decanoate (15 mg/kg day) or oil vehicle (sterile arachidis oleum) during 14 days. The effect on the densities of the neurokinin NK1 receptor in brain was examined with autoradiography. An overall tendency of attenuation of NK1 receptor density was observed after completed treatment with nandrolone decanoate. The density of the NK1 receptor was found to be significantly lower compared to control animals in the nucleus accumbens core (37% density reduction), in dentate gyrus (26%), in basolateral amygdaloid nucleus (23%), in ventromedial hypothalamic nucleus (36%), in dorsomedial hypothalamic nucleus (43%) and finally in the periaqueductal gray (PAG) (24%). In the cortex region, no structures exhibited any significant reduction of NK1 receptor density. This result provides additional support to the hypothesis that substance P and the NK1 receptor may be involved as important components that participate in mediating physiological responses including the adverse behaviors often associated with chronically administrated anabolic androgenic steroids in human.

Chronic stress decreases the number of parvalbumin-immunoreactive interneurons in the hippocampus: prevention by treatment with a substance P receptor (NK1) antagonist

Previous studies have demonstrated that stress may affect the hippocampal GABAergic system. Here, we examined whether long-term psychosocial stress influenced the number of parvalbumin-containing GABAergic cells, known to provide the most powerful inhibitory input to the perisomatic region of principal cells. Adult male tree shrews were submitted to 5 weeks of stress, after which immunocytochemical and quantitative stereological techniques were used to estimate the total number of hippocampal parvalbumin-immunoreactive (PV-IR) neurons. Stress significantly decreased the number of PV-IR cells in the dentate gyrus (DG) (-33%), CA2 (-28%), and CA3 (-29%), whereas the CA1 was not affected. Additionally, we examined whether antidepressant treatment offered protection from this stress-induced effect. We administered fluoxetine (15 mg/kg per day) and SLV-323 (20 mg/kg per day), a novel neurokinin 1 receptor (NK1R) antagonist, because the NK1R has been proposed as a possible target for novel antidepressant therapies. Animals were subjected to a 7-day period of psychosocial stress before the onset of daily oral administration of the drugs, with stress continued throughout the 28-day treatment period. NK1R antagonist administration completely prevented the stress-induced reduction of the number of PV-IR interneurons, whereas fluoxetine attenuated this decrement in the DG, without affecting the CA2 and CA3. The effect of stress on interneuron numbers may reflect real cell loss; alternatively, parvalbumin concentration is diminished in the neurons, which might indicate a compensatory attempt. In either case, antidepressant treatment offered protection from the effect of stress and appears to modulate the hippocampal GABAergic system. Furthermore, the NK1R antagonist SLV-323 showed neurobiological efficacy similar to that of fluoxetine.

Neurokinin-1 receptors in the rat nucleus tractus solitarius: pre- and postsynaptic modulation of glutamate and GABA release

Neurokinins such as substance P and neurokinin A have long been thought to act as neurotransmitters or modulators in the nucleus tractus solitarius. However, the role and location of the receptors for these peptides have remained unclear. We examined the consequences of activation of the neurokinin-1 (NK1) receptor subtype in the rat nucleus tractus solitarius using whole-cell patch clamp recordings in brain slices. Application of delta-Ala-Phe-Phe-Pro-MeLeu-D-Pro[spiro-gamma-lactam]-Leu-Trp-NH2 (a specific NK1 agonist) or neurokinin A resulted in depolarization, evident as a slow inward current, mediated by direct postsynaptic NK1 receptor activation. The effect was conserved in the presence of tetrodotoxin, and protein kinase C-dependent since it was blocked by 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide, a specific protein kinase C inhibitor. In addition, an increase in the frequency and amplitude of spontaneous excitatory postsynaptic currents was observed, reflecting increased glutamate release induced by NK1 receptor activation. This effect was abolished by tetrodotoxin, suggesting that it resulted from increased firing in afferent neurons, subsequent to somatodendritic excitation via NK1 receptors. Furthermore, spontaneous inhibitory postsynaptic currents were increased in frequency and amplitude showing that GABA release was promoted by NK1 receptor activation. However, amplitude of miniature inhibitory postsynaptic currents was unaltered by NK1 receptor activation, but the increase in frequency persisted. These findings suggest that NK1 receptors are located on presynaptic terminals as well as at somatodendritic sites of GABAergic neurons. The increase in GABA release was also shown to be protein kinase C-dependent. The data presented here show NK1 receptors in the rat nucleus tractus solitarius are present both excitatory and inhibitory neurons. Activation of these receptors can result in increases in release of both GABA and glutamate, suggesting a crucial modulatory role for NK1 receptors in the rat nucleus tractus solitarius.

Increased neurogenesis and brain-derived neurotrophic factor in neurokinin-1 receptor gene knockout mice

It has previously been shown that chronic treatment with antidepressant drugs increases neurogenesis and levels of brain-derived neurotrophic factor in the hippocampus. These changes have been correlated with changes in learning and long-term potentiation and may contribute to the therapeutic efficacy of antidepressant drug treatment. Recently, antagonists at the neurokinin-1 receptor, the preferred receptor for the neuropeptide substance P, have been shown to have antidepressant activity. Mice with disruption of the neurokinin-1 receptor gene are remarkably similar both behaviourally and neurochemically to mice maintained chronically on antidepressant drugs. We demonstrate here that there is a significant elevation of neurogenesis but not cell survival in the hippocampus of neurokinin-1 receptor knockout mice. Neurogenesis can be increased in wild-type but not neurokinin-1 receptor knockout mice by chronic treatment with antidepressant drugs which preferentially target noradrenergic and serotonergic pathways. Hippocampal levels of brain-derived neurotrophic factor are also two-fold higher in neurokinin-1 receptor knockout mice, whereas cortical levels are similar. Finally, we examined hippocampus-dependent learning and memory but found no clear enhancement in neurokinin-1 receptor knockout mice. These data argue against a simple correlation between increased levels of neurogenesis or brain-derived neurotrophic factor and mnemonic processes in the absence of increased cell survival. They support the hypothesis that increased neurogenesis, perhaps accompanied by higher levels of brain-derived neurotrophic factor, may contribute to the efficacy of antidepressant drug therapy.

Changes in the levels of neuropeptides and their metabolizing enzymes in the brain regions of nucleus basalis magnocellularis-lesioned rats

The regulation mechanism of the interrelation between neuropeptides and their metabolizing enzymes in in vivo tissues is still not clear. In the present report, we attempted to measure the levels of neuropeptides and their enzymes in the frontal cortex, hippocampus, and striatum of the rat that had been bilaterally lesioned by the infusion of ibotenic acid or amyloid beta-peptide 25 - 35 (Abeta25 - 35) into the nucleus basalis magnocellularis. In the drug-treated rats, at two weeks after the infusion, the decrease of somatostatin-like immunoreactivity (SS-LI) and the increase of cholecystokinin-8S-LI were found in some brain regions relative to vehicle-treated rats. The immunoreactivities of endopeptidase 24.15 and puromycin-sensitive aminopeptidase and the leucine aminopeptidase- and aminopeptidase B-like enzyme activities did not change in the three brain regions, suggesting that the levels of those peptide-degrading enzymes do not correlate with the changes of the neuropeptide levels. The decrease of subtilisin-like proprotein convertase (SPC)-like enzyme activity was found in the hippocampus of the Abeta25 - 35-treated rats. The SS mRNA level decreased in the hippocampus in parallel with decreases in the SS-LI level and SPC-like enzyme activity. The present data indicate that some of the neuropeptide-processing enzymes may contribute to the control of neuropeptide levels.

Biphasic effects of substance P on respiratory activity and respiration-related neurones in ventrolateral medulla in the neonatal rat brainstem in vitro

The effects of substance P (SP) on respiratory activity in the brainstem-spinal cord preparation from neonatal rats (0-4 days old) were investigated. The respiratory activity was recorded from C4 ventral roots and intracellularly from three types of respiration-related neurones, i.e. pre-inspiratory (or biphasic E), three subtypes of inspiratory; expiratory and tonic neurones in the ventrolateral medulla (VLM). After the onset of SP bath application (10 nM-1 microM) a dose-dependent decline of burst rate (by 48%) occurred, followed by a weaker dose-dependent increase (by 17.5%) in burst rate. The biphasic effect of SP on inspiratory burst rate was associated with sustained membrane depolarization (in a range of 0.5-13 mV) of respiration-related and tonic neurones. There were no significant changes in membrane resistance in any type of neurones when SP was applied alone or when synaptic transmission was blocked with tetrodotoxin (TTX). The initial depolarization was associated with an increase in inspiratory drive potential (by 25%) as well as in bursting time (by 65%) and membrane excitability in inspiratory and pre-inspiratory neurones, which corresponded to the decrease in burst rate (C4 activity). The spiking frequency of expiratory and tonic neurones was also increased (by 36 and 48%). This activation was followed by restoration of the synaptic drive potential and bursting time in inspiratory and to a less extent in pre-inspiratory neurones, which corresponded to the increase in burst rate. The discharge frequency of expiratory and tonic neurones also decreased to control values. This phase followed the peak membrane depolarization. At the peak depolarization, SP reduced the amplitude of the action potential by 4-8% in all types of neurones. Our results suggest that SP exerts a general excitatory effect on respiration-related neurones and synaptic coupling within the respiratory network in the VLM. The transient changes in neuronal activity in the VLM may underlie the biphasic effect of SP in the brainstem respiration activity recorded in C4 roots. However, the biphasic effect of SP on inspiratory burst rate seems to be also defined by the balance in activity of other SP-sensitive systems and neurones in the respiratory network in the brainstem and spinal cord, which can modify the activity of medullary respiratory rhythm generator.

Distribution and regulation of substance P-related peptide in the frog visual system

Modulation of visual signal activity has consequences for both signal processing and for activity-dependent structuring mechanisms. Among the neuromodulatory agents found in visual areas are substance P (SP)-related peptides. This article reviews what is known about these substances in the amphibian retina and optic tectum with special emphasis on the leopard frog, Rana pipiens. It is found that the distribution of these SP-related peptides is remarkably similar to that seen in mammals. This suggests that study of model amphibian systems may significantly enhance our understanding of how neuropeptides contribute to visual system function and organization.

Modulation of hypoglossal motoneuron excitability by NK1 receptor activation in neonatal mice in vitro

1. The effects of substance P (SP), acting at NK1 receptors, on the excitability and inspiratory activity of hypoglossal (XII) motoneurons (MNs) were investigated using rhythmically active medullary-slice preparations from neonatal mice (postnatal day 0-3). 2. Local application of the NK1 agonist [SAR(9),Met (O(2))(11)]-SP (SP(NK1)) produced a dose-dependent, spantide- (a non-specific NK receptor antagonist) and GR82334-(an NK1 antagonist) sensitive increase in inspiratory burst amplitude recorded from XII nerves. 3. Under current clamp, SP(NK1) significantly depolarized XII MNs, potentiated repetitive firing responses to injected currents and produced a leftward shift in the firing frequency-current relationships without affecting slope. 4. Under voltage clamp, SP(NK1) evoked an inward current and increased input resistance, but had no effect on inspiratory synaptic currents. SP(NK1) currents persisted in the presence of TTX, were GR82334 sensitive, were reduced with hyperpolarization and reversed near the expected E(K). 5. Effects of the alpha(1)-noradrenergic receptor agonist phenylephrine (PE) on repetitive firing behaviour were virtually identical to those of SP(NK1). Moreover, SP(NK1) currents were completely occluded by PE, suggesting that common intracellular pathways mediate the actions of NK1 and alpha(1)-noradrenergic receptors. In spite of the similar actions of SP(NK1) and PE on XII MN responses to somally injected current, alpha(1)-noradrenergic receptor activation potentiated inspiratory synaptic currents and was more than twice as effective in potentiating XII nerve inspiratory burst amplitude. 6. GR82334 reduced XII nerve inspiratory burst amplitude and generated a small outward current in XII MNs. These observations, together with the first immunohistochemical evidence in the newborn for SP immunopositive terminals in the vicinity of SP(NK1)-sensitive inspiratory XII MNs, support the endogenous modulation of XII MN excitability by SP. 7. In contrast to phrenic MNs (Ptak et al. 2000), blocking NMDA receptors with AP5 had no effect on the modulation of XII nerve activity by SP(NK1). 8. In conclusion, SP(NK1) modulates XII motoneuron responses to inspiratory drive primarily through inhibition of a resting, postsynaptic K+ leak conductance. The results establish the functional significance of SP in controlling upper airway tone during early postnatal life and indicate differential modulation of motoneurons controlling airway and pump muscles by SP.

Adaptive plasticity in tachykinin and tachykinin receptor expression after focal cerebral ischemia is differentially linked to gabaergic and glutamatergic cerebrocortical circuits and cerebrovenular endothelium

To test the hypothesis of an involvement of tachykinins in destabilization and hyperexcitation of neuronal circuits, gliosis, and neuroinflammation during cerebral ischemia, we investigated cell-specific expressional changes of the genes encoding substance P (SP), neurokinin B (NKB), and the tachykinin/neurokinin receptors (NK1, NK2, and NK3) after middle cerebral artery occlusion (MCAO) in the rat. Our analysis by quantitative in situ hybridization, immunohistochemistry, and confocal microscopy was concentrated on cerebrocortical areas that survive primary infarction but undergo secondary damage. Here, SP-encoding preprotachykinin-A and NK1 mRNA levels and SP-like immunoreactivity were transiently increased in GABAergic interneurons at 2 d after MCAO. Coincidently, MCAO caused a marked expression of SP and NK1 in a subpopulation of glutamatergic pyramidal cells, and in some neurons SP and NK1 mRNAs were coinduced. Elevated levels of the NKB-encoding preprotachykinin-B mRNA and of NKB-like immunoreactivity at 2 and 7 d after MCAO were confined to GABAergic interneurons. In parallel, the expression of NK3 was markedly downregulated in pyramidal neurons. MCAO caused transient NK1 expression in activated cerebrovenular endothelium within and adjacent to the infarct. NK1 expression was absent from activated astroglia or microglia. The differential ischemia-induced plasticity of the tachykinin system in distinct inhibitory and excitatory cerebrocortical circuits suggests that it may be involved in the balance of endogenous neuroprotection and neurotoxicity by enhancing GABAergic inhibitory circuits or by facilitating glutamate-mediated hyperexcitability. The transient induction of NK1 in cerebrovenular endothelium may contribute to ischemia-induced edema and leukocyte diapedesis. Brain tachykinin receptors are proposed as potential drug targets in stroke.

Neuropeptides in hippocampus and cortex in transgenic mice overexpressing V717F beta-amyloid precursor protein--initial observations

Immunohistochemistry was used to analyse 18- and 26-month-old transgenic mice overexpressing the human beta-amyloid precursor protein under the platelet-derived growth factor-beta promoter with regard to presence and distribution of neuropeptides. In addition, antisera/antibodies to tyrosine hydroxylase, acetylcholinesterase, amyloid peptide, glial fibrillary acidic protein and microglial marker OX42 were used. These mice have been reported to exhibit extensive amyloid plaques in the hippocampus and cortex [Masliah et al. (1996) J. Neurosci. 16, 5795-5811]. The most pronounced changes were related to neuropeptides, whereas differences between wild-type and transgenic mice were less prominent with regard to tyrosine hydroxylase and acetylcholinesterase. The main findings were of two types; (i) involvement of peptide-containing neurites in amyloid beta-peptide positive plaques, and (ii) more generalized changes in peptide levels in specific layers, neuron populations and/or subregions in the hippocampal formation and ventral cortices. In contrast, the parietal and auditory cortices were comparatively less affected. The peptide immunoreactivities most strongly involved, both in plaques and in the generalized changes, were galanin, neuropeptide Y, cholecystokinin and enkephalin. This study shows that there is considerable variation both with regard to plaque load and peptide expression even among homozygotes of the same age. The most pronounced changes, predominantly increased peptide levels, were observed in two 26-month-old homozygous mice, for example, galanin-, enkephalin- and cholecystokinin-like immunoreactivities in stratum lacunosum moleculare, and galanin, neuropeptide Y, enkephalin and dynorphin in mossy fibers. Many peptides also showed elevated levels in the ventral cortices. However, decreases were also observed. Thus, galanin-like immunoreactivity could not any longer be detected in the diffusely distributed (presumably noradrenergic) fiber network in all hippocampal and cortical layers, and dynorphin-like immunoreactivity was decreased in stratum moleculare, cholecystokinin-like immunoreactivity in mossy fibers and substance P-like immunoreactivity in fibers around granule cells. The significance of generalized peptide changes is at present unclear. For example, the increase in the mainly inhibitory peptides galanin, neuropeptide Y, enkephalin and dynorphin and the decrease in the mainly excitatory peptide cholecystokinin in mossy fibers (and of substance P fibers around granule cells) indicate a shift in balance towards inhibition of the input to the CA3 pyramidal cell layer. Moreover, it may be speculated that the increase in levels of some of the peptides represents a reaction to nerve injury with the aim to counteract, in different ways, the consequences of injury, for example by exerting trophic actions. Further studies will be needed to establish to what extent these changes are typical for Alzheimer mouse models in general or are associated with the V717F mutation and/or the platelet-derived growth factor-beta promoter.

Cooperation of NMDA and tachykinin NK(1) and NK(2) receptors in the medullary transmission of vagal afferent input from the acid-threatened rat stomach

Noxious challenge of the rat gastric mucosa by hydrochloric acid (HCl) is signaled to the nucleus tractus solitarii (NTS) and area postrema (AP). This study examined the participation of glutamate and tachykinins in the medullary transmission process. Activation of neurons was visualized by in situ hybridization autoradiography of c-fos messenger RNA (mRNA) 45 min after intragastric (IG) administration of 0.5 M HCl or saline. IG HCl caused many neurons in the NTS and some neurons in the AP to express c-fos mRNA. The NMDA glutamate receptor antagonist MK-801 (2 mg/kg), the NK(1) tachykinin receptor antagonist GR-205,171 (3 mg/kg) and the NK(2) receptor antagonist SR-144,190 (0.1 mg/kg) failed to significantly reduce the NTS response to IG HCl, whereas the triple combination of MK-801, GR-205,171 and SR-144,190 inhibited it by 45--50%. Only in rats that had been preexposed IG to HCl 48 h before the experiment was MK-801 alone able to depress the NTS response to IG HCl. In contrast, the c-fos mRNA response in the AP was significantly augmented by MK-801, an action that was prevented by coadministration of GR-205,171 plus SR-144,190. Inhibition of neuronal nitric oxide synthase with 7-nitroindazole (45 mg/kg) was without effect on the IG HCl-evoked c-fos mRNA expression in the NTS and AP. Our data show that glutamate acting via NMDA receptors and tachykinins acting via NK(1) and NK(2) receptors cooperate in the vagal afferent input from the acid-threatened stomach to the NTS and participate in the processing of afferent input to the AP in a different and complex manner. These opposing interactions in the AP and NTS and the increase in NMDA receptor function in the NTS after a gastric acid insult are likely to have a bearing on the neuropharmacology of dyspepsia.

Intracerebroventricular injection of the N-terminal substance P fragment SP(1-7) regulates the expression of the N-methyl-D-aspartate receptor NR1, NR2A and NR2B subunit mRNAs in the rat brain

Studies have implicated substance P (SP) in the regulation of affective behaviour, memory function, pain influx, stress and opioid reward. All these dimensions are known to involve glutamate transmission mediated through the N-methyl-D-aspartate (NMDA) receptor. The SP N-terminal fragment SP(1-7) is shown to share some but oppose other effects of the parent compound. We have examined the effect of intracerebroventricular injections of SP(1-7) on the expression of the NMDA receptor subunits NR1, NR2A and NR2B mRNAs in the spinal cord and in discrete areas of the male rat brain. The results indicated that the heptapeptide induced a dose-dependent upregulation of the NR2A transcript in hippocampus, periaqueductal grey and ventral tegmental area, already within a few hours. The level of the NR2B mRNA was increased in hippocampus and nucleus accumbens. The expression of the transcript of the NR1 was enhanced in hippocampus and nucleus accumbens but attenuated in spinal cord. The observed effects of the SP(1-7) fragment are in agreement with what could be expected from the known effects of the heptapeptide on various behaviours involving glutamate transmission.

Neuropeptides in drug research

Neuropeptides have been a subject of considerable interest in the pharmaceutical industry over the last 20 years or more. Many drug discovery teams have contributed to our understanding of neuropeptide biology but no significant drugs that act selectively upon neuropeptide receptors have yet emerged from the clinic. There are, however, a plethora of clinically useful drugs that act at other classes of neurotransmitter and neuromodulator receptors, many of them discovered over the last 20 years. Nevertheless, we think that the future for the discovery of novel drugs acting at neuropeptide receptors looks bright for two reasons: (1) there has been a substantial increase in our understanding of the function of neuropeptides; and (2) high-throughput screening (HTS) against neuropeptide receptors has now begun to yield many interesting drug-like molecules, rather than peptides, that have the potential to become clinically useful drugs. The objective of this review is to summarise our current understanding of specific areas of neuropeptide biology and pharmacology in the CNS as well as the PNS. We will also speculate on where we think the new generation of neuropeptide agonists and antagonists could emerge from the clinic.

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.

Cellular and synaptic effect of substance P on neonatal phrenic motoneurons

Experiments were carried out on the in vitro brainstem-spinal cord preparation of the newborn rat to analyse the effects of substance P (SP) on phrenic motoneuron (PMN) activity. In current-clamp mode, SP significantly depolarized PMNs, increased their input resistance, decreased the rheobase current and shifted the firing frequency-intensity relationships leftwards, but did not affect spike frequency adaptation or single spike configuration. The neurokinin receptor agonist NK1 had SP-mimetic effects, whereas the NK3 and NK2 receptor agonists were less effective and ineffective, respectively. In a tetrodotoxin-containing aCSF, only SP or the NK1 receptor agonist were still active. No depolarization was observed when the NK1 receptor agonist was applied in the presence of muscarine. In voltage-clamp mode, SP or the NK1 receptor agonist produced an inward current (ISP) which was not significantly reduced by extracellular application of tetraethylammonium, Co2+, 4-aminopyridine or Cs+. In aCSF containing tetrodotoxin, Co2+ and Cs+, ISP was blocked by muscarine. No PMN displayed any M-type potassium current but only a current showing no voltage sensitivity over the range -100 to 0 mV, reversing near the expected EK +, hence consistent with a leak current. SP application to the spinal cord only (using a partitioned chamber) significantly increased the phrenic activity. Pretreatment with the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP5) decreased the C4 discharge duration and blocked the effect of SP, thus exhibiting an NMDA potentiation by SP. In conclusion, SP modulates postsynaptically the response of phrenic motoneurons to the inspiratory drive through the reduction of a leak conductance and the potentiation of the NMDA component of the synaptic input.

Distribution of Fos-like immunoreactivity in guinea-pig brain following administration of the neurokinin-1 receptor agonist, [SAR9,MET(O2)11]substance P

The tachykinins are a family of peptides with putative neurotransmitter roles in the nervous system. They mediate their effects via neurokinin-1, neurokinin-2 and neurokinin-3 receptors. There has been increasing interest in the therapeutic application of the tachykinin neurokinin-1 receptor antagonists in the treatment of pain and emesis, and more recently in depression. However, the central role of neurokinin-1 receptors is not well understood. The aims of the present study were to determine the behavioural responses of guinea-pigs, and the distribution of Fos-like immunoreactivity in the guinea-pig brain, following intracerebroventricular administration of the neurokinin-1 receptor-selective agonist, [Sar9,Met(O2)11]substance P. The effects of pretreatment with the neurokinin-1 receptor antagonist, SR 140333, were also investigated. Administration of [Sar9,Met(O2)11]substance P induced increased locomotor activity, as well as face washing, grooming and wet-dog shake behaviours, all of which were inhibited by the neurokinin-1 receptor antagonist, SR 140333, indicating the involvement of neurokinin-1 receptors. In order to localize the brain regions activated by [Sar9,Met(O2)11]substance P, the distribution of neurons expressing Fos-like immunoreactivity was examined. [Sar9,Met(O2)11]substance P induced increased Fos-like immunoreactivity in widespread areas, including the frontal cortex, hippocampus, amygdala, thalamus, hypothalamus, periaqueductal gray, area postrema and nucleus of the solitary tract. SR 140333 reduced Fos-like immunoreactivity induced by [Sar9,Met(O2)11]substance P in most areas. Thus, brain regions associated with emotion, sensation, learning and memory, autonomic regulation and emesis were activated by stimulation of neurokinin-1 receptors. The present data have added a functional domain to previous neurokinin-1 receptor localization studies by describing the extensive regions of the CNS that may be activated by stimulation of these receptors, and the potential of neurokinin-1 receptor antagonists to inhibit activation of these regions.

Regulation of Excitability in the Epileptic Human Dentate Gyrus

Numerous anatomical and physiological changes occur in the dentate gyrus of patients with medial temporal lobe sclerosis, a specific form of temporal lobe epilepsy. Although many of the reported changes are potentially preconvulsive, patients do not seize continuously. We hypothesize that neuromodulatory systems present in the epileptic dentate gyrus may help limit neuronal hyperexcitability and/or hypersynchronization. Three such systems are described in detail, including GABA, zinc, and adenosine. In addition, we briefly discuss several other modulatory systems that have not been studied extensively in the epileptic human hippocampus but that are also well suited to controlling neuronal excitability.

Resistance to excitotoxin-induced seizures and neuronal death in mice lacking the preprotachykinin A gene

Epileptic seizures are associated with increases in hippocampal excitability, but the mechanisms that render the hippocampus hyperexcitable chronically (in epilepsy) or acutely (in status epilepticus) are poorly understood. Recent evidence suggests that substance P (SP), a peptide that has been implicated in cardiovascular function, inflammatory responses, and nociception, also contributes to hippocampal excitability and status epilepticus, in part by enhancing glutamate release. Here we report that mice with disruption of the preprotachykinin A gene, which encodes SP and neurokinin A, are resistant to kainate excitoxicity. The mice show a reduction in the duration and severity of seizures induced by kainate or pentylenetetrazole, and both necrosis and apoptosis of hippocampal neurons are prevented. Although kainate induced the expression of bax and caspase 3 in the hippocampus of wild-type mice, these critical intracellular mediators of cell death pathways were not altered by kainate injection in the mutant mice. These results indicate that the reduction of seizure activity and the neuroprotection observed in preprotachykinin A null mice are caused by the extinction of a SP/neurokinin A-mediated signaling pathway that is activated by seizures. They suggest that these neurokinins are critical to the control of hippocampal excitability, hippocampal seizures, and hippocampal vulnerability.

The tachykinin NK1 receptor in the brain: pharmacology and putative functions

After its discovery in 1931, substance P (SP) remained the only mammalian member of the family of tachykinin peptides for several decades. Tachykinins thus refer to peptides sharing the common C-terminal amino acid sequence Phe-X-Gly-Leu-Met x NH2. In recent years the family of mammalian tachykinins has grown with the isolation of two novel peptides from bovine and porcine central nervous system (CNS), neurokinin A and neurokinin B. In parallel with the identification of multiple endogenous tachykinins several classes of tachykinin receptors were discovered. The receptors described so far are named tachykinin NK1 receptor, tachykinin NK2 receptor and tachykinin NK1 receptor, respectively. The present review focuses on the pharmacology and putative function of tachykinin NK1 receptors in brain. The natural ligand with the highest affinity for the tachykinin NK1 receptor is SP itself. The C-terminal sequence is essential for activity, the minimum length of a fragment with reasonable affinity for the tachykinin NK1 receptor is the C-terminal hexapeptide. A rapid advance of knowledge was caused by development of non-peptidic tachykinin NK1 receptor antagonists. This area is under rapid development and a variety of different chemical classes of compounds are involved. Species-dependent affinities of tachykinin NK1 receptor antagonists reveal two clusters of compounds, targeting the tachykinin NK1 receptor subtype found in guinea pig, human or ferret or the one in rat or mouse, respectively. The most recently developed compounds are highly selective, enter the brain and are orally bioavailable. Distinct behavioural effects in experimental animals suggest the involvement of tachykinin NK1 receptors in nociceptive transmission, basal ganglia function or anxiety and depression. Recent clinical trials in man showed that tachykinin NK1 receptor antagonists are effective in treating depression and chemotherapy-induced emesis. Therefore, it is well possible that tachykinin NK1 receptor antagonists will be clinically used for treatment of specific CNS disorders within a short period of time.