Postsynaptic actions of substance P on rat periaqueductal grey neurons in vitro

Substance P's Impact on Chronic Pain and Psychiatric Conditions-A Narrative Review

Substance P (SP) plays a crucial role in pain modulation, with significant implications for major depressive disorder (MDD), anxiety disorders, and post-traumatic stress disorder (PTSD). Elevated SP levels are linked to heightened pain sensitivity and various psychiatric conditions, spurring interest in potential therapeutic interventions. In chronic pain, commonly associated with MDD and anxiety disorders, SP emerges as a key mediator in pain and emotional regulation. This review examines SP's impact on pain perception and its contributions to MDD, anxiety disorders, and PTSD. The association of SP with increased pain sensitivity and chronic pain conditions underscores its importance in pain modulation. Additionally, SP influences the pathophysiology of MDD, anxiety disorders, and PTSD, highlighting its potential as a therapeutic target. Understanding SP's diverse effects provides valuable insights into the mechanisms underlying these psychiatric disorders and their treatment. Further research is essential to explore SP modulation in psychiatric disorders and develop more effective treatment strategies.

Ionic signalling mechanisms involved in neurokinin-3 receptor-mediated augmentation of fear-potentiated startle response in the basolateral amygdala

The tachykinin peptides include substance P (SP), neurokinin A and neurokinin B, which interact with three G-protein-coupled neurokinin receptors, NK1Rs, NK2Rs and NK3Rs, respectively. Whereas high densities of NK3Rs have been detected in the basolateral amygdala (BLA), the functions of NK3Rs in this brain region have not been determined. We found that activation of NK3Rs by application of the selective agonist, senktide, persistently excited BLA principal neurons. NK3R-elicited excitation of BLA neurons was mediated by activation of a non-selective cation channel and depression of the inwardly rectifying K+ (Kir) channels. With selective channel blockers and knockout mice, we further showed that NK3R activation excited BLA neurons by depressing the G protein-activated inwardly rectifying K+ (GIRK) channels and activating TRPC4 and TRPC5 channels. The effects of NK3Rs required the functions of phospholipase Cβ (PLCβ), but were independent of intracellular Ca2+ release and protein kinase C. PLCβ-mediated depletion of phosphatidylinositol 4,5-bisphosphate was involved in NK3R-induced excitation of BLA neurons. Microinjection of senktide into the BLA of rats augmented fear-potentiated startle (FPS) and this effect was blocked by prior injection of the selective NK3R antagonist SB 218795, suggesting that activation of NK3Rs in the BLA increased FPS. We further showed that TRPC4/5 and GIRK channels were involved in NK3R-elicited facilitation of FPS. Our results provide a cellular and molecular mechanism whereby NK3R activation excites BLA neurons and enhances FPS. KEY POINTS: Activation of NK3 receptors (NK3Rs) facilitates the excitability of principal neurons in rat basolateral amygdala (BLA). NK3R-induced excitation is mediated by inhibition of GIRK channels and activation of TRPC4/5 channels. Phospholipase Cβ and depletion of phosphatidylinositol 4,5-bisphosphate are necessary for NK3R-mediated excitation of BLA principal neurons. Activation of NK3Rs in the BLA facilitates fear-potentiated startle response. GIRK channels and TRPC4/5 channels are involved in NK3R-mediated augmentation of fear-potentiated startle.

Cannabinoids in the descending pain modulatory circuit: Role in inflammation

The legalization of cannabis in some states has intensified interest in the potential for cannabis and its constituents to lead to novel therapeutics for pain. Our understanding of the cellular mechanisms underlying cannabinoid actions in the brain have lagged behind opioids; however, the current opioid epidemic has also increased attention on the use of cannabinoids as alternatives to opioids for pain, especially chronic pain that requires long-term use. Endogenous cannabinoids are lipid signaling molecules that have complex roles in modulating neuronal function throughout the brain. In this review, we discuss cannabinoid functions in the descending pain modulatory pathway, a brain circuit that integrates cognitive and emotional processing of pain to modulate incoming sensory inputs. In addition, we highlight areas where further studies are necessary to understand cannabinoid regulation of descending pain modulation.

Tibetan medicine Ru-yi-Zhen-bao Pills exhibits anti-migraine effect through mediating PAG anti-nociceptive channel

ETHNOPHARMACOLOGICAL RELEVANCE

Migraine is a disabling neurovascular disorder, which increases risk of cardiovascular events and is a social burden worldwide. The present first-line anti-migraine medications can cause overwhelming side-effects, of which one includes the onset of cardiovascular disease. As one of the marketed Tibetan drugs, Ru-yi-Zhen-bao Pills (RYZBP) have been clinically used to treat cardiovascular disorders and as anti-migraine medication. However, there is currently no research exploring the anti-migraine actions of RYZBP.

AIM OF THE STUDY

The current research was designed to assess the anti-migraine roles of RYZBP and explore the underlying mechanisms in a nitroglycerin (NTG)-induced migraine rat model trial.

MATERIALS AND METHODS

120 rats were randomly divided into the following six groups of 20 rats each: normal control group, model control group, positive control group, and RYZBP high/medium/low-dose groups (Ru-yi-Zhen-bao Pills; TH 1.00 g/kg, TM 0.50 g/kg and TL 0.25 g/kg). All rats were administered intragastrically for 7 consecutive days, which were subcutaneously injected with the NTG (10 mg/kg) after the last gavage (except in the normal control group). 3min after NTG treatment, 30 rats (5 rats from each group) were anesthetized and devoted to electroencephalogram(EEG) testing, which was used to evaluate the analgesic effect of RYZBP. One hour after NTG treatment, the rest of the 90 rats (15 rats from each group) were anesthetized and midbrain tissue sample was dissected. The dissection was then washed with physiological saline and collected. The histopathological changes in the periaqueductal gray(PAG) of 5 tissue samples were determined by aematoxylin-eosin (H&E) staining, as well as an estimation of substance P (SP) and neurokinin 1 receptor (NK1R) expression through immunohistochemically staining(IHC). Another 5 midbrain preparations were carried out to evaluate calcitonin gene-related peptide (CGRP), proenkephalin (PENK), SP, and cholecystokinin (CCK) expressions by real-time quantitative polymerase chain reaction (RT-qPCR). The rest of the 5 brainstem tissues were then used to measure CCK, CGRP, and opioid peptide receptor (DORR) levels by western blotting(WB).

RESULTS

In the EEG test, RYZBP (TM 0.50 g / kg) treatment transformed the EEG pain-wave of the NTG-induced migraine model rats in different time period. In the mechanism assay, compared with the model control group, RYZBP pretreatment reduced inflammatory cell infiltration, fibrosis and vacuolation of neuronal cells of PAG tissue seen by HE staining. IHC experiments further showed that RYZBPTM up-regulated SP expression levels and enhanced NK1R levels in the NTG-induced migraine rats (P < 0.05). Therapeutic administration of RYZBP also increased PENK mRNA expression and DORR protein level. Both RT-qPCR and western blotting trials indicated that RYZBP treatment significantly decreased CCK and CGRP expression levels (P < 0.01 or P < 0.05) in the NTG-induced migraine rats.

CONCLUSIONS

RYZBP has the potential to be an effective anti-migraine treatment through suppressing the EEG pain-wave, increasing the levels of SP, PENK, DORR and reducing expression of CCK and CGRP. Mediating the PAG anti-nociceptive channel and inhibiting central sensitization were the two potential mechanisms, which offers further evidence for clinical therapy.

Substance P and pain chronicity

Substance P (SP) is a highly conserved member of the tachykinin peptide family that is widely expressed throughout the animal kingdom. The numerous members of the tachykinin peptide family are involved in a multitude of neuronal signaling pathways, mediating sensations and emotional responses (Steinhoff et al. in Physiol Rev 94:265–301, 2014). In contrast to receptors for classical transmitters, such as glutamate (Parsons et al. in Handb Exp Pharmacol 249–303, 2005), only a minority of neurons in certain brain areas express neurokinin receptors (NKRs) (Mantyh in J Clin Psychiatry 63:6–10, 2002). SP is also expressed by a variety of non-neuronal cell types such as microglia, as well as immune cells (Mashaghi et al. in Cell Mol Life Sci 73:4249–4264, 2016). SP is an 11-amino acid neuropeptide that preferentially activates the neurokinin-1 receptor (NK1R). It transmits nociceptive signals via primary afferent fibers to spinal and brainstem second-order neurons (Cao et al. in Nature 392:390–394, 1998). Compounds that inhibit SP’s action are being investigated as potential drugs to relieve pain. More recently, SP and NKR have gained attention for their role in complex psychiatric processes. It is a key goal in the field of pain research to understand mechanisms involved in the transition between acute pain and chronic pain. The influence of emotional and cognitive inputs and feedbacks from different brain areas makes pain not only a perception but an experience (Zieglgänsberger et al. in CNS Spectr 10:298–308, 2005; Trenkwaldner et al. Sleep Med 31:78–85, 2017). This review focuses on functional neuronal plasticity in spinal dorsal horn neurons as a major relay for nociceptive information.

Substance P excites GABAergic neurons in the mouse central amygdala through neurokinin 1 receptor activation

Substance P (SP) is implicated in stress regulation and affective and anxiety-related behavior. Particularly high expression has been found in the main output region of the amygdala complex, the central amygdala (CE). Here we investigated the cellular mechanisms of SP in CE in vitro, taking advantage of glutamic acid decarboxylase-green fluorescent protein (GAD67-GFP) knockin mice that yield a reliable labeling of GABAergic neurons, which comprise 95% of the neuronal population in the lateral section of CE (CEl). In GFP-positive neurons within CEl, SP caused a membrane depolarization and increase in input resistance, associated with an increase in action potential firing frequency. Under voltage-clamp conditions, the SP-specific membrane current reversed at -101.5 ± 2.8 mV and displayed inwardly rectifying properties indicative of a membrane K(+) conductance. Moreover, SP responses were blocked by the neurokinin type 1 receptor (NK1R) antagonist L-822429 and mimicked by the NK1R agonist [Sar(9),Met(O2)(11)]-SP. Immunofluorescence staining confirmed localization of NK1R in GFP-positive neurons in CEl, predominantly in PKCδ-negative neurons (80%) and in few PKCδ-positive neurons (17%). Differences in SP responses were not observed between the major types of CEl neurons (late firing, regular spiking, low-threshold bursting). In addition, SP increased the frequency and amplitude of GABAergic synaptic events in CEl neurons depending on upstream spike activity. These data indicate a NK1R-mediated increase in excitability and GABAergic activity in CEl neurons, which seems to mostly involve the PKCδ-negative subpopulation. This influence can be assumed to increase reciprocal interactions between CElon and CEloff pathways, thereby boosting the medial CE (CEm) output pathway and contributing to the anxiogenic-like action of SP in the amygdala.

Neurokinin B activates arcuate kisspeptin neurons through multiple tachykinin receptors in the male mouse

Kisspeptin neurons located in the arcuate nucleus (ARN) coexpress dynorphin and neurokinin B (NKB) and may interact to influence gonadotropin secretion. Using a kisspeptin-green fluorescent protein mouse model, the present study examined whether the neuropeptides kisspeptin, dynorphin, and NKB modulate the electrical activity of ARN kisspeptin neurons in the adult male mouse. Cell-attached recordings showed that kisspeptin itself had no effect on kisspeptin neuron firing. Dynorphin and the κ-opioid receptor agonist U50-488 evoked a potent suppression of all ARN kisspeptin neuron firing that was blocked completely by the κ-opioid receptor antagonist nor-Binaltorphimine. Both NKB and Senktide, a neurokinin 3 receptor agonist, exerted a potent stimulatory action on ∼95% of ARN kisspeptin neurons. Although the selective neurokinin 3 receptor antagonists SB222200 and SR142801 blocked the effects of Senktide on kisspeptin neurons, they surprisingly had no effect on NKB activation of firing. Studies with selective neurokinin 1 receptor (SDZ-NKT343) and neurokinin 2 receptor (GR94800) antagonists revealed that the activation of kisspeptin neurons by NKB was only blocked completely by a cocktail of antagonists against all 3 tachykinin receptors. Whole-cell recordings revealed that individual kisspeptin neurons were activated directly by all 3 tachykinins substance, P, neurokinin A, and NKB. These experiments show that dynorphin and NKB have opposing actions on the electrical activity of kisspeptin neurons supporting the existence of an interconnected network of kisspeptin neurons in the ARN. However, the effects of NKB result from an unexpected activation of multiple tachykinin receptors.

Substance P induces gastric mucosal protection at supraspinal level via increasing the level of endomorphin-2 in rats

The aim of the present study was to analyze the potential role of substance P (SP) in gastric mucosal defense and to clarify the receptors and mechanisms that may be involved in it. Gastric ulceration was induced by oral administration of acidified ethanol in male Wistar rats. Mucosal levels of calcitonin gene-related peptide (CGRP) and somatostatin were determined by radioimmunoassay. For analysis of gastric motor activity the rubber balloon method was used. We found that central (intracerebroventricular) injection of SP (9.3-74 pmol) dose-dependently inhibited the formation of ethanol-induced ulcers, while intravenously injected SP (0.37-7.4 nmol/kg) had no effect. The mucosal protective effect of SP was inhibited by pretreatment with neurokinin 1-, neurokinin 2-, neurokinin 3- and μ-opioid receptor antagonists, while δ- and κ-opioid receptor antagonists had no effect. Endomorphin-2 antiserum also antagonized the SP-induced mucosal protection. In the gastroprotective dose range SP failed to influence the gastric motor activity. Inhibition of muscarinic cholinergic receptors, or the synthesis of nitric oxide or prostaglandins significantly reduced the effect of SP. In addition, centrally injected SP reversed the ethanol-induced reduction of gastric mucosal CGRP content. It can be concluded, that SP may induce gastric mucosal protection initiated centrally. Its protective effect is likely to be mediated by endomorphin-2, and vagal nerve may convey the centrally initiated protection to the periphery, where both prostaglandins, nitric oxide and CGRP are involved in mediating this effect.

Postsynaptic mGluR mediated excitation of neurons in midbrain periaqueductal grey

Metabotropic glutamate (mGlu) receptors modulate pain from within the midbrain periaqueductal grey (PAG). In the present study, the postsynaptic mGlu receptor mediated effects on rat PAG neurons were examined using whole-cell patch-clamp recordings in brain slices. The selective group I agonist DHPG (10 μM) produced an inward current in all PAG neurons tested which was associated with a near parallel shift in the current-voltage relationship. By contrast, the group II and III mGlu receptor agonists DCG-IV (1 μM) and l-AP4 (3 μM) produced an outward current in only 10-20% of PAG neurons tested. The DHPG induced current was concentration dependent (EC(50) = 1.4 μM), was reduced by the mGlu1 antagonist CPCCOEt (100 μM), and was further reduced by CPCCOEt in combination with the mGlu5 antagonist MPEP (10 μM). The glutamate transport blocker TBOA (30 μM) also produced an inward current, however, this was largely abolished by CNQX (10 μM) plus AP5 (25 μM). Slow EPSCs were evoked following train, but not single shock stimulation, which were enhanced by TBOA (30 μM). The TBOA enhancement of slow EPSCs was abolished by MPEP plus CPCCOEt. These findings indicate that endogenously released glutamate, under conditions in which neurotransmitter spill-over is enhanced, activates group I mGlu receptors to produce excitatory currents within PAG. Thus, postsynaptic group I mGlu receptors have the potential to directly modulate the analgesic, behavioural and autonomic functions of the PAG. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Involvement of midbrain tectum neurokinin-mediated mechanisms in fear and anxiety

Electrical stimulation of midbrain tectum structures, particularly the dorsal periaqueductal gray (dPAG) and inferior colliculus (IC), produces defensive responses, such as freezing and escape behavior. Freezing also ensues after termination of dPAG stimulation (post-stimulation freezing). These defensive reaction responses are critically mediated by _Y-aminobutyric acid and 5-hydroxytryptamine mechanisms in the midbrain tectum. Neurokinins (NKs) also play a role in the mediation of dPAG stimulation-evoked fear, but how NK receptors are involved in the global processing and expression of fear at the level of the midbrain tectum is yet unclear. The present study investigated the role of NK-1 receptors in unconditioned defensive behavior induced by electrical stimulation of the dPAG and IC of male Wistar rats. Spantide (100 pmol/0.2 µL), a selective NK-1 antagonist, injected into these midbrain structures had anti-aversive effects on defensive responses and distress ultrasonic vocalizations induced by stimulation of the dPAG but not of the IC. Moreover, intra-dPAG injections of spantide did not influence post-stimulation freezing or alter exploratory behavior in rats subjected to the elevated plus maze. These results suggest that NK-1 receptors are mainly involved in the mediation of defensive behavior organized in the dPAG. Dorsal periaqueductal gray-evoked post-stimulation freezing was not affected by intra-dPAG injections of spantide, suggesting that NK-1-mediated mechanisms are only involved in the output mechanisms of defensive behavior and not involved in the processing of ascending aversive information from the dPAG.

Rizatriptan benzoate influences the endogenous pain modulatory system in a rat model of migraine

The present study utilized a nitroglycerin-induced rat model of migraine to detect the effects of rizatriptan benzoate on proenkephalin and substance P gene expression in the midbrain using real-time quantitative polymerase chain reaction and investigate whether rizatriptan benzoate can regulate the endogenous pain modulatory system. The results showed that rizatriptan benzoate significantly reduced expression of the mRNAs for proenkephalin and substance P. Rizatriptan benzoate may inhibit the analgesic effect of the endogenous pain modulatory system.

Effects of neurokinin-1 and 3-receptor antagonists on the defensive behavior induced by electrical stimulation of the dorsal periaqueductal gray

The dorsal periaqueductal gray (dPAG) is the main output structure for the defensive response to proximal aversive stimulation. Panic-like responses, such as freezing and escape behaviors, often result when this structure is electrically stimulated. Freezing also ensues after termination of the dPAG stimulation (post-stimulation freezing (PSF)). GABA and 5-HT have been proposed as the main neuromediators of these defense reactions. Neurokinins (NKs) also play a role in the defense reaction; however, it is unclear how the distinct types of NK receptors are involved in the expression of these fear responses. This study investigated the role of NK-1 and NK-3 receptors in the unconditioned defensive behaviors induced by electrical stimulation of the dPAG of rats, with and without previous experience with contextual fear conditioning (CFC). Spantide (100 ρmol/0.2 μl) and SB 222200 (50 and 100 ρmol/0.2 μl), selective antagonists of NK-1 and NK-3 receptors, respectively, were injected into the dPAG. Injection of spantide had antiaversive effects as determined by stimulation of the dPAG in naive animals and in animals subjected previously to CFC. SB 222200 also increased these aversive thresholds but only at doses that caused a motor deficit. Moreover, neither spantide nor SB 222200 influenced the PSF. The results suggest that NK-1 receptors are mainly involved in the mediation of the defensive behaviors organized in the dPAG. Because dPAG-evoked PSF was not affected by intra-dPAG injections of either spantide or SB 222200, it is suggested that neurokinin-mediated mechanisms are not involved in the processing of ascending aversive information from the dPAG.

Cholecystokinin exerts an effect via the endocannabinoid system to inhibit GABAergic transmission in midbrain periaqueductal gray

Cholecystokinin modulates pain and anxiety via its functions within brain regions such as the midbrain periaqueductal gray (PAG). The aim of this study was to examine the cellular actions of cholecystokinin on PAG neurons. Whole-cell patch clamp recordings were made from rat midbrain PAG slices in vitro to examine the postsynaptic effects of cholecystokinin and its effects on synaptic transmission. Sulfated cholecystokinin-(26-33) (CCK-S, 100-300 nM), but not non-sulfated cholecystokinin-(26-33) (CCK-NS, 100-300 nM) produced an inward current in a sub-population of opioid sensitive and insensitive PAG neurons, which did not reverse over a range of membrane potentials. The CCK-S-induced current was abolished by the CCK1 selective antagonist devazepide (100 nM), but not by the CCK2 selective antagonists CI988 (100 nM, 1 μM) and LY225910 (1 μM). CCK-S, but not CCK-NS produced a reduction in the amplitude of evoked GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) and an increase in the evoked IPSC paired-pulse ratio. By contrast, CCK-S had little effect on the rate and amplitude of TTX-resistant miniature IPSCs under basal conditions and when external K(+) was elevated. The CCK-S-induced inhibition of evoked IPSCs was abolished by the cannabinoid CB1 receptor antagonist AM251 (3 μM), the mGluR5 antagonist MPEP (10 μM) and the 1, 2-diacylglycerol lipase (DAGLα) inhibitor tetrahydrolipstatin (10 μM). In addition, CCK-S produced an increase in the rate of spontaneous non-NMDA-mediated, TTX-dependent excitatory postsynaptic currents (EPSCs). These results suggest that cholecystokinin produces direct neuronal depolarisation via CCK1 receptors and inhibits GABAergic synaptic transmission via action potential-dependent release of glutamate and mGluR5-induced endocannabinoid signaling. Thus, cholecystokinin has cellular actions within the PAG that can both oppose and reinforce opioid and cannabinoid modulation of pain and anxiety within this brain structure.

Control of enteric neuromuscular functions by purinergic A(3) receptors in normal rat distal colon and experimental bowel inflammation

BACKGROUND AND PURPOSE

Adenosine A(3) receptors mediate beneficial effects in experimental colitis, but their involvement in enteric neuromuscular functions during bowel inflammation is undetermined. This study investigated the regulatory role of A(3) receptors on colonic motility in the presence of experimental colitis.

EXPERIMENTAL APPROACH

Colitis was induced in rats by 2,4-dinitrobenzenesulfonic acid. A(3) receptors and adenosine deaminase (ADA, adenosine catabolic enzyme) mRNA were examined by RT-PCR. Tissue distribution of A(3) receptors was detected by confocal immunofluorescence. The effects of 2,3-ethyl-4,5-dipropyl-6-phenylpyridine-3-thiocarboxylate-5-carboxylate (MRS1523) (MRS, A(3) receptor antagonist), 2-chloro-N(6) -(3-iodobenzyl)-adenosine-5'-N-methyluronamide (2Cl-IB-MECA) (CIB, A(3) receptor agonist), dipyridamole (DIP, adenosine transport inhibitor) and ADA were assayed on contractile responses evoked by electrical stimulation (ES) or carbachol in colonic longitudinal muscle preparations (LMP).

KEY RESULTS

RT-PCR showed A(3) receptors and ADA mRNA in normal colon and their increased level in inflamed tissues. Immunofluorescence showed a predominant distribution of A(3) receptors in normal myenteric ganglia and an increased density during colitis. MRS enhanced ES-induced cholinergic contractions in normal LMP, but was less effective in inflamed tissues. After pretreatment with dipyridamole plus ADA, to reduce extracellular adenosine, CIB decreased cholinergic motor responses of normal LMP to ES, with enhanced efficacy in inflamed LMP. A(3) receptor ligands did not affect carbachol-induced contractions in LMP from normal or inflamed colon.

CONCLUSIONS AND IMPLICATIONS

Normally, adenosine modulated colonic cholinergic motility via activation of A(3) receptors in the myenteric plexus. A(3) receptor-mediated tonic inhibitory control by adenosine was impaired in inflamed bowel, despite increased density of functioning and pharmacologically recruitable A(3) receptors.

Substance P drives endocannabinoid-mediated disinhibition in a midbrain descending analgesic pathway

Substance P is thought to play an essential role in several forms of supraspinally mediated analgesia. The actions of substance P on synaptic transmission within descending analgesic pathways, however, are largely unknown. Here, we used whole-cell recordings from rat midbrain slices to examine the effects of substance P on GABAergic and glutamatergic transmission within the periaqueductal gray (PAG), a key component of a descending analgesic pathway that projects via the rostral ventromedial medulla (RVM) to the spinal cord dorsal horn. We found that substance P reversibly decreased the amplitude and increased the paired-pulse ratio of evoked IPSCs recorded from identified PAG-RVM projection neurons and from unidentified PAG neurons. Substance P had no effect on miniature IPSCs, implying an indirect mode of action. The effects of substance P were abolished by metabotropic glutamate type 5 and cannabinoid CB1 receptor antagonists, but unaltered by NMDA, GABA(B), mu,delta-opioid, adenosine A(1), and 5HT(1A) receptor antagonists. Consistent with a role for endogenous glutamate in this process, substance P increased the frequency of action potential-dependent spontaneous EPSCs. Moreover, the effect of substance P on evoked IPSCs was mimicked and occluded by a glutamate transport inhibitor. Finally, these effects were dependent on postsynaptic G-protein activation and diacylglycerol lipase activity, suggesting the requirement for retrograde signaling by the endocannabinoid 2-arachidonoylglycerol. Thus, substance P may facilitate descending analgesia in part by enhancing glutamate-mediated excitation and endocannabinoid-mediated disinhibition of PAG-RVM projection neurons.

Neurokinin 1 receptor activates transient receptor potential-like currents in noradrenergic A7 neurons in rats

Noradrenergic (NAergic) A7 neurons are involved in modulating nociception by releasing noradrenaline in the dorsal spinal cord. Since NAergic A7 neurons receive dense Substance P (Sub-P) releasing terminals from ventromedial medulla, here we tested the effect of Sub-P on them. Bath application of Sub-P induced an inward current (I(Sub-P)) in NAergic neurons, which was significantly blocked by Neurokinin 1 (NK1) receptor antagonist. The I(Sub-P) was reversed at approximately -20 mV, blocked by several TRP channel blockers, enhanced by OAG and negatively regulated by PKC. Immunohistochemistry staining showed that NAergic A7 neurons express high level of TRPC6 channel proteins, which is consistent with pharmacological properties of I(Sub-P) shown above, as TRPC6 channel is shown to be augmented by OAG and inhibited by PKC. In conclusion, the above results provide mechanism underlying postsynaptic action of Sub-P on NAergic A7 neurons and a role for TRPC6 channel in NAergic pain modulation.

Evidence for mediation of nociception by injection of the NK-3 receptor agonist, senktide, into the dorsal periaqueductal gray of rats

RATIONALE

Ultrasound vocalizations (USVs) at approximately 22 kHz are usual components of the defensive response of rats. However, depending on the neural substrate that is activated, such as the dorsal periaqueductal gray (dPAG), USV emissions may be reduced. Activation of neurokinin-1 (NK-1)-mediated mechanisms of the dPAG causes analgesia, reduced 22 kHz USVs, and anxiogenic-like effects in rats exposed to the elevated plus maze (EPM). Involvement of other types of neurokinin receptors in this activation has not yet been evaluated.

OBJECTIVES

The present study examined whether local injections of the selective NK-3 agonist senktide (1-100 pmol/0.2 microL) into the dPAG can (1) cause anxiogenic effects in the EPM, (2) influence novelty-induced 22 kHz USVs, or (3) change nociceptive reactivity in the tail-flick test.

RESULTS

Senktide elicited a significant increase in exploratory behavior, an effect accompanied by hyperalgesia and an increase in the number of 22 kHz USVs. The nociceptive effects, increased locomotor activity, and USV emissions elicited by local injections of senktide (50 pmol/0.2 microL) were reduced by prior injections of the selective NK-3 receptor antagonist SB222200 (50 pmol/0.2 microL) into the dPAG.

CONCLUSIONS

These findings show that NK-3 receptors in the dPAG mediate nociceptive responses in this area, contrasting with the known fear-related processes mediated by NK-1 receptors in the dPAG. Both hyperalgesia and fear-related processes are accompanied by emissions of 22 kHz USVs.

Neurotensin inhibition of GABAergic transmission via mGluR-induced endocannabinoid signalling in rat periaqueductal grey

Neurotensin modulates pain via its actions within descending analgesic pathways which include brain regions such as the midbrain periaqueductal grey (PAG). The aim of this study was to examine the cellular actions of neurotensin on PAG neurons. Whole cell patch clamp recordings were made from rat midbrain PAG slices in vitro to examine the postsynaptic effects of neurotensin and its effects on GABA(A) mediated inhibitory postsynaptic currents (IPSCs). Neurotensin (100-300 nM) produced an inward current in subpopulations of opioid sensitive and insensitive PAG neurons which did not reverse over membrane potentials between -50 and -130 mV. The neurotensin induced current was abolished by the NTS1 and NTS1/2 antagonists SR48692 (300 nM) and SR142948A (300 nM). Neurotensin also produced a reduction in the amplitude of evoked IPSCs, but had no effect on the rate and amplitude of TTX-resistant miniature IPSCs. The neurotensin induced inhibition of evoked IPSCs was reduced by the mGluR5 antagonist MPEP (5microM) and abolished by the cannabinoid CB(1) receptor antagonist AM251 (3 microM). These results suggest that neurotensin produces direct neuronal depolarisation via NTS1 receptors and inhibits GABAergic synaptic transmission within the PAG. The inhibition of synaptic transmission is mediated by neuronal excitation and action potential dependent release of glutamate, leading to mGluR5 mediated production of endocannabinoids which activate presynaptic CB(1) receptors. Thus, neurotensin has cellular actions within the PAG which are consistent with both algesic and analgesic activity, some of which are mediated via the endocannabinoid system.

An NK1 receptor antagonist microinjected into the periaqueductal gray blocks lateral hypothalamic-induced antinociception in rats

Substantial data are accumulating that implicate the lateral hypothalamus (LH) as part of the descending pain modulatory system. The LH modifies nociception in the spinal cord dorsal horn partly through connections with the periaqueductal gray (PAG), an area known to play a central role in brainstem modulation of nociception. Early work demonstrated a putative substance P connection between the LH and the PAG, but the connection is not fully defined. To determine whether LH-induced antinociception mediated by the PAG is neurokinin1 (NK1) receptor-dependent, we conducted behavioral experiments in which the cholinergic agonist carbachol (125 nmol) was microinjected into the LH of lightly anesthetized female Sprague-Dawley rats (250-350 g) and antinociception was obtained on the tail flick or foot withdrawal tests. Cobalt chloride (100 nM), which reversibly blocks synaptic activation, blocked LH-induced antinociception. In another set of experiments, the specific NK1 receptor antagonist L-703,606 (5 microg) was microinjected in the PAG following LH stimulation with carbachol abolished LH-induced antinociception as well. Microinjection of cobalt chloride or L-703,606 in the absence of LH stimulation had no effect. These behavioral experiments coupled with earlier work provide converging evidence to support the hypothesis that antinociception produced by activating neurons in the LH is mediated in part by the subsequent activation of neurons in the PAG by NK1 receptors.

Central mechanisms I: plasticity of central pathways

Cough is the most common symptom for which individuals seek medical attention and spend health-care dollars. Despite the burden induced by cough, the current treatments for cough are only partially effective. Delineating the sites and mechanisms in the cough central network for changes in the cough reflex could lead to new therapeutic strategies and drug target sites for more effective treatments. The first synaptic target in the CNS for the cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS); these neurons reorganize the primary sensory information into a coherent output. The NTS neurons have been shown to undergo neuroplasticity under a variety of conditions, such as respiratory disorders, stress, and exposures to environmental pollutants. The NTS contains a rich innervation of substance P immunoreactive nerve terminals, suggesting that substance P might be important in altered cough reflex response. This chapter summarizes our current findings on the role of substance P in enhanced cough reflex as well as the potential NTS targets for the action of substance P.

Nocistatin and nociceptin exert opposite effects on the excitability of central amygdala nucleus-periaqueductal gray projection neurons

Central amygdala nucleus (CeA)-periaqueductal gray (PAG) pathway is the component of descending antinociceptive circuitry. Nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) produce supraspinal pronociceptive and antinociceptive effects, respectively. We hypothesized that opposite effects of N/OFQ and NST on supraspinal pain modulation result from their opposing effects on the excitability of CeA-PAG projection neurons. This hypothesis was tested by investigating electrophysiological effects of N/OFQ and NST on medial CeA neurons that project to PAG (CeA(M)-PAG). N/OFQ hyperpolarized CeA(M)-PAG projection neurons by enhancing inwardly rectifying potassium conductance. In contrast, NST depolarized CeA(M)-PAG neurons by causing the opening of TRPC cation channels via G(alphaq/11)-PLC-PKC pathway. CeA(M)-PAG neurons hyperpolarized by N/OFQ express CRF or neurotensin mRNA. NST-responsive CeA(M)-PAG neurons contain CRF or substance P mRNA. Our study provides the evidence that the molecular and cellular basis for opposite effects of N/OFQ and NST on supraspinal pain regulation is their opposing effects on the excitability of peptidergic CeA(M)-PAG neurons.

Modulation of lymphatic muscle contractility by the neuropeptide substance P

Substance P (SP) is a neuropeptide associated with sensory innervation of lymphoid tissue and a suspected modulator of lymphatic function in inflammation. Only a few studies have examined the effects of SP on lymphatic contraction, and it is not clear to what extent SP acts directly on the lymphatic muscle and/or endothelium or indirectly through changes in intraluminal filling pressure secondary to increases in capillary permeability/filtration. We tested the effects of SP on the spontaneous contractions of rat isolated mesenteric lymphatic vessels under isometric and isobaric conditions, hypothesizing that low concentrations would stimulate lymphatic pumping by enhancing lymphatic muscle contraction in a manner complementary to the effect of increased preload. Under isometric conditions, SP (10 nM) dramatically enhanced lymphatic chronotropy and inotropy. Unlike guinea pig lymphatics, SP actions were not blocked by cyclooxygenase or PLA(2) inhibition. In the absence of SP, ramp increases in isometric preload resulted in x approximately 1.6 increases in contraction amplitude (Amp) and x approximately 1.7 increases in frequency (Freq). SP increased Freq by x approximately 2.4, Amp by x approximately 1.9, and the Amp-Freq product (AFP) by x approximately 3.5. Under isobaric conditions, the pressure elevation from 0.5 to 10 cmH(2)O in the absence of SP decreased Amp by x approximately 0.6 and increased Freq by x approximately 1.8. SP caused a modest increase in Amp, a robust increase in Freq at all pressures, and shifted the AFP-pressure relationship upward and leftward. Therefore, SP has substantial positive inotropic and chronotropic effects on rat lymphatic muscle, improving pump efficiency independent of the effects of preload and broadening of the working range of the lymphatic pump.

Neurokinin-1 receptors modulate the excitability of expiratory neurons in the ventral respiratory group

We studied the role of neurokinin-1 receptors (NK1-R) on the excitability of expiratory (E) neurons (tonic discharge, E(TONIC); augmenting, E(AUG); decrementing, E(DEC)) throughout the ventral respiratory group, including Bötzinger Complex (BötC) using extracellular single-unit recording combined with pressurized picoejection in decerebrate, arterially perfused juvenile rats. Responses evoked by picoejection of the NK1-R agonist, [Sar9-Met(O2)11]-substance P (SSP) were determined before and after the selective NK1-R antagonist, CP99,994. SSP excited 20 of 35 expiratory neurons by increasing the number of action potentials per burst (+33.7 +/- 6.5% of control), burst duration (+20.6 +/- 7.9% of control), and peak firing frequency (+16.2 +/- 4.8% of control; means +/- SE). Pretreatment with CP99,994 completely blocked SSP-evoked excitation in a subset of neurons tested, supporting the notion that SSP excitation was mediated through NK1-R activation. Because we had previously shown that E(AUG) neurons were crucial to locomotor-respiratory coupling (LRC), we reasoned that blockade of NK1-R would alter LRC by preventing somatic-evoked excitation of E(AUG) neurons. Blockade of NK1-Rs by CP99,994 in the BötC severely disrupted LRC and prevented somatic-evoked excitation of E(AUG) neurons. These findings demonstrate that LRC is dependent on endogenous SP release acting via NK1-Rs on E(AUG) neurons of the BötC. Taken together with our earlier finding that inspiratory off-switching by the Hering-Breuer Reflex requires endogenous activation of NK1-Rs through activation of NK1-Rs on E(DEC) neurons, we suggest that endogenous release of substance P in the BötC provides a reflex pathway-dependent mechanism to selectively modulate respiratory rhythm.

Glutamate-stimulated ATP release from spinal cord astrocytes is potentiated by substance P

ATP has recently emerged as a key molecule mediating pathological pain. The aim of this study was to examine whether spinal cord astrocytes could be a source of ATP in response to the nociceptive neurotransmitters glutamate and substance P. Glutamate stimulated ATP release from these astrocytes and this release was greatly potentiated by substance P, even though substance P alone did not elicit ATP release. Substance P also potentiated glutamate-induced inward currents, but did not cause such currents alone. When glutamate was applied alone it acted exclusively through alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate receptors to stimulate Ca(2+) influx-dependent ATP release. However, when substance P was co-applied with glutamate, ATP release could be elicited by activation of NMDA and metabotropic glutamate receptors. Activation of neurokinin receptor subtypes, protein kinase C and phospholipases A(2), C and D were needed for substance P to bring about its effects. These results suggest that astrocytes may be a major source of ATP in the spinal cord on activation of nerve fibres that release substance P and glutamate.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).