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

TRPC-mediated Ca2+ signaling and control of cellular functions

Canonical members of the TRP superfamily of ion channels have long been recognized as key elements of Ca²⁺ handling in a plethora of cell types. The emerging role of TRPC channels in human physiopathology has generated considerable interest in their pharmacological targeting, which requires detailed understanding of their molecular function. Although consent has been reached that receptor-phospholipase C (PLC) pathways and generation of lipid mediators constitute the prominent upstream signaling process that governs channel activity, multimodal sensing features of TRPC complexes have been demonstrated repeatedly. Downstream signaling by TRPC channels is similarly complex and involves the generation of local and global cellular Ca²⁺ rises, which are well-defined in space and time to govern specific cellular functions. These TRPC-mediated Ca²⁺ signals rely in part on Ca²⁺ permeation through the channels, but are essentially complemented by secondary mechanisms such as Ca²⁺ mobilization from storage sites and Na⁺/Ca²⁺ exchange, which involve coordinated interaction with signaling partners. Consequently, the control of cell functions by TRPC molecules is critically determined by dynamic assembly and subcellular targeting of the TRPC complexes. The very recent availability of high-resolution structure information on TRPC channel complexes has paved the way towards a comprehensive understanding of signal transduction by TRPC channels. Here, we summarize current concepts of cation permeation in TRPC complexes, TRPC-mediated shaping of cellular Ca²⁺ signals and the associated control of specific cell functions.

Cholinergic and glutamatergic transmission at synapses between pedunculopotine tegmental nucleus axonal terminals and A7 catecholamine cell group noradrenergic neurons in the rat

We characterized transmission from the pedunculopotine tegmental nucleus (PPTg), which contains cholinergic and glutamatergic neurons, at synapses with noradrenergic (NAergic) A7 neurons. Injection of an anterograde neuronal tracer, biotinylated-dextran amine, into the PPTg resulted in labeling of axonal terminals making synaptic connection with NAergic A7 neurons. Consistent with this, extracellular stimulation using a train of 10 pulses at 100 Hz evoked both fast and slow excitatory synaptic currents (EPSCs) that were blocked, respectively, by DNQX, a non-N-methyl-d-aspartate receptor blocker, or atropine, a cholinergic muscarinic receptor (mAChR) blocker. Interestingly, many spontaneous-like, but stimulation-dependent, EPSCs, were seen for up to one second after the end of stimulation and were blocked by DNQX and decreased by EGTA-AM, a membrane permeable form of EGTA, showing they are glutamatergic EPSCs causing by asynchronous release of vesicular quanta. Moreover, application of atropine or carbachol, an mAChR agonist, caused, respectively, an increase in the number of asynchronous EPSCs or a decrease in the frequency of miniature EPSCs, showing that mAChRs mediated presynaptic inhibition of glutamatergic transmission of the PPTg onto NAergic A7 neurons. In conclusion, our data show direct synaptic transmission of PPTg afferents onto pontine NAergic neurons that involves cooperation of cholinergic and glutamatergic transmission. This dual-transmitter transmission drives the firing rate of NAergic neurons, which may correlate with axonal and somatic/dendritic release of NA.

Noradrenergic Locus Coeruleus pathways in pain modulation

The noradrenergic system is crucial for several activities in the body, including the modulation of pain. As the major producer of noradrenaline (NA) in the central nervous system (CNS), the Locus Coeruleus (LC) is a nucleus that has been studied in several pain conditions, mostly due to its strategic location. Indeed, apart from a well-known descending LC-spinal pathway that is important for pain control, an ascending pathway passing through this nucleus may be responsible for the noradrenergic inputs to higher centers of the pain processing, such as the limbic system and frontal cortices. Thus, the noradrenergic system appears to modulate different components of the pain experience and accordingly, its manipulation has distinct behavioral outcomes. The main goal of this review is to bring together the data available regarding the noradrenergic system in relation to pain, particularly focusing on the ascending and descending LC projections in different conditions. How such findings influence our understanding of these conditions is also discussed.

Inhibition of TRPC6 by protein kinase C isoforms in cultured human podocytes

Transient receptor potential canonical‐6 (TRPC6) ion channels, expressed at high levels in podocytes of the filtration barrier, are recently implicated in the pathogenesis of various forms of proteinuric kidney diseases. Indeed, inherited or acquired up‐regulation of TRPC6 activities are suggested to play a role in podocytopathies. Yet, we possess limited information about the regulation of TRPC6 in human podocytes. Therefore, in this study, we aimed at defining how the protein kinase C (PKC) system, one of the key intracellular signalling pathways, regulates TRPC6 function and expression. On human differentiated podocytes, we identified the molecular expressions of both TRPC6 and several PKC isoforms. We also showed that TRPC6 channels are functional since the TRPC6 activator 1‐oleoyl‐2‐acetyl‐sn‐glycerol (OAG) induced Ca²⁺‐influx to the cells. By assessing the regulatory roles of the PKCs, we found that inhibitors of the endogenous activities of classical and novel PKC isoforms markedly augmented TRPC6 activities. In contrast, activation of the PKC system by phorbol 12‐myristate 13‐acetate (PMA) exerted inhibitory actions on TRPC6 and suppressed its expression. Importantly, PMA treatment markedly down‐regulated the expression levels of PKCα, PKCβ, and PKCη reflecting their activation. Taken together, these results indicate that the PKC system exhibits a ‘tonic’ inhibition on TRPC6 activity in human podocytes suggesting that pathological conditions altering the expression and/or activation patterns of podocyte‐expressed PKCs may influence TRPC6 activity and hence podocyte functions. Therefore, it is proposed that targeted manipulation of certain PKC isoforms might be beneficial in certain proteinuric kidney diseases with altered TRPC6 functions.

Morphological and physiological evidence of a synaptic connection between the lateral parabrachial nucleus and neurons in the A7 catecholamine cell group in rats

BACKGROUND

The descending noradrenergic (NAergic) system is one of the important endogenous analgesia systems. It has been suggested that noxious stimuli could activate descending NAergic system; nevertheless, the underlying neuronal circuit remains unclear. As NAergic neurons in the A7 catecholamine cell group (A7) are a part of the descending NAergic system and the lateral parabrachial nucleus (LPB) is an important brainstem structure that relays ascending nociceptive signal, we aimed to test whether LPB neurons have direct synaptic contact with NAergic A7 neurons.

RESULTS

Stereotaxic injections of an anterograde tracer, biotinylated dextran-amine (BDA), were administered to LPB in rats. The BDA-labeled axonal terminals that have physical contacts with tyrosine hydroxylase-positive (presumed noadrenergic) neurons were identified in A7. Consistent with these morphological observations, the excitatory synaptic currents (EPSCs) were readily evoked in NAergic A7 neurons by extracellular stimulation of LPB. The EPSCs evoked by LPB stimulation were blocked by CNQX, a non-NMDA receptor blocker, and AP5, a selective NMDA receptor blocker, showing that LPB-A7 synaptic transmission is glutamatergic. Moreover, the amplitude of LPB-A7 EPSCs was significantly attenuated by DAMGO, a selective μ-opioid receptor agonist, which was associated with an increase in paired-pulse ratio.

CONCLUSIONS

Taken together, the above results showed direct synaptic connections between LPB and A7 catecholamine cell group, the function of which is subject to presynaptic modulation by μ-opioid receptors.

Substance P Activates Ca2+-Permeable Nonselective Cation Channels through a Phosphatidylcholine-Specific Phospholipase C Signaling Pathway in nNOS-Expressing GABAergic Neurons in Visual Cortex

To understand the functions of the neocortex, it is essential to characterize the properties of neurons constituting cortical circuits. Here, we focused on a distinct group of GABAergic neurons that are defined by a specific colocalization of intense labeling for both neuronal nitric oxide synthase (nNOS) and substance P (SP) receptor [neurokinin 1 (NK1) receptors]. We investigated the mechanisms of the SP actions on these neurons in visual cortical slices obtained from young glutamate decarboxylase 67-green fluorescent protein knock-in mice. Bath application of SP induced a nonselective cation current leading to depolarization that was inhibited by the NK1 antagonists in nNOS-immunopositive neurons. Ruthenium red and La(3+), transient receptor potential (TRP) channel blockers, suppressed the SP-induced current. The SP-induced current was mediated by G proteins and suppressed by D609, an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), but not by inhibitors of phosphatidylinositol-specific PLC, adenylate cyclase or Src tyrosine kinases. Ca(2+) imaging experiments under voltage clamp showed that SP induced a rise in intracellular Ca(2+) that was abolished by removal of extracellular Ca(2+) but not by depletion of intracellular Ca(2+) stores. These results suggest that SP regulates nNOS neurons by activating TRP-like Ca(2+)-permeable nonselective cation channels through a PC-PLC-dependent signaling pathway.

Continuous infusion of substance P into rat striatum alleviates nociceptive behavior via phosphorylation of extracellular signal-regulated kinase 1/2

Intraplantar injection of 0.4% formalin into the rat hind paw leads to a biphasic nociceptive response; an 'acute' phase (0-15 min) and 'tonic' phase (16-120 min), which is accompanied by significant phosphorylation of extracellular signal-regulated kinase (ERK)1/2 in the contralateral striatum at 120 min post-formalin injection. To uncover a possible relationship between the slow-onset substance P (SP) release and increased ERK1/2 phosphorylation in the striatum, continuous infusion of SP into the striatum by reverse microdialysis (0.4 μg/mL in microdialysis fiber, 1 μL/min) was performed to mimic volume neurotransmission of SP. Continuous infusion for 3 h of SP reduced the duration of 'tonic' phase nociception, and this SP effect was mediated by neurokinin 1 (NK1) receptors since pre-treatment with NK1 receptor antagonist CP96345 (10 μM) blocked the effect of SP infusion. However, formalin-induced 'tonic' phase nociception was significantly prolonged following acute injection of the MAP/ERK kinase 1/2 inhibitor PD0325901 (100 pmol) by microinjection. The coinfusion of SP and PD0325901 significantly increased the 'tonic' phase of nociception. These data demonstrate that volume transmission of striatal SP triggered by peripheral nociceptive stimulation does not lead to pain facilitation but a significant decrease of tonic nociception by the activation of the SP-NK1 receptor-ERK1/2 system. Noxious stimulation induces a slow-onset substance P (SP) release as a volume transmitter, activating extra-synaptic NK1 receptors, and evokes phosphorylation of extracellular signal-regulated kinase (ERK) 1/2. The SP-NK1-ERK1/2 system in the striatum decreases tonic nociception.

Physiological Function and Characterization of TRPCs in Neurons

Ca²⁺ entry is essential for regulating vital physiological functions in all neuronal cells. Although neurons are engaged in multiple modes of Ca²⁺ entry that regulates variety of neuronal functions, we will only discuss a subset of specialized Ca²⁺-permeable non-selective Transient Receptor Potential Canonical (TRPC) channels and summarize their physiological and pathological role in these excitable cells. Depletion of endoplasmic reticulum (ER) Ca²⁺ stores, due to G-protein coupled receptor activation, has been shown to activate TRPC channels in both excitable and non-excitable cells. While all seven members of TRPC channels are predominately expressed in neuronal cells, the ion channel properties, mode of activation, and their physiological responses are quite distinct. Moreover, many of these TRPC channels have also been suggested to be associated with neuronal development, proliferation and differentiation. In addition, TRPCs also regulate neurosecretion, long-term potentiation and synaptic plasticity. Similarly, perturbations in Ca²⁺ entry via the TRPC channels have been also suggested in a spectrum of neuropathological conditions. Hence, understanding the precise involvement of TRPCs in neuronal function and in neurodegenerative conditions would presumably unveil avenues for plausible therapeutic interventions for these devastating neuronal diseases.

Differential regulation of TRPC4 in the vasopressin magnocellular system by water deprivation and hepatic cirrhosis in the rat

Transient receptor potential canonical subtype 4 (TRPC4) is expressed in the magnocellular paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus. In this study, the regulation of TRPC4 expression was investigated in water deprivation and hepatic cirrhosis. We used laser capture microdissection technique for precise dissection of pure AVP cell population in the PVN and SON followed by quantitative real-time RT-PCR, and immunodetection techniques by Western blot analysis and immunofluorescence. Bile duct ligation elevated TRPC4 transcripts in the SON but not PVN with correlated changes in the protein expression in these regions, as well as increased colocalization with AVP in the SON, with no changes in the PVN. Water deprivation resulted in increased TRPC4 mRNA expression in the PVN, while it decreased channel expression levels in the SON. In both of these regions, protein expression measured from tissue punches were unaltered following water deprivation, with no changes in the number of TRPC4-positive cells. Thus, TRPC4 expression is differentially regulated in physiological and pathophysiological models of vasopressin release.

Regulated norepinephrine transporter interaction with the neurokinin-1 receptor establishes transporter subcellular localization

Neurokinin-1 receptor (NK1R) mediates down-regulation of human norepinephrine (NE) transporter (hNET) via protein kinase C (PKC). However, native NET regulation by NK1R and the mechanism by which NK1R targets NET among other potential effectors are unknown. Effect of NK1R activation on native NET regulation and NET/NK1R interaction were studied using rat brain synaptosomes expressing native NET and NK1R as well as human placental trophoblast (HTR) cells coexpressing WT-hNET or NK1R/PKC-resistant hNET-T258A,S259A double mutant (NET-DM) and hNK1R. The selective NK1R agonist, GR73632, and Substance-P (SP) inhibited NE transport and reduced plasma membrane expression of NET and NK1R. Pretreatment with the NK1R antagonist, EMEND (aprepitant) prevented these NK1R-mediated effects. Immunoprecipitation experiments showed that NET forms stable complexes with NK1R. In HTR cells, combined biotinylation and immunoprecipitation studies revealed plasma membrane localization of NET·NK1R complexes. Receptor activation resulted in the internalization of NET·NK1R complexes. Lipid raft and immunoprecipitation analyses revealed the presence of NET·NK1R complexes exclusively in non-raft membrane fractions under basal/unstimulated conditions. However, NK1R activation led to translocation of NET·NK1R complexes to raft-rich membrane fractions. Importantly, PKCα was found in association with raft-localized NET following SP treatment. Similar to WT-NET, PKC-resistant NET-DM was found in association with NK1R exclusively in non-raft fractions. However, SP treatment failed to translocate NET-DM·NK1R complexes from non-raft fractions to raft fractions. Collectively, these results suggest that NK1R forms physical complexes with NET and that the receptor-mediated Thr(258) + Ser(259) motif-dependent translocation of NET·NK1R complexes into raft-rich microdomains facilitates NET/NK1R interaction with PKCα to coordinate spatially restricted NET regulation.

Noradrenaline regulates substance P release from rat dorsal root ganglion neurons in vitro

OBJECTIVE

To determine whether activation and/or inhibition of α-adrenoreceptors influences substance P (SP) release from dorsal root ganglion (DRG) primary sensory neurons in vitro.

METHODS

DRGs were dissected from 15-day embryonic Wistar rats. DRG neurons were dissociated and cultured for 2 d and then exposed to noradrenaline (NA) alone (1×10(-4) mol/L), or along with the α1-adrenoreceptor antagonist prazosin (1×10(-6) mol/L) or the α2-adrenoreceptor antagonist yohimbine (1×10(-5) mol/L) for 4 d. Then, RT-PCR was used to determine the levels of preprotachykinin (PPT) mRNA encoding for SP and Western blot to assess the protein levels of SP. Basal and capsaicin (CAP)-evoked SP release were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

CAP-evoked SP release was sensitized by NA and this effect was inhibited by pre-incubation with prazosin but not with yohimbine. The levels of PPT mRNA, SP peptide, and basal SP release did not change significantly in any of the experimental conditions.

CONCLUSION

NA may significantly increase CAP-evoked SP release through activation of α-adrenoreceptors, which may contribute to noradrenergic pain modulation.

Emerging roles of canonical TRP channels in neuronal function

Ca(2+) signaling in neurons is intimately associated with the regulation of vital physiological processes including growth, survival and differentiation. In neurons, Ca(2+) elicits two major functions. First as a charge carrier, Ca(2+) reveals an indispensable role in information relay via membrane depolarization, exocytosis, and the release of neurotransmitters. Second on a global basis, Ca(2+) acts as a ubiquitous intracellular messenger to modulate neuronal function. Thus, to mediate Ca(2+)-dependent physiological events, neurons engage multiple mode of Ca(2+) entry through a variety of Ca(2+) permeable plasma membrane channels. Here we discuss a subset of specialized Ca(2+)-permeable non-selective TRPC channels and summarize their physiological and pathological role in the context of excitable cells. TRPC channels are predominately expressed in neuronal cells and are activated through complex mechanisms, including second messengers and store depletion. A growing body of evidence suggests a prime contribution of TRPC channels in regulating fundamental neuronal functions. TRPC channels have been shown to be associated with neuronal development, proliferation and differentiation. In addition, TRPC channels have also been suggested to have a potential role in regulating neurosecretion, long term potentiation, and synaptic plasticity. During the past years, numerous seminal discoveries relating TRPC channels to neurons have constantly emphasized on the significant contribution of this group of ion channels in regulating neuronal function. Here we review the major groundbreaking work that has uniquely placed TRPC channels in a pivotal position for governing neuronal Ca(2+) signaling and associated physiological responses.

GABAB receptor-mediated tonic inhibition of noradrenergic A7 neurons in the rat

Noradrenergic (NAergic) A7 neurons that project axonal terminals to the dorsal horn of the spinal cord to modulate nociceptive signaling are suggested to receive tonic inhibition from local GABAergic interneurons, which are under the regulation of descending analgesic pathways. In support of this argument, we presently report GABA(B) receptor (GABA(B)R)-mediated tonic inhibition of NAergic A7 neurons. Bath application of baclofen induced an outward current (I(Bac)) in NAergic A7 neurons that was blocked by CGP 54626, a GABA(B)R blocker. The I(Bac) was reversed at about -99 mV, displayed inward rectification, and was blocked by Ba(2+) or Tertipian-Q, showing it was mediated by G protein-activated inward-rectifying K(+) (GIRK) channels. Single-cell RT-PCR results suggested that GIRK1/3 heterotetramers might dominate functional GIRK channels in NAergic A7 neurons. Under conditions in which GABA(A) and glycine receptors were blocked, bath application of GABA inhibited the spontaneous firing of NAergic A7 neurons in a dose-dependent manner. Interestingly, CGP 54626 application not only blocked the effect of GABA but also increased the firing rate to 126.9% of the control level, showing that GABA(B)Rs were constitutively active at an ambient GABA concentration of 2.8 μM and inhibited NAergic A7 neurons. GABA(B)Rs were also found at presynaptic excitatory and inhibitory axonal terminals in the A7 area. Pharmacological activation of these GABA(B)Rs inhibited the release of neurotransmitters. No physiological role was found for GABA(B)Rs on excitatory terminals, whereas those on the inhibitory terminals were found to exert autoregulatory control of GABA release.

Roles of A-type potassium currents in tuning spike frequency and integrating synaptic transmission in noradrenergic neurons of the A7 catecholamine cell group in rats

We investigated voltage-dependent K(+) currents (I(K)) in noradrenergic (NAergic) A7 neurons. The I(K) evoked consisted of A-type I(K) (I(A)), which had the characteristics of a low threshold for activation (approximately -50 mV), fast activation/inactivation, and rapid recovery from inactivation. Since the I(A) were blocked by heteropodatoxin-2 (Hptx-2), a specific Kv4 channel blocker, and the NAergic A7 neurons were shown to be reactive with antibodies against Kv4.1/Kv4.3 channel proteins, we conclude that the I(A) evoked in NAergic neurons are mediated by Kv4.1/Kv4.3 channels. I(A) were also evoked using voltage commands of a single action potential (AP), a subthreshold voltage change between two consecutive APs, or excitatory postsynaptic potential (EPSP) activity recorded in current-clamp mode (CCM). Blockade of the I(A) by 4-AP, a broad spectrum I(A) blocker, or by Hptx-2 increased the half-width and spontaneous firing of APs and reduced the amount of synaptic drive needed to elicit APs in CCM, showing that the I(A) play important roles in regulating the shape and firing frequency of APs and in synaptic integration in NAergic A7 neurons. Since these neurons are the principal projection neurons to the dorsal horn of the spinal cord, these results also suggest roles for Kv4.1/4.3 channels in descending NAergic pain regulation.