Two parallel signaling pathways couple 5HT1A receptors to N- and L-type calcium channels in C-like rat dorsal root ganglion cells

Serotonin-Mediated Activation of Serotonin Receptor Type 1 Oppositely Modulates Voltage-Gated Calcium Channel Currents in Rat Sensory Neurons Innervating Hindlimb Muscle

Serotonin (5-HT) is a multifaceted neurotransmitter that has been described to play a role as a peripheral inflammatory mediator when released in ischemic or injured muscle. Dorsal root ganglia (DRG) neurons are key sensors of noxious stimuli that are released under inflammatory conditions or mechanical stress. Little information is available on the specific 5-HT receptor subtypes expressed in primary afferents that help regulate reflex pressor responses. In the present study, the whole-cell patch-clamp technique was employed to examine the modulation of voltage-gated calcium channel (CaV) 2.2 currents by 5-HT and to identify the 5-HT receptor subtype(s) mediating this response in acutely dissociated rat DRG neurons innervating triceps surae muscle. Our results indicate that exposure of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labeled DRG neurons to 5-HT can exert three modulatory effects on CaV currents: high inhibition, low inhibition, and enhancement. Both 5-HT-mediated inhibition responses were blocked after pretreatment with pertussis toxin (PTX), indicating that 5-HT receptors are coupled to CaV2.2 via Gα i/o protein subunits. Application of selective serotonin receptor type 1 (5-HT1) agonists revealed that modulation of CaV2.2 currents occurs primarily after 5-HT1A receptor subtype stimulation and minimally from 5-HT1D activation. Finally, the intrathecal administration of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), significantly (P < 0.05) decreased the pressor response induced by intra-arterial administration of lactic acid. This suggests that 5-HT1A receptors are expressed presynaptically on primary afferent neurons innervating triceps surae muscle. Our findings indicate that preferential stimulation of 5-HT1 receptors, expressed on thin fiber muscle afferents, serves to regulate the reflex pressor response to metabolic stimuli. SIGNIFICANCE STATEMENT: The monoamine serotonin (5-HT), released under ischemic conditions, can contribute to the development of inflammation that negatively affects the exercise pressor reflex. The 5-HT receptor subtype and signaling pathway that underlies calcium channel modulation in dorsal root ganglia afferents, innervating hindlimb muscles, are unknown. We show that 5-HT can either block (primarily via serotonin receptor type 1 (5-HT1)A subtypes) or enhance voltage-gated calcium channel (CaV2.2) currents. Our findings suggest 5-HT exhibits receptor subtype selectivity, providing a complexity of cellular responses.

Involvement of Serotonergic System in Oxaliplatin-Induced Neuropathic Pain

Oxaliplatin is a chemotherapeutic agent widely used against colorectal and breast cancers; however, it can also induce peripheral neuropathy that can rapidly occur even after a single infusion in up to 80-90% of treated patients. Numerous efforts have been made to understand the underlying mechanism and find an effective therapeutic agent that could diminish pain without damaging its anti-tumor effect. However, its mechanism is not yet clearly understood. The serotonergic system, as part of the descending pain inhibitory system, has been reported to be involved in different types of pain. The malfunction of serotonin (5-hydroxytryptamine; 5-HT) or its receptors has been associated with the development and maintenance of pain. However, its role in oxaliplatin-induced neuropathy has not been clearly elucidated. In this review, 16 in vivo studies focused on the role of the serotonergic system in oxaliplatin-induced neuropathic pain were analyzed. Five studies analyzed the involvement of 5-HT, while fourteen studies observed the role of its receptors in oxaliplatin-induced allodynia. The results show that 5-HT is not involved in the development of oxaliplatin-induced allodynia, but increasing the activity of the 5-HT1A, 5-HT2A, and 5-HT3 receptors and decreasing the action of 5-HT2C and 5-HT6 receptors may help inhibit pain.

Serotonin enhances depolarizing spontaneous fluctuations, excitability, and ongoing activity in isolated rat DRG neurons via 5-HT4 receptors and cAMP-dependent mechanisms

Ongoing activity in nociceptors, a driver of spontaneous pain, can be generated in dorsal root ganglion neurons in the absence of sensory generator potentials if one or more of three neurophysiological alterations occur - prolonged depolarization of resting membrane potential (RMP), hyperpolarization of action potential (AP) threshold, and/or increased amplitude of depolarizing spontaneous fluctuations of membrane potential (DSFs) to bridge the gap between RMP and AP threshold. Previous work showed that acute, sustained exposure to serotonin (5-HT) hyperpolarized AP threshold and potentiated DSFs, leading to ongoing activity if a separate source of maintained depolarization was present. Cellular signaling pathways that increase DSF amplitude and promote ongoing activity acutely in nociceptors are not known for any neuromodulator. Here, isolated DRG neurons from male rats were used to define the pathway by which low concentrations of 5-HT enhance DSFs, hyperpolarize AP threshold, and promote ongoing activity. A selective 5-HT₄ receptor antagonist blocked these 5-HT-induced hyperexcitable effects, while a selective 5-HT₄ agonist mimicked the effects of 5-HT. Inhibition of cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (EPAC), attenuated 5-HT's hyperexcitable effects, but a blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels had no significant effect. 5-HT₄-dependent PKA activation was specific to DRG neurons that bind isolectin B4 (a nonpeptidergic nociceptor marker). 5-HT's effects on AP threshold, DSFs, and ongoing activity were mimicked by a cAMP analog. Sustained exposure to 5-HT promotes ongoing activity in nonpeptidergic nociceptors through the G_s-coupled 5-HT₄ receptor and downstream cAMP signaling involving both PKA and EPAC.

Role of 5-HT1A and 5-HT3 receptors in serotonergic activation of sensory neurons in relation to itch and pain behavior in the rat

Serotonin (5-hydroxytryptamine, 5-HT) released by platelets, mast cells, and immunocytes is a potent inflammatory mediator which modulates pain and itch sensing in the peripheral nervous system. The serotonergic receptors expressed by primary afferent neurons involved in these sensory functions are not fully identified and appear to be to a large extent species dependent. Moreover, the mechanisms through which 5-HT receptor activation is coupled to changes in neuronal excitability have not been completely revealed. Using a combination of in vitro (calcium and voltage imaging and patch-clamp) and in vivo behavioral methods, we used both male and female Wistar rats to provide evidence for the involvement of two 5-HT receptor subtypes, 5-HT1A and 5-HT3, in mediating the sustained and transient effects, respectively, of 5-HT on rat primary afferent neurons involved in pain and itch processing. In addition, our results are consistent with a model in which sustained serotonergic responses triggered via the 5-HT1A receptor are due to closure of background potassium channels, followed by membrane depolarization and action potentials, during which the activation of voltage-gated calcium channels leads to calcium entry. Our results may provide a better understanding of mammalian serotonergic itch signaling.

Nociceptor Signalling through ion Channel Regulation via GPCRs

The prime task of nociceptors is the transformation of noxious stimuli into action potentials that are propagated along the neurites of nociceptive neurons from the periphery to the spinal cord. This function of nociceptors relies on the coordinated operation of a variety of ion channels. In this review, we summarize how members of nine different families of ion channels expressed in sensory neurons contribute to nociception. Furthermore, data on 35 different types of G protein coupled receptors are presented, activation of which controls the gating of the aforementioned ion channels. These receptors are not only targeted by more than 20 separate endogenous modulators, but can also be affected by pharmacotherapeutic agents. Thereby, this review provides information on how ion channel modulation via G protein coupled receptors in nociceptors can be exploited to provide improved analgesic therapy.

Understanding responder neurobiology in schizophrenia using a quantitative systems pharmacology model: application to iloperidone

The concept of targeted therapies remains a holy grail for the pharmaceutical drug industry for identifying responder populations or new drug targets. Here we provide quantitative systems pharmacology as an alternative to the more traditional approach of retrospective responder pharmacogenomics analysis and applied this to the case of iloperidone in schizophrenia. This approach implements the actual neurophysiological effect of genotypes in a computer-based biophysically realistic model of human neuronal circuits, is parameterized with human imaging and pathology, and is calibrated by clinical data. We keep the drug pharmacology constant, but allowed the biological model coupling values to fluctuate in a restricted range around their calibrated values, thereby simulating random genetic mutations and representing variability in patient response. Using hypothesis-free Design of Experiments methods the dopamine D4 R-AMPA (receptor-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor coupling in cortical neurons was found to drive the beneficial effect of iloperidone, likely corresponding to the rs2513265 upstream of the GRIA4 gene identified in a traditional pharmacogenomics analysis. The serotonin 5-HT3 receptor-mediated effect on interneuron gamma-aminobutyric acid conductance was identified as the process that moderately drove the differentiation of iloperidone versus ziprasidone. This paper suggests that reverse-engineered quantitative systems pharmacology is a powerful alternative tool to characterize the underlying neurobiology of a responder population and possibly identifying new targets.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

Role of protein kinase C in agonist-induced desensitization of 5-HT₁A receptor coupling to calcium channels in F11 cells

The 5-Hydroxytriptamine 1A receptor (5-HT1A) is expressed both as a pre- and post-synaptic receptor in neurons. The presynaptic receptor preferentially desensitizes compared to post-synaptic receptors, suggesting different underlying mechanisms of agonist-induced desensitization. Using F11 cells as a model of post-synaptic neurons, the present study examined the role of protein kinase C (PKC) and protein kinase A (PKA) in desensitization of the 5-HT1A-receptor by agonist. Desensitization in whole cell experiments was dependent on internal [Ca(2+)] and was blocked by chelation of intracellular Ca(2+). Using the perforated patch technique, desensitization was reduced when Ba(2+) was used as the conducting cation. Selective inhibitors of conventional PKC isoforms prevented 5-HT-induced desensitization, whereas an inhibitor of PKA did not. In cells in which 3 PKC/PKA sites located in the third intracellular loop (i3) of the 5-HT1A receptor were mutated (i3, T229A-S253G-T343A), 5-HT-mediated desensitization was reduced (and abolished in the absence of intracellular Ca(2+)). In cells in which a fourth mutation was added (T149 in the second i2 loop), the cells responded similarly to the triple mutants suggesting that phosphorylation of T149 does not contribute greatly to the desensitization induced by 5-HT-mediated activation of PKC. Thus agonist-induced uncoupling of the 5-HT1A-receptor is PKC-dependent, but requires a different set of phosphorylation sites than phorbol ester-mediated PKC activation, suggesting differential recruitment of PKC. Furthermore, these studies reveal that 5-HT1A-receptor desensitization utilizes a different kinase in F11 cells and serotonergic neurons, which may in part account for their differential sensitivity in vivo.

A-kinase anchoring protein 150 expression in a specific subset of TRPV1- and CaV 1.2-positive nociceptive rat dorsal root ganglion neurons

Modulation of phosphorylation states of ion channels is a critical step in the development of hyperalgesia during inflammation. Modulatory enhancement of channel activity may increase neuronal excitability and affect downstream targets such as gene transcription. The specificity required for such regulation of ion channels quickly occurs via targeting of protein kinases and phosphatases by the scaffolding A-kinase anchoring protein 79/150 (AKAP79/150). AKAP79/150 has been implicated in inflammatory pain by targeting protein kinase A (PKA) and protein kinase C (PKC) to the transient receptor potential vanilloid 1 (TRPV1) channel in peripheral sensory neurons, thus lowering threshold for activation of the channel by multiple inflammatory reagents. However, the expression pattern of AKAP150 in peripheral sensory neurons is unknown. Here we identify the peripheral neuron subtypes that express AKAP150, the subcellular distribution of AKAP150, and the potential target ion channels in rat dorsal root ganglion (DRG) slices. We found that AKAP150 is expressed predominantly in a subset of small DRG sensory neurons, where it is localized at the plasma membrane of the soma, axon initial segment, and small fibers. Most of these neurons are peripherin positive and produce C fibers, although a small portion produce Aδ fibers. Furthermore, we demonstrate that AKAP79/150 colocalizes with TRPV1 and Ca(V) 1.2 in the soma and axon initial segment. Thus AKAP150 is expressed in small, nociceptive DRG neurons, where it is targeted to membrane regions and where it may play a role in the modulation of ion channel phosphorylation states required for hyperalgesia.

Electrophysiological and chemical properties in subclassified acutely dissociated cells of rat trigeminal ganglion by current signatures

In the present study, we subclassified acutely dissociated trigeminal ganglion (TRG) cells of rats using a current signature method in whole cell patch-clamp recordings. Using modified criteria for cell classification for the dorsal root ganglion (DRG), TRG cells were subclassified into nine cell types: 1-5, 7-9, and 13. Types 1, 3, and 7 were in the small cell groups (15-24 μm); types 4, 5, and 8-13 were in the medium cell groups (25-38 μm); and type 2 was a mixed group of both cell sizes. Types 1-3, 5, and 7 showed high-input resistance and types 1, 2, and 7 showed more depolarized resting membrane potentials. Types 1, 2, and 5-13 expressed long-duration action potentials (APs), but types 3 and 4 expressed short-duration APs. Sensitivities to capsaicin, protons, and adenosine 5'-triphosphate (ATP) in TRG cell types largely corresponded to DRG cell types. However, different from the matched DRG types, half of TRG type 1 cells were capsaicin insensitive, showing desensitizing proton-induced currents, and types 5, 7, and 9 exhibited slow-desensitizing ATP-induced currents. Types 4, 5, and 8-13 had nicotine sensitivity, but the other cell types were insensitive. These results indicate that the "current signatures" classification is a useful means to separate TRG cells into internally homogeneous subpopulations that were distinct from other cell types. Furthermore, the data suggest some specific differences in the chemical responsiveness of some cell types between the TRG and DRG.

Serotonin receptors are involved in the spinal mediation of descending facilitation of surgical incision-induced increase of Fos-like immunoreactivity in rats

Background

Descending pronociceptive pathways may be implicated in states of persistent pain. Paw skin incision is a well-established postoperative pain model that causes behavioral nociceptive responses and enhanced excitability of spinal dorsal horn neurons. The number of spinal c-Fos positive neurons of rats treated intrathecally with serotonin, noradrenaline or acetylcholine antagonists where evaluated to study the descending pathways activated by a surgical paw incision.

Results

The number of c-Fos positive neurons in laminae I/II ipsilateral, lamina V bilateral to the incised paw, and in lamina X significantly increased after the incision. These changes: remained unchanged in phenoxybenzamine-treated rats; were increased in the contralateral lamina V of atropine-treated rats; were inhibited in the ipsilateral lamina I/II by 5-HT_1/2B/2C (methysergide), 5-HT_2A (ketanserin) or 5-HT_{1/2A/2C/5/6/7} (methiothepin) receptors antagonists, in the ipsilateral lamina V by methysergide or methiothepin, in the contralateral lamina V by all the serotonergic antagonists and in the lamina X by LY 278,584, ketanserin or methiothepin.

Conclusions

We conclude: (1) muscarinic cholinergic mechanisms reduce incision-induced response of spinal neurons inputs from the contralateral paw; (2) 5-HT_1/2A/2C/3 receptors-mediate mechanisms increase the activity of descending pathways that facilitates the response of spinal neurons to noxious inputs from the contralateral paw; (3) 5-HT_1/2A/2C and 5-HT_1/2C receptors increases the descending facilitation mechanisms induced by incision in the ipsilateral paw; (4) 5-HT_2A/3 receptors contribute to descending pronociceptive pathways conveyed by lamina X spinal neurons; (5) α-adrenergic receptors are unlikely to participate in the incision-induced facilitation of the spinal neurons.

Arachidonic acid inhibition of L-type calcium (CaV1.3b) channels varies with accessory CaVbeta subunits

Arachidonic acid (AA) inhibits the activity of several different voltage-gated Ca²⁺ channels by an unknown mechanism at an unknown site. The Ca²⁺ channel pore-forming subunit (Ca_Vα₁) is a candidate for the site of AA inhibition because T-type Ca²⁺ channels, which do not require accessory subunits for expression, are inhibited by AA. Here, we report the unanticipated role of accessory Ca_Vβ subunits on the inhibition of Ca_V1.3b L-type (L-) current by AA. Whole cell Ba²⁺ currents were measured from recombinant channels expressed in human embryonic kidney 293 cells at a test potential of −10 mV from a holding potential of −90 mV. A one-minute exposure to 10 µM AA inhibited currents with β_1b, β₃, or β₄ 58, 51, or 44%, respectively, but with β_2a only 31%. At a more depolarized holding potential of −60 mV, currents were inhibited to a lesser degree. These data are best explained by a simple model where AA stabilizes Ca_V1.3b in a deep closed-channel conformation, resulting in current inhibition. Consistent with this hypothesis, inhibition by AA occurred in the absence of test pulses, indicating that channels do not need to open to become inhibited. AA had no effect on the voltage dependence of holding potential–dependent inactivation or on recovery from inactivation regardless of Ca_Vβ subunit. Unexpectedly, kinetic analysis revealed evidence for two populations of L-channels that exhibit willing and reluctant gating previously described for Ca_V2 channels. AA preferentially inhibited reluctant gating channels, revealing the accelerated kinetics of willing channels. Additionally, we discovered that the palmitoyl groups of β_2a interfere with inhibition by AA. Our novel findings that the Ca_Vβ subunit alters kinetic changes and magnitude of inhibition by AA suggest that Ca_Vβ expression may regulate how AA modulates Ca²⁺-dependent processes that rely on L-channels, such as gene expression, enzyme activation, secretion, and membrane excitability.

Developmental expression and serotonergic regulation of relaxin 3/INSL7 in the nucleus incertus of rat brain

Relaxin 3 or insulin like peptide 7 has been identified as a new member of the insulin/relaxin superfamily. We recently reported that relaxin 3 was dominantly expressed in the brain, particularly in neurons of the nucleus incertus (NI) of the median dorsal tegmental pons and that it might act as a neurotransmitter. In the present study we investigated the developmental expression and serotonergic regulation of relaxin 3 gene in the rat brain. Relaxin 3 mRNA appeared at embryonic day 18 in the near region of the fourth ventricle, and was shown to have increased its density and the number of expressing neurons by in situ hybridization and RT-PCR examination. Relaxin 3 peptide was detected after birth by immunocytochemistry. Since the NI is located just caudal to the dorsal raphe nucleus where abundant serotonin (5-HT) neurons are present, we examined if 5-HT effects on the expression of relaxin 3. Relaxin 3 gene expression in the NI significantly increased after 5-HT depletion by p-chlorophenylalanine (PCPA) administration. We also observed the 5-HT1A receptor localization in relaxin 3 positive neurons of the NI. This result suggests that 5-HT negatively regulates the expression of relaxin 3 gene in the NI. The function of relaxin 3 neurons in the brain is influenced by the serotonergic activity.

Modification of antiallodynic and antinociceptive effects of morphine by peripheral and central action of fluoxetine in a neuropathic mice model

We have previously reported that serotonin concentration was reduced in the brain of mice with neuropathic pain and that it may be related to reduction of morphine analgesic effects. To further prove this pharmacological action, we applied fluoxetine, a selective serotonin reuptake inhibitor, to determine whether it suppressed neuropathic pain and examined how its different administration routes would affect antinociceptive and antiallodynic effects of morphine in diabetic (DM) and sciatic nerve ligation (SL) mice, as models of neuropathic pain. Antiallodynia and antinociceptive effect of drugs were measured by using von Frey filament and tail pinch tests, respectively. Fluoxetine given alone, intracerebroventicularly (i.c.v., 15 microg/mouse) or intraperitoneally (i.p., 5 and 10 mg/kg) did not produce any effect in either model. However, fluoxetine given i.p. enhanced both antiallodynic and antinociceptive effects of morphine. Administration of fluoxetine i.c.v., slightly enhanced only the antiallodynic effect of morphine in SL mice. Ketanserine, a serotonin 2A receptor antagonist (i.p., 1 mg/kg) and naloxone, an opioid receptor antagonist (i.p., 3 mg/kg), blocked the combined antinociceptive effect of fluoxetine and morphine. Our data show that fluoxetine itself lacks antinociceptive properties in the two neuropathy models, but it enhances the analgesic effect of morphine in the periphery and suggests that co-administration of morphine with fluoxetine may have therapeutic potential in treatment of neuropathic pain.

Sensitization and Activation of Intracranial Meningeal Nociceptors by Mast Cell Mediators

Intracranial headaches such as migraine are thought to result from activation of sensory trigeminal pain neurons that supply intracranial blood vessels and the meninges, also known as meningeal nociceptors. Although the mechanism underlying the triggering of such activation is not completely understood, our previous work indicates that the local activation of the inflammatory dural mast cells can provoke a persistent sensitization of meningeal nociceptors. Given the potential importance of mast cells to the pain of migraine it is important to understand which mast cell-derived mediators interact with meningeal nociceptors to promote their activation and sensitization. In the present study, we have used in vivo electrophysiological single-unit recording of meningeal nociceptors in the trigeminal ganglion of anesthetized rats to examine the effect of a number of mast cell mediators on the activity level and mechanosensitivity of meningeal nociceptors. We have found that that serotonin (5-HT), prostaglandin I(2) (PGI(2)), and to a lesser extent histamine can promote a robust sensitization and activation of meningeal nociceptors, whereas the inflammatory eicosanoids PGD(2) and leukotriene C(4) are largely ineffective. We propose that dural mast cells could promote headache by releasing 5-HT, PGI(2), and histamine.

Neuronal 5-HT metabotropic receptors: fine-tuning of their structure, signaling, and roles in synaptic modulation

Serotonin (5-hydroxytryptamine, 5-HT) is, without doubt, the neurotransmitter for which the number of receptors is the highest. Fifteen genes encoding functional 5-HT receptors have been cloned in mammalian brain. 5-HT(3) receptors are ionotropic receptors, whereas all the others are metabotropic G-protein-coupled receptors (GPCRs). 5-HT receptor diversity is further increased by post-genomic modifications, such as alternative splicing (up to 10 splice variants for the 5-HT(4) receptor) or by mRNA editing in the case of 5-HT(2C) receptors. The cellular and behavioral implications of 5-HT(2C) receptor editing are of great physiological importance. Signaling of 5-HT receptors involves a great variety of pathways, but only some of these have been demonstrated in neurons. The classical view of neurotransmitter receptors localized within the synaptic cleft cannot be applied to 5-HT receptors, which are mostly (but not exclusively) localized at extra-synaptic locations either pre- or post-synaptically. 5-HT receptors are engaged in pre- or post-synaptic complexes composed of many GPCR-interacting proteins. The functions of these proteins are starting to be revealed. These proteins have been implicated in targeting, trafficking to or from the membrane, desensitization, and fine-tuning of signaling.

Substance P initiates NFAT-dependent gene expression in spinal neurons

Persistent hyperalgesia is associated with increased expression of proteins that contribute to enhanced excitability of spinal neurons, however, little is known about how expression of these proteins is regulated. We tested the hypothesis that Substance P stimulation of neurokinin receptors on spinal neurons activates the transcription factor nuclear factor of activated T cells isoform 4 (NFATc4). The occurrence of NFATc4 in spinal cord was demonstrated with RT-PCR and immunocytochemistry. Substance P activated NFAT-dependent gene transcription in primary cultures of neonatal rat spinal cord transiently transfected with a luciferase DNA reporter construct. The effect of Substance P was mediated by neuronal neurokinin-1 receptors that coupled to activation of protein kinase C, l-type voltage-dependent calcium channels, and calcineurin. Interestingly, Substance P had no effect on cyclic AMP response element (CRE)-dependent gene expression. Conversely, calcitonin gene-related peptide, which activated CRE-dependent gene expression, did not activate NFAT signaling. These data provide evidence that peptides released from primary afferent neurons regulate discrete patterns of gene expression in spinal neurons. Because the release of Substance P and calcitonin gene-related peptide from primary afferent neurons is increased following peripheral injury, these peptides may differentially regulate the expression of proteins that underlie persistent hyperalgesia.

TRPV1 function in mouse colon sensory neurons is enhanced by metabotropic 5-hydroxytryptamine receptor activation

Using whole-cell patch-clamp methods, we examined the hypothesis that serotonin [5-hydroxytryptamine (5-HT)] receptor activation enhances TRPV1 function in mouse colon sensory neurons in lumbosacral dorsal root ganglia, which were identified by retrograde labeling with DiI (1,1'-dioctadecyl-3,3,3',3-tetramethlindocarbocyanine methanesulfonate) injected into multiple sites in the wall of the descending colon. 5-HT increased membrane excitability at a temperature below body temperature in response to thermal ramp stimuli in colon sensory neurons from wild-type mice, but not from TRPV1 knock-out mice. 5-HT significantly enhanced capsaicin-, heat-, and proton-evoked currents with an EC50 value of 2.2 microm. 5-HT (1 microm) significantly increased capsaicin-evoked (100 nm) and proton-evoked (pH 5.5) currents 1.6- and 4.7-fold, respectively, and significantly decreased the threshold temperature for heat current activation from 42 to 38 degrees C. The enhancement of TRPV1 by 5-HT was significantly attenuated by selective 5-HT2 and 5-HT4 receptor antagonists, but not by a 5-HT3 receptor antagonist. In support, 5-HT2 and 5-HT4 receptor agonists mimicked the facilitating effects of 5-HT on TRPV1 function. Downstream signaling required G-protein activation and phosphorylation as intracellularly administered GDP-beta-S [guanosine 5'-O-(2-thiodiphosphate], protein kinase A inhibitors, and an A-kinase anchoring protein inhibitor significantly blocked serotonergic facilitation of TRPV1 function; 5-HT2 receptor-mediated facilitation was also inhibited by a PKC inhibitor. We conclude that the facilitation of TRPV1 by metabotropic 5-HT receptor activation may contribute to hypersensitivity of primary afferent neurons in irritable bowel syndrome patients.

A review of the neuroprotective properties of the 5-HT1A receptor agonist repinotan HCl (BAYx3702) in ischemic stroke

Repinotan HCl (repinotan, BAYx3702), a highly selective 5-HT1A receptor agonist with a good record of safety was found to have pronounced neuroprotective effects in experimental models that mimic various aspects of brain injury. Repinotan caused strong, dose-dependent infarct reductions in permanent middle cerebral artery occlusion, transient middle cerebral artery occlusion, and traumatic brain injury paradigms. The specific 5-HT1A receptor antagonist WAY 100635 blocked these effects, indicating that the neuroprotective properties of repinotan are mediated through the 5-HT1A receptor. The proposed neuroprotective mechanisms of repinotan are thought to be the result of neuronal hyperpolarization via the activation of G protein-coupled inwardly rectifying K+ channels upon binding to both pre- and post-synaptic 5-HT1A receptors. Hyperpolarization results in inhibition of neuron firing and reduction of glutamate release. These mechanisms, leading to protection of neurons against overexcitation, could explain the neuroprotective efficacy of repinotan per se, but not necessarily the efficacy by delayed administration. The therapeutic time window of repinotan appeared to be at least 5 h in in vivo animal models, but may be even longer at higher doses of the drug. Experimental studies indicate that repinotan affects various mechanisms involved in the pathogenesis of brain injury. In addition to the direct effect of repinotan on neuronal hyperpolarization and suppression of glutamate release this compound affects the death-inhibiting protein Bcl-2, serotonergic glial growth factor S-100beta and Nerve Growth Factor. It also suppresses the activity of caspase-3 through MAPK and PKCalpha; this effect may contribute to its neuroprotective efficacy. The dose- and time-dependent neuroprotective efficacy of repinotan indicates that the drug is a promising candidate for prevention of secondary brain damage in brain-injured patients suffering from acute ischemic stroke. Unfortunately, however, the first, randomized, double blind, placebo-controlled clinical trial did not demonstrate the efficacy of repinotan in acute ischemic stroke.

Posttranslational mechanisms of peripheral sensitization

The sensation of pain can be dramatically altered in response to injury or disease. This sensitization can occur at the level of the primary sensory neuron, and can be mediated by multiple biochemical mechanisms, including, but not limited to, changes in gene transcription, changes in translation, stability, or subcellular localization of translated proteins, and posttranslational modifications. This review focuses on posttranslational modifications to ion channels expressed in primary sensory neurons that form the machinery driving peripheral sensitization and pain hypersensitivity. Studies published to date show strong evidence for modulation of ion channels involved in transduction and transmission of nociceptive inputs coincident with biophysical and behavioral sensitization. The roles of phosphorylation and oxidation/reduction reactions of voltage-dependent sodium, potassium, and calcium channels are discussed, as well as phosphorylation-mediated modulation of sensory transduction channels.

Nicotinic AChR in subclassified capsaicin-sensitive and -insensitive nociceptors of the rat DRG

Nociceptive cells of the dorsal root ganglion (DRG) were subclassified, in vitro, according to patterns of voltage-activated currents. The distribution and form of nicotinic ACh receptors (nAChRs) were determined. nAChRs were present on both capsaicin-sensitive and -insensitive nociceptors but were not universally present in unmyelinated nociceptors. In contrast, all A delta nociceptors (types 4, 6, and 9) expressed slowly decaying nAChR. Three major forms of nicotinic currents were identified. Specific agonists and antagonists were used to demonstrate the presence of alpha7 in two classes of capsaicin-sensitive, unmyelinated nociceptors (types 2 and 8). In type 2 cells, alpha7-mediated currents were found in isolation. Whereas alpha7 was co-expressed with other nAChR in type 8 cells. These were the only classes in which alpha7 was identified. Other nociceptive classes expressed slowly decaying currents with beta4 pharmacology. Based on concentration response curves formed by nicotinic agonists [ACh, nicotine, dimethyl phenyl piperazinium (DMPP), cytisine] evidence emerged of two distinct nAChR differentially expressed in type 4 (alpha3beta4) and types 5 and 8 (alpha3beta4 alpha5). Although identification could not be made with absolute certainty, patterns of potency (type 4: DMPP > cytisine > nicotine = ACh; type 5 and type 8: DMPP = cytisine > nicotine = ACh) and efficacy provided strong support for the presence of two distinct channels based on an alpha3beta4 platform. Studies conducted on one nonnociceptive class (type 3) failed to reveal any nAChR. After multiple injections of Di-I (1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) into the hairy skin of the hindlimb, we identified cell types 2, 4, 6, 8, and 9 as skin nociceptors that expressed nicotinic receptors. We conclude that at least three nicotinic AChR are diversely distributed into discrete subclasses of nociceptors that innervate hairy skin.

Differential effects of CB1 and opioid agonists on two populations of adult rat dorsal root ganglion neurons

Inhibition of primary afferent neurons contributes to the antihyperalgesic effects of opioid and CB1 receptor agonists. Two bioassays were used to compare the effects of the CB1 receptor agonist CP 55,940 and morphine on dissociated adult rat DRG neurons. Both agonists inhibited the increase in free intracellular Ca2+ concentration evoked by depolarization; however, effects of CP 55,940 occurred primarily in large neurons (cell area, >800 microm2), whereas morphine inhibited the response in smaller neurons. Cotreatment with selective blockers of L-, N-, and P/Q-type voltage-dependent Ca2+ channels indicated that CB1 receptors on DRG neurons couple solely with N-type channels but opioid receptors couple with multiple subtypes. Experiments with selective agonists and antagonists of opioid receptors indicated that mu and delta, but not kappa, receptors contributed to the inhibitory effect of morphine on voltage-dependent Ca2+ influx. Because Ca2+ channels underlie release of transmitters from neurons, the effects of opioid agonists and CP 55,940 on depolarization-evoked release of calcitonin gene-related peptide (CGRP) were compared. Morphine inhibited release through delta receptors but CP 55,940 had no effect. Colocalization of CGRP with delta-opioid but not mu-opioid or CB1 receptor immunoreactivity in superficial laminae of the dorsal horn of the spinal cord was consistent with the data for agonist inhibition of peptide release. Therefore, CB1 and opioid agonists couple with different voltage-dependent Ca2+ channels in different populations of DRG neurons. Furthermore, differences occur in the distribution of receptors between the cell body and terminals of DRG neurons. The complementary action of CB1 and opioid receptor agonists on populations of DRG neurons provides a rationale for their combined use in modulation of somatosensory input to the spinal cord.

Voltage-dependent Ca2+ currents in rat cardiac dorsal root ganglion neurons

This study presents the kinetic and pharmacological properties of voltage-gated Ca(2+) currents in anatomically defined cardiac dorsal root ganglion (DRG) neurons in rats. The neurons were labelled by prior injection of fluorescent tracer Fast Blue into the pericardial sack. There were three distinct groups of neurons with respect to cell size: small (27% of total; cell capacitance <30 pF), medium (65% of total; capacitance 30-80 pF) and large neurons (8% of total; capacitance >80 pF). The properties of Ca(2+) currents were tested in small and medium-sized neurons. In large neurons currents could not be adequately controlled and were not analysed. Ca(2+) currents did not completely inactivate during 100 ms depolarising voltage steps. The activation thresholds in small (-36.9+/-1.3 mV) and medium (-39.0+/-1.3 mV) size neurons were similar. Current densities were 105.8 pA/pF in small and 97.4 pA/pF in large neurons and also did not differ. The kinetic properties of activation and inactivation did not differ between small and medium-sized cardiac DRG neurons. At membrane potentials between -50 and -60 mV (the expected resting membrane potential in these neurons) 55 to 70% of Ca(2+) currents in small and medium-sized neurons were available for activation. Both, small and medium-sized neurons expressed similar proportions of L (7.5%), N (25%) and P/Q (36%) type Ca(2+) currents. We conclude that small and medium-sized cardiac DRG neurons are homogeneous with respect to the expression and properties of voltage-gated Ca(2+) currents. Voltage-gated Ca(2+) currents probably play an important role in action potential generation in cardiac DRG neurons due to their availability for activation at resting membrane potential, their high density and voltage threshold close to the threshold for voltage-gated Na(+) currents.

Arachidonic acid mediates muscarinic inhibition and enhancement of N-type Ca2+ current in sympathetic neurons

N-type Ca(2+) channels participate in acute activity-dependent processes such as regulation of Ca(2+)-activated K(+) channels and in more prolonged events such as gene transcription and long-term depression. A slow postsynaptic M(1) muscarinic receptor-mediated modulation of N-type current in superior cervical ganglion neurons may be important in regulating these processes. This slow pathway inhibits N-type current by using a diffusible second messenger that has remained unidentified for more than a decade. Using whole-cell patch-clamp techniques, which isolate the slow pathway, we found that the muscarinic agonist oxotremorine methiodide not only inhibits currents at positive potentials but enhances N-type current at negative potentials. Enhancement was also observed in cell-attached patches. These findings provide evidence for N-type Ca(2+)-current enhancement by a classical neurotransmitter. Moreover, enhancement and inhibition of current by oxotremorine methiodide mimics modulation observed with direct application of a low concentration of arachidonic acid (AA). Although no transmitter has been reported to use AA as a second messenger to modulate any Ca(2+) current in either neuronal or nonneuronal cells, we nevertheless tested whether a fatty acid signaling cascade was involved. Blocking phospholipase C, phospholipase A(2), or AA but not AA metabolism minimized muscarinic modulation of N-type current, supporting the participation of these molecules in the slow pathway. A role for the G protein G(q) was also confirmed by blocking muscarinic modulation of Ca(2+) currents with anti-G(qalpha) antibody. Our finding that AA participates in the slow pathway strongly suggests that it may be the previously unknown diffusible second messenger.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Calcium influx into dendrites of the leech Retzius neuron evoked by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is a ubiquitous neurotransmitter and neuromodulator that affects neural circuits and behaviours in vertebrates and invertebrates. In the present study, we have investigated 5-HT-induced Ca(2+) transients in subcellular compartments of Retzius neurons in the leech central nervous system using confocal laser scanning microscopy, and studied the effect of 5-HT on the electrical coupling between the Retzius neurons. Bath application of 5-HT (50mM) induced a Ca(2+) transient in axon, dendrites and cell body of the Retzius neuron. This Ca(2+) transient was significantly faster and larger in dendrites than in axon and cell body, and was half-maximal at a 5-HT concentration of 5-12mM. The Ca(2+) transient was suppressed in the absence of extracellular Ca(2+) and by methysergide (100mM), a non-specific antagonist of metabotropic 5-HT receptors, and was strongly reduced by bath application of the Ca(2+) channel blocker Co(2+) (2mM). Injection of the non-hydrolysable GTP analogue GTPgammaS increased and prolonged the dendritic 5-HT-induced Ca(2+) transient. The non-selective protein kinase inhibitor H7 (100mM) and the adenylate cyclase inhibitor SQ22536 (500 mM) did not affect the Ca(2+) transient, and the membrane-permeable cAMP analogue dibutyryl-cAMP (500 mM) did not mimic the effect of 5-HT application. 5-HT reduced the apparent electrical coupling between the two Retzius neurons, whereas suppression of the Ca(2+) influx by removal of external Ca(2+) improved the transmission of action potentials at the electrical synapses which are located between the dendrites of the adjacent Retzius neurons. The results indicate that 5-HT induces a Ca(2+) influx through calcium channels located primarily in the dendrites, and presumably activated by a G protein-coupled 5-HT receptor. The dendritic Ca(2+) increase appears to modulate the excitability of, and the synchronization between, the two Retzius neurons.

5-HT(1A) receptor-mediated inhibition and 5-HT(2) as well as 5-HT(3) receptor-mediated excitation in different subdivisions of the rat amygdala

The techniques of extracellular single cell recording and microiontophoresis were used to study the effects of serotonin (5-HT) and of 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptor agonists on the spontaneous activity of amygdaloid neurons in rats anesthetized with urethane. The background discharge rate was modified by 5-HT as well as by 5-HT agonists in about two-thirds of neurons tested in different nuclei of the amygdaloid complex. Whereas the 5-HT(2) and 5-HT(3) agonists significantly increased the neuronal discharge rate in nearly all subdivisions of the amygdala, the 5-HT(1A) agonist significantly inhibited the firing rate. Co-administration of bicuculline and 5-HT receptor agonists prevented the 8-OH-DPAT-induced increases in the firing rate in most cases tested, as well as the inhibitory effects of DOI or 2-methyl-5HT. Therefore, GABAergic interneurons seem to be involved in the mediation of serotonergic effects. The action of 5-HT agonists on the neuronal discharge rate was blocked by different receptor-specific antagonists. The results support the hypothesis that 5-HT exerts control throughout the amygdala by acting at least on 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptors seemingly located both on projection and interneurons.

Subclassified acutely dissociated cells of rat DRG: histochemistry and patterns of capsaicin-, proton-, and ATP-activated currents

We used a "current signature" method to subclassify acutely dissociated dorsal root ganglion (DRG) cells into nine subgroups. Cells subclassified by current signature had uniform properties. The type 1 cell had moderate capsaicin sensitivity (25.9 pA/pF), powerful, slowly desensitizing (tau = 2,300 ms), ATP-activated current (13.3 pA/pF), and small nondesensitizing responses to acidic solutions (5.6 pA/pF). Type 1 cells expressed calcitonin gene-related peptide immunoreactivity (CGRP-IR), manifested a wide action potential (7.3 ms), long duration afterhyperpolarization (57.0 ms), and were IB4 positive. The type 2 cell exhibited large capsaicin activated currents (134.9 pA/pF) but weak nondesensitizing responses to protons (15.3 pA/pF). Currents activated by ATP and alphabeta-m-ATP (51.7 and 44.6 pA/pF, respectively) had fast desensitization kinetics (tau = 214 ms) that were distinct from all other cell types. Type 2 cells were IB4 positive but did not contain either substance P (SP) or CGRP-IR. Similar to capsaicin-sensitive nociceptors in vivo, the afterhyperpolarization of the type 2 cell was prolonged (54.7 ms). The type 3 cell expressed, amiloride-sensitive, rapidly desensitizing (tau = 683 ms) proton-activated currents (127.0 pA/pF), and was insensitive to ATP or capsaicin. The type 3 cell was IB4 negative and contained neither CGRP nor SP-IR. The afterhyperpolarization (17.5 ms) suggested nonnociceptive function. The type 4 cell had powerful ATP-activated currents (17.4 pA/pF) with slow desensitization kinetics (tau = 2, 813 ms). The afterhyperpolarization was prolonged (46.5 ms), suggesting that this cell type might belong to a capsaicin-insensitive nociceptor population. The type 4 cell did not contain peptides. The type 7 cell manifested amiloride-sensitive, proton-activated currents (45.8 pA/pF) with very fast desensitization kinetics (tau = 255 ms) and was further distinct from the type 3 cell by virtue of a nondesensitizing amiloride-insensitive component (6.0 pA/pF). Capsaicin and ATP sensitivity were relatively weak (4.3 and 2.9 pA/pF, respectively). Type 7 cells were IB4 positive and contained both SP and CGRP-IR. They exhibited an exceptionally long afterhyperpolarization (110 ms) that was suggestive of a silent (mechanically insensitive) nociceptor. We concluded that presorting of DRG cells by current signatures separated them into internally homogenous subpopulations that were distinct from other subclassified cell types.

Sequential activation of the 5-HT1(A) serotonin receptor and TrkB induces the serotonergic neuronal phenotype

Serotonin (5-HT) is an important factor controlling survival, differentiation, and plasticity of neurons in serotonergic target regions of the brain and has been implicated in major psychiatric and autonomic disorders. Relatively little is known, however, of factors controlling differentiation and plasticity of developing and adult 5-HT neurons. We show now that 5-HT, the 5-HT1(A) receptor, brain-derived neurotrophic factor (BDNF), and its receptor, trkB, form an auto/paracrine loop for the regulation of the serotonergic phenotype. Serotonin applied to cultures from E14 rat raphe increased numbers of neurons expressing serotonergic markers in a dose-dependent manner. Agonists of the 5-HT1(A) receptor, BP-554 and 8-OH-DPAT, but not agonists of the 5-HT1(B) and 5-HT1(D) receptors, mimicked this effect, while the specific 5-HT1(A) antagonist, WAY-100635, inhibited it. Serotonin also increased BDNF mRNA and protein in embryonic raphe cultures. Induction of serotonergic markers by serotonin was suppressed by a trkB-IgG fusion protein but not by trkC-IgG. Taken together, our data indicate that serotonin acts on 5-HT1(A) autoreceptors, causing up-regulation of BDNF, which activates trkB to promote serotonergic phenotype-specific markers.

Mechanical activation of dorsal root ganglion cells in vitro: comparison with capsaicin and modulation by kappa-opioids

The aim of this study was to characterize plasma membrane pathways involved in the intracellular calcium ([Ca(2+)](i)) response of small DRG neurons to mechanical stimulation and the modulation of these pathways by kappa-opioids. [Ca(2+)](i) responses were measured by fluorescence video microscopy of Fura-2 labeled lumbosacral DRG neurons obtained from adult rats in short-term primary culture. Transient focal mechanical stimulation of the soma, or brief superfusion with 300 nM capsaicin, resulted to [Ca(2+)](i) increases which were abolished in Ca(2+)-free solution, but unaffected by lanthanum (25 microM) or tetrodotoxin (10(-6) M). 156 out of 465 neurons tested (34%) showed mechanosensitivity while 55 out of 118 neurons (47%) were capsaicin-sensitive. Ninty percent of capsaicin-sensitive neurons were mechanosensitive. Gadolinium (Gd(3+); 250 microM) and amiloride (100 microM) abolished the [Ca(2+)](i) transient in response to mechanical stimulation, but had no effect on capsaicin-induced [Ca(2+)](i) transients. The kappa-opioid agonists U50,488 and fedotozine showed a dose-dependent inhibition of mechanically stimulated [Ca(2+)](i) transients but had little effect on capsaicin-induced [Ca(2+)](i) transients. The inhibitory effect of U50,488 was abolished by the kappa-opioid antagonist nor-Binaltorphimine dihydrochloride (nor-BNI; 100 nM), and by high concentrations of naloxone (30-100 nM), but not by low concentrations of naloxone (3 nM). We conclude that mechanically induced [Ca(2+)](i) transients in small diameter DRG somas are mediated by influx of Ca(2+) through a Gd(3+)- and amiloride-sensitive plasma membrane pathway that is co-expressed with capsaicin-sensitive channels. Mechanical-, but not capsaicin-mediated, Ca(2+) transients are sensitive to kappa-opioid agonists.

delta opioid receptor modulation of several voltage-dependent Ca(2+) currents in rat sensory neurons

Endogenous enkephalins and delta opiates affect sensory function and pain sensation by inhibiting synaptic transmission in sensory circuits via delta opioid receptors (DORs). DORs have long been suspected of mediating these effects by modulating voltage-dependent Ca(2+) entry in primary sensory neurons. However, not only has this hypothesis never been validated in these cells, but in fact several previous studies have only turned up negative results. By using whole-cell current recordings, we show that the delta enkephalin analog [D-Ala(2), D-Leu(5)]-enkephalin (DADLE) inhibits, via DORs, L-, N-, P-, and Q-high voltage-activated Ca(2+) channel currents in cultured rat dorsal root ganglion (DRG) neurons. The percentage of responding cells was remarkably high (75%) within a novel subpopulation of substance P-containing neurons compared with the other cells (18-35%). DADLE (1 microM) inhibited 32% of the total barium current through calcium channels (I(Ba)). A delta (naltrindole, 1 microM), but not a mu (beta-funaltrexamine, 5 microM), antagonist prevented the DADLE response, whereas a DOR-2 subtype (deltorphin-II, 100 nM), but not a DOR-1 (DPDPE, 1 microM), agonist mimicked the response. L-, N-, P-, and Q-type currents contributed, on average, 18, 48, 14, and 16% to the total I(Ba) and 19, 50, 26, and 20% to the DADLE-sensitive current, respectively. The drug-insensitive R-type current component was not affected by the agonist. This work represents the first demonstration that DORs modulate Ca(2+) entry in sensory neurons and suggests that delta opioids could affect diverse Ca(2+)-dependent processes linked to Ca(2+) influx through different high-voltage-activated channel types.

Predominant presence of beta-arrestin-1 in small sensory neurons of rat dorsal root ganglia

Reverse transcription-polymerase chain reaction and western immunoblot analyses were performed to demonstrate the presence of beta-arrestin-1 in rat dorsal root ganglion. beta-Arrestin-1 existed as two alternatively spliced variants, although predominantly in its untruncated form. Several factors affected the visualization of the truncated version on a sodium dodecyl sulfate-polyacrylamide gel; however, the isoform was clearly detected on a two-dimensional gel. We further localized beta-arrestin-1 immunoreactivity in the sensory neurons of the 5th lumbar dorsal root ganglia. Beta-arrestin-1-immunoreactive neurons accounted for approximately 60% of the sensory neurons, and approximately 88% of the beta-Arrestin-1 immunoreactive neurons fell into a category of small neurons having a diameter of 10-30 microm. Members of the arrestin superfamily play crucial roles in the desensitization of G protein-coupled receptors. Our data demonstrating the presence of beta-arrestin-1 in the rat dorsal root ganglion at both messenger RNA and protein levels support the idea that beta-arrestin- participates in receptor desensitization in the sensory neurons. Furthermore, because small-size neurons of dorsal root ganglion are often implicated in nociception, the predominant presence of beta-arrestin-1 immunoreactivity in small-size sensory neurons suggests that beta-arrestin-1 may have a role modulating nociceptive signals.

The primary afferent nociceptor as pattern generator

One of the most important advances in our understanding of the pain experience was the introduction of the 'gate control' theory which stimulated analysis of activity pattern in nociceptive pathways and its modulation. Advances in cellular and molecular biology have recently begun to provide detailed information on the mechanisms of stimulus transduction within primary afferent nociceptors as well as mechanisms that modulate the transduction process. From these new insights into the sensory physiology of the nociceptive nerve ending emerges a concept of the primary afferent as the first site of pattern generation in the nociceptive pathway, in which dynamic tuning of gain in the mosaic of inputs to individual primary afferents occurs. The electrical properties of the nociceptor membrane that converts the generator potential to a pattern of action potentials is also actively adjusted. Our present understanding of the intracellular mechanisms that modulate the pattern of activity in nociceptive primary afferents is summarized, and implications for future efforts to unravel the meaning of patterning in nociceptor activity are discussed.

Serotonergic modulation of hyperpolarization-activated current in acutely isolated rat dorsal root ganglion neurons

1. The effect of serotonin (5-HT) on the hyperpolarization-activated cation current (IH) was studied in small-, medium- and large-diameter acutely isolated rat dorsal root ganglion (DRG) cells, including cells categorized as type 1, 2, 3 and 4 based on membrane properties. 5-HT increased IH in 91 % of medium-diameter DRG cells (including type 4) and in 67 % of large-diameter DRG cells, but not other DRG cell types. 2. The increase of IH by 5-HT was antagonized by spiperone but not cyanopindolol, and was mimicked by 5-carboxyamidotryptamine, but not (+)-8-hydroxydipropylaminotetralin (8-OH-DPAT) or cyanopindolol. These data suggested the involvement of 5-HT7 receptors, which were shown to be expressed by medium-diameter DRG cells using RT-PCR analysis. 3. 5-HT shifted the conductance-voltage relationship of IH by +6 mV without changing peak conductance. The effects of 5-HT on IH were mimicked and occluded by forskolin, but not by inactive 1,9-dideoxy forskolin. 4. At holding potentials negative to -50 mV, 5-HT increased steady-state inward current and instantaneous membrane conductance (fast current). The 5-HT-induced inward current and fast current were blocked by Cs+ but not Ba2+ and reversed at -23 mV, consistent with the properties of tonically activated IH. 5. In medium-diameter neurons recorded from in the current clamp mode, 5-HT depolarized the resting membrane potential, decreased input resistance and facilitated action potential generation by anode-break excitation. 6. The above data suggest that in distinct subpopulations of DRG neurons, 5-HT increases cAMP levels via activation of 5-HT7 receptors, which shifts the voltage dependence of IH to more depolarized potentials and increases neuronal excitability.

The pharmacology of ion channels: with particular reference to voltage-gated Ca2+ channels

Ion channels are molecular machines that serve as principal integrating and regulatory devices for the control of cellular excitability. They are also major targets for drug action. The basic aspects of ion channel structure and pharmacological control are reviewed and illustrated with specific reference to a major class of therapeutic agents and molecular tools--the clinically available Ca2+ channel antagonists.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Muscarine modulates Ca2+ channel currents in rat sensorimotor pyramidal cells via two distinct pathways

We used the whole cell patch-clamp technique and single-cell reverse transcription-polymerase chain reaction (RT-PCR) to study the muscarinic receptor-mediated modulation of calcium channel currents in both acutely isolated and cultured pyramidal neurons from rat sensorimotor cortex. Single-cell RT-PCR profiling for muscarinic receptor mRNAs revealed the expression of m1, m2, m3, and m4 subtypes in these cells. Muscarine reversibly reduced Ca2+ currents in a dose-dependent manner. The modulation was blocked by the muscarinic antagonist atropine. When the internal recording solution included 10 mM ethylene glycol-bis(beta-aminoethyl ether)-N, N,N',N'-tetraacetic acid (EGTA) or 10 mM bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA), the modulation was rapid (tauonset approximately 1.2 s). Under conditions where intracellular calcium levels were less controlled (0.0-0.1 mM BAPTA), a slowly developing component of the modulation also was observed (tauonset approximately 17 s). Both fast and slow components also were observed in recordings with 10 mM EGTA or 20 mM BAPTA when Ca2+ was added to elevate internal [Ca2+] ( approximately 150 nM). The fast component was due to a reduction in both N- and P-type calcium currents, whereas the slow component involved L-type current. N-ethylmaleimide blocked the fast component but not the slow component of the modulation. Preincubation of cultured neurons with pertussis toxin (PTX) also greatly reduced the fast portion of the modulation. These results suggest a role for both PTX-sensitive G proteins as well as PTX-insensitive G proteins in the muscarinic modulation. The fast component of the modulation was reversed by strong depolarization, whereas the slow component was not. Reblock of the calcium channels by G proteins (at -90 mV) occurred with a median tau of 68 ms. We conclude that activation of muscarinic receptors results in modulation of N- and P-type channels by a rapid, voltage-dependent pathway and of L-type current by a slow, voltage-independent pathway.