Pharmacological characterization of amine receptors on embryonic chick sensory neurones

Reframing a debate in chiropractic

The chiropractic profession is 125 years old and has evolved a culture beset with internal conflict. The internal ructions have been particularly noticeable during the last 20 years. The recent resignation of the entire World Federation of Chiropractic Research Committee has again focussed the conflicting views and goals of the “wellness” and “evidence” factions within the profession. These polarising viewpoints are worsening to the degree that there are calls for the profession to break into two separate entities. Key to the recognition of the differences within the profession is the recognition of title for particular sub populations of patients presenting to chiropractors. For many of the sub populations such as sport or paediatrics there has grown appropriate post professional specialist educational training sometimes leading to a protected title. However, this is not occurring in that group of practitioners that choose to focus on wellness care. A recommendation is made that wellness chiropractic be viewed as a post professional specialty program within chiropractic, as it is in medicine and elsewhere, and that recognition follow after appropriate post professional educational programs have been completed, as is customary in the other special interest groups. In order to do so, consensus will be required from all stakeholders within the profession on the level, scope and depth of such programs. Furthermore, it is possible that different jurisdictions around the world may require different post graduate educational levels based on local competitive, legal and professional circumstances. In such cases, transitioning to the higher level over a period of time may be undertaken. Recognition of the wellness specialty by the profession would allow for vertical integration with other healthcare providers as well as help bridge a gap between the entrepreneur and academic groups that would be responsible for creating these programs at tertiary education institutions. Finally, should these programs acquire evidence to underpin them, a process that would be taught within the programs, it is likely that recognition of an extended scope of practice would occur increasing the appeal of chiropractic to the public.

Adrenoceptors Modulate Cholinergic Synaptic Transmission at the Neuromuscular Junction

Adrenoceptor activators and blockers are widely used clinically for the treatment of cardiovascular and pulmonary disorders. More recently, adrenergic agents have also been used to treat neurodegenerative diseases. Recent studies indicate a location of sympathetic varicosities in close proximity to neuromuscular junctions. The pressing question is whether there could be any effects of endo- or exogenous catecholamines on cholinergic neuromuscular transmission. It was shown that the pharmacological stimulation of adrenoceptors, as well as sympathectomy, can affect both acetylcholine release from motor nerve terminals and the functioning of postsynaptic acetylcholine receptors. In this review, we discuss the recent data regarding the effects of adrenergic drugs on neurotransmission at the neuromuscular junction. The elucidation of the molecular mechanisms by which the clinically relevant adrenomimetics and adrenoblockers regulate quantal acetylcholine release from the presynaptic nerve terminals and postsynaptic sensitivity may help in the design of highly effective and well-tolerated sympathomimetics for treating a number of neurodegenerative diseases accompanied by synaptic defects.

Sympathomimetics regulate quantal acetylcholine release at neuromuscular junctions through various types of adrenoreceptors

The studies of the interaction between the sympathetic and motor nervous systems are extremely relevant due to therapy for many neurodegenerative and cardiovascular disorders involving adrenergic compounds. Evidences indicate close contact between sympathetic varicosities and neuromuscular synapses. This raises questions about the effects of catecholamines on synaptic transmission. The currently available information is contradictory, and the types of adrenoreceptors responsible for modulation of neurotransmitter release have not been identified in mammalian neuromuscular synapses. Our results have shown that the α1A, α1B, α2A, α2B, α2C, and β1 adrenoreceptor subtypes are expressed in mouse diaphragm muscle containing neuromuscular synapses and sympathetic varicosities. Pharmacological stimulation of adrenoreceptors affects both spontaneous and evoked acetylcholine quantal secretion. Agonists of the α1, α2 and β1 adrenoreceptors decrease spontaneous release. Activation of the α2 and β1 adrenoreceptors reduces the number of acetylcholine quanta released in response to a nerve stimulus (quantal content), but an agonist of the β2 receptors increases quantal content. Activation of α2 and β2 adrenoreceptors alters the kinetics of acetylcholine quantal release by desynchronizing the neurosecretory process. Specific blockers of these receptors eliminate the effects of the specific agonists. The action of blockers on quantal acetylcholine secretion indicates possible action of endogenous catecholamines on neuromuscular transmission. Elucidating the molecular mechanisms by which clinically utilized adrenomimetics and adrenoblockers regulate synaptic vesicle release at the motor axon terminal will lead to the creation of improved and safer sympathomimetics for the treatment of various neurodegenerative diseases with synaptic defects.

Slow inhibition of N-type calcium channels with GTP gamma S reflects the basal G protein-GDP turnover rate

The inhibition of N-type Ca channels via a G protein pathway is a rapid mechanism for modulating Ca influx. It has been noted, however, that when G proteins are activated by guanosine 5'- O-(3-thiotriphosphate) (GTPgammaS), the speed of inhibition is greatly reduced, despite the pathway having fewer molecular steps. We explored this anomaly in chick dorsal root ganglion neurons by comparing Ca current inhibition using GTPgammaS with application of the G protein receptor agonist noradrenaline. Noradrenaline caused rapid Ca channel inhibition (tau~5 s), contrasting greatly with the ~70-fold slower rate observed with GTPgammaS. Additionally, the slow rate with GTPgammaS could be accelerated to near agonist-induced rates by application of noradrenaline, demonstrating that the inhibition with GTPgammaS was not perfusion limited and that the rate-limiting step was upstream from GTPgammaS binding. Our results suggest that in the absence of noradrenaline, G protein activation by GTPgammaS is impeded by the slow resting turnover of GDP/GTP. The rate at which inhibition develops with GTPgammaS (tau~350 s) is thus a direct and sensitive measure of resting GDP turnover.

Activation of phosphoinositide turnover and protein kinase C by neurotransmitters that modulate calcium channels in embryonic chick sensory neurons

Gamma aminobutyric acid (GABA) and norepinephrine modulate the excitability of primary chick sensory neurons by decreasing the voltage dependent Ca current. Although previous electrophysiological studies indicate that neurotransmitter modulation of the Ca current in these neurons involves protein kinase C, the biochemical aspects of this mechanism have not been examined directly. We find that both norepinephrine (via a unique alpha receptor subtype) and GABA (via GABAb receptors) linked to pertussis toxin sensitive pathways, stimulate the metabolism of membrane phosphatidylinositol phospholipids in primary chick sensory neurons. In addition, norepinephrine causes the rapid translocation of C kinase activity from cytosolic to membrane associated distribution, consistent with its rapid activation in response to applied neurotransmitter. The pharmacology, pertussis toxin sensitivity and time course of the biochemical changes due to neurotransmitter treatment parallel the effects of these transmitters on calcium current modulation. These biochemical studies confirm the hypothesis that activation of protein kinase C is critically involved in calcium channel modulation in embryonic chick sensory neurons.

Serotonin in the inferior colliculus

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

Monoclonal antibodies reveal receptor specificity among G-protein-coupled receptor kinases

Guanine nucleotide-binding regulatory protein (G protein)-coupled receptor kinases (GRKs) constitute a family of serine/threonine kinases that play a major role in the agonist-induced phosphorylation and desensitization of G-protein-coupled receptors. Herein we describe the generation of monoclonal antibodies (mAbs) that specifically react with GRK2 and GRK3 or with GRK4, GRK5, and GRK6. They are used in several different receptor systems to identify the kinases that are responsible for receptor phosphorylation and desensitization. The ability of these reagents to inhibit GRK- mediated receptor phosphorylation is demonstrated in permeabilized 293 cells that overexpress individual GRKs and the type 1A angiotensin II receptor. We also use this approach to identify the endogenous GRKs that are responsible for the agonist-induced phosphorylation of epitope-tagged beta2- adrenergic receptors (beta2ARs) overexpressed in rabbit ventricular myocytes that are infected with a recombinant adenovirus. In these myocytes, anti-GRK2/3 mAbs inhibit isoproterenol-induced receptor phosphorylation by 77%, while GRK4-6-specific mAbs have no effect. Consistent with the operation of a betaAR kinase-mediated mechanism, GRK2 is identified by immunoblot analysis as well as in a functional assay as the predominant GRK expressed in these cells. Microinjection of GRK2/3-specific mAbs into chicken sensory neurons, which have been shown to express a GRK3-like protein, abolishes desensitization of the alpha2AR-mediated calcium current inhibition. The intracellular inhibition of endogenous GRKs by mAbs represents a novel approach to the study of receptor specificities among GRKs that should be widely applicable to many G-protein-coupled receptors.

G protein-coupled receptor kinase mediates desensitization of norepinephrine-induced Ca2+ channel inhibition

G protein-coupled receptors are essential signaling molecules at sites of synaptic transmission. Here, we explore the mechanisms responsible for the use-dependent termination of metabotropic receptor signaling in embryonic sensory neurons. We report that the inhibition of voltage-dependent Ca2+ channels mediated by alpha2-adrenergic receptors desensitizes slowly with prolonged exposure to the transmitter and that the desensitization is mediated by a G protein-coupled receptor kinase (GRK). Intracellular introduction of recombinant, purified kinases or synthetic blocking peptides into individual neurons demonstrates the specific involvement of a GRK3-like protein. These results suggest that GRK-mediated termination of receptor-G protein coupling is likely to regulate synaptic strength and, as such, may provide one effective mechanism for depression of synaptic transmission.

Interaction of convergent pathways that inhibit N-type calcium currents in sensory neurons

Norepinephrine and GABA inhibit omega-conotoxin GVIA-sensitive (N-type) calcium current in embryonic sensory neurons by separate pathways. We have investigated the mechanisms that limit the modulation of N current by varying the level of activation for a single pathway or simultaneously activating multiple pathways. Calcium currents were measured with tight-seal, whole-cell recording methods. Simultaneous application of the two transmitters at saturating concentrations produced a larger inhibition of the current than either transmitter by itself, but the maximal inhibition was not linearly additive. Maximal, direct activation of GTP-binding proteins by intracellular application of guanosine 5'-(3-O-thio)-triphosphate (GTP gamma S) resulted in a similar limit to the inhibition; furthermore, GTP gamma S did not enhance the maximal inhibition produced by co-application of transmitters. Interventions downstream in the modulatory pathway (e.g. direct activation of protein kinase C or inhibition of protein phosphatases) were also unable to alter the maximal limit for inhibition. These results suggest that transmitter-mediated inhibition is not limited by receptor number, levels of G-protein or protein kinase C activation, or degree of phosphorylation; rather, the extent of inhibition may be limited by the structural properties of the N channels themselves.

Transmitter-mediated inhibition of N-type calcium channels in sensory neurons involves multiple GTP-binding proteins and subunits

The modulation of voltage-activated Ca2+ channels by neurotransmitters and peptides is very likely a primary means of regulating Ca(2+)-dependent physiological functions such as neurosecretion, muscle contraction, and membrane excitability. In neurons, N-type Ca2+ channels (defined as omega-conotoxin GVIA-sensitive) are one prominent target for transmitter-mediated inhibition. This inhibition is widely thought to result from a shift in the voltage independence of channel gating. Recently, however, voltage-independent inhibition has also been described for N channels. As embryonic chick dorsal root ganglion neurons express both of these biophysically distinct modulatory pathways, we have utilized these cells to test the hypothesis that the voltage-dependent and -independent actions of transmitters are mediated by separate biochemical pathways. We have confirmed this hypothesis by demonstrating that the two modulatory mechanisms activated by a single transmitter involve not only different classes of G protein but also different G protein subunits.

Serotonin inhibits Ca2+ currents in porcine melanotrophs by activating 5-HT1C and 5-HT1A receptors

1. We have investigated the effect of serotonin (5-HT) on Ca2+ currents in cultured porcine pituitary intermediate lobe (IL) cells. Electrophysiological recordings were performed in the whole-cell configuration of the patch-clamp technique. All membrane currents other than Ca2+ currents were blocked pharmacologically and by ionic substitution. 2. Two types of Ca2+ currents were recorded in IL cells, differing by their activation and inactivation properties. The first type of Ca2+ current was activated at membrane potentials more positive than -60 mV and had a transient time course during the 100 ms depolarizing voltage steps. The properties of this current correspond to those of the T-type or low-voltage-activated Ca2+ current. The second type of Ca2+ current had a threshold for activation between -30 and -20 mV and showed no sign of inactivation with time during the voltage steps. The properties of this current are similar to those of the L-type or high-voltage-activated Ca2+ current. 3. Current to voltage (I-V) relationships obtained either by conventional 100 ms voltage steps from a holding potential (VH) of -100 mV to various test potentials or by 800 ms voltage ramps from -100 to +50mV matched one another closely and showed two inward current humps corresponding to the activation of the T-type and L-type Ca2+ currents respectively. The ramp protocol was used to characterize the effect of 5-HT on the Ca2+ current I-V relationship. 4. 5-HT (100nM to 50 microM) reversibly inhibited the amplitude of the Ca2+ current triggered by 100 ms voltage jumps from a Vh of -100 mV to a test potential of 0 mV. 5. The effect of 5-HT was dose dependent with a threshold between 10 and 100 nM and a maximal effect at 10 microM. At a concentration of 10 microM, the average inhibition of Ca2+ current by 5-HT was 18.3 +/- 6.5% (n = 27). 5-HT inhibited Ba2+ current in a similar fashion. 6. When examining the effect of 5-HT on Ca2+ current I-V relationships, we observed a reversible inhibition of the high-threshold component corresponding to the L-type Ca2+ current. We never observed any effect of 5-HT on the T-type current. 7. The effect of 5-HT (10 microM) was antagonized to various extents by mianserin (1 microM) but not by ketanserin (0.1 microM), suggesting the involvement of 5-HT1C receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of sympathetic nerves exerts an inhibitory influence on afferent nerve-induced vasodilation unrelated to vasoconstriction in rat dental pulp

In order to elucidate a possible influence of the sympathetic nervous system on afferent nerve function, rat mandibular incisors were electrically stimulated and blood flow changes monitored in the incisor pulp of untreated and sympathectomized animals by a laser Doppler flowmeter. Monopolar electrical stimulation of the tooth (200 microA, 5 ms, 40 Hz, 1 s) in normal animals resulted in a transient reduction in pulpal blood flow (PBF) (16% reduction, n = 10) followed by a small but long-lasting increase (11% increase). After administration of phenoxybenzamine or phentolamine (3 mg kg-1, i.v.) the initial dip in PBF was reduced by 59% (P < 0.001) while the subsequent increase was enhanced by 185% (P < 0.001). Similarly, infusion of prazosin (50 micrograms kg-1, i.v.) and idazoxan (0.5 mg kg-1, i.v.) significantly enhanced the increase in PBF by 118 and by 79%, respectively. In chronically sympathectomized animals the increase in PBF was 250% larger than that seen in untreated animals (P < 0.001). This increase in PBF was not further enhanced after alpha-adrenergic blockade. Acute resection of the superior cervical sympathetic ganglion, also resulted in some enhancement (by 56%) of the stimulation-induced increase in PBF (P < 0.01, n = 6). The increase in PBF was unaffected by infusion of timolol (150 micrograms kg-1) and atropine (1 mg kg-1) but was totally abolished by intravenous pre-treatment with capsaicin (1-3 mg kg-1). The present results suggest that activation of sympathetic nerves exerts inhibitory effects on the afferent nerve-induced vasodilation in the rat incisor pulp unrelated to sympathetic vasoconstriction.

GABAB-mediated modulation of the voltage-gated Ca2+ channels

1. The amino acid, gamma-aminobutyric acid (GABA), activates two different receptor types (Bowery et al., 1980; reviewed by Ogata, 1990a). 2. GABAA receptors are bicuculline-sensitive and are coupled to Cl- channels, while activation of bicuculline-insensitive GABAB receptors has been implicated in the modulation of Ca2+ (Dunlap and Fischbach, 1981) and K+ (Gahwiler and Brown, 1985; Inoue et al., 1985a,b; reviewed by Ogata, 1990b) channels. 3. Baclofen is a specific agonist for GABAB receptors (Bowery et al., 1980). In rat sensory neurones, baclofen suppresses the membrane Ca2+ current (ICa) by a mechanism involving a partussis toxin-sensitive G protein (Holz et al., 1986; Scott and Dolphin, 1986). 4. It has been shown that the inhibitory effect of baclofen is more potent on the early portion of ICa than on the later portion and consequently the rate of ICa activation is slowed (Deisz and Lux, 1985; Dolphin and Scott, 1986). 5. The mechanisms underlying these GABAB-mediated modulation of ICa is not fully understood. This article reviews the inhibitory action of baclofen on ICa in sensory neurones.

The role of neuroeffector mechanisms in cerebral hyperperfusion syndromes

Cerebral hyperperfusion, a state in which blood flow exceeds the metabolic needs of brain, may complicate a number of neurological and neurosurgical conditions. It may account for the propensity with which hemorrhage, cerebral edema, or seizures follow embolic stroke, carotid endarterectomy, or the excision of large arteriovenous malformations, and for some of the morbidity that accompanies acute severe head injury, prolonged seizures, and acute severe hypertension. Hyperperfusion syndromes have in common acute increases in blood pressure, vasodilatation, breakdown of the blood-brain barrier, and the development of cerebral edema. These common features suggest the possibility that they share the same pathogenic mechanisms. It was believed until recently that reactive hyperemia was caused primarily by the generation of vasoactive metabolites, which induced vasodilatation through relaxation of vascular smooth muscle. However, the authors have recently established that the release of vasoactive neuropeptides from perivascular sensory nerves via axon reflex-like mechanisms has a significant bearing upon a number of hyperperfusion syndromes. In this article, the authors summarize their data and discuss possible therapeutic implications for blockade of these nerves or their constituent neuropeptides.

Modulation of vertebrate neuronal calcium channels by transmitters

A large number of neurotransmitters have now been shown to reduce the amplitude and slow the activation kinetics of whole cell HVA ICa in a great diversity of neurons. These transmitters include L-glutamate (AMPA/kainate, metabotropic and NMDA receptors), GABA (via GABAB receptors, NA (via alpha 2 receptors), 5-HT, NA (via alpha 2 receptors), DA and several peptides. Both whole-cell and single-channel studies have demonstrated that the N-channel is the most common channel type to be blocked by transmitters, although an inhibition of the L-type channel has also occasionally been reported. The suppression of the N-type Ca current was commonly shown to be voltage-dependent, with a relief at large positive voltages. Strong evidence has been put forward showing that the transmitter action is mediated by a G-protein, with GDP-beta-S blocking transmitter action, and GTP-gamma-S directly inhibiting the Ca channel. Moreover, pertussis toxin blocked the transmitter action in most neurons, and following such block, injection of the G-protein Go restored transmitter action. A direct link between the G-protein and the Ca channel has been widely theorized to mediate the action of transmitters on certain neurons. There is also some evidence that certain transmitters in specific neurons mediate calcium channel inhibition through a 2nd messenger, perhaps protein kinase C. Transmitters have also been found, although uncommonly, to inhibit HVA L-type and LVA T-type channels. In addition, an enhancement of both HVA and LVA Ca currents by transmitters has been demonstrated, and substantial evidence exists for mediation of this action by cAMP.

Inhibition of calcium currents by noradrenaline, somatostatin and opioids in guinea-pig submucosal neurones

1. Whole-cell recordings were made from submucosal neurones acutely dissociated from guinea-pigs. The actions of noradrenaline, somatostatin and [Met5]enkephalin on currents carried by calcium ions were studied. 2. On depolarization from a holding potential of -70 mV, an inward current activated at -40 mV, reached its peak amplitude at 10 mV and reversed to outward at 72 mV (with external calcium of 5 mM and internal caesium of 160 mM). 3. Cadmium, nickel and cobalt reversibly blocked the calcium current; concentrations causing 50% block were 2.5, 500 and 2000 microM respectively. The calcium current (holding at -70 or -30 mV) was reversibly blocked by omega-conotoxin (100 nM), and unaffected by Bay K 8644 (0.1-10 microM) and nifedipine (1 microM). Cadmium caused an outward shift in holding current at -30 mV, implying that there was a persistent inward calcium current at this potential. 4. Noradrenaline, somatostatin and [Met5]enkephalin decreased the calcium current. The maximal inhibition observed with any one agonist, or with a combination of two agonists, did not exceed 50%; concentrations giving half-maximal inhibition were 5.5 microM for noradrenaline, 4 nM for somatostatin and 1 microM for [Met5]enkephalin. The inhibition was independent of membrane potential. All three agonists also reduced the persistent calcium current at -30 mV. 5. Inhibition of the calcium current by noradrenaline occurred with a latency of not less than 175 ms; cadmium applied by the same method depressed the current within 5-45 ms. 6. Experiments with selective agonists and antagonists indicated that the receptor types involved in calcium current inhibition were alpha 2-adrenoceptors and delta-opioid receptors. Somatostatin acted at a distinct receptor. 7. Calcium currents were also inhibited by intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S). Agonists were ineffective in cells pre-treated with pertussis toxin, but their action was restored when purified GTP-binding proteins (Go or Gi) were included in the intracellular recording solution. 8. It is concluded that noradrenaline, somatostatin and [Met5]enkephalin act at their respective receptors on guinea-pig submucosal neurones to inhibit a voltage-dependent calcium current. Activation of the same receptors also increases a potassium conductance in these cells: in both cases a pertussis-sensitive G protein is involved.

Norepinephrine blocks a calcium current of adult rat sympathetic neurons via an alpha 2-adrenoceptor

The effects of alpha-adrenoceptor agonist and antagonist drugs on the Ca2+ current of acutely isolated adult rat superior cervical ganglion (SCG) neurons were investigated to characterize the adrenoceptor which mediates a catecholamine-induced decrease of the Ca2+ current. Ca2+ currents were recorded using the whole-cell variant of the patch-clamp technique from neurons isolated enzymatically from adult rat SCG. Norepinephrine (1 microM) produced a rapid, reversible, and concentration-dependent decrease in Ca2+ current amplitude and slowed the rising phase of the Ca2+ current. These effects could be mimicked by clonidine (1 microM), an alpha 2-agonist but not by the alpha 1-agonist phenylephrine (1 microM). The norepinephrine-induced decrease in Ca2+ current amplitude was attenuated in the presence of idazoxan (1 microM), an alpha 2-antagonist, but was unaffected in the presence of the alpha 1-antagonist prazosin (1 microM). Neither antagonist displayed any Ca2+ current blocking activity. These results suggest that the alpha-receptor which mediates the norepinephrine-induced decrease of the Ca2+ current in adult rat SCG neurons is of the alpha 2-subtype.

Pulse duration and alpha 2-adrenoceptors modify noradrenaline release from field-stimulated atria

In guinea-pig atria preloaded with 10 muCi [3H]noradrenaline, field stimulation during the refractory period increased the release of radioactivity and the force of contraction. Both effects were dependent on the extracellular Ca2+ concentration and were abolished by 3 x 10(-8) mol/l tetrodotoxin. When applied during each refractory period, two short (0.05 ms) pulses released significantly more radioactivity than one pulse of 0.1 ms duration applied during each refractory period. Similarly, a train of four pulses (0.05 ms each) was more effective than one pulse lasting 0.2 ms. The radioactivity released by single, long-lasting pulses in each refractory period was increased by phentolamine, idazoxan and N-ethylmaleimide. A small increase was also obtained with prazosin. The effect of phentolamine was antagonized by clonidine but not by phenylephrine. Clonidine did not prevent the effect of N-ethylmaleimide (all drugs 3 x 10(-5) mol/l; atropine 10(-7) mol/l and cocaine 3 x 10(-6) mol/l present in all experiments). It is concluded that activation of prejunctional alpha 2-adrenoceptors reduces the amount of noradrenaline release by one long field pulse in each refractory period. In most experiments, the inotropic effect of the two stimulation protocols was not significantly different. However, when atria were stimulated with four pulses immediately before stimulation with one pulse, the autoinhibition thus caused was strong enough to also reduce the inotropic effect of this stimulation protocol.

Alpha-adrenergic inhibition of sympathetic neurotransmitter release mediated by modulation of N-type calcium-channel gating

In sympathetic neurons, catecholamines interact with prejunctional alpha-adrenergic receptors to reduce delivery of transmitter to postjunctional target organs. This autoinhibitory feedback is a general phenomenon seen in diverse neurons containing a variety of transmitters. The underlying mechanisms of alpha-adrenergic inhibition are not clear, although decreases in cyclic AMP and cAMP-mediated phosphorylation have been implicated. We have studied depolarization-induced catecholamine release and calcium-channel currents in frog sympathetic neurons. Here we show that alpha-adrenergic inhibition of transmitter release can be explained by inhibition of Ca2+-channel currents and not by modulation of intracellular proteins. Noradrenaline strongly reduces the activity of N-type Ca2+ channels, the dominant calcium entry pathway triggering sympathetic transmitter release, whereas L-type Ca2+ channels are not significantly inhibited. The down-modulation of N-type channels involves changes in rapid gating kinetics but not in unitary flux. This is the first detailed description of inhibition of a high-voltage activated neuronal Ca2+ channel at the single-channel level. The coupling between alpha-adrenergic receptors and N-type channels involves a G protein, but not a readily diffusible cytoplasmic messenger or protein kinase C, and may be well suited for rapid and spatially localized feedback-control of transmitter release.

G proteins couple alpha-adrenergic and GABAb receptors to inhibition of peptide secretion from peripheral sensory neurons

Regulation of neuronal calcium channels by GTP-binding proteins (G proteins) is likely to be an important mechanism by which inhibitory transmitters influence excitation-secretion coupling in presynaptic nerve endings. Here, we report that in peripheral sensory neurons from embryonic chick dorsal root ganglia (DRG), the G protein-mediated inhibition of voltage-dependent calcium channels may best explain how norepinephrine (NE) and GABA inhibit the electrically evoked, calcium-dependent release of substance P (SP). As is the case for the previously reported inhibitory actions of these transmitters on DRG cell calcium channels, we demonstrate that NE and GABA inhibit peptide secretion through activation of alpha-adrenergic and GABAb receptors that are functionally coupled to pertussis toxin (PTX)-sensitive G proteins. Pretreatment of DRG cell cultures with PTX blocked the ability of NE and GABA to inhibit the release of SP, an action correlated with PTX-catalyzed ADP-ribosylation of membrane proteins with apparent molecular weight (Mr) of 40-41 kDa. Western immunoblot analysis of chick DRG cell membrane proteins using antisera directed against synthetic peptides corresponding to amino acid sequences predicted from cDNAs for PTX-sensitive G protein alpha subunits revealed a minimum of 2 Gi-like proteins (Mr 40 and 41 kDa) and a third Go-like protein (Mr 40 kD). Significantly, these findings implicate Gi- and/or Go-like GTP-binding proteins as mediators of presynaptic inhibition in peripheral sensory neurons.

Noradrenaline-Induced Inhibition of Voltage-Sensitive Calcium Currents in NG108-15 Hybrid Cells

The effect of noradrenaline (NA) on voltage-sensitive calcium currents in neuroblastoma x glioma hybrid (NG108-15) cells has been studied using a whole-cell clamp technique. NA inhibited calcium current. The EC50 for NA induced inhibition was 177 nM. The NA receptor involved had a similar pharmacological profile to alpha2 adrenoreceptors but did not respond to clonidine. NA inhibited calcium current by an amount which dependend on both membrane potential and current amplitude. When current-dependent inactivation of calcium current was maximal, NA was without effect. The data are consistent with a mechanism where NA enhances current-dependent inactivation.

G proteins couple alpha-adrenergic and GABAb receptors to inhibition of peptide secretion from peripheral sensory neurons

Regulation of neuronal calcium channels by GTP-binding proteins (G proteins) is likely to be an important mechanism by which inhibitory transmitters influence excitation-secretion coupling in presynaptic nerve endings. Here, we report that in peripheral sensory neurons from embryonic chick dorsal root ganglia (DRG), the G protein-mediated inhibition of voltage-dependent calcium channels may best explain how norepinephrine (NE) and GABA inhibit the electrically evoked, calcium-dependent release of substance P (SP). As is the case for the previously reported inhibitory actions of these transmitters on DRG cell calcium channels, we demonstrate that NE and GABA inhibit peptide secretion through activation of alpha-adrenergic and GABAb receptors that are functionally coupled to pertussis toxin (PTX)-sensitive G proteins. Pretreatment of DRG cell cultures with PTX blocked the ability of NE and GABA to inhibit the release of SP, an action correlated with PTX-catalyzed ADP-ribosylation of membrane proteins with apparent molecular weight (Mr) of 40-41 kDa. Western immunoblot analysis of chick DRG cell membrane proteins using antisera directed against synthetic peptides corresponding to amino acid sequences predicted from cDNAs for PTX-sensitive G protein alpha subunits revealed a minimum of 2 Gi-like proteins (Mr 40 and 41 kDa) and a third Go-like protein (Mr 40 kD). Significantly, these findings implicate Gi- and/or Go-like GTP-binding proteins as mediators of presynaptic inhibition in peripheral sensory neurons.

Inhibition of Ca-spikes in rat preganglionic cervical sympathetic nerves by sympathomimetic amines

1. Propagated Ca-spikes were recorded from isolated cervical sympathetic nerve trunks of the rat when bathed in a solution containing 5 mM Ca2+, 0.5 or 1 microM tetrodotoxin (to block Na currents) and 1 mM 4-aminopyridine (to reduce K currents). 2. Spikes persisted when external Ca2+ was replaced with Sr2+ or Ba2+, but were blocked by the addition of the following inorganic Ca-channel blockers (in descending order of potency): Cd2+ greater than La3+ greater than Ni2+ greater than Co2+ greater than Mn2+ greater than Mg2+. 3. Ca-spike amplitude was reduced by up to 90% by (-)-noradrenaline (IC50 1.5 microM). The following sympathomimetic amines imitated this effect (in descending order of potency): clonidine greater than or equal to (-)-adrenaline greater than or equal to [(-)-noradrenaline] greater than or equal to dopamine greater than (-)-phenylephrine greater than or equal to (+/-)-amidephrine. 4. Ca-spike inhibition by (-)-noradrenaline was antagonized by phentolamine (pA2 6.5). Yohimbine was about 10 times weaker than phentolamine; (+/-)-propranolol (1 microM) and prazosin (10 microM) had no clear effect. 5. (-)-Noradrenaline reduced the amplitude of the compound action potential recorded from the superior cervical sympathetic ganglion following supramaximal preganglionic trunk stimulation when recorded in normal Krebs solution and hyperpolarized the ganglion with respect to the post-ganglionic trunk. Depression of the transmitted ganglionic action potential was antagonized by phentolamine (5 microM) but not by yohimbine (1 microM); in contrast 1 microM yohimbine completely prevented the ganglionic hyperpolarization. (-)-Noradrenaline did not hyperpolarize the preganglionic cervical sympathetic nerve trunk under these recording conditions. 6. It is suggested that inhibition of transmitter release from sympathetic preganglionic fibres produced by noradrenaline results from a depression of the voltage-gated Ca current in the fibres and/or their terminals, and that this action is mediated by an alpha-adrenoceptor which does not fully conform to either alpha 1 or alpha 2 subtypes.

Mechanism of synaptic inhibition by noradrenaline acting at alpha 2-adrenoceptors

The actions of agonists at alpha 2-adrenoceptors were investigated on single cells of the submucous plexus of the guinea pig small intestine. Intracellular recordings were made from neurons in vitro, and noradrenaline and other agonists were applied by adding them to the superfusion solution. The actions of noradrenaline released from terminals of sympathetic nerves was also studied by stimulating the nerves and recording the inhibitory postsynaptic current; this current can be mimicked by brief applications of noradrenaline from a pipette tip positioned within 50 micron of the neuron. The alpha 2-adrenoceptor-bound noradrenaline with an apparent dissociation constant of 15 microM, determined by the method of partial irreversible receptor inactivation: clonidine and 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304) had dissociation constants of 36 nM and 2.5 microM respectively. Noradrenaline and UK 14304 caused maximal hyperpolarizations, or outward currents; clonidine was a full agonist in only 4 of 35 cells, a partial agonist in 25 cells, and without effect in 4 cells. Clonidine acted as a competitive antagonist of noradrenaline in those cells in which it lacked agonist action; its dissociation equilibrium constant determined by Schild analysis was about 20 nM. The potassium conductance increased by the alpha 2-adrenoceptor agonists, whether they were applied exogenously or released by stimulation of presynaptic nerves, showed marked inward rectification. The neurons showed inward rectification also in the absence of agonist; both types of rectification were eliminated by rubidium (2 mM), barium (3-30 microM) and caesium (2 mM). When the recording electrodes contained the nonhydrolysable derivative of guanosine 5'-triphosphate (GTP), guanosine 5'-O-(3-thiotriphosphate, GTP-gamma-S), the effects of applied alpha 2-adrenoceptor agonists did not reverse when they were washed from the tissue, implying that GTP hydrolysis is necessary for the termination of agonist action. Pretreatment with pertussis toxin abolished the inhibitory synaptic potential (IPSP) and agonist-induced hyperpolarizations. Phorbol 12,13-dibutyrate, forskolin, cholera toxin and sodium fluoride did not affect the responses to alpha 2-adrenoceptor agonists. The synaptic hyperpolarization resulting from sympathetic nerve stimulation, or the hyperpolarization evoked by a brief (3-5 ms) application of noradrenaline, began after a latency of about 30 and 60 ms respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Alpha-2 adrenergic modulation of norepinephrine secretion in the perfused rabbit iris-ciliary body

Clonidine and other selective alpha-2 adrenergic agonists have been found to lower intraocular pressure in the eyes of rabbits and primates, including humans. It has been suggested that the ocular hypotensive response to alpha-2 agonists may be mediated, in part, by prejunctional inhibition of norepinephrine secretion at intraocular synapses. In this study, we have investigated the effects of adrenergic agonists and antagonists on field-stimulated, Ca++-dependent release of 3H-norepinephrine (3H-NE) from isolated, perfused rabbit iris-ciliary bodies and have utilized radioligand binding methods to identify prejunctional adrenoceptors in this tissue. Clonidine (10(-9)-10(-5) M) produced a dosage-dependent inhibition of stimulation-evoked 3H-NE secretion (EC50 approximately equal to 3 X 10(-8) M), but did not alter basal secretion. Other adrenergic agonists capable of activating alpha-2 adrenoceptors (e.g., epinephrine, norepinephrine and xylazine) also significantly depressed 3H-NE secretion, whereas selective alpha-1 adrenergic or beta adrenergic agonists were without effect. Clonidine-mediated inhibition of 3H-NE release was reversed by the selective alpha-2 antagonist yohimbine (10(-7) M), but was unaffected by prazosin or timolol. Yohimbine alone markedly enhanced 3H-NE secretion, indicating tonic activation of prejunctional alpha-2 adrenoceptors by endogenous released norepinephrine. Forskolin or 8-bromo-cAMP, which alone enhanced norepinephrine secretion, failed to attenuate the inhibitory responses to alpha-2 agonists. 3H-rauwolscine binding measurements showed a small decrease in alpha-2 receptor sites in iris-ciliary body membranes following surgical sympathetic denervation. It is concluded that the rabbit iris-ciliary body contains functional, prejunctional alpha-2 adrenoceptors which may play an autoregulatory role in vivo and contribute to the ocular effects of adrenergic drugs.

The effect of unilateral dorsal root ganglionectomies or ventral rhizotomies on alpha 2-adrenoceptor binding to, and the substance P, enkephalin, and neurotensin content of, the cat lumbar spinal cord

The density of alpha 2-adrenoceptor binding sites and the content of substance P, enkephalins, and neurotensin were determined in quadrants of the lumbar spinal enlargement of control cats and of cats upon which either unilateral dorsal root ganglionectomies or unilateral ventral rhizotomies had been performed. The performance of unilateral dorsal root ganglionectomies resulted in a significant decrease (45-55%) of substance P content in the ipsilateral dorsal horn 7 and 21 days postoperatively. The concentration of alpha 2-adrenoceptor binding sites ([3H]rauwolscine Bmax) in the ipsilateral dorsal horn was consistently and significantly decreased at these same postganglionectomy times (20% reduced relative to the contralateral dorsal horn). Enkephalin content 7 and 21 days after ganglionectomies was not significantly different from control, whereas the neurotensin content of the ipsilateral dorsal horn was significantly increased in the 21-day survival cats. The performance of unilateral ventral rhizotomies did not produce any statistically significant changes in the density of alpha 2-adrenoceptor binding sites or in the substance P or enkephalin content of any spinal quadrant. The neurotensin content of both the ipsilateral dorsal and ipsilateral ventral quadrants of the ventral rhizotomized cats was significantly increased. The significant decrease of alpha 2-adrenoceptor binding site concentration in the ipsilateral dorsal horn after unilateral dorsal root ganglionectomies suggests that approximately 20% of the alpha 2-adrenoceptors present within the cat lumbar spinal dorsal gray are located on the axons or terminals of primary sensory afferents. Consistent with this interpretation of the ganglionectomy results, we found significant levels of saturable [3H]rauwolscine binding to homogenates of the cat L4-Sl spinal dorsal root ganglia. Because alpha 2-adrenoceptor binding sites in the ipsilateral ventral lumbar spinal gray were not significantly reduced after unilateral ventral rhizotomies, our results provide no evidence for the location of alpha 2-adrenoceptor on lumbar spinal motoneurons.

Pertussis toxin blocks depressant effects of opioid, monoaminergic and muscarinic agonists on dorsal-horn network responses in spinal cord-ganglion cultures

After chronic exposure of mouse spinal cord-ganglion explants to morphine, the acute depressant effects of opioids on sensory-evoked dorsal-horn network responses are markedly attenuated, and characteristic cord discharges can then occur even in the presence of greater than 100-fold higher opioid concentrations. The present study demonstrates that a remarkably similar degree of tolerance to opioids develops in these cord-ganglion explants after exposure to pertussis toxin (PTX). The usual acute depressant effects of serotonin, norepinephrine and oxotremorine on dorsal-horn discharges are also similarly attenuated in PTX-treated cultures. PTX is known to interfere with the guanine nucleotide protein Gi that is required for opioid, alpha 2-adrenergic and muscarinic receptor-mediated inhibition of adenylate cyclase in various cells. We have previously found that in cord-dorsal root ganglion explants agents which elevate intracellular cAMP also attenuate opioid depressant effects. Furthermore, these explants contain an opioid-inhibited adenylate cyclase system, and chronic exposure to morphine as well as PTX increases adenylate cyclase activity. These findings together with the present results suggest that the neuromodulatory effects of opioid, monoaminergic and muscarinic agonists on primary afferent networks in the spinal cord may be mediated by binding to neuronal receptor subtypes that are negatively coupled via Gi to a common pool of adenylate cyclase.

Noradrenaline modulates calcium channels in avian dorsal root ganglion cells through tight receptor-channel coupling

Averaged ensemble Ba currents were recorded from tissue cultured embryonic chick dorsal root ganglion (d.r.g.) cells using the cell-attached patch-clamp technique. Noradrenaline (NA) applied to extrapatch membrane had no clear consistent effect on drug-free patch currents. This finding supports a previous suggestion that second messengers may not be involved in NA-mediated decreases in Ca currents in sensory neurones (Forscher & Oxford, 1985). Cell-attached patch currents sometimes increased slowly after extrapatch application of NA, but were not reversibly decreased by drug treatment. Large patch currents were used to trigger cellular action potentials. NA reversibly decreased action potential duration as reflected in extracellularly recorded patch action currents. Simultaneously recorded inward patch currents were not affected. D.r.g. cell adenylate cyclase activity was assayed. NA did not affect intracellular cyclic AMP levels at concentrations which cause 30-70% decreases in gCa in dialysed cells (Forscher & Oxford, 1985). Treatment with forskolin (50 microM) or isoprenaline (10 microM) resulted in 60- and 2-fold increases respectively in adenylate cyclase activity over basal levels. These results suggest that NA decreases Ca currents by direct NA interactions with the Ca channel or a molecule tightly coupled to channel function in d.r.g. cells.

Morphine and norepinephrine but not 5-hydroxytryptamine and gamma-aminobutyric acid inhibit the potassium-stimulated release of substance P from rat spinal cord slices

We studied whether morphine, norepinephrine (NE), 5-hydroxytryptamine (5-HT) and gamma-aminobutyric acid (GABA) inhibit the potassium-stimulated release of substance P (SP) from rat spinal cord slices. Male Sprague-Dawley rats were decapitated and a 2-cm segment of lumbosacral spinal cord was removed, chopped into 0.5 X 0.5 mm pieces, weighed, placed in a perfusion chamber and perfused at 37 degrees C with a modified Krebs bicarbonate buffer. Perfusate was collected, lyophilized, then assayed for SP using radioimmunoassay. Exposure of spinal cord tissue to 50 mM KCl for 8 min produced a calcium-dependent increase in the release of SP from a basal level of approximately 0.1 pg/mg tissue/min to 0.3 pg/mg tissue/min. Morphine and NE at concentrations of 10(-4) and 10(-5) M did not alter basal release but caused a significant reduction in the potassium-stimulated release of SP. Naloxone (10(-5) M) and phentolamine (10(-5) M) did not affect SP release but attenuated the effects of morphine and NE, respectively. Naloxone did not antagonize the inhibition of release produced by NE nor did phentolamine block the effect of morphine, suggesting that the actions of the agonists are independent. In contrast, 5-HT and GABA at concentrations of 10(-4) M and 10(-5) M did not significantly alter the basal or potassium-stimulated release of SP. These results demonstrate a differential regulation of SP release in the spinal cord and support the hypothesis that morphine and NE may modify nociception, in part, by inhibiting the release of SP in the spinal cord.

Serotonin decreases the duration of action potentials recorded from tetraethylammonium-treated bullfrog dorsal root ganglion cells

Neurotransmitter effects on calcium currents activated by sensory neuron action potentials have been previously studied in embryonic or neonatal dorsal root ganglion (DRG) cells in culture. In the present study we examined the effects of serotonin (5-HT) on the shape of action potentials recorded from fully differentiated primary afferent neurons in isolated DRG of adult bullfrogs. Intracellular recordings were obtained from cell bodies of type A and C neurons. Concentrations of 5-HT that had no effect on membrane potential or input resistance had little or no effect on action potential shape. Treatment with 5-20 mM tetraethylammonium ion (TEA) led to the appearance of a plateau phase on the falling limb of the spike. This plateau phase appears to result from calcium influx, as it was dramatically reduced in amplitude and duration by solutions containing low concentrations of calcium or the calcium channel blocker, manganese. In preparations treated with 7.5 mM TEA, low concentrations of 5-HT (10 nM-1 microM) produced a dose-dependent narrowing of the calcium-dependent plateau phase of the mixed sodium/calcium spike. A decrease in spike afterhyperpolarization was also noted. The decrease in spike duration was recorded from 74% of type A neurons and 57% of type C neurons, and was not secondary to a change in resting potential or input resistance. The 5-HT receptor antagonists methysergide and metergoline did not block the response to 5-HT. Instead, they exhibited weak agonist-like actions. Serotonin also reduced the rate of rise and peak amplitude of calcium spikes recorded in the presence of tetrodotoxin and TEA.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin decreases the duration of action potentials recorded from tetraethylammonium-treated bullfrog dorsal root ganglion cells

Neurotransmitter effects on calcium currents activated by sensory neuron action potentials have been previously studied in embryonic or neonatal dorsal root ganglion (DRG) cells in culture. In the present study we examined the effects of serotonin (5-HT) on the shape of action potentials recorded from fully differentiated primary afferent neurons in isolated DRG of adult bullfrogs. Intracellular recordings were obtained from cell bodies of type A and C neurons. Concentrations of 5-HT that had no effect on membrane potential or input resistance had little or no effect on action potential shape. Treatment with 5-20 mM tetraethylammonium ion (TEA) led to the appearance of a plateau phase on the falling limb of the spike. This plateau phase appears to result from calcium influx, as it was dramatically reduced in amplitude and duration by solutions containing low concentrations of calcium or the calcium channel blocker, manganese. In preparations treated with 7.5 mM TEA, low concentrations of 5-HT (10 nM-1 microM) produced a dose-dependent narrowing of the calcium-dependent plateau phase of the mixed sodium/calcium spike. A decrease in spike afterhyperpolarization was also noted. The decrease in spike duration was recorded from 74% of type A neurons and 57% of type C neurons, and was not secondary to a change in resting potential or input resistance. The 5-HT receptor antagonists methysergide and metergoline did not block the response to 5-HT. Instead, they exhibited weak agonist-like actions. Serotonin also reduced the rate of rise and peak amplitude of calcium spikes recorded in the presence of tetrodotoxin and TEA.(ABSTRACT TRUNCATED AT 250 WORDS)

GTP-binding proteins mediate transmitter inhibition of voltage-dependent calcium channels

The modulation of voltage-dependent calcium channels by hormones and neurotransmitters has important implications for the control of many Ca2+-dependent cellular functions including exocytosis and contractility. We made use of electrophysiological techniques, including whole-cell patch-clamp recordings from dorsal root ganglion (DRG) neurones, to demonstrate a role for GTP-binding proteins (G-proteins) as signal transducers in the noradrenaline- and gamma-aminobutyric acid (GABA)-induced inhibition of voltage-dependent calcium channels. This action of the transmitters was blocked by: (1) preincubation of the cells with pertussis toxin (a bacterial exotoxin catalysing ADP-ribosylation of G-proteins); or (2) intracellular administration of guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S), a non-hydrolysable analogue of GDP that competitively inhibits the binding of GTP to G-proteins. Our findings provide the first direct demonstration of the G-protein-mediated inhibition of voltage-dependent calcium channels by neurotransmitters. This mode of transmitter action may explain the ability of noradrenaline and GABA to presynaptically inhibit Ca2+-dependent neurosecretion from DRG sensory neurones.

Modulation of calcium channels by norepinephrine in internally dialyzed avian sensory neurons

Modulation of voltage-dependent Ca channels by norepinephrine (NE) was studied in chick dorsal root ganglion cells using the whole-cell configuration of the patch-clamp technique. Cells dialyzed with K+ and 2-10 mM EGTA exhibited Ca action potentials that were reversibly decreased in duration and amplitude by NE. Ca channel currents were isolated from other channel contributions by using: (a) tetrodotoxin (TTX) to block gNa, (b) internal K channel impermeant ions (Cs or Na/N-methylglucamine mixtures) as K substitutes, (c) external tetraethylammonium (TEA) to block K channels, (d) internal EGTA to reduce possible current contribution from Ca-activated channels. A marked decline (rundown) of Ca conductance was observed during continual dialysis, which obscured reversible NE effects. The addition of 2-5 mM MgATP to the intracellular solutions greatly retarded Ca channel rundown and permitted a clear assessment of modulatory drug effects. The inclusion of an intracellular creatine phosphate/creatine phosphokinase nucleotide regeneration system further stabilized Ca channels, which permitted recording of Ca currents for up to 3 h. NE reversibly decreased both steady state Ca currents and Ca tail currents in Cs/EGTA/MgATP-dialyzed cells. A possible role of several putative intracellular second messengers in NE receptor-Ca channel coupling was investigated. Cyclic AMP or cyclic GMP added to the intracellular solutions at concentrations several orders of magnitude higher than the Kd for activation of cyclic nucleotide-dependent protein kinases did not block or mask the expression of the NE-mediated decrease in gCa. Addition of internal EGTA to a final concentration of 10 mM also did not affect the expression of the NE response. These results suggest that neither cyclic AMP nor cyclic GMP nor Ca is acting as a second messenger coupling the NE receptor to the down-modulated Ca channel population.

Forskolin prolongs action potential duration and blocks potassium current in embryonic chick sensory neurons

To determine if alterations in internal cyclic adenosine monophosphate (cAMP) play a role in modulation of voltage-dependent channels in embryonic chick sensory neurons in vitro, forskolin (a direct activator of adenylate cyclase) was tested on the cells. Forskolin, in concentrations between 1 and 100 microM, produced dose-dependent, reversible increases in action potential duration. This effect of forskolin was blocked by incubation of the neurons in 1 mM 2',5'-dideoxyadenosine, an inhibitor of forskolin-induced activation of cyclase in other cells. This suggests that the increase in action potential duration is likely to be mediated by activation of adenylate cyclase. Cholera toxin, another cyclase activator, also increased action potential duration when applied to the sensory neurons in a concentration of 10 micrograms/ml. Forskolin applied to voltage-clamped neurons decreased a voltage-dependent outward current, a result consistent with its effect on the action potential. These effects of forskolin are mimicked by capsaicin, but are in marked contrast to those previously reported for norepinephrine on the action potential and membrane currents (Dunlap and Fischbach 1981). Furthermore, forskolin does not block (or attenuate) the effects of norepinephrine, suggesting that increases in adenylate cyclase activity are most likely not involved in norepinephrine's action on the calcium channel.