Pharmacological discrimination between muscarinic receptor signal transduction cascades with bethanechol chloride

1. Muscarinic agonist specificity is limited, making it difficult to match receptor subtypes with signal transduction cascades that mediate ion channel modulation. We have characterized the inhibitory effects of two muscarinic agonists, oxotremorine-M (Oxo-M) and bethanechol chloride (BeCh), on Ca(2+) currents in neonatal rat superior cervical ganglion neurons. 2. Oxo-M-mediated (10 micro M) inhibition occurred via two signaling pathways. The first pathway inhibited whole cell peak currents, consisting primarily of N-type current, but not FPL 64176-induced, long-lasting tail currents, comprised entirely of L-type current. Inhibited currents displayed slowed activation kinetics and voltage dependence, characteristics of membrane-delimited inhibition. Current inhibition was blocked by the selective M(2) receptor antagonist, methoctramine (METH; 100 nM), or following pertussis toxin (PTX) pretreatment. 3. Activation of the second pathway inhibited both peak and long-lasting tail currents. This pathway was voltage-independent, PTX-insensitive, but sensitive to internal Ca(2+) chelator concentration. Muscarinic toxin 7 (MT-7, 100 nM), an irreversible M(1) receptor antagonist, eliminated this inhibition. Oxo-M (100 micro M) decreased L- and N-type channel activities in cell-attached patches, indicating that a diffusible second messenger is involved. 4. BeCh (100 micro M) also inhibited whole cell currents via the membrane-delimited pathway. Blocking M(4) receptors with 100 nM pirenzepine (in the presence of MT-7) had no effect, while antagonizing M(2) receptors with METH abolished inhibition. Concentrations of BeCh as high as 3 mM failed to inhibit either peak or long-lasting tail currents following PTX pretreatment. 5. These results indicate that BeCh may be an effective tool for selectively activating M(2) receptor stimulation of the membrane-delimited pathway.

cGMP/protein kinase G-dependent inhibition of N-type Ca2+ channels induced by nitric oxide in human neuroblastoma IMR32 cells

Although data from our laboratory and others suggest that nitric oxide (NO) exerts an overall inhibitory action on high-voltage-activated Ca2+ channels, conflicting observations have been reported regarding its effects on N-type channels. We performed whole-cell and cell-attached patch-clamp recordings in IMR32 cells to clarify the functional role of NO in the modulation of N channels of human neuronal cells. During depolarizing steps to +10 mV from V(h) = -90 mV, the NO donor, sodium nitroprusside (SNP; 200 microm), reduced macroscopic N currents by 34% (p < 0.01). The magnitude of inhibition was similar at all voltages tested (range, -40 to +50 mV). No significant inhibition was observed when SNP was applied together with the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium salt (300 microm), or after cell treatment with the guanylate cyclase inhibitor, 1H-[1,2,4] oxadiazole [4,3-a] quinoxalin-1-one (10 microm). 8-bromoguanosine-cGMP (8-Br-cGMP) (400 microm) mimicked the effects of SNP, reducing Ba2+ currents by 37% (p < 0.001). Cell treatment with the protein kinase G (PKG) inhibitor KT5823 (1 microm) or guanosine 3',5'-cyclic monophosphorothioate, 8-(4-chloro-phenylthio)-Rp-isomer, triethylammonium salt (20 microm) virtually abolished the effects of 8-Br-cGMP. At the single-channel level, 8-Br-cGMP reduced the channel open probability by 59% and increased both the mean shut time and the null sweep probability, but it had no significant effects on channel conductance, mean open time, or latency of first openings. These data suggest that NO inhibits N-channel gating through cGMP and PKG. The consequent decrease in Ca2+ influx through these channels may affect different neuronal functions, including neurotransmitter release.

Cardiovascular and autonomic effects of omega-conotoxins MVIIA and CVID in conscious rabbits and isolated tissue assays

1. The effects of a novel N-type voltage-operated calcium channel antagonist, omega-conotoxin CVID, were compared with omega-conotoxin MVIIA on sympathetic-evoked activation of right atria (RA), small mesenteric arteries (MA) and vasa deferentia (VD) isolated from the rat. Their effects were also compared on blood pressure and cardiovascular reflexes in conscious rabbits. 2. The pIC(50) values for MVIIA and CVID, respectively, for inhibiting sympathetic-evoked responses were equivalent in RA (8.7 and 8.7) and VD (9.0 and 8.7); however, in MA the values were 8.4 and 7.7. The cardiac to vascular (RA/MA) potency ratios, antilog (plog RA - plog MA), for MVIIA and CVID were 2 and 10. The offset rates for CVID and MVIIA were rapid, and peptide reapplication caused rapid onset of blockade, suggesting limited desensitization. 3. In the conscious rabbit, CVID and MVIIA (100 microg kg(-1) i.v.) caused a similar fall in blood pressure and a tachycardia that rapidly reached maximum. Both peptides decreased the vagal- and sympathetic-mediated components of the baroreflex, but had no effect on the vagal nasopharyngeal reflex. The orthostatic reflex to 90 degrees tilt was blocked by MVIIA with sustained postural hypotension for > or = 90 min after administration. In contrast, CVID caused postural hypotension at 30 min which recovered rapidly. 4. Neither CVID nor MVIIA (3 microg kg(-1) i.t.) significantly altered cardiovascular variables or autonomic reflexes. 5. In conclusion, CVID appears to be relatively weak at inhibiting the reflex response to tilt consistent with its weaker inhibition of rat mesenteric artery constriction to perivascular nerve stimulation. This may point to subtype N-type calcium channel selectivity.

Syntaxin modulation of slow inactivation of N-type calcium channels

Syntaxin, a membrane protein vital in triggering vesicle fusion, interacts with voltage-gated N- and P/Q-type Ca(2+) channels. This biochemical association is proposed to colocalize Ca(2+) channels and presynaptic release sites, thus supporting rapid and efficient initiation of neurotransmitter release. The syntaxin channel interaction may also support a novel signaling function, to modulate Ca(2+) channels according to the state of the associated release machinery (Bezprozvanny et al., 1995; Wiser et al., 1996; see also Mastrogiacomo et al., 1994). Here we report that syntaxin 1A (syn1A) coexpressed with N-type channels in Xenopus oocytes greatly promoted slow inactivation gating, but had little or no effect on the onset of and recovery from fast inactivation. Accordingly, the effectiveness of syntaxin depended strongly on voltage protocol. Slow inactivation was found for N-type channels even in the absence of syntaxin and could be distinguished from fast inactivation on the basis of its slow kinetics, distinct voltage dependence (voltage-independent at potentials higher than the level of half-inactivation), and temperature independence (Q(10), approximately 0.8). Trains of action potential-like stimuli were more effective than steady depolarizations in stabilizing the slowly inactivated condition. Agents that stimulate protein kinase C decreased the inhibitory effect of syntaxin on N-type channels. Application of BoNtC1 to cleave syntaxin sharply attenuated the modulatory effects on Ca(2+) channel gating, consistent with structural analysis of syntaxin modulation, supporting use of this toxin to test for the impact of syntaxin on Ca(2+) influx in nerve terminals.

The phosphatidylinositol 4-kinase inhibitor phenylarsine oxide blocks evoked neurotransmitter release by reducing calcium entry through N-type calcium channels

The effects of the phosphatidylinositol 4-kinase inhibitor, phenylarsine oxide (PAO), on acetylcholine (ACh) release and on prejunctional Ca(2+) currents were studied at the frog neuromuscular junction using electrophysiological recording techniques. Application of PAO (30 microM) increased both spontaneous ACh release reflected as miniature end-plate potential (mepp) frequencies and evoked ACh release reflected as end-plate potential (epp) amplitudes with a similar time course. Following the initial increase in epp amplitudes produced by PAO, epps slowly declined and were eventually abolished after approximately 20 min. However, mepp frequencies remained elevated over this time period. PAO (30 microM) also inhibited the perineural voltage change associated with Ca(2+) currents through N-type Ca(2+) channels (prejunctional Ca(2+) currents) at motor nerve endings. Addition of British anti-lewisite (BAL, 1 mM), an inactivator of PAO, partially reversed both the inhibition of epps and the inhibition of the prejunctional Ca(2+) current. The effects of PAO on N-type Ca(2+) channels were investigated more directly using the whole cell patch clamp technique on acutely dissociated sympathetic neurons. Application of PAO (30 - 40 microM) to these neurons decreased the voltage-activated calcium currents through N-type Ca(2+) channels, an effect that was partially reversible by BAL. In combination, these results suggest that inhibition of neurotransmitter release by PAO occurs as a consequence of the inhibition of Ca(2+) entry via N-type calcium channels. The relationship between the effects of PAO on N-type Ca(2+) channels in motor nerve endings and in neuronal soma is discussed.

Developmental changes in calcium channel types mediating central synaptic transmission

Multiple types of high-voltage-activated Ca(2+) channels trigger neurotransmitter release at the mammalian central synapse. Among them, the omega-conotoxin GVIA-sensitive N-type channels and the omega-Aga-IVA-sensitive P/Q-type channels mediate fast synaptic transmission. However, at most central synapses, it is not known whether the contributions of different Ca(2+) channel types to synaptic transmission remain stable throughout postnatal development. We have addressed this question by testing type-specific Ca(2+) channel blockers at developing central synapses. Our results indicate that N-type channels contribute to thalamic and cerebellar IPSCs only transiently during early postnatal period and P/Q-type channels predominantly mediate mature synaptic transmission, as we reported previously at the brainstem auditory synapse formed by the calyx of Held. In fact, Ca(2+) currents directly recorded from the auditory calyceal presynaptic terminal were identified as N-, P/Q-, and R-types at postnatal day 7 (P7) to P10 but became predominantly P/Q-type at P13. In contrast to thalamic and cerebellar IPSCs and brainstem auditory EPSCs, N-type Ca(2+) channels persistently contribute to cerebral cortical EPSCs and spinal IPSCs throughout postnatal months. Thus, in adult animals, synaptic transmission is predominantly mediated by P/Q-type channels at a subset of synapses and mediated synergistically by multiple types of Ca(2+) channels at other synapses.

Omega-conotoxin GVIA-resistant neurotransmitter release from postganglionic sympathetic nerves in the guinea-pig vas deferens and its modulation by presynaptic receptors

1 Intracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals in the guinea-pig isolated vas deferens. 2 Recently, a component of action potential-evoked release which is insensitive to high concentrations of the selective N-type calcium channel blocker omega-conotoxin GVIA termed 'residual release' has been described. Under these conditions, release of the neurotransmitter ATP evoked by trains of low frequency stimuli is abolished, but at higher frequencies a substantial component of release is revealed. 3 'Residual release' was studied with trains of 5 or 10 stimuli at stimulation frequencies of 10, 20 and 50 Hz. The alpha2-adrenoceptor agonist clonidine (30-100 nM) inhibited 'residual release', the degree of inhibition being most marked at the beginning of a train. 4 The alpha2-adrenoceptor antagonist yohimbine (1 microM) induced a marked increase in 'residual release' which was dependent on both the frequency of stimulation and the number of stimuli in a train. 5 Prostaglandin E2 (30 nM) and neuropeptide Y (100 nM) caused a rapid inhibition of 'residual release' at all stimulation frequencies examined. 6 4-Aminopyridine (100 microM) induced a powerful potential of 'residual release' and could reverse the inhibition of omega-conotoxin GVIA. 7 'Residual release' was modulated through presynaptic alpha2-adrenoceptors suggesting that (i) residual release of ATP is subject to alpha-autoinhibition through the co-release of noradrenaline, (ii) noradrenaline release can be triggered by calcium channels other than the N-type and (iii) when presynaptic receptors are activated, inhibition of transmitter release can occur by mechanisms other than modulation of calcium-entry through N-type calcium channels in postganglionic sympathetic nerves. Prostaglandin E2 and neuropeptide Y also modulated neurotransmitter release.

Identification of the single channels that underlie the N-type and L-type calcium currents in bullfrog sympathetic neurons

Most of the whole-cell calcium current of frog sympathetic neurons is an N-type current, blocked by omega-conotoxin GVIA (omegaCGVIA). Thus, these cells should be an excellent system to study the properties of single N-type channels. However, a channel that is active near -10 mV in isotonic Ba2+, originally identified as "N-type," corresponds more closely to a omegaCGVIA-resistant component of the whole-cell current observed in 100 mM Ba2+. That conclusion would imply that the true single-channel correlate of the macroscopic N-current remains to be identified in frog sympathetic neurons. I report here recordings from cell-attached patches of a calcium channel that activates in the appropriate voltage range (>0 mV, in isotonic Ba2+) and is blocked by omegaCGVIA. This channel has a slope conductance of 20 pS (range, 17-25 pS) and a single-channel current of -1.3 pA at 0 mV. Other channels active in the same voltage range (24 pS, -1.3 pA at 0 mV) were identified as L-type channels because they exhibited long openings after repolarization in the presence of 1 microM Bay K 8644 and were resistant to omegaCGVIA. A third channel type (13-19 pS) was distinguished by current amplitude (-0.6 pA at 0 mV) and strong inactivation at -40 mV. The similarity in slope conductance among these channels demonstrates that distinguishing them requires the consideration of additional properties. The omegaCGVIA-sensitive channel can be identified as an N-type calcium channel.