Forskolin reduces the activity of the rat muscle embryonic type acetylcholine receptor channel

Modulation by 5-hydroxytryptamine of nicotinic acetylcholine responses recorded from an identified cockroach (Periplaneta americana) motoneuron

Recordings from the soma of the cockroach (Periplaneta americana) fast coxal depressor motoneuron (Df) were made while acetylcholine (ACh) was regularly pressure-applied locally from a micropipette. The modulatory effects upon these nicotinic ACh responses of bath-applied 5-hydroxytryptamine (5-HT, serotonin), dopamine and octopamine were investigated under either current-clamp or voltage-clamp conditions. The biogenic amines reversibly suppressed, but never totally abolished, ACh responses, 5-HT being the most potent, with a threshold near 10(-6) m (EC50 = 5 x 10(-5) m). Occlusion experiments indicate that the amines share a common mechanism at the level of either receptors or second messenger pathways. The amines also modulated responses to nicotine or carbachol (each of which resists hydrolysis by acetylcholinesterases), indicating that the amines did not act by accelerating ACh degradation. Pharmacological antagonists were used in an attempt to characterize the receptor responsible for amine-mediated modulation. Although a number of antagonists mimicked the action of amines rather than producing blockade, the antagonistic actions of LSD and RS23597 pointed strongly to a receptor-mediated mechanism, but did not allow receptor identification. The magnitude of the modulatory effect of 5-HT was significantly reduced by intracellular guanosine-5'-O-(2-thiodiphosphate) (GDP-beta-S), indicating involvement of a G-protein. Intracellular injection of the calcium chelator BAPTA did not block the modulatory effect of 5-HT, showing that the amines do not operate through the calcium-dependent pathway by which muscarinic receptors act on nicotinic currents. The adenylate cyclase inhibitor dideoxyadenosine (DDA), on the other hand, did attenuate the action of 5-HT, suggesting involvement of cyclic AMP.

Intracellular regulation of neuronal nicotinic cholinoreceptors

Experiments on isolated superior cervical ganglia from rats were used to study the effects of substances affecting intracellular second messengers on membrane currents evoked by iontophoretic application of acetylcholine (ACh currents) and on excitatory postsynaptic currents (EPSC) induced by single discharges of preganglionic nerve fibers. These studies showed that the adenylate cyclase activator forskolin, the phosphodiesterase inhibitor isobutylmethylxanthine (IMBX), and the protein kinase C activator phorbol ester decreased the amplitude of the ACh current. Neither IMBX nor phorbol ester had any effect on the amplitude or decay time constant of EPSC, while forskolin increased the amplitude of EPSC without altering its decay time constant. Thapsigargin, which liberates intracellular calcium, not only decreased the amplitude of the ACh current, but also decreased EPSC amplitude without affecting its decay time constant. These results suggest that intracellular signaling via protein kinases A and C may affect neuronal nicotinic cholinoceptors (nAChR) only by altering receptor desensitization and not affecting receptor sensitivity to transmitters released from nerves or the kinetics of receptor ion channels. At the same time, neuronal nAChR are influenced by intracellular calcium, which decreases their ability to be activated by exogenous (perhaps acting via desensitization) and nerve-released acetylcholine without affecting the kinetics of ion channel function.

Suppression of the nicotinic acetylcholine response in rat superior cervical ganglionic neurons by steroids

The effects of various types of steroids on the nicotinic acetylcholine (ACh) receptor (nAChR)-mediated responses were investigated in superior cervical ganglionic neurons acutely dissociated from rats using nystatin perforated patch recording. ACh induced a peak followed by a gradual decrease in the inward current at a holding potential of -40 mV. Nicotine, but not muscarine, mimicked ACh. Hydrocortisone at a concentration of >10(-6) M reversibly suppressed both the peak and steady-state nicotine-induced currents (Inic) in a noncompetitive manner. The inhibition of Inic by hydrocortisone did not show any voltage dependency and persisted in the presence of either cyclic AMP modulators, forskolin and 3-isobutyl-1-methylxanthine, or a protein kinase A inhibitor, N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89). Beta-estradiol, androsterone, aldosterone, and 17alpha-estradiol mimicked hydrocortisone in its inhibitory action on ACh-induced currents (I(ACh)). The potency for the inhibitory actions on I(ACh) was as follows: androsterone > beta-estradiol > hydrocortisone > or = aldosterone = 17alpha-estradiol. Cholesterol had no effect on the I(ACh). In conclusion, the structural characteristics of a steroid are thus considered to be necessary to block nicotinic I(ACh) in rat superior cervical ganglionic cells, whereas the cholesterol side chain might disturb the inhibitory action of the steroid skeleton on nAChRs.

Modulation of nicotinic acetylcholine receptor activity in submucous neurons by intracellular messengers

The effects on acetylcholine-induced membrane currents (ACh currents), produced by agents known to modify the activity of intracellular messengers, were studied in the neurons of the guinea-pig ileum submucous plexus (SMP) using a whole-cell patch clamp recording method. The ACh currents were not affected by forskolin, the adenylate cyclase activator, regardless of whether or not ATP and GTP were present in the intracellular solution, and by phorbol 12-myristate 13-acetate, the protein kinase C activator. The ACh currents were strongly suppressed by thapsigargin, the microsomal calcium ATPase inhibitor, and genistein, the tyrosine protein kinase inhibitor. They were also suppressed by 3-isobutyl-1-methylxanthine, the cyclic-AMP phosphodiesterase inhibitor, regardless of the presence of forskolin in the extracellular solution and ATP and GTP in the intracellular solution. In addition, the currents were suppressed by activation of P2 purinoceptors with ATP, which could not be explained by a direct effect of ATP on nicotinic acetylcholine receptors (nAChRs). Reactive blue 2, the P2y purinoceptor antagonist, did not abolish inhibition of the ACh current by ATP. Alpha,beta-Imido-ATP and adenosine caused no membrane current responses and did not influence the ACh currents. These results suggest that the activity of the nAChRs in the SMP neurons is strongly suppressed by raised intracellular Ca2+ level, without involvement of protein kinases A and C, and may involve the participation of tyrosine kinase. The activity of nAChRs is also influenced by the activity of P2 purinoceptors; the mechanisms responsible for this influence are not yet clear. So, the activity of the SMP neuronal nAChRs is relatively independent on the intracellular signaling known to influence many other groups of transmitter-gated receptors of neuronal membrane.