strychnine and subecholine

Ionotropic, nicotinic receptors have previously been shown to mediate both inhibitory (Cl-dependent) and excitatory (cationic) cholinergic responses in Aplysia neurons. We have used fast perfusion methods of agonist and antagonist application to reevaluate the effects on these receptors of a wide variety of cholinergic compounds, including a number of recently isolated and/or synthesized alpha toxins [alpha-conotoxin (alphaCTx)] from Conus snails. These toxins have been shown in previous studies to discriminate between the many types of nicotinic receptors now known to be expressed in vertebrate muscle, neuroendocrine, and neuronal cells. One of these toxins (alphaCTx ImI from the worm-eating snail Conus imperialis) revealed that two kinetically and pharmacologically distinct elements underlie the ACh-induced Cl-dependent response in Aplysia neurons: one element is a rapidly desensitizing current that is blocked by the toxin; the other is a slowly desensitizing current that is unaffected by the toxin. The two kinetically defined elements were also found to be differentially sensitive to different agonists. Finally, the proportion of the rapidly desensitizing element to the sustained element was found to be cell-specific. These observations led to the conclusion that two distinct nicotinic receptors mediate Cl currents in Aplysia neurons. The receptor mediating the rapidly desensitizing Cl-dependent response shows a strong pharmacological resemblance to the vertebrate alpha-bungarotoxin-sensitive, alpha7-containing receptor, which is permeable to calcium and mediates a rapidly desensitizing excitatory response.

Topics: Acetylcholine; Aconitine; Alkaloids; alpha7 Nicotinic Acetylcholine Receptor; Animals; Aplysia; Azocines; Bungarotoxins; Cations; Chlorides; Choline; Cholinergic Antagonists; Conotoxins; Dihydro-beta-Erythroidine; Electric Conductivity; Glycine Agents; Insecticides; Ion Channel Gating; Mollusk Venoms; Nicotinic Agonists; Nicotinic Antagonists; Oligopeptides; Patch-Clamp Techniques; Peptides; Quinolizines; Receptors, Nicotinic; Strychnine; Vertebrates

Acetylcholine (ACh) is the major neurotransmitter released from the efferent fibers in the cochlea onto the outer hair cells (OHCs). The type of ACh receptor on OHCs and the events subsequent to receptor activation are unclear. Therefore we studied the effect of agonists and antagonists of the ACh receptor on isolated OHCs from the guinea pig. OHCs were recorded from in whole cell voltage and current clamp configuration. ACh induced an increase in outward K+ current (IACh) which hyperpolarized the OHCs. No desensitization to ACh application was observed. Cs+ replaced K+ in carrying the IACh. The IACh is Ca(2+)-dependent, time and voltage sensitive, and different from the IKCa induced by depolarization of the membrane potential. When tested at 100 microM, several agonists also induced outward current responses (acetylcholine > suberyldicholine > or = carbachol > DMPP) whereas nicotine, cytisine and muscarine did not. The IACh response to 10 microM ACh was blocked by low concentrations of traditional and non-traditional-nicotinic antagonists (strychnine > curare > bicuculline > alpha-bungarotoxin > thimethaphan) and by higher concentrations of muscarinic antagonists (atropine > 4-DAMP > AF-DX 116 > pirenzepine). Pharmacologically, the ACh receptor on OHCs is nicotinic.

Topics: Acetylcholine; Alkaloids; Animals; Atropine; Azocines; Bungarotoxins; Carbachol; Choline; Cholinergic Antagonists; Curare; Dimethylphenylpiperazinium Iodide; Guinea Pigs; Hair Cells, Auditory, Outer; In Vitro Techniques; Membrane Potentials; Nicotine; Piperidines; Pirenzepine; Quinolizines; Receptors, Cholinergic; Strychnine; Trimethaphan