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The influence of changes in the ionic composition of the bathing medium on responses of the retinula cell of the honeybee drone to light was examined by means of intraceUular microelectrodes. The resting potential of the cell was influenced mainly by the concentration of K. The peak of the receptor potential (the transient), which in a normal solution and with strong light approaches zero membrane potential, overshot this level in a K-rich solution. An increase in the concentration of K also raised the level of the steady-state phase of the receptor potential (the plateau). The amplitude of the receptor potential was decreased and the spike potential rapidly abolished when Na was replaced by either sucrose, choline, or Tris. In a Ca-free solution the amplitude of the response and especially that of the plateau, was increased. An increase in Ca had the opposite effects. All these changes were reversible. An attempt was made to interpret the receptor and spike potentials in terms of passive movements of Na and K across the membrane of the retinnla cell. The major difficulty encountered was to find an explanation for the persistence of an appreciable fraction of the transient and the plateau in preparations kept up to 12 hr in a solution in which all the Na had been replaced by choline, Tris, or sucrose.

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The observation that neurotoxins from elapid venoms are powerful curarizing agents [l] provides a class of potent and specific reagents that can be used to purify nicotinic receptors. Thus, the isolation of such receptors is the subject of many investigations [2-S] but the nature and the characteristics of the molecule remain uncertain. We report here the isolation from the electric organ of the eel, Elecrruphorus electricus, of a protein based on its ability to bind specifically and with high affinity a neurotoxin from cobra venom. We describe some of the properties of this molecule and of its complexes with cobra toxin and pharmacologically active ligands. Materials and methodsThe main neurotoxic component from the venom of the cobra, Naja naja siamensis (supplied by Miami Serpentarium Inc., Florida), was purified and tritiated according to the technique of Cooper and Reich 161. A specific activity of 3 Ci/mmole was obtained by this method. The preparation of the membrane fragments from the cephalic part of the electric organ was carried out following the technique of Changeux et al. Preparation of soluble extracts [S]A nonionic detergent, Tween 80 (10% w/v water) was added to the suspension of membrane fragments to a final concentration of 1%. The resulting solution was dialysed against I mM Tris-HCl. pH 7.4, at 4' for 5 hr, then centrifuged at 100,OOOg for 30 min. The receptor activity was assayed in the supernatant. Assay for nicotinic receptor [S]An assay was developed from the observation [8] that the toxin, a strongly cationic molecule, is not adsorbed to DEAE-cellulose at pH near neutral whereas the receptor, an acidic protein, adsorbs to DEN-cellulose, as does the toxin-receptor complex. When the material solubilized from membrane fragments is incubated with the tritiated cobra toxin and then filtered through discs of DEAE-cellulose paper, only those [" HItoxin molecules complexed to receptor adsorb to the paper. These discs are dried and adsorbed toxin measured by scintillation counting using a toluene PPO-POPOP mixture. Purification by gel permeation chromatographyGel permeation chromatography on Sepharose 4B was performed with soluble extracts containing 5-6 mg/ml of protein. The profile in fig.1 shows that 331 _ __