Momentary alteration of the postsynaptic membrane during transmission of a single nerve impulse.

Abstract

Transmission of a nerve impulse at neuromuscular and other synapses is an extremely brief event. By using rapid-freezing and cryofracture techniques in the electric organ of Torpedo, synaptic transmission was found to be accompanied by significant changes affecting the postsynaptic membrane for a few milliseconds. In the replicas, the protoplasmic leaflet of this membrane was seen to contain intramembrane particles (IMPs) of two different forms, globular and elongated. Globular IMPs had a mean diameter of 8.8 nm; they were the most frequently found (80% in unstimulated specimens). Elongated IMPs had a major diameter of 17.9 nm, about twice that of globular IMPs. Transmission of a single nerve impulse was accompanied by a marked decrease in the number of globular IMPs and by an increase in the number of elongated IMPs, as if there were a coalescence of two adjacent round particles to form an elongated one. These changes started soon after the electrical stimulus and lasted for --3 ms. IMPs in the postsynaptic protoplasmic face are thought to correspond to a certain proportion of nicotinic acetylcholine receptors that were extracted with this leaflet during the fracture process. The phenomenon described here reflects an abrupt change in the membrane, probably linked to activation of the acetylcholine nicotinic receptors.During transmission of a nerve impulse, a brief chemical impulse of acetylcholine is released from the axon terminals. On the postsynaptic membrane, the neurotransmitter activates the nicotinic receptor ion channel, which opens for 1 or 2 ms. An intriguing question is whether receptor activation is accompanied by any ultrastructural change in the membrane. The difficulty in answering such a question comes from the fugacity of activation. On the other hand, on continued acetylcholine exposure the receptor becomes inactivated, and further ion fluxes across the membrane are blocked, a process called desensitization. Attempts to soak postsynaptic membrane fragments (1) or tubular crystals of receptors (2) in a solution containing acetylcholine or a related compound did not reveal any significant change until recently when Unwin and co-workers observed that desensitization is accompanied by a quaternary rearrangement of the acetylcholine receptor (3).The present work describes a change caught by rapidfreezing techniques in intact tissue during the passage of a nerve impulse under the most physiological conditions possible. No drug was present, and the tissue was not treated by chemical fixatives or cryoprotectants. The data were obtained from recent experiments where we found that a characteristic change occurs abruptly, for 2-3 ms, in the membrane of axon terminals when acetylcholine is released (4, 5). In addition to the presynaptic membrane pictures, the freeze-fracture replicas often exposed areas of the postsynaptic membranes that were easily identified by their high density of intramembrane particles (IMPs) (see Fig. 1). We were surprised to find that the repartition between globular and e...

Evidence for Cooperativity Between Nicotinic Acetylcholine Receptors in Patch Clamp Records

“…Possible physical mechanisms for the cooperative effect could include a direct protein-protein interaction whereby opening of one channel causes binding sites in the neighboring closed channel to be more accessible (perhaps sterically compatible or electrically attractive) to the positively charged acetylcholine molecule. Consistent with the present finding of interaction between (at least) two channels, it is interesting that structural evidence of transient dimer formation in response to acetylcholine release has been obtained using rapid-freezing and cryofracture techniques in the electric organ of Torpedo (Dunant et al, 1989). Similarly, coupled gating in ryanodine receptors has been explained by dimer formation (Marx et al, 1998).…”

Quantal acetylcholine release: Vesicle fusion or intramembrane particles?

“…This abrupt change had the time course expected from synaptic transmission. It was most probably linked to activation of nicotinic ACh receptors (Dunant et al, 1989).…”

Major histocompatibility complex class I protein conformation altered by transmembrane potential changes

“…Transmembrane potential fields in living cells influence the charge distribution of individual membrane-bound molecules, in particular, those of transmembrane proteins (3,19,25,30). Changes in this potential can thereby lead to conformational changes in such macromolecules (10,15,22,31), whose biological function may depend on or may be altered simultaneously with such conforma-tional changes, for example, in protein translocation through membranes (3), in postsynaptic membrane structures (10,22), and in transport processes governed by band 3 protein (31).…”