Unmasking of cryptic binding sites for alpha -bungarotoxin in membrane fractions from chick retina

Effect of soluble factors from nerve and muscle on alpha-bungarotoxin binding sites in isolated sarcolemmal membranes of the rat

1. Sarcolemmal membranes were isolated from normal and from denervated muscles and the specific binding of [125I]-alpha-bungarotoxin to the membranes was determined. 2. Cytosol prepared from either slow (soleus) or fast (extensor digitorum longus) muscle increased the toxin binding. Similar effects were seen with cytosol from muscles which had been denervated 5 to 7 days previously and with cytosol from sciatic nerve. A purified preparation of a bacterial phospholipase C also produced this effect. 3. Bathing medium in which normal or denervated muscles had been incubated, increased the toxin binding to normal membranes, but the normal bathing medium decreased the binding to membranes prepared from denervated muscles. 4. The possibility that pre-existing latent acetylcholine receptors can be activated by intracellular factors released from muscles is discussed.

Acetylcholine as a neurotransmitter in the vertebrate retina

The evidence for the existence of acetylcholine as a neurotransmitter in the vertebrate retina is reviewed. There is evidence for the existence of a cholinergic system in every retina studied to date; therefore, it appears that acetylcholine is both essential and ubiquitous at this level of the visual system. Particular attention is directed to descriptions of the possible functions of acetylcholine in the retina, and formation of testable models which will serve to elucidate some of the details of cholinergic neurotransmission in the retina.

Flight induced by microinjection of D-tubocurarine or alpha-bungarotoxin into medial hypothalamus or periaqueductal gray matter: cholinergic or GABAergic mediation?

Microinjections of various doses (50-300 ng) of the nicotinic antagonist D-tubocurarine (TUBO) into the rat's medial hypothalamus (MH) or dorsal periaqueductal gray (PAG) produced flight reactions characterized by jumps. Two different types of flight reactions were produced depending on whether the drug was injected into the MH or into the PAG. MH injections provoked an increase in both locomotor activity and rearing together with well-oriented jumps. PAG injections provoked either freezing reactions or running with explosive jumps, but no increase in rearing. In addition, the rat exhibited an asymmetry in responsiveness to tactile stimulation. These reactions also differed depending on whether the drug was injected into the dorsal or ventral PAG. Behavioral reactions similar to those produced by TUBO were also produced by microinjection of the GABA receptor antagonist bicuculline into the same brain sites. Among the 4 putative cholinergic antagonists tested under the same conditions only alpha-bungarotoxin produced effects that were qualitatively similar to those induced by D-tubocurarine or bicuculline. Gallamine and hemicholinium produced tremor when injected into sites located near the ventricular system at either the MH or the PAG level, while vocalizations were only produced by PAG injections. Hexamethonium produced no marked effect. The hypothesis that flight reactions induced by D-tubocurarine or alpha-bungarotoxin do not result from their antinicotinic action but rather from a direct effect on GABAergic transmission is discussed.

Cholinergic systems and multiple cholinergic receptors in ocular tissues

Acetylcholine (ACh), choline acetyltransferases and cholinesterases occur in cornea, iris-ciliary body complex and retina of several vertebrates. In cornea, ACh may serve as a sensory transmitter as well as a local hormone, the function of which is not delineated. The function of ACh as the parasympathetic neurotransmitter at the iris and ciliary body is well established. The muscarinic receptors on the iris smooth muscle are similar to the muscarinic receptors (M2 type in two way classification) at other smooth muscles towards their interaction with agonists and antagonists. Binding studies using radiolabeled antagonists and their displacement by agonists indicate that muscarinic receptors in membranes of iris-ciliary body complex are heterogeneous indicating more than one subtype of muscarinic receptors. A subtype other than M2 receptors may occur at the presynaptic sites of parasympathetic nerves, which have yet to be investigated using specific agonists and antagonists. Cholinergic markers, choline acetyltransferase and acetylcholinesterase, differ quantitatively and qualitatively in retinas of different species. However, amacrine cells are cholinergic in all vertebrate species. Although they make up 1% of retinal neurons, they influence the activity of a majority of ganglion cells. Cholinergic effects in ganglia are mediated through nicotinic and muscarinic receptors. Both of these types of cholinergic receptors are heterogeneous. They have yet to be investigated for their subtypes using specific agonists and antagonists. Although the role of cholinergic retinal neurons in the processing of visual information is not known, their input to ganglion cells generally increases the rate of spontaneous activity or the number of action potentials in light-evoked responses. Thus, the cholinergic input seems to modify the overall neuronal input to the ganglion cells from the receptive fields. Endothelial cells of blood vessels contain muscarinic receptors, which are activated by ACh to cause relaxation. Although retinal blood vessels provide recognizable characteristic signs in diabetes mellitus and hypertensive disease, no information is available on the muscarinic receptors of these vessels.

Extra-synaptic localization of alpha-bungarotoxin receptors in cultured chick ciliary ganglion neurons

Surface [125I]alpha-bungarotoxin receptors were localized on dissociated cultures of embryonic chick ciliary ganglion neurons by electron microscopic autoradiography. Grain density was 4-5-fold higher on cell body membrane versus neuritic process membrane, and there was no apparent concentration of grains at morphologically identifiable inter-neuronal synapses.