Surface AChE in the chick ciliary ganglion neurons: Ultrastructural localization and possible relations to ?-bungarotoxin receptors

Rapid synaptic transmission in the avian ciliary ganglion is mediated by two distinct classes of nicotinic receptors

We analyzed the kinetics and pharmacology of EPSCs in two kinds of neurons in the embryonic avian ciliary ganglion. Whole-cell voltage-clamp recordings revealed that the singly innervated ciliary neurons had large-amplitude (1.5-8.0 nA) EPSCs that could be classified according to the kinetics of their falling phases. Most of the neurons responded with an EPSC the falling phase of which followed a double exponential time course with time constants of approximately 1 and 10 msec. The EPSCs of the remaining ciliary neurons followed a single time constant ( approximately 8 msec). Multiple innervated choroid neurons had smaller-amplitude responses (0.2-1.5 nA when all inputs were activated) that appeared to contain only a slowly decaying component (tau = 12 msec). The fast and slow components of EPSC decay seen in most ciliary neurons could be pharmacologically isolated with two toxins against nicotinic acetylcholine receptors (AChRs). The fast component was blocked by 50 nM alpha-bungarotoxin (alpha-BuTx), which binds alpha7-subunit-containing AChRs. The slow component was selectively blocked by 50 nM alpha-conotoxin MII (alpha-CTx-MII), which blocks mammalian AChRs containing an alpha3/beta2 subunit interface. A combination of both alpha-BuTx and alpha-CTx-MII abolished nearly all evoked current. Similar pharmacological results were found for ciliary neurons with monoexponentially decaying EPSCs and for choroid neurons. These results suggest that nerve-evoked transmitter acts on at least two different populations of AChRs on autonomic motor neurons in the ciliary ganglion.

Localization of alpha-bungarotoxin binding sites in the ciliary ganglion of the embryonic chick: an autoradiographic study at the light and electron microscopic level

Binding sites for [125I]alpha-bungarotoxin were localized in the chick ciliary ganglion by light and electron microscopic autoradiography. Groups of four ganglia were incubated for 4 h with 20 nM [125I]alpha-bungarotoxin alone or with radioactive alpha-bungarotoxin plus either 1 microM unlabeled alpha-bungarotoxin or 100 microM d-tubocurarine. Specific binding to various morphological regions was determined by subtracting the densities of autoradiographic grains in the presence of competing nonradioactive ligands from the densities in the absence of those ligands. Most of the specific binding in the ganglion (91%) was associated with neurons. Seventy-four per cent was found within 1.2 micron of neuronal plasma membranes and 17% was found overlying neuronal cell bodies. Analysis of the specific binding associated with neurons, but not with the neuronal plasma membranes, revealed that lysosomes and multivesicular bodies were 9- and 32-fold more heavily labeled than other cellular organelles. The grain density over choroid cell bodies was significantly higher than that over ciliary neurons. Most (greater than 75%) of the autoradiographic grains within 0.25 micron of neuronal plasma membranes were found in "complex" contact regions of the membranes, which are characterized by extensive membrane evaginations. However, after correcting for the amount of plasma membrane present in the various regions of the membrane studied, alpha-bungarotoxin binding was found to be uniform. Few (less than 10%) of the specialized membranes between pre- and postsynaptic neurons were found in these "complex" contact regions suggesting that the bulk of alpha-bungarotoxin binding to neuronal membranes is located at some distance from the sites of transmitter release. The density of alpha-bungarotoxin binding sites in the neuronal plasma membrane was low (approximately 100-200 sites/micron 2 of neuronal membrane) compared to published values of the density of binding sites at the neuromuscular junction. Since alpha-bungarotoxin does not bind preferentially to specialized synaptic membranes, it seems unlikely that the binding sites for this toxin are the neuronal nicotinic receptors.

Acetylcholinesterase localization at synapses in chick embryo ciliary ganglion

The cytochemical localization of acetylcholinesterase (AChE) at calyciform synapses of the chick ciliary ganglion during embryonic development has been investigated. AChE activity is present at the surface membrane of newly formed calyciform synapses and closely follows the progressive enlargement of the synaptic area. The occurrence of a retrograde iris-dependent influence on ganglionic AChE is considered. AChE seems to be a suitable marker for synaptic maturation.

Developmental distributive pattern of acetylcholinesterase in chick embryo ciliary ganglion

AChE cytochemistry was performed in the chick ciliary ganglion (CG) during various embryonic stages. AChE first appeared in the RER of the neurons at 5 days of incubation (d.i.). Synaptic AChE appeared only later, parallely to the appearance of the calyciform synapses, i.e. at 9 d.i. At first AChE was mainly localized at the calyx side facing the satellite cell, thereafter extending to the neuronal side and especially labeling synaptic contacts occurring at points along the presynaptic membrane. Finally, at 15 d.i., i.e. when the calyx is morphologically mature, AChE reaction labels the whole contour of the calyciform nerve terminal. At 10 d.i., limited AChE-positive extrasynaptic areas of the ciliary neurons surface first appeared, thereafter extending up to affect at 15 d.i. large neuronal surfaces. Some hypotheses can be drawn from our results: (i) the earliest appearance of cytoplasmic AChE seems somehow independent of the establishing of functional synaptic contacts; (ii) the pattern of development of neuronal AChE suggests the existence of a sort of transynaptic control by presynaptic nerve terminals. However, it is possible that concomitant retrograde iris-dependent influences on ganglionic AChE concur in modulating neuronal AChE; (iii) the functional role possibly played by AChE localized at extra-synaptic level still remains to be clarified.