Synaptic organization of amphibian sympathetic ganglia

Abstract

The synaptic organization of the amphibian sympathetic ganglia was studied, especially in the last two abdominal paravertebral ganglia of the frog. These ganglia appear to form a monosynaptic relay, not containing interneurons. They consist of two systems working in parallel: the principal neurons, by far the most numerous, and a small number of chromaffin (i.e., SIF) cells, usually arranged in clusters. Each principal neuron is innervated by a preganglionic branch forming a set of cholinergic synapses which exhibit classical ultrastucture. The only peculiarity is the presence of a subsynaptic apparatus in a variable percentage of synaptic complexes. Electro‐physiological studies have demonstrated that synaptic transmission is due to ACh release and involves several postsynaptic potentials. Moreover, the principal neurons are of two types, B and C, whose preganglionic axons and their own axons have different conduction velocities. C neurons tend to be small in diameter, and B neurons are larger, but the size distribution of the two populations overlaps. More recently, it was demonstrated that these two neuronal systems have different immunocytochemical features. The C preganglionic fibers contain an LHRH‐like peptide, which is responsible for late synaptic events. The B preganglionic fibers contain CGRP, whose role has not yet been established. The principal neurons all contain adrenaline, but neuropeptide Y is also present in C neurons and could be a second transmitter at peripheral junctions. SP‐containing fibers also pass through the ganglia, but give rise to intraganglionic synapses only rarely, except in the celiac plexus. Galanin can coexist with neuropeptide Y in certain C neurons. Numerous principal neurons are immunoreactive for VIP. Chromaffin cells contain noradrenaline and metenkephalin, and some contain SP or LHRH; they are endocrine cells controlled by preganglionic fibers and can have a modulatory effect on principal neurons endowed with appropriate receptors. The accessibility of frog abdominal ganglia and the anatomical separation of B and C preganglionic fiber pathways provide interesting systems in which to carry out experimentation on the stability and specificity of synaptic contacts. After postganglionic axotomy, the majority of synapses disappear by disruption of synaptic contacts. There is a certain discrepancy between the recovery of synaptic transmission and the reappearance of morphologically identifiable synapses, suggesting that a certain amount of transmission is possible at contacts devoid of synaptic complexes. The selective deafferentation of B or C neurons showed that the subsynaptic apparati are mainly found at B neuron synapses. The course of reinnervation following selective deafferentation reveals the existence of different specificities at B and C synapses: C neurons are easily reinnervated by B preganglionic fibers, whereas C fibers appear fairly ineffective at reinnervating B neurons, even after a long interval. Attempts were made to reinnervate ganglionic neurons...

Structure of peripheral synapses: autonomic ganglia

“…Post-synaptic sites commonly have conspicuous electron-dense sub-synaptic "bars" (Watanabe and Burnstock 1976;Watanabe 1983;Lascar et al 1996) or "bandeletes sous synaptiques" (see Taxi 1976). The true nature of these structures has not been identified.…”

Neuro-Endocrinology of Amphibians and Reptiles: An Overview

“…In addition to the fact that in amphibians some isoforms of GnRH are located in the nervous system -the hypothalamus, pituitary, neurons of the nasal cavity, and in the spinal cord and sympathetic ganglia (Lascar et al, 1996;Troskie et al, 1997;Chartel et al, 1998 a, b;), it can be detected in the interstitial cells and spermatocytes and spermatozoa of the testis; theca interna and granulosa cells, and perinuclear zone of oocytes, and the placenta (Cariello et al, 1989;Di Matteo et al, 1990Fasano et al, 1990;Battisti et al, 1994Battisti et al, , 1997Kikuyama et al, 2003).…”