Aspects of the chemical embryology of the electromotor system of Torpedo marmorata with special reference to synaptogenesis

Abstract

The functional development of the cholinergic electromotor system of Torpedo marmorata has been followed by measuring four proteins, choline acetyltransferase, acetylcholinesterase, myosin and nicotinic acetylcholine receptor, and the discharge of the electric organ in response to presynaptic stimulation in embryos from 28mm in length to new-born (~ 130 mm). The first two, as indicators of cholinergic function, were followed, both in the electric lobes, sites of the perikarya of the electromotor neurones, and in the electric organ; the last three, indicative respectively of the myofibrillar structures of the myotubes which are the precursors of the electrocytes, the cholinoceptive function of the differentiating postsynaptic membrane, and the existence of functional synapses, were followed in the electric organ. A fall in the tissue concentration of myosin heavy chain protein in the electric organ between 37 ± 2 and 57 ± 2 mm of embryo length correlated well with the transition from vertically orientated myotubes to horizontally flattened electrocytes, with loss in myofibrillar structure. The finding that receptor protein had reached almost 100% of the adult value at an embryo length at which discharges of the electric organ were barely detectable showed that synaptogenesis, seen morphologically as a continuous process commencing with the ingrowth of axons at 55 mm, occurs functionally in two stages in which the formation of a receptor- (and esterase-) rich postsynaptic membrane precedes that of transmitting synapses. Choline acetyltransferase activity was first detected in the lobe at about 40mm embryo length, 4–6 weeks after the electromotor perikarya had stopped dividing. It rose steadily to 125% of the adult value at birth. By contrast the rise in the electric organ was much slower; it reached only 50% of the adult value at birth and showed evidence of two phases, an initial small rise coincident with and perhaps due to the invasion of the electric organ by cholinergic electromotor axons and a more pronounced one from 90 mm accompanied by the formation of transmitting synapses. Significant (1 V) discharges of electric organs in response to single stimuli were first detected at 90mm and rose exponentially to reach adult values at birth. However, the organ remained more fatiguable than that of the adult perhaps due to limited transmitter synthesis by the sub-adult concentrations of choline acetyltransferase at the nerve terminals.