Normal distribution and denervation changes of neurotransmitter related enzymes in cholinergic neurones

Abstract

The activities of choline acetyltransferase (CAT) [EC 2.3.1.6] and acetylcholinesterase (AChE) [EC 3.1.1.7] were assayed in adult pigeon [Columba livia] ciliary ganglia, in the post-synaptic ciliary and choroid nerves, and in ciliary nerve iris terminals isolated from control birds and from animals from which the oculomotor nerve was previously transected. Enzyme activity levels were also measured in the iris terminals after surgical section of the ciliary nerves. From differences in enzyme activity between control and 3 day denervated tissues, the localization of CAT and AChE in pre- and post-synaptic elements of the ganglia and at the iris neuromuscular junctions was estimated. The fate of the preganglionic nerve terminals after denervation was investigated by EM examination of ganglia after surgical section of the oculomotor nerve. Distribution of CAT activity in the ganglion was 60% in presynaptic elements, 31% in cell somas, and 9% in intraganglionic post-synaptic axons; 98% of the activity in iris junctions was present in the ciliary nerve terminals. For AChE: 20% was present in the preganglionic terminals, 69% in ganglion cell somas and the remaining 11% in post-ganglionic axons; at the neuromuscular iris junctions, 20% was found in the ciliary nerve terminals and 80% in the iris striated muscle. The 1st changes in the fine structure of the nerve terminals were observed 14 h after surgery, and by 24 h marked alteration of the synaptic structure were clearly recognized. There was a positive correlation between CAT activity in the control iris nerve terminals and in ganglia. After denervation, when the activity of the enzyme decreased in ganglion cell somas, there was a corresponding decrease in the post-synaptic nerves. CAT slow axoplasmic transport is probably related to its perikarial concentration. There was a 60% reduction of CAT activity in the post-synaptic elements, assayed in the 10 day denervated ganglia, which was accompanied by a 30% decrease in activity in the iris nerve terminals. Post-synaptic AChE decreased about 30% in the ganglion and about 30% in the iris 10 days after section of the oculomotor nerve. CAT activity decreased in the nerve trunks, 70% at the ciliary nerve and 40% at the choroid. AChE showed smaller changes. No differences were found in ganglionic protein content, or lactate dehydrogenase (LDH) [EC 1.1.1.27], co-enzyme A (CoA) and monoamine oxidase (MAO) [EC 1.4.3.4] levels between short-term (3 days) and long-term (10 days) denervated ganglia. The later decrease of CAT and AChE activity in the cell somas, axons and nerve terminals after long-term preganglionic transection suggests that the activity of these enzymes is regulated across the synapses. Probably, AChE regulation is part of a general trophic interaction between neurons, but transsynaptic modulation of CAT is specific for cholinergic cells.