Ultrastructural analysis of enkephalinergic terminals in parasympathetic ganglia innervating the urinary bladder of the cat

Abstract

Leucine enkephalin immunoreactivity was identified in axons and varicosities in parasympathetic ganglia located in the pelvic plexus and on the surface of the urinary bladder of the cat. Electron microscopic immunohistochemical studies revealed that varicosities containing leucine enkephalin exhibited large dense core vesicles and small, clear, spherical vesicles, which were similar to those found in cholinergic terminals. Leucine enkephalin immunoreactivity was primarily associated with large dense core vesicles. The varicosities formed axodendritic and axosomatic synapses with principal ganglion cells. Axoaxonic synapses were not detected. Some axosomatic enkephalinergic synapses were detected embedded within or invaginating the principal ganglion cells. Varicosities containing flattened and/or small dense core vesicles did not exhibit enkephalin immunoreactivity. Bladder ganglion cells identified by retrograde HRP tracing from the urinary bladder exhibited similar leucine enkephalinergic synapses. These observations, coupled with previous reports that leucine enkephalin is present in sacral preganglionic neurons and released by preganglionic nerve stimulation, suggest that leucine enkephalin and acetylcholine are cotransmitters stored and released from the same nerve terminals in bladder parasympathetic ganglia.