Recent Developments on the Ultrastructural Aspect of Adrenergic Nerve Endings in Various Experimental Conditions

Abstract

This chapter reviews the work on the precise and direct ultrastructural localization of neurotransmitter substances in the vesiculated part of postganglionic adrenergic nerves in various organs of the cat and rat. Adrenergic vesiculated nerve endings contain both small dense-core vesicles and small empty-looking vesicles. The dual population of vesicles is observed when the tissues are prepared for electron microscopy by conventional methods. The dense core of the vesicles represents the physiological transmitter noradrenaline (NA). The empty vesicles in adrenergic nerve endings might contain acetylcholine (Ach) and represent the morphological correlate of a cholinergic link in postganglionic sympathetic transmission. In adrenergic-vesiculated nerve endings, biogenic amines are stored not only in the small but also in the large dense-core (LDC) vesicles. The direct and precise localization of false adrenergic transmitters with the electron microscope requires as a prerequisite that they be rendered insoluble and electron dense during the processing of the tissues for electron microscopy. The small vesicles of some nerves of the central nervous system (CNS) are able to take up and store amines in a way similar to that in the adrenergic nerves of the periphery. The possibility of a direct, visual localization of this neurotransmitter with a high degree of resolution opened a new field of investigation in the physiology and pharmacology of the adrenergic nervous system.