Anterograde tracing in the brain using autoradiography and HRP‐histochemistry. A comparison at the ultrastructural level

Abstract

K E Y WORDS. Analysis, anterograde tracing, autoradiography, comparison, electron microscopy, horseradish peroxidase histochemistry , neuro-anatomy , 3,3 ', 5,5 ' -tet ramethylbenzidine. S U M M A R YThe present paper decribes the use of 'H-leucine and WGA-HRP as anterograde tracers in the brain at the ultrastructural level. In addition the different aspects of the two tracing techniques are compared, including the analysis of the results, the reliability and relative efficiency. I N T R O D U C T I O NThe nervous system is a most complex organ, involved in such diverse functions as monitoring the outside world, organizing and directing movement, learning and memory, maintaining the internal environment of the body, etc. For these purposes internal and external sensory information are integrated within the central nervous system and may eventually result in the initiation and execution of a particular movement by the appropriate motor areas. In order to fulfil these complex tasks it is a prerequisite that the various parts of the brain are functionally interconnected. Functional interconnection within the brain necessarily implies structural connections between distinct neuronal cell groups or areas. It is the aim of anterograde tracing studies to unravel the intricate afferent connections (input) of the various neuronal cell groups. However, for the understanding of the integrative function of neurons it is not sufficient to determine from which area(s) of the brain afferent input is conveyed to a particular neuronal cell group. It is of equal importance to determine which transmitters are used in a particular system and whether their action is excitatory or inhibitory. For the latter purpose electro-physiologica1 techniques are most appropriate. However, in general they measure the overall effect of a specific input to a neuron without detailed knowledge of the local circuitry. Anterograde tracing techniques at the ultrastructural level are meant to fill this gap by studying the local characteristics of the afferent systems, i.e. the strategic location of their terminals on the neuronal membrane. It is determined whether synaptic contacts are established and whether they are located on a cell soma (axo-somatic), on a dendrite (axodendritic) or on another terminal (axo-axonic). The size and shape of the terminals and their synaptic vesicles as well as the proximity of other synapses may be important parameters all of which determine the functional properties of a particular afferent connection. At present the *To whom all correspondence should be addressed. 0 1988 The Royal Microscopical Society 233 234 J . C . Holsrege and G . F . J . M . Vrensentwo main techniques for anterograde tracing at the ultrastructural level are the autoradiographic technique and the horseradish peroxidase (HRP) technique. In this paper a critical comparison of these two methods at the electron microscopical (EM) level is presented, including a discussion on the analysis of the EM autoradiographs of brain tissue. Fig. 1. Electron micro...

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