Immunocytochemical demonstration of GABA in physiologically characterized, HRP-filled neurons and their postsynaptic targets

The combination of intracellular recording and marking of neurons with horseradish peroxidase (HRP) has greatly extended the understanding of neuronal circuits. This approach is suitable for the reconstruction of the axonal and dendritic arborizations of neurons over several millimeters, thus providing a more complete picture of individual cells than was possible with previous methods. In addition the direct correlation of electrophysiological and structural data becomes possible. In most cases the interpretation of the possible role of identified neurons would greatly benefit from a knowledge of their biochemical characteristics, especially their transmitters. The identification of the transmitters of the postsynaptic cells would also be useful because each neuron or afferent system is likely to contact several neurochemically, morphologically and physiologically distinct neuronal populations. In the vertebrate nervous system, with few exceptions (Aghajanian and Vandermalen, 1982; Bornstein et aI., 1986; Grace and Bunney, 1983; Reaves et aI., 1983; Weiler and Ammermuller, 1986), information about the transmitters of physiologically characterized cells and their postsynaptic targets has largely been predicted from separate neurochemical, pharmacological or histochemical ments. In order to provide direct information £lbout the neurochemical characteristics of intracellularly recorded cells, we combined intracellular HRP filling with light and electron microscopic post-embedding immunocytochemistry. Following intracellular recording, the tissue containing the HRP-filled cells is processed for electron microscopic analysis. Thus the afferent and efferent synaptic connections of the physiologically characterized, HRP-filled neurons can be studied at the ultrastructural level. In addition, the connections of these same cells can also be