ANALYSIS OF SYNAPTIC INPUTS AND TARGETS OF PHYSIOLOGICALLY CHARACTERIZED NEURONS IN RAT FRONTAL-CORTEX - COMBINED IN-VIVO INTRACELLULAR-RECORDING AND IMMUNOLABELING

Ultrastructural immunocytochemical identification of transmitters in a fferent terminals and targets of individual physiologically characteri zed neurons is essential for understanding the complex circuitry withi n the mammalian neocortex. For this type of analysis, we examined the utility of combining in vivo intracellular recording and biocytin inje ctions with silver intensified 1 nm immunogold labeling of GABA and th e catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). These transmitters are found in local neurons and afferents known to promine ntly modulate the activity of pyramidal neurons in the neocortex. Indi vidual neurons were physiologically characterized and filled with bioc ytin in the frontal cortex of anesthetized rats. The brains were then preserved by vascular perfusion with aldehydes. Single vibratome secti ons through the recording site were reacted (1) for immunoperoxidase d etection of biocytin and (2) for immunogold labeling of GABA or TH. Du ally labeled sections were processed for light microscopy or embedded in plastic for electron microscopy. The dense peroxidase product for b iocytin was detected in pyramidal neurons. These were located in super ficial as well as deep cortical laminae, and were readily distinguishe d from immunogold silver labeling. GABA labeled terminals formed symme tric synapses with larger biocytin filled dendrites, whereas the TH la beled terminals contacted distal dendrites and spines. Peroxidase labe ling for biocytin also was seen in a few axon terminals forming synaps es with unlabeled and with GABA immunoreactive dendrites. These result s suggest that single pyramidal neurons of the rat frontal cortex rece ive dual input from both GABA and catecholamine terminals. Additionall y, this study demonstrates the usefulness of silver enhancement of 1 n m colloidal gold prior to plastic embedding for electron microscopic d etection of neurotransmitters within afferents and targets of neurons physiologically characterized in vivo. (C) 1994 Wiley-Liss, Inc.