Intracellular horseradish peroxidase injection for correlation of light and electron microscopic anatomy with synaptic physiology of cultured mouse spinal cord neurons.

Synaptic interactions between spinal cord neurons grown in dissociated cell culture were studied electrophysiologically, and presynaptic cells were subsequently injected by intracellular iontophoresis with horseradish peroxidase (HRP). Following histochemical processing, injected cells were filled with dense reaction product which facilitated the light and electron microscopic identification of the individual physiologically typed neurons. This technique applied to neurons in monolayer culture allowed the visualization of complex intercellular relationships in essentially two dimensions. The number and distribution of morphologically defined synaptic contacts was determined for correlation with individual evoked postsynaptic potentials. HRP-filling of inhibitory and excitatory neurons revealed differences with respect to cellular geometry, axonal projection, and the number, location and ultrastructure of synaptic contacts.