Cytochemical and pharmacological studies on polysensory neurons in the primate frontal cortex

Neurons of the squirrel monkey postarcuate cortex respond to a number of different sensory modalities. The pharmacological responses of these neurons to putative neurotransmitters, administered by microiontophoresis, was studied. Norepinephrine and serotonin had a powerful inhibitory action on the vast majority of cells at relatively low iontophoretic currents. The inhibitory action of norepinephrine was potentiated by low doses of desmethylimipramine; higher doses produced a direct slowing of discharge. Acetylcholine had mixed effects, exciting and inhibiting approximately equal numbers of neurons. The threshold current for acetylcholine responses was considerably higher than that for norepinephrine or serotonin. Treatment of freeze-dried slices of postarcuate cortex with paraformaldehyde vapor, after uptake of alpha-methyl norepinephrine or 6-hydroxytryptamine, showed an extensive plexus of fine fluorescent axons with characteristic varicosities, throughout the cortex. Many of the varicosities surrounded the soma of neurons. Light microscopic autoradiography of polysensory cortex, after subdural injection or incubation with tritiated norepinephrine or serotonin, revealed many grains along neuronal soma and major dendrites. With electron microscopy after this same treatment, autoradiographic grains were localized largely to fine unmyelinated axons and to small terminals making synaptic contact with the soma and large dendrites. After chronic treatment with 6-hydroxydopamine, fibers within the polysensory cortex which take up alpha-methyl norepinephrine or tritiated norepinephrine are greatly reduced in number, whereas fibers which take up 6-hydroxytryptamine or tritiated serotonin are still plentiful. These electrophysiological and cytochemical studies support the existence of norepinephrine and serotonin-containing inhibitory pathways to neurons of the postarcuate cortex of squirrel monkey.