Serotonergic axons in the monkey's lateral geniculate nucleus

Abstract

AbstractAxons containing serotonin (5-hydroxytryptamine, 5-HT) in the lateral geniculate nucleus of Macaca monkeys were stained using light or electron microscopic (EM) immunocytochemical labeling techniques. 5-HT labeled axons were sparsely and homogeneously distributed throughout the entire nucleus. Small varicosities were located irregularly along each axon giving them a beaded appearance. Examination of these axons at the EM level revealed that the varicosities contained synaptic vesicles that were variable in size and shape. Nearly all of the varicosities lacked synaptic contacts; only two out of over two hundred labeled varicosities appeared to form a synaptic contact, and serial sections through ten varicosities showed no discernible pre- or postsynaptic membrane specializations. 5-HT labeled axon varicosities were not observed to be preferentially located in proximity to any specific neuronal structures at either the light or EM level. 5-HT varicosities were estimated to form about 1% of the profiles containing synaptic vesicles in the neuropil of the lateral geniculate nucleus.We conclude that axons containing serotonin in the lateral geniculate nucleus release this neurotransmitter to act in a neuromodulatory manner throughout the neuropil, probably to set a general level of neuronal excitability.