GABA and neuropeptide Y co-exist in axons innervating the neurointermediate lobe of the pituitary of Xenopus laevis--an immunoelectron microscopic study.

The neural innervation of the neurointermediate lobe of the pituitary of the amphibian Xenopus laevis has been studied at the light and electron microscopic level. In the pars intermedia melanotropes and stellate cells are abutted by varicosities originating from GABA- and neuropeptide Y-producing neurons. The varicosities contain two types of vesicle: electron-lucent vesicles (mean diameter 50 nm) which are immunopositive for GABA and larger (80 nm) electron-dense vesicles which are immunopositive for neuropeptide Y. Double immunogold labeling established that GABA and neuropeptide Y co-exist within the varicosities. In the pars nervosa similar varicosities, though low in number, occur. They are associated with neurosecretory nerve terminals, pituicytes and blood vessels. The possible significance of GABA and neuropeptide Y for the neural regulation of melanophore stimulating hormone-release from the pars intermedia is discussed.