CLOSE MICROTOPOGRAPHICAL RELATIONSHIPS BETWEEN SYMPATHETIC-NERVE TERMINALS AND BULBOUS PROCESS ENDINGS OF PINEALOCYTES IN THE PINEAL-GLAND OF THE MONGOLIAN GERBIL

Previous studies have shown that pinealocytes of the gerbil pineal gla nd exhibit processes that form terminal swellings filled with abundant electron-lucent microvesicles. The membrane of these presumptive secr etory microvesicles is known to contain synaptophysin, a major integra l glycoprotein of neuronal synaptic vesicles. The present study was co nducted to evaluate the microtopographical relationships between the v esicle-rich process swellings and intra-pineal nerve terminals. For th is purpose, both nerve terminals and pinealocyte process endings were visualized immunohistochemically in the same semi-thin sections of pla stic-embedded gerbil pineals, using antibodies directed against synapt ophysin. This approach consistently revealed close spatial association s of punctate immunopositive nerve endings with intensely stained bulb ous process terminals of pinealocytes in or near the perivascular spac es. The light-microscopic observations of intimate neuronal-pinealocyt ic relationships were corroborated at the electron-microscopic level. Perivascular varicosities with ultrastructural features characteristic of sympathetic nerve terminals were frequently juxtaposed to vesicle- filled process endings of pinealocytes. Analysis of serial thin sectio ns showed that multiple point-to-point contacts are encountered betwee n noradrenergic nerve terminals and pinealocytic process swellings. Ou r morphological findings imply that bulbous process terminals, at leas t in the gerbil pineal gland, are major targets for the neuronal contr ol of the secretory activity of pinealocytes.