Vasopressinergic innervation of the bovine pineal gland: is there a local source for arginine vasopressin?

Although the presence of arginine vasopressin (AVP) in the mammalian pineal has been characterized biochemically, the source of this nonapeptide hormone remains enigmatic. Most earlier data pointed to an extrapineal origin, although some recent evidence suggests intrapineal synthesis of AVP. The present study examined this issue using a combination of immunohistochemistry with antibodies against both the AVP and neurophysin moieties of the AVP precursor polypeptide, together with polymerase chain reaction (PCR) assay for the specific mRNA. Furthermore, the effects of AVP on melatonin production by monolayer cultures of bovine pinealocytes were examined. Bovine pineal glands possessed numerous neurophysin- and AVP-immunopositive nerve fibers, mainly in the distal part of the gland. However, no positively stained perikarya were observed. As a positive control perikarya of AVP cells were easily identifiable in the magnocellular cells of the bovine hypothalamus. Nevertheless, a highly sensitive PCR assay specific for full-length AVP mRNA did indicate the presence of AVP gene transcripts in both bovine and ovine pineal glands, using two different primer combinations. This suggests either that there are AVP perikarya in the pineal whose peptide contents are below the level of detection by immunohistochemistry or that gene transcripts may be present in AVP nerve axons, as in the posterior pituitary. AVP had significant inhibitory effects on noradrenaline-provoked melatonin secretion in vitro. These results indicate that AVP released by nonsympathetic nerve fibers terminating in the bovine pineal gland may act to modulate melatonin production.