Tachykinins are vasoactive peptides present mainly in nervous tissues. These peptides were found to be present in the pineal gland of several species. Tachykinins do not seem to be synthesized in the pineal gland, but rather they are contained in nervous fibers coming from other areas of the nervous system. Most of the tachynergic fibers in the pineal gland seem to be originated in the trigeminal ganglia, although some of them are likely of central origin. Tachykinin receptors were found to be present in the pineal gland, where they have been localized in the interstitial cells. The pinealocytes, which secrete melatonin, do not have tachykinin receptors nor have they synaptic connections with tachynergic fibers. Thus, it is apparent that tachykinins may not have a major influence on the melatonin-producing pinealocytes. The main target for tachykinins in the pineal gland seems to be the interstitial cells. As the interstitial cells synthesize cytokines and are therefore involved in some immune functions, the most likely role of tachykinins in the pineal gland seems to be related to the immune processes that take place in the pineal gland. Gonadal steroids affect the levels of tachykinins in the pineal gland, and therefore an interaction between hormonal steroids and tachykinins may exist in the gland.
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