Calmodulin immunoreactivity in the chicken pineal gland: Comparison with calbindin‐D28k, calretinin, and S100

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Abstract
Calmodulin distribution in the chicken pineal organ was investigated by immunohistochemistry. Calmodulin immunoreactivity was detected in ependymocytes in the follicular zone and in interstitial cells in the parafollicular zone. No calmodulin immunoreactivity was detected in pinealocytes. Lack of calmodulin immunoreactivity in pinealocytes raises questions about its proposed function in melatonin synthesis as suggested by pharmacological studies using calmodulin antagonists. The calmodulin distribution was comparable to that of S100, a glial cell marker. Two other markers, calbindin‐D28k and calretinin, which in neuroanatomical studies give excellent cytoarchitectonic staining, in the chick pineal permitted the detection of two subclasses of pinealocytes. One was darkly stained by calbindin‐D28k and rare. The other was very abundant and calretinin positive. In the parafollicular zone, calbindin‐D28k and/or calretinin antibodies allowed us to visualize cells presenting a neuron‐like morphology. Calretinin immunoreactivity was detected in nearly all pinealocytes in which hydroxy‐indol‐O‐methyl transferase was also located. Comparison between the lack of calmodulin and the presence of calretinin, belonging to the same calcium‐binding protein family, in chick pinealocytes raises the hypothesis about a possible role of calretinin in melatonin synthesis.