Immunoelectronmicroscopic localization of vasopressin in the rat suprachiasmatic nucleus

The classical areas for arginine-vasopressin (AVP) synthesis are the magnocellular supraoptic (SON) and paraventricular nuclei. More recently AVP was also demonstrated in neurons of the parvocellular suprachiasmatic nucleus (SCN) of the rat. This result was substantiated in the present study by means of immunoelectron microscopy, by subjecting sections to antivasopressin plasma. Conventional electron microscopy revealed dense-core vesicles in the SCN cell bodies and fibres (mean diameter 94.7 +/- 0.9 nm and 84.0 +/- 1.1 nm respectively). These vesicles were infrequent within the cell bodies and could not be accumulated by ethanol administration. Immunoelectron microscopy showed a positive reaction in the cell bodies and fibres within vesicles of 96.7 +/- 1.1 nm and 98.5 +/- 1.1 nm and 98.5 +/- 1.2 nm respectively. By comparison, the cell bodies and fibres of the SON showed immunoreactive granules of 143.0 +/- 1.8 and 147.3 +/- 1.8 nm respectively. The presence in the SCN of AVP in vesicles of different size than those in the SON suggests that synthesis of this substance is indeed occurring within the SCN cells.