Aims: 

Agonists of a2-adrenoceptors in the CNS inhibit vasopressinergic (AVP) and stimulate oxytocinergic (OXY) neurons in the hypothalamic supraoptic nucleus (SON). We studied the impact of xylazine (XYL), an a2-adrenoceptor agonist, on the activity of OXY and tyrosine hydroxylase (TH) synthesizing neurons in SON of control Long Evans rats (+/+), heterozygous (di/+), and AVP-deficient homozygous (di/di) Brattleboro rats.

Methods: 

The animals were injected with XYL (10 mg/kg, i.p.) and 90 min later sacrificed by transcardial perfusion with fixative. Activity of OXY and TH neurons was signalized by the presence of Fos protein visualised by dual immunohistochemistry. Co-labelings were analyzed on 30mm thick coronal sections using computerized light microscope.

Results: 

In +/+ rats no TH immunoreactive perikarya occurred but +/+ rats showed 4% activated OXY neurons. In these rats XYL raised the amount of Fos/ OXY cells up to 67%. In di/+ rats 18% of OXY cells was activated and XYL increased their number up to 60%. No TH neurons occurred in the SON of saline di/+ animals, however, 1 from 6 animals injected with XYL displayed TH immunoreactivity with 25% of Fos co-labelings. Di/di rats displayed many activated OXY (61%) as well as TH (35%) producing neurons. XYL potentiated the number of Fos/OXY neurons up to 83% and lowered the number of activated TH neurons to 5%.

Conclusion: 

The present data indicate that stimulation of a2-adrenoceptors activates OXY and inhibits TH neurons in the SON of Brattleboro rats. Supported by VEGA 2/7003/7.