Effects of thyroid status on clonidine-induced hypoactivity responses in the developing rat.

Clonidine-induced hypoactivity was studied in rats made neonatally hypo- and hyperthyroid (PTU or T4 administered between 1-27 days postnatally) as an indication of central presynaptic alpha 2-adrenoceptor function. Paradoxically, at 28 days postnatally both treatments caused an increase in clonidine-hypoactivity compared with untreated control animals, which was not due to sex differences amongst littermates in the various groups. It is proposed that whereas the hyperthyroid neonate data perhaps reflect an enhancement of presynaptic alpha 2 function similar to that seen in the adult rat brain, the result obtained for the hypothyroid neonates could perhaps be due to a retardation of the ontogeny of central noradrenergic neurones.