The physiological role of thyroid hormone in the hypothalamic ventromedial nucleus
John R Counsell, Errol Richardson & James V Gardiner
The hypothalamo-pituitarythyroid (HPT) axis serves to maintain appropriate systemic levels of thyroid hormone (TH) through a negative feedback pathway via the hypothalamic arcuate and paraventricular nuclei. However, the effects of TH in other hypothalamic regions are poorly understood. Triiodothyronine (T3) administration to the hypothalamic ventromedial nucleus (VMN) induces a potent hyperphagic response, although it is unclear whether this is part of a novel physiological pathway or simply an additional component of the HPT axis.
This research investigates the physiological role of TH in the VMN through the use of a recombinant adeno-associated virus (rAAV) designed to locally inactivate TH. Activation and inactivation of TH is mediated by the iodothyonine deiodinases (D1, D2, and D3), where D2 functions to deiodinate tetraiodothyronine (T4) to produce the active T3 hormone, and D3 is the inactivating enzyme.
A plasmid encoding D3 cDNA under the control of a CMV promoter was packaged into rAAV to generate a viral titre capable of over-expressing D3 (rAAV-D3). Anaesthetised male Wistar rats received bilateral stereotactic injections of either rAAV-D3 (n=13) or rAAV-GFP (n=11) into the VMN. Both groups were dissected 78 days after surgery.
Over-expression of D3 in rAAV-D3 treated rats was confirmed by QPCR, which demonstrated a 10-fold increase in hypothalamic D3 mRNA (P≤0.001) in comparison to rAAV-GFP controls. Furthermore, the enzymatic activity of D3 was increased in whole hypothalamic samples (P=0.31), whilst D2 activity was unaffected (0.92). This was accompanied by up-regulation of the T3 transporter, MCT8 (P=0.15), suggesting a reduction in neuronal T3 content. Plasma levels of free T3 (P=0.95) and free T4 (P=0.82) were not altered, however plasma insulin (P=0.38) and leptin (P=0.2) were reduced by rAAV-D3 treatment, thus suggesting that local inactivation of T3 in the VMN directly modulates peripheral metabolism without affecting the HPT axis.