Increased GABA shunt activity in VMN of three hyperphagic rat models

Metabolic signals have been proposed as controls of energy balance. Glucose utilization for gamma-aminobutyric acid (GABA) synthesis was evaluated in diet-restricted and streptozotocin-diabetic rats. Rates for glucose flux through the GABA shunt and glutamate decarboxylase (GAD), the rate-limiting enzyme in GABA synthesis, were measured in ventro-medial nucleus (VMN) and lateral hypothalamic (LHA) homogenates. Rates of GAD activity were elevated in the VMN of both restricted and diabetic rats but did not change in the LHA of either model. The rate of glucose flux through the GABA shunt in the VMN was increased in restricted rats but were decreased in the VMN of diabetic rats. Restricted rats allowed to feed ad libitum initially ate 160% of the amount the control rats ate. GAD activity rates in the VMN returned to near control levels after 2 days of refeeding. Obese Zucker rats (fa/fa) also had higher rates of GAD activity and glucose flux through the GABA shunt in the VMN but not in the LHA. The increased rate of GABA synthesis may represent increased GABAergic activity in the VMN and may possibly be involved in feeding behavior and energy balance.