Effects of oral glucose administration on preproinsulin mRNA in rats in vivo

Negative feedback regulation of hypophysiotropic thyrotropin-releasing hormone (TRH) synthesizing neurons: role of neuronal afferents and type 2 deiodinase

Hypophysiotropic thyrotropin-releasing hormone (TRH): synthesizing neurons reside in the hypothalamic paraventricular nucleus (PVN) and are the central regulators of the hypothalamic-pituitary-thyroid (HPT) axis. TRH synthesis and release from these neurons are primarily under negative feedback regulation by thyroid hormone. Under certain conditions such as cold exposure and fasting, however, inputs from neurons in the brainstem and hypothalamic arcuate and dorsomedial nuclei alter the set point for negative feedback through regulation of CREB phosphorylation. Thus, during cold exposure, adrenergic neurons stimulate the HPT axis, while fasting-induced central hypothyroidism is mediated through an arcuato-paraventricular pathway. Feedback regulation of TRH neurons may also be modified by local tissue levels of thyroid hormone regulated by the activation of type 2 iodothyronine deiodinase (D2), the primary enzyme in the brain that catalyzes T4 to T3 conversion. During infection, endotoxin or endotoxin induced cytokines increase D2 activity in the mediobasal hypothalamus, which by inducing local hyperthyroidism, may play an important role in infection-induced inhibition of hypophysiotropic TRH neurons.

Differential effects of central leptin, insulin, or glucose administration during fasting on the hypothalamic-pituitary-thyroid axis and feeding-related neurons in the arcuate nucleus

The reductions in circulating levels of leptin, insulin, and glucose with fasting serve as important homeostasis signals to neurons of the hypothalamic arcuate nucleus that synthesize neuropeptide Y (NPY)/agouti-related protein (AGRP) and alpha-MSH/cocaine and amphetamine-regulated transcript. Because the central administration of leptin is capable of preventing the inhibitory effects of fasting on TRH mRNA in hypophysiotropic neurons primarily through effects on the arcuate nucleus, we determined whether the continuous administration of 30 mU/d insulin or 648 microg/d glucose into the cerebrospinal fluid by osmotic minipump might also have similar effects on the hypothalamic-pituitary-thyroid axis. As anticipated, the intracerebroventricular infusion of leptin reduced fasting-induced elevations in NPY and AGRP mRNA and increased proopiomelanocortin and cocaine and amphetamine-regulated transcript mRNA in the arcuate nucleus. In addition, leptin prevented fasting-induced reduction in pro-TRH mRNA levels in the paraventricular nucleus and in circulating thyroid hormone levels. In contrast, whereas insulin increased proopiomelanocortin mRNA and both insulin and glucose reduced NPY mRNA in arcuate nucleus neurons, neither prevented the fasting-induced suppression in hypophysiotropic TRH mRNA or circulating thyroid hormone levels. We conclude that insulin and glucose only partially replicate the central effects of leptin and may not be essential components of the hypothalamic-pituitary-thyroid regulatory system during fasting.

Increase in liver glucose transporter mRNA levels during rat liver regeneration

Gene expression of liver facilitated glucose transporter was rapidly induced during the liver regenerating process in rats. It reached maximum of 2.7 times at 8 hr of the regenerating course and returned to normal by 48 hr. The protein synthesis inhibitor, cycloheximide, did not interfere with the increased gene expression of liver facilitated glucose transporter. By contrast, erythrocyte/brain-type glucose transporter mRNA could not be detected in the livers of partially hepatectomized rats and sham-operated rats. The plasma glucose levels were transiently increased within 2 hr of the regenerative course and then decreased to a nadir at 4 hr. These results suggest that the increased gene expression of liver facilitated glucose transporter contributes to the decrease in plasma glucose levels.