Fasting enhances pyroglutamyl peptidase II activity in tanycytes of the mediobasal hypothalamus of male adult rats

IVÁN LAZCANO; AGUSTINA CABRAL; ROSA MARÍA URIBE; LORRAINE JAIMES-HOY; MARIO PERELLO; PATRICIA JOSEPH-BRAVO; EDITH SÁNCHEZ-JARAMILLO; JEAN-LOUIS CHARLI

ENDOCRINOLOGY

ENDOCRINE SOC

Año: 2015

0013-7227

Fasting down-regulates the hypothalamus-pituitary-thyroid (HPT) axis activity through a reductionof thyrotropin releasing hormone (TRH) synthesis in neurons of the parvocellular paraventricularnucleus of the hypothalamus (PVN). These TRH neurons project to the median eminence (ME),where TRH terminals are close to the cytoplasmic extensions of 2 tanycytes. Tanycytes expresspyroglutamyl peptidase II (PPII), the TRH-degrading ectoenzyme that controls the amount of TRHthat reaches the anterior pituitary. We tested the hypothesis that regulation of ME PPII activity isanother mechanism by which fasting affects the activity of the HPT axis. Semi-quantitative in situhybridization histochemistry data indicated that PPII and deiodinase 2 mRNA levels increased intanycytes after 48 h of fasting. This increase was transitory, followed by an increase of PPII activityin the ME, and a partial reversion of the reduction in PVN pro-TRH mRNA levels and the numberof TRH neurons detected by immunohistochemistry. In fed animals, adrenalectomy and corticosterone treatment did not change ME PPII activity 72 h hour later. Methimazole-induced hypothyroidism produced a profound drop in tanycytes PPII mRNA levels, which was reverted by 3 days of treatment with T4. The activity of thyroliberinase, the serum isoform of PPII, was increased at most fasting time points studied. We conclude that delayed increases in both the ME PPII as well as the thyroliberinase activities in fasted male rats may facilitate the maintenance of the deep down-regulation of the HPT axis function, in spite of a partial reactivation of TRH expression in the PVN.