Regulation of adrenomedullary preproenkephalin mRNA: effects of hypoglycemia during development

Abstract

To further evaluate whether transsynaptic mechanisms account for stress-induced changes in adrenomedullary preproenkephalin mRNA (ppEnk mRNA), neonatal rats were made hypoglycemic at a time when synapses are non-functional (<10 days postnatal age). While ppEnk mRNA in medullae from adult rats increased as much as 60-fold in this paradigm (insulin 10 U/kg), ppEnk mRNA levels in the newborn increased only 1.6-fold (insulin 20 U/kg). To evaluate whether postsynaptic cholinergic pathways of the neonatal adrenal medulla were functional, we treated 5-day-old pups with cholinergic agonists (nicotine [1 mg/kg, s.c., q 12 h]+carbachol [1.7 μmol/kg, s.c., q 12 h × 4 days]). Combined cholinergic agonist treatment augmented enkephalin prohormone and peptide levels up to 3-fold ( P < 0.05). To determine whether the blunted response to hypoglycemia in the newborn resulted from a deficiency in functional transsynaptic activity, synapses were matured using thyroid hormone pretreatment (postnatal days 2 and 3) before hypoglycemic stress. Hypoglycemia now caused a 40-fold increase in adrenomedullary ppEnk mRNA levels only in the T3/insulin treated group. To exclude other secondary effects of hypoglycemia (eg. hormonal, or insulin treatment-dependent), intracellular glycopenia was produced in the presence of secondary hyperglycemia by injecting adult rats or pups with 2-deoxyglucose (500 mg/kg). Similar to the insulin-hypoglycemia group, a large increase in adrenomedullary ppEnk mRNA resulted in the adult but not in the 5-day-old neonatal adrenal medullae. We conclude that enkephalin biosynthesis, like co-stored catecholamines, is induced by a transsynaptic process. However, since decreasing transsynaptic activity can also increase ppEnk mRNA, a bimodal, activity-dependent mechanism of ppEnk gene regulation appears to exist.