GLYCOGEN-PHOSPHORYLASE ACTIVITY IN THE CEREBRAL-CORTEX OF RATS DURINGDEVELOPMENT - EFFECT OF HOMOCYSTEINE-INDUCED SEIZURES

The aim of the present study was to examine the total, as well as the active form of glycogen phosphorylase in the rat cerebral cortex durin g development, and to assess the response of the enzyme to induced sei zures. Seizures were induced in 7-, 12- and 18-day-old male Wistar rat s by i.p. administration of DL-homocysteine thiolactone HCl. Total gly cogen phosphorylase activity increased from 54.76 +/- 2.33 to 181.14 /- 5.79 mu mol/g/h and phosphorylase a from 3.45 +/- 0.45 to 63.73 +/- 1.41 mu mol/g/h, from postnatal day 7 to 18, respectively. In 7-day-o ld pups phosphorylase a corresponded to only 6% of total activity. At the onset of seizures a marked rise (34-90%) in active phosphorylase o ccurred in all age groups. Thus, in the brains of immature animals a r apid conversion of phosphorylase b to a can occur in response to incre ased cellular activity. However, in 7-day-old rats, in spite of marked activation, phosphorylase a remained very low (6.0 +/- 0.42 mu mol/g/ h) and can thus explain the slow onset of glycogenolysis in this age g roup. Cyclic AMP levels remained unchanged at the onset of seizures in 7- and 12-day-old pups, and only a mild (+25%) rise was observed in 1 8-day-old rats. The marked increase of phosphorylase a in 7- and 12-da y-old rats thus occurred in the presence of unchanged levels of cAMP, suggesting the involvement of cAMP-independent mechanism of activation , in which Ca2+ most probably plays a role.