USE OF A NEWLY DEVELOPED TECHNIQUE TO ISOLATE RAT PINEALOCYTES AND STUDY THE EFFECTS OF ADENOSINE AGONISTS ON MELATONIN PRODUCTION

Recent studies have suggested a role for adenosine in the regulation o f rat pineal melatonin synthesis. The data, however, are conflicting a nd therefore the aim of this study was to characterize adenosine recep tors more fully in vitro by using a range of selective adenosine agoni sts and the adenosine antagonist 8-sulphophenyltheophylline (8-SPT). A simple method for the mechanical separation of rat pinealocytes was d eveloped. Pinealocytes were briefly (15 min) incubated with drugs foll owed by a 4 hr drug-free incubation period after which melatonin conce ntrations in the incubation medium were measured by radioimmunoassay. The beta-adrenoceptor agonist isoprenaline gave a dose-related increas e in melatonin production, demonstrating that this pinealocyte prepara tion technique is suitable to evaluate the effect of drugs on pineal m elatonin synthesis. Our results show that adenosine, N-6-(phenylisopro pyl)adenosine (R-PIA) and 2p-(2-carboxethyl) phenethylamino-5'-N-ethyl carboxamidoadenosine (CGS21680) did not affect melatonin synthesis alo ne or in combination with isoprenaline. However 5'-N-ethylcarboxamidoa denosine (NECA) (100 mu M) potentiated the stimulatory effect of isopr enaline (3 mu M) on pineal melatonin production and this effect appear ed to be antagonized by 8-SPT (50 mu M). These results are consistent with activation by NECA of an A(2b) adenosine receptor.