[H-3] ADENOSINE TRANSPORT IN RAT DORSAL BRAIN-STEM USING A CRUDE SYNAPTOSOMAL PREPARATION

The neuromodulator adenosine, has been shown to have the highest densi ty of central uptake sites in the nucleus tractus solitarius in the do rsal brain stem. The nucleus tractus solitarius is involved in the cen tral regulation of reflex cardiovascular activity suggesting that aden osine may also be involved in central cardiovascular control. Thus, th e present study has characterized the transport of [H-3]adenosine into rat dorsal brain stem synaptosomes. The process was found to be satur able and highly dependent on temperature. Furthermore, [H-3]adenosine transport in rat dorsal brain stem synaptosomes was Ear more sensitive to the removal of sodium ions than has been previously reported for r at cortical synaptosomes. In addition transport was rapid, being linea r for at least 30 s at 37 degrees C, reaching equilibrium within 1 min and had an apparent K-m, value of 2.7+/-0.2 mu M (n = 4) and a V-max value of 135.5+/-17.8 pmol/mg protein/min (n = 4). These kinetic param eters are within an order of magnitude of adenosine uptake processes f ound in rat cerebral cortical synaptosomes. Transport of [H-3]adenosin e was significantly inhibited by an excess of unlabelled adenosine (1 mM) and the adenosine uptake inhibitor S(4-nitrobenzyl)-6-thioinosine (100 mu M) while morphine (150 mu M) and flurazepam (150 mu M) were la rgely ineffective as inhibitors of the process, in contrast with previ ous findings in rat cortical synaptosomes. The present findings demons trate the presence of a high affinity transport system for adenosine i n the rat dorsal brain stem which appears to differ in some properties to uptake processes found in rat cortex.