[3H]adenosine transport in rat dorsal brain stem using a crude synaptosomal preparation

The neuromodulator adenosine, has been shown to have the highest density of central uptake sites in the nucleus tractus solitarius in the dorsal brain stem. The nucleus tractus solitarius is involved in the central regulation of reflex cardiovascular activity suggesting that adenosine may also be involved in central cardiovascular control. Thus, the present study has characterized the transport of [³H]adenosine into rat dorsal brain stem synaptosomes. The process was found to be saturable and highly dependent on temperature. Furthermore, [³H]adenosine transport in rat dorsal brain stem synaptosomes was far more sensitive to the removal of sodium ions than has been previously reported for rat cortical synaptosomes. In addition transport was rapid, being linear for at least 30 s at 37°C, reaching equilibrium within 1 min and had an apparent K_m value of 2.7 ± 0.2 μM (n = 4) and a V_max value of 135.5 ± 17.8 pmol/mg protein/min (n = 4). These kinetic parameters are within an order of magnitude of adenosine uptake processes found in rat cerebral cortical synaptosomes. Transport of [³H]adenosine was significantly inhibited by an excess of unlabelled adenosine (1 mM) and the adenosine uptake inhibitor S(4-nitrobenzyl)-6-thioinosine (100 μM) while morphine (150 μM) and flurazepam (150 μM) were largely ineffective as inhibitors of the process, in contrast with previous findings in rat cortical synaptosomes. The present findings demonstrate the presence of a high affinity transport system for adenosine in the rat dorsal brain stem which appears to differ in some properties to uptake processes found in rat cortex.