The effect of adenosine and caffeine on paragigantocellularis (PGi) nucleus neurons in morphine-dependent rats

In this study the effect of adenosine and caffeine on spontaneous activity of paragigantocellularis (PGi) neurons was investigated. The spontaneous activity of PGi neurons was significantly decreased by microinjection of adenosine (10 nM, 0.5 µl) into PGi nucleus of both control and morphine-dependent rats. The decrease in firing rate of PGi neurons of morphine-dependent rats was greater than that of control. There was also significant enhancement of spontaneous activity of PGi neurons 8-15 min after caffeine administration (50 mg/kg i.p.) in both control and morphine-dependent rats. However, the effect of caffeine in morphine-dependent rats was higher than that of control. These data suggest that there is an increase in the sensitivity to chemicals, which interact with adenosine receptors in morphine- dependent rats. Nevertheless, considering a common second messenger system (cAMP) for adenosine (A1) and opioid (µ) receptor, it is proposed that up-regulation and hypersensitivity of A1 adenosine receptors are responsible for these results in morphine-dependent rats.