Theophylline concentration in the extracellular space of the rat brain: measurement by microdialysis and relation to behaviour.

The free extracellular concentration of theophylline in the brain was estimated from microdialysis samples. Two different methods were used to estimate extracellular concentrations by microdialysis, the perfusion rate method and the difference method. Theophylline 20 mg/kg (s.c.) gave a sufficiently stable level of theophylline in the brain 60 min after injection and lasting over the observation period to allow application of the two methods in vivo. The relation between dose and dialysate concentration was linear. It was found that doses of 20-24 mg/kg theophylline corresponded to a free extracellular concentration of 60-90 microM. The behaviour of theophylline-treated rats was assessed in parallel experiments by means of a holeboard apparatus. Behavioural activation was observed in the dose-range 3-30 mg/kg. It is concluded that behavioural effects of theophylline can be induced at a concentration well below that required to inhibit phosphodiesterase but within the range in which adenosine receptor blockade may be observed, suggesting that the latter mechanism is responsible for the behavioural effects of theophylline.