Background: The objective of this study was to determine time-course changes in in vivo ethanol (EtOH) concentrations using a novel subcutaneous (s.c.) microdialysis sampling technique. The hypothesis to be tested was that EtOH concentrations in the s.c. fluid would reflect blood EtOH concentrations. If this is the case, then s.c. microdialysis could allow a more detailed analysis of changes in in vivo levels of EtOH under different drinking paradigms.
Methods: Adult male and female Wistar rats and male alcohol-preferring (P) rats were used in this study. A loop-style microdialysis probe was designed for s.c. applications. After initial in vitro characterization, probes were implanted under the skin between the shoulder blades. Animals were allowed to recover 4 to 24 hours prior to microdialysis collection (2.0 microl/min flow rate with isotonic saline). In vivo microdialysis experiments were then conducted to determine (i) the extraction fraction (or clearance) using EtOH no-net-flux (NNF) coupled with the alcohol clamp method, (ii) the dose-response and time-course effects after systemic EtOH administration and to compare with blood EtOH levels, and (iii) the time-course changes in EtOH levels during and after an EtOH drinking episode.
Results: In vivo probe recovery (extraction fraction) obtained using the alcohol clamp method was 69 +/- 3%, and was comparable to the in vitro recovery of 73 +/- 2%. For the EtOH dose-response experiment, rats injected i.p. with 0.5, 1.0, or 2.0 g/kg EtOH showed a clear dose-response effect in the s.c. dialysate samples. Peak concentrations (70, 123, and 203 mg%, respectively) were reached by 15 minutes after injection. In an experiment comparing levels of EtOH in s.c. dialysis and arterial blood samples in rats administered 1.0 g/kg EtOH, similar time-course changes in in vivo EtOH concentrations were observed with both i.g. and i.p. EtOH administration. In P rats drinking 15% EtOH during a 1-hour scheduled access period, EtOH levels in s.c. microdialysates rose rapidly over the session and peaked at approximately 50 mg% at 60 to 80 minutes.
Conclusions: Overall, these experiments indicate that s.c. EtOH and blood EtOH concentrations follow a similar time course. Moreover, s.c. microdialysis can be useful as an experimental approach for determining detailed time-course changes in in vivo EtOH concentrations associated with alcohol drinking episodes.