Operant behavior and alcohol levels in blood and brain of alcohol-dependent rats

Abstract

Background: The purpose of the present investigation was to more clearly define blood-alcohol parameters associated with alcohol dependence produced by alcohol vapor inhalation and alcohol-containing liquid diet.

Methods: Alcohol levels in blood and brain were compared during and after 4 hours of acute alcohol vapor exposure; also, brain-alcohol levels were assessed in alcohol-exposed (14-day alcohol vapor) and alcohol-naïve rats during and after 4 hours of acute alcohol vapor exposure. A separate group of rats were implanted with i.v. catheters, made dependent on alcohol via vapor inhalation, and tested for operant alcohol responding; blood-alcohol levels (BALs) were measured throughout operant alcohol drinking sessions during alcohol withdrawal. A final group of rats consumed an alcohol-liquid diet until they were dependent, and those rats were then tested for operant behavior at various withdrawal time points; BALs were measured at different withdrawal time points and after operant sessions.

Results: Blood- and brain-alcohol levels responded similarly to vapor, but brain-alcohol levels peaked at a higher point and more slowly returned to zero in alcohol-naïve rats relative to alcohol-exposed rats. Alcohol vapor exposure also produced an upward shift in subsequent operant alcohol responding and resultant BALs. Rats consumed large quantities of alcohol-liquid diet, most of it during the dark cycle, sufficient to produce high blood-alcohol levels and elevated operant alcohol responding when tested during withdrawal from liquid diet.

Conclusions: These results emphasize that the key determinants of excessive alcohol drinking behavior are the BAL range and pattern of chronic high-dose alcohol exposure.

Fig. 1

Mean ± SEM alcohol levels (mg%) in brain (open circles) and blood (closed circles) of previously alcohol-naïve rats (n=5) 30 minutes prior to alcohol vapor exposure, during 4 hours of alcohol vapor exposure (gray area), and 8 hours following termination of alcohol vapor exposure.

Fig. 2

Mean ± SEM brain-alcohol levels (mg%) in previously alcohol-naïve (n=5; open circles) or alcohol-exposed (14 days of intermittent vapor; n=5; closed circles) rats 30 minutes prior to alcohol vapor exposure, during 4 hours of alcohol vapor exposure (gray area), and 8 hours following termination of alcohol vapor exposure. * p<0.05, # p<0.01 significant difference from alcohol-naïve group.

Fig. 3

(A) Mean ± SEM operant responses for 10% w/v alcohol by dependent (n=6; black bars) and non-dependent (n=8; white bars) rats during pre-vapor baseline (average of final three pre-vapor sessions) and following chronic alcohol vapor exposure (average of first two post-vapor sessions). Post-vapor operant test sessions always occurred ~6 hrs following the termination of vapor exposure. (B) Mean ± SEM operant responses for 10% w/v alcohol by high alcohol responders (>12 responses/30 min during baseline period) in 30-min test sessions during the baseline period and also 0, 2, 4, and 6 hrs into withdrawal from liquid diet. Alcohol-dependent rats consumed alcohol-liquid diet (n=5; closed circles) and non-dependent rats consumed control diet (n=5; open circles). Liquid diet bottles were removed from the cage lids of all rats at various time points preceding operant tests.

Fig. 4

(A) Mean ± SEM cumulative operant responses for 10% w/v alcohol by dependent (n=6; black circles) and non-dependent (n=8; white circles) rats in 5-minute bins across the 30-min test session that occurred 6–8 hrs into withdrawal from alcohol vapor, and (B) mean ± SEM resultant blood-alcohol levels (mg%) in those rats 0, 15, 30, and 45 minutes following the start of the operant test session. (C) Mean ± SEM cumulative operant responses for 10% w/v alcohol by dependent (n=5; black circles) and non-dependent (n=5; white circles) rats in 5-minute bins across the 30-min test session that occurred 4 hrs into withdrawal from alcohol liquid-diet, and (D) mean ± SEM resultant blood-alcohol levels (mg%) in those rats 0 and 30 minutes following the start of the operant test session.

Fig. 5

(A) Scatter plot of alcohol-liquid diet intake by alcohol-dependent rats during the first two hrs of the dark cycle and resultant BALs measured two hrs into the dark cycle; (B) mean BALs in alcohol-dependent and –non-dependent rats measured 8 hrs into the light cycle; and (C) mean BALs in alcohol-dependent and –non-dependent rats measured at the start of the dark cycle, following 4 hrs with no liquid diet (i.e. the time point when 4-hr withdrawal operant sessions occurred on behavioral test days).

Fig. 6

Scatter plot of operant responses for 10% w/v alcohol and resultant BALs for (A) dependent and (B) non-dependent rats during the 30-min test session that occurred 4 hrs into withdrawal from alcohol-liquid diet. The scatter plot for the 4-hr time point is representative of scatter plots at the 2-hr and 6-hr time points. Operant alcohol responses were significantly correlated with BALs at all three time points (see text)