Imaging of alcohol-induced dopamine release in rats:preliminary findings with [(11) C]raclopride PET

Abstract

Microdialysis studies report that systemic alcohol increases extracellular dopamine (DA) in the rat striatum. The present study examined whether changes in striatal DA could be detected in rats using small animal positron emission tomography (PET). PET images were acquired in 44 alcohol-naïve male Wistar and alcohol-preferring (P) rats. Subjects received up to three [(11) C]raclopride scans (rest, alcohol, and saline). Animals were anesthetized with isoflurane and secured on a stereotactic-like holder during all scans. Blood samples were collected from the tail or lateral saphenous vein of 12 animals 10 min after tracer injection for determination of blood alcohol concentration (BAC). Time activity curves were extracted from the striatum and the cerebellum and binding potential (BP(ND) ) was calculated as a measure of D(2) receptor availability. Wistars given 1.0 g kg(-1) alcohol (20%v/v) i.v. or 3.0 g kg(-1) alcohol (20%v/v) i.p. showed significant alcohol-induced decreases in BP(ND) . In P rats (given 1.5, 2.25, or 3.0 g kg(-1) alcohol), no individual group showed a statistical effect of alcohol on BP(ND) , but taken together, all P rats receiving i.p. alcohol had significantly lower BP(ND) than rest or saline scans. Large decreases in BP(ND) were primarily observed in rats with BAC above 200 mg%. Also, a significant difference was found between baseline BP(ND) of Wistars who had undergone jugular catheterization surgery for i.v. alcohol administration and those who had not. Preliminary results suggest that alcohol-induced DA release in the rat striatum is detectable using small animal PET given sufficiently large cohorts and adequate blood alcohol levels.

Fig. 1

Experimental timeline. Once anesthetized, animals were secured in a stereotactic-like holder. Anesthesia was maintained for the 75 minute scan. Once the animal was secured to the holder, rat and holder were placed in the scanner and anesthesia was reduced (from 5% to 1.5% isoflurane). For alcohol scans, animals received either an I.P. injection 5 minutes before the scan started or a 5 minute I.V. infusion 5 minutes after the scan started.

Fig. 2

(Top) Axial slice of an [¹¹C]raclopride image of a rat brain showing left and right striata. (Bottom) Time activity curves from one animal given 3g/kg alcohol. Striatum curves shown are an average of the left and right striata. This animal received similar activity and mass doses of [¹¹C]raclopride in each scan (rest and alcohol) and did not gain an appreciable amount of weight between scans (< 8% increase in body weight).

Fig. 3

Effect of alcohol and saline injections on striatal DA concentration in Wistar and P rats. Bars are average ${\text{BP}}_{\text{ND}}^{\text{Condition}}$ of each group + SEM. A significant effect of alcohol was seen in WIS-IV-1G and WIS-IP-3G (**, p ≤ 0.01). Saline injection did not produce a significant $\mathrm{\Delta }{\text{BP}}_{\text{ND}}^{\text{Saline}}$ in any group. No data is available for WIS-IV-1G because no saline scans were taken.

Fig. 4

Effect of alcohol and saline injections on striatal DA concentration in P rats. Bars are average ${\text{BP}}_{\text{ND}}^{\text{Condition}}$ of all P rats regardless of alcohol dose (n = 27) + SEM. Alcohol significantly (*, p ≤ 0.05) decreased BP_ND.

Fig. 5

Comparison between BAC and alcohol dose in Wistar rats (n = 2, open diamonds) and P rats (n = 10, filled diamonds) given various doses of alcohol I.P. (1.5g/kg, 2.25g/kg, 3g/kg). Pearson correlation was not significant (p = 0.18).

Fig. 6

Correlation between $\mathrm{\Delta }{\text{BP}}_{\text{ND}}^{\text{Alcohol}}$ and BAC of Wistars (n = 2, open diamonds) and P rats (n = 10, filled diamonds) given various doses of alcohol I.P. (1.5g/kg, 2.25g/kg, 3.0g/kg). Correlation coefficient for all rats (n = 12) was 0.54 (p = 0.07).

Fig. 7

Comparison of average rest BP_ND for (Wistars) with surgically implanted ports (n = 7) and (Wistars) without ports (n = 11). Error bars are SEM (**, p ≤ 0.01, two-tailed homoscedastic t-test).

Fig. 8

Comparison of average rest BP_ND for Wistars (n = 11) and P rats (n = 28). Error bars are SEM (*, p ≤ 0.05, two-tailed homoscedastic t-test).