Relations between stimulation of mesolimbic dopamine and place conditioning in rats produced by cocaine or drugs that are tolerant to dopamine transporter conformational change

Abstract

Rationale: Dopamine transporter (DAT) conformation plays a role in the effectiveness of cocaine-like and other DAT inhibitors. Cocaine-like stimulants are intolerant to DAT conformation changes having decreased potency in cells transfected with DAT constructs that face the cytosol compared to wild-type DAT. In contrast, analogs of benztropine (BZT) are among compounds that are less affected by DAT conformational change.

Methods: We compared the displacement of radioligand binding to various mammalian CNS sites, acute stimulation of accumbens shell dopamine levels, and place conditioning in rats among cocaine and four BZT analogs with Cl substitutions on the diphenyl-ether system including two with carboalkoxy substitutions at the 2-position of the tropane ring.

Results: Binding assays confirmed high-affinity and selectivity for the DAT with the BZT analogs which also produced significant stimulation of mesolimbic dopamine efflux. Because BZT analogs produced temporal patterns of extracellular dopamine levels different from those by cocaine (3-10 mg/kg, i.p.), the place conditioning produced by BZT analogs and cocaine was compared at doses and times at which both the increase in dopamine levels and rates of increase were similar to those produced by an effective dose of cocaine. Despite this equilibration, none of the BZT analogs tested produced significant place conditioning.

Conclusions: The present results extend previous findings suggesting that cocaine-like actions are dependent on a binding equilibrium that favors the outward conformational state of the DAT. In contrast, BZT analogs with reduced dependence on DAT conformation have reduced cocaine-like behavioral effects and may prove useful in development of medications for stimulant abuse.

Figure 1

Chemical structures of cocaine and the BZT analogs studied.

Figure 2

Forebrain sections, redrawn from Paxinos and Watson, 1998, showing the limits of the positions of the dialyzing portions of the microdialysis probes (superimposed rectangles) resulting from histological analysis of brain sections. On each section the anterior coordinate (A) (measured from bregma) is indicated. CPU=caudate putamen; shell=nucleus accumbens shell.

Figure 3

Place conditioning and stimulation of extracellular levels of DA in the NAC shell produced by various doses of cocaine. Ordinates: on the left, time allocated to the compound-paired side during the post-conditioning session expressed as a difference from that during the last preconditioning session. Ordinates on the right also show average changes in extracellular DA levels expressed as a percentage of baseline during the first 30-min after compound injection, corresponding to the time at which place conditioning sessions were conducted with cocaine. Abscissae: dose of compound in mg/kg. The saline and 10.0 mg/kg cocaine data are presented as means and SEM values determined in 52 subjects used over the course of all of the place-conditioning studies. The place conditioning data for the remainder of the cocaine doses are means and SEM values from six subjects. Basal DA values (fmoles/sample) and group size (n) were: 65.3 ± 8.3 (4), 39.8 ± 6.8 (7), and 58.9 ± 6.1 (6), for saline, 3.0 or 10.0 mg/kg cocaine groups, respectively.

Figure 4

Time course of effects of systemic administration of 4,4-diCl-BZT (panel A), MFZ 4-86 (Panel B) and MFZ 4-87 (Panel C) on extracellular levels of DA in dialysates from the NAC shell. Results are means, with vertical bars representing SEM, of the amount of DA in 10-min dialysate samples, expressed as percentage of basal values, uncorrected for probe recovery. Basal DA values (fmoles/sample) and group size (n) were: 46.0 ± 1.8 (7), 47.9 ± 8.7 (7), and 51.4 ± 7.6 (8) for 4,4-diCl-BZT doses of 1, 3 and 10 mg/kg, respectively; 48.7 ± 12.3 (6), 51.3 ± 5.1 (8), and 41.9 ± 4.3 (12) for MFZ 4-86 doses of 1, 3 and 10 mg/kg, respectively; 43.4 ± 2.3 (4), 33.3 ± 3.9 (6), and 36.0 ± 4.8 (7) for MFZ 4-87 doses of 1, 3 and 10 mg/kg, respectively.

Figure 5

Place conditioning results for the chloro-substituted BZT analogs administered to rats at different times before placement in the conditioning chamber. Ordinates: time allocated to the compound-paired side during the post-conditioning session expressed as a difference from that during the last preconditioning session. Abscissae: dose of compound in mg/kg. Panels A, D, G, and K show results from place conditioning when the compounds were given immediately before conditioning sessions. Panels B, E, H, and L show results from the 45-min pretreatment before conditioning sessions. Panels C, F, J, and M show results for the 90-min pretreatment conditioning sessions. Data are presented as means and SEM values of from 8 or 16 subjects. The results of place conditioning with saline or 10.0 mg/kg of cocaine administered immediately before testing are shown by the open and filled bars, respectively. Only the effects of cocaine were statistically significant.

Figure 6

Extracellular levels of DA as a function of dose of the chloro-substituted BZT analogs, during the place-conditioning time periods. Ordinates: extracellular DA levels in the NAC shell as a percentage of baseline during 30-min periods corresponding to the times after compound administration during which place conditioning sessions were conducted (pretreatment times of 0, 45, or 90 min). Abscissae: dose of compound in mg/kg, log scale. For comparison, the dashed horizontal line shows the effect (± SEM) of cocaine on extracellular DA levels as a percentage of baseline at the lowest dose that produced a significant place conditioning (3.0 mg/kg) during the time after injection (0-30 min) at which the positive effect was obtained. Each point represents the average effect (± SEM) determined in four to 12 rats.

Figure 7

Rates of increase in DA extracellular levels in dialysates from the NAC shell produced by the chloro-substituted BZT analogs. Ordinates: rates of increase, percentage/minute in DA levels during the two phases identified on the time course of effects on DA levels described in Figure 4. Abscissae: dose of compound in mg/kg, log scale. For comparison, the dashed horizontal line shows the effect (± SEM) of cocaine on rate of increase in extracellular DA levels during phase 1 at the lowest dose tested that produced significant place-conditioning. Each point represents the average effect (±SEM) determined in four to 12 rats.