In Vivo Occupancy of D2 Dopamine Receptors by Nafadotride

Abstract

Nafadotride has been proposed as a selective antagonist for the D3 dopamine receptor. This drug has been shown to exhibit selectivity between D2 and D3 dopamine receptors in in vitro assay systems; however, the in vivo D2/D3 selectivity of the compound has not been determined. In this study, protection against inactivation by EEDQ was used as a measure of in vivo occupancy of D2 receptors by behaviorally relevant doses of nafadotride (0.1-10 mg/kg, s.c. and i.p.) in adult, male Sprague-Dawley rats. Ex vivo [3H]spiperone binding was then determined in striatal membranes, l-Nafadotride (10 mg/kg) protected 71% of D2 receptors after s.c. administration; 40% after i.p. administration. Protection of 13% of D2 receptors was observed at a dose of 3 mg/kg (s.c.). These data suggest that blockade of D2 receptors contributes to the pharmacological effects of nafadotride when administered at doses above 1 mg/kg (s.c.) or 3 mg/kg (i.p.).

Altered Behavioral Response to Dopamine D3 Receptor Agonists 7-OH-DPAT and PD 128907 Following Repetitive Amphetamine Administration

“…In dose-ranging studies of 7-OH-DPAT and PD 128907 in rat, we have been unable to detect locomotor inhibition at doses lower than those used in the current study (Richtand et al, unpublished observations). This suggests that dose ranges for mice and rats are comparable, consistent with previously published studies evaluating D3-selective doses delivered via subcutaneous injection in adult male Sprague-Dawley rats (Levant et al, 1996). Animals were individually housed and treated in home cages for 5 consecutive days with either saline or AMPH (1 mg/kg) ( Table 2).…”

Behavioral and neuropharmacological characterization of nicotine as a conditional stimulus

“…Because we wanted to parse apart the potential dopamine D 2 and D 3 receptor effects, we did not use doses higher than 3 mg/kg nafadotride. In male Sprague-Dawley rats, doses higher than 3 mg/kg cause a loss of dopamine D 3 receptor specifi city due to increased dopamine D 2 receptor binding (Audinot et al, 1998 andLevant andVansell, 1997). These fi ndings are similar to operant drug discrimination work in which dopamine has a minimal role in the function of nicotine as a discriminative stimulus , Le Foll et al, 2005, Mansbach et al, 1998, Reavill and Stolerman, 1987and Shoaib, 1998.…”

“…Dopamine D 3 receptor antagonists have been shown to block expression of nicotine-conditioned place preference (Le Foll et al, 2005) and nicotine-conditioned hyperactivity (Le Foll et al, 2003). Although the effects of the dopamine D 3 receptor antagonist nafadotride have not been explicitly studied with nicotine, its specifi city in vivo makes it a useful tool for studying potential dopamine D 3 -mediated behaviors (Audinot et al, 1998 andLevant andVansell, 1997). In brief, given the clear effects of nicotine on dopamine, we examined the potential role of dopamine D 1 , D 2 , and D 3 receptors in mediating nicotine's ability to function as a conditional stimulus.…”

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

“…For example, behavioral characterizations of both 7-OH-DPAT and PD128907 up to 10 mg/kg revealed U-shaped dose-response curves for both compounds [2,71,228], suggesting activation of D 3 receptors at low doses and increasing D 2 receptor occupancy at higher doses. This hypothesis is further supported by studies based upon D 2 receptor protection from N-ethoxycarbonyl-2-ethoxyl-1,2-dihydro-quinilone (EEDQ) alkylation, suggesting that 7-OH-DPAT doses below 0.3 mg/kg are devoid of significant D 2 receptor occupancy [181]. Similarly, studies using DA D 3 knockout mice showed that intraperitoneal (ip) doses of PD128907 in the range of 0.03-0.1 mg/kg affect DA release in wild type, but not knockout mice, suggesting D 3 -mediated effects of PD128907 when given at sufficiently low dose [314].…”

“…In support of the first argument (lack of selectivity) are the findings described in the previous section (i.e., radioligand binding studies vs. in vitro functional assays). In addition to lack of D 3 receptor selectivity under in vitro assay conditions, both 7-OH-DPAT and PD128907 may activate D 2 receptors in vivo in rats as a function of the doses used in different behavioral paradigms [2,19,36,71,73,95,146,159,180,181,203,228,293]. For example, behavioral characterizations of both 7-OH-DPAT and PD128907 up to 10 mg/kg revealed U-shaped dose-response curves for both compounds [2,71,228], suggesting activation of D 3 receptors at low doses and increasing D 2 receptor occupancy at higher doses.…”