Selective actions of central μ and κ opioid antagonists upon sucrose intake in sham-fed rats

Intake of a palatable sucrose solution in real-fed rats is mediated in part by central μ and κ opioid receptors. Since general opioid antagonists still inhibit sucrose intake in sham-fed rats, the present study examined whether centrally administered μ (β-funaltrexamine: 5, 20 μg), mu₁ (naloxonazine: 50 μg), κ (nor-binaltorphamine: 1, 5, 20 μg), σ (naltrindole: 20 μg) or σ₁ (DALCE: 40 μg) opioid subtype antagonists altered sucrose intake in sham-fed rats in a similar manner to systemic naltrexone (0.01-1 mg/kg) and whether such effects were equivalent to altering the sucrose concentration. Sucrose (20%) intake in sham-fed rats was significantly and dose-dependently reduced by naltrexone (59%), β-funaltrexamine (44%) and nor-binaltorphamine (62%), but not by naloxonazine, naltrindole or DALCE. The reductions in sham sucrose (20%) intake by general, μ and κ antagonism were similar in pattern and magnitude to diluting sucrose concentration from 20% to 10% in untreated sham-fed rats. Since both real-fed and sham-fed rats share similar patterns of specificity of opioid effects, magnitudes and potencies of inhibition, it suggests that central μ and κ antagonism acts on orosensory mechanisms supporting sucrose intake.