Behavioral and neurobiological effects of the enkephalinase inhibitor RB101 relative to its antidepressant effects

Abstract

Nonpeptidic delta-opioid receptor agonists produce antidepressant-like effects in rodents, and compounds that inhibit the breakdown of endogenous opioid peptides have antidepressant-like effects in animal models. In this study, the behavioral effects of the enkephalinase inhibitor, RB101 (N-[(R, S)-2-benzyl-3-[(S)(2-amino-4-methyl-thio)-butyldithio]-1-oxopropyl]-l-phenylalanine benzyl ester), were examined. Specifically, the effects of RB101 on convulsive activity, locomotor activity, and antidepressant-like effects in the forced swim test were studied in Sprague-Dawley rats, and the opioid receptor types mediating these effects were examined by antagonist studies. In addition, the effects of RB101 on brain-derived neurotrophic factor (BDNF) mRNA expression were evaluated in relation to its antidepressant effects. RB101 produced delta-opioid receptor-mediated antidepressant effects (32 mg/kg i.v. and 100 mg/kg i.p.) and increased locomotor activity (32 mg/kg i.v.) in rats. RB101 did not produce convulsions or seizures and did not alter BDNF mRNA expression. In conclusion, RB101 has the potential to produce antidepressant effects without convulsions.

Fig. 1

The effects of RB101 (i.v.) in the forced swim test in Sprague–Dawley rats (n = 6/dose). A single dose of RB101 was injected 30 min before the forced swim test. The columns and vertical lines represent the mean and S.E.M. for immobility (open columns), swimming counts (single-hatched columns), and climbing counts (double-hatched columns). **P < 0.001 compared with vehicle as determined by Dunnett's post hoc test.

Fig. 2

The effects of RB101 (i.p.) in the forced swim test in Sprague–Dawley rats (n = 5–6/dose). A single dose of RB101 was injected 60 min before the forced swim test. The columns and vertical lines represent the mean and S.E.M. for immobility (open columns), swimming counts (single-hatched columns), and climbing counts (double-hatched columns).

Fig. 3

The effects of naltrindole (A), mu-selective doses of naltrexone (B), or nor-BNI (C) administered (s.c.) prior to 32 mg/kg RB101 (i.v.) in the forced swim test in Sprague–Dawley rats (n = 6/condition). The columns and vertical lines represent the mean and S.E.M. for immobility (open columns), swimming counts (single-hatched columns), and climbing counts (double-hatched columns).

Fig. 4

(A) The effects of RB101 (i.v.) alone on locomotor activity in Sprague–Dawley rats (n = 6/dose). Locomotor activity counts before time 0 were baseline levels of activity before injection that occurred at time 0. (B) The effects of naltrindole or mu-selective doses of naltrexone administered (s.c.) prior to 32 mg/kg RB101 (i.v.) on locomotor activity.

Fig. 5

Representative traces showing the effects of i.v. administration of 3.2 mg/kg SNC80 (A) and 32 mg/kg RB101 (B) on electroencephalographic recordings. The asterisks indicate the time of injection, and the time lines indicate 2-min intervals. Baseline amplitude is 150–200 μV.

Fig. 6

The effects of RB101 (i.v.) administered 10 min prior to a single dose of SNC80 (i.v.) on the convulsive (A) and antidepressant-like effects (B) in Sprague–Dawley rats. For measure of antidepressant-like effects, data are represented as counts of immobility only.

Fig. 7

The effect of 32 mg/kg RB101 (i.v.) ( ), 10 mg/kg naltrindole (s.c.) (( ), and 10 mg/kg naltrindole given 15 min prior to 32 mg/kg RB101 (( ) on BDNF mRNA expression, in comparison to vehicle controls (( ). Quantification of BDNF mRNA signal was obtained from photomicrographs of X-ray films exposed for 14 days after in situ hybridization with the antisense cRNA probe to rat BDNF mRNA. BDNF mRNA levels were obtained from frontal cortex, and three hippocampal subfields, the CA1, CA3, and dentate gyrus regions (panel A), and from the basolateral amygdale (BLA), endopiriform nucleus (EN), and olfactory cortex (Olf. Ctx.) (panel B). Data are expressed as mean±S.E.M.