Increases in doublecortin immunoreactivity in the dentate gyrus following extinction of heroin-seeking behavior

Abstract

Adult-generated neurons in the dentate gyrus (DG) of the hippocampus play a role in various forms of learning and memory. However, adult born neurons in the DG, while still at an immature stage, exhibit unique electrophysiological properties and are also functionally implicated in learning and memory processes. We investigated the effects of extinction of drug-seeking behavior on the formation of immature neurons in the DG as assessed by quantification of doublecortin (DCX) immunoreactivity. Rats were allowed to self-administer heroin (0.03 mg/kg/infusion) for 12 days and then subjected either to 10 days of extinction training or forced abstinence. We also examined extinction responding patterns following heroin self-administration in glial fibrillary acidic protein thymidine kinase (GFAP-tk) transgenic mice, which have been previously demonstrated to show reduced formation of immature and mature neurons in the DG following treatment with ganciclovir (GCV). We found that extinction training increased DCX immunoreactivity in the dorsal DG as compared with animals undergoing forced abstinence, and that GCV-treated GFAP-tk mice displayed impaired extinction learning as compared to saline-treated mice. Our results suggest that extinction of drug-seeking behavior increases the formation of immature neurons in the DG and that these neurons may play a functional role in extinction learning.

Figure 1

Timelines of procedures in Experiments 1 and 2.

Figure 2

Extinction training increases DCX immunoreactivity in the DG following heroin self-administration (Experiment 1). (a) Active and (b) inactive lever presses during heroin self-administration and extinction. Animals undergoing extinction (n = 15) were subjected to 10 days of extinction training following the last day of heroin self-administration, while animals undergoing forced abstinence (n = 13) remained in the home cage. *P < 0.05 versus the average of the last two days of heroin self-administration. (c) and (d) Representative DCX immunostaining at 400x magnification in the DG (~3.8 mm posterior to bregma) from animals subjected to extinction (c) or abstinence (d). Scale bar = 35 μm. (e) Quantification of DCX immunoreactivity along the DG neuraxis in a subset of animals subjected to extinction or abstinence (n = 8 per group). *P < 0.05 versus the abstinence group at the same coronal plane relative to bregma.

Figure 3

Heroin self-administration and extinction in GFAP-tk mice implanted with minipumps containing either saline or GCV. (a) Active lever presses during 10 days of heroin self-administration followed by 5 days of extinction. Mice treated with GCV (n = 11) displayed increased extinction responding during the first 3 extinction days (E1–E3) as compared to mice that were treated with saline (n = 9). *P < 0.05 versus saline-treated mice on the same day of extinction. (b) Inactive lever presses during 10 days of heroin self-administration followed by 5 days of extinction.