Behavioral control over shock blocks behavioral and neurochemical effects of later social defeat

Abstract

Experience with behavioral control over tailshock (escapable shock, ES) has been shown to block the behavioral and neurochemical changes produced by later uncontrollable tail shock (inescapable shock, IS). The present experiments tested, in rats, whether the protective effect of control over tailshock extends beyond reducing the behavioral and neurochemical impact of a subsequent tailshock experience to stressors that are quite different. Social defeat (SD) was chosen as the second stress experience because it has few if any cues in common with tailshock. SD produced shuttlebox escape learning deficits ("learned helplessness") and reduced juvenile social investigation 24 h later, as does IS. IS is notable for inducing a large increase in dorsal raphe nucleus (DRN) serotonergic (5-HT) activity as measured by extracellular levels of 5-HT within the DRN, and SD did so as well. ES occurring 7 days before SD blocked this SD-induced DRN activation, as well as the SD-induced interference with shuttlebox escape and reduction in social investigation. Prior exposure to yoked IS did not reduce the DRN 5-HT activation or later behavioral effects produced by SD, and thus the proactive stress-blunting effects of ES can be attributed to it's controllability. Thus, ES confers a very general protection to the impact of a subsequent stress experience.

Figure 1

Schematic representation of microdialysis cannula placements within the DRN (black bars). Numerals indicate in mm distance from bregma.

Figure 2

Attack/defeat Index (ADI) during social defeat (SD). HCC, ES and IS were treatments administered 7 days earlier. *Indicates that HCC is different from IS (P<0.02).

Figure 3

Mean shuttlebox escape latencies across blocks of 5 shuttlebox FR-2 escape trials 24 h after SD or HC, in groups of rats expose 8 days earlier to ES (ES/SD), IS (IS/SD), HC (HC/HC) or HC (HC/SD). ES/SD and HC/HC are different from IS/SD and HC/SD (P<0.05).

Figure 4

Juvenile social exploration in sec, 24 h after SD or HC, in groups of rats exposed 8 days earlier to ES (ES/SD), IS (IS/SD), HC (HC/HC) or HC (HC/SD). ES/SD and HC/HC are different from IS/SD and HC/SD (P<0.05).

Figure 5

5-HT in the DRN as a percentage of baseline before, during and after social encounters either involving social defeat (SD, squares) or without SD (No Defeat, circles). **And * indicate that SD is different from No Defeat with P<0.005 and P<0.05 respectively.

Figure 6

5-HT in the DRN as a percentage of baseline before, during and after social defeat (SD) in groups of rats exposed 7 days earlier to ES (ES/SD), IS (IS/SD) or HC (HC/SD). ** And * indicate that IS/SD and HC/SD are different from ES/SD with P<0.01 and P<0.02 respectively.