Aggressive behavior, increased accumbal dopamine, and decreased cortical serotonin in rats

Abstract

Dopamine (DA) and serotonin have been implicated in the regulation of aggressive behavior, but it has remained challenging to assess the dynamic changes in these neurotransmitters while aggressive behavior is in progress. The objective of this study was to learn about ongoing monoamine activity in corticolimbic areas during aggressive confrontations in rats. Male Long-Evans rats were implanted with a microdialysis probe aimed at the nucleus accumbens (NAC) or medial prefrontal cortex (PFC); next, 10 min samples were collected before, during, and after a 10 min confrontation. Rats continued to display aggressive behavior while being sampled, and they performed two to six attack bites as well as 140 sec of aggressive acts and postures. Dopamine levels in NAC were significantly increased up to 60 min after the confrontation. Peak levels of 140% were achieved approximately 20-30 min after the confrontation. No concurrent changes in accumbal serotonin levels were seen during or after the confrontation. Dopamine and serotonin levels in PFC changed in the opposite direction, with a sustained decrease in serotonin to 80% of baseline levels during and after the confrontation and an increase in dopamine to 120% after the confrontation. The temporal pattern of monoamine changes, which followed rather than preceded the confrontation, points to a significant role of accumbal and cortical DA and 5-hydroxytryptamine in the consequences as opposed to the triggering of aggressive acts. The increase in accumbal DA in aggressive animals supports the hypothesis that this neural system is linked to the execution of biologically salient and demanding behavior.

Fig. 1.

Histological representation of probe placements in NAC. Coronal sections are reproduced from Paxinos and Watson (1997).Vertical bars represent the 2 mm exposed membrane of each microdialysis probe. Probes were implanted at random in the left or right hemisphere. Asterisks indicate probe placements that were excluded from data analysis.

Fig. 2.

Dopamine and serotonin extracellular concentrations in NAC (A; n = 18) or PFC (B; n = 15) of male resident rats. Ten minute samples were collected 50 min before, during, and 80 min after a confrontation with a smaller male intruder. Thevertical gray bar indicates the 10 min period of actual physical confrontation. Filled diamonds, Serotonin;open circles, dopamine. Asterisksindicate a significant change from baseline levels, as assessed by planned paired t tests (*p < 0.05; **p < 0.01).

Fig. 3.

Histological representation of probe placements in PFC. Coronal sections are reproduced from Paxinos and Watson (1997).Vertical bars represent the 3 mm exposed membrane of each microdialysis probe. See Figure 1.