ReviewHigh-anxiety rats are less sensitive to the rewarding affects of amphetamine on 50Â kHz USV

- Amphetamine enhances appetitive 50 kHz vocalisation in rats.
- Amphetamine's effect is stronger in low anxiety (LR) than in high anxiety rats (HR).
- LR had increased basal levels of dopamine in the basolateral amygdala (BLA).
- LR had increased dopamine metabolism in the BLA, in response to aversive context.

This study assessed behaviour, as measured by 50 kHz calls related to positive affect, in rats with different fear conditioned response strengths: low-anxiety rats (LR) and high-anxiety rats (HR), after amphetamine injection in a two-injection protocol (TIPS). The results showed that the first dose of amphetamine evoked similar behavioural effects in frequency-modulated (FM) 50 kHz calls in the LR and HR groups. The second injection of amphetamine resulted in stronger FM 50 kHz calls in LR compared with HR rats. The biochemical data ('ex vivo' analysis) showed that the LR rats had increased basal levels of dopamine in the amygdala, and increased homovanilic acid (HVA), dopamine's main metabolite, in the amygdala and prefrontal cortex compared with HR rats. The 'in vivo' analysis (microdialysis study) showed that the LR rats had increased HVA concentrations in the basolateral amygdala in response to an aversively conditioned context. Research has suggested that differences in dopaminergic system activity in the amygdala and prefrontal cortex may be one of the biological factors that underlie individual differences in response to fear stimuli, which may also affect the rewarding effects of amphetamine.