Fast dopamine release events in the nucleus accumbens of early adolescent rats

Abstract

Subsecond fluctuations in dopamine (dopamine transients) in the nucleus accumbens are often time-locked to rewards and cues and provide an important learning signal during reward processing. As the mesolimbic dopamine system undergoes dynamic changes during adolescence in the rat, it is possible that dopamine transients encode reward and stimulus presentations differently in adolescents. However, to date no measurements of dopamine transients in awake adolescents have been made. Thus, we used fast scan cyclic voltammetry to measure dopamine transients in the nucleus accumbens core of male rats (29-30 days of age) at baseline and with the presentation of various stimuli that have been shown to trigger dopamine release in adult rats. We found that dopamine transients were detectable in adolescent rats and occurred at a baseline rate similar to adult rats (71-72 days of age). However, unlike adults, adolescent rats did not reliably exhibit dopamine transients at the unexpected presentation of visual, audible and odorous stimuli. In contrast, brief interaction with another rat increased dopamine transients in both adolescent and adult rats. While this effect habituated in adults at a second interaction, it persisted in the adolescents. These data are the first demonstration of dopamine transients in adolescent rats and reveal an important divergence from adults in the occurrence of these transients that may result in differential learning about rewards.

Figure 1

Dopamine release events in adolescent and adult rats during the presentation of unexpected stimuli. (A) In adolescent rats dopamine transient frequency did not change from basal rates during presentation of stimuli. In contrast, the frequency of dopamine transients increased at stimulus presentations in adult rats (* p < 0.05 versus baseline). (B) The amplitude of dopamine transients during stimulus presentations did not significantly change from baseline in either age group. Bin size is 0.01 μM. (C) Frequency of dopamine transients in individual adolescent and adult rats before (“Basal”) and during presentations of stimuli (“Stim”). Fewer adolescent rats than adult rats exhibited increased dopamine transients to unexpected stimuli.

Figure 2

Dopamine release events in adolescent and adult rats during brief interaction with a size-matched rat. (A) Dopamine transients were more frequent in both adolescent and adult rats during the first interaction with another rat (“Rat”) versus immediately before (“Basal”). However, only adolescents exhibited a second significant increase in dopamine transient rate during a second interaction 10 min later (bottom). (B) The distribution of dopamine transient amplitudes shifted to higher concentrations during conspecific interaction versus baseline in both age groups. Bin size is 0.01 μM.

Figure 3

Dopaminergic recording sites within the nucleus accumbens core of adolescent and adult rats, shown on representative coronal slices at 1.2 and 1.6 mm anterior to bregma (adapted from Paxinos and Watson, 1986).