Long-term memory for pavlovian fear conditioning requires dopamine in the nucleus accumbens and basolateral amygdala

Abstract

The neurotransmitter dopamine (DA) is essential for learning in a pavlovian fear conditioning paradigm known as fear-potentiated startle (FPS). Mice lacking the ability to synthesize DA fail to learn the association between the conditioned stimulus and the fear-inducing footshock. Previously, we demonstrated that restoration of DA synthesis to neurons of the ventral tegmental area (VTA) was sufficient to restore FPS. Here, we used a target-selective viral restoration approach to determine which mesocorticolimbic brain regions receiving DA signaling from the VTA require DA for FPS. We demonstrate that restoration of DA synthesis to both the basolateral amygdala (BLA) and nucleus accumbens (NAc) is required for long-term memory of FPS. These data provide crucial insight into the dopamine-dependent circuitry involved in the formation of fear-related memory.

Figure 1. Selective restoration of TH in virally-rescued DD mice.

A–E) Immunohistochemistry (IHC) results for a control mouse. A) Tyrosine hydroxylase (TH) stain in the nucleus accumbens (NAc). B) 40x magnification of TH stain. C) Stain for dopamine transporter (DAT) in NAc. D) Merged image of TH and DAT stain showing extensive overlap of the two signals. E) TH stain in basolateral amygdala (BLA) shown at 20x. F–J) IHC results for a non-rescued DD mouse. F) Complete absence of TH in NAc. G) 40x magnification demonstrating lack of TH in NAc. H) DAT stain. I) Merged image of TH and DAT illustrating lack of TH in the NAc. J) TH in BLA (20x) is almost undetectable. K–O) Representative IHC from NAc-rescued DD mouse. K) TH was largely restored in the NAc. L) 40x magnification illustrating rescue of TH. M) DAT staining in NAc-rescued DD mouse. N) Merged image of TH and DAT illustrating large extent of TH restoration in NAc. O) TH in BLA (20x) remains at low, non-injected levels. P–T) Representative IHC from DD mouse injected into the NAc and BLA. P) Robust restoration of TH to the NAc. Q) 40x magnification demonstrating TH rescue. R) DAT staining. S) Merged image of TH and DAT showing extensive restoration of TH to NAc. T) TH is restored to higher levels in NAc and BLA rescued DD mice.

Figure 2. Restoration of DA to both the NAc and BLA is sufficient for LTM for FPS.

A) Startle response curves for all three groups of mice (control, n = 24; NAc alone, n = 13; Nacc+BLA, n = 9) illustrating intact startle responses in all rescue groups. B) Prepulse inhibition is intact in all groups of mice. C) LTM for fear-potentiated startle is restored to control levels in NAc and BLA-rescued DD mice but not in NAc-alone rescues. * = p<0.05, Bonferroni post-test. D) Behavioral response to footshock was the same in all groups.