Chemogenetic inhibition of the ventral hippocampus but not its direct projection to the prelimbic cortex attenuates context-specific operant responding

Abstract

Previous work has demonstrated the importance of the prelimbic cortex (PL) in contextual control of operant behavior. However, the associated neural circuitry responsible for providing contextual information to the PL is not well understood. In Pavlovian fear conditioning the ventral hippocampus (vH) and its projection to the PL have been shown to be important in supporting the effects of context on learning. The present experiments used chemogenetic inhibition of the direct vH-PL projection or the vH to determine involvement in expression of context-specific operant behavior. Rats were injected with an inhibitory DREADD (hM4D_i) or mCherry-only into the vH, and subsequently trained to perform a lever press response for a food pellet in a distinct context. The DREADD ligand clozapine-n-oxide (CNO) was then delivered directly into the PL (experiment 1) and then systemically (experiment 2) prior to tests of the response in the training context as well as an equally familiar but untrained context. vH (systemic CNO) but not vH-PL (intra-PL CNO) inhibition was found to attenuate operant responding in its acquisition context. A third experiment, using the same rats, showed that chemogenetic inhibition of vH also reduced Pavlovian contextual fear. The present results suggest that multisynapatic connections between the vH and PL may be responsible for integration of contextual information with operant behavior.

Keywords: DREADD; Pavlovian; context; operant; prelimbic; ventral hippocampus.

Figure 1

Representative images of DREADD expression within the High DREADD expression group. (A) ventral hippocampus expression (score = 3) at 2.5x magnification with 500 μM scale bar. (B) prelimbic cortex (A32V) expression at 4x magnification with 300 μM scale bar. Cannula tract shown in yellow. (C) prelimbic cortex expression at 40x magnification with 50 μM scale bar.

Figure 2

Representative images of mCherry control expression. (A) Ventral hippocampus expression at 2.5x magnification with 500 μM scale bar. (B) prelimbic cortex (A32V) expression at 2.5x magnification with 500 μM scale bar.

Figure 3

Cannula tip placements of all rats included in analysis. Treatment group is indicated by shape and fill (High DREADD – open square, Low DREADD – open circle, mCherry control – closed circle). Atlas panels adapted from Paxinos and Watson (2014).

Figure 4

Operant Response Acquisition. Response rates (per min) for each group during the initial six and two additional sessions of training. Dotted line indicates when vH-PL inhibition operant tests occurred. Error bars indicate standard error of the mean.

Figure 5

Operant Test Results. vH-PL inhibition (A) and vH inhibition (B) operant test responding as a proportion of baseline (response rate during the acquisition session occurring on the day prior to each test). Error bars indicate standard error of the mean.

Figure 6

Context Fear Test Results. Mean percent time freezing during 4-min bins of the context fear test. Error bars indicate standard error of the mean.