Active projections to medullary raphe during conditioned fear in the rat

Abstract

Psychological stress leads to sympathetic and behavioural responses that are mediated by the medullary raphe nuclei (MR). These nuclei receive direct projections from the hypothalamus, which drive the tachycardia, and the periaqueductal gray (PAG), which drives behaviour. To confirm that these projections are active during conditioned fear, we combined fear-induced Fos expression with cholera toxin B retrograde tracing from the MR. Rats were either placed for 30 min in a context where they had previously experienced footshocks (conditioned fear, N=5) or left undisturbed in their homecages (control, N=5) prior to sacrifice. Relative to the total number of double labelled neurons, most of these were found in the hypothalamus (24.5±6.8%) and PAG (34.2±8.8%). Within the PAG, double labelling was found in the dorsomedial (14.9±5.0%), lateral (9.9±2.4%) and ventrolateral columns (9.4±1.9%). The anterior hypothalamus had about the same number of double labelled neurons (5.6±2.1%) as the dorsomedial (5.8±1.9%), paraventricular (5.8±1.3%) and perifornical nuclei (4.4±2.0%). The prefrontal cortex was the third most labelled region (15.0±3.8%). Double labelling was also found in the midbrain tegmentum (11.3±4.6%), nucleus cuneiformis (5.3±2.4%) and A5 (2.0±0.7%) area. These results confirm that the most important active afferents to MR during conditioned fear are in the hypothalamus and PAG.