Rostral ventromedial medulla modulates nociception and tonic immobility behavior through connections with the A7 catecholaminergic region.

Cholinergic stimulation of the rostral ventromedial medulla (RVM) produces antinociception and reduces the duration of tonic immobility (TI) behavior in guinea pigs. Previous studies indicated that cholinergic antinociception in the RVM is mediated through connections with the A7 catecholaminergic cell group (A7). In the current study, we tested the role of the A7 in both the antinociception and reduction of TI duration mediated by cholinergic stimulation of the RVM. In addition, we used biotinylated dextran amines (BDA) to evaluate the connections between the RVM and A7. The microinjection of the cholinergic agonist carbachol into the RVM produced antinociception and reduced TI behavior duration. These effects were blocked by prior administration of lidocaine to the A7. However, the microinjection of lidocaine into the A7 prior to saline injection into the RVM had no effect on either the nociceptive or TI responses. The microinjection of the neurotracer BDA into the RVM positively stained fibers and synaptic boutons in the A7, indicating that there are direct projections from the RVM to the A7. Taken together, our results indicate that the antinociception and reduction of TI behavior duration after cholinergic stimulation of the RVM depends on connections with the A7.