Ethanol-induced c-Fos expression in catecholamine- and neuropeptide Y-producing neurons in rat brainstem

Background: Previous studies have used c-Fos-like immunoreactivity (cFLI) to examine the neuroanatomical location of cells that are activated in response to ethanol administration. However, the use of cFLI alone fails to reveal the phenotypical identity of cells. Tn the present study we used double-labeling procedures to identify the neurochemical phenotype of neurons that showed ethanol-induced cFLI in the rat brainstem. Methods: Individual groups of rats received intraperitoneal injection of ethanol (1.5 g/kg or 3.5 g/kg) or isotonic saline (23 ml/kg). To assess the specificity of cFLI induced by ethanol, we injected other rats with the drug lithium chloride (LiCl; 76 mg/kg).Two hours after injection, rats were killed and their brains were processed for immunohistochemistry. Results: Both doses of ethanol promoted cFLI in several brainstem regions, including the nucleus of the solitary tract (NTS), the locus coeruleus (LC), and the ventrolateral medulla (VLM). Although LiCl caused significant cFTI in the NTS, this drug promoted only minimal cFLI in the VLM and no significant activation in the LC. We found that a significant proportion of tyrosine hydroxylase (TH)-positive neurons coexpressed ethanol-induced cFLI in the VLM (similar to 75-85%), the NTS (similar to 65-75%), and the LC (similar to 30-65%). Additionally, a significant proportion of neuropeptide Y (NPY)-producing neurons in the VLM coexpressed ethanol-induced cFLI (similar to 60-75%). On the other hand, LiCl promoted activation of TW-positive neurons in the VLM and the NTS but failed to stimulate cFLI in TH-producing neurons in the LC or in NPY-producing neurons of the VLM Conclusions: Neurons in the rat brainstem that show ethanol-induced c-Fos expression produce catecholamines and NPY. This research demonstrates the usefulness of double-labeling immunohistochemistry procedures for identifying the neurochemical identity of neurons that are activated after ethanol administration.