Endomorphin‐2‐immunoreactive fibers selectively appose serotonergic neuronal somata in the rostral ventral medial medulla

The rostral portion of the ventral medial medulla (RVM) is a crucial site for the supraspinal antinociceptive actions of opioids. Previous studies have reported that serotonergic antagonists block the analgesia induced by microinjection of morphine into the RVM (Hammond and Yaksh [1984] Brain Res 298:329–337) and that spinally projecting serotonergic RVM neurons express μ‐opioid receptors (MOR) (Kalyuzhny et al. [1996] J Neurosci 16:6490–6503; Wang and Wessendorf [1999] J Comp Neurol 404:183–196). In addition, axons immunoreactive for the endogenous MOR ligand endomorphin‐2 (Tyr‐Pro‐Phe‐Phe‐NH2) (EM‐2) have been reported to be in the RVM (Martin‐Schild et al. [1999] J Comp Neurol 405:450–471; Pierce and Wessendorf [2000] J Chem Neuroanat 18:181–207). In the present study we examined the relationship of EM‐2‐immunoreactive (EM‐2‐ir) axons to serotonergic and nonserotonergic RVM neurons in rats, including neurons projecting to the dorsal spinal cord. We also examined the origins of EM‐2‐ir in the RVM. Using unbiased methods we estimated the total number of cells in the RVM to be 1.50 × 10 4 and of these up to 70% were retrogradely labeled from the dorsal spinal cord. EM‐2‐ir fibers apposed both serotonergic and nonserotonergic RVM neuronal profiles. However, serotonergic profiles were significantly more likely to be apposed than nonserotonergic profiles. Thus, although serotonergic neurons comprise a minority of RVM neurons (23% of the total RVM neurons), they appear to be selectively apposed by EM‐2‐ir fibers. We also found that hypothalamic EM‐2‐ir neurons, but not EM‐2‐ir neurons, in the nucleus of the solitary tract projected their axons to the RVM. J. Comp. Neurol. 502:701–713, 2007. © 2007 Wiley‐Liss, Inc.