Purinergic actions on neurons that modulate nociception in the rostral ventromedial medulla

The rostral ventromedial medulla (RVM) serves as a critical link in bulbo-spinal nociceptive modulation. Within the RVM, 'off-cells' pause and 'on-cells' discharge immediately prior to a nocifensive reflex. These neurons are also activated and inactivated, respectively, by local or systemic application of opioids. Off-cell activation leads to behavioral anti-nociception and on-cell activation to hyperalgesia. Thus, on- and off-cell populations allow bi-directional modulation of nociception by the RVM. A third neuronal population, neutral cells, shows no reflex-related change in discharge. The role of neutral cells in nociception, if any, is unknown. We investigated the responses of on-, off- and neutral cells to the iontophoretic application of purinergic ligands in lightly anesthetized rats. On-cell firing increased rapidly in response to application of ATP and to the P2X-receptor agonist, α,β-methylene ATP. Off-cell firing increased gradually in response to ATP and to the P2Y-receptor agonist, 2-methylthio-ATP. All of these responses were attenuated or reversed by the non-specific P2-receptor antagonists, suramin and pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS). Activation of off-cells was preferentially antagonized by the relatively selective P2Y antagonist, MRS2179. By contrast with activation of on- and off-cells by ATP, neutral cell firing was depressed by ATP, adenosine and the P1-receptor agonist, 5′-(N-ethylcarboxamido) adenosine (NECA). Neutral cell responses to these agonists were at least partially reversed by the adenosine-receptor antagonist, 8-phenyltheophylline (8PT). These data imply that on-cells preferentially express P2X-receptors, off-cells P2Y-receptors and neutral cells P1-receptors. Immunohistochemical localization of purinergic receptors confirms the presence of some subtypes of P2X, P2Y and A1 receptors on neuronal cell bodies and fibers within the RVM. The differential responses of on-, off- and neutral-cells to purinergic ligands highlight the value of pharmacological signatures in further delineation of the anatomy, connectivity and function of this therapeutically important system.