Neurotensin excites periaqueductal gray neurons projecting to the rostral ventromedial medulla

Microinjection of neurotensin into the midbrain periaqueductal gray (PAG) produces a potent and naloxone-insensitive analgesic effect. To test the hypothesis that neurotensin induces the analgesic effect by activating the PAG-rostral ventromedial medulla (RVM) descending antinociceptive pathway, PAG neurons that project to RVM (PAG-RVM) were identified by microinjecting DiI_C18, a retrograde tracing dye, into the rat RVM. Subsequently, fluorescently labeled PAG-RVM projection neurons were acutely dissociated and selected for whole cell patch-clamp recordings. During current-clamp recordings, neurotensin depolarized retrogradely labeled PAG-RVM neurons and evoked action potentials. Voltage-clamp recordings indicated that neurotensin excited PAG-RVM neurons by opening the voltage-insensitive and nonselective cation channels. Both SR 48692, a selective NTR-1 antagonist, and SR 142948A, a nonselective antagonist of NTR-1 and NTR-2, failed to prevent neurotensin from exciting PAG-RVM neurons. Neurotensin failed to evoke cationic currents after internally perfusing PAG-RVM projection neurons with GDP-β-S or anti-G_αq/11 antiserum. Cellular Ca²⁺ fluorescence measurement using fura-2 indicated that neurotensin rapidly induced Ca²⁺ release from intracellular stores of PAG-RVM neurons. Neurotensin-evoked cationic currents were blocked by heparin, an IP₃ receptor antagonist, and 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA), a fast chelator of Ca²⁺. These results suggest that by activating a novel subtype of neurotensin receptors, neurotensin depolarizes and excites PAG-RVM projection neurons through enhancing Ca²⁺-dependent nonselective cationic conductance. The coupling mechanism via G_αq/11 proteins is likely to involve the production of IP₃, and subsequent IP₃-evoked Ca²⁺ release leads to the opening of nonselective cation channels.