Neurotensin Excites Periaqueductal Gray Neurons Projecting to the Rostral Ventromedial Medulla

1 Department of Anesthesiology and 3 Department of Neurology, Chang Gung Memorial Hospital; and 2 Department of Anatomy and 4 Department of Physiology, Chang Gung University School of Medicine, Kwei-San, Tao-Yuan, Taiwan, R.O.C. Li, Allen H., Hwa-Min Hwang, Peter P. Tan, Tony Wu, and Hung-Li Wang. Neurotensin Excites Periaqueductal Gray Neurons Projecting to the Rostral Ventromedial Medulla. J. Neurophysiol. 85: 1479-1488, 2001. 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- -S or anti-G q/11 antiserum. Cellular Ca 2+ fluorescence measurement using fura-2 indicated that neurotensin rapidly induced Ca 2+ release from intracellular stores of PAG-RVM neurons. Neurotensin-evoked cationic currents were blocked by heparin, an IP 3 receptor antagonist, and 1,2-bis(2-aminophenoxy)ethane- N,N,N',N' -tetraacetic acid (BAPTA), a fast chelator of Ca 2+ . These results suggest that by activating a novel subtype of neurotensin receptors, neurotensin depolarizes and excites PAG-RVM projection neurons through enhancing Ca 2+ -dependent nonselective cationic conductance. The coupling mechanism via G q/11 proteins is likely to involve the production of IP 3 , and subsequent IP 3 -evoked Ca 2+ release leads to the opening of nonselective cation channels.