Hayar, Abdallah and Patrice G. Guyenet. Pre-and postsynaptic A likely site for inhibition of sympathetic tone by endogeinhibitory actions of methionine-enkephalin on identified bulbonous opioids is the rostral ventrolateral medulla (RVL), a spinal neurons of the rat RVL. J. Neurophysiol. 80: 2003Neurophysiol. 80: -2014 region that harbors the main excitatory projection to sympa-1998. The effects of methionine-enkephalin (ME) on visualized thetic vasomotor preganglionic neurons (for reviews, see

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... bulbospinal neurons of the rostral ventrolateral medulla (RVL) Guyenet et al. 1996; Sun 1995 Sun , 1996 . Exogenous applicawere characterized in thin slices at 32ЊC using the whole cell patchtion of opioids either by intravenous administration or by clamp technique. Thirty-five percent of the recorded neurons were direct injection into the RVL induces potent hypotensive found to be tyrosine hydroxylase immunoreactive (C1 neurons). effects (Punnen and Sapru 1986; Rhee et al. 1992 ; Sun et al. In voltage-clamp recordings, ME (3 mM) induced an outward cur-1996; White et al. 1995). Moreover, some RVL bulbospinal rent in 66% of RVL bulbospinal neurons. A similar percentage of C1 and non-C1 neurons were opioid sensitive. The current induced neurons with presumed sympathoexcitatory function are inby ME was inwardly rectifying, reversed close to the potassium hibited by iontophoretically applied morphine (Baraban et equilibrium potential, and was blocked by barium. Most spontaneal. 1995). In addition, the RVL contains a dense plexus ous postsynaptic currents recorded in these neurons were tetrodoof enkephalinergic fibers, and the catecholaminergic (C1) toxin (TTX)-resistant miniature postsynaptic currents (mPSCs). neurons constitute one of the primary targets of this innerva-Approximately, 75% of mPSCs had rapid kinetics (decay time Å tion (Milner et al. 1989 (Milner et al. , 1990 . The enkephalin-containing 4.7 ms) and were glutamatergic [miniature excitatory postsynaptic terminals present in RVL may originate from local neurons currents (mEPSCs)] because they were blocked by 6-cyano-7-(FITC) microspheres (0.3-0.5 ml; Lumafluor) was injected bilater-2003 receptors because injection of the specific m receptor agonist DAMGO in the RVL also elicited hypotensive effects. Fur-Physiological consequences tamine and opioids. Neuroscience 77: 155-166, 1997. CAPOGNA, M., GAHWILER, B. H., AND THOMPSON, S. M. Mechanism of m-Glutamate and GABA appeared to be the only transmitters opioid receptor-mediated presynaptic inhibition in the rat hippocampus in vitro.