Inhibition of Glutamatergic Synaptic Input to Spinal Lamina IIo Neurons by Presynaptic 2-Adrenergic Receptors

Pan, Yu-Zhen, De-Pei Li, and Hui-Lin Pan. Inhibition of glutamatergic synaptic input to spinal lamina IIo neurons by presynaptic 2-adrenergic receptors. J Neurophysiol 87: 1938–1947, 2002; 10.1152/jn.00575.2001. Activation of spinal 2-adrenergic receptors by the descending noradrenergic system and 2-adrenergic agonists produces analgesia. However, the sites and mechanisms of the analgesic action of spinally administered 2-adrenergic receptor agonists such as clonidine are not fully known. The dorsal horn neurons in the outer zone of lamina II (lamina IIo) are important for processing nociceptive information from C-fiber primary afferents. In the present study, we tested a hypothesis that activation of presynaptic 2-adrenergic receptors by clonidine inhibits the excitatory synaptic input to lamina IIo neurons. Whole cell voltage-clamp recordings were performed on visualized lamina IIo neurons in the spinal cord slice of rats. The miniature excitatory postsynaptic currents (mEPSCs) were recorded in the presence of tetrodotoxin, bicuculline, and strychnine. The evoked EPSCs were obtained by electrical stimulation of the dorsal root entry zone or the attached dorsal root. Both mEPSCs and evoked EPSCs were abolished by application of 6-cyano-7-nitroquinoxaline-2,3-dione. Clonidine (10 M) significantly decreased the frequency of mEPSCs from 5.8 0.9 to 2.7 0.6 Hz (means SE) without altering the amplitude and the decay time constant of mEPSCs in 25 of 27 lamina IIo neurons. Yohimbine (2 M, an 2-adrenergic receptor antagonist), but not prazosin (2 M, an 1adrenergic receptor antagonist), blocked the inhibitory effect of clonidine on the mEPSCs. Clonidine (1–20 M, n 8) also significantly attenuated the peak amplitude of evoked EPSCs in a concentration-dependent manner. The effect of clonidine on evoked EPSCs was abolished in the presence of yohimbine (n 5). These data suggest that clonidine inhibits the excitatory synaptic input to lamina IIo neurons through activation of 2-adrenergic receptors located on the glutamatergic afferent terminals. Presynaptic inhibition of glutamate release from primary afferents onto lamina IIo neurons likely plays an important role in the analgesic action produced by activation of the descending noradrenergic system and 2-adrenergic agonists.