Inhibition of glutamatergic synaptic input to spinal lamina II_o neurons by presynaptic α₂-adrenergic receptors

Activation of spinal α₂-adrenergic receptors by the descending noradrenergic system and α₂-adrenergic agonists produces analgesia. However, the sites and mechanisms of the analgesic action of spinally administered α₂-adrenergic receptor agonists such as clonidine are not fully known. The dorsal horn neurons in the outer zone of lamina II (lamina II_o) are important for processing nociceptive information from C-fiber primary afferents. In the present study, we tested a hypothesis that activation of presynaptic α₂-adrenergic receptors by clonidine inhibits the excitatory synaptic input to lamina II_o neurons. Whole cell voltage-clamp recordings were performed on visualized lamina II_o 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 μM) 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 II_o neurons. Yohimbine (2 μM, an α₂-adrenergic receptor antagonist), but not prazosin (2 μM, an α₁-adrenergic receptor antagonist), blocked the inhibitory effect of clonidine on the mEPSCs. Clonidine (1-20 μM, 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 II_o neurons through activation of α₂-adrenergic receptors located on the glutamatergic afferent terminals. Presynaptic inhibition of glutamate release from primary afferents onto lamina II_o neurons likely plays an important role in the analgesic action produced by activation of the descending noradrenergic system and α₂-adrenergic agonists.