Fig. 4.

Activation of the α₂ adrenergic receptor does not directly affect the Ca PIC but instead inhibits EPSPs. A: amplitude of LLR recorded in the isolated in vitro spinal cord of chronic spinal rat and measured repeatedly over time (LLR quantified 0.5–4 s post stimulus, as in Fig. 2). Control values are shown on left, followed by activation of α₂ adrenergic receptors with the agonist clonidine (top bar 0.1 μM) and subsequent application of the α₂ receptor antagonist RX821002 (0.3 μM; bottom bar). B: same format as A with application of RX821002 alone (0.5 μM). C: intracellular motoneuron recording of long-latency polysynaptic EPSP (abbreviated EPSP) evoked by dorsal root stimulation (0.1 ms at 3 × T) of chronic spinal rat (quantified at 200 ms post stimulus) during hyperpolarizing bias current before (top plot) and after blocking the α₂ receptor with bath application of clonidine (0.3 μM, bottom plot). D: normalized group mean for LLR in chronic spinal rats in vitro with application of the α₂ receptor agonists UK14304 (0.03 μM; n = 8), and clonidine (0.1 μM; n = 5), and application α₂ antagonist RX821002 (0.3 μM) after UK14304 (abbreviated UK14 + RX; n = 8) and after clonidine (abbreviated Clon + RX; n = 8). Normalized group mean of intracellulary recorded polysynaptic EPSP (E) evoked by 3 × T dorsal root stimulation, PIC (F), and resting membrane potential (V_m; G) before and after application of clonidine in chronic spinal rats (0.1–1 μM; n = 5). *P < 0.05, **P < 0.01. Error bars, SE.