Adenosine A1 Receptors Reduce Release from Excitatory But Not Inhibitory Synaptic Inputs onto Lateral Horn Neurons

Abstract

Although adenosine is an important neuromodulator in the CNS, its role in modulating sympathetic outflow at the level of the spinal cord has not been studied. Because very little is known about adenosine A1 receptors (A1Rs) in the spinal cord, we determined their location and role with particular reference to the control of sympathetic preganglionic activity and interneuronal activity in the rat. High levels of immunoreactivity for A1Rs were observed throughout the spinal cord. Immunostaining was dense in the intermediolateral cell column (IML) and intercalated nucleus, regions containing retrogradely labeled sympathetic preganglionic neurons (SPNs). Electron microscopy revealed A1R immunoreactivity (A1R-IR) within presynaptic terminals and (to a lesser extent) postsynaptic structures in the IML, as well as the luminal membrane of endothelial cells lining capillaries. Using double-labeling techniques, some presynaptic terminals were observed to synapse onto SPNs. To investigate the effects of activating these A1Rs, visualized whole-cell patch-clamp recordings were made from electrophysiologically and morphologically identified SPNs and interneurons. Applications of the A1R agonist cyclopentyladenosine (CPA) reduced the amplitude of EPSPs elicited by stimulation of the lateral funiculus, an effect blocked by the A1R antagonist 8-cyclopentyl-1,3-dipropylxanthine. These effects were attributable to adenosine acting at a presynaptic site because CPA application increased the paired-pulse ratio. CPA did not affect evoked IPSPs. These data show that activating A1Rs reduces fast excitatory, but not inhibitory, transmission onto SPNs and interneurons in the IML and that A1Rs may play a protective role on neurons involved in the control of sympathetic outflow.

Input-Specific Modulation of Neurotransmitter Release in the Lateral Horn of the Spinal Cord via Adenosine Receptors

“…Adenosine A1 and A2 A receptors are located on excitatory and inhibitory terminals, respectively synapsing onto the same neurons in the IML As previously reported, A1Rs are located exclusively on excitatory, not inhibitory, terminals, which synapse onto SPNs and interneurons in the IML (Deuchars et al, 2001b). In this study, we have shown that A2 A Rs are functionally located exclusively on inhibitory terminals, which also synapse onto SPNs and interneurons.…”

“…Hence, for some experiments, CGS 21680 was applied in the presence of the A1R antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (50 nM; n ϭ 7) or CPT (10 M; n ϭ 25). Administration of DPCPX (50 nM) or CPT (10 M) alone had no effect on the evoked EPSP as expected (results not shown; Deuchars et al, 2001b). No significant effect was observed on the evoked EPSP in interneurons after the application of CGS 21680 alone (25 nM, 6.5 Ϯ 0.4 to 6.5 Ϯ 0.4; n ϭ 5 ; Fig.…”

“…This was somewhat surprising because intrathecal administration of A1R agonists caused similar cardiovascular effects (Koh et al, 1996). In a previous study, we revealed that the A1R-mediated effects on sympathetic outflow could be attributable to reduction in activity in SPNs because activation of A1Rs in the lateral horn of the spinal cord decreased excitatory transmitter release onto SPNs and interneurons without effects on inhibitory transmission (Deuchars et al, 2001b). Considering the diversity of responses observed as a result of activation of A2 A Rs at a cellular level, we were prompted to investigate how A2 A Rs may exert an influence on sympathetic nervous activity.…”

“…The lf was chosen because this contains descending axons that make monosynaptic connections onto SPNs, whereas inputs from the dorsal horn are polysynaptic and thus more difficult to analyze (Dembowsky et al, 1985;Shen et al, 1990). Despite being shown to be selective for the A2 A R, it has been reported that CGS 21680 at micromolar concentrations may weakly activate the A1R (Li and Henry, 1998), which does appear to be functional in this region of the spinal cord (Deuchars et al, 2001b). Hence, for some experiments, CGS 21680 was applied in the presence of the A1R antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (50 nM; n ϭ 7) or CPT (10 M; n ϭ 25).…”

“…If an IPSP was not observed in control conditions, NBQX and AP-5 were administered to uncover an IPSP. DPCPX and CPT were not included in the bathing medium because activation of A1Rs with CPA had no effect on IPSP amplitude in SPNs or interneurons (Deuchars et al, 2001b).…”