Centralization of noxious stimulus-induced analgesia (NSIA) is related to activity at inhibitory synapses in the spinal cord

Abstract

The duration of noxious stimulus-induced antinociception (NSIA) has been shown to outlast the pain stimulus that elicited it, however, the mechanism that determines the duration of analgesia is unknown. We evaluated the role of spinal excitatory and inhibitory receptors (NMDA, mGluR(5), mu-opioid, GABA(A), and GABA(B)), previously implicated in NSIA initiation, in its maintenance. As in our previous studies, the supraspinal trigeminal jaw-opening reflex (JOR) in the rat was used for nociceptive testing because of its remoteness from the region of drug application, the lumbar spinal cord. NSIA was reversed by antagonists for two inhibitory receptors (GABA(B) and mu-opioid) but not by antagonists for either of the two excitatory receptors (NMDA and mGluR(5)), indicating that NSIA is maintained by ongoing activity at inhibitory synapses in the spinal cord. Furthermore, spinal administration of the GABA(B) agonist baclofen mimicked NSIA in that it could be blocked by prior injection of the mu-opioid receptor antagonist H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP) in nucleus accumbens. CTAP also blocked baclofen antinociception when administered in the spinal cord. We conclude that analgesia induced by noxious stimulation is maintained by activity in spinal inhibitory receptors.

Figure 1. Effect of excitatory receptor antagonists

Capsaicin (Cap) was administered immediately after the last baseline recording. The antagonist or vehicle (PBS) was administered intrathecally immediately after the recording at 30 minutes. Compared to PBS (n=8), neither the competitive NMDA receptor antagonist LY367385 (n=5) nor the mGluR₅ receptor antagonist MPEP (n=5) significantly affected NSIA. In this and subsequent figures data are presented as percentage attenuation of the JOR EMG (mean ± s.e.m.); see Methods for additional details regarding data presentation and analysis.

Figure 2. Effect of inhibitory receptor antagonists

Capsaicin (Cap) was administered immediately after the last baseline recording. Follow capsaicin administration, the antagonist or vehicle (PBS, n=8) was administered intrathecally immediately after the recording at 30 minutes. A. The selective μ-opioid receptor antagonist CTAP (n=4) significantly reversed capsaicin-induced attenuation of the JOR. B. The selective GABA_B-receptor antagonist CGP35348 (n=8) significantly reversed capsaicin-induced attenuation of the JOR. C. The GABA_A-receptor antagonist bicuculline (n=6) did not significantly reverse capsaicin-induced attenuation of the JOR.

Figure 3. Effect of GABA_B receptor agonist baclofen

Baclofen injected i.t. as a single agent significantly attenuated the JOR (n=6), an effect that was blocked by prior administration of CTAP into nucleus accumbens (n=6) or by intrathecal co-administration of CTAP with baclofen (n=6).

Figure 4. Schematic diagram of proposed spinal NSIA circuit

Top panel. Under basal conditions, in the absence of noxious stimulation, tonically active neurons (red) project supraspinally to inhibit accumbens (“NAc”)-mediated antinociception. Note lack of activity in the incoming primary afferent nociceptor (“PAN”) and the inhibitory interneuron (both black). The activity in the ascending neuron results from ongoing glutamatergic synaptic input arising from an unknown intrinsic source [30]. Middle panel. When noxious stimulation, depicted as a red chili pepper, is applied, activating the PAN (now red), the inhibitory interneuron (also now red) is activated by various glutamate receptors. Activity in this neuron in turn inhibits activity in the ascending neuron (now black), thereby disinhibiting accumbens-mediated antinociception (i.e., NSIA). Bottom panel. After cessation of primary afferent nociceptor (“PAN”) activity (now black), the inhibitory interneuron remains active (red), maintaining tonic inhibition of the ascending neuron (black). This decoupling of the inhibitory interneuron from peripheral input is what we refer to as “centralization.”