Acute nociceptive somatic stimulus sensitizes neurones in the spinal cord to colonic distension in the rat

Abstract

The common co-existence of fibromyalgia and chronic abdominal pain could be due to sensitization of spinal neurones (SNs), as a result of viscero-somatic convergence. The objective of this study is to explore the influence of acute nociceptive somatic stimulation in the form of acid injections, into the ipsilateral somatic receptive field of neurones responsive to colorectal distension (CRD), and the potential role of ionotropic glutamate receptors on sensitization. Action potentials of CRD-sensitive SNs were recorded extracellularly from the lumbar (L(2)-L(5)) spinal cord. Stimulus-response functions (SRFs) to graded CRD (10-80 mmHg, 30 s) were constructed before and 30 min after ipsilateral injection of low pH (4.0, 100 microl) saline into the somatic receptive fields. In some experiments, cervical (C(1)-C(2)) spinalization was performed to eliminate supraspinal influence. The selective NMDA receptor antagonist CGS 19755 and AMPA receptor antagonist NBQX were injected (25 micromol kg(-1), i.v.) to examine their influence on sensitization. Three types of neurones were characterized as short-latency abrupt (SLA, n = 24), short latency sustained (SLS, n = 12), and long-latency (LL, n = 6) to CRD. Ipsilateral injection of low pH (4.0) in the somatic receptive field, but not the contralateral gastrocnemius (GN) or front leg muscles, sensitized responses of these neurones to CRD. Spinalization had no influence on the development of low pH-induced sensitization. Both CGS 19755 and NBQX significantly attenuated the sensitized response to CRD in intact and spinalized animals. Acute nociceptive somatic stimulus sensitizes CRD-sensitive SNs receiving viscero-somatic convergence. The sensitization occurs at the spinal level and is independent of supraspinal influence. Ionotropic glutamate receptors in the spinal cord are involved in sensitization.

Figure 1. Three types of CRD-sensitive SNs recorded in the lumbar spinal cord

In each panel, the top trace is the frequency histogram (1 s bin width) of the firing of the neurone, the middle trace is the nerve action potentials and the lower trace is the distending pressure (60 mmHg for 30 s). The depths of recordings of these neurones were 1142 μm for SLA, 1224 μm for SLS and 1434.5 μm for LL ventral from the dorsal surface.

Figure 2. Response of a CRD-sensitive neurone with a convergent somatic receptive field in the thigh muscle

The recording depth of the neurone was 718.5 μm. Each panel in the columns shows frequency histogram of firing of the neurone and action potentials. A shows the intensity-dependent response of the neurone to graded (20, 40 and 80 mmHg) CRD. B shows responses of the neurone to brush, skin retraction with a forceps and muscle pinch, respectively. C shows the response of the neurone to low pH injection into the thigh muscle, brush and muscle pinch after low pH injection, respectively. The baseline ongoing firing of the neurone increased after low pH injection. The neurone exhibited greater response to muscle pinch, but not to brush. Arrows indicate the point of application of mechanical or chemical stimulus.

Figure 3. Response of CRD-sensitive neurons to saline injection into the somatic receptive field

A, response of a CRD-sensitive SN to an injection of 0.1 ml of pH 4.0-buffered saline to its somatic receptive field in the ipsilateral GN muscle. The neurone was recorded 1142 μm from the dorsal surface of the spinal cord. The top trace is the activity of the nerve represented as frequency histogram (1 s bin width), and the lower trace is the nerve action potentials. The neurone had low ongoing firing, which markedly increased following low pH injection. The enhanced firing of the neurone lasted for the 2 h tested. B, response of a CRD-sensitive SN to an injection of 0.1 ml of pH 7.4-buffered saline to its somatic receptive field in the ipsilateral GN muscle. The neurone was recorded 1027 μm from the dorsal surface of the spinal cord. A transient increase in firing occurred at the time of injection. However, the increased firing progressively declined. The initial burst of firing was due to mechanical stimulus occurred during needle prick. C, summary data of neuronal response to injection to the somatic receptive field, contralateral GN muscle and anti-brachial region of the forelimb. Only low pH injections given to the ipsilateral somatic receptive field produced sustained increase in firing on the CRD-sensitive SNs. (*P < 0.001 versus baseline).

Figure 4. Response of a CRD-sensitive SLA neurone to graded CRD before and after low pH (4.0) injection to its somatic receptive field (GN muscle)

The electrode position was 1224 μm from the dorsal surface of the spinal cord. Both baseline spontaneous firing and response to distensions significantly increased following the low pH 4.0 injection. Post-low pH recording was made 30 min post low pH injection.

Figure 5. Summary data of the mean SRFs of three subtypes of SNs before and 30 min following low pH 4.0 injection to the somatic receptive field

SLA (n = 15), SLS (n = 9) and LL (n = 5) neurones showed sensitization to CRD following the low pH injection. Buffered saline injection (pH 7.4) to the somatic receptive field of three CRD-sensitive neurones did not produce change in response to graded CRD (*P < 0.05 versus baseline).

Figure 6. Effect of NMDA and AMPA receptor antagonists on sensitized responses of CRD-sensitive SNs

A, responses of a SLA neurone (depth: 1035 μm) to graded CRD before and after low pH (4.0) injection and followed by intravenous injection of NMDA antagonist CGS 19755 (25 μmol kg⁻¹). CGS 19755 attenuated the post-acid-enhanced spontaneous firing and sensitized response to CRD. B, summary data of the mean SRFs of six neurones tested with CGS 19755. C, responses of a LL neurone (depth: 1133 μm) to graded CRD before and after low pH (4.0) and followed by intravenous injection of the AMPA receptor antagonist NBQX (25 μmol kg⁻¹). NBQX attenuated the post-acid-enhanced spontaneous firing and sensitized response to CRD. D, summary data of the mean SRFs of six neurones tested with NBQX. (*post pH 4.0 versus baseline, ^#post pH 4.0 versus post drug in B and D).

Figure 7. Summary data of the mean SRFs of seven neurones tested before and after cervical (C₁–C₂) spinal transection

All SNs exhibited greater responses to graded CRD after spinal transection. Low pH (4.0) produced further increase in response of these neurones under spinalized condition. Both NMDA receptor antagonist CGS 19755 (25 μmol kg⁻¹) and AMPA receptor antagonist NBQX (25 μmol kg⁻¹) significantly attenuated the enhanced spontaneous firing and the sensitized response to CRD (*P < 0.05).