Neurotransmission in CNS regions involved in pain modulation : neurochemical effects of analgesic drugs and spinal cord stimulation in the spinal cord and midbrain periaqueductal grey of the rat

Abstract

The dorsal hom of the spinal cord and the midbrain penaqueductal grey matter (PAG) are important regions for pain modulation. In this thesis the effects of opioids and spinal cord stimulation (SCS) on the release of gamma-amino butyric acid (GABA), substance P (SP) and neurotensin (NT) were investigated in these regions of the rat by in vivo microdialysis. In addition, the effect of the adenosine A1 analogue R-phenyl-isopropyl-adenosine (R-PIA), on the binding of SP to its receptor was studied in vitro in the dorsal horn by autoradiography. R-PIA has previously been shown to inhibit the pain like behaviour induced by spinal administration of SP in rodents, and may also alleviate pain in man R-PIA (0.1 myM) was found to increase the affinity of the SP-receptor in the rat dorsal hom via a GTP dependent mechanism, while the number of available binding sites remained unchanged in the presence of R-PIA. These results suggest that effects of R-PIA and other adenosine analogues on SP-binding may be of importance for the previously reported effects of these substances on pain related behaviour.

Significantly decreased extracellular levels of the inhibitory neurotransmitter GABA were detected in the dorsal hom of rats that had lesions of the sciatic nerve and showed signs of allodynia (i.e. hypersensitivity to light touch). SCS, a method used for alleviation of neuropathic pain in man, induced a significant increase of the extracellular GABA level in the dorsal horn of neuropathic rats that responded to SCS with a reversal of allodynia. In contrast, no change in GABA release was detected in other nerve lesioned rats, that did not respond behaviourally to SCS. In the PAG, where GABA is regarded to mediate a tonic suppression of descending pain inhibitory pathways, a decreased extracellular GABA level was observed following repeated SCS. This finding may indicate a supraspinal mechanism contributing to the analgesic effect of SCS. The effect of morphine on the release of GABA in the PAG was monitored by microdialysis combined with capillary electrophoresis with laser induced fluorescence detection (CE-LIF). In a separate experiment CE-LIF was found to have a higher sensitivity for GABA than HPLC with electrochernical detection. Local administration of an analgesic dose of morphine (100 myM) in the PAG, via the dialysis probe, decreased the extracellular GABA level in the PAG by almost 50 percent in a naloxone reversible manner. These results support the hypothesis that opioids induce activation of descending pain inhibitory pathways by an inhibition of tonically active GABA neurons in the PAG. In other experiments, morphine (10 myM) and the my-opioid selective agonist DAGO (1myM) administered via a microdialysis probe in the ventromedial PAG, were found to induce a significant, naloxone-reversible and calcium dependent increase of the extracellular level of neurotensin-like immunoreactivity in microdialysates obtained at the same site. Since NT induces a potent antinociception upon microinjection in the PAG, these findings may suggest that an increased release of NT in the PAG contributes to opioid analgesia.

In conclusion, several neurochemical effects at spinal and supraspinal level were demonstrated in response to treatment with opioids and SCS. These effects may indicate that opiates and SCS act via previously unknown mechanisms to alleviate pain.