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Gabapentin is a useful agent for the relief of trigeminal neuralgia and orofacial phantom pain. However, there is scarce information on the gabapentin analgesic effect in orofacial pain models. We tested the analgesic action of gabapentin on the formalin-induced face grooming in the rat, an orofacial pain paradigm. IP Gabapentin (10 mg/kg), induced a drastic reduction in face grooming during phase I and II, indicating a clear-cut antinociceptive effect. However, at 1 mg/kg, gabapentin had an analgesic effect only on phase I. D-serine (100 microg, ICV) was silent when given alone and did not antagonize the antinociceptive effect of gabapentin. On the contrary, gabapentin 1 mg/kg plus D-serine significantly reduced face grooming in phase II. These results show a difference between gabapentin induced orofacial analgesia and previous studies showing gabapentin-induced hind paw analgesia in the formalin test, only during phase II, as well as D-serine antagonism of gabapentin. The results are discussed in terms of different pain processing of hind paw, versus orofacial nociceptive stimulation.

The aim of this research was to find out the effects of ip morphine pretreatment in the extracellular content of the arginine, glutamate, aspartate and GABA levels in the anterior cingulate cortex in rats, during the formalin test (phase I). A combination of micro dialysis and Capillary Electrophoresis Zone and laser-induced fluorescence detection (CZE-LIFD) technique was used to measure the extracellular levels of amino acids in microdialized zones. The microdialysis probes were unilaterally implanted in the left anterior cingulate cortex of freely moving rats. The samples were collected every 30 seconds and derivatized with fluorescein isothiocianate. The arginine, glutamate, aspartate and GABA levels were measured in the CZE-LIFD device. Arginine (p0.001) and glutamate levels (p0.012) were significantly increased in the first few minutes following the formalin test (phase 1). Pretreatment with morphine suppressed the glutamate increase. A transient GABA level increase (p0.001) was also detected. These experiments suggest that rapid changes in neurotransmitters levels were detected in the first few minutes of acute pain as revealed by the glutamate and arginine level increases in the anterior cingulate cortex. These changes could be related to the emotion of pain processing (fear and aversion). Morphine pretreatment produced an increase in GABA levels and a decrease in glutamate levels in the first few minutes. These findings may be related to euphoria and/or analgesia.

In the present experiments extracellular arginine, glutamate and aspartate were studied in the basolateral nucleus of the amygdala and core of the nucleus accumbens during the formalin test (phase I). A combination of capillary zone electrophoresis with laser induced fluorescence detection and microdialysis in freely moving rats was used. Glutamate and arginine significantly increased in the nucleus accumbens after formalin injection; glutamate, arginine and aspartate significantly increased in the basolateral nucleus of the amygdala, after formalin injection. These experiments suggest that rapid neurotransmitters changes observed in the nucleus accumbens and amygdala, are possibly related to immobility and emotional states such as anxiety, aversion and/or depression caused by pain.