Effects of serotonin and morphine on spontaneous and evoked firing of nociceptive neurons in the trigeminal spinal nucleus of rats

Spontaneously firing neurons that were responsive to noxious face pinch or noxious heat were studied in the trigeminal spinal nucleus of the rat brain. These eurons responded with either an increase or decrease in firing rate. In these neurons serotonin (5-hydroxytryptamine; 5-HT) apparently acts through two mechanisms to attenuate the response to a noxious stimulus. One mechanism is mimicked by morphine; these two drugs block the response to the noxious stimuli without having a consistent effect on spontaneous firing. The effects of the two drugs were somewhat selective depending on the noxious stimulus used and the effect of the noxious stimulus; morphine and 5-HT were more effective in blocking the increase in firing rate evoked by the face pinch but 5-HT and morphine were more effective in blocking the decrease in firing rate evoked by the noxious heat stimulus. Interestingly, the direction of the response to a particular noxious stimulus frequently predicted whether or not both morphine and 5-HT would act on the same or different neurons. A second mechanism by which 5-HT, but not morphine, acted was to change the spontaneous firing in a direction opposite that evoked by the noxious stimulus. This type of effect apparently modulated the response to a noxious stimulus by changing the spontaneous firing rate such that a noxious stimulus had to be more intense before it could significantly alter the neuronal firing in the opposite direction. Morphine occasionally produced a change in firing pattern in neurons; this effect remains to be documented more extensively.