Monoaminergic mechanisms in aversive brain stimulation.

In these experiments we have examined the role of brain monoamines in the fearlike aversive responses produced by the electrical stimulation of the dorsal midbrain tegmentum (DMT). Chronic bipolar stimulating electrodes were bilaterally implanted into the DMT of 77 rats. Electrical stimulation via 34 of these electrodes produced fearlike, escape seeking responses. These animals were then trained for stable stimulus escape using a decremental bar pressing paradigm. In this paradigm, each bar press reduced the stimulation current by a predetermined fraction (5 percent) of the initial current level. Perceived aversive strength of the initial stimulus current was thereby represented by an increasing function of the number of bar presses to escape. Administration of the catecholamine depleting drug alpha-methyl-para-tyrosine yielded no change in bar pressing relative to saline-injected controls. However, the serotonin depleting drub para-chlorophenylalanine produced a marked increase in decremental bar pressing compared to saline-injected controls. These results suggest that fearlike responses to DMT stimulation may be dependent upon brain serotonin levels and relatively insensitive to levels of brain catecholamines.