Electrical activity of the reticular formation during aversive and appetitive conditioning in rats.

Evoked potentials (EP) to a conditioned stimulus (CS) were recorded from several parts of the reticular formation (RF), the midbrain central gray (CG), the occipital cortex (OC), the basolateral amygdala and the ventrobasal thalamus during appetitive and aversive classical conditionings in rats. In addition, single unit activity to rewarding CS (+) and punishing CS (-) in a single discriminative classical conditioning was recorded from RF, OC, CG and the tectum. In the EP study, peak-to-peak (p-p) amplitudes, especially of the pontine RF, increased in the initial phase of pre-conditioning, in aversive conditioning and in appetitive conditioning but not in aversive pseudo-conditioning. The p-p amplitude in the pontine RF might reflect an amount of stimulus value the animal received. Slow negative potentials after CS presentation in RF was a reflection of body movements, indicating heavy involvement of RF in motor functions. In the single unit studies, cells which selectively responded to the CS (-) were more frequently found in the medial tegmental area close to the median raphe nucleus as compared to other areas. These results were considered to indicate high sensitivity of RF to the significance of the stimulus, and also the involvement of the medial tegmental area close to the median raphe nucleus in aversive emotional processes. Although many studies employing stimulation or lesion techniques have reported the steady relation between aversive emotional responses and CG, no supporting result was obtained in the present experiments.