Shuttlebox self-stimulation in the rat: an anatomical analysis and the effect of morphine with two current levels

A shuttlebox paradigm was used to train rats to turn electrical stimulation ON and OFF by crossing back and forth in a stabilimeter cage. Two experiments are presented. In the first experiment a threshold current level was used in testing four electrode sites: the lateral hypothalamic area (LHA), lateral septal nucleus (LSN), periaqueductal grey (PAG), and the mesencephalic reticular formation (MRF). In the second experiment, a suprathreshold current level was used to explore two electrode sites: the PAG and the MRF. Stimulation with electrodes in the MRF produced an aversive behavioral response; animals shuttled mainly to turn electrical stimulation OFF. At the other electrode sites, both rewarding and aversive properties were apparent: animals shuttled to turn the stimulation ON as well as OFF. Systemic morphine (10 mg/kg) injections nonselectively increased both average ON and OFF times for the three rewarding sites (minimum p less than 0.05) at the threshold current level. Systemic morphine injections (10 mg/kg) in animals stimulated at a suprathreshold current level in the PAG selectively increased time spent with stimulation ON (p less than 0.05) as opposed to time spent with stimulation OFF. No significant behavioral change due to morphine was seen in the aversive MRF at either current level. Animal behavior also was found to vary as a function of site of stimulation (p less than 0.05). The use of suprathreshold currents appears necessary to produce selective reward facilitation effects of morphine such as those found in the PAG or LHA.