Brain site variations in effects of morphine on electrical self-stimulation

Pairs of bipolar electrodes were stereotaxically aimed at two of three sites: the locus coeruleus (LC), the substantia nigra, pars compacta (SNC), and the median forebrain bundle (MFB). Rats were shaped to bar-press for trains of intracranial electrical stimulation presented as pairs of monophasic pulses. The first pulse of a pair (the C, conditioning pulse) was followed by a second pulse (the T, test pulse) after a parametrically varied interval. The effects of chronic morphine administration were tested in a paradigm of 7 days saline, 7 days morphine, 1 day morphine+naloxone, and 6 days post-drug saline. High doses of morphine (5 mg/kg) depressed response rates for intracranial self-stimulation (ICSS). LC placements and those just lateral or ventral to the LC showed large increases in ICSS rates under morphine (2.5 mg/kg). This area was delimited on either side by tips that showed response rate depressions under morphine. MFB placements yielded response rate facilitations under morphine. Sites medial to the MFB and ventral within the MFB showed rate depressions under morphine. Dorsal substantia nigra placements showed facilitated rates, whereas placements ventral within the SNC and substantia nigra, pars reticulata (SNR) produced more variable results, with rates tending to be depressed by morphine. The ICSS procedure may be a useful animal model for detecting the abuse potential of drugs.