Intracranial self-stimulation derived from entorhinal cortex

Abstract

An ipsilateral projection from the A10 cell group of the ventral tegmental area provides the source of islands of dopamine innervation found in lateral entorhinal cortex2,3,7,1°. Since dopamine has been linked to intracranial serf-stimulation (ICSS) 4, 13,15, and lateral entorhinal cortex receives fiber connections from ventral midbrain tegmental2,3,14 and frontal cortical areas 16,19 which support self-stimulation, the present study investigated the possibility that lateral entorhinal cortex itself may support ICSS*. Subjects were male albino rats (Holtzman Research, Madison, Wisc.) weighing 250-300 g at the time of surgery. Animals were housed in individual cages on a 12 h light (7 a.m.-7 p.m.), 12 h dark cycle. Our initial investigation of ICSS in entorhinal cortex employed standard bipolar electrodes constructed of nichrome wire 254 # M in diameter. In the second study, electrodes were constructed using finer nichrome wire 78.7 # M in diameter. Electrodes were stereotaxically implanted unilaterally with 4 days allowed for recovery following surgery. Coordinates for implantation were 5.3-5.8 mm posterior to bregma, 3.3 mm lateral to the midline, and 9.8 mm below the skull surface. Electrodes were implanted perpendicular to the skull in the anteriorposterior plane, and at an angle of 16-18 ° laterally in the medial-lateral plane. Following recovery, animals were placed overnight in operant conditioning boxes with access to rewarding stinmlation for a period of 14 h. The animals were allowed to acquire the operant task without shaping or priming by the experimenter. Testing procedures and the test apparatus used have been described previously 14. Following the initial exposure to the test situation, the animals were given access each evening at about 9 p.m. to rewarding stimulation in a 15 min test session. Test sessions were run for a minimum of 10 days. Stimulation consisted of a 0.5 sec 60 Hz train of sine wave current, at an intensity of 25 /~A when using the standard diameter electrodes, and 15 /~A when using the fine wire electrodes. Upon completion of behavioral testing, routine histological procedures were used to enable electrode localization.