Rats with electrodes in the ventral tegmental area were tested for the threshold of intracranial reward using a rate insensitive selfstimulation procedure. It was found that an electrolytic lesion of a part of the dorsal hippocampus induced a marked decrease in the variation of thresholds across rats, while the mean reward level did not change. This indicates that a factor is removed, by the hippocampal lesion, which causes differences in reward between individual rats. It is suggested that the mesolimbic dopaminergic system is involved in this modulatory influence of the hippocampus on reward.It has been suggested that mechanisms present in the hippocampus interact with brain reward systemsl, 2. An exaggerated reaction to signalled administration of food reward has been described in rats with extensive bilateral hippocampal damage 3. This reaction, which includes behavioral stereotypy and increased locomotor activity, bears resemblance with amphetamine-induced behavioral changes and is blocked by the dopamine antagonist haloperidol. These observations were interpreted as indicating that the intact hippocampus moderates the effect of reward, probably by opposing catecholamine mechanisms3. Indeed, hippocampal damage has been found to influence dopaminergic ascending systems as assessed by behavioral and biochemical methods s.~0. These systems include the mesolimbic dopaminergic system with cell bodies in the ventral tegmental area and terminals in a number of limbic structures, e.g. the nucleus accumbens 11. This system is thought to be important for reward, especially as assessed with electrical intracranial self-stimulation (ICSS)I-L However, the interaction between the hippocampus and the dopaminergic systems is not well understood, since hippocampal damage has been reported to result in both increased and decreased dopaminergic activity 1°,14,15. Moreover, experimental evidence for a direct interaction between the hippocampus and brain reward is scarce. The present experiment was designed to investigate whether damage of the hippocampus directly affects brain reward elicited in the mesolimbic system and assessed with ICSS. The performance of well trained rats with an electrode in the ventral tegmental area was compared before and after electrolytic hippocampal lesions using a response rate insensitive self-stimulation schedule 9.~2.Ten male Wistar rats (TNO, Zeist, The Netherlands) from our own breeding stock weighing approximately 230 g at the time of the operation were kept in single transparent cages under standard conditions with laboratory food and tap water available ad libiturn throughout the experiment. Under Hypnorm anesthesia (0.7 ml/kg of the solution containing 0.2 mg/ml fentanyl and 10 mg/ml fluanison) they were implanted with a bipolar stainless steel stimulation electrode of 200 ,urn thickness that was insulated except at the cross section at the tip, in the ventral tegmental area (coordinates: A 2.6; D -3.7: L 1.0 according to De Groot, (see ref. 7) and a monopolar lesion electrode of 200 ~tm ...
