Probable independence between the genetic determinants of approach and escape responses induced by lateral hypothalamic stimulation in the mouse

Differential effects of naloxone on approach and escape responses induced by electrical stimulation of the lateral hypothalamus or the mesencephalic central gray area in mice

BALB/c mice implanted with a bipolar electrode were trained in a shuttle-box to initiate and to terminate a continuous electrical stimulation applied in the lateral hypothalamus (LH) or in the mesencephalic central gray area (CG). Following stabilization of the baseline response latencies, the subjects were subcutaneously injected with isotonic NaCl or with naloxone HCl (0.5, 2 or 10 mg/kg) 15 min or 45 min before a test session. In LH-stimulated animals no modification of the behavioral responses was observed after injection of 0.5 mg/kg of naloxone. The 2 mg/kg dose increased the value of escape latency (ON time) but had no effect on approach latency (OFF time). The 10 mg/kg dose also increased ON time. At this dose, an increase of OFF time was simultaneously observed but only 15 min after the injection. In CG-stimulated mice an increase of OFF time and a reduction of ON time were recorded 15 min after the injection of 0.5 mg/kg. Only the reduction of ON time was detected for the 45-min delay. The 2 mg and 10 mg/kg doses simultaneously increased OFF time and reduced ON time for the two delays. These results demonstrate 1) that the effects of naloxone on self-stimulation varied as a function of the structure considered 2) that the predominant characteristic of the considered structure (essentially "rewarding" as the LH or "aversive" as the CG) governs the modulations induced by naloxone.

Electrical self-stimulation in the medial and lateral septum as compared to the lateral hypothalamus: differential intervention of reward and learning processes?

The characteristics of the electrical self-stimulation behavior elicited from the lateral hypothalamus (LH) and from both medial (MS) and lateral (LS) parts of the septal nucleus have been compared in male mice of the BALB/c strain. Using two different experimental situations (the lever-press box and a spatial discrimination test in a Y-maze) the self-stimulation rate-current intensity relations and the performances during both acquisition and reversal of a spatial discrimination were tested successively. In the lever-press box, it was observed that highest self-stimulation rates were obtained from LH placements while both MS and LS rates were lower. However, MS animals showed higher self-stimulation rates and lower self-stimulation thresholds than LS animals. Acquisition of the spatial discrimination in the Y-maze was achieved by all 3 groups with similar time courses. However, when tested for the reversal of the discrimination, the LS implanted mice were much more perturbed than the two other groups and exhibited marked perseveration. The incidence of convulsive episodes was more frequent in LS mice than in either MS or LH implanted animals during both phases of the discrimination task. These differences in the self-stimulation behavior elicited from medial and lateral parts of the septal complex are discussed in relation to the operation of positive reinforcement mechanisms and to internal inhibition processes operating during acquisition and reversal of the spatial discrimination.

Comparison of deficits in electrical self-stimulation after ibotenic acid lesion of the lateral hypothalamus and the medial prefrontal cortex

The aim of the present study was to compare the self-stimulation deficit produced by a unilateral injection of the neurotoxin, ibotenic acid, in the lateral hypothalamus (LH) to the deficit produced by the same unilateral injection in the medial prefrontal cortex (MPC). Four groups of adult male Sprague-Dawley rats were used: in two control groups, electrodes were bilaterally implanted in the LH (5 rats) or in the MPC (6 rats) and self-stimulation (ICSS) was obtained separately with the right and left electrodes. In the two experimental groups the intrinsic neurons of the LH (8 rats) or of the MPC (10 rats) were destroyed unilaterally by local injection of ibotenic acid (4 micrograms in 0.5 microliter); the other side served as the sham-lesioned control. Ten days later ICSS electrodes were implanted bilaterally, one in the lesioned area, the other in the contralateral region. As in the case of the control rats, ICSS was determined separately for each electrode, first by a rate dependent test (nose-poke) then by a 'rate-free' test (shuttle-box). In the LH and MPC control rats, ICSS responses were the same with stimulation on either side. In the LH-lesioned rats, the ICSS rates measured with the nose-poke test were significantly decreased with stimulation on the lesioned side, whereas rates with stimulation of the non-lesioned LH were normal. Likewise, while shuttle responses with stimulation of the non-lesioned LH were normal, the OFF-time was increased and the ON-time was decreased with stimulation of the lesioned LH. In the MPC-lesioned rats, ICSS (nose-poke) was totally suppressed and the shuttle responses were disorganized since neither the ON- nor the OFF-times changed in response to increasing current intensities. Nose-poke responses with stimulation of the non-lesioned MPC were just about normal. These results show that in the two brain regions studied local neurons are involved in ICSS. The difference in the magnitude of the deficit observed suggests, that the neuronal circuits involved in MPC self-stimulation are poorly represented whereas in the LH many neuronal circuits involved in these mechanisms overlap.