Interactions between amygdaloid and hypothalamic self-stimulation: a re-examination.
The function relating bar-pressing rate to the frequency of cathodal pulses was obtained in rats self-stimulating with amygdaloid (AMY) and lateral hypothalamic (LH) electrodes. The maximum self-stimulation (SS) rates in the AMY was found to be very low, compared to the LH. Concurrent stimulation with pairs of AMY-LH pulses did not shift the rate-frequency functional laterally, indicating the absence of summation of the two rewarding effects. In a second experiment, concurrent AMY-LH stimulation (using sub-threshold intensity LH pulses) facilitated bar-pressing for AMY stimulation (it increased the slope of the AMY rate-frequency function) without shifting this function laterally. In a third experiment, subjects were given a choice between a pulse frequency yielding maximal AMY rate and a series of higher pulse frequencies. Subjects consistently preferred the higher frequency values, attesting that the maximum AMY rates were not constrained by a saturating reinforcing effect. In a fourth experiment, subjects were given a choice between AMY stimulation and concurrent AMY-LH stimulation, using low intensity LH pulses. Subjects showed no preference for either stimulation condition, although rates were higher for the latter condition. These findings suggest that the maximum rate for AMY stimulation was constrained by factors interfering with bar-pressing and that the effect of these factors was attenuated by co-activation of the LH. In a fifth experiment, pre-treatment with phenobarbital mimicked the rate-enhancing effect of concurrent AMY-LH stimulation for 2 of the 4 subjects tested. This finding suggests that the LH pulses contributed to attenuate seizure activity accompanying AMY SS. In a final experiment, AMY SS rates were also increased by co-activation of rewarding sites in the rostral MFB but not the dorsal raphe, suggesting an anatomical specificity of this effect.