REPRODUCTIVE BEHAVIOR 



1237 



of the stimulus. Olds & Critchlow (personal communi- 

 cation) have recently found that the stimulus from 

 certain electrodes, presumably hypothalamic, in cas- 

 trate male rats is positively reinforcing only when 

 exogenous testosterone is supplied. An excess of tes- 

 tosterone is inhibitory; but as the hormone level falls 

 to within physiological limits, the bar pressing is re- 

 sumed. An inverse relationship was obserxed between 

 the responses to androgen and to hunger, implying 

 differential centers for these two fundamental drives. 



Rhinemcplialw anil Rituiilm Mechanisms 



The rhincncephalon or limbic system has been 

 associated with emotion ever since Papez proposed 

 his now famous circuit in 1937. Papez (67) suggested 

 that the circuitous sequence, involving the hippo- 

 campus, fornix, mammillarv Ijody, anterior thalamic 

 nucleus, cingulate cortex and entorhinal cortex back 

 to the hippocampus, was more likely associated with 

 emotion than with olfaction. Gastaut (33) and Pri- 

 bram & Kruger (70) have reviewed and clarified the 

 interrelations of the limbic system, including the 

 amygdala, septum and frontotemporal cortex. Gloor 

 (34) and Green & Adey (39) have studied the de- 

 tailed neuronal organization by electrophysiological 

 methods. Green & Arduini (40), MacLean {60) and 

 Adey et at. (1,2) have stressed functional connections 

 between the limbic lobe and the ascending reticular 

 activating center (63). According to Green & Arduini 

 (40), afferent connections to the hippocampus include 

 pathways from the reticular actixating svstem, hy- 

 pothalamus, preoptic region and septum while Adey 

 et al. (i, 2) present e\'idence of reverse connections: 

 fornix — hippocampus — entorhinal cortex — septum — 

 stria medullaris — midbrain tegmentum. (See also 

 Chapters LV, LVI and LVIII by Kaada, Green and 

 Gloor in this Handbook which deal with these cortical 

 areas.) Lindsley (58, 59) has suggested that the ener- 

 gizing aspects of emotion, motivation and drive mav 

 be supplied via the reticular activating system. From 

 these anatomical and functional connections and inter- 

 relationships, the limbic and activating systems are in 

 excellent positions to exert a collaborative influence on 

 sexual behavior. 



A dramatic demonstration of a rhinencephalic in- 

 fluence on a type of sexual behavior has been re- 

 ported by MacLean (60). Local seizures in the cortex 

 above the posterior cingulate gyrus in the male cat, 

 induced by local chemical or electrical stimulation, 

 were accompanied by penile erections and secretion. 



A similar tendency toward penile erection was evoked 

 by electrical stimulation of the superior hippocampus 

 in the rat. von Bechterew (93) may have stimulated 

 one of these areas in his dog experiments in which 

 electrical stimulation of the cortex produced erection 

 and ejaculation. 



Many locations within the rhincncephalon and 

 midbrain have been found by Olds (65) to be posi- 

 tively reinforcing in the self-stimulation experiments 

 described above. In these experiments the depend- 

 ence of the rewarding nature of the stimulus on the 

 intactncss of the hypotlialamus and upon hormone 

 le\els has yet to be ascertained. 



Several studies have been made of the effects of 

 hormones on the electroencephalograms (EEG) of 

 rabbits and other species by Faure et al. (28, 29). These 

 authors report differential changes in the EEG's of 

 various rhinencephalic and hypothalamic loci within 

 minutes after intramu.scular injections of large doses 

 of sex and adrenal steroids and placental gonado- 

 trophins. The EEG changes occur .so rapidly as to 

 make one suspect nonspecific effects, and the records 

 have not been correlated directly with reproductive 

 behavior. However, further work with chronically 

 implanted electrodes may prove these localized al- 

 terations in spontaneous electrical activity to be sig- 

 nificant. 



Within the first few hours after subcutaneous treat- 

 ment with progesterone, the estrogen-primed rabbit 

 is highly estrous in its mating behavior. During this 

 period (fig. 9) Kawakami and Sawyer have found 

 that its threshold of EEG arousal on direct electrical 

 stimulation of the midbrain reticular formation is 

 much redviced from its preprogesterone threshold (80). 

 Vaginal stimulation or low-frequency stimulation of 

 the ventromedial Inpothalamus induces the EEG 

 'after-reaction' mentioned above, consisting of a 

 sleep-like record followed by 'pseudo-arousal' — 

 the latter is characterized by high amplitude 

 theta or faster activitv, especially in rhinencephalic 

 leads: amygdala, hippocampus and limbic cortex 

 (fig. 8). Twenty-four hours after progesterone treat- 

 ment, the rabbit is usually anestrous. Its threshold of 

 EEG arousal is markedly elevated (fig. 9) and vaginal 

 or hypothalamic stimulation is not followed by the 

 typical estrous EEG after-reaction. Hypothalamic 

 stimulation, at higher voltage, may induce sleep 

 spindles but not ordinarily the complete reaction with 

 'pseudo-arousal.' Estrus reappears with a return 

 of the EEG arousal threshold to preprogesterone 

 levels. 



