|
3. A female sex motivational mechanism; suppressed perinatally by androgen and activated in adulthood by estrogen, progestin, and luteinizing hormone-releasing factor. The existence of a female sex motivational mechanism may be postulated on the basis of a distinction between two types of stimuli that elicit both motor patterns of female sexual receptivity, lordosis and darting locomotion. Lordosis occurs in all muroid rodents, while darting locomotion occurs only in species without a locking copulatory pattern (Dewsbury, 1974). On the one hand, there are releasing and directing stimuli which directly affect the motor pattern of lordosis; these consist of tactile stimulation of the flank and pelvic region (Kow and Pfaff, 1976). On the other hand, there are motivating stimuli (called "priming" stimuli by some authors) that consist of stimulation of the genital tract in rats (Komisaruk, 1974) and ultrasound in hamsters (Beach et aI, 1976; F1oody and Pfaff, 1971b)and rats (Geyer et al, 1978) that affect a central excitatory state and may increase the female's readiness to show lordosis (and in the case of the rat, darting locomotion) for minutes or even hours after the stimulation has ceased. There may also be olfactory motivating stimuli: in the female rat, darting is enhanced following "priming" with ultrasound and male urine odors, but not following ultrasound alone (Geyer et aI, 1978); in the hamster, lordosis is enhanced following "priming'. which consists of exposure to an anesthetized male (Noble, 1973); and in the mouse (Thompson and Edwards, 1972) and hamster (Murphy, 1974), lordosis is greatly reduced following destruction of the olfactory bulbs. The dependence of lordosis upon estrogen may be taken as evidence that estrogen activates a female sex motivational mechanism (figure 1, site 3). Lordosis only appears under natural circumstances during behavioral estrus, at which time estrogen levels are high. It does not appear in ovariectomized rats, but may be obtained by systemic injections of estrogen (Ball, 1939; ])avidson et al, 1968). Estrogen injections into the spinal cord do not facilitate lordosis (Hart, 1969), while estrogen injections in the brain may be effective (Barfield and Chen, 1977). Progestin has both an excitatory and inhibitory effect upon female sex, depending upon both the strength and duration of its concentration. In small doses progestin combines with estrogen to facilitate lordosis in the rat (Beach, 1942), gerbil (Kuehn and Zucker, 1968), and hamster (Carter and Porges, 1974), although it is not always necessary for estrogen-induced lordosis (Bermant and Davidson, 1974). In high doses, progestin may be inhibitory {Powers and Zucker, 1969; Marrone et aI, 1977). Lisk (1978) has summarized the data and suggested that progestin has at least three separate actions; (1 sensitization of' target neurons for estrogen; (2) initiation of' estrus in the estrogen-primed animal; and (3) termination of heat. Since the dose levels used in experiments that demonstrate inhibition have been very high, the inhibitory effect of progestin and its role in termination of heat has recently been questioned (Baum et al, 1979). The hypothalamic releasing factor for the luteinizing hormone has been shown to facilitate female sexual behavior when injected into estrogen-primed rats from which both the pituitary and ovary have been removed (Pfaff, 1973; Moss and McCann, 1973). The significance of this effect under normal circumstances is not known. The location of the hypothetical female sex motivational mechanism may be in the ventromedial hypothalamus. Lesions of this structure abolish lordosis in the rat (Matthews and Edwards, 1977) and hamster (Kow et aI, 1974). Estrogen facilitates lordosis when implanted in this structure in the rat (Barfield and Chen, 1977; Davis et al, 1979) and hamster (Ma1sbury et al, 1978). Radioactive estrogen is taken up by this region in the rat (Pfaff and Keiner, 1973) and hamster (F1oody and Pfaff, 1974). The facilitating effects of progestin also appear to take place in this region (Powers, 1972J Blaustein and Wade, 1978), although earlier studies had implicated the midbrain reticular formation instead (Gorski, 1976). The neural substrate of the female sex motivational mechanism is apparently organized around the time of birth in females and suppressed in males by androgen at that time. This has been shown in the rat (Barraclough and Gorski, 1962), mouse (Edwards and Burge, 1971), and hamster (Crossley and Swanson, 1968). Under certain circumstances, lordosis may be obtained in adult male rats (Komissruk, 1974), and it is readily obtained in males which have been neonatally castrated to remove the suppressing effects of androgens (Feder and Whalen, 1965). The neonatal androgen apparently exerts its effect upon hypothalamic neurons (Nadler, 1968), presumably the same neurons which are critical for the female sex motivational mechanism in the adult.
|