Hypothalamic progesterone implants and facilitation of lordosis behavior in estrogen-primed ovariectomized guinea pigs

Brain Research, 70 (1974) 81-93 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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HYPOTHALAMIC PROGESTERONE IMPLANTS AND FACILITATION OF LORDOSIS BEHAVIOR IN ESTROGEN-PRIMED OVARIECTOMIZED GUINEA PIGS

L. P. M O R I N AND H. H. F E D E R

Institute of Animal Behavior, Rutgers University, Newark, N.J. 07102 (U.S.A.) (Accepted October 18th, 1973)

SUMMARY

Adult ovariectomized Hartley guinea pigs were bilaterally implanted in the basal hypothalamus or anterior hypothalamic-preoptic area with stainless steel cannulae with removable inserts. In experiment 1, animals received inserts containing crystalline progesterone, 17a-hydroxyprogesterone or cholesterol 36 h after subcutaneous injection of 3.3 #g estradiol benzoate. Regardless ofcannula content, about 43 ~o of animals with implants aimed at the basal hypothalamus displayed heat. Lesions or irritation associated with implants in this region may have increased behavioral responsiveness to estrogen. However, control implanted animals (17a-hydroxyprogesterone and cholesterol) tended to have shorter heat durations and more scattered facilitatory implant sites than progesterone implanted animals, suggesting that progesterone might be acting in basal hypothalamus to facilitate lordosis. Progesterone placed in basal anterior hypothalamic-preoptic area did not facilitate lordosis. Progesterone in basal hypothalamus or basal anterior hypothalamic-preoptic area did not produce inhibitory effects on behavior. In experiment 2, a revised procedure was used to control for sexual behavior resulting from estrogen treatment alone. The control procedures revealed that most of the animals with cannulae in the ventromedial-arcuate area displayed lordosis after estradiol benzoate alone. When animals not displaying estrus in response to estradiol benzoate alone were utilized for further implantation, it was clear that progesterone placed in the ventromedial-arcuate-premammillary area facilitated the expression of short latency, long duration heat. However, animals with cannulae in the basal hypothalamus, regardless of cannula content, had short maximum lordoses. No inhibitory effects of progesterone implants (in ventromedial-arcuate-premammillary area) on lordosis were observed. These results suggest that brain sites selectively responsive to the facilitatory actions of progesterone exist in the ventromedial-arcuate-premammil-

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lary region and that the whole basal hypothalamus participates in the normal, full expression o f guinea pig behavioral estrus.

INTRODUCTION

An early investigation of the hormonal action on the central nervous system employed intraventricular infusion of progesterone into estrogen-primed hamsters 8. This procedure facilitated lordosis in some individuals within 10 min. In recent studies with rats it has been reported that progesterone implants in the basal hypothalamus, midbrain reticular formation or basal preoptic area facilitate lordosis a8,2°,24. Guinea pig midbrain concentrates [3H]progesterone relative to hypothalamus, hippocampus and cortex2a, 22, but progesterone implants in this region inhibit, and do not facilitate, the display of guinea pig lordosis 16. In view of these results, and the demonstration that basal hypothalamic lesions in guinea pigs abolish the display of lordosis 7, the present experiments were designed to investigate the effects of diencephalic progesterone placements on the display of lordosis in female guinea pigs. MATERIALS AND METHODS

Adult Hartley strain, albino female guinea pigs were obtained from Camm Research Laboratories (Wayne, N.J.). Animals arrived at the laboratory weighing 350-450 g and were ovariectomized under Equi-Thesin (Jensen-Salsbery, St. Louis, Mo. ; 0.6 ml per animal) and Innovar-vet (Pittman-Moore, Washington, Crossing, N.J. ; 0.05 ml per animal). Throughout the experiment, animals were housed 6-8 per cage and were provided with Purina guinea pig chow and water ad libitum. Fresh lettuce was provided 3 times weekly. Lights were on from 05.00 until 19.00 and laboratory temperature was maintained at approximately 23 °C. Ten to 14 days after ovariectomy, all animals were given estradiol benzoate (EB) followed 36 h later by progesterone (P). Animals were then given a 'screening' test for lordosis behavior. Only those animals which responded on two or more consecutive hours with lordoses which were held for more than 1 sec duration were utilized during the subsequent experiments. Testing during this screening test and the experiments was by the manual stimulation method of Young et al. 26. Within 2-4 days after screening, all animals were implanted with bilateral double-barreled stainless steel cannulae according to a procedure described previously 16. The surgery was conducted with the animal placed in a K o p f stereotaxic instrument with the incisor bar at --7.5 mm. The sites aimed at were the preoptic area (POA), anterior hypothalamus (AH) and ventromedial hypothalamus (VMH). The coordinates were: POA: 1.5 mm lateral, 8.5 mm vertical, 12 mm anterior to the interaural line; AH: 1.0 mm lateral, 10.0 mm vertical, 10.4 mm anterior; VMH: 1.0 mm lateral, 9.5 mm vertical, 9.2 mm anterior 13. After all behavioral tests were completed, animals were killed with Equi-Thesin. Histology was performed according to methods described previously 16. Implant sites were verified by microscope.

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EXPERIMENT 1

The method of examining neural loci responsive to intracranial hormone implantation traditionally employs an experimental group with cannulae containing the critical steroid and a control group cannulated with a control substance, usually the steroid precursor, cholesterol4,12,16,18. The purpose of the second group is ostensibly to control for possible lesion or irritative effects caused by the placement of a foreign body in the brain. If the two treatment groups differ in spite of their common properties, then it may be concluded on a statistical basis that the difference reflects the treatment difference namely, the content of the cannulae. The present experiments are based on the foregoing rationale and attempt to demonstrate facilitatory effects of anterior hypothalamic-preoptic area (AH-POA) or basal hypothalamic progesterone placements on the lordosis behavior of estrogen-primed female guinea pigs. Methods

Twelve days after the 'screening' test, all animals were injected subcutaneously with EB. Thirty-six hours later, animals were tested for lordosis. Any animal showing a lordosis response for longer than 1 sec was considered as being in heat in response to estrogen and no further tests were made on that animal. The remaining animals were randomly assigned to one of three groups and their blank cannulae were replaced bilaterally with clean ones containing crystalline P, cholesterol (chol) or 17a-hydroxyprogesterone (17-OHP). Animals were then tested hourly until all were out of heat. Some of the animals received a systemic injection of P 8 h after implantation of steroid or sterol, in pursuance o f a procedure established previously as a method of testing for inhibitory effects of intracranial P on lordosis 16. Heat duration, latency and maximum lordosis were measured and have been defined previously 1~. Results

Progesterone aimed at the suprachiasmatic A H - P O A did not facilitate sexual behavior in estrogen-primed guinea pigs during the 8 h period of testing prior to systemic P injection. Similarly, control animals with implants containing cholesterol did not exhibit any signs of sexual behavior. The sites of implantation are displayed in Fig. 1. Although this area in guinea pigs is responsive to the priming actions of intracranial estrogen 17, there is no evidence from this experiment that P exerts an effect on lordosis behavior via this neural locus. Of the 34 implanted animals with cannulae aimed more caudally (into the A H VMH-arcuate area) than the above group, 15 displayed lordosis responses which met the criteria for estrus. However, the proportion of animals responding to P implants did not differ significantly from the proportion of control animals responding (8/19; 4/8 with chol and 4/11 with 17-OHP implants; for the purpose of all further analysis, chol and 17-OHP implanted animals are treated as identical controls). Heat durations following basal hypothalamic implants of P tended to be longer than heat durations in animals receiving control implants in basal hypothalamus (P -----12.2 ± 1.4 vs. controls -----8.4 ± 2.4 h; 0.1 > P > 0.05, one-tailed t-test). Histological analysis also suggested

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Fig. 1. Sagittal section o f guinea pig brain a b o u t 1 m m lateral to the midline indicating the location o f implant sites which facilitated lordosis behavior in estrogen-primed animals. The only control animals s h o w n arc those with implants a i m e d at the basal h y p o t h a l a m u s . Control implants aimed at the basal A H - P O A (N = 4) did not facilitate lordosis. Abbreviations: A H -- anterior h y p o t h a l a m u s ; D M H -- dorsomedial h y p o t h a l a m u s ; O C ~ optic c h i a s m ; P O A -- preoptic area; V M H -- ventromedial h y p o t h a l a m u s .

a difference between P and control implanted animals. Positive brain sites from control animals tended to be more widely scattered than P positive sites. Of the 15 P responders, all but two had implants within the VMH-arcuate area, while 5/8 control responders had implant sites outside the VMH vicinity (Fig. 1). The results suggest that P implanted in the basal hypothalamus may facilitate lordosis, but the role of P is made ambiguous by the finding that animals bearing control implants also displayed lordosis. It is unlikely that the control substances had direct facilitatory effects. Rather, the lordosis responses obtained from control implanted animals are probably attributable to the priming dose of EB. Placement of cannulae in basal hypothalamus may have produced lesions or irritative stimulation which rendered these animals more sensitive to EB and resulted in 'estrogen heats '2. In order to control for this possibility, and to more clearly delineate the role of progesterone in facilitating lordosis, a different experimental design was devised. This is the subject of experiment 2. Additional manipulations of P implanted animals (VMH-arcuate area) in experiment 1 were performed. Six P implanted animals injected subcutaneously with P 8 h after hormone placement and after the beginning of heat in response to the implant had a mean duration of 12.67 :k 2.12 h. This did not represent a prolongation of heat beyond that caused by the P implant alone (12.22 ~: 1.44 h). However, it should be noted that the approximately 12 h heat duration shown by P implanted animals (whether or not they received a supplementary P injection) is considerably longer than that which is normally obtained in unimplanted, spayed females given EB followed 36 h later by a subcutaneous P injection (no implants; 6.8 ± 0.6 hl~). These results suggest, first, that continued presence of P in basal hypothalamus prolongs the duration of heat. Second, injection of P at 8 h after implantation of P in the VMH-arcuate area does not lengthen heat beyond what could be expected from the

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TABLE I EFFECTS OF INTRACRANIALPROGESTERONE PLACEMENTAT 36 h AFTER EB PRIMING UPON RESPONSE TO 0.6 m g PROGESTERONEADMINISTEREDSYSTEMICALLYAT 44 h The data include only those guinea pigs which were not in heat following cannula manipulation at 36 h. There were no significant differences between progesterone and cholesterol implants at corresponding locations.

Cannula location

Cannula content

Proportion in heat

Duration

Maximum lordosis

Latency

AH-POA

Prog Chol

13/13 4/4

7.38 4- 0.60 6.75 4- 0.95

20.15 4- 3.82 17.50 4- 4.44

3.54 ~ 0.24 4.00 i 0.00

Basal H t h

Prog Chol

11/17 10/11

4.80 4- 0.98 6.44 4- 1.02

6.50 4- 0.86 8.11 4- 2.59

4.10 4- 0.53 3.67 4- 0.65

P implant alone. The fact that heat is not prolonged even beyond a 12 h duration by intracranial P or intracranial P plus systemic P suggests that the VMH becomes unresponsive to further facilitatory effects o f P after about 12 h. A final manipulation of the P implanted animals in experiment 1 was designed to test for the inhibitory effects of the hormone on lordosis 16. The test was performed on animals which did not display lordosis in response to control or P implants in A H - P O A or basal hypothalamus placed 36 h after systemic EB injection. These animals were given 0.6 mg P systemically 8 h after steroid or sterol placement. All animals with P or control implants in the A H - P O A responded to the P injection with apparently normal heats in terms of duration, maximum lordosis and latency (P = 13/13; control = 4/4; Table I). In the basal hypothalamus, there also was no difference between P and control implanted animals (P = ll/17; control = 10/ll; P = 0.13, Fisher's exact probability test). There were no significant differences in duration, maximum lordosis or latency between the two groups (Table I). Although these data are biased in the sense that information from responders to intracranial P is necessarily excluded, the results clearly indicate that P does not act in the A H - P O A to exert inhibitory effects on lordosis. Basal hypothalamic P implanted animals tended to show some degree of inhibition, but this did not attain statistical significance. EXPERIMENT

2

The first experiment suggested that the facilitatory effects of P implanted in basal hypothalamus were confounded by cannula induced lesions or stimulative irritation associated with the process of cannula placement. The present experiment sought to control for these effects by (a) testing carefully for estrogen responses before manipulating the cannulae; (b) implanting all animals with blank or cholesterol cannulae; and (c) not placing P in the brain unless it was clear that the animal was not responding to the control implants. In addition, the first experiment did not provide sufficient information concerning possible inhibitory actions of P within the VMH area. The

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Method All the animals in experiment 2 had implants aimed at the VMH. Twelve days after the 'screening' test, the animals were injected with EB. All animals were tested for lordosis 36, 37 and 38 h later. I f no response was obtained, a blank or chol-containing set of cannulae was placed in the brain. I f a lordosis response was elicited, implantation was deferred until two consecutive hours of non-response occurred; then implantation proceeded as it otherwise would. If, 8 h after the first implantation, the animal had not shown lordosis for 6 h, the control set of cannulae was removed and replaced with a clean set containing either P or chol. Animals were tested hourly throughout the entire procedure and the tests continued until all animals were out of heat (Fig. 2). Four guinea pigs were anesthetized with Equi-Thesin 4 h after the end o~

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heat and a plasma sample was withdrawn by cardiac puncture. Plasma P levels were measured by the radioimmunoassay method of Abraham et ai. 1. A second group of guinea pigs was tested at 36, 37 and 38 h after EB, cannulated with chol at 38 h, then tested again at 43 and 44 h. They were recannulated with chol or P at 44 h, and tested again at 45.5 and 47 h. Thus, these animals were tested 7 times scattered over 11 h, instead of hourly for about 20 h as in the foregoing group. By use of this testing procedure, we sought to prevent any debilitating effects of prolonged repeated testing on the likelihood of response to systemic P 60 h after EB (test for inhibitory effects of P implants). Because of the limited testing, no information from this group was used to indicate the effects of VMH-P facilitation of sexual behavior, although the procedure allowed observation of estrogen responses, if they appeared. Sixty hours after EB injection all animals (except those bled for plasma samples) were tested for lordosis and, if the test was negative, they were injected with 0.6 mg P. If positive responses were obtained, injection was deferred until two consecutive hours of non-response occurred. Testing was performed hourly from 60 h after EB for about 12 h. A third set of control animals was ovariectomized (but not implanted), given 3.3 #g EB at h 0, and either 0.6 mg P systemically or oil at 44 h. Testing for lordosis began at 36 h and continued until all animals were out of heat following the P injection. At 60 h, all animals received 0.6 mg P and were tested hourly for 9 h. This group provided normative data on response to P administered 60 h after EB to unimplanted animals which had been tested for about 20 h, in the same manner as the first group of implanted animals described in experiment 2. Cannulae loaded with P were removed from 6 animals 68 h after placement in the brain and inspected under a dissecting microscope. There was no obvious loss of P from the lumen o f any insert. Results The revised cannulation procedure for controlling against the possibility of

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Fig. 3. Sagittal section of guinea pig brain about 1 mrn lateral to the midline indicating the location of progesterone implants which facilitated lordosis in estrogen-primed guinea pigs. See Fig. 1 for abbreviations.

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Fig. 4. Facilitatory effect of progesterone implants in premammillary-VMH-arcuate region on heat duration, latency and maximum lordosis of estrogen-primed guinea pigs (solid bars). Data from control implanted animals injected with progesterone (striped bars, from experiment 1) and unimplanted, chronically tested controls injected with progesterone (open bars) are provided for comparison. Animals were tested for lordosis beginning at 44 b. See Fig. 1 for abbreviations. 'estrogen heats' clearly revealed a facilitatory effect of intracranial P on lordosis behavior in estrogen-primed guinea pigs. The results indicate that 9/16 P implanted animals displayed heat as compared with only 1/9 chol implanted animals (P -- 0.034; Fisher's exact probability test; Fig. 3). In Fig. 4, values for several characteristics of heat in response to P implants in the V M H area are related to comparable data obtained from non-responding control (chol or 17-OHP) implanted animals injected with 0.6 mg P at 44 h (from experiment 1). Data from unimplanted animals which received 0.6 mg P at 44 h are also shown for comparison. As compared with control implanted, P injected animals, the duration of heat in response to the P implant was longer, as expected (9.11 ~ 1.06 vs. 6.44 q- 1.02 h; t -- 1.82, d f ~ 16, P < 0.05, one-tail); the latency to heat after P cannulation was shorter than after P injection (1.44 ± 0.34 vs. 3.67 ~ 0.65 h; t ~ 3.05, d f ~ 16, P < 0.01, two-tail); there was no difference with regard to the length of maximum lordosis (6.44 ~- 0.67 vs. 8.11 ~- 2.59; t -- 0.62, d f - - 16, P ~ 0.25, one-tail). Although the maximum lordoses exhibited by both P implanted and control implanted, P injected

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LORDOSIS AND HYPOTHALAMIC PROGESTERONE

animals were not different, both groups tended to have short lordoses relative to an unimplanted control group (Fig. 4). This result indicates that the presence of cannulae in the V M H area may have had a detrimental influence on the intensity of guinea pig sexual behavior. It may be noteworthy that one control implanted, P injected animal had a 24 sec m a x i m u m lordosis. This was the only animal in either group with implants outside of subfornical hypothalamus; placement was in the posterior hypothalamus. In experiment 1 the suggestion was made that bilateral cannulae placed in the vicinity of the V M H might cause a lesion and thereby increase behavioral sensitivity to estrogen. Data from experiment 2 substantiate the suggestion because 52.5 ~ of all implanted animals (N = 42) exhibited lordosis responses before any steroids or sterols were placed in the cannulae (Fig. 5). This is compared to 1/66 animals implanted in either the midbrain 16 or A H - P O A vicinity. Also, among 13 EB injected, unimplanted controls tested for 8 h, none showed an estrogen response. It seems likely, therefore, that the bilateral cannulation procedure increased sensitivity to estrogen via a localized lesion effect. In a previous communication, we reported that P could act in the midbrain to inhibit sexual behavior following systemic hormone administration 16. Experiment 1 demonstrated that a similar effect did not occur following P implantation in A H - P O A . In addition, experiment 1 suggested that P did not have significant inhibitory effects on lordosis when implanted into VMH. Experiment 2 extends this observation. Because the limited testing procedure employed with some animals did not affect behavioral responsiveness, the data f r o m all animals injected with P about 60 h after EB were combined according to the substance placed intracranially. The results indicate no difference in the likelihood of heat as a function of P or chol placed in the brain (6/16 P vs. 5/15 chol). Thus, P implants in V M H are as ineffective in inducing an inhibitory state as chol implants in VMH. However, there is evidence in these two groups of a detrimental effect of cannulation on display of sexual behavior in response to exogenous steroids. There was 100 K heat in spayed, but unimplanted, animals injected systemically with P 60 h after EB as compared to the 33-38 ~ observed in the cannu-

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Fig. 5. Sagittal section of guinea pig brain about 1 mm lateral to the midline showing the relationship of basal hypothalamic implant sites and occurrence of lordosis behavior in estrogen-primed guinea pigs prior to cannula manipulation or hormone injection. See Fig. 1 for abbreviations.

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L. P. M O R I N A N D H . H . F E D E R

lated animals. Unexpectedly, 3/6 P implanted animals came into heat within 1 h after P injection. Latencies of less than 2 h were not seen in 5 control animals. Brain sites responsive to P implantation are illustrated in Fig. 3. Positive P sites are scattered throughout a region which cannot be localized to any particular hypothalamic nucleus. Short latency, long duration responses were obtained from sites variously located in basal VMH-arcuate, ventral dorsomedial hypothalamus (DMH) and ventral premammillary nucleus (VPM). This histological analysis does not contradict, but rather extends, the information obtained in experiment 1. In that study no control or P implants were placed in the VPM. In the present study, the lone animal which responded with chol in the brain had implant sites about 2 mm below the dura. The other chol implant sites, which were all negative, were scattered throughout the P sensitive area. Among 4 progesterone implanted animals, radioimmunoassay indicated values of 0.026, 0.026, 0.051 and 0.102/~g progesterone/100 ml plasma. These values were not strikingly different from 'blank' values from plasma of spayed, uninjected guinea pigs (about 0.041/~g/100 ml). Thus, progesterone from the intracerebral implants was not entering the general circulation in significant quantities. Even in the one animal having a progesterone value of 0.102 #g progesterone/100 ml, it must be noted that the quantity above the 'blank' level is only about one-sixth the concentration of circulating progesterone during early heat in intact guinea pigs 6. DISCUSSION

The present data demonstrate that progesterone acts directly on diencephalic tissues to facilitate, but not to inhibit, the expression of lordosis behavior in EB-primed guinea pigs. The area for such facilitation extends basally from rostral VMH into the premammillary region. On the other hand, progesterone implants in basal AHPOA had neither facilitatory nor inhibitory effects on sex behavior, and progesterone implants in midbrain had inhibitory effects only16. The estrous receptivity obtained after progesterone implantation in VMHpremammillary region was generally of short latency and long duration. In terms of latencies, 7 of the 9 responsive animals in experiment 2 had latencies of 1 h. This was the minimum possible latency under the testing procedure used, and is the first demonstration that heat latency in guinea pigs can be significantly shortened as a function of route of administration of progesterone. Previous attempts to shorten heat latency by means of intravenous or intraventricular infusion of progesterone were unsuccessful in guinea pigs (W.D. Joslyn, unpublished data), but successful in hamsters and rats 8,11,14. In terms of heat duration, progesterone implantation in the VMH-premammillary area resulted in prolonged durations (about 12 h in experiment 1 and 9 h in experiment 2) in comparison with durations seen after a single systemic injection of progesterone (about 7 h) 15, or during the occurrence of a naturally occurring estrus (about 8 h) ~,2~. This prolongation of estrous receptivity in the presence of continued high levels of progesterone in brain is consistent with the finding that multiple systemic injections of progesterone also prolong heat (by about 2.7 h)lL The data are

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also consistent with the previously established idea that there is an inverse relationship between heat latency and heat durationL The effective sites for progesterone facilitation of lordosis reported here correspond well with those reported for rats by Powers TM. However, they are at variance with other experiments showing that progesterone implanted in midbrain or basal POA facilitates lordosis in rats20,24. These inconsistencies may represent true species differences. It is more difficult to account for the fact that no inhibitory effects of basal hypothalamic progesterone implants were seen in the present study, because Wallen e t al. 23 have implicated the VMH region as a mediator of progesterone induced inhibitory effects on sex behavior in guinea pigs. These investigators found that placement of cycloheximide (a protein synthesis blocker) in the VMH area abolished progesterone-induced inhibition of lordosis. The major objective of the present experiment was to determine brain sites mediating the facilitatory actions of progesterone on female behavior. However, some of the implantation procedures used to accomplish this goal had unexpected effects. For example, nearly all animals receiving control implants in hypothalamic and premammillary areas displayed short maximum lordoses. In contrast, cannulae with tips situated above the optic chiasm or in posterior hypothalamus were not associated with any behavioral deficit. In what may be an analogous phenomenon, Goy and Phoenix7 described decreases in maximum lordosis in guinea pigs with electrolytic lesions in the rostral VMH-arcuate area. Because estradiol can act in the vicinity of the lesions 17, while the progesterone sensitive area extends more caudally, an estradiol responsive system was probably disrupted by the lesions. A second behavioral deficit found in the course of this investigation was the inability of EB-primed, cannulated animals to show heat after a systemic progesterone injection at 60 h. Only 35 ~o of animals thus treated displayed lordosis. Therefore, this behavioral deficit appears related to the presence of cannula induced lesions or irritations in the basal hypothalamus rather than to inhibitory effects of progesterone on lordosis. The deficit may also be related to the time since estrogen-priming because among cannulated animals injected with exogenous progesterone at 44 h (experiment 1, Table I), 75~ displayed lordosis (21/28 vs. 11/31 which received P at 60 h). Although the foregoing results were unexpected, the most surprising outcome was an apparent increase in sensitivity to estrogen resulting in approximately 50 ~ of implanted animals (experiment 2, Fig. 5) showing lordosis prior to any cannula manipulation. In spite of the 9-11 day recovery period after implantation, persistent mechanical stimulation associated with cannula placement may have facilitated lordosis in the EB-primed animals. L.G. Clemens (personal communication) has obtained such responses with blank cannulae placed in the midbrain of rats. In the present study, increased responsiveness to EB seems to have occurred largely among animals with cannulae in the VMH-arcuate vicinity, generally excluding the premammillary area. The region affected by the cannulae therefore appears to be separable from the more caudally situated areas mediating facilitatory actions of progesterone. These observations and the chronic behavioral estrus, observed in two guinea pigs with rostral basal hypothalamic lesions 7 and several rats with premammillary lesions1°, suggest

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some f o r m o f disinhibition resulting f r o m lesions in the basal area. This is n o t an unlikely effect because estradiol can facilitate lordosis by acting in the basal h y p o t h a l a m u s o f b o t h rats s a n d guinea p i g s l L Indeed, several investigators have h y p o t h e sized t h a t a p r i m a r y facilitatory function o f estradiol is to induce disinhibition o f neural systems regulating lordosis behaviorg, 19. ACKNOWLEDGEMENTS S u p p o r t e d by R e s e a r c h Scientist D e v e l o p m e n t A w a r d MH-29006 (to H. H. F e d e r ) f r o m the N a t i o n a l Institute o f M e n t a l H e a l t h a n d by Research G r a n t H D 04467 (to H. H. F e d e r ) f r o m the N a t i o n a l Institute o f Child H e a l t h a n d H u m a n D e v e l o p m e n t a n d a G r a n t f r o m the A l f r e d P. Sloan F o u n d a t i o n to Dr. D. S. L e h r m a n , Director, Institute o f A n i m a l Behavior. L. P. M o r i n was s u p p o r t e d by T r a i n i n g G r a n t G M - 1 1 3 5 f r o m the N a t i o n a l Institute o f M e n t a l Health. P r o g y n o n - B a n d P r o l u t o n were p r o v i d e d by the Schering C o r p o r a t i o n . W e t h a n k Ms. M. Buntin for the histology, Mr. Ch. R e b o u l l e a u for p e r f o r m i n g the r a d i o i m m u n o a s s a y s a n d Ms. J. R u b i n for her help with the manuscript. C o n t r i b u t i o n No. 175 o f the Institute o f A n i m a l Behavior.

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