Mating-induced inhibition of receptivity in the female golden hamster (Mesocricetus auratus): III. Stimuli mediating long-term effects

Physiology & Behavior. Vol. 39, pp. 421428. Copyright©PergamonJournals Ltd., 1987.Printedin the U.S.A.

0031-9384/87$3.00 + .00

Mating-Induced Inhibition of Receptivity in the Female Golden Hamster (Mesocricetus auratus): III. Stimuli Mediating Long-Term Effects U. W I L L I A M H U C K / R O B E R T D. L I S K , E R I C J. P A R E N T E A N D C H R I S T I N A L. G U Y T O N

D e p a r t m e n t o f Biology, Princeton University, Princeton, N J 08544 R e c e i v e d 28 J u l y 1986 HUCK, U. W., R. D. LISK, E. J. PARENTE AND C. L. GUYTON. Mating-induced inhibition of receptivity in the female golden hamster (Mesocricetus auratus): IlL Stimuli mediating hmg-term effects, PHYSIOL BEHAV 39(4) 421-428, 1987.--In the golden hamster, mating with an intact male results in both a reduction in receptivity shown toward a second male (short-term effect) as well as an absolute abbreviation of the receptive period (long-term effect). The present study examined the components of the mating situation responsible for the long-term effect. In experiment 1 we showed that the paradigm used to determine receptivity (i.e., brief hourly exposures to an intact male) did not affect the duration of receptivity whereas an ad lib mating early during the receptive period resulted in a 40% reduction in the length of behavioral estrus. In experiment 2 females received various types of mating stimuli, e.g., mounts from an apronned male, intromissions from a vasectomized male, a simulated copulatory plug, manually-induced lordosis, or several of these stimuli in combination. Only intromissions from an intact or vasectomized male significantly reduced the duration of behavioral estrus. Although as few as 10 intromissions from a vasectomized male were effective in significantly shortening the receptive period, higher levels of vaginocervical stimulation from a single male (experiment 3) or from additional males (experiment 4) effected significantly greater reductions in the length of behavioral estrus. Our results further demonstrated that only those females that subsequently became pseudopregnant showed significant abbreviations of behavioral estrus. The association of these two phenomena is of obvious adaptive value since it provides a mechanism to terminate a behavior once its function has been achieved. Hamster

Inhibition of receptivity

Behavioral estrus

M A T I N G - I N D U C E D inhibition of receptivity has been conceptualized as a mechanism to terminate a behavioral function once that function has served its,purpose--in this case providing the sperm and vaginocervical stimuli necessary for a successful pregnancy. However, we believe that this view is overly simple and may have inhibited rather than facilitated understanding of the underlying mechanisms as well as the adaptive significance of this phenomena. Specifically, in earlier studies of mating-induced inhibition of receptivity, little distinction was made between short-term and long-term effects of copulatory stimuli. Recently, we have shown that female golden hamsters which have actively terminated mating with one male will continue mating if given access to a novel male [19, 24, 25]. Thus, the inhibition of receptivity noted in some earlier studies (cf., [4]) was probably mate specific rather than general. We have also shown that although females which have terminated mating with one male will respond to others, the copulatory behavior

Intromissions

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of the first male resulted in both a temporary, short-term inhibition of receptivity (within 5 min) as well as a long-term (after 8 hr) reduction in receptivity [18]. Copulation with a single male was also shown to effect a 40% shortening of behavioral estrus [18]. The present communication is part of a series designed to identify the components of the mating situation responsible for short- and long-term effects on receptivity. In a previous study [21], it was shown that the short-term inhibition of receptivity observed in recently-mated female hamsters was a function of lordosis duration with the original male rather than of male intromissive stimulation as had been previously supposed [3]. Indeed, by varying lordosis time and intromission frequency independently, intromissions were shown to have no appreciable effect on a female hamster's reaction to a second male presented 5 min after the first. The objective of the present study was to examine the specific copulatory stimuli responsible for the long-term reduction in receptivity.

1Supported by the Department of Biology of Princeton University and Grant BNS-8607258 from the National Science Foundation. 2Requests for reprints should be addressed to U. William Huck, Biology Program, Sangamon State University, Springfield, IL 62794.

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In experiment 1 we established a population graph for the onset and termination of receptivity in mated and unmated virgin and primiparous females under the conditions of our study. We also determined whether the testing paradigm used in the remainder of this study would itself have any effect on the duration of the receptive period. Experiment 2 was designed to clarify the relative importance of vaginocervical stimulation, sperm deposition, the mere presence of an intact male, as well as of lordosis per se for reducing the overall length of behavioral estrus. The role of intromissions in reducing the duration of behavioral estrus was further investigated in experiment 3 by providing females with a varying number of intromissions from a vasectomized male. In addition, the possibility of a correlation between the number of intromissions received, duration of the receptive period, and the incidence of pseudopregnancy was also examined. Lastly, experiment 4 was designed to determine whether mating with more than one male would reduce the length of behavioral estrus beyond the reduction observed as a result of mating with a single male. METHOD

Animals and Husbandry

Subjects and stimulus animals were laboratory-born descendants of the randomly bred LVG strain obtained from Charles River, Inc. A total of 150 virgin and 537 primiparous females were used. The virgins were 128--171 days of age (mean=143.7) and weighed 120-158 g (mean=132.0); the primiparous females were 130-195 days of age (mean= 149.3) and weighed 126--174 g (mean=140.8). All were in natural estrus as determined by extravaginal examination [28] and

each was used only once. The 78 males used in this study were 170-315 days of age (mean=228.9) and weighed 123162 g (mean= 134.7). All had sired at least two previous litters and each was allowed at least 2 weeks between tests. All animals were maintained in windowless, airconditioned rooms on a reversed 14 L:10 D photoperiod. Each was individually housed in a 35×20×17.5-cm rackmounted cage provided with commercial laboratory chow and water ad lib. Males and females were maintained in separate rooms. All tests were begun at 12:30 (30 min after the start of the dark phase of the photoperiod). Apparatus

All tests and pre-tests were conducted in a 35 cmdiameter Plexiglas cylinder. Behavior was recorded in real time with a microcomputer (IBM PC or Aim 45). The arena was cleaned between tests. EXPERIMENT

1

Previous estimates of the duration of receptivity in the unmated golden hamster range from 12 to 20 hr [7, 15, 22]. More recently we [18] showed that when maintained under a 14 L:10 D photoperiod, the mean duration of estrus was significantly, longer for virgins than for older, multiparous females (20 hr, 34 min versus 18 hr, 17 rain). This effect was due both to an earlier onset and later termination of receptivity in the virgin females. Since the data reported from these different laboratories show considerable variance in the duration of receptivity, it was first necessary to determine for our conditions of animal maintenance the duration of receptivity in virgin and primiparous females. In addition,

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since the procedure used to determine the length of behavioral estrus involved exposing test females to stud males repeatedly at hourly intervals, this experiment was also designed to determine whether this procedure itself would affect the duration of receptivity in mated and unmated females. Procedure A total of 150 virgin and 150 primiparous females were randomly assigned to one of 12 treatment groups (see Fig. I; n =25 per group). In half of the treatment groups, the females were paired with a stud male and allowed to copulate until they attacked the male or until 15 min elapsed without copulatory activity. These females received 31-73 short intromissions (mean=42.6), 11-17 ejaculations (mean= 13.9), and 5-27 long intromissions (mean= 12.8). To determine the effects of repeated exposures to a stud male on the duration of receptivity, three test paradigms were employed. In the first (paradigm A) females were removed from their home cages and placed into the Plexiglas arena containing an habituated (at least 5 min) stud male. This procedure began at 1300 hr (6 hr before the onset of the dark phase of the photoperiod) on the day of proestrus and continued at 1-hr intervals until 1700 hr the next day (estrus). Females were scored as " r e c e p t i v e " if they assumed the lordotic posture within 15 min or " n o n r e c e p t i v e " if they failed to show lordosis during that time or if they attacked the male. Females were not exposed to the same male twice. Intromissions were prevented by removing the male whenever he mounted while the female was displaying lordosis. In paradigm B, females were similarly exposed to males only at the beginning and end of their receptive period. Specifically, virgin and primiparous females were tested beginning at 1300 hr on proestrus and at hourly intervals until they were judged to be receptive. No additional receptivity tests were conducted until 3--4 hr before the expected termination of receptivity

(0600 hr of estrus for nonmated females and 2300 hr for mated females). In paradigm C, receptivity tests were conducted only near the end of the receptive period (beginning at 0600 hr on estrus for nonmated females and 2300 hr on proestrus for mated females). Although the latter testing paradigm precluded determination of the length of the receptive period, it did allow us to determine whether repeated exposures to a stud male at the onset of receptivity would affect the time at which the receptive period ended. Results The results of this experiment are summarized in Fig. 1. Scores for duration of the receptive period were analyzed using a 3-way analysis of variance test (ANOVA) with only Treatments A and B included in the analysis. There were significant main effects due to parity, F(1,192)=12.83, p<0.001, and mating, F(1,192)= 168.35, p<0.001, indicating that behavioral receptivity was longer for virgin than for primiparous females and that mating significantly reduced the duration of estrus. There was no significant effect due to treatment, F(1,192)=0.88, nor were any of the interaction effects statistically significant. The reductions in the length of estrus observed in this experiment due to mating (32% in virgins and 37% in multiparous females) are similar to those reported by us in an earlier study [18]. The results of a 3-way A N O V A (with all three treatments included) on the time of estrus termination yielded similar results. Estrus termination occurred significantly earlier in primiparous females than in virgins, F(1,288)=8.98, p<0.001, and earlier in mated than in nonmated females, F(1,288)=94.54, p<0.001. There was no significant effect due to treatment, F(2,288)=0.97, however, nor were any of the interaction effects statistically significant. Clearly, then, determining the length of the receptive period by briefly exposing females to a series of stud males at 1-hr intervals does not itself significantly affect the length of estrus.

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Changes in the duration of behavioral estrus following copulation have been observed in several species (rats: [l]; cattle: [30]; cats: [31]; golden hamsters: [lS]). In guinea pigs either mating or mechanical vaginocervical stimulation is adequate to abbreviate estrus [16] while in ewes the mere presence of the male may alter the length of estrus [29]. In several species of insects, chemical cues from the ejaculate are important for postcopulatory declines in female receptivity [8, 23, 271 and Buntin. Ciaccio and Lisk [2] found that in the hamster, introduction into the vagina of a piece of silastic tubing (intended to mimic a seminal plug) plus mounting in the absence of intromissive stimuli significantly hastens the postcopulatory decline in receptivity. In view of the foregoing and our previous finding that the short-term inhibitory effect of a previous mating was a function of lordosis duration with the original male, we decided to test a wide variety of stimuli which might be effective in altering the length of the female’s receptive period. These included mount stimuli from an apronned male, intromissions from a vasectomized male, a simulated copulatory plug, manuallyinduced lordosis, the presence of a male within a hardware cloth cylinder, or several of these stimuli in combination. Females receiving intromissions and ejaculations from an intact male or which were placed into an empty test arena (i.e., no male stimuli) provided a baseline against which the previous treatment groups were compared.

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A total of 193 primiparous females were used in this experiment. Beginning at 130 hr on proestrus, all females were tested for receptivity using the procedure described in the previous experiment (test paradigm A). At 1930 hr subjects were assigned to 1 of 9 treatment groups (n=20-30 per group). Nonmated, control females (Treatment C) were placed into an empty arena for 30 min. Females in Treatment M received 50 mounts from an apronned male capable of mounting but not in&omitting. The apron was similar to that used by Carter 131and consisted of adhesive tape lined with gauze and applied to the lower half of the body. Females in Treatment I received 2-6 mounts and 4&48 intromissions from a vasectomized male. Females in Treatment L were placed into the test arena and maintained in lordosis for 30 min by gently fingering the female’s flanks and occasionally touching her tail (see [26]). Females in Treatment P received a simulated copulatory plug (5 x 3 mm section of silastic tub-

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ing) lubricated with glycerine and inserted into the vagina immediately before being placed into an empty arena for 30 min. Females in Treatment Lp received a simulated copulatory plug as well as 30 rain of manually-induced lordosis. Females in Treatments Mp and Ip each received a simulated copulatory plug and 50 mounts from an apronned male or 2-5 mounts and 45-48 intromissions from a vasectomized male, respectively. Lastly, females in Treatment E were paired with an intact male and received 4-6 mounts and 44-46 intromissions, 12-14 of which resulted in ejaculations. At the end of each treatment, females were returned to their home cages where they remained undisturbed until hourly receptivity tests were resumed at 2100 hr. These tests were continued until females were non-receptive for two consecutive tests. Vasectomies were performed under Nembutal anesthesia by making a small incision in the lower abdominal wall anterior to the scrotal sacs and tying offeach ductus deferens in two places. The section between the ties was then removed. Seminal vesicles were also tied and removed at this time. Two weeks after surgery, vaginal lavages were taken from females which had received intromissions and behavioral "ejaculations" from a vasectomized male. The absence of sperm and coagulated seminal fluids was used to validate the surgical procedure. Results The results of this experiment are summarized in Fig. 2. A l-way A N O V A indicated a significant treatment effect on the duration of behavioral receptivity, F(8,184)=114.39, p<0.001. Further analysis with Student-Newman-Keuls multiple range comparison tests (SNK, p<0.05) indicated that the nine treatment groups fell into two distinct statistical categories, i.e., those treatments in which females received intromissive stimulation (e.g., Treatments I, Ip, and E) and those in which females did not receive intromissions (e.g., the six remaining treatments). In all three of the former treatments, estrus duration was reduced by about 40% compared to nonmated control animals (Treatment C) whereas the duration of estrus in the treatments in the latter category did not differ significantly from that of control females. Clearly, then, mating-induced abbreviation of estrus in the golden hamster is mediated by vaginocervical stimulation from male intromissions rather than mounts, a simulated plug, maintenance of the lordotic posture, or the mere presence of a male.

EXPERIMENT 3

The purpose of this experiment was two-fold. First, since vaginocervical stimuli received from male intromissions were shown to be responsible for the abbreviation of estrus which normally follows copulation by intact males (previous experiment), we wished to determine how many intromissions were required to produce this effect and whether the effect was an " a l l " or " n o n e " phenomenon. Second, insofar as females receiving intromissions (without ejaculations) are likely to become pseudopregnant [17], we wished to determine whether a correlation existed between the initiation of pseudopregnancy and the abbreviation of behavioral estrus. Specifically, would females that became pseudopregnant as a result of vaginocervical stimulation also show a shortening of the period of behavioral receptivity.9 Procedure A total of 109 primiparous females were used in this experiment. Beginning at 1300 hr on proestrus all were tested for receptivity at 1-hr intervals (experiment 1, test paradigm A). At 1930 hr subjects were assigned to 1 of 4 treatment groups (n=20-46 per group). Nonmated control females (Treatment C) were placed into an empty arena for 30 min. Females in the remaining treatment conditions received either 10 (Treatment 10 I), 25 (Treatment 25 I), or 50 (Treatment 50 I) intromissions from a vasectomized male. At the end of each treatment, females were returned to their home cages until the hourly receptivity tests were resumed at 2100 hr. Pseudopregnancies were detected behaviorally by placing females with males at 4 day intervals to check for the display of lordosis. Females which were nonreceptive on day 4 but receptive again on day 8 or 12 were categorized as having become pseudopregnant. Results Results of this experiment are summarized in Fig. 3. A 1-way A N O V A indicated a significant treatment effect on the duration of behavioral receptivity, F(3,105)=34.70, p<0.001. S N K tests indicated that 10 intromissions significantly reduced the duration of receptivity relative to nonmated controls and that 25 intromissions effected a further shortening of estrus beyond the level of 10 intromissions. However, 50 intromissions produced only a marginal reduction (p<0.08) in the duration of estrus beyond the level produced by 25 intromissions.

426

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The incidence of pseudopregnancy was related to the number of intromissions received. All of the females receiving 25 or 50 intromissions subsequently became pseudopregnant versus a 52% pseudopregnancy rate for females receiving 10 intromissions. None of the nonmated controls became pseudopregnant. An interesting pattern emerges when individual scores for estrus duration are plotted (Fig. 4). Whereas distributions approximating a normal curve are evident for nonmated controls and for females receiving 25 or 50 intromissions, there is a distinct bimodal distribution for the 10 intromissions group. Furthermore, all of the females showing shorter estrus duration scores subsequently became pseudopregnant whereas most of the females showing longer estrus duration scores did not become pseudopregnant. Put another way, the mean duration of estrus for females that became pseudopregnant was 782_+28 min whereas that for the females that did not become pseudopregnant was 1105_+1.3 min, t(45)= 10.04, p<0.001. The duration of behavioral estrus of females receiving 10 intromissions but which did not subsequently become pseudopregnant was nearly identical to that of the control females (see Fig. 6). Clearly, the reduced estrus duration of the 10 intromission group vis-a-vis nonmated controls was due entirely to those females which subsequently became pseudopregnant. This apparent correlation between pseudopregnancy and the length of the receptive period may have important implications regarding the physiological mechanisms underlying mating-induced abbreviations of the receptive period (see below). EXPERIMENT 4

Mating to satiety with one male resulted in a 413% shortening of the period of behavioral receptivity [18] and similar reductions in the duration of behavioral estrus were noted in females receiving intromissive stimuli in experiments 2 and 3 in this study (see Figs. 2 and 3). The present experiment was designed to determine whether mating with more than one male would further reduce the duration of receptivity. Procedure

Eighty-five primiparous females were randomly assigned to 1 of 4 treatment groups (n=20-25 each). Beginning at 1300 hr on the day of proestrus all females were tested for receptivity at l-hr intervals. At 1930 hr subjects of a nonmated, control group (Treatment C) were placed into an empty arena for 60 min. Females in the remaining groups received intromissive stimuli from either 1, 2 or 3 vasectomized males. All matings were terminated either when a male engaged in his first long intromission or when the female attacked the male. In treatments involving more than one male, females were returned to their home cages for 5 rain before exposure to another male. Females received 48-71 intromissions (mean=59.0) in the 1-male group, 61-110 intromissions (mean=88.7) in the 2-male group, and 75-132 intromissions (mean= 102.9) in the 3-male group. At the end of each treatment, all females were returned to their home cages until the hourly receptivity tests were resumed at 2100 hr. Results

The results of this experiment are summarized in Fig. 5. Not surprisingly, treatment had a significant effect on the

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length of the receptive period, F(3,81)=130.81, p<0.001. S N K tests further revealed that mating with a second male resulted in a significant reduction in the duration of estrus beyond the level seen in females mating with a single male. Mating with a third male did not significantly reduce the duration of estrus beyond the 2-male level although the effect is in the expected direction. However, due to the substantial overlap in the number of intromissions received by females paired with 2 or 3 males, these groups were combined and the resulting sample of 40 animals subjected to Pearson correlation analysis. There was a highly significant correlation (r=0.49; p <0.001) between the total number of intromissions received from the stimulus males and the length of the receptive period. This relationship is illustrated in Fig. 6. DISCUSSION

The major question of the present study was what stimuli are necessary for an absolute shortening of the receptive period. It was first important to demonstrate that the method employed to detect receptivity (i.e., repeated exposures to a series of males) did not by itself have a significant effect on the parameter being measured, namely, the total duration of receptivity. In experiment 1 females were exposed to a male at 1-hr intervals and scored as receptive if they assumed the lordotic posture within 15 min (Fig. 1, paradigm A). By exposing other groups of females only about the time of onset and termination of receptivity (paradigm B) or only about the time of termination of receptivity (paradigm C), we showed that exposure to a series of males which were not allowed to mate, did not effect any significant change in the total length of receptivity. However, if early during the receptive period the female was permitted to mate until she attacked the male or no longer displayed lordosis, both virgin and multiparous females showed an approximately 35% reduction in the duration of receptivity. Thus, it is clear that while the mere

M A T I N G - I N D U C E D I N H I B I T I O N OF RECEPTIVITY presence of a male for brief periods did not reduce the length of the receptive period, mating stimuli were effective in significantly reducing the total length of receptivity. Copulatory stimuli have been documented to produce a significant reduction in the total duration of receptivity in a wide variety of mammals including both reflex and spontaneous ovulators (cf., [18]). Despite the widespread occurrence of this phenomenon, however, relatively little is known about either the stimuli responsible for the effect or of the underlying mechanism(s). In the hamster, we have demonstrated that neither lordosis p e r se nor sensory stimuli provided by males in the absence of intromissive stimuli have any significant effect on the duration of the receptive period. Even a vaginal plug plus mounting stimulation had no effect on the duration of receptivity in the absence of intromissive stimulation (Fig. 2). (In contrast, lordosis p e r s e - - n o t intromissions--provided the critical stimuli for the short-term inhibition of receptivity shown by recently m a t e d female hamsters toward subsequent males [21].) The exclusive role of intromissive stimuli for effecting a decrease in the duration of receptivity appears to contradict the findings of Buntin, Ciaccio and Lisk [2]. They reported that the presence of a simulated copulatory plug plus mounting stimuli in the absence of any intromissive stimuli was sufficient to produce a significant decrease in receptivity. In the latter study animals were given 10 min tests at hourly intervals over the first 8 hr of the receptive period. Differences in receptivity were expressed as the relative percent of the 10 min test during which lordosis was displayed. In the present study the measure was the presence or absence of lordosis. The females were placed with males for 15 rain each hour and the test terminated when lordosis occurred. Furthermore, testing was not arbitrarily terminated after 8 hr but continued until all females were scored as nonreceptive. Employing this ___ criterion the presence of a simulated copulatory plug was not found to shorten the duration of the receptive period. Thus while a simulated copulatory plug in the absence of intromissive stimuli does not significantly shorten the period over which a brief display of lordosis can be induced in the presence of a male it does appear to affect the relative degree of receptivity measured as duration of lordosis in a limited time test. This interpretation is supported by the findings in Huck, Lisk and Thierjung [21] which examined short term effects of mating stimuli. For this study females having a simulated copulatory plug and receiving mounting stimuli also showed a significantly shorter total lordosis duration towards a second male than females receiving similar mounting stimuli in the absence of a simulated copulatory plug. Therefore the presence of the copulatory plug appears to affect the duration of mating displayed towards an individual male but not the duration of the receptive period. Examination of intromissive stimuli for graduated effects on the reduction of the duration of receptivity showed that as few as 10 intromissions were effective in producing a shortened period of receptivity but significantly greater decrements were effected by a greater number of intromissions--inchiding those from several males (Fig. 6). For example, whereas 35-50 intromissions from a single, sexually-rested male effected a 38% abbreviation of estrus, 111-132 intromissions from three stimulus males produced a 68% reduction. In an earlier study Buntin, Ciaccio and Lisk [2] failed to find any relationship between the amount of vaginocervical stimulation and decline in receptivity. The seeming con-

427 tradictory conclusions between that report and the present one are the result of both the nature of the measurements employed and the arbitrary termination of testing after 8 hr in the Buntin, Ciaccio and Lisk study. The latter study showed that the rate over time at which relative receptivity (lordosis duration/unit of time) declines during the first 8 hr of the receptive period is independent of the amount of intromissive stimuli per hour. Since in the present study animals always remained receptive for at least 8 hr regardless of the number of intromissions provided by up to 3 males there is no conflict between the data from the two studies. However, beyond 8 hr the number of hours lopped off of the 20 hr potential receptive period is a function of the amount of intromissive stimuli. This study is the first to demonstrate a relationship between the amount of vaginocervical stimulation required to initiate the neuroendocrine events of pregnancy/ pseudopregnancy and a significant shortening in the duration of the receptive period. Earlier [17] we showed that intromissions were necessary for the induction of pseudopregnancy; here we showed that only those females in which vaginocervical stimulation was sufficient to initiate a pregnancy/pseudopregnancy would also show a significant decrease in the duration of receptivity (Fig. 4). The tying together of these phenomena is of obvious adaptive value since it provides a mechanism to terminate a behavior (i.e., copulation) once its goal (pregnancy) has been achieved. The amount of copulatory stimulation necessary to initiate pregnancy/pseudopregnancy has been shown to vary widely not only among species but also within a species depending on age and previous reproductive history [9--13]. In the golden hamster, for example, stimulus requirements necessary to initiate pregnancy/pseudopregnancy change not only with parity [17] but also with time of mating relative to the onset of the receptive period [20]. As one proceeds later into the receptive period, significantly more copulatory stimulation is required. We have demonstrated that the addition of more sperm does not enhance pregnancy rates but, rather, it is the addition of vaginocervical stimulation which is necessary for pregnancy initiation. In pairings with a sexually-rested male, a nearly 100% pregnancy rate is obtainable over the first half of the receptive period. During the last 25% of receptivity, however, this same level of vaginocervical stimulation failed to produce even a single pregnancy or pseudopregnancy [20]. The latter females continued to cycle without interruption. If we recall that in the present study, intromissions received from a single, sexually-rested male early during the receptive period shortened the total duration of receptivity by about 40%, we see that this shortening of the receptive period corresponds to a period during which mating would normally be ineffective for induction of pregnancy/ pseudopregnancy. Thus, the portion of the receptive period eliminated corresponds to a non-fertile period. Furthermore, the finding that increasing numbers of intromissions effect an increasing shortening of the receptive period is also not without possible adaptive value since there appears to be a correlation between an increased probability of pregnancy initiation (with increased number of intromissions) and the decrease in the duration of the receptive period. The physiological basis for the effects of intromissions on the duration of the receptive period, however, remain unclear at present. Removal of a variety of endocrine glands including ovaries, adrenals, pituitary or pineal has been reported not to affect this phenomenon [4]. Also, attempts to

428

HUCK, LISK, PARENTE AND GUYTON

b l o c k m a t i n g - i n d u c e d c h a n g e s in r e c e p t i v i t y by m e a n s of p h a r m a c o l o g i c a l m a n i p u l a t i o n s h a v e also b e e n generally uns u c c e s s f u l [5,6]. A likely m e c h a n i s m for t h e s e matingi n d u c e d effects is a s u m m a t i o n o f stimuli at t h e n e u r a l level. T h i s , in t u r n , effects a c h a n g e in the level o f t r a n s m i t t e r r e q u i r e d to facilitate the n e u r a l p a t h w a y s w h i c h m e d i a t e this b e h a v i o r a l r e s p o n s e . N u m e r o u s r e p o r t s suggest t h a t activity in s e r o t o n e r g i c n e u r o n s n o r m a l l y inhibits t h e e x p r e s s i o n o f lordosis (e.g., [14]). T h e r e f o r e , females w h i c h s h o w a de-

c r e a s e in r e c e p t i v i t y as a result o f c o p u l a t o r y s t i m u l a t i o n would be e x p e c t e d to s h o w e l e v a t e d levels o f s e r o t o n i n in b r a i n regions i m p o r t a n t for display of lordosis. H o w e v e r , n o c h a n g e in s e r o t o n i n c o n c e n t r a t i o n s w e r e f o u n d in t h e c o r t e x , h y p o t h a l a m u s or b r a i n s t e m o f rats w h i c h s h o w e d matingi n d u c e d d e c r e m e n t s in lordosis [14]. T h u s , the n e u r a l m e c h anism(s) b y w h i c h i n t r o m i s s i o n stimuli result in a d e c r e a s e in lordosis r e s p o n s i v e n e s s r e m a i n to be identified.

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