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Facilitation of lordosis behavior in the ovariectomized estrogen primed rat by dibutyryl cAMP
Physiology&Behavior, Vol. 26, pp. 249-251. Pergamon Press and Brain Research Publ., 1981. Printed in the U.S.A.
Facilitation of Lordosis Behavior in the Ovariectomized Estrogen Primed Rat by Dibutyryl cAMP C. B E Y E R , 1 E . C A N C H O L A
AND K. LARSSON
Centro de Investigaci6n en Reproducci6n Animal, Universidad Autonoma de Tlaxcala, Tlaxcala and Secci6n de Neuroendocrinologia, Subjefatura de lnvestigaci6n Cientffica, Instituto Mexicano del Seguro Social, Apartado Postal 73-032, Mexico 73, D. F. R e c e i v e d 15 A u g u s t 1980 BEYER, C., E. CANCHOLA AND K. LARSSON. Facilitation of lordosis behavior in the ovariectomized estrogen primed rat by dibutyrylcAMP. PHYSIOL. BEHAV. 26(2) 249-151, 1981.--The effect of systemic administration of various dosages of db cAMP (0.5, 1, 2, 8 and 16 mg) on the sexual behavior of ovariectomized estrogen primed rats (2/zg of EB administered 44 hours before dibutyryl cAMP) was studied. All but the low dose of db cAMP elicited lordosis in at least 50% of the rats. The response to db cAMP varied greatly among subjects. No dose response relationship was observed within the dose range ofdb cAMP employed. Tbeophylline (10 mg) administered simultaneously to db cAMP increased and prolonged the action of the nucleotide. Bilateral infusion of 50/zg of db cAMP into the medial preoptic area of estrogen primed rats elicited lordosis behavior in four out of eight animals. Dibutyryl cAMP did not produce sequential inhibition. The results suggest that a rise in the intracellular level of cAMP is involved in the hormone facilitation of sexual behavior in estrogen primed rats. Sexual behavior
cAMP and lordosis
Cellular mechanisms in sexual behavior
L H - R H [7,8], prostaglandin E2 [12] and clonidine [4] facilitate lordosis behavior in ovariectomized estrogen primed rats and guinea pigs. These drugs share the property of raising intracellular c A M P levels in a number of effectors [5,11]. Therefore, we considered it of interest to study the effect o f dibutyryl cyclic A M P (db cAMP) on the sexual behavior of ovariectomized estrogen primed rats. Some preliminary data of this study were reported in abstract [1]. METHOD Young sexually inexperienced Wistar female rats (approximately 250 g) were used in this study. They were maintained at 23°C under a controlled dark light cycle (14 hr-10 hr), and fed with Purina and water ad lib. Ovariectomies were performed under ether anesthesia. Two to three weeks after ovariectomy, rats were primed with 2/~g estradiol benzoate (time 0) and 44 hr later they were subjected to the following treatments (see also Table 1): Group 1, oil, 6 rats; Group 2, 2 mg progesterone (P), 6 rats; Group 3, 0.5 mg db cAMP, 6 rats; Group 4, 1.0 mg db cAMP, 6 rats; Group 5, 2 mg db cAMP, 55 rats (this large number of rats was used to ascertain the variability of the response to db cAMP); Group 6, 8 mg db cAMP, 11 rats; Group 7, 16 mg db cAMP, 6 rats; Group 8, 2 mg db c A M P plus two injections of 10 mg o f
theophylline, one given immediately before db cAMP (44 hr) and the other 8 hours later (52 hr), 12 rats; Group 9, theophylline 10 mg, 6 rats; Group 10, 50/zg db c A M P (5/xl) bilaterally infused into the medial preoptic area, 8 rats; Group 11, saline (5 /zl) bilaterally infused into the medial preoptic area, 6 rats. The quantity of db c A M P was selected on the basis o f previous studies o f intrabrain implantation of this nucleotide [3]. All rats, but those o f group 11, received an injection o f 2 mg of P at 72 hr to test for sequential inhibition. All injections were given subcutaneously. EB and P were dissolved in sesame oil and injected in a volume of 0.2 ml. Dibutyryl c A M P was dissolved in saline and injected in a volume of 0.2 ml. Theophylline was dissolved in warm saline and injected in a volume of 0.5 ml. All injections were given subcutaneously. Rats in groups 10 and 11 were bilaterally implanted with two cannulas (28 ga) into the medial preoptic area, one week before testing. Implants were placed under pentobaribital anesthesia. Infusion of db cAMP into the brain was made under very light ether anesthesia. Animals were tested for sexual behavior with experienced males. Ten mounts were allowed to each female and their LQ (lordosis/number of mounts) was calculated. Rats were tested immediately before the administration of db c A M P or
~Please send reprint requests to Doctor Carlos Beyer, Secci6n de Neuroendocrinologia, Subjefatura de Investigaci6n Cientifica, Instituto Mexicano del Seguro Social, Apartado Postal 73-032, M6xico 73, D.F.
C o p y r i g h t © 1981 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/81/020249-03502.00/0
250
BEYER, CANCHOLA AND LARSSON TABLE 1 EFFECT OF DIBUTYRYL CYCLIC AMP ON THE SEXUAL BEHAVIOR OF OVARIECTOMIZED ESTROGEN PRIMED RATS (2/a,g ESTRADIOL BENZOATE)
Group 1
No. of Ss
Treatment* (44 hours)
6
oil
45 hr
Percentage of responders and mean lordosis quotient +_ SD 48 hr 52 hr 72 hr
76 hr
0%
0%
0%
0%
0.000
0.000
0.000
0.000
100% 0.960 _+ 0.023
2
6
2 mg P
0% 0.000
10~. 0.950 _+ 0.076
100% 0.833 _+ 0.235
33% 0.100 _+ 0.152
0% 0.000
3
6
0.5 mg db cAMP
0% 0.000
0% 0.000
0% 0.000
0% 0.000
83% 0.833 -+ 0.772
4
6
1.0 mg db cAMP
17% 0.033 _+ 0.074
50% 0.150 _+ 0.160
33% 0.066 _+ 0.110
17% 0.083 _+ 0.186
100% 0.966 _+ 0.047
5
55
2.0 mg db cAMP
59% 0.296 _+ 0.326
56% 0.303 _+ 0.358
41% 0.181 _+ 0.245
9% 0.014 _+ 0.060
96% 0.850 _+ 0.226
6
11
8.0 mg db cAMP
36% 0.145 _+ 0.227
73% 0.227 _+ 0.106
55% 0.244 _+ 0.267
36% 0.081 _+ 0.119
91% 0.754 _+ 0.339
7
6
16.0 mg db cAMP
50% 0.333 _+ 0.334
66% 0.433 _+ 0.349
66% 0.183 _+ 0.211
17% 0.050 _+ 0.111
100% 0.883 -+ 0.186
8
12
2.0 mg db cAMP + theophylline 10 mg
50% 0.216 _+ 0.251
83% 0.558 _+ 0.383
83% 0.625 _+ 0.422t
50% 0.142 + 0.170t
100% 0.975 -+ 0.059
9
6
Theophylline 10 mg
0%. 0.000
0% 0.000
0% 0.000
0% 0.000
100% 0.916 -+ 0.076
10
8
50/xg ofdb cAMP in preoptic area
13% 0.013 _+ 0.033
50% 0.213 _+ 0.340
50% 0.113 -+ 0.231
0% 0.000
100% 0.730 -+ 0.171
11
6
Saline (5 ~1) in preoptic area
0% 0.000
0% 0.000
0% 0.000
0% 0.000
--
*All groups also received an injection of 2 mg of P at 72 hr to test for sequential inhibition. tStatistical comparisons were made between groups 5 (2 mg db cAMP) and 8 (2 mg db cAMP plus theophylline), t test p<0.01.
theophylline (around 44 hr after estrogen) and 1 hr (45 hr), 4 hr (48 hr), and 8 hr (52 hr) after P or db c A M P . T w e n t y one out o f the 55 rats o f group 4 (2 mg db c A M P ) were only tested 4 hours after the nucleotide. Yet, all rats were tested the following day immediately before (72 hr) and 4 hr after P (76 hr). RESULTS Table 1 summarizes the results obtained in these experiments. All but the low dose of db c A M P stimulated lordosis in a high proportion of rats, i.e., 51)% or more. The m a x i m u m level o f activity was usually o b s e r v e d 4 hours after db c A M P , but many rats displayed lordosis earlier, i.e., one hour after the nucleotide. Although the highest mean L Q was o b s e r v e d in group 7, receiving also the highest dose of db c A M P (16 mg), no clear dose-response relationship was observed within the range o f doses e m p l o y e d . Figure 1 presents the L Q values o b s e r v e d four hours after 2 mg o f db c A M P in 55 rats. It can be seen from this figure that the response to db c A M P was strikingly variable and that a high proportion of rats did not respond to the treatment. Interestingly, no correlation between the response to db c A M P and the subsequent response to P was noted. A peculiar feature of the behavior
stimulated by db c A M P was the f r e q u e n t performance o f back kicking and the adoption of aggressive postures towards the male. This rejection b e h a v i o r was rarely displayed by rats treated with P. Theophylline alone failed to synergize with estrogen but significantly prolonged the effect exerted by 2 mg o f db c A M P . Thus, rats receiving theophylline plus db c A M P showed significantly higher L Q ' s at 8 and at 28 hr after db c A M P than those receiving only the nucleotide. P administration at 72 hr elicited in all animals intense estrous behavior, indicating that no sequential inhibition resuited from the previous db c A M P treatment. Infusion of db c A M P into the brain induced lordosis behavior in half o f the rats (see Table 1). The histological study revealed that most o f the cannulas w e r e placed in the preoptic area slightly anterior to the hypothalamus. DISCUSSION Our results show that db c A M P either infused into the brain or systemically injected stimulated lordosis b e h a v i o r in estrogen primed rats. This last finding was surprising considering that systemic administration o f db c A M P usually fails to induce behavioral changes due to its inability to penetrate into the brain [11]. Our data do not contradict the a b o v e
CYCLIC AMP ON S E X U A L BEHAVIOR E F F E C T OF 2 r a g db c A M P
251
ON LORDOSIS
BEHAVIOR.
20.
(/)
rr
10.
LL
0 tY w m
5-
:E
z 0
LO
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
R D 0
S
I S
I
Q UOTIENT
FIG. 1. Effect of 2 mg of db cAMP on the intensity of lordosis behavior of 55 estrogen primed ovariectomized rats. Note the great variability in the individual responsivity of the rats to db cAMP. mentioned reports, but rather suggest that db cAMP acted at the level of the ventral diencephalon to facilitate lordosis behavior. This region lacks a well constituted blood brain barrier [10] and it has been shown to be related to the integration of lordosis behavior in the rat [6,9]. The behavioral response to db cAMP differed from that observed with P. Thus, the response to the nucleotide appeared earlier, but was more variable and generally weaker than that to P. Moreover, in contrast to P, db cAMP neither induced proceptivity nor sequential inhibition, i.e. did not
inhibit the subsequent response to P administration. These data suggest that the effect exerted by db cAMP was not mediated by the secretion of adrenal P. Administration of db cAMP alone to ovariectomized rats, even for prolonged periods, fails to stimulate lordosis behavior (Beyer, unpublished results). Therefore, db cAMP is only effective when administered to estrogen primed rats. It is only possible to speculate on the mechanisms through which cAMP interacts with estrogen to induce lordosis behavior. Estrogen participates in the production of estrous behavior by stimulating the synthesis of proteins presumably involved in neural transmission [2]. Yet, estrogen alone, unless administered in very large dosages, fails to stimulate lordosis in ovariectomized rats. This suggests that most of the protein molecules induced by estrogen are in an "inactive" form, thus requiring a stimulus to become "active" and facilitate lordosis. It appears likely that this stimulus is provided by cAMP which is known to alter the functional properties of proteins by phosphorylating them through phosphokinases [5, 13, 14]. Protein phosphorylation may result in dramatic changes in neuronal functioning and recent studies have demonstrated its role in complex behavioral processes such as learning and memory formation [13,14]. Estrous behavior induced by db cAMP peaks between 4 to 8 hours and lasts less than 24 hr. Duration of the effect is related to the availability of cAMP molecules since theophylline, a phosphodiesterase inhibitor, prolongs estrous behavior. An additional mechanism possibly regulating the duration of estrus induced by db cAMP could be dephosphorylation, a step catalized by phosphoprotein phosphatases [5]. A high concentration of these enzymes are known to occur in brain tissue [5]. Dephosphorylation may inactivate but apparently does not degrade the estrogen induced proteins since P administration 24 hr after db cAMP elicits intense sexual behavior. In summary, our results suggest that a rise in cAMP may be involved in the facilitatory effect of some hormones and neural modulators (P, LH-RH, prostaglandin E2 and noradrenergic agonists) on the sexual behavior of estrogen primed rats. The possibility that cAMP also plays a role in other species of rodents is supported by the recent observation that intraventricular infusion of db cAMP facilitates lordosis in estrogen primed guinea pigs (Feder and Nock, personal communication). On the other hand, cAMP is not involved in the sequential inhibition of sexual behavior elicited by P.
REFERENCES 1. Beyer, C., E. Canchola, M. L. Cruz and K. Larsson. A model for explaining estrogen-progesterone interactions in the induction of lordosis behavior. Book o f Symposia, V1 International Congress o f Endocrinology. Australian Academy of Science, in press, 1980. 2. Beyer, C., K. Larsson, M. L. Cruz. Neuronal mechanisms probably related to the effect of sex steroids on sexual behavior. In: Endocrine Control o f Sexual Behavior, edited by C. Beyer. New York: Academic Press, 1979, p. 365. 3. Breckenridge, B. McL. and R. D. Lisk. Cyclic adenylate and hypothalamic regulatory functions. Proc. Soc. exp. Biol. Med. 131: 934-935, 1969. 4. Crowley, W. R., B. L. Nock and H. H. Feder. Facilitation of lordosis behavior by clonidine in female guinea pigs. Pharmac. Biochem. Behav. 8: 207-209, 1978. 5. Greengard, P. Cyclic Nucleotides, Phosphorylated Proteins, and Neuronal Function. New York: Raven Press, 1978. 6. Mathews, D. and D. E. Edwards. The ventromedial nucleus of the hypothalamus and the hormonal arousal of sexual behaviors in the female rat. Hormones Behav. 8: 40-51, 1977. 7. Moss, R. L. and S. M. McCann. Induction of mating behavior in rats by luteinizing hormone-releasing factor. Science 181: 177179, 1973.
8. Pfaff, D. W. Luteinizing hormone-releasing factor potentiates lordosis behavior in hypophysectomized ovariectomized female rats. Science 182: 1148--1149, 1973. 9. Powers, J. B. Facilitation of lordosis in ovariectomized rats by intracerebral progesterone implants. Brain Res. 48: 311-325, 1971. 10. Rapoport, S. I. Blood-Brain Barrier in Physiology and Medicine. New York: Raven Press, 1976. 11. Robison, G. A., R. W. Butcher and E. W. Sutherland. Cyclic AMP. New York: Academic Press, 1971. 12. Rodriquez-Sierra, J. F. and B. R. Komisaruk. Effect of prostaglandin Ez and indomethacin on sexual behavior in the female rat. Hormones Behav. 9: 281-289, 1977. 13. Routtenberg, A. Anatomical localization of phosphoprotein and glycoprotein substrates of memory. Prog. Neurobiol. 12: 85114, 1979. 14. Williams, M. and R. Rodnight. Protein phosphate in nervous tissue: Possible involvement in nervous tissue function and relationship to cyclic nucleotide metabolism. Prog. Neurobiol. 8: 183-250, 1977.
Facilitation of Lordosis Behavior in the Ovariectomized Estrogen Primed Rat by Dibutyryl cAMP C. B E Y E R , 1 E . C A N C H O L A
AND K. LARSSON
Centro de Investigaci6n en Reproducci6n Animal, Universidad Autonoma de Tlaxcala, Tlaxcala and Secci6n de Neuroendocrinologia, Subjefatura de lnvestigaci6n Cientffica, Instituto Mexicano del Seguro Social, Apartado Postal 73-032, Mexico 73, D. F. R e c e i v e d 15 A u g u s t 1980 BEYER, C., E. CANCHOLA AND K. LARSSON. Facilitation of lordosis behavior in the ovariectomized estrogen primed rat by dibutyrylcAMP. PHYSIOL. BEHAV. 26(2) 249-151, 1981.--The effect of systemic administration of various dosages of db cAMP (0.5, 1, 2, 8 and 16 mg) on the sexual behavior of ovariectomized estrogen primed rats (2/zg of EB administered 44 hours before dibutyryl cAMP) was studied. All but the low dose of db cAMP elicited lordosis in at least 50% of the rats. The response to db cAMP varied greatly among subjects. No dose response relationship was observed within the dose range ofdb cAMP employed. Tbeophylline (10 mg) administered simultaneously to db cAMP increased and prolonged the action of the nucleotide. Bilateral infusion of 50/zg of db cAMP into the medial preoptic area of estrogen primed rats elicited lordosis behavior in four out of eight animals. Dibutyryl cAMP did not produce sequential inhibition. The results suggest that a rise in the intracellular level of cAMP is involved in the hormone facilitation of sexual behavior in estrogen primed rats. Sexual behavior
cAMP and lordosis
Cellular mechanisms in sexual behavior
L H - R H [7,8], prostaglandin E2 [12] and clonidine [4] facilitate lordosis behavior in ovariectomized estrogen primed rats and guinea pigs. These drugs share the property of raising intracellular c A M P levels in a number of effectors [5,11]. Therefore, we considered it of interest to study the effect o f dibutyryl cyclic A M P (db cAMP) on the sexual behavior of ovariectomized estrogen primed rats. Some preliminary data of this study were reported in abstract [1]. METHOD Young sexually inexperienced Wistar female rats (approximately 250 g) were used in this study. They were maintained at 23°C under a controlled dark light cycle (14 hr-10 hr), and fed with Purina and water ad lib. Ovariectomies were performed under ether anesthesia. Two to three weeks after ovariectomy, rats were primed with 2/~g estradiol benzoate (time 0) and 44 hr later they were subjected to the following treatments (see also Table 1): Group 1, oil, 6 rats; Group 2, 2 mg progesterone (P), 6 rats; Group 3, 0.5 mg db cAMP, 6 rats; Group 4, 1.0 mg db cAMP, 6 rats; Group 5, 2 mg db cAMP, 55 rats (this large number of rats was used to ascertain the variability of the response to db cAMP); Group 6, 8 mg db cAMP, 11 rats; Group 7, 16 mg db cAMP, 6 rats; Group 8, 2 mg db c A M P plus two injections of 10 mg o f
theophylline, one given immediately before db cAMP (44 hr) and the other 8 hours later (52 hr), 12 rats; Group 9, theophylline 10 mg, 6 rats; Group 10, 50/zg db c A M P (5/xl) bilaterally infused into the medial preoptic area, 8 rats; Group 11, saline (5 /zl) bilaterally infused into the medial preoptic area, 6 rats. The quantity of db c A M P was selected on the basis o f previous studies o f intrabrain implantation of this nucleotide [3]. All rats, but those o f group 11, received an injection o f 2 mg of P at 72 hr to test for sequential inhibition. All injections were given subcutaneously. EB and P were dissolved in sesame oil and injected in a volume of 0.2 ml. Dibutyryl c A M P was dissolved in saline and injected in a volume of 0.2 ml. Theophylline was dissolved in warm saline and injected in a volume of 0.5 ml. All injections were given subcutaneously. Rats in groups 10 and 11 were bilaterally implanted with two cannulas (28 ga) into the medial preoptic area, one week before testing. Implants were placed under pentobaribital anesthesia. Infusion of db cAMP into the brain was made under very light ether anesthesia. Animals were tested for sexual behavior with experienced males. Ten mounts were allowed to each female and their LQ (lordosis/number of mounts) was calculated. Rats were tested immediately before the administration of db c A M P or
~Please send reprint requests to Doctor Carlos Beyer, Secci6n de Neuroendocrinologia, Subjefatura de Investigaci6n Cientifica, Instituto Mexicano del Seguro Social, Apartado Postal 73-032, M6xico 73, D.F.
C o p y r i g h t © 1981 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/81/020249-03502.00/0
250
BEYER, CANCHOLA AND LARSSON TABLE 1 EFFECT OF DIBUTYRYL CYCLIC AMP ON THE SEXUAL BEHAVIOR OF OVARIECTOMIZED ESTROGEN PRIMED RATS (2/a,g ESTRADIOL BENZOATE)
Group 1
No. of Ss
Treatment* (44 hours)
6
oil
45 hr
Percentage of responders and mean lordosis quotient +_ SD 48 hr 52 hr 72 hr
76 hr
0%
0%
0%
0%
0.000
0.000
0.000
0.000
100% 0.960 _+ 0.023
2
6
2 mg P
0% 0.000
10~. 0.950 _+ 0.076
100% 0.833 _+ 0.235
33% 0.100 _+ 0.152
0% 0.000
3
6
0.5 mg db cAMP
0% 0.000
0% 0.000
0% 0.000
0% 0.000
83% 0.833 -+ 0.772
4
6
1.0 mg db cAMP
17% 0.033 _+ 0.074
50% 0.150 _+ 0.160
33% 0.066 _+ 0.110
17% 0.083 _+ 0.186
100% 0.966 _+ 0.047
5
55
2.0 mg db cAMP
59% 0.296 _+ 0.326
56% 0.303 _+ 0.358
41% 0.181 _+ 0.245
9% 0.014 _+ 0.060
96% 0.850 _+ 0.226
6
11
8.0 mg db cAMP
36% 0.145 _+ 0.227
73% 0.227 _+ 0.106
55% 0.244 _+ 0.267
36% 0.081 _+ 0.119
91% 0.754 _+ 0.339
7
6
16.0 mg db cAMP
50% 0.333 _+ 0.334
66% 0.433 _+ 0.349
66% 0.183 _+ 0.211
17% 0.050 _+ 0.111
100% 0.883 -+ 0.186
8
12
2.0 mg db cAMP + theophylline 10 mg
50% 0.216 _+ 0.251
83% 0.558 _+ 0.383
83% 0.625 _+ 0.422t
50% 0.142 + 0.170t
100% 0.975 -+ 0.059
9
6
Theophylline 10 mg
0%. 0.000
0% 0.000
0% 0.000
0% 0.000
100% 0.916 -+ 0.076
10
8
50/xg ofdb cAMP in preoptic area
13% 0.013 _+ 0.033
50% 0.213 _+ 0.340
50% 0.113 -+ 0.231
0% 0.000
100% 0.730 -+ 0.171
11
6
Saline (5 ~1) in preoptic area
0% 0.000
0% 0.000
0% 0.000
0% 0.000
--
*All groups also received an injection of 2 mg of P at 72 hr to test for sequential inhibition. tStatistical comparisons were made between groups 5 (2 mg db cAMP) and 8 (2 mg db cAMP plus theophylline), t test p<0.01.
theophylline (around 44 hr after estrogen) and 1 hr (45 hr), 4 hr (48 hr), and 8 hr (52 hr) after P or db c A M P . T w e n t y one out o f the 55 rats o f group 4 (2 mg db c A M P ) were only tested 4 hours after the nucleotide. Yet, all rats were tested the following day immediately before (72 hr) and 4 hr after P (76 hr). RESULTS Table 1 summarizes the results obtained in these experiments. All but the low dose of db c A M P stimulated lordosis in a high proportion of rats, i.e., 51)% or more. The m a x i m u m level o f activity was usually o b s e r v e d 4 hours after db c A M P , but many rats displayed lordosis earlier, i.e., one hour after the nucleotide. Although the highest mean L Q was o b s e r v e d in group 7, receiving also the highest dose of db c A M P (16 mg), no clear dose-response relationship was observed within the range o f doses e m p l o y e d . Figure 1 presents the L Q values o b s e r v e d four hours after 2 mg o f db c A M P in 55 rats. It can be seen from this figure that the response to db c A M P was strikingly variable and that a high proportion of rats did not respond to the treatment. Interestingly, no correlation between the response to db c A M P and the subsequent response to P was noted. A peculiar feature of the behavior
stimulated by db c A M P was the f r e q u e n t performance o f back kicking and the adoption of aggressive postures towards the male. This rejection b e h a v i o r was rarely displayed by rats treated with P. Theophylline alone failed to synergize with estrogen but significantly prolonged the effect exerted by 2 mg o f db c A M P . Thus, rats receiving theophylline plus db c A M P showed significantly higher L Q ' s at 8 and at 28 hr after db c A M P than those receiving only the nucleotide. P administration at 72 hr elicited in all animals intense estrous behavior, indicating that no sequential inhibition resuited from the previous db c A M P treatment. Infusion of db c A M P into the brain induced lordosis behavior in half o f the rats (see Table 1). The histological study revealed that most o f the cannulas w e r e placed in the preoptic area slightly anterior to the hypothalamus. DISCUSSION Our results show that db c A M P either infused into the brain or systemically injected stimulated lordosis b e h a v i o r in estrogen primed rats. This last finding was surprising considering that systemic administration o f db c A M P usually fails to induce behavioral changes due to its inability to penetrate into the brain [11]. Our data do not contradict the a b o v e
CYCLIC AMP ON S E X U A L BEHAVIOR E F F E C T OF 2 r a g db c A M P
251
ON LORDOSIS
BEHAVIOR.
20.
(/)
rr
10.
LL
0 tY w m
5-
:E
z 0
LO
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
R D 0
S
I S
I
Q UOTIENT
FIG. 1. Effect of 2 mg of db cAMP on the intensity of lordosis behavior of 55 estrogen primed ovariectomized rats. Note the great variability in the individual responsivity of the rats to db cAMP. mentioned reports, but rather suggest that db cAMP acted at the level of the ventral diencephalon to facilitate lordosis behavior. This region lacks a well constituted blood brain barrier [10] and it has been shown to be related to the integration of lordosis behavior in the rat [6,9]. The behavioral response to db cAMP differed from that observed with P. Thus, the response to the nucleotide appeared earlier, but was more variable and generally weaker than that to P. Moreover, in contrast to P, db cAMP neither induced proceptivity nor sequential inhibition, i.e. did not
inhibit the subsequent response to P administration. These data suggest that the effect exerted by db cAMP was not mediated by the secretion of adrenal P. Administration of db cAMP alone to ovariectomized rats, even for prolonged periods, fails to stimulate lordosis behavior (Beyer, unpublished results). Therefore, db cAMP is only effective when administered to estrogen primed rats. It is only possible to speculate on the mechanisms through which cAMP interacts with estrogen to induce lordosis behavior. Estrogen participates in the production of estrous behavior by stimulating the synthesis of proteins presumably involved in neural transmission [2]. Yet, estrogen alone, unless administered in very large dosages, fails to stimulate lordosis in ovariectomized rats. This suggests that most of the protein molecules induced by estrogen are in an "inactive" form, thus requiring a stimulus to become "active" and facilitate lordosis. It appears likely that this stimulus is provided by cAMP which is known to alter the functional properties of proteins by phosphorylating them through phosphokinases [5, 13, 14]. Protein phosphorylation may result in dramatic changes in neuronal functioning and recent studies have demonstrated its role in complex behavioral processes such as learning and memory formation [13,14]. Estrous behavior induced by db cAMP peaks between 4 to 8 hours and lasts less than 24 hr. Duration of the effect is related to the availability of cAMP molecules since theophylline, a phosphodiesterase inhibitor, prolongs estrous behavior. An additional mechanism possibly regulating the duration of estrus induced by db cAMP could be dephosphorylation, a step catalized by phosphoprotein phosphatases [5]. A high concentration of these enzymes are known to occur in brain tissue [5]. Dephosphorylation may inactivate but apparently does not degrade the estrogen induced proteins since P administration 24 hr after db cAMP elicits intense sexual behavior. In summary, our results suggest that a rise in cAMP may be involved in the facilitatory effect of some hormones and neural modulators (P, LH-RH, prostaglandin E2 and noradrenergic agonists) on the sexual behavior of estrogen primed rats. The possibility that cAMP also plays a role in other species of rodents is supported by the recent observation that intraventricular infusion of db cAMP facilitates lordosis in estrogen primed guinea pigs (Feder and Nock, personal communication). On the other hand, cAMP is not involved in the sequential inhibition of sexual behavior elicited by P.
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