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Hypothalamic adrenergic activity precedes the preovulatory luteinizing hormone surge in the rat
Neuroscience Letters, 43 0983) 221-226
221
Elsevier Scientific Publishers Ireland Ltd.
HYPOTHALAMIC ADRENERGIC ACTIVITY PRECEDES THE PREOVULATORY LUTEiNIZING HORMONE SURGE IN THE RAT
P.C.B. MacKINNON, E.M. CLEMENT, C. CLARK and R, SHEAVES
Department of Human Anatomy, Oxford University Oxford OXI 3QX (U.K.) (Received September 6th, 1983; Revised version received October 17th, 1983; Accepted October 21st, 1983)
Key words: arcuate nucleus - median eminence - adrenaline turnover - critical period - luteinizing hormone
A preovulatory surge of luteinizing hormone (LH) is stimulated by an increase in plasma oestrogen (E2) and a proestrous neural signal which occurs during the 'critical period' (CP). Adrenergic activity during the CP in brain areas concerned with LH release and receiving adrenergic nerve terminals, increased in the arcuate nucleus-median eminence area (AN-ME) during the CP (14.30-16.30 h), but not in the adjacent perifornical area. Pentobarbital injected prior to the CP inhibited the effect as did ovariectomy, while E2 replacement restored it. Thus increased adrenergic activity in the AN-ME may represent the neural signal that triggers the preovulatory LH surge [5].
Although much past evidence has implicated central catecholamines in the elicitation of an oestrogen (E2)-stimulated luteinizing hormone (LH) surge in the rat [22], only recently have studies, based on pharmacological manipulations, demonstrated the essential role of adrenaline [4, 6, 12]. We have therefore studied adrenaline (A) turnover in hypothalamic nuclei during the critical period (CP) in groups (n = 6) of proestrous 4-day cyclic Wistar rats (220-280 g) maintained under regulated lighting (on: 06.00-20.00 h). They were either decapitated at 13.00, 14.30 or 15.00 h, or were injected (i.p.) at the same times with an inhibitor of phenylethanolamine-Nmethyltransferase, (SKF 64139 [9], 50 mg/kg b.wt; Smith, Kline and French, Welwyn Garden City) and killed 2 h later. Brains were removed, coronal sections (250 ~m) cut in a cryostat (-12°C), and specific areas (arcuate nucleus-median eminence, (AN-ME) and perifornical area (PFA)) removed with a stainless steel punch (int. diam. 0.85 mm) [15]. Tissue was homogenized in 0.1 M HCI containing I mM diethylene-triaminepentaacetic acid and 0.1e/o N a 2 S 2 0 5 at 0-4°C. After centrifugation at 10,000 g (30 min) the A content of the supernatant was determined by radioenzymatic assay [18] (sensitivity 5.4 pg; intra- and interassay coefficients of variation :~e/0 and 13~0 respectively). Data was corrected for crossover of 0.46% noradrenaiine and 0.7¢/0 dopamine into counts associated with A. Counts were ex0304-3940/83/$ 03.00 © 1983 Elsevier Scientific Publishers Ireland Ltd.
222
pressed in te:ans of tissue protein estimated by the Lowry et al. method. Assessment of A turnover was based on the rate of decline in its concentration 2 h after injection of SKF 64139 [7]. Data were analyzed by Analysis of Variance and the Newman-Keul's multiple range test. To define the CP in accordance with earlier work [8], Sagatai (pentobarbital; 36 mg/100 g b.wt) was injected (i.p.) into groups (n =6) of rats at 30min intervals from 12.00 to 18.00 h. Ovulation was blocked next day in 5-6 rats/group injected between 14.30 and 17.00 h; higher rates of ovulation occurring in animals injected before and after these times. A further Stoup (n = 10) of proestrous animals was implanted with intracardiac catheters. The following proestrous afternoon animals were either injected (14.25 h) with Sagatai or saline and serial blood samples (0.3 ml) taken (with Sa
i
-!-
]o c
14. O0
i
16.®
15.O0
l,:®
18'00
lime~hl
,,,5O z/t
"Critical period"
2¢
_= 40 z
2O
'°I _+ '1
.00
. . . . . . .
16 ®
I
1,00
~
I
)8®
•
,+.®
Time,hi
Fig. 1. Serial plasma LH concentrations obtained from two groups of proestrous rats over the period of the preovulatory LH surge. The animals had been injected (i.p.) either with saline (S) or Sagatal (Sa) at the start of the critical period (black bar).
223
saline replacement) at regular 30 win intervals. Plasma samples were stored ( - 2 0 ° C ) until LH concentrations were measured by radioimmunoassay [20]. Only saline-injected rats showed typical LH surge prof'fles (Fig. 1). Turnover of A in the AN-ME was increased in proestrous rats between 14.30 and 16.30 h when compared with turnover between either 13,00 and 15.00 h or 15.30 and 17.30 h. No differences in activity were found in the PFA (Fig. 2). The turnover rate of catecholamines in the brain depends on the rate of nerve impulse flow, more decfine in the concentration of transmitter following the relevant enzyme inhibitor being associated with increased neuronal activity [l ]. Therefore increased turnover in the AN-ME at proestrous (14.30-16.30 h), which is consistent with an increase in multiple unit activity during the CP [13], may be attributable to increased activity of brainstem adrenergic neurones, the axons of which terminate in the AN-ME [14]. That such an increase in adrenergic activity represents at least a part of the mechanisms which initiate the LH surge is suggested by past work [22]; while recent evidence derived from pharmacological manipulation of central adrenaline and its synthesising enzyme also strongly supports a role for A in preovulatory LH release [4, 6, 12]. An increase in plasma prolaain concentrations occurs at about the same time as the LH surge, and this too might be attributed to increased A activity in the AN-ME except that SKF 64139, in spite of preventing
0.7
AN-ME
b (17)
PFA
0.6
0.5 "2 0.4
E gO.3 0.2
fll~
z) O.l
I
ill)
I
g//////A 1300h
1500h
1700h
1300h
1500h
1700h
Fig. 2. 2-Hourly (13.00-15.00 h; 14.30-16.30 h; 15.30-17.30 h) turnover rates of adrenaline in the ANME and PFA around the time of the proestrous critical period. Numbers of rats used are shown in brackets and S.E. of the mean is indicated, a:b, P<0.001; b:c, P<0.025.
224
a steroid-stimulated LH surge, has no effect on Ez-stimulated prolactin release [4]. Ez replacement restores LH surges in ovariectomized rats [3], so to substantiate our findings in intact animals we measured A turnover in groups of rats ovariectomized at dioestrous (12.00 h) and injected (s.c.) with either oestradiol benzoate (OB; 10/~g in 0.2 ml arachis oil) or oil vehicle. The following day the animals were either decapitated or injected (i.p.) with SKF 64139 and treated in the same way as the previous groups. Turnover of A in the AN-ME of the ovariectomized oil-treated group at expected procstrous 04.30-16.30 h) was significantly lower than that found in either the intact untreated animals or in the ovariectomized Ez-treated group, whereas A turnover was similar in the latter group and in intact untreated animals. The turnover of A in the PFA, however, was unaffected by either treatment (Table I). Early experiments with barbiturates injected at the start of a 2 h period on the afternoon of proestrous showed that ovulation was delayed by 24 h. This led to the concept of a daily neural event (CP) on which the LH surge and ovulation depends [8]. Our results show that injection of barbiturate at the start of the CP 04.25 h) almost immediately inhibited A turnover in the AN-ME, but had no effect on activity in the PFA (Fig. 3). Although the action of barbiturates on LH release is not understood, they alter activity of nerve terminals [2, 211 and depress the reticular activating system [14], a site of catecholamine cell bodies which concentrate Ez [ 19]. Increased turnover of noradrenaline (NA) and dopamine (DA) at proestrous (12.00-15.00 h) concomitant with a fall in the hypothalamic content of gonadotrophin-releasing hormone (Gn-RH) and an increase in plasma LH levels, have also been reported and interpreted as representing processes involved with the initiation of the LH surge [171. But a barbiturate injected in an ovulatory blocking dose at the start of this period (12.00 h) failed to affect NA or DA turnover in any of the areas examined until some 2 h later, when NA turnover in the ME was significantly reduced [16]. Bearing in mind that only the initial 13070 of LH surge TABLE 1 MEAN ± S.E. C O N C E N T R A T I O N S OF A (pg/ag protein) A N D A T U R N O V E R IN THE AN-ME A N D PFA IN G R O U P S OF INTACT OR E X P E R I M E N T A L RATS ( E X P L A N A T I O N IN TEXT) Rats were either killed at 14.30 h proestrous or injected ~vith SKF 64139 and killed 2 h later. Numbers of animals in parentheses: a:b, P < 0 . 0 2 5 ; c:b, P < 0 . 0 2 5 ; a:c, n.s. Intact
OVEX + oil
OVEX + E2
Baseline {A cone.)
AN-ME PFA
1.99 + 0.14 (!1) 0.57 _+ 0.03 ( 6 )
!.23 _+ 0.12 ( 6 ) 0.46 _+ 0.04 ( 6 )
i.58 _+ 0.19 ( 7 ) 0.61 _+ 0.05 ( 8 )
SKF 64139 {Aconc.)
AN-ME PFA
0.85 _+ 0.10 ( 6 ) 0.34 _+ 0.03 ( 6 )
0.89 _+ 0.12 ( 5 ) 0.26 _+ 0.06 ( 5 )
0.76 +_ 0.09 ( 5 ) 0.21 _+ 0.02 ( 6 )
Turnover (pg/~g protein/h)
AN-ME PFA
0.57 _+ 0.09 (17) ~ 0.14 _+ 0.02 (12)
0.17 _+ 0.08 (11) b 0.10 _+ 0.03 (11)
0.41 + 0.11 (12) ~ 0.20 +_ 0.02 (14)
225 AN-ME
PFA
0.7
0.6
0.5
0.4 ¢.-
d Gll
"~
0.3
~'
0.2
~11)
0.1
Fig. 3. Turnover rates of adrenaline in the AN-ME and PFA between 14.30 and 16.30 h (during the critical period) in proestrous rats which had been injected either with saline (1~) or Sagatal ( m ) at the start of the critical period. Numbers of rats used are shown in brackets and S.E. of the means are indicated, a:b, P<0.001. d:c, n.s.
output is required for successful ovulation [10] it is difficult to reconcile this data with initiation of the surge. Our results, therefore, suggest that while NA and DA may have a role to play in LH output, increased adrenergic activity in the AN-ME precedes and is associated with initiation of the preovulatory LH surge. Whether as a result of the preovulatory rise of E2 adrenaline activity acts as a direct or indirect trigger for the release of Gn-RH, or whether it mediates the effects of E2 at a hypothalamic and/or pituitary level, are questions currently being studied. We thank the MRC for financial support (G8215236N to P.C.B.M.); Smith Kline and French for gifts of SKF 64139; and particularly Mr. R. Laynes for excellent technical assistance. I And~n, M.-E., Corrodi, H., Dahlstr6m, A., Fuxe, K. and H6kfelt, T., Effects of tyrosine hydroxylase inhibition on the amine levels of central monoamine neurons, Life Sci., 5 (1966) 561-568. 2 Blaustein, M.P., Barbiturates block calcium uptake by stimulated and potassium-depolarized rat :,ympathetic ganglia, J. Pharmacol. exp. Ther., 196 (1976) 80-86. 3 Caligaris, L., Astrada, J.J. and Taleisnik, S., Release of LH induced by estrogen injection into ovariectomised rats, Endocrinology, 88 (I 971) 810--815. 4 Cass Terry, L., Crowley, W.R., Lynch, C., Longserre, C. and Johnson, M.D., Role of central epinephrine in regulation of anterior pituitary hormone secretion, Peptides, 3 (1982) 311-318. 5 Clark, C.R., Clement, M. and MacKinnon, P.C.B., Hypothalamic adrenaline in the prooestrous 'critical period' in the rat, J. Physiol. (Lond.), 334 (1982) 128P.
226 6 Co-n, C.W. and Coombs, M.C., Effects of manipulating catecholamines on the incidence of the pr~vulatory surge of luteinising hormone and ovulation in the rat: evidence for a necessary involveme,tt of adrenaline in the normal or 'midnight" occurrence of the surge, Neuroscience, I0, 187-206. 7 Co ta, E. and Neff, N.H., Mechanisms in the nervous system, In A. Lajtha 0Ed.), Handbook of Ne trochemical Control, Vol. 4, Plenum Press, NY, 1977, pp. 45-90. 8 Everett, J.W. and Sawyer, C.H., A 24 hour periodicity in the LH release apparatus of female Wistar rats disclosed by barbiturate sedation, Endocrinology, 47 (1950) 198-218. 9 Fuller, R.W., Pharmacology of brain epinephrine neurons, Ann. Rev. Pharmacol. Toxicol., 22 (1982) 31-55. I0 Gr~ig. F. and Weiss, J., Preovulatory levels of luteinising hormone, the critical period and ovulation in the rat, J. Endocr., 57 (1973) I-1 I. I ! H~kfelt, T., Goldstein, M., Fuxe, K., Johansson, O., Verhofstad, A., Steinbusch, H., Penke, B. and Vargas, J. Histochemical identification of adrenaline containing cells with special reference to ne'Jrons, in K. l-'uxe, M. Goldstein, B. H6kfelt and T. H6kfelt (Eds.), Central Adrenaline Neurons, Pergamon Press, Oxford. 1976, pp. 19-47. 12 Kalra, S.P. and McCann, S.M., Effects of drugs modifying catecholamine synthesis on plasma LH and ovulation in the rat, Neuroendocrinology, 15 (1973) 79-91. 13 Kawakami, M.. Terasawa, E. and lbuki, T., Changes in multiple unit activity of the brain during the estrous cycle. Neuroendocrinology, 6 (1970) 30-48. 14 KiUam, K.K., Drug action on the brain-stem reticular formation, Pharmacoi. Rev.. 14 {1962) 175-223. 15 Paikovits, M., Isolated removal of hypothalamic or other brain nuclei of the rat, Brain Res., 59 (1983) 449-450. 16 gance, N. and Barraclough, C.A.. Effects of phenobarbital on hypothalamic I.HRH and catecholamine turnover rates in proestrous rats, Proc. Soc. exp. Biol. Med., 166 (1981) 425-431. 17 Rance, N., Wise, P.M., Selmanoff, M.K. and Barraclough, C.A., Catecholamine turnover rates in discrete hypothalamic areas and associated changes in median eminence luteinising hormone-releasing hormone and serum gonadotrophins on proestrous and diestrous day I, Endocrinology, 108 (1981) ! 795-1802. 18 Sailer, C.F. and Zigmond, M.J., A radioenzymatic assay for catecholamines and dihydroxyphenylacetic acid, Life Sci., 23 (1978) I 117-1130. 19 Sar, M. and Stumpf, W.E., Central noradrenergic neurones concentrate ~H-oestradiol, Nature (Lond.), 289 (1981) 500-502. 20 Ter Haar, M.B. and MacKinnon, P.C.B., Changes in serum gonadotrophin levels and in protein levels and in-vivo incorporation of methionine into protein of di,~crete blain areas and the anterior pituitary of the rat during the oestrous cycle, J. E-~docr., 58 (1983) 563-57. 21 Weakly, J.N., Effects of barbiturates on 'quantal' synaptic transmission in spinal motoneurones, J. Physiol. (Lond.L 204 (1969) 63-77. 22 Weiner, R.l, and Ganong, W.F., Role of brain monoamines and histamine in regulatien of anterior pituitary secretion, Physiol. Rev., 58 (1978) 905-976.
221
Elsevier Scientific Publishers Ireland Ltd.
HYPOTHALAMIC ADRENERGIC ACTIVITY PRECEDES THE PREOVULATORY LUTEiNIZING HORMONE SURGE IN THE RAT
P.C.B. MacKINNON, E.M. CLEMENT, C. CLARK and R, SHEAVES
Department of Human Anatomy, Oxford University Oxford OXI 3QX (U.K.) (Received September 6th, 1983; Revised version received October 17th, 1983; Accepted October 21st, 1983)
Key words: arcuate nucleus - median eminence - adrenaline turnover - critical period - luteinizing hormone
A preovulatory surge of luteinizing hormone (LH) is stimulated by an increase in plasma oestrogen (E2) and a proestrous neural signal which occurs during the 'critical period' (CP). Adrenergic activity during the CP in brain areas concerned with LH release and receiving adrenergic nerve terminals, increased in the arcuate nucleus-median eminence area (AN-ME) during the CP (14.30-16.30 h), but not in the adjacent perifornical area. Pentobarbital injected prior to the CP inhibited the effect as did ovariectomy, while E2 replacement restored it. Thus increased adrenergic activity in the AN-ME may represent the neural signal that triggers the preovulatory LH surge [5].
Although much past evidence has implicated central catecholamines in the elicitation of an oestrogen (E2)-stimulated luteinizing hormone (LH) surge in the rat [22], only recently have studies, based on pharmacological manipulations, demonstrated the essential role of adrenaline [4, 6, 12]. We have therefore studied adrenaline (A) turnover in hypothalamic nuclei during the critical period (CP) in groups (n = 6) of proestrous 4-day cyclic Wistar rats (220-280 g) maintained under regulated lighting (on: 06.00-20.00 h). They were either decapitated at 13.00, 14.30 or 15.00 h, or were injected (i.p.) at the same times with an inhibitor of phenylethanolamine-Nmethyltransferase, (SKF 64139 [9], 50 mg/kg b.wt; Smith, Kline and French, Welwyn Garden City) and killed 2 h later. Brains were removed, coronal sections (250 ~m) cut in a cryostat (-12°C), and specific areas (arcuate nucleus-median eminence, (AN-ME) and perifornical area (PFA)) removed with a stainless steel punch (int. diam. 0.85 mm) [15]. Tissue was homogenized in 0.1 M HCI containing I mM diethylene-triaminepentaacetic acid and 0.1e/o N a 2 S 2 0 5 at 0-4°C. After centrifugation at 10,000 g (30 min) the A content of the supernatant was determined by radioenzymatic assay [18] (sensitivity 5.4 pg; intra- and interassay coefficients of variation :~e/0 and 13~0 respectively). Data was corrected for crossover of 0.46% noradrenaiine and 0.7¢/0 dopamine into counts associated with A. Counts were ex0304-3940/83/$ 03.00 © 1983 Elsevier Scientific Publishers Ireland Ltd.
222
pressed in te:ans of tissue protein estimated by the Lowry et al. method. Assessment of A turnover was based on the rate of decline in its concentration 2 h after injection of SKF 64139 [7]. Data were analyzed by Analysis of Variance and the Newman-Keul's multiple range test. To define the CP in accordance with earlier work [8], Sagatai (pentobarbital; 36 mg/100 g b.wt) was injected (i.p.) into groups (n =6) of rats at 30min intervals from 12.00 to 18.00 h. Ovulation was blocked next day in 5-6 rats/group injected between 14.30 and 17.00 h; higher rates of ovulation occurring in animals injected before and after these times. A further Stoup (n = 10) of proestrous animals was implanted with intracardiac catheters. The following proestrous afternoon animals were either injected (14.25 h) with Sagatai or saline and serial blood samples (0.3 ml) taken (with Sa
i
-!-
]o c
14. O0
i
16.®
15.O0
l,:®
18'00
lime~hl
,,,5O z/t
"Critical period"
2¢
_= 40 z
2O
'°I _+ '1
.00
. . . . . . .
16 ®
I
1,00
~
I
)8®
•
,+.®
Time,hi
Fig. 1. Serial plasma LH concentrations obtained from two groups of proestrous rats over the period of the preovulatory LH surge. The animals had been injected (i.p.) either with saline (S) or Sagatal (Sa) at the start of the critical period (black bar).
223
saline replacement) at regular 30 win intervals. Plasma samples were stored ( - 2 0 ° C ) until LH concentrations were measured by radioimmunoassay [20]. Only saline-injected rats showed typical LH surge prof'fles (Fig. 1). Turnover of A in the AN-ME was increased in proestrous rats between 14.30 and 16.30 h when compared with turnover between either 13,00 and 15.00 h or 15.30 and 17.30 h. No differences in activity were found in the PFA (Fig. 2). The turnover rate of catecholamines in the brain depends on the rate of nerve impulse flow, more decfine in the concentration of transmitter following the relevant enzyme inhibitor being associated with increased neuronal activity [l ]. Therefore increased turnover in the AN-ME at proestrous (14.30-16.30 h), which is consistent with an increase in multiple unit activity during the CP [13], may be attributable to increased activity of brainstem adrenergic neurones, the axons of which terminate in the AN-ME [14]. That such an increase in adrenergic activity represents at least a part of the mechanisms which initiate the LH surge is suggested by past work [22]; while recent evidence derived from pharmacological manipulation of central adrenaline and its synthesising enzyme also strongly supports a role for A in preovulatory LH release [4, 6, 12]. An increase in plasma prolaain concentrations occurs at about the same time as the LH surge, and this too might be attributed to increased A activity in the AN-ME except that SKF 64139, in spite of preventing
0.7
AN-ME
b (17)
PFA
0.6
0.5 "2 0.4
E gO.3 0.2
fll~
z) O.l
I
ill)
I
g//////A 1300h
1500h
1700h
1300h
1500h
1700h
Fig. 2. 2-Hourly (13.00-15.00 h; 14.30-16.30 h; 15.30-17.30 h) turnover rates of adrenaline in the ANME and PFA around the time of the proestrous critical period. Numbers of rats used are shown in brackets and S.E. of the mean is indicated, a:b, P<0.001; b:c, P<0.025.
224
a steroid-stimulated LH surge, has no effect on Ez-stimulated prolactin release [4]. Ez replacement restores LH surges in ovariectomized rats [3], so to substantiate our findings in intact animals we measured A turnover in groups of rats ovariectomized at dioestrous (12.00 h) and injected (s.c.) with either oestradiol benzoate (OB; 10/~g in 0.2 ml arachis oil) or oil vehicle. The following day the animals were either decapitated or injected (i.p.) with SKF 64139 and treated in the same way as the previous groups. Turnover of A in the AN-ME of the ovariectomized oil-treated group at expected procstrous 04.30-16.30 h) was significantly lower than that found in either the intact untreated animals or in the ovariectomized Ez-treated group, whereas A turnover was similar in the latter group and in intact untreated animals. The turnover of A in the PFA, however, was unaffected by either treatment (Table I). Early experiments with barbiturates injected at the start of a 2 h period on the afternoon of proestrous showed that ovulation was delayed by 24 h. This led to the concept of a daily neural event (CP) on which the LH surge and ovulation depends [8]. Our results show that injection of barbiturate at the start of the CP 04.25 h) almost immediately inhibited A turnover in the AN-ME, but had no effect on activity in the PFA (Fig. 3). Although the action of barbiturates on LH release is not understood, they alter activity of nerve terminals [2, 211 and depress the reticular activating system [14], a site of catecholamine cell bodies which concentrate Ez [ 19]. Increased turnover of noradrenaline (NA) and dopamine (DA) at proestrous (12.00-15.00 h) concomitant with a fall in the hypothalamic content of gonadotrophin-releasing hormone (Gn-RH) and an increase in plasma LH levels, have also been reported and interpreted as representing processes involved with the initiation of the LH surge [171. But a barbiturate injected in an ovulatory blocking dose at the start of this period (12.00 h) failed to affect NA or DA turnover in any of the areas examined until some 2 h later, when NA turnover in the ME was significantly reduced [16]. Bearing in mind that only the initial 13070 of LH surge TABLE 1 MEAN ± S.E. C O N C E N T R A T I O N S OF A (pg/ag protein) A N D A T U R N O V E R IN THE AN-ME A N D PFA IN G R O U P S OF INTACT OR E X P E R I M E N T A L RATS ( E X P L A N A T I O N IN TEXT) Rats were either killed at 14.30 h proestrous or injected ~vith SKF 64139 and killed 2 h later. Numbers of animals in parentheses: a:b, P < 0 . 0 2 5 ; c:b, P < 0 . 0 2 5 ; a:c, n.s. Intact
OVEX + oil
OVEX + E2
Baseline {A cone.)
AN-ME PFA
1.99 + 0.14 (!1) 0.57 _+ 0.03 ( 6 )
!.23 _+ 0.12 ( 6 ) 0.46 _+ 0.04 ( 6 )
i.58 _+ 0.19 ( 7 ) 0.61 _+ 0.05 ( 8 )
SKF 64139 {Aconc.)
AN-ME PFA
0.85 _+ 0.10 ( 6 ) 0.34 _+ 0.03 ( 6 )
0.89 _+ 0.12 ( 5 ) 0.26 _+ 0.06 ( 5 )
0.76 +_ 0.09 ( 5 ) 0.21 _+ 0.02 ( 6 )
Turnover (pg/~g protein/h)
AN-ME PFA
0.57 _+ 0.09 (17) ~ 0.14 _+ 0.02 (12)
0.17 _+ 0.08 (11) b 0.10 _+ 0.03 (11)
0.41 + 0.11 (12) ~ 0.20 +_ 0.02 (14)
225 AN-ME
PFA
0.7
0.6
0.5
0.4 ¢.-
d Gll
"~
0.3
~'
0.2
~11)
0.1
Fig. 3. Turnover rates of adrenaline in the AN-ME and PFA between 14.30 and 16.30 h (during the critical period) in proestrous rats which had been injected either with saline (1~) or Sagatal ( m ) at the start of the critical period. Numbers of rats used are shown in brackets and S.E. of the means are indicated, a:b, P<0.001. d:c, n.s.
output is required for successful ovulation [10] it is difficult to reconcile this data with initiation of the surge. Our results, therefore, suggest that while NA and DA may have a role to play in LH output, increased adrenergic activity in the AN-ME precedes and is associated with initiation of the preovulatory LH surge. Whether as a result of the preovulatory rise of E2 adrenaline activity acts as a direct or indirect trigger for the release of Gn-RH, or whether it mediates the effects of E2 at a hypothalamic and/or pituitary level, are questions currently being studied. We thank the MRC for financial support (G8215236N to P.C.B.M.); Smith Kline and French for gifts of SKF 64139; and particularly Mr. R. Laynes for excellent technical assistance. I And~n, M.-E., Corrodi, H., Dahlstr6m, A., Fuxe, K. and H6kfelt, T., Effects of tyrosine hydroxylase inhibition on the amine levels of central monoamine neurons, Life Sci., 5 (1966) 561-568. 2 Blaustein, M.P., Barbiturates block calcium uptake by stimulated and potassium-depolarized rat :,ympathetic ganglia, J. Pharmacol. exp. Ther., 196 (1976) 80-86. 3 Caligaris, L., Astrada, J.J. and Taleisnik, S., Release of LH induced by estrogen injection into ovariectomised rats, Endocrinology, 88 (I 971) 810--815. 4 Cass Terry, L., Crowley, W.R., Lynch, C., Longserre, C. and Johnson, M.D., Role of central epinephrine in regulation of anterior pituitary hormone secretion, Peptides, 3 (1982) 311-318. 5 Clark, C.R., Clement, M. and MacKinnon, P.C.B., Hypothalamic adrenaline in the prooestrous 'critical period' in the rat, J. Physiol. (Lond.), 334 (1982) 128P.
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