Ontogeny of tyrosine hydroxylase and dopamine-beta-hydroxylase activity in discrete limbic and hypothalamic structures of female rats

Tyrosine hydroxylase (TOH) and dopamine-ß-hydroxylase (DBH) activities were measured in the mediocortical amygdala (AMY), hippocampus (HPC), nucleus accumbens (ACB), medial preoptic area (MPO), and anterior and posterior parts of the mediobasal hypothalamus (AMBH and PMBH) in female rats of various postnatal ages. Serum LH, FSH and prolactin levels were also measured. In a previous study it was shown that dopamine (DA) and norepinephrine (NE) turnover rates in the MPO and the MBH increase between days 20 and 30 after birth and decrease thereafter. The cause for increased DA turnover was shown to be increasing serum prolactin levels. The activity of TOH in the ACB, MPO, AMBH and PMBH did not parallel DA turnover rates. Around day 20 after birth, when serum prolactin levels and preoptic and hypothalamic DA turnover increase, TOH activity also increased in the ACB and PMBH. In adult, diestrous animals, however, which have low serum prolactin levels and low preoptic and hypothalamic DA turnover, TOH activity was higher than in immature rats. The activity of DBH in the MPO, AMBH, AMY and HPC was high in 15-day-old animals, decreased between days 20 and 30 and then increased again to adult values. This pattern is just opposite to the one observed for serum prolactin levels and for preoptic and hypothalamic NE turnover. It is concluded that activity of both enzymes is not a good measure for neuronal activity of those NE and DA cells which innervate limbic and hypothalamic structures.     

Involvement of catecholamines in eliciting LH peaks in 15-day-old female rats: effects of treatment with prolactin

In the serum of 15-day-old female rats, treated either with -methyl-p-tyrosine (-MPT) or saline, serum LH levels were determined. From the brains of all animals the medial preoptic areas (MPO) and the anterior and posterior mediobasal hypothalami (AMBH and PMBH) were punched out and stored frozen in perchloric acid. In the punches of those animals with very high or basal LH levels catecholamine concentrations were measured radioenzymatically and on the basis of the degree of catecholamine depletion after -MPT the turnover rates of dopamine (DA) and norepinephrine (NE) were calculated. NE turnover in the MPO was significantly higher in the animal with high LH levels as compared to those with low LH values. In the PMBH an inverse correlation between LH levels and NE turnover rates could be demonstrated. NE turnover in the AMBH, and DA turnover in all 3 structures did not correlate with serum LH levels. Serum FSH and prolactin levels were higher in the animals with high LH levels. These changes are similar to those observed in adult proestrous rats and it is concluded that the LH peaks in 15-day-old animals reflect a positive feedback action of estradiol.

In a second experiment prolactin was administered to determine if it could prevent the occurrence of these LH peaks. Ovine prolactin (2 × 0.5 μg/g body weight) injected from day 8–15 suppressed serum LH levels to almost undetectable values. Catecholamine turnover measurements in these animals revealed increased DA turnover in the MPO, AMBH and PMBH, whereas NE turnover was not affected. These results indicate that DA turnover of the incertohypothalamic and of the tuberoinfundibular DA neurons is accelerated by prolactin and that an increased DA turnover in one of these systems is responsible for inhibition of LH release.     

Oestrogen and noradrenaline modulate endogenous GABA release from slices of the rat medial preoptic area

Endogenous gamma-aminobutyric acid (GABA) release from the rat medial preoptic area (MPOA) was measured in an in vitro slice technique with sensitive HPLC analysis. Oestrogen is demonstrated to increase GABA activity in the ovariectomised, oestrogen-primed (OVX-EB) rat prior to the luteinising hormone (LH) surge compared with ovariectomised (OVX) animals. Noradrenaline (NA) at a concentration of 10 microM was found to significantly enhance GABA release in response to 30 mM potassium stimulation in both OVX and OVX-EB animals. A significantly greater response to NA was observed in the OVX-EB animal. No effect of NA on basal GABA release was detected. The effects of NA were blocked by the alpha-adrenergic receptor blocker phenoxybenzamine (PB). These data suggest that GABA activity is modulated both by oestrogens and noradrenergic-mediated input in the MPOA.     

Ontogeny of catecholamine turnover rates in limbic and hypothalamic structures in relation to serum prolactin and gonadotropin levels

Norepinephrine (NE) and dopamine (DA) concentrations and turnover rates have been studied in the n. accumbens, medial preoptic area (MPO) and anterior and posterior parts of the mediobasal hypothalamus of developing rats. Turnover rates are determined by injection of -methyl-p-tyrosine 30 and 90 min prior to decapitation. NE concentrations and turnover in the n. accumbens were low in all age groups with slightly increased values between days 20 and 35 after birth whereas DA concentrations and turnover rates were low at day 15 and 20 and at high adult values by day 25 after birth. Medial preoptic and anterior mediobasal hypothalamic catecholamines exhibited a rather unique pattern. Concentrations and turnover rates were low in 15-day-old animals and increased between days 20 and 30 to very high values. Such high values were never observed in adult diestrous animals. The same pattern was also observed in the posterior mediobasal hypothalamus for NE concentrations and turnover rates whereas the respective values for DA showed relatively large fluctuations. On the basis of catecholamine measurements 30 and 90 min after blockade of tyrosine hydroxylase an attempt was also made to differentiate turnover rates of the functional and of the storage pool.

Serum LH levels in the 15-day-old animals showed large fluctuations. FSH levels were high and prolactin levels were low. At the time of increased preoptic and hypothalamic NE and DA turnover rates, serum prolactin levels were also high whereas serum LH levels were lowest between days 20 and 30 and then slightly increased. Serum FSH levels were uniformly low. The possibility is discussed that high NE turnover may stimulate pituitary LH and prolactin release by hypothalamic mechanisms. High serum prolactin levels may stimulate DA turnover which is inhibitory to pituitary LH release, thus counteracting the stimulatory effect of NE on LH-RH release. The dopaminergic inhibition of LH may be relieved at the time of puberty partially because the DA receptors become desensitized to the action of DA.     

Cellular mechanisms of acute estrogen negative feedback on LH secretion: norepinephrine, dopamine and 5-hydroxytryptamine metabolism in discrete regions of the rat brain

Rats on diestrous day 1 were ovariectomized (OVX) and killed 10 days later. LH was measured by RIA and the metabolism of NE, DA and 5-HT were assayed concurrently in the suprachiasmatic (SCN), medial preoptic (MPO), dorsomedial (DMN), rostral (ANr) and caudal (ANc) arcuate nuclei as well as the median eminence (ME) utilizing HPLC with electrochemical detection. Serum LH increased 10-12 fold 10 days following OVX compared to diestrous controls. The injection of estradiol benzoate (Eb, 20 micrograms in corn oil/rat, SC) did not affect LH concentrations at 30 minutes but decreased serum LH both 60 and 180 min following its administration. OVX caused an increased NE metabolism (estimated by the concentration of the NE metabolite, 3-methoxy-4-hydroxyphenylethylene glycol) in the SCN, MPO, ME, and DMN and a decreased NE metabolism in the ANc compared to diestrous control values. All of these changes were reversed or attenuated 180 minutes following Eb treatment. Observed changes in the DA and 5-HT neuronal systems were more restricted and less dramatic with the largest effects on DA metabolism occurring in the DMN and ME and the clearest changes in 5-HT metabolism occurring in the MPO, ANr, and ANc. The results demonstrate that the inhibition of LH secretion following the injection of Eb to OVX rats is accompanied by changes in metabolism in NE neurons in preoptic (SCN and MPO) and medial (ME, DMN, and ANc) hypothalamic areas, as well as in DA neurons in the DMN and ME, and in 5-HT neurons in the MPO, ANr, and ANc.     

Gonadotropin responses to naloxone may depend upon spontaneous activity in noradrenergic neurons at the time of treatment

The opiate system is thought to modulate gonadotropin secretion by its effect on catecholamine secretion. This action may be produced by opiates regulating the amount of catecholamine released from presynaptic terminals at a given frequency of depolarization and/or by increasing the rate of impulse traffic within catecholamine neurons. We examined the effects of naloxone, an opiate receptor antagonist, on luteinizing hormone (LH) and prolactin (Prl) secretion in 3 sex steroid-treated, gonadectomized rat models in which we have considerable information on the rates of turnover of norepinephrine (NE) and dopamine (DA) in the hypothalamus. In 7 day ovariectomized rats treated for 2 days with estradiol (E2), the injection of naloxone (10 mg/kg) at 09.15 h produced a small 3-fold rise in LH and a short-lived decline in Prl. In contrast, naloxone, given at 12.15 h, markedly amplified (10-fold) and advanced the time of the LH surge but did not affect afternoon Prl surges. Hypothalamic NE turnovers are low in the morning and high in the afternoon for such animals. Other ovariectomized (OVX) rats received E2 for 2 days and progesterone (P4) on day 2. Such treatment extinguishes the LH surges which normally occur the next day (day 3) but does not affect phasic Prl secretion. Naloxone, given at 09.15 h to E2P4-treated rats on day 3, did not affect basal LH levels but serum Prl declined for about 1 h. When given at 12.15 h, naloxone produced a small 3-fold rise in LH but did not affect phasic Prl release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Monoamine Levels and Turnover in Brain: Relationship to Priming Actions of Estrogen

Norepinephrine (NE), dopamine (DA) and serotonin (5HT) levels and turnover rates were studied in 8 discrete brain nuclei of ovariectomized rats 24 hours after the administration of 5 microgram of estradiol benzoate (EB) or sesame oil vehicle. This estrogen paradigm, by itself, did not induce sexual behavior or alter LH levels at the time these parameters were evaluated. However, combined with progesterone, the estrogen treatment was sufficient to generate an LH surge and induce sexual receptivity. Steady state concentrations of NE were significantly higher in the diagonal band of Broca (NDB) and the periventricular nucleus (PVE2; anterior hypothalamic level) following EB treatment. In addition, 5-hydroxyindole acetic acid (5HIAA) concentrations were elevated in the dorsal raphe of EB treated animals. Estrogen did not affect steady state concentrations of DA or 5HT in any of the brain nuclei studied. Turnover rates (K, pg/microgram protein/hr) and rate constants (k, hr-1) for NE were increased in the lateral septum (K, 140%; k, 120%), NDB (K, 160%; k, 130%) and the PVE2 (K, 140%; k, 70%) in EB treated animals. Estrogen decreased the rate constant for NE by 30% in the medial preoptic area. In contrast, DA and 5HT turnover rates were not significantly affected by estrogen. These results localize sites where estrogen induces changes in noradrenergic activity and suggest that these changes may be involved in the priming action of the steroid in inducing sexual behavior and/or gonadotropin secretion.     

The effect of intraventricular gamma aminobutyric acid (GABA) on hypothalamic catecholamines and LHRH and on pituitary hormone release

The present experiments were designed to evaluate the role of catecholamines in mediating the actions of GABA on pituitary hormone release following its intraventricular injection. GABA was administered intraventricularly, at doses of 0.1 and 4 μMol, to chronically ovariectomized rats. Five and 15 min after intraventricular injection, the animals were sacrificed by decapitation. Blood was collected for LH and PRL determination by RIA, and brains were frozen for subsequent determination of LHRH by RIA, and dopamine (DA) and norepinephrine (NE) by radioenzymatic assay. Brain areas analyzed included the median eminence (ME), medial basal hypothalamus (MBH) and suprachiasmatic-medial preoptic area (Sch-PO). Controls received an equal volume (2 μl) of saline. As previously observed, GABA had opposite effects on prolactin (PRL) release depending on the dose employed. A decline in serum PRL was induced by a low (0.1 μMol) dose, whereas a larger (4 μMol) dose induced an increase in PRL levels. LH levels were increased only with the 4 μMol dose. The elevated LH values were paralleled by an increase in LHRH levels in the Sch-PO region 15 min after GABA. Marked changes in catecholamines, particularly in DA, were seen after GABA injection. Significant increments in DA levels in the ME were seen 5 and 15 min after the 0.1 μMol dose of GABA. Similarly, this low dose of GABA induced a marked increase in DA levels in the anterior-pituitary (AP) gland at 15 min. No changes in DA were seen in either MBH or Sch-PO areas. The 4 μMol dose of GABA induced only a small increase in AP levels of DA, without altering the amine levels in any of the brain areas examined. NE levels in the ME were elevated 5 min after the administration of either dose of GABA. No changes in NE were seen in either the MBH or Sch-PO areas. These results indicate that intraventricular GABA injection not only alters pituitary hormone release but also release of DA and NE from terminals in the ME. The released catecholamines may be important in mediating the effects of GABA on releasing factor discharge. In addition, the DA released may have acted directly on the AP to inhibit release of PRL.     

Effects of morphine on luteinizing hormone secretion and catecholamine turnover in the hypothalamus of estrogen-treated rats

This study examined the effects of morphine sulfate and naloxone alone or in combination on phasic luteinizing hormone (LH) secretion in estrogen-treated ovariectomized rats. Thereafter, the effects of morphine on initial concentrations, rate constants and rates of turnovers of norepinephrine and dopamine were evaluated in untreated or morphine-injected, estrogen-primed rats. Morphine, when given at 12.30 h, completely suppressed the spontaneous LH surges which occur in steroid-treated rats. The opiate antagonist, naloxone, (12.15 h) markedly amplified and advanced the time of LH release and a combination of morphine and naloxone produced peak afternoon LH values which were intermediate between those obtained in controls and in rats receiving only naloxone. Norepinephrine (NE) and dopamine (DA) turnover were calculated from data obtained in groups of rats sacrificed 0,45 or 90 min after administering 300 mg/kg b. wt. i.p. ofα-methyl-p-tyrosine (α-MPT) at 10.00 or 15.00 h. In these experiments, the medial preoptic nucleus (MPN) and the median eminence (ME) were microdissected and analyzed for changes in NE and DA concentrations by a radioenzymatic procedure. In estrogen-treated rats, NE rate constants and turnover significantly increased at 15.00 vs 10.00 h in MPN and ME concomitant with increases in serum LH. Morphine blocked both increases in rate constants and turnover in the MPN and ME and also significantly reduced initial concentrations of NE in the MPN. None of the DA parameters measured in MPN or ME changed in estrogen-treated controls between morning and afternoon. Further, while morphine did not affect DA turnover in the MPN, DA turnover declined in the ME. These data add to accumulating evidence which demonstrates an important modulatory role for hypothalamic opiate neurons in regulating catecholamine activity and gonadotropin secretion.     

Hypothalamic monoamines and their catabolites in relation to the estradiol-induced luteinizing hormone surge

Monoamines and non-conjugated catabolites (serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA), 3,4-dihydroxyphenyl-acetic acid (DOPAC), homovanillic acid (HVA), 4-hydroxy-3-methoxyphenylethyleneglycol (MHPG), norepinephrine (NE), and dopamine (DA] were measured in the medial basal hypothalamus (MBH) and preoptic area (POA) of ovariectomized (OVX) and OVX estradiol (E2)-treated rats using high-performance liquid chromatography with electrochemical detection. These E2 treatments were sufficient to induce an LH surge. The use of MHPG/NE ratios as estimates of NE release was validated in the rat hypothalamus by the major decreases of MHPG after injection of the alpha 2-adrenergic agonist, clonidine, and by MHPG increases after the alpha 2-antagonist, yohimbine. The ratio, MHPG/NE, decreased between morning and afternoon in the MBH but not in the POA; there were no differences between OVX and E2-treated rats. Previous studies using a variety of methods indicate that NE turnover increases during LH surges. The present data suggest that unconjugated MHPG is not a sensitive measure of NE release in the rat hypothalamus, but can detect the large changes produced by stimulating or inhibiting the alpha 2-adrenergic autoreceptor. The ratios of DOPAC/DA and 5-HIAA/5-HT in the MBH decreased consistently between morning and afternoon in OVX rats, with or without E2 treatment. This suggests that the release of DA and 5-HT decreases during the day regardless of steroidal milieu.     

Studies on dopamine turnover in ovariectomized or hypo-physectomized female rats. effects of 17β-estradiol benzoate, ethynodioldiacetate and ovine prolactin

The effects of 17 beta-estradiol benzoate (EB), ethynodioldiacetate and ovine prolactin on dopamine (DA) turnover have been studied. As an index for a change in turnover, differences in DA depletion following tyrosine hydroxylase inhibition with alpha-methyl-tyrosine methylester were observed. DA was measured by means of mass fragmentographical analysis in the rat median eminence, the olfactory tubercle and the striatum. The actions of ethynodioldiacetate and ovine prolactin on DA turnover in various subregions of the rat median eminence were analysed by quantitative microfluorimetry. Repeated injections of EB to ovariectomized rats resulted in a significant increase of DA turnover in the median eminence. Administration of ethynodioldiacetate to ovariectomized rats almost significantly increased DA turnover in the olfactory tubercle. In the median eminence DA turnover was significantly increased only in the lateral palisade zone. In male hypophysectomized rats ovine prolactin increased DA turnover in both the lateral and the medial palisade zone of the median eminence. The results support the involvement of DA neurons in the control of prolactin and luteinizing hormone secretion. It is suggested that the tubero-infundibular DA neurons are involved in mediating the central inhibitory feedback actions of prolactin and gonadal steroids on prolactin and LH secretion, respectively.     

Gamma aminobutyric acid (GABA), a modulator of anterior pituitary hormone secretion by hypothalamic and pituitary action

We have evaluated the role of GABA in the control of anterior pituitary (AP) hormone secretion by injecting it into the third ventricle of ovariectomized, ovariectomized-steroid primed and male rats. Specificity of the effects was determined by injecting the GABA blocker, bicuculline. The action of GABA directly on the pituitary was evaluated in vitro. The results indicate that intraventricular GABA can stimulate LH, growth hormone (GH) and, at high doses, prolactin (Prl) release, whereas low doses inhibit Prl and al doses inhibit TSH release. All of these actions are blocked by bicuculline. Intraventricular GABA administration is followed by an elevation of hypothalamic norepinephrine (NE) and median eminence dopamine (DA) levels and AP DA levels, which indicates that the compound stimulates both NE and DA release. The actions on GH and LH appear to proceed independently of DA, since the DA receptor blocker, pimozide, did not interfere with these effects, whereas the action to elevate Prl and to lower TSH was blocked by DA receptor blockade. Anterior pituitary hormone release by AP's incubated with GABA in vitro was unaltered except for an inhibition of Prl release by very high GABA doses, which could be blocked by bicuculline. Intravenous injection of bicuculline to assess the physiological significance of GABA in control of AP hormone secretion revealed no effect on FSH but a delayed rise in LH, an initial rise in Prl, followed by a fall, a tendency for GH values to rise and dramatic fall in TSH levels. These results suggest the possibility that GABA plays a physiological role in the control of AP hormone secretion, mainly via a hypothalamic action.     

Regulation of glutamic acid decarboxylase 65 and 67 gene expression by ovarian steroids: identification of two functionally distinct populations of GABA neurones in the preoptic area

GABA neurones in the preoptic area (POA) are critical for oestradiol (E2)-dependent surge release of luteinizing hormone (LH); however, it is not clear which population(s) of POA GABA neurones is involved. The goals of the present studies were: (i) to determine whether E2 regulates GABA neurones similarly in two subdivisions of the POA that play a role in LH surge release, the rostral POA region that contains the organum vasculosum of the lamina terminalis (rPOA/OVLT), and the region containing the anteroventral periventricular nucleus (AVPV) and medial preoptic nucleus (MPN) and (ii) to determine whether GABA neurones in either or both regions exhibit temporal changes consistent with a role in the regulation of LH surge release. To accomplish these goals, we measured glutamic acid decarboxylase (GAD) 65 and 67 mRNA levels at several time points in ovariectomized (OVX), E2-treated OVX rats exhibiting LH surge release, and in E2-treated OVX rats in which LH surge release was blocked by prior administration of progesterone (P4). Our findings demonstrate that, despite their close proximity, GABA neurones in the AVPV/MPN region are regulated differently from those in the rPOA/OVLT. Only neurones in the AVPV/MPN region show temporal changes in GAD 67 mRNA expression that appear to be linked to positive-feedback effects of E2 on luteinizing hormone-releasing hormone (LHRH) and LH release. Our findings also indicate that a morning rise and an afternoon fall in GAD 67 mRNA levels marks two E2-dependent signals required for LHRH and LH surge release. Finally, our results suggest that there are distinct E2-induced signals to the rPOA/OVLT and AVPV/MPN regions and that these signals differentially regulate GAD 65 and 67 gene expression.     

Effects of estrogen and progesterone on plasma gonadotropins and on catecholamine levels and turnover in discrete brain regions of ovariectomized rats.

Estradiol benzoate (EB) was administered, either alone or followed 48 h later by progesterone to ovariectomized rats. Plasma gonadotropins (FSH and LH) and steady state levels of norepinephrine (NE) and dopamine (DA) in 17 individual brain nuclei were assayed. In addition, catecholamines were measured after administration of the synthesis inhibitor alpha-methyltyrosine (alpha-MT) in order to assess hormonal influences on turnover. Treatment with EB, which lowered plasma FSH and LH, reduced the depletion of NE produced by alpha-MT in the lateral septum, interstitial nucleus of the stria terminalis, and central gray catecholamine area, and reduced the depletion of DA in the nucleus of the tractus diagonalis. EB enhanced NE depletion in the periventricular and anterior hypothalamic nuclei, and raised steady state levels of NE in the medial amygdaloid nucleus. These effects were reversed by subsequent treatment with progesterone, which stimulated FSH and LH release. EB plus progesterone enhanced the alpha-MT-induced depletion of NE over that observed with EB alone in the arcuate nucleus, and similarly enhanced DA depletion in the interstitial nucleus of the stria terminalis. EB plus progesterone prevented the depletion of NE by alpha-MT in the paraventricular and ventromedial nuclei, and also lowered resting NE levels in the paraventricular nucleus. The results suggest that catecholamine neurons in several discrete brain regions participate in the stimulatory and inhibitory feedback effects of ovarian hormones on gonadotropin secretion, and perhaps also on the hormonal induction of sexual receptivity.     

Release and synthesis rates of catecholamines in hypothalamic, limbic and midbrain structures following intraventricular injection of β-endorphin in male rats

Serum hormone levels and turnover rates of dopamine (DA), norepinephrine (NE) and epinephrine (E) in tissue of pontine midbrain, limbic caudate, and hypothalamic structures were measured following intraventricular injection of 20 microgram beta-endorphin. Turnover of DA was increased in the locus coeruleus and in structures innervated by the nigrostriatal and mesolimbic DA systems while it decreased in the posterior hypothalamus. The NE turnover was reduced in the locus coeruleus and posterior mediobasal hypothalamus. In all tissues examined, beta-endorphin treatment had no significant effect on E turnover. Serum levels of prolactin and corticosterone increased following beta-endorphin treatment whereas LH, FSH and TSH levels decreased. The observed effects of beta-endorphin on preoptic and hypothalamic DA and NE turnover rates may explain these hormonal changes. The reduced NE turnover in the locus coeruleus may be induced by increased DA turnover. The observation, that NE turnover in structures innervated by locus coeruleus neurons may go in different direction suggest that NE turnover is regulated independently from the activity of locus coeruleus neurons, hence that the regulation must occur at the terminals by a direct or indirect interaction with endorphinergic neurons.     

Morphine but not Naloxone Enhances Luteinizing Hormone-Releasing Hormone Neuronal Responsiveness to Norepinephrine

Some axon terminals of hypothalamic opiate neurons directly synapse on luteinizing hormone-releasing hormone (LHRH) neurons. To determine whether such synaptic connections affect LHRH neuronal activity, we have examined the profiles and concentrations of LH released in response to intracerebroventricular (icv) norepinephrine (NE, 45 μg) infusions alone or following medial preoptic area (MPOA) electrochemical stimulation (ECS) in estrogen-treated ovariectomized rats. Similar studies were performed in rats treated with naloxone (5 mg/kg ip) or morphine (20 mg/kg sc) given 15 min prior to MPOA-ECS or 30 min prior to icv NE. Naloxone neither augmented nor suppressed the LH response obtained with NE alone. MPOA-ECS evoked a significant increase in plasma LH. When the preoptic area was stimulated (0 min) and NE was infused at 30 min, a significant amplification of LH release occurred. Prior treatment of rats (-15 min) with naloxone had no effect on LH responses elicited by either preoptic stimulation alone or combined with icv NE. In the second study, morphine was given sc and had no effect on basal plasma LH levels. However, when morphine was given (-15 min) and icv NE infusions were made (30 min), the rise in plasma LH induced by NE was significantly enhanced. Preoptic ECS (0 min) evoked a rise in plasma LH and this response was also enhanced by morphine pretreatment. The major effect on LH release occurred when sc morphine injections (-15 min) were combined with MPOA-ECS (0 min) followed by icv NE (30 min). In these rats, a remarkable and highly significant release of LH occurred which reached peak levels even greater than those observed during spontaneous LH surges (2,392 versus 16 to 1,800 ng/ml). Since morphine has profound effects on the serotonergic system, in the third series of studies, morphine was infused into the dorsal raphe nucleus (DRN) and LH responses to MPOA-ECS or icv NE alone or following combined ECS + NE were examined. DRN morphine did not affect basal LH release but it produced a rapid and highly significant rise in plasma prolactin. When DRN morphine was given (-15 min) and NE was infused icv (30 min), there was marked amplification in LH release compared to those values observed after only NE. However, there were no appreciable differences in LH values obtained after sc versus DRN morphine injections in response to NE. Similarly, the amplification of LH release which occurred following DRN morphine (-15 min) + MPOA-ECS (0 min) was not different from that obtained after sc morphine. In the final group of rats, DRN morphine was given (-15 min), the preoptic area was stimulated (0 min) and NE was infused (30 min). Following this treatment, plasma LH release was also markedly enhanced and did not differ appreciably (except at 60 and 120 min) from the levels of LH released after sc morphine. Prolactin concentrations rose slowly after icv NE to reach peak levels 75 min after treatment. Combinations of morphine + MPOA-ECS without or with NE neither augmented nor suppressed the high prolactin concentrations achieved after only DRN morphine infusions. We conclude from these data that: 1) those opiate neuronal terminals which synapse directly on LHRH neurons do not affect LHRH neuronal responsiveness to either NE, to MPOA-ECS or to combined preoptic stimulation+ NE, and 2) morphine has profound effects on LHRH neuronal responsiveness to both NE, to MPOA-ECS and, in particular, to combined ECS + NE. Since amplification of LH release occurs after treatment of rats with morphine either by sc injections or DRN infusions, the augmented LH and prolactin responses observed are most likely due to the morphine-induced release of serotonin and not to direct morphine effects on LHRH neurons.     

Age-related alteration in catecholamine activity within microdissected brain regions of ovariectomized fischer 344 rats

The effects of increasing age on catecholamine (CA) metabolism in microdissected brain regions and on serum and pituitary hormone levels were examined in ovariectomized Fischer 344 rats. Young (4 to 5 months old) and middle-aged (9 to 10 months old) normally cycling and old repeated pseudopregnant rats (21–22 months old, PP) were ovariectomized to eliminate the complicating effects of cyclic gonadal steroid fluctuations. CA metabolism was examined 2 weeks later. To determine CA turnover rates, each agegroup was subdivided into three groups, which were killed by decapitation 0, 45, or 90 min after administration of α-methyl-para-tyrosine (α-mpt). Dopamine (DA) and norepinephrine (NE) concentrations were determined in microdissected brain regions by radioenzymatic assay, and turnover rates were estimated. Steady-state concentrations of NE were not altered in middle-aged rats, but NE turnover rates increased in middle-aged rats in five of the six areas examined. While NE concentrations did not change with age in the median eminence (ME), NE turnover rates increased significantly in the two older age groups. These data indicate that the age-related decline in NE concentrations in several ventral diencephalic nuclei is preceded by a period of hyperactivity in noradrenergic neurons. DA concentrations were generally decreased in most areas examined in old versus young rats, with dramatic DA depletions (42–78%) observed in five regions. However, no consistent relationship between DA concentrations and turnover rates was seen either in regions with stable DA levels or in those which showed an age-associated decrease in DA concentrations. In the ME, a 42% decline in DA concentration was associated with an increase in the DA turnover rate in the oldest group of rats. Serum luteinizing hormone (LH) levels were similar in all three age groups of ovariectomized rats, while serum prolactin was elevated four-fold in old compared to younger animals. These data indicate that a complex pattern of regional alterations in CA metabolism accompanies the aging process and these may be related to the pseudopregnant state and hormone secretory capacity of aging Fischer 344 rats.     

puberty of female rats may in part be explained by decreased hypothalamic dopamine receptor sensitivity

Dopamine (DA)-stimulated adenylate cyclase activity in the hypothalamus and preoptic area is highest at day 20 after birth and declines gradually to reach adult values at the time of puberty. Previously it was shown that serum prolactin levels and hypothalamic DA turnover show an inverse pattern. It is suggested that increasing prolactin levels stimulate DA turnover which results in inhibition of luteinizing hormone (LH) release. The constantly high DA turnover at later prepuberal ages desensitizes DA receptors which gradually relieves the inhibitory action of DA on LH release.     

Effects of estrogen on dopamine turnover, glutamic acid decarboxylase activity and lordosis behavior in septal lesioned female rats

Estradiol benzoate (EB) treatment (2.0 μg/day × 3) of septal lesioned adult female rats produces a marked increase in lordosis behavior over similarly treated sham operated animals. In these animals endogenous dopamine (DA) and norepinephrine (NE) levels were measured and turnover rates were estimated following tyrosine hydroxylase inhibition with α-methyltyrosine, in both EB and oil (0.05 ml × 3 days) treated groups. No consistent pattern of change in the level or rate of turnover of NE was noted for any of the treatment groups. No differences in either the level or turnover of DA were seen in the oil treated septal lesioned group, relative both to EB and oil treated sham operated groups. However, in the amygdala, corpus striatum and nucleus accumbens septi of the septal lesioned animals primed with EB there was both a reduction in endogenous levels (20–50%) and a decrease in synthesis rates (30–70%) of DA relative to both EB and oil treated sham operated groups and the oil treated septal lesioned group. The EB treated septal lesioned animals showed a significant negative correlation between DA levels in the corpus striatum and the level of lordosis behavior (r = −.850), supporting the concept of a possible inhibitory role for DA on female sexual receptivity. Glutamic acid decarboxylase (GAD; the rate limiting enzyme in the synthesis of γ-aminobutyric acid (GABA)) activity was measured in the substantia nigra and ventral tegmental region from the same animals. GAD activity was reduced in both areas in the EB primed sham operated animals, while the EB and oil treated septal lesioned groups remained comparable to the oil treated sham operated animals. Because of the proposed existence of a GABA mediated inhibitory neuronal feedback on DA cell bodies in the midbrain, the reduction in DA turnover in the EB primed septal lesioned female rat and thus the increase in behavioral receptivity could be due in part to the failure of the septal lesioned animal to show a compensatory decrease in GAD activity.     

Locus ceruleus lesions block pulsatile LH release in ovariectomized rats

Luteinizing hormone (LH) secretion during the reproductive cycle and in ovariectomized (OVX) rats is pulsatile and this pattern of secretion is determined by intermittent discharges of LH-releasing hormone (LHRH) into the hypophysial portal vessels. LHRH secretion is probably controlled by prior pulsatile norepinephrine (NE) release. The locus ceruleus (LC) is an important source of NE to the LHRH neurons. We have shown previously that LC lesions block the preovulatory LH surge and ovulation and also cause a decrease in plasma LH concentrations in OVX rats. The possible role of the LC in regulating pulsatile LH release has not been explored. Therefore, the aim of this work was to investigate, in OVX rats, the effects of LC lesions on pulsatile LH secretion. LC lesions were produced in adult female rats three weeks after OVX. On the next morning, the jugular vein was catheterized and, on the afternoon of the same day, blood samples (0.3 ml) were withdrawn every 5 min, during 90 min, from conscious freely moving rats. Plasma LH was measured by radioimmunoassay. LC lesions greatly suppressed pulsatile LH secretion by decreasing both LH pulse frequency and amplitude. The basal as well as total secretion of LH were also decreased. This inhibitory effect of the lesions was observed only when at least 50% of the nucleus was destroyed. Data from sham-operated animals as well as those with less than 50% destruction of the LC did not differ from those of the control rats without brain lesions. Since LC lesions induce a decrease in NE content in the preoptic area and median eminence, the inhibition of pulsatile LH release in ovariectomized rats with LC lesions occurs presumably as result of decreased pulsatile NE release into these areas of the brain that decreases both the frequency and the amount of LHRH released per pulse.