Norepinephrine Release In Hypothalamus Research Articles

Tyrosine hydroxylase and norepinephrine transporter mRNA levels increase in locus coeruleus after coitus in rabbits.

Hypothalamic norepinephrine (NE) plays an important role in the control of sexual behavior and in the secretion of gonadotropin. Our previous study showed that coitus induced simultaneous increases in hypothalamic NE and GnRH releases in female but not in male rabbits. To investigate the activities in noradrenergic neurons during the coitus-induced process of an LH surge, we measured tyrosine hydroxylase (TH, the rate-limiting enzyme in NE synthesis) and NE transporter (NET, a key protein for NE cellular reuptake) mRNA levels in locus coeruleus (LC) noradrenergic cells in female New Zealand White rabbits. Changes in LC-TH and LC-NET mRNA levels were also measured in males as controls. Female rabbits were killed before coitus and at 15, 30, 60, 120, and 240 min after coitus (n = 6-7/time point); males were killed before and at 30, 60, and 120 min after coitus (n = 3/time). Individual brainstems were sectioned, the LC neurons punched, and TH and NET mRNAs were quantified by ribonuclease protection assay (RPA). Rabbit-specific TH (330 bp) and NET (503 bp) cDNAs were used as probes in the RPA for gene-specific signals. A rabbit 'house-keeping' cDNA (cyclophilin, 158 bp) was also cloned and used as an internal marker for tissue RNA content. Trunk blood was collected to determine serum LH levels. In female rabbits, serum LH levels rose by 15 min after coitus, reached peak concentrations at 1-2 h, and declined thereafter. The time interval for changes in TH and NET mRNA levels in females was similar to that in serum LH levels. Both TH and NET mRNAs increased significantly by 15 min (73% and 85% respectively) and were elevated for 2 h (87% and 111% respectively). TH mRNA levels returned to basal levels by 4 h after coitus, whereas NET mRNA values were elevated throughout the 4 h of observation. In contrast, LH, TH and NET mRNA levels did not change after coitus in males. The enhanced gene expression of both TH and NET in the LC in females, in accord with our previous demonstration of increased hypothalamic NE release, suggests that regulation of NE synthesis and reuptake is an integral part of the coitus-induced NE/GnRH/LH surge process that includes the initiation, sustenance or recovery of the release and/or storage of these neurochemicals.

GABA augments basal and electrically stimulated 3H-norepinephrine release in hypothalamic, preoptic area and cortical slices of female rats

These studies examined the regulation by GABA of norepinephrine release from hypothalamus, preoptic area and frontal cortex. Using superfused brain slicesfrom female rats, we show that 100 μM GABA enhances both basal and electrically stimulated release of 3H-norepinephrine in all three brain regions. The GABA A agonist muscimol (100 μM) significantly augments 3H-norepinephrine release, but it is somewhat less effective than GABA. The GABA B agonist baclofen has little or no effect on basal 3H-norepinephrine efflux. GABA also augments both the magnitude and duration of electrically evoked 3H-norepinephrine release in slices from all three brain regions. GABA facilitation of electrically stimulated 3H-norepinephrine release is mediated through GABA A receptors as evidenced by its blockad by 10 μM bicuculline, a GABA A antagonist, but not by 200 μM 2-OH-saclofen, a GABA B antagonist. These data show that the inhibitory amino acid neurotransmitter GABA enhances both basal and evoked release of 3H-norepinephrine in brain slices from female rats. These effects are predominantly mediated by GABA A receptors. GABA modulation of hypothalamic norepinephrine release may play a role in the regulation of gonadotropin secretion and reproductive behaviors such as lordosis.

Centtral Interactions Between Noradrenaline and Neuropeptide Y in the Rat: Implications for Blood Pressure Control

Neuropeptide Y (NPY) and noradrenaline are co-localised in central neurones and both transmitters exert cardiovascular effects. Using microdialysis and push-pull techniques to measure transmitter release in vivo and microinjection studies, we examined the role(s) of central noradrenaline and NPY in blood pressure regulation in the hypothalamus and nucleus tractus solitarius (NTS) of the rat. Hypothalamic noradrenaline release was increased following haemorrhage and reduced after phenylephrine infusion. Ageing is associated with markedly reduced NPY concentrations in the hypothalamus. 18-month old animals showed a reduced ability to release both NPY and noradrenaline to a potassium depolarisation stimulus. NTS administration of NPY induced dose-dependent decreases in blood pressure and heart rate. The depressor but not the bradycardic response was attenuated by prior administration of yohimbine. NTS microinjection of 23 pmol NPY induced similar cardiovascular effects in spontaneously hypertensive and Wistar Kyoto rats. NPY and noradrenaline appear to interact at several sites in the brain known to be important for blood pressure control.

Sexual dimorphism in secretion of hypothalamic gonadotropin-releasing hormone and norepinephrine after coitus in rabbits.

Coital activation of the hypothalamo-hypophyseal ovarian axis (HOA) is well documented in rabbits, but coital excitation of the hypothalamo-hypophyseal testicular axis (HTA) is less well described. We and others have postulated that the response of the HOA to coitus, as characterized by a dramatic release of hypothalamic GnRH, may be mediated by an increase in norepinephrine (NE) neuronal activity. Herein, we studied selective HOA and HTA responses in New Zealand White rabbits before, during, and after coitus. Firstly, we determined the effects of microdialysis (mu D) and blood-sampling methods on plasma LH and testosterone (T) patterns in male rabbits during sexual performance. Subsequently, we compared the patterns of release in GnRH and norepinephrine (NE) from the arcuate nucleus-median eminence (AME) at 10-min intervals with changes in plasma LH levels in copulating male and female rabbits. Lastly, in 2.5-min AME mu D samples from females immediately after coitus, we measured NE and GnRH concentrations to determine whether NE release precedes that of GnRH. Tethered, freely moving rabbits were exposed to their partners for 10 min at the end of the third (10-min sampling for 5-7 h) or second (2.5-min sampling for 4 h) hour. Data from individuals that did not mate during the 10-min of pairing in the 3- to 7-h sampling trials were included as a control group (sham-mated). The results showed no changes (P > 0.05) in plasma LH and T in either mated (LH: pre, 0.13 +/- 0.08 ng/ml; post, 0.15 +/- 0.03 ng/ml; T: pre, 2.39 +/- 1.20 ng/ml; post, 0.85 +/- 0.26 ng/ml) or sham-mated males (LH: pre, 0.21 +/- 0.08; post, 0.25 +/- 0.10 ng/ml; T: pre, 1.46 +/- 0.51 ng/ml; post, 1.40 +/- 0.38 ng/ml). Likewise, coitus did not alter patterns of AME-NE (pre, 0.47 +/- 0.25; post, 0.56 +/- 0.25 ng/ml) and GnRH (pre, 0.61 +/- 0.45; post, 0.74 +/- 0.32 pg/ml) in mated or sham-mated males. The constant HTA activity during coitus in males appears to be independent of experimental manipulation per se because LH and T levels between mu D (0.18 +/- 0.05 and 1.72 +/- 0.85 ng/ml, respectively) and non-mu D (0.16 +/- 0.05 and 1.52 +/- 0.36 ng/ml, respectively) rabbits were not different (P > 0.05). In contrast to males, females displayed unambiguous and simultaneous increases in NE (P < 0.05) and GnRH (P < 0.01) release from the AME within 10-20 min after coitus; these elevated concentrations in mu D samples lasted for 3-4 h. Microdialysis NE levels averaged 0.02 +/- 0.01 ng/ml before mating, whereas postcoital values averaged 0.09 +/- 0.01 ng/ml. GnRH levels were 1.04 +/- 0.56 and 11.78 +/- 5.06 pg/ml before and after coitus, respectively. Concomitant increases in plasma LH levels were also observed after coitus in these female rabbits. Moreover, measurements of NE and GnRH in 2.5-min mu D samples revealed that the postcoital increase in NE preceded that in GnRH by 2.5-7.5 min (P < 0.05). The results suggest that neuroendocrine circuits in the two sexes of the New Zealand White rabbit respond differently to genital stimulation. In male rabbits, coitus does not alter patterns of AME NE or GnRH secretion, nor does it change the circulating levels of plasma LH or T. Conversely, in females, coitus induces the rapid release of hypothalamic NE, GnRH, and pituitary LH. The increase in coitally induced NE occurs before the rise in GnRH, which supports the hypothesis that NE is a critical neurochemical in coital activation of GnRH neurons.

S-63-2 - Bipolar mixed states

(−)-Nicotine activates the hypothalamic–pituitary–adrenal axis via an activation of the brainstem catecholaminergic neurons in rats. The present study was undertaken to clarify the mechanisms involved in the (−)-nicotine-induced activation of brainstem catecholaminergic neurons in anesthetized rats. Physostigmine (a cholinesterase inhibitor) (0.31 and 0.77 μmol/animal, i.p.) dose-dependently elevated plasma corticosterone in the presence of scopolamine (a muscarinic receptor antagonist) (2.3 μmol/animal, i.p.). (−)-Nicotine (250 and 500 nmol/animal, i.c.v.) dose-dependently elevated plasma corticosterone with concomitant noradrenaline release in the hypothalamic paraventricular nucleus. The (−)-nicotine (500 nmol/animal, i.c.v.)-induced elevation of corticosterone was abolished by phentolamine (an α-adrenoceptor antagonist) (0.66 μmol/animal, i.c.v.), and attenuated by (±)-sotalol (a β-adrenoceptor antagonist) (0.97 μmol/animal, i.c.v.). The (−)-nicotine-induced increases of plasma corticosterone and hypothalamic noradrenaline release were abolished either by hexamethonium (a nicotinic acetylcholine receptor antagonist) (1.8 μmol/animal, i.c.v.), CP-154,526 (butyl-ethyl-[2,5-dimethyl-7-(2,4,6-trimethylphenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-yl]amine) (a selective CRF-1 receptor antagonist) (1.3 μmol/animal, i.c.v.) or indomethacin (a cyclooxygenase inhibitor) (1.2 μmol/animal, i.c.v.). These results suggest that (−)-nicotine elevates plasma corticosterone by CRF-1 receptor- and prostaglandin-mediated noradrenaline release in the paraventricular nucleus in rats.

Effects of sevoflurane and halothane on the hypothalamic noradrenaline release in rat brain

Previous studies suggest that the posterior hypothalamus is closely related to arousal from volatile anaesthetics. This study was designed to investigate the release of noradrenaline from the posterior hypothalamus during emergence from sevoflurance anaesthesia, compared with halothane anaesthesia. Earlier clinical researches have shown more rapid emergence after sevoflurane than after halothane, when they are used in equivalent concentrations. Twenty four Wistar rats were used. A microdialysis probe was inserted in the posterior hypothalamus, using the diagram of paxinos, in each rat 24 h before general anaesthesia. Twenty minute period samples of 60 μL dialysate were collected into a high performance liquid chromatograph with electrochemical detector. The remaining ten rats were used to determine blood gas analysis during sevoflurane (five rats) and halothane (five rats) anaesthesia. Seven rats received 2.6% sevoflurane in air for 1 h and seven rats receive 1.0% halothane in air for the same drug in the microdialysis system. Noradrenaline released from the posterior hypothalamus as significantly increased in both groups of rats during the emergence from the anaesthesia. The magnitude of the increase was similar in both groups, however, a longer response was observed in the halothane group than in the sevoflurane group. No significant differences between anaesthetic groups were detected by blood gas analysis. We have found that the increased noradrenaline release persisted for longer following halothane anaesthesia than following sevoflurane anaesthesia indicating that the recovery time is shorter after sevoflurane anaesthsia than after halothane anaesthesia. These findings are consistent with clinical experience using these agents.

Effects of two volatile anesthetics (sevoflurane and halothane) on the hypothalamic noradrenaline release in rat brain

There are marked increases in noradrenaline (NA) release during emergence from general anesthesia induced with volatile anesthetics. These changes in NA in the posterior hypothalamus of the rat were assessed by intracranial microdialysis. Sevoflurane and halothane in equipotent concentrations were used to obtain the same depth of anesthesia. NA release increased similarly with the two agents during recovery. However, NA release remained elevated longer with halothane, from which recovery was also slower.

Peripheral administration of interleukin-1 increases extracellular concentrations of norepinephrine in rat hypothalamus: comparison with plasma corticosterone.

Peripheral administration of interleukin-1 (IL-1) activates the hypothalamo-pituitary-adrenocortical (HPA) axis and also activates norepinephrine (NE) metabolism in the hypothalamus. Because there is evidence that hypothalamic NE can activate CRF secretion, it has been proposed that changes in hypothalamic noradrenergic activity are instrumental in the IL-1-induced activation of the HPA axis. We have examined this hypothesis by assessing the release of hypothalamic NE using in vivo microdialysis following intravenous (IV) or intraperitoneal (IP) injection of human IL-1 beta and comparing the responses with those in plasma corticosterone. The results indicate that extracellular concentrations of NE in the hypothalamus increased following IV and IP administration of IL-1 beta. The elevation was more rapid following IV IL-1 beta, and reached a peak at 1 h, whereas the slower and smaller increase following IP IL-1 beta did not reach a peak until 2 h. The results were similar in both anesthetized and unanesthetized rats. In unanesthetized rats, plasma corticosterone increased shortly following IV or IP IL-1 beta administration, but the peak concentration following IV IL-1 beta was significantly earlier (around 1 h) than that following IP IL-1 beta (around 2 h). The hypothalamic noradrenergic responses followed a pattern similar to that of plasma corticosterone with either route of IL-1 beta administration, but they were not identical. The results are consistent with the possibility that a central noradrenergic mechanism mediates the activation of the HPA axis by peripherally administered IL-1 beta. However, appreciable differences in the time courses of the responses may indicate the involvement of other factors.

Noradrenaline release in hypothalamus and ACTH secretion induced by central interleukin-1beta.

The effect of interleukin-1beta (IL-1beta, 10 ng per 5 mu l i.c.v.) on noradrenaline release in the hypothalamus of freely moving rats and on plasma ACTH levels has been studied, respectively by intracerebral microdialysis and radioimmunoassay. IL-1beta induced an early increase in noradrenaline (NA) release in the paraventricular nucleus (PVN) followed by a second long-lasting enhancement starting from 80 min, with a maximum at 140 min (187 +/- 14%). Heat-denatured IL-1beta induced only the early increase without any other change at later times. IL-1beta did not significantly affect NA release in the area surrounding the PVN at any time. IL-1beta increased ACTH levels at 2 and 3 h. Indomethacin (10 mg kg(-1), i.p.) prevented the noradrenergic and ACTH responses. The NA system in the PVN plays a part in the mechanism controlling the ACTH response induced by IL-1.

Oxytocin does not act locally in the hypothalamus to elicit norepinephrine release

An in vitro brain slice preparation was used to determine whether facilitatory effects of systemically administered oxytocin on hypothalamic norepinephrine release result from direct action of the neuropeptide on noradrenergic terminals. When oxytocin was superfused onto hypothalamic slices from ovariectomized control and ovariectomized, hormone-treated female rats, the neuropeptide failed to influence either basal or electrically evoked release of 3H-norepinephrine. Thus, it is likely that oxytocin facilitates hypothalamic norepinephrine release by a peripheral mechanism, perhaps vaginocervical contraction.

Posterior Hypothalamic Noradrenaline Release During Emergence from Sevoflurane Anesthesia in Rats

Posterior Hypothalamic Noradrenaline Release During Emergence from Sevoflurane Anesthesia in Rats

Posterior hypothalamic noradrenaline release during emergence from sevoflurane anesthesia in rats.

Posterior hypothalamic noradrenaline release during emergence from sevoflurane anesthesia in rats.

Synergistic effects of Bay K 8644 and bradykinin on norepinephrine release in the hypothalamus of spontaneously hypertensive rats.

1. In the present study, we examined the effects of Bay K 8644, a dihydropyridine (DHP)-sensitive Ca2+ channel agonist, and bradykinin on norepinephrine release in the hypothalamus of spontaneously hypertensive rats (SHR). 2. In the preliminary studies using Sprague-Dawley rats, Bay K 8644 by itself had no significant effects on the stimulation-evoked [3H]-norepinephrine release from hypothalamic slices. Bradykinin increased the stimulation-evoked [3H]-norepinephrine release in a dose-related fashion. The facilitatory effects of bradykinin on norepinephrine release were potentiated by Bay K 8644. 3. In SHR, Bay K 8644 significantly increased the stimulation-evoked norepinephrine release from hypothalamic slices. However, exposure of slices to Bay K 8644 caused no significant effects on norepinephrine release in Wistar-Kyoto (WKY) rats. The effects of Bay K 8644 in combination with bradykinin on the stimulation-evoked norepinephrine release were also greater in SHR than in WKY rats. 4. These results demonstrate that Bay K 8644 significantly potentiated the facilitatory effects of bradykinin on norepinephrine release in rat hypothalamus. The finding indicates a possible interaction of bradykinin with DHP-sensitive Ca2+ channels in the central nervous system. Furthermore, the pronounced effects of Bay K 8644 and bradykinin in SHR suggest that bradykinin-related Ca2+ channels might have a role in the regulation of norepinephrine release in the hypothalamus of SHR.

Maternal hormonal manipulations in rats cause obesity and increase medial hypothalamic norepinephrine release in male offspring

In previous work it has been shown that adult male, but not female, offspring of rats that have either been injected with Prolamine Zinc Insulin on days 15–20 of gestation, or undernourished during the first 2 weeks of gestation, develop significant obesity commencing at about 50 days of age. The present experiment examines the question of whether rats with these two forms of obesity display neurochemical abnormalities in areas of the brain known to influence food intake and body weight. Twenty-one gauge stainless steel guide shafts were surgically implanted using standard stereotaxic procedures. One week later 26 ga microdialysis probes were lowered into the medial hypothalamus. Dialysates collected from male offspring in the two experimental conditions contain significantly higher norepinephrine (NE) levels than did controls. It would appear that in addition to sharing a similar time course of onset and a sex dependent expression of obesity, both of these models are also characterized by elevated medial hypothalamic NE. Since this obesity appears only in males, and at a time when testosterone levels are rapidly rising in males, and since testosterone has been shown to elevate food intake and body weight in rats, we also investigated whether gonadal weights or circulating testosterone levels were differentially elevated by our manipulations.

Effect of development of habituation to restraint stress on hypothalamic noradrenaline release and adrenocorticotropin secretion.

The effect of repeated stress has been studied on noradrenaline release in the hypothalamic paraventricular nucleus and on adrenocorticotropin levels. Rats were stressed by 20-min immobilization once a day for 5 days. On day 6 they were exposed to the same stress or to a different one (ether vapors for 2 min). Immobilization and ether stress increased noradrenaline release in naive rats (271 +/- 43 and 197 +/- 9%, respectively) and raised adrenocorticotropin levels, showing activation of the hypothalamus-pituitary axis. Repeated daily restraint did not modify basal noradrenaline or adrenocorticotropin levels. The further immobilization session on day 6 did not change noradrenaline levels at any observation time (20-120 min). The adrenocorticotropin response was still present, although significantly reduced. In repeatedly restrained rats, exposure to ether vapors induced a maximal increase in noradrenaline level similar to that observed in naive rats, although prolonged. In these rats the adrenocorticotropin response did not differ from that in acutely stressed rats. These results suggest that habituation may develop to a stressful stimulus leading to suppression of the hypothalamic noradrenergic response and that this phenomenon is stress specific. Moreover, modifications of noradrenaline release in the paraventricular nucleus are not solely responsible for the adrenocorticotropin response during stress, suggesting that other pathways and/or neurotransmitters are involved too.

Basal and stress-induced release of noradrenaline in hypothalamus of spontaneously hypertensive rats at different ages

Basal and stress-induced noradrenaline (NA) release was studied by intracerebral microdialysis in the hypothalamic paraventricular nucleus of spontaneously hypertensive rats (SHR) at different ages (9 weeks, 6, 18 and 24 months). NA was measured in 20-min dialysate samples by high performance liquid chromatography with electrochemical detection. Microdialysis sampling was done at baseline, during a 20-min immobilization stress and for the next 100 min. Basal NA levels decreased with age, showing a highly significant correlation. Immobilization stress raised NA similarly in the four age groups (respectively 281%, 235%, 243%, 251% of baseline at 9 weeks, 6, 18, 24 months), indicating that the response to stress is maintained at all these ages and is not affected by the development of hypertension or by aging.

Characteristics of voltage-dependent calcium uptake and norepinephrine release in hypothalamus of SHR

The characteristics of voltage-dependent45Calcium uptake and norepinephrine release as factors controlling neural activities in the hypothalamus which is an important regulatory site for cardiovascular function were studied. Two groups of animals: male spontaneously hypertensive rat (SHR) and age-matched normotensive wistar rat (NW) were used in this study. Animals at 4, 6 and 16 weeks of age were sacrificed by decapitation and the hypothalamus was dissected out. Voltage-dependent calcium uptake and norepinephrine release were determined from hypothalamic synaptosomes either in low potassium (5 mM) or high potassium (41 mM) stimulatory conditions by using45Ca isotope and HPLC-ECD techniques. Degrees of voltage-dependent45calcium uptake and norepinephrine release evoked by calcium uptake in the hypothalamus of prehypertensive phase (4 weeks old) of SHR were significantly smaller than those in NW of the same age. However, in the developmental phase (6 weeks old) and the established phase (16 weeks old) of hypertension in SHR, degrees of voltage-dependent45calcium uptake and norepinephrine release were similar to those of age-matched normotensive wistar rats. These data imply that the deficit in hypothalamic norepinephrine release might be an important underlying factor for the development of hypertension in SHR.

Effects of bradykinin on [3H]-norepinephrine release in rat hypothalamus.

1. We examined the regulatory actions of bradykinin on norepinephrine release in the hypothalamus of rats. 2. Bradykinin increased the stimulation-evoked [3H]-norepinephrine release from hypothalamic slices of Sprague-Dawley rats in a dose-dependent manner (1 Hz: S2/S1 ratio, mean +/- s.e.m., control 0.868 +/- 0.016, n = 6; bradykinin 1 x 10(-6) mol/L 1.039 +/- 0.018, n = 6, P < 0.05; bradykinin 3.3 x 10(-6) mol/L 1.130 +/- 0.064, n = 6, P < 0.05). The basal release of [3H]-norepinephrine was not affected by the peptide. 3. Bay K 8644, a dihydropyridine-sensitive calcium channel agonist, significantly potentiated the facilitatory effect of bradykinin on norepinephrine release, although Bay K 8644 by itself had no significant effect. By contrast, nicardipine, a dihydropyridine-sensitive calcium channel blocker, reversed the increase in norepinephrine release induced by bradykinin and Bay K 8644. 4. These results indicate that bradykinin may increase norepinephrine release in rat hypothalamus, partially mediated by interactions with dihydropyridine-sensitive calcium channels.

Cardiovascular change and hypothalamic norepinephrine release in response to environmental stress.

The major objective of this study was to compare the magnitude and duration of cardiovascular (CV) responses to acute environmental stresses with the associated patterns of noradrenergic activity in the paraventricular nucleus (PVN) and posterior nucleus (PH) of the hypothalamus. Simultaneous microdialysis samples of extracellular norepinephrine (NE) were collected at 5-min intervals from PVN and PH and the CV responses were recorded before, during, and for 15 min after acute shaker (cage oscillation) stress or inhalation of ether vapor in freely moving rats. Five minutes of shaker stress, 60 and 150 cycles/min, elicited pressor responses coupled with increases in dialysate NE from both PVN and PH in a frequency-dependent manner. Tachycardia occurred at 150 but not 60 cycles/min. Ten minutes after 60 cycles/min and 15 min after 150 cycles/min, NE efflux in PH was still increased, whereas in PVN it returned to control as had arterial pressure and heart rate. Ether vapor elicited a transient CV response but a continuing efflux of NE in PH and PVN. Urethan anesthesia raised baseline values of dialysate NE in PH and PVN but significantly attenuated cardiovascular and dialysate NE responses to shaker stress. We conclude that acute environmental stress simultaneously elicits CV responses and the efflux of NE from PVN and PH but, during or after stress, CV values may return to control levels while NE efflux remains elevated in PVN and/or PH.

Tyrosine Hydroxylase Messenger Ribonucleic Acid Levels Increase in A1 but not Locus Ceruleus Noradrenergic Neurons in Proestrous Rats but not in Diestrous or Androgen‐Sterilized Animals

Norepinephrine (NE) turnovers (an index of secretion) increase in the hypothalamus of proestrous rats concomitant with luteinizing hormone surges, whereas, neither of these events are observed in diestrous nor in androgen-sterilized rats. Increased hypothalamic NE release may occur as a consequence of the withdrawal of local inhibitory γ-aminobutyric acid and opiate controls on specific presynaptic NE terminals and/or as a result of an increase in activity within noradrenergic neurons. Tyrosine hydroxylase (TH) is the rate-limiting enzyme for the synthesis of NE and our earlier studies revealed that increases in TH mRNA in A1 and locus ceruleus (LC) neurons can serve as an index of increased activity within these cells. In the present study, we evaluated whether TH message levels change in A1 and LC neurons prior to and during the hours when luteinizing hormone surges and increased NE turnovers are observed. As controls, TH mRNA levels in A1 and LC neurons were evaluated at the same hours of day in diestrous day 2 and in androgen-sterilized rats. In situ hybridization histochemistry and quantitative image analysis methods were used to measure changes in TH mRNA levels. Luteinizing hormone surges in proestrous rats began at 1500 h, peaked between 1600 and 1700 h and declined, thereafter, to 2000 h. In contrast, plasma luteinizing hormone remained basal throughout the day in diestrous and androgen-sterilized rats. While A1 neuronal TH mRNA levels did not differ in the three groups of rats during the morning (0930 to 1030 h), these message levels were significantly elevated in proestrous rats during the afternoon (1645 to 1715 h) and remained high at 2000 to 2030 h. In contrast, no changes in TH mRNA levels were observed in A1 neurons throughout the afternoon in diestrous animals or androgen-sterilized rats. TH mRNA levels in the LC did not differ in the three groups of rats and they remained unchanged throughout the afternoon hours we examined. From these observations we conclude that concomitant with afternoon proestrous luteinizing hormone surges and the accompanying increase in hypothalamic NE secretion, there is an increase in activity within A1 but not LC neurons. These data suggest that the proestrous increase in hypothalamic NE turnover we previously observed is not due solely to withdrawal of local inhibitory controls of presynaptic NE release but it also involves an increase in activity within A1 but not LC neurons.