Catecholamine Synthesis Inhibitor Research Articles

Involvement of adrenaline in diazepam-induced hyperglycemia in mice

Diazepam-elicited hyperglycemia in mice was inhibited by adrenalectomy and the catecholamine synthesis inhibitor α-methyl-p-tyrosine, although it was unaffected by pretreatment with the corticosterone synthesis inhibitor dexamethasone. Diazepam-induced hyperglycemia was prevented by the α 2 adrenoceptor antagonist idazoxan, while the β 1 and β 2 adrenoceptor antagonists, metoprolol and ICI 118 551, did not affect it. Furthermore, diazepam increased plasma adrenaline levels in mice. These results suggest that diazepam-induced hyperglycemia is closely related to adrenaline release from the adrenal gland.

Effects of inhibitors of the synthesis of catecholamines and spiperone on sea urchin and murine embryos

We studied the effects of three inhibitors of catecholamine synthesis on the development of sea urchinsSphaerechinus granularis andParacentrotus lividus. These drugs affected the early embryogenesis, which was expressed in inhibition of the cleavage divisions, appearance of abnormal embryos, and developmental arrest. The addition of arachidonic acid amide and dopamine to the incubation medium weakened the effects of the inhibitors. Spiperone induced developmental defects in preimplantation mouse embryos and sea urchin embryos. Arachidonic acid amide with dopamine exerted a protective effect against spiperone when introduced to sea urchin embryos at the blastula or late gastrula stages, rather than after fertilization. In murine embryos, this amide induced developmental defects and arrest itself, and its effect was reversible. Possible mechanisms underlying the effects of these drugs are discussed.

A study of the activity of enzymes of the antioxidant system in ontogenesis ofArabidopsis thaliana mutants tolerant to norflurazone

We studied the effects of three inhibitors of catecholamine synthesis on the development of sea urchins Sphaerechinus granularis and Paracentrotus lividus. These drugs affected the early embryogenesis, which was expressed in inhibition of the cleavage divisions, appearance of abnormal embryos, and developmental arrest. The addition of arachidonic acid amide and dopamine to the incubation medium weakened the effects of the inhibitors. Spiperone induced developmental defects in preimplantation mouse embryos and sea urchin embryos. Arachidonic acid amide with dopamine exerted a protective effect against spiperone when introduced to sea urchin embryos at the blastula or late gastrula stages, rather than after fertilization. In murine embryos, this amide induced developmental defects and arrest itself and its effect was reversible. Possible mechanisms underlying the effects of these drugs are discussed.

Effects of red ginseng saponins and nootropic drugs on impaired acquisition of ethanol-treated rats in passive avoidance performance

Effects of single and repeated administration of red ginseng total saponins (ROTS) and nootropic drugs were examined on impairment of acquisition induced by single oral administration of 3 g/kg ethanol (EtOH) in a step through test. The inhibitory effect of EtOH on acquisition was significantly reduced following single or repeated RGTS administration. The nootropic drugs, piracetam and N-methyl- d-glucamine, given orally significantly reduced impairment of acquisition induced by EtOH. On the other hand, the inhibitory effect of repeated RGTS on the EtOH-induced amnesia was blocked by the pretreatment of α-methyl-ρ-tyrosine (α-MT), an inhibitor of catecholamine synthesis, in a dose-dependent manner but not ρ-chlorophenylalanine (PCPA), an inhibitor of serotonin synthesis, whereas the inhibitory effect of repeated N-methyl- d-glucamine on the EtOH-induced amnesia was blocked neither by α-MT nor PCPA. These results suggest that repeated RGTS and N-methyl- d-glucamine ameliorate the impairing effect of EtOH on acquisition, and the effect of RGTS on EtOH-induced amnesia is dependent on the catecholaminergic but not serotonergic neuronal activity, while RGTS and N-methyl- d-glucamine seem to have a different mechanism on EtOH-induced amnesia.

Sympathetic inhibition, leptin, and uncoupling protein subtype expression in normal fasting rats.

To further investigate neural effects on leptin and uncoupling proteins (UCPs), we studied in vivo perturbations intended to block adrenergic input to peripheral tissues. We examined plasma leptin, leptin mRNA, and adipose and muscle UCP subtype mRNA in rats treated with alpha-methyl-p-tyrosine methyl ester (AMPT-ME), which inhibits catecholamine synthesis and 6-hydroxydopamine (6HDA), which is toxic to catecholinergic nerve terminals but, unlike AMPT-ME, does not enter the central nervous system. Intraperitoneal AMPT-ME, 250 mg/kg, was administered at 1800 and 0700 the following day, and rats were killed at 1200-1400. All rats were fasted with free access to water during this time. Intraperitoneal AMPT-ME increased plasma leptin by 15-fold, increased interscapular brown adipose tissue (IBAT) and epididymal fat leptin mRNA by 2- to 2.5-fold, and also increased plasma insulin and glucose concentrations. Intraperitoneal AMPT-ME decreased IBAT UCP-3 mRNA to 40% of control, while it increased epididymal adipose UCP-3 mRNA approximately twofold. Intravenous AMPT-ME, 250 mg/kg, administered to conscious rats for 5 h decreased lumbar sympathetic nerve activity, increased plasma leptin (5.89 +/- 1.43 compared with 2.75 +/- 0.31 ng/ml in vehicle-treated rats, n = 7, P < 0.05), and decreased cardiac rate with no sustained change in blood pressure. Intraperitoneal 6HDA, 100 mg/kg, as a single dose at 1800, increased plasma leptin approximately twofold after 18-20 h, increased IBAT (but not epididymal fat) leptin mRNA by two- to threefold, and decreased IBAT UCP-3 mRNA to 30-40% of control. Neither AMPT-ME nor 6HDA significantly altered mRNA encoding gastrocnemius muscle UCP-3, IBAT UCP-1, or IBAT and epididymal UCP-2. In summary, AMPT-ME and 6HDA increased plasma leptin and upregulated leptin mRNA expression. AMPT-ME also resulted in complex tissue and subtype-specific modulation of adipose UCP mRNA. These data are consistent with interaction between leptin and sympathetic nerve activity (SNA) in regulation of fat cell energy utilization. However, the in vivo modulation of leptin and UCPs appears complex and, beyond a causal effect of SNA per se, may depend on concurrent changes in plasma insulin, glucose, and circulatory hemodynamics.

The effects of near-ultraviolet light on serotonin N-acetyltransferase activity in the chick pineal gland.

Effects of near-ultraviolet light (UV-A; 325-390 nm, peak at 365 nm) on the activity of the pineal serotonin N-acetyltransferase (NAT; a key regulatory enzyme in melatonin biosynthesis) were examined in chicks. Acute exposure of dark-adapted animals to UV-A radiation produced a marked decline in NAT activity of the pineal gland. The magnitude of this suppression was dependent upon duration of the light pulse and the age of the animals. The decrease in the nighttime NAT activity evoked by a 5 min pulse of UV-A light applied during the fourth hour of the dark phase of the 12 hr light:12 hr dark cycle (LD) gradually deepened during the first 40 min after the return of animals to constant darkness, then the enzyme activity began to rise, reaching control values by 2 hr. Exposure of chicks to a 5 min pulse of UV-A light during the ninth hour of the dark phase produced a marked decline in pineal NAT activity, which was reversible after 15 min of darkness. Pretreatment of animals with an inhibitor of catecholamine synthesis, alpha-methyl-p-tyrosine (300 mg/kg, i.p.), or with a blocker of alpha2-adrenergic receptors, yohimbine (2 mg/kg, i.p.), antagonized the suppressive effect of UV-A light on nighttime NAT activity of the chick pineal gland. It is concluded that UV-A irradiation, similar to visible light, potently suppresses melatonin biosynthesis in the chick pineal gland, with an alpha2-noradrenergic signal playing the role of an intermediate in this action.

High and low responders to novelty: effects of a catecholamine synthesis inhibitor on novelty-induced changes in behaviour and release of accumbal dopamine

The purpose of the present study was two-fold: (i) investigating to what extent novelty, i.e. a novel cage with slightly larger dimensions than the home cage and lacking the floor covering that was originally present, produced behavioural effects in high responders to novelty and low responders to novelty that could be correlated with the extracellular amount of accumbal dopamine, using the microdialysis technique, and (ii) establishing the ability of the catecholamine synthesis inhibitor α-methyl- p-tyrosine to inhibit the novelty-induced responses in high and low responders. The difference in the behavioural response to novelty between the high and low responders was limited to walking, which lasted significantly longer in high responders than in low responders. The novelty-induced increase in extracellular concentration of accumbal dopamine was significantly greater in high responders than in low responders; moreover, the shape of the growth curves differed between high and low responders. The behavioural changes did not correlate with the neurochemical effects, which outlasted the duration of the novelty-induced behavioural arousal. It is hypothesized that this long-lasting increase in accumbal dopamine produces “adaptive changes” that help and/or allow the animal to incorporate knowledge about the condition that it experienced. When the nucleus accumbens was perfused with α-methyl- p-tyrosine for a period of 40 min, given at the same time as the transfer of the rat to the novel cage, it reduced the novelty-induced increase in walking in the high responders, but did not alter the novelty-induced behaviour of low responders. Finally, α-methyl- p-tyrosine reduced the novelty-induced increase in the release of accumbal dopamine in high responders, but enhanced it in low responders. The present neurochemical data are discussed in view of the outcome of earlier reported pharmaco-behavioural studies on the neurochemical state of the nucleus accumbens of non-challenged versus challenged high and low responders. It is hypothesized that, in the high responder, exposure to novelty enhances the release of accumbal dopamine from reserpine-resistant, α-methyl- p-tyrosine-sensitive pools that are under the stimulatory control of β-adrenergic receptors in the nucleus accumbens, and that, in the low responder, exposure to novelty enhances the release of accumbal dopamine from reserpine-sensitive, α-methyl- p-tyrosine-resistant pools that are under the inhibitory control of α-adrenergic receptors in the nucleus accumbens.

The role of catecholamines in desmopressin induced locomotor stimulation.

Central and peripheral administration of DDAVP increase locomotor activity in rats in doses that alter brain dopamine neurochemistry. In order to delineate the role of catecholamines in this behavioural effect of DDAVP, the effects of different catecholamine manipulating agents on DDAVP-induced locomotor stimulation were studied in rats. The catecholamine depleting agent reserpine (5 mg/kg), administered alone or together with the catecholamine synthesis inhibitor alpha-methyltyrosine (250 mg/kg), completely prevented the locomotor stimulatory effect of DDAVP. The dopamine D1 receptor antagonist Sch-23390 (0.01 and 0.03 mg/kg) significantly antagonized the DDAVP-induced locomotor stimulation when administered in the higher dose, that also produced a significant reduction of locomotor activity per se, whereas the dopamine D2 receptor antagonist raclopride (0.08 and 0.16 mg/kg) had no significant effect. The two dopamine blockers administered together produced a significant, dose-dependent reduction of DDAVP-induced locomotor stimulation, while controls were not significantly affected. Also the alpha-adrenoceptor antagonist phenoxybenzamine decreased the DDAVP-induced locomotor stimulation in a dose (20 mg/kg) that did not influence locomotor activity in controls, and, finally, administration of Sch-23390, raclopride and phenoxybenzamine antagonised the DDAVP-induced effect in a dose combination that failed to influence locomotor activity per se. In vivo microdialysis experiments in awake, freely moving rats indicated that DDAVP increases dopamine overflow in the nucleus accumbens, a brain area of importance for initiation of locomotor activity, by approximately 25%, as compared to baseline levels. Taken together, these results indicate that the central stimulatory action of DDAVP involves granula-mediated dopamine release and subsequent activation of dopamine D1 and D2 receptors, and that alpha-adrenoceptors possibly also are involved.

Neuropeptide Y inhibition of calcium channels in PC-12 pheochromocytoma cells.

We previously demonstrated, using rat PC-12 pheochromocytoma cells differentiated to a sympathetic neuronal phenotype with nerve growth factor (NGF), that neuropeptide Y (NPY) inhibits catecholamine synthesis as well as release. Inquiry into the mechanisms of these inhibitions implicated distinct pathways involving reduction of Ca2+ influx through voltage-activated Ca2+ channels. In the present investigation the effects of NPY on whole cell Ba2+ currents were examined to obtain direct evidence supporting the mechanisms suggested by those studies. NPY was found to inhibit the voltage-activated Ba2+ current in NGF-differentiated PC-12 cells in a reversible fashion with an EC50 of 13 nM. This inhibition was pertussis toxin sensitive and resulted from NPY modulation of L- and N-type Ca2+ channels. The inhibition of L-type channels was not seen with < 1 nM free intracellular Ca2+ or when protein kinase C (PKC) was inhibited by chelerythrine or PKC-(19-31). Furthermore, the effect of NPY on L-type channels was mimicked by the PKC activator phorbol 12-myristate 13-acetate. These studies demonstrate that, in addition to inhibition of N-type Ca2+ channels, in NGF-differentiated PC-12 cells NPY inhibits L-type Ca2+ channels via an intracellular Ca(2+)- and PKC-dependent pathway.

Release of Pituitary Gonadotrophins GtH I and GtH II in the Rainbow Trout (Oncorhynchus mykiss): Modulation by Estradiol and Catecholamines

The present study focused on the role of catecholaminergic neurons and estrogens on the release of gonadotropins I and II in immature and early vitellogenic female rainbow trout. The ovariectomy-induced increase of GtH I blood levels (from about 10 to 15 ng/ml) was prevented in vitellogenic fish by E2 supplementation. E2 implantation of immature fish decreased blood GtH I levels (from about 6 to 1 ng/ml). Blood levels of GtH II were low (about 0.5 ng/ml) and not altered by ovariectomy and E2 treatment. These data demonstrate that estrogens exert a negative feedback on the release of GtH I in trout. A treatment with α-methyl-p-tyrosine (MPT), an inhibitor of catecholamine synthesis, increased blood GtH II levels of sham-operated vitellogenic fish and ovariectomized fish implanted with E2, but had no effects in ovariectomized fish. MPT did not modify blood GtH I levels in any experimental group. A treatment of E2-implanted immature or vitellogenic fish with the dopamine antagonist pimozide also increased blood GtH II levels, but did not significantly change blood GtH I levels. These data demonstrate that release of GtH II, but not of GtH I, depends on an E2-activated DA inhibitory tone.

O-69 - Effects of a catecholamine synthesis inhibitor on novelty- induced changes in behaviour and accumbal dopamine release in high and low responders to novelty

O-69 - Effects of a catecholamine synthesis inhibitor on novelty- induced changes in behaviour and accumbal dopamine release in high and low responders to novelty

Catecholamines and dihydroxyphenylalanine in metamorphosing larvae of the nudibranch Phestilla sibogae Bergh (Gastropoda: Opisthobranchia).

The content of catecholamines and dihydroxyphenylalanine in larvae of the nudibranch Phestilla sibogae was analyzed by high-performance liquid chromatography with electrochemical detection. Dihydroxyphenylalanine, norepinephrine and dopamine were identified in larvae of all ages examined (5 through 12 days post-fertilization). Dihydroxyphenylalanine could be accurately quantified only in larvae of ages 8 through 12 days, when its average concentration increased from 0.62 to 6.71 x 10(-2) pmol micrograms protein-1. Between ages 5 and 12 days dopamine rose from 0.081 to 0.616 pmol microgram protein-1, and norepinephrine from 0.45 to 2.17 x 10(-2) pmol micrograms protein-1. Dihydroxyphenylalanine, dopamine and norepinephrine were also measured at different stages of metamorphic progress in 10- to 12-day larvae. Dihydroxyphenylalanine increased by a factor of 2.4 between the onset and completion of metamorphosis, but levels of dopamine and norepinephrine remained stable. One millimolar alpha-methyl-DL-m-tyrosine, an inhibitor of catecholamine synthesis, inhibited natural metamorphosis and depleted endogenous norepinephrine and especially dopamine, respectively, to 75% and 35% of control values. The existence of unexpectedly high levels of catecholamines in metamorphically competent larvae, and the association of catecholamine depletion with inhibition of metamorphosis, indicate that these compounds may participate in the control of gastropod development.

Blockade of ovulation in the rat by systemic and ovarian intrabursal administration of the fungicide sodium dimethyldithiocarbamate

Dithiocarbamates, acting as inhibitors of catecholamine synthesis, have been reported to block ovulation in female rats following systemic administration by suppressing the neural noradrenergic signaling involved in triggering the ovulatory surge of luteinizing hormone. The ovaries also synthesize norepinephrine and receive noradrenergic input via sympathetic innervation, and it has been suggested that such input may play a role in follicular maturation and ovulation. The current experiments investigated whether the dithiocarbamate fungicide dimethyldithiocarbamate (DMDTC) would block oocyte release in normally cycling rats when administered systemically during the proestrous presurge period, and if so, would the compound also have a comparable direct ovarian effect on ovulation in response to a local intrabursal exposure of one ovary late on the day of vaginal proestrus. The results showed that a dose-related suppression of oocyte release was present in response to both intraperitoneal and intrabursal (IB) injections. But these effects appear to be mediated through different mechanisms. The unilateral IB injections were effective only on the exposed side for each ovarian pair, while no alterations were seen in ovarian norepinephrine. IB administration 24 h earlier blocked ovulation on both sides, while hCG injections were able to restore ovulation on the noninjected side only, implying that diestrous DMDTC was inhibiting the LH surge. The data indicate that while an effect on hypothalamic catecholamine synthesis may underlie the ovulatory blockade following intraperitoneal DMDTC administration, it does not appear to be involved in the response to local ovarian exposure. Moreover, the blockade in response to the diestrous IB exposure likely involves two separate mechanisms, one attributable to an alteration in ovarian hormonal feedback to the brain (or pituitary), inhibiting the LH surge, and the other associated with a direct, as yet undetermined, effect on local preovulatory events within the ovary.

Metyrosine and Pheochromocytoma

Severe hemodynamic instability may occur during surgery for removal of pheochromocytoma, unless there is preoperative pharmacological treatment. To evaluate the effects of metyrosine (alpha-methyl-p-tyrosine), a catecholamine synthesis inhibitor, and alpha-blockade with prazosin or phenoxybenzamine on cardiovascular morbidity during surgery for pheochromocytoma. A retrospective analysis was made of patients followed up at the Medical College of Georgia, Augusta, during 28 years who received metyrosine and prazosin (n = 6), metyrosine and phenoxybenzamine alone (n = 14), phenoxybenzamine alone (n = 6), or no medication (n = 7) during 3 weeks before tumor removal. The percentage of patients not requiring pressors or phentolamine during the intraoperative period as well as the perioperative peak systolic pressures and peak heart rates were estimated in each group. There was a significant (P < .05) increase in intraoperative peak systolic pressures without preoperative treatment (mean +/- SD, 243 +/- 40 mm Hg) vs metyrosine (mean +/- SD, 168 +/- 27 mm Hg). Ninety-five percent of patients who received metyrosine did not require pressors intraoperatively vs 50% with phenoxybenzamine alone. Eighty-one percent of patients pretreated with metyrosine did not require intraoperative phentolamine vs 33% with phenoxybenzamine alone and 29% without medications. Two patients in the no medication group died as a results of hypertensive crisis. The combination of alpha-metyrosine and alpha-blockade results in better blood pressure control and less need for use of antihypertensive medication or pressors during surgery, compared with the classical method of single-agent adrenergic blockade. Preoperative treatment with metyrosine along with an alpha-blocker is a useful strategy for decreasing the surgical morbidity in patients with pheochromocytoma and assumes greater importance as long as the availability of phentolamine for intraoperative use is a problem.

Pheochromocytoma

To characterize the symptoms associated with pheochromocytoma and discuss the diagnosis and management of this tumor. We review the clinical manifestations in patients with pheochromocytoma, the biochemical and imaging studies recommended for diagnosis and localization of the tumor, and the available strategies for treatment. Pheochromocytoma is a tumor of chromaffin cells that originates in either the adrenal medulla or the extra-adrenal sympathetic tissues. It is usually unilateral and benign. Frequent initial symptoms include headache, sweating, and palpitations, with or without increased blood pressure. In many patients, hypertension is accelerated during a paroxysm. Pheochromocytoma may also occur as a part of multiple endocrine neoplasia type IIA and B. Several common syndromes, such as panic disorders and hyperthyroidism, may mimic pheochromocytoma; however, pheochromocytoma should be suspected in the presence of hypertension, tachycardia, and throbbing headache, especially occurring as paroxysmal episodes. The physiologic diagnosis of pheochromocytoma is established by biochemical tests of levels of plasma and urinary catecholamines or their metabolites (or both). In most patients, anatomic localization is achieved with computed tomography or magnetic resonance imaging, metaiodobenzylguanidine scintigraphy, labeled somatostatin scans, or positron emission tomography. The management preferentially includes surgical removal of the pheochromocytoma after preparation with appropriate medical therapy to avoid hypertensive crisis during the perioperative period. Patients with contraindications to a major surgical procedure or with malignant pheochromocytoma and metastatic disease, however, may be treated with multiple drugs--for example, alpha- and beta-adrenergic blockers and direct vasodilators to neutralize the effects of high levels of circulating catecholamines and alpha-methyl-metatyrosine to inhibit catecholamine synthesis. The presence of suggestive clinical features in patients with hypertension should prompt clinicians to undertake appropriate diagnostic testing because surgical resection of a pheochromocytoma will yield a cure in many cases.

Impaired distribution of α-methyl-l-p-tyrosine in diabetic rats

Although α-methyl-l-p-tyrosine (α-MPT), an inhibitor of catecholamine synthesis, has been used to study catecholamine turnover in diabetic animals, effects of diabetes on metabolism of the drug have not been investigated. In this study, administration of a standard dose of α-MPT (250 mg/kg initially and 125 mg/kg at 2 h intervals) resulted in lower plasma and tissue levels of α-MPT and its metabolites in streptozocin-diabetic rats than in controls. Two to six hours after the initial dose of α-MPT, concentrations of α-MPT were 2–8-fold lower in the hypothalamus, medulla/pons, and plasma of diabetic animals than in controls. Brain and plasma levels of the α-MPT metabolite, α-methyl DOPA (α-MD) were 2–10-fold lower in tissues of diabetic animals. Levels of the α-MPT metabolite α-methyl norepinephrine (α-MNE), measured only in the hypothalamus, were 4-fold lower in diabetic rats than in controls. There were no differences in the ratio of free/conjugated α-MPT in plasma. Treatment of diabetic rats with insulin restored α-MPT and α-MD to control levels. These findings indicate that i.p. administration of α-MPT does not result in equivalent levels of the drug in diabetic and control rats and suggest caution in the use of α-MPT to compare catecholamine turnover in diabetic and healthy animals.

Mechanism of catecholamine synthesis inhibition by neuropeptide Y: role of Ca2+ channels and protein kinases.

We have previously demonstrated that neuropeptide Y (NPY) inhibits depolarization-stimulated catecholamine synthesis in rat pheochromocytoma (PC12) cells differentiated to a sympathetic neuronal phenotype with nerve growth factor (NGF). The present study uses multiple selective Ca2+ channel and protein kinase agonists and antagonists to elucidate the mechanisms by which NPY modulates catecholamine synthesis as determined by in situ measurement of DOPA production in the presence of the decarboxylase inhibitor m-hydroxybenzylhydrazine (NSD-1015). The L-type Ca2+ channel blocker nifedipine inhibited the depolarization-induced stimulation of DOPA production by approximately 90% and attenuated the inhibitory effect of NPY. In contrast, the N-type Ca2+ channel blocker omega-conotoxin GVIA inhibited neither the stimulation of DOPA production nor the effect of NPY. Antagonism of Ca2+/calmodulin-dependent protein kinase (CaM kinase) greatly inhibited the stimulation of DOPA production by depolarization and prevented the inhibitory effect of NPY, whereas alterations in the cyclic AMP-dependent protein kinase pathway modulated DOPA production but did not prevent the effect of NPY. Stimulation of Ca2+/phospholipid-dependent protein kinase (PKC) with phorbol 12-myristate 13-acetate (PMA) did not affect the basal rate of DOPA production in NGF-differentiated PC12 cells but did produce a concentration-dependent inhibition of depolarization-stimulated DOPA production. In addition, NPY did not produce further inhibition of DOPA production in the presence of PMA, and the inhibition by both PMA and NPY was attenuated by the specific PKC inhibitor chelerythrine. These results indicate that NPY inhibits Ca2+ influx through L-type voltage-gated Ca2+ channels, possibly through a PKC-mediated pathway, resulting in attenuation of the activation of CaM kinase and inhibition of depolarization-stimulated catecholamine synthesis.

Paraventricular norepinephrine release mediates glucoprivic suppression of pulsatile luteinizing hormone secretion.

Restriction of glucose availability by 2-deoxyglucose (2DG) suppresses pulsatile LH release. The aim of the present study was to determine whether norepinephrine (NE) release in the paraventricular nucleus (PVN) is involved in the glucoprivic suppression of LH secretion in ovariectomized rats. Twelve days after ovariectomy, animals were stereotaxically implanted with a guide cannula for microdialysis in the PVN. Two days later, the PVN was perfused continuously with Ringer's solution or Ringer's solution containing a catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine (100 microM), through a microdialysis probe inserted in the guide cannula 2 h before the beginning of sampling, which lasted 3 h. Blood samples were collected every 6 min through an atrial cannula, and dialysates were collected every 20 min. One hour after the beginning of sampling, 2DG (400 mg/kg BW) was administered iv through the atrial cannula. Paraventricular NE levels significantly increased immediately after 2DG injection (P < 0.05), and both mean LH concentrations and the frequency of LH pulses decreased. By contrast, when alpha-methyl-p-tyrosine was administered into the PVN, 2DG did not produce an increase in paraventricular NE, and no depression of LH secretion occurred. These results suggest that the PVN mediates the glucoprivic suppression of LH pulses via the release of NE.

Effects of 5-HT 3 receptor-selective agents on locomotor activity in rats following injection into the nucleus accumbens and the ventral tegmental area

5-Hydroxytryptamine (5-HT) is involved in the modulation of dopaminergic activity in the mesolimbic system, but its sites of action and the receptors involved are not well understood. Locomotor activity responses in rats were monitored in Animex automated activity boxes following injection of 5-HT 3 receptor-selective agents directly into two mesolimbic nuclei, the nucleus accumbens and the ventral tegmental area, via stereotactically implanted injection guide cannulae. Neither spontaneous nor dexamphetamine-stimulated locomotor activity was changed by bilateral intra-nucleus accumbens injection of the selective agonist 2-methyl-5-HT or the selective antagonists ondansetron or granisetron. In contrast, intra-ventral tegmental area injection of 2-methyl-5-HT produced significant long-lasting (approximately 240 min) increases in locomotor activity; intra-ventral tegmental area injection of ondansetron elicited an initial inhibition of spontaneous and dexamphetamine-stimulated locomotor activity (for the 0–30 min period), but granisetron had no effect. The hyperlocomotor response to intra-ventral tegmental area 2-methyl-5-HT was abolished by pretreatment with the catecholamine synthesis inhibitor α-methyl- p-tyrosine, or by pretreatment with ondansetron. Methiothepin pretreatment had no effect on the hyperlocomotor response to 2-methyl-5-HT, although methiothepin itself produced an initial increase in spontaneous locomotor activity (for the 60–120 min period). Intra-ventral tegmental area injection of 5-carboxamidotryptamine, α-methyl-5-HT or renzapride produced no changes in spontaneous locomotor activity. In some of the ventral tegmental area experiments, other behaviours were also monitored. 2-Methyl-5-HT produced forward locomotion, rearing, and increased wakefulness, but did not appreciably alter circling, grooming or sniffing. Ondansetron alone had no effect on any of these behaviours, but it opposed the 2-methyl-5-HT-induced changes. Methiothepin alone increased forward locomotion and wakefulness but did not alter the other behaviours; it had no effect on the responses to 2-methyl-5-HT. These observations show that 5-HT 3 receptors may mediate increased locomotor activity by modulating firing of mesolimbic dopaminergic cell bodies in the ventral tegmental area rather than terminals in the nucleus accumbens.

314. - The effect of catecholamine depletion on thyrotropin secretion

Complex interaclions exisl belween psychiatric disorders and the hypoIhalamic·pituitnry-thyroid axis. Endogcnou.~ catccholamines, in particular tubcroinfundibular dopamine, arc believed to be involved in thyroid slimulating hormone (TSH) secretion. Alpha-methyl.par::a-tyrosine (AMPT) is ~I catecholamine synthesis inhibitor which is frcquenlly used in psychiatric and neuroendocrine research. The purpose of this sludy was to evaluate lhe effeCI of AMPT on circadian TSH secretion. Eight heallhy subjecls (4 women. 4 men) were given AMPT (5 x. Ig) ~r an :lctive placebo (promethazine, 5 x 50 mg) o\'cr a two·day pcnod uSing II double blind crossover design. Slaning at 08:00h day two. serllm WitS colkclcd ot 18 time points over a 24-hour time period including hourly sampling ovenlighl. At all lime points TSH wos lower during lhe aelive experimenl. From 22:00h until 08:00h a significant difference existed (p< a.OS} excellt at 02:00 h which showed a trend (p<O.06). These data indicnlc that AMPT blunted the physiologic nocturnal rise in 15H. In contraslto nprevious study daytime, TSH did nol differ belween the two groups. In summary, studies using the AMPT probe in psychiulrie and neuroendocrine rescllrch hnve to con.ducr Ihat AMPT nller~ nocturnal TSH secretion :trlll therefore might in(luencc Ihyroid function.