Distribution of 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylglycol (DOPEG) in microdissected brain structures and the pituitary gland: metabolite changes in the median eminence in response to hyperprolactinemia and suckling

Chronic unpredictable stress augments +3,4-methylenedioxymethamphetamine-induced monoamine depletions: the role of corticosterone

Exposure to stress alters the behavioral and neurochemical effects of drugs of abuse. However, it is unknown if chronic stress can affect the serotonergic depletions induced by the psychostimulant drug 3,4-methylenedioxymethamphetamine (MDMA). Rats were exposed to 10 days of chronic unpredictable stress (CUS) which resulted in the predicted elevation of basal plasma corticosterone concentrations. On the 11th day, rats received four challenge doses of MDMA (5 mg/kg every 2 h, i.p.) or saline. Five days later, rats were killed and serotonin (5-HT) and dopamine content were measured in the striatum, hippocampus, and frontal cortex. MDMA produced greater depletions of 5-HT in all three brain regions of rats pre-exposed to CUS compared to rats not exposed to CUS. CUS-exposed rats also had an augmented acute hyperthermic response but a similar increase in plasma corticosterone after challenge injections of MDMA compared with non-stressed rats similarly challenged with MDMA. Moreover, CUS-exposed rats exhibited an MDMA-induced depletion of striatal dopamine that was absent in non-stressed rats that received MDMA. To investigate the role of corticosterone in these effects, the corticosterone synthesis inhibitor, metyrapone (50 mg/kg i.p.), was administered prior to each stressor on each of the 10 days of CUS. Metyrapone blocked the chronic stress-induced elevation in basal plasma corticosterone, prevented the enhancement of MDMA-induced hyperthermia, and blocked the enhanced depletions of 5-HT and dopamine in CUS-exposed rats, but had no effect on the acute MDMA-induced increases in plasma corticosterone. These findings suggest that CUS alone can increase the basal level of corticosterone that in turn, plays an important role in enhancing the sensitivity of both 5-HT and dopamine terminals to the hyperthermic and monoamine depleting effects of MDMA without altering the acute corticosterone response to an MDMA challenge.

Neurotensin-induced activation of hypothalamic dopaminergic neurons is accompanied by a decrease in pituitary secretion of prolactin and alpha-melanocyte-stimulating hormone

The effects of neurotensin on the activity of hypothalamic tuberoinfundibular and periventricular-hypophysial dopaminergic (DA) neurons, and on the secretion of pituitary hormones that are tonically regulated by these neurons (i.e. prolactin and alpha-melanocyte-stimulating hormone [alpha MSH], respectively) were examined in estrogen-primed ovariectomized rats. The activity of tuberoinfundibular and periventricular-hypophysial DA neurons was estimated by measuring concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the terminals of these neurons in the median eminence and intermediate lobe of the posterior pituitary, respectively. Intracerebroventricular administration of neurotensin caused a dose- and time-related increase in DOPAC concentrations in both the median eminence and intermediate lobe, and a concurrent decrease in plasma levels of prolactin and alpha MSH. These results suggest that neurotensin-induced inhibition of secretion of prolactin and alpha MSH from the pituitary may be due to the stimulatory action of this neuropeptide on the release of dopamine from tuberoinfundibular and periventricular-hypophysial neurons.

Age- and dose-dependent effects of neonatal monosodium glutamate (MSG) administration to female rats

The age- and dose-dependent effects of neonatal MSG were evaluated in pre- and postpubertal female rats. The neurotoxic action of MSG was assessed by examining monoamine content in microdissected regions of the mediobasal hypothalamus. MSG was administered at a dose of 4 mg/g on postnatal days 2 and 4 (MSG-Lo) or on postnatal days 2, 4, 6 and 8 (MSG-Hi). MSG-Hi treatment significantly reduced dopamine (DA) content in the arcuate nucleus (ANH) and lateral median eminence (LME) on postnatal day 21 when compared to NaCl-injected controls. DA content relative to controls was not altered in the ANH or LME postnatal or postnatal day 60 in MSG-Hi, however, norepinephrine (NE) was significantly (p less than 0.05) decreased on both postnatal day 21 and 60 in the LME. MSG-Lo treatment significantly (p less than 0.05) reduced ANH NE content on postnatal day 60 compared to controls. Both MSG-Hi and MSG-Lo treatment increased 5-hydroxyindoleacetic acid content in the preoptic area (POA) on postnatal day 60 relative to the controls. Developmental changes independent of MSG treatment were noted in the hypothalamus. DA and 3,4-dihydroxyphenylacetic acid (DOPAC) content in the POA were 2-3-fold higher on postnatal day 21 compared to postnatal day 60. In contrast, DA content increased with age in the ANH, LME and medial ME. NE content in the ANH increased as a function of age in controls, but not in MSG-treated rats. The effects of MSG treatment on the postnatal development and maturation of neurons in the mediobasal hypothalamus were discussed in relation to the direct neurotoxicity of MSG.

Lack of effect of hyperprolactinemia on serotonin turnover in ovariectomized and ovariectomized estrogen-treated rats

Hyperprolactinemia suppresses luteinizing hormone (LH) and prolactin (PRL) secretion under a variety of experimental conditions. The secretion of both of these hormones is regulated at the hypothalamic level by several neurotransmitters, including serotonin (5-HT). Therefore, we examined the effect of hyperprolactinemia on 5-HT neuronal activity in key hypothalamic areas that are rich in 5-HT terminals and which are known to regulate the release of LH and PRL. Young cycling virgin rats were ovariectomized (day 0). From days 11-16, animals were injected with ovine prolactin (oPRL, 4 mg/kg, s.c.) or vehicle every 8 h. On day 14, one-half of the oPRL- and vehicle-treated rats were implanted with 20-mm long Silastic capsules containing estradiol (180 micrograms/ml). On day 16, animals were killed at 08.00, 12.00 or 18.00 h or treated with pargyline (75 mg/kg) and killed 10 min later. Trunk blood was collected and serum was radioimmunoassayed for LH and endogenous rat PRL (rPRL). Brains were removed, frozen, sectioned and the medial preoptic, suprachiasmatic, and arcuate nuclei, median eminence and globus pallidus were microdissected. Serotonin was measured using high pressure liquid chromatographic methodology. We were unable to detect any feedback effect of hyperprolactinemia on 5-HT turnover in any brain area of ovariectomized or ovariectomized estradiol-treated rats at any time of day that we examined. Several potential reasons for the absence of an effect of hyperprolactinemia on serotonergic function are discussed.

The metabolism of dopamine in the median eminence reflects the activity of tuberoinfundibular neurons

The purpose of the present study was to characterize the metabolism of dopamine (DA) in tuberoinfundibular (TI) neurons terminating in the median eminence and to examine the effects of procedures that alter the synthesis and turnover of DA in these neurons on the concentrations of dihydroxyphenylacetic acid (DOPAC) in the median eminence. The DA uptake inhibitor nomifensine (25 mg/kg, i.p.; 30 min) failed to alter median eminence DOPAC concentrations indicating that very little released DA is recaptured and metabolized by TIDA neurons. Within 5 min following the administration of the monoamine oxidase inhibitor pargyline (50 mg/kg, i.v.) median eminence DOPAC concentrations declined to 15% of control demonstrating that this metabolite has a high turnover rate and is rapidly removed from the median eminence. Median eminence DOPAC concentrations in diestrous female rats, whose TIDA neuronal activity is higher than in the male, were two-fold greater than in male rats. Prolactin (10 micrograms/rat, i.c.v.; 12 h), which increases TIDA neuronal activity, produced a corresponding increase in median eminence DOPAC concentrations in male rats. Restraint stress (30 min), which decreases TIDA neuronal activity, produced a corresponding decrease in median eminence DOPAC concentrations in diestrous female rats. The results from the present study suggest that DOPAC concentrations in the median eminence can be used as an index of TIDA neuronal activity.