Decreased expression of fos-related antigens (FRAs) in the hypothalamic dopaminergic neurons after immunoneutralization of endogenous prolactin.

In our previous studies we found that administration of exogenous prolactin increased dopamine turnover in the terminal areas of the hypothalamic dopaminergic neurons controlling prolactin secretion from pituitary lactotrophs. In this study we investigated the effect of immunoneutralization of endogenous prolactin on the expression of FRAs in the tuberoinfundibular dopaminergic (TIDA), tuberohypophysial dopaminergic (THDA), and periventricular hypothalamic dopaminergic (PHDA) subpopulations of the hypothalamic dopaminergic neurons. Female rats were ovariectomized on d 0 of the experiment. At 1000 h of d 10, all animals were injected with 20 microg of 17-beta-estradiol sc to induce a proestrous-like surge of prolactin at 1700 h the next day. At 1000 h on d 11, half of the animals were injected with 200 microL of rabbit anti-rat prolactin antiserum ip, while the controls received normal rabbit serum. Groups of animals were sacrificed for immunocytochemistry in 2 h intervals between 1300 and 2100 h. Double-label immunocytochemistry for FRAs and tyrosine hydroxylase (TH) was performed and the results are presented as percentage of TH-immunoreactive neurons expressing FRAs. In the control animals, expression of FRAs decreased at 1500 h, gradually increased by 1900 h, but was lower than the basal levels by 2100 h. Expression of FRAs was significantly lower at 1900 h in the PHDA, THDA and TIDA neurons of prolactin antiserum treated rats than in the controls. These results indicate that elimination of endogenous prolactin from the circulation lowers the activity and/or prevents the reactivation of neuroendocrine dopaminergic neurons at the beginning of the dark phase.

Dopamine transporters participate in the physiological regulation of prolactin.

Three populations of hypothalamic neuroendocrine dopaminergic (NEDA) neurons, arising from the arcuate and periventricular nuclei of the hypothalamus release dopamine (DA) that acts at the pituitary gland to regulate the secretion of PRL. It is generally accepted that NEDA neurons lack functional DA transporters (DATs), which are responsible for uptake of DA from the synaptic cleft into the presynaptic axon terminal. This study localized DATs to the hypothalamo-pituitary axis and evaluated the effect of DAT blockade on the hypothalamo-pituitary regulation of PRL. After 7 days of treatment with cocaine (a nonspecific amine transporter blocker) or mazindol (a specific DAT blocker), the relative abundance of PRL messenger RNA (mRNA) in the anterior lobe (AL) of OVX rats was significantly decreased, whereas the relative abundance of tyrosine hydroxylase mRNA in the hypothalamus was significantly increased. The effect of cocaine or mazindol administration on DA turnover and serum PRL concentration was examined in estradiol (E2)-treated OVX rats. E2 administration (i.v.) resulted in a significant increase in serum PRL within 4 h; however, cocaine or mazindol administration abolished the E2-induced increase of PRL. Cocaine or mazindol significantly increased the concentration of DA at the site of the axon terminals within the median eminence (ME), intermediate lobe (IL) and neural lobe (NL), indicating blockade of uptake. Because formation of DOPAC requires uptake of DA, concentrations of DOPAC in the ME, IL and NL decreased following treatment with either cocaine or mazindol. These data, together with the presence of immunopositive DAT in the ME, pituitary stalk, IL, and NL, suggest that a functional DAT system is present within all three populations of NEDA neurons. Moreover, similarity between the effects of cocaine and mazindol treatment indicate that blockade of the DAT, but not other amine transporters, is responsible for suppression of PRL gene expression and secretion. Blockade of DATs prevent uptake of DA into NEDA neurons and consequently increases the amount of DA that diffuses into the portal vasculature and reaches the AL. These data provide evidence that DATs play a physiological role in the regulation of DA release from and TH expression in NEDA neurons and consequently PRL secretion and PRL gene expression and further support our previous observation that the regulation of PRL secretion involves all three populations of NEDA neurons.

Prolactin activates all three populations of hypothalamic neuroendocrine dopaminergic neurons in ovariectomized rats.

Prior studies suggest that prolactin (PRL) stimulates release of dopamine (DA) from tuberoinfundibular dopaminergic (TIDA) neurons. In the present study, the time course over which PRL exerts its effects on all three populations of neuroendocrine dopaminergic (DAergic) neuron populations [TIDA, tuberohypophyseal (THDA) and periventricular-hypophyseal (PHDA)] was determined. Ten days following ovariectomy (OVX), groups of female rats were injected either with 15 microg of ovine PRL (oPRL) or saline at 0900 h. Rats were decapitated every 30 min from 0830 h-1100 h and hourly from 1200 h-1500 h. Trunk blood was assayed for rat PRL (rPRL) and oPRL using species-specific radioimmunoassays (RIAs). The concentration of DA and 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence (ME), as well as the anterior (AL), intermediate (IL) and neural (NL) lobes of the pituitary gland were determined by HPLC-EC. The concentration of rPRL in oPRL-treated animals, compared to saline-treated animals, was diminished by 1000 h and again between 1200 h-1500 h. DOPAC/DA ratio, an indicator of dopaminergic neuronal activity, increased spontaneously in the ME, IL, and NL during the afternoon in OVX rats. In animals injected with oPRL at 0900 h, the DOPAC/DA ratio increased in the ME, IL and NL within 1 h. Moreover, a secondary increase in the DOPAC/DA ratio in the IL and NL occurred during the afternoon in oPRL-treated rats. However, the second increase of DA turnover present in the ME of control animals never occurred in oPRL-treated animals. Furthermore, there were two increases in the concentration of DA in the AL: the first coincided with the increased turnover of DA in all three terminal areas and the second with increased DA turnover in the IL and NL. These data suggest that all three populations of hypothalamic neuroendocrine DAergic neurons are activated by PRL and that PHDA/THDA neurons have a second 'delayed' activation.

Free zinc increases at the site of injury after cortical stab wounds in mature but not immature rat brain.

The accumulation of free zinc (Zn2+) appears to play a role in the neuronal degeneration that occurs after brain injury. Given that neonates respond to brain injury with increased plasticity compared to adults, this study compared the effect of age on free Zn2+ and the Zn2+-binding protein metallothionein-3 (MT-3) after injury. Unilateral cortical stab wounds were produced in 3-day-old and adult rats. Four weeks later, brains were removed for in situ visualization of free Zn2+ and measurement of MT-3 mRNA. Free Zn2+ and MT-3 mRNA accumulated after 4 weeks at the site of injury site when injury occurred in adults. However, 4 weeks after neonatal injury there was no increase in free Zn2+ or MT-3 mRNA in or around the site of injury.