Sandor Arancibia

New insights into brain BDNF function in normal aging and Alzheimer disease

The decline observed during aging involves multiple factors that influence several systems. It is the case for learning and memory processes which are severely reduced with aging. It is admitted that these cognitive effects result from impaired neuronal plasticity, which is altered in normal aging but mainly in Alzheimer disease. Neurotrophins and their receptors, notably BDNF, are expressed in brain areas exhibiting a high degree of plasticity (i.e. the hippocampus, cerebral cortex) and are considered as genuine molecular mediators of functional and morphological synaptic plasticity. Modification of BDNF and/or the expression of its receptors (TrkB.FL, TrkB.T1 and TrkB.T2) have been described during normal aging and Alzheimer disease. Interestingly, recent findings show that some physiologic or pathologic age-associated changes in the central nervous system could be offset by administration of exogenous BDNF and/or by stimulating its receptor expression. These molecules may thus represent a physiological reserve which could determine physiological or pathological aging. These data suggest that boosting the expression or activity of these endogenous protective systems may be a promising therapeutic alternative to enhance healthy aging.

Endogenous opiates block dopamine inhibition of prolactin secretion in vitro.

OPIATES stimulate prolactin secretion in vivo; plasma prolactin levels are increased by morphine administration in normal1 and steroid-primed male rats2, as well as in immature3 and oestrogen-treated female rats4. Met-enkephalin and β-endorphin were also stimulatory1–6 as were enkephalin analogues3,7,8. Naloxone or naltrexone, specific opiate antagonists, reduce basal prolactin levels when given alone1,3,9, and block, at least partially, the effect of opiate agonists1–4,7,8. None of these drugs is active in vitro when tested alone on anterior pituitaries3 or dispersed pituitary cells2,4. However, as we have previously shown in a preliminary experiment, the inhibitory effect of dopamine on prolactin secretion in vitro was antagonised by the addition of morphine to the incubation medium10. In the present work, we have attempted to clarify further the mode of interaction of opiates with prolactin secretion. We confirmed that morphine, Met-enkephalin and β-endorphin do not affect the spontaneous release of prolactin in vitro, but found that they suppress the inhibitory effect of dopamine on prolactin release. We have also characterised the specificity and the kinetics of this interaction.

Stimulation of in vitro Prolactin Release by Vasoactive Intestinal Peptide

VIP stimulated prolactin secretion from incubated rat hemipituitaries. Under the same conditions, the secretion of GH, LH, FSH was not affected. The stimulation of prolactin was dose-dependent, with an apparent affinity of VIP of 10.9 +/- 3.1 nM and a maximal stimulation of 57.7 +/- 4.2%. Secretin, a structurally related peptide, was also active at higher concentrations whereas another partial analogue, glucagon, was ineffective. The effect of VIP was not blocked by alpha-flupentixol, a potent dopaminergic antagonist, at concentrations which antagonized the dopamine inhibition of prolactin secretion. Stimulation by VIP and TRH was additive. Neither Met-enkephalin nor naloxone interfered with the response to VIP. It thus seems that specific VIP receptors are present on pituitary prolactin cells. VIP, present in the mediobasal hypothalamus and detected in the hypothalamo-hypophyseal portal blood therefore is a good candidate as a physiological PRF.

Expression of mRNAs encoding BDNF and its receptor in adult rat hypothalamus.

WE used a digoxigenin-UTP-labeled cRNA probe with in situ hybridization and Northern blot analysis to investigate the localization of brain derived neurotrophic factor (BDNF) mRNA and expression of its different transcripts in adult rat hypothalamus. As the BDNF gene is under the control of alternative multiple promoters, which provide tissue-specific gene expression, we studied whether these transcripts were expressed in adult hypothalamus. Our data revealed two novel sources of hypothalamic BDNF mRNA: the supraoptic and periventricular nuclei. In addition, we observed the expression of transcripts from exons I, II and III as well as the presence of 1.6 and 4.2 kb BDNF mRNAs. Finally, our findings confirmed expression of mRNA encoding neurotrophins receptors in the hypothalamus.

Neurotensin Stimulation of Prolactin Secretion in vitro

Neurotensin stimulated prolactin (PRL) secretion from incubated rat hemipituitaries. Under the same conditions, the secretion of growth hormone, luteinizing hormone and follicle-stimulating hormone was not affected. The stimulation of PRL was dose dependent, with an apparent affinity of neurotensin of 0.56 +/- 0.12 nM and a maximal stimulation of 56.5 +/- 6.7%. The effect of neurotensin seemed to be independent of that of other PRL releasing factors. In fact, the stimulation of neurotensin and thyrotropin-releasing hormone (TRH) and also of neurotensin and vasoactive intestinal peptide were additive. The action of neurotensin on PRL cells does not appear to involve either dopamine or gamma-aminobutyric acid receptors, since antagonists to these transmitters were found ineffective on PRL stimulation by neurotensin. PRL-releasing factor activity distinct from TRH has been described in fractions of hypothalamic extracts. Neurotensin, which is present in high amounts in the median eminence and has been measured in the adenohypophysis, is a candidate as a physiological PRL-releasing factor distinct from TRH.

Effect of aging on the expression of BDNF and TrkB isoforms in rat pituitary

Brain-derived neurotrophic factor (BDNF) is a key regulator of neuronal plasticity in adult rat brain and its effects are mediated through TrkB receptors. BDNF and its receptors are also localized in the pituitary, but their expressions throughout the rat lifespan are poorly known. Here we analyzed levels of BDNF and the different subtypes of TrkB receptors (mRNA and proteins) in the rat pituitary at different stages of life. BDNF immunoreactivity was expressed in folliculo-stellate cells from the anterior pituitary and in the intermediate lobe. TrkB.FL and TrkB.T1 receptors were strongly and essentially expressed in the intermediate lobe similar to the alpha-MSH localization pattern. These receptors begun decreasing at middle-age but TrkB.T2 was not detected in the pituitary at any age. Finally, in vitro alpha-MSH release from the intermediate lobe was correlated with the receptor content throughout the lifespan. The present results demonstrate the presence of BDNF in folliculo-stellate cells and indicated that receptors, rather than BDNF itself, are impaired with aging. These changes can contribute to explain age-dependent endocrine changes.

Neuroactive steroids modulate GABA inhibition of hypothalamic somatostatin release

The reduced steroids 3α-hydroxy-5α-pregnan-20-one (allopregnanolone) and 3α,21-di-hydroxy-5α-pregnan-20-one (allotetrahydroDOC) are potent ligands of GABAA receptors. This study examined the possible modulatory effect of these metabolites on GABA inhibition of somatostatin release in cultured hypothalamic neurones. Allopregnanolone potentiates GABA inhibition, and reversed picrotoxin and bicuculline-induced augmentation of somatostatin release in a dose-dependent manner. AllotetrahydroDOC also inhibits the stimulated release induced by the antagonists, but did not modify that induced by depolarizing concentrations of K+. Pregnenolone sulphate had no effect on picrotoxin-induced somatostatin release. These findings clearly establish that 3α-hydroxysteroids modulate GABA inhibition of hypothalamic somatostatin release.

Rapid reduction in somatostatin mRNA expression by hypothalamic neurons induced by dexamethasone

We have previously reported that peripherally administered dexamethasone induces a rapid increase in hypothalamic somatostatin release. Here we investigated whether somatostatin synthesis could also be affected by this treatment and the potential involvement of glutamate in this effect. Male rats received a saline or a dexamethasone injection (300 μg/100 g body weight) and were killed 30 min later. Thirty minutes prior to dexamethasone treatment, another group received an i.p. injection of MK-801, a NMDA receptor antagonist. Cells expressing somatostatin mRNA in the periventricular nucleus were analyzed by in situ hybridization using digoxigenin-labeled somatostatin oligonucleotide probe. Dexamethasone decreased the number of digoxigenin-labeled cells expressing somatostatin mRNA in the periventricular nucleus as compared to the same histological sections from control rats. The dexamethasone effect was reversed by pretreatment with MK-801, which alone also decreased the number of cells expressing somatostatin mRNA. In summary, dexamethasone administration induces a significant rapid decrease in periventricular cells expressing somatostatin mRNA and this effect is partly abolished by MK-801.

Pharmacological Studies of Pituitary Receptors Involved in the Control of Prolactin Secretion

The major action of the hypothalamus on mammalian prolactin secretion is inhibitory. Ectopic pituitary grafts (Everett, 1954; Meites et al., 1961), electrolytic lesion of the median eminence (Chen et al., 1970; Bishop et al., 1971) and in vitro incubation of adenohypophyseal tissue (Meites and Nicoll, 1966) result in increased prolactin secretion. It was postulated that the hypothalamus contained an unidentified factor that inhibited the release of prolactin at the hypophyseal level (Pasteels, 1961, 1967; Talwalker et al., 1963) and that this factor could reach the pituitary directly across the hypothalamo-hypophyseal portal system. This was substantiated by experiments in which hypothalamic extracts infused into a portal blood vessel induced a dose-dependent inhibition of prolactin release (Kamberi et al., 1971b). This inhibitory effect of hypothalamic extracts was found in many in vivo and in vitro preparations.