Age-related modifications of the morphological organization of pituicytes are associated with alteration of the GABAergic and dopaminergic innervation afferent to the neurohypophysial lobe.

Ageing is known to induce a marked activation of astrocytes within various regions of the central nervous system. To date, the age-related factors responsible for these modifications are unknown. The neural lobe of the hypophysis (NL) is a particular brain region which does not contain neurons but does contain specialized astrocytes, called pituicytes, and numerous terminals of afferent axons, including (i) peptidergic neurohypophysial axons which terminate on the NL blood vessels, and (ii) axons containing both gamma amino-butyric acid (GABA) and dopamine (DA) which form contacts with pituicytes. Because evidence has recently been provided that GABA signalling mediates the morphological organization of astrocytes, the present study was designed to determine whether modifications of pituicytes during ageing were associated with modifications of the GABAergic axons innervating the NL. We show here that, in adult rats, GABA/DA axons form preferential synaptic-like contacts with pituicytes which express both GABAA and D2 dopamine receptors. We then show that, during ageing, pituicytes undergo dramatic modifications of their morphology, correlatively with marked modifications of the GABA/DA fibres innervating the NL. Lastly, in vitro experiments indicate that modifications of the morphology of pituicytes similar to those observed during ageing were obtained by incubating isolated NL of adult rats with a GABAA receptor agonist and/or a D2 dopamine receptor antagonist, whereas inverse modifications were observed when NL of aged rats were incubated with a GABAA receptor antagonist and a D2 dopamine receptor agonist. Taken together, these data suggest that the age-related morphological changes of pituicytes result from the alteration of the GABA/DAergic innervation of the NL.

Osmoregulation of vasopressin secretion via activation of neurohypophysial nerve terminals glycine receptors by glial taurine.

Osmotic regulation of supraoptic nucleus (SON) neuron activity depends in part on activation of neuronal glycine receptors (GlyRs), most probably by taurine released from adjacent astrocytes. In the neurohypophysis in which the axons of SON neurons terminate, taurine is also concentrated in and osmo-dependently released by pituicytes, the specialized glial cells ensheathing nerve terminals. We now show that taurine release from isolated neurohypophyses is enhanced by hypo-osmotic and decreased by hyper-osmotic stimulation. The high osmosensitivity is shown by the significant increase on only 3.3% reduction in osmolarity. Inhibition of taurine release by 5-nitro-2-(3-phenylpropylamino)benzoic acid, niflumic acid, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid suggests the involvement of volume-sensitive anion channels. On purified neurohypophysial nerve endings, activation of strychnine-sensitive GlyRs by taurine or glycine primarily inhibits the high K(+)-induced rise in [Ca(2+)](i) and subsequent release of vasopressin. Expression of GlyRs in vasopressin and oxytocin terminals is confirmed by immunohistochemistry. Their implication in the osmoregulation of neurohormone secretion was assessed on isolated whole neurohypophyses. A 6.6% hypo-osmotic stimulus reduces by half the depolarization-evoked vasopressin secretion, an inhibition totally prevented by strychnine. Most importantly, depletion of taurine by a taurine transport inhibitor also abolishes the osmo-dependent inhibition of vasopressin release. Therefore, in the neurohypophysis, an osmoregulatory system involving pituicytes, taurine, and GlyRs is operating to control Ca(2+) influx in and neurohormone release from nerve terminals. This elucidates the functional role of glial taurine in the neurohypophysis, reveals the expression of GlyRs on axon terminals, and further defines the role of glial cells in the regulation of neuroendocrine function.

Pharmacological characterization of volume-sensitive, taurine permeable anion channels in rat supraoptic glial cells.

To characterize the volume-sensitive, osmolyte permeable anion channels responsible for the osmodependent release of taurine from supraoptic nucleus (SON) astrocytes, we investigated the pharmacological properties of the [(3)H]-taurine efflux from acutely isolated SON. Taurine release induced by hypotonic stimulus (250 mosmol l(-1)) was not antagonized by the taurine transporter blocker guanidinoethyl sulphonate, confirming the lack of implication of the transporter. The osmodependent release of taurine was blocked by a variety of Cl(-) channel inhibitors with the order of potency: NPPB>niflumic acid>DPC>DIDS>ATP. On the other hand, release of taurine was only weakly affected by other compounds (dideoxyforskolin, 4-bromophenacyl bromide, mibefradil) known to block volume-activated anion channels in other cell preparations, and was completely insensitive to tamoxifen, a broad inhibitor of these channels. Although the molecular identity of volume-sensitive anion channels is not firmly established, a few genes have been postulated as potential candidates to encode such channels. We checked the expression in the SON of three of them, ClC(3), phospholemman and VDAC(1), and found that the transcripts of these genes are found in SON neurons, but not in astrocytes. Similar observation was previously reported for ClC(2). In conclusion, the osmodependent taurine permeable channels of SON astrocytes display a particular pharmacological profile, suggesting the expression of a particular type or subtype of volume-sensitive anion channel, which is likely to be formed by yet unidentified proteins.

Tyrosine phosphorylation modulates the osmosensitivity of volume-dependent taurine efflux from glial cells in the rat supraoptic nucleus.

1. In the supraoptic nucleus, taurine, selectively released in an osmodependent manner by glial cells through volume-sensitive anion channels, is likely to inhibit neuronal activity as part of the osmoregulation of vasopressin release. We investigated the involvement of various kinases in the activation of taurine efflux by measuring [3H]taurine release from rat acutely isolated supraoptic nuclei. 2. The protein tyrosine kinase inhibitors genistein and tyrphostin B44 specifically reduced, but did not suppress, both the basal release of taurine and that evoked by a hypotonic stimulus. Inhibition of tyrosine phosphatase by orthovanadate had the opposite effect. 3. The tyrosine kinase and phosphatase inhibitors shifted the relationship between taurine release and medium osmolarity in opposite directions, suggesting that tyrosine phosphorylation modulates the osmosensitivity of taurine release, but is not necessary for its activation. 4. Genistein also increased the amplitude of the decay of the release observed during prolonged hypotonic stimulation. Potentiation of taurine release by tyrosine kinases could serve to maintain a high level of taurine release in spite of cell volume regulation. 5. Taurine release was unaffected by inhibitors and/or activators of PKA, PKC, MEK and Rho kinase. 6. Our results demonstrate a unique regulation by protein tyrosine kinase of the osmosensitivity of taurine efflux in supraoptic astrocytes. This points to the presence of specific volume-dependent anion channels in these cells, or to a specific activation mechanism or regulatory properties. This may relate to the particular role of the osmodependent release of taurine in this structure in the osmoregulation of neuronal activity.

Properties and glial origin of osmotic-dependent release of taurine from the rat supraoptic nucleus.

1. Taurine, prominently concentrated in glial cells in the supraoptic nucleus (SON), is probably involved in the inhibition of SON vasopressin neurones by peripheral hypotonic stimulus, via activation of neuronal glycine receptors. We report here the properties and origin of the osmolarity-dependent release of preloaded [3H]taurine from isolated whole SO nuclei. 2. Hyposmotic medium induced a rapid, reversible and dose-dependent increase in taurine release. Release showed a high sensitivity to osmotic change, with a significant enhancement with less than a 5% decrease in osmolarity. Hyperosmotic stimulus decreased basal release. 3. Evoked release was independent of extracellular Ca2+ and Na+, and was blocked by the Cl- channel blockers DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) and DPC (N-phenylanthranilic acid), suggesting a diffusion process through volume-sensitive Cl- channels. 4. Evoked release was transient for large osmotic reductions (> or = 15%), probably reflecting regulatory volume decrease (RVD). However, it was sustained for smaller changes, suggesting that taurine release induced by physiological variations in osmolarity is not linked to RVD. 5. Basal and evoked release were strongly inhibited by an incubation of the tissue with the glia-specific toxin fluorocitrate, but were unaffected by a neurotoxic-treatment with NMDA, demonstrating the glial origin of the release of taurine in the SON. 6. The high osmosensitivity of taurine release suggests an important role in the osmoregulation of the SON function. These results strengthen the notion of an implication of taurine and glial cells in the regulation of the whole-body fluid balance through the modulation of vasopressin release.

The kinin B1 receptor antagonist des-Arg9-[Leu8]bradykinin: an antagonist of the angiotensin AT1 receptor which also binds to the AT2 receptor.

1. Agonists and antagonists of kinin B1 and B2 receptors were evaluated in vitro for their effects against angiotensin II (AII)-induced contractile responses in the rabbit aorta and for their binding properties to angiotensin AT1 and AT2 receptors from purified membrane of rat liver and lamb uterus respectively. 2. In aortic rings, the kinin B1 receptor antagonist, des-Arg9-[Leu8]bradykinin (BK) (3-100 microM) caused a concentration-dependent decrease in sensitivity and a depression of the maximum response to AII. Des-Arg10-[Leu9]kallidin (KD), des-Arg9-BK, des-Arg10-KD, BK or KD at 3 microM had no effect against AII-induced contractions. 3. Des-Arg9-[Leu8]BK (3 or 100 microM) did not affect contractions of aortic rings to histamine, potassium chloride, endothelin-1, 5-hydroxytryptamine, noradrenaline and the thromboxane A2-mimetic, U46619. 4. Des-Arg9-[Leu8]BK displaced [125I]-Sar1-AII binding to the AT1 subtype in rat liver membranes with a Ki value of 1.1 +/- 0.4 microM. Values of Ki for des-Arg9-BK and KD were 45 +/- 13 microM and 25 +/- 22 microM, respectively. The other kinin derivatives des-Arg10-KD, BK and des-Arg10-[Leu9]KD at concentrations up to 100 microM did not bind to the AT1 receptor. 5. All the kinin derivatives except BK bound to AT2 receptors in lamb uterus membranes. Values of Ki for des-Arg9-[Leu8]BK, des-Arg10-[Leu9]KD, des-Arg9-BK, des-Arg 10-KD and KD were 0.3 +/- 0.1, 0.7 +/- 0.1, 1.2 +/- 0.3, 1.5 +/- 0.3 and 7.0 +/- 1.6 microM, respectively. 6. In conclusion, des-Arg9-[Leu8]BK is an insurmountable antagonist of AII-induced contractions in the rabbit aorta and also binds with a relatively high affinity to AT1 and AT2 receptors in isolated membrane fractions. These additional properties of des-Arg9-[Leu8]BK should be considered when it is used as an antagonist to characterize kinin B1 receptors.

Upregulation of V1a vasopressin receptors by glucocorticoids.

WRK1 cells (a rat mammary tumor cell line) exhibit a vasopressinergic receptor of V1a subtype tightly coupled to phospholipase C. Addition of dexamethasone to the culture medium principally potentiated the vasopressin-sensitive accumulation of inositol phosphates and to a lesser extent the NaF-sensitive phospholipase C activity. On the opposite, such treatment was without effect on the basal level of intracellular inositol phosphates or on bradykinin- or serotonin-sensitive phosphoinositide metabolisms. Glucocorticoid receptors were probably involved in these actions since dexamethasone was found to be more potent than aldosterone or corticosterone. Dexamethasone treatment also increased the number of vasopressin binding sites without affecting its affinity for vasopressin or other specific vasopressin analogues. These results strongly suggest that dexamethasone principally acts at the vasopressin receptor level by affecting its synthesis and/or the translation of its mRNA and also affects the G protein that couples the V1a receptor to the phospholipase C. These results explain how glucocorticoids may regulate the transduction mechanisms involved in vasopressin actions on WRK1 cells. They provide explanations for understanding the cross talk between adrenal steroids and hormones, which mobilize intracellular calcium.