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. 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. 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. Evoked release was transient for large osmotic reductions (≥ 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. 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. 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.

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.

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. 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. 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. 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. Taurine release was unaffected by inhibitors and/or activators of PKA, PKC, MEK and Rho kinase. 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.

1 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 e ux from acutely isolated SON. 2 Taurine release induced by hypotonic stimulus (250 mosmol l 71 ) was not antagonized by the taurine transporter blocker guanidinoethyl sulphonate, con®rming the lack of implication of the transporter. 3 The osmodependent release of taurine was blocked by a variety of Cl 7 channel inhibitors with the order of potency: NPPB4ni¯umic acid4DPC4DIDS4ATP. On the other hand, release of taurine was only weakly a ected 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. 4 Although the molecular identity of volume-sensitive anion channels is not ®rmly 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 . 5 In conclusion, the osmodependent taurine permeable channels of SON astrocytes display a particular pharmacological pro®le, suggesting the expression of a particular type or subtype of volume-sensitive anion channel, which is likely to be formed by yet unidenti®ed proteins.