It has been found that cells in the region of the supraoptic nucleus (SON) of the hypothalamus are sensitive to osmotic stimuli in a physiological range, and have studied the mechanism by which the osmotic sensitivity arises. An in vitro hypothalamic brain slice preparation has been used to make intracellular recordings from the SON. Cells lying in the SON respond to small increases (9--40 mosmol l-1) in the osmolarity of their external environment with a marked increase in firing rate. They respond to NaCl and mannitol although not to glucose. The osmotic sensitivity of SON neurones has a complex origin at both a pre- and a postsynaptic level, being composed first of depolarization of the primary SON neurone by an increase of extracellular osmolarity and second by an increased rate of occurrence of excitatory synaptic events which markedly augment firing rate. These results are consistent with Jewell and Verney's suggestion that osmoreception in the mammalian brain occurs in the region of the anterior hypothalamus, and extends this localization by indicating that the SON neurones are themselves both directly osmosensitive and part of an osmoreceptive complex.

Low-molecular-weight GTP-binding proteins (small G proteins) of the Rab family have been proposed to act as central regulators of vesicular traffic, and proteins of the Rab3 subfamily (Rab3A, B, C and D) are thought to be associated with membrane vesicles or granules undergoing exocytotic fusion with the plasma membrane. Rab3A is highly expressed in brain, whereas Rab3B is the major form found in rat anterior pituitary gland. We report here that antisense oligonucleotides against Rab3B, introduced into pituitary cells using the whole-cell patch clamp technique, specifically and reversibly block expression of Rab3B. We find that calcium-dependent exocytosis is inhibited without affecting endocytosis. Antisense oligonucleotides directed against Rab3A have no effect. Our results indicate that Rab3B is likely to be a key intracellular signalling molecule which can control exocytosis downstream of other calcium-dependent processes in anterior pituitary cells.

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