IDENTIFICATION OF OSMORESPONSIVE NEURONS IN THE FOREBRAIN OF THE RAT - A FOS STUDY AT THE ULTRASTRUCTURAL LEVEL

The aims of this study are twofold. The first is to describe the ultra structural morphology of putative osmoreceptors concentrated in the ve ntral aspect of the lamina terminalis in the rat forebrain. The second is to determine whether or not these neurons lie within an area which lacks a blood-brain barrier, i.e. the organum vasculosum lamina termi nalis. The results describe a compact population of neurons in the ven tral part of the lamina terminalis which both respond to an osmotic ch allenge and project directly to the supraoptic nucleus. Injection of h orseradish peroxidase into the circulation, as a marker to define area s of the brain without a blood-brain barrier, indicates that these neu rons are in the dorsal aspect of the organum vasculosum of the lamina terminalis. An ultrastructural analysis of the neurons in this area, w hich respond to an osmotic challenge with an elevation of Fos protein, show them to have no specific morphological characteristics which dif ferentiate them from other, non-responsive neurons in the organum vasc ulosum of the lamina terminalis. However, one possible exception is th at osmotically sensitive neurons have a less indented nucleus, suggest ing that they are in a more active state than their non-osmotically se nsitive neighbours. It is concluded that neurons in this region of the brain are candidate structures for the ''receptors'' which mediate va sopressin release in response to an osmotic challenge. The response of only a subset of neurons in the organum vasculosum of the lamina term inalis to an osmotic stimulus, despite an apparent morphological homog eneity and the ability of blood borne agents to reach all parts of the structure suggests that osmoresponsiveness is conferred by unique mem brane properties or intracellular processing events. The presence of s ynaptic input to osmoresponsive cells indicates a potential for integr ation of other inputs at this level.