Dye coupling in hypothalamic slices: dependence on in vivo hydration state and osmolality of incubation medium

Electrotonic coupling is one mechanism which may coordinate the electrophysiological activity of a population of neurons. By measuring the incidence of dye coupling, we have investigated whether conditions that stimulate hormone secretion by hypothalamic magnocellular neuroendocrine cells affect coupling between these neurons. Neurons in the magnocellular regions of the paraventricular nucleus (PVN), in slices prepared from normally hydrated or chronically dehydrated male rats, were intracellularly injected with the fluorescent dye Lucifer Yellow CH. The dye coupling index (DCI), the ratio of the number of dye-coupled neurons to the total number of filled cells, was determined for each treatment group. The DCI for slices from dehydrated animals incubated in 310 milliosmoles/kg of medium (0.121) was significantly lower than that for slices for hydrated animals incubated in medium of the same osmolality (0.333). This decrease was reversed when slices from dehydrates were incubated in medium having an osmolality of 340 milliosmoles/kg (DCI = 0.307). There was also evidence for an interaction between slices incubated in the same chamber: the DCI in slices from dehydrated animals was significantly higher (0.475) when slices from normally hydrated rats were also present in the incubation chamber. Based on these data and on cited evidence, we suggest that the osmolality of the extracellular fluid and the local concentration of sex steroid hormones may influence dye coupling in the PVN.