Lateral hypothalamic neurones: osmosensitivity and the influence of activating magnocellular neurosecretory neurones

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Abstract
The activity of single cells in the supraoptic nucleus and the lateral hypothalamus of urethane-anesthetized rats was studied during electrical stimulation of the neural stalk. Neurons activated antidromically from the neural stalk were either unaffected or, rarely, slightly affected by single shocks or by brief trains of shocks presented at an intensity just below that necessary for antidromic invasion. There does not appear to be a strong synaptic coupling between rat supraoptic neurons. Many lateral hypothalamic neurons, and particularly those in the perinuclear zone, were strongly affected by neural stalk stimulation, being either orthodromically excited or inhibited. Thus, rat magnocellular neurosecretory neurons appear to project, directly or indirectly, to the lateral hypothalamus. Lateral hypothalamic neurons (31) were studied following an i.p. injection of 1 ml 1.5 M-NaCl. Neurons (18) responded to this osmotic stimulus with a change of at least 1 spike/s in mean firing rate. Of the 31 neurons 15 were strongly affected by neural stalk stimulation (8 inhibited, 7 excited). Of these 15 neurons 10 were affected in the same way by osmotic stimulation. The remaining 16 of 31 neurons were unresponsive to neural stalk stimulation, and of these, 10 were also unresponsive to osmotic stimulation. Most lateral hypothalamic neurons responded in a similar way to neural stalk stimulation and to systemic osmotic stimulation. The osmosensitivity of some neurons in the lateral hypothalamus may be mediated by synaptic input arising from the magnocellular neurosecretory neurons. The osmosensitivity of some lateral hypothalamic neurons cannot be explained in this way, and probably derives from osmoreceptors other than the magnocellular neurons.