Systems neuroscienceA study of the role of neuro-glial remodeling in the oxytocin system at lactation

Under conditions of strong secretion of neurohypophysial hormone, such as during parturition, lactation and dehydration, the hypothalamic oxytocin-system displays a remarkable morphological plasticity such that astrocytic coverage of its neurones diminishes, their surfaces become directly juxtaposed and contacted by an increased number of synapses. A growing body of evidence indicates that these anatomical changes have an impact on glutamatergic neurotransmission in the supraoptic nucleus, and may be therefore of physiological consequence. We here evaluated the consequences of the inhibition of such plasticity on the overall activity of the oxytocin system during lactation. Remodeling was prevented by performing hypothalamic microinjections in gestating rats of endoneuraminidase, an enzyme that removes polysialic acid from the neural cell adhesion molecule. Our earlier studies established that the presence of polysialic acid is a prerequisite for remodeling of the oxytocin system in the supraoptic and paraventricular nuclei. In dams in which polysialic acid was absent in all magnocellular nuclei after bilateral endoneuraminidase injections, parturition was normal and neither the frequency nor the amplitude of suckling-induced reflex milk ejections was different from vehicle-treated dams. The weight gain of pups was also normal as was water intake by the dams. We then assessed the electrical activity of antidromically identified magnocellular neurones in the polysialic acid-free supraoptic nucleus of isoflurane-anesthetized lactating rats. Basal and bursting activity characteristic of oxytocin neurones before each reflex milk ejection was not significantly different from that recorded in the supraoptic nucleus of rats with normal levels of polysialic acid. Our results indicate that neuro-glial remodeling, despite its role on fine modulation of oxytocin neuronal activity, is not essential to parturition and lactation.