THE GLUTAMATERGIC INNERVATION OF OXYTOCIN-SECRETING AND VASOPRESSIN-SECRETING NEURONS IN THE RAT SUPRAOPTIC NUCLEUS AND ITS CONTRIBUTION TOLACTATION-INDUCED SYNAPTIC PLASTICITY

The present ultrastructural study analysed the distribution of glutama tergic synapses on oxytocin- and vasopressin-secreting neurons in the rat supraoptic nucleus (SON) after post-embedding immunogold labelling for glutamate and GABA, oxytocin or vasopressin. About 20% of SON axe -somatic synapses were enriched in glutamate immunoreactivity, visible over synaptic-like vesicles, mitochondria and synaptic densities. Dou ble labelling for glutamate and GABA showed that putative glutamatergi c terminals were distinct from GABAergic terminals. In ultrathin secti ons stained for glutamate and either oxytocin or vasopressin, the prop ortion of glutamatergic synapses was similar on oxytocinergic and vaso pressinergic somata in virgin rats under basal conditions of peptide r elease as well as in lactating rats, in which oxytocin secretion is en hanced. Cross-sectional soma areas were significantly increased in lac tating rats: oxytocinergic profiles were, on average, similar to 40% l arger than in virgin rats. However, the incidence of axe-somatic gluta matergic synapses (assessed as mean number of synapses per 100 mu m of plasmalemma or proportion of somatic surface apposed to synaptic acti ve zones) did not diminish, indicating that there was a compensatory i ncrease of synapses during lactation. Also, we found an increase in th e number of glutamatergic terminals making synaptic contact simultaneo usly onto two or more oxytocinergic elements in the same plane of sect ion. Our observations therefore indicate that SON oxytocinergic and va sopressinergic neurons are innervated to a similar extent by a relativ ely large proportion of glutamatergic synapses. They reveal, moreover, that glutamatergic afferents participate in the lactation-induced syn aptic plasticity of the oxytocinergic system.