Supraoptic nucleus neurons: Evidence for a glutamatergic phenotype and purinergic control

Neurons of the supraoptic nucleus (SON) are part of the magnocellular neuroendocrine system and regulate many physiological processes including lactation, dehydration, and water balance. Research focused on investigating mechanisms controlling this system is important for both neuroscience and general animal physiology. Described in this volume are studies investigating the purinergic regulation and chemical phenotype of SON neurons.; It was found that activation of adenosine A1 receptors causes a large hyperpolarization of SON neurons leading to a lasting halt in spontaneous firing. Both spontaneous and singly evoked action potentials were reduced in duration by A1 receptor activation via an increase in the falling slope. Though adenosine A1 receptors are found presynaptically in the SON, these effects were found to occur through activation of postsynaptic A1 receptors. Activation of adenosine A2A receptors caused a depolarization which often initiated firing activity in neurons with sub-threshold membrane potentials. An increase in high-frequency postsynaptic currents---both inhibitory and excitatory---was seen and an A 2A receptor-mediated increase of a particular population of EPSCs was also observed. When the effects of endogenous adenosine were increased throughout the nucleus, the A1 effects dominated. The source of this endogenous adenosine is likely through bidirectional adenosine transporters and more importantly, the breakdown of the cotransmitter ATP into adenosine.; Electron micrographs of the neural lobe showed magnocellular terminals to contain microvesicles (MVs) in addition to the hormone-containing large dense-core vesicles (LDCVs), and the possibility and contents of dendritically and somatically-located MVs were investigated here. Specific vesicular glutamate transporter (VGLUT) proteins, signifying a glutamatergic phenotype are expressed by magnocellular neurons of the supraoptic nucleus.; All three known VGLUTs were investigated. The somas of SON neurons were strongly immunoreactive for VGLUT-3. VGLUT-3 immunoreactivity was also seen in astrocytes and meningeal cells. VGLUT-2 immunoreactivity was also seen in the dendrites of SON neurons. VGLUT-1 immunoreactivity was absent from these cells, though strong labeling did occur just lateral to the SON. These finding suggest that magnocellular neurons are glutamatergic and are capable of dendritic release of glutamate.