Intranuclear coupling of hypothalamic magnocellular nuclei by glutamate synaptic circuits.

Magnocellular neurons of the supraoptic nucleus (SON) and paraventricular nucleus (PVN) display bursting activity that is synchronized under certain conditions. They receive excitatory synaptic inputs from intrahypothalamic glutamate circuits, some of which are activated by norepinephrine. Ascending noradrenergic afferents and intrahypothalamic glutamate circuits may be responsible for the generation of synchronous bursting among oxytocin neurons and/or asynchronous bursting among vasopressin neurons located in the bilateral supraoptic and paraventricular nuclei. Here, we tested whether magnocellular neurons of the PVN receive excitatory synaptic input from the contralateral PVN and the region of the retrochiasmatic SON (SONrx) via norepinephrine-sensitive internuclear glutamate circuits. Whole cell patch-clamp recordings were performed in PVN magnocellular neurons in coronal hypothalamic slices from male rats, and the ipsilateral SONrx region and contralateral PVN were stimulated using electrical and chemical stimulation. Electrical and glutamate microdrop stimulation of the ipsilateral SONrx region or contralateral PVN elicited excitatory postsynaptic potentials/currents (EPSP/Cs) in PVN magnocellular neurons mediated by glutamate release, revealing internuclear glutamatergic circuits. Microdrop application of norepinephrine also elicited EPSP/Cs, suggesting that these circuits could be activated by activation of noradrenergic receptors. Repetitive electrical stimulation and drop application of norepinephrine, in some cases, elicited bursts of action potentials. Our data reveal glutamatergic synaptic circuits that interconnect the magnocellular nuclei and that can be activated by norepinephrine. These internuclear glutamatergic circuits may provide the functional architecture to support burst generation and/or burst synchronization in hypothalamic magnocellular neurons under conditions of activation.

Activity-dependent release and actions of endocannabinoids in the rat hypothalamic supraoptic nucleus.

Exogenous cannabinoids have been shown to significantly alter neuroendocrine output, presaging the emergence of endogenous cannabinoids as important signalling molecules in the neuroendocrine control of homeostatic and reproductive functions, including the stress response, energy metabolism and gonadal regulation. We showed recently that magnocellular and parvocellular neuroendocrine cells of the hypothalamic paraventricular nucleus and supraoptic nucleus (SON) respond to glucocorticoids by releasing endocannabinoids as retrograde messengers to modulate the synaptic release of glutamate. Here we show directly for the first time that both of the main endocannabinoids, anandamide (AEA) and 2-arachidonoyl glycerol (2-AG), are released in an activity-dependent fashion from the soma/dendrites of SON magnocellular neurones and suppress synaptic glutamate release and postsynaptic spiking. Cannabinoid reuptake blockade increases activity-dependent endocannabinoid levels in the region of the SON, and results in the inhibition of synaptically driven spiking activity in magnocellular neurones. Together, these findings demonstrate an activity-dependent release of AEA and 2-AG that leads to the suppression of glutamate release and that is capable of shaping spiking activity in magnocellular neurones. This activity-dependent regulation of excitatory synaptic input by endocannabinoids may play a role in determining spiking patterns characteristic of magnocellular neurones under stimulated conditions.

Increased tonic activation of presynaptic metabotropic glutamate receptors in the rat supraoptic nucleus following chronic dehydration.

Chronic dehydration induces structural changes in the hypothalamic supraoptic nucleus (SON), including increased glutamate synapses and retraction of astroglial processes. We performed whole-cell recordings in acute hypothalamic slices to determine whether these changes increase tonic activation of presynaptic metabotropic glutamate receptors (mGluRs) by increasing ambient glutamate in the SON. Activation of presynaptic group III mGluRs caused a decrease in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in SON neurones that was significantly attenuated in slices from dehydrated rats (-27.8 %) compared with untreated rats (-41.7 %), suggesting a higher basal occupancy of mGluRs by ambient glutamate during dehydration. Blocking group III mGluRs caused an increase in the frequency of mEPSCs that was significantly higher in slices from dehydrated rats (+42.8 %) than untreated rats (+31.4 %), suggesting greater tonic activation of presynaptic mGluRs by ambient glutamate during dehydration. Increasing ambient glutamate levels by inhibiting astrocyte glutamate uptake resulted in a decrease in mEPSC frequency due to increased activation of presynaptic mGluRs. This was attenuated in slices from dehydrated rats (-35.4 %) compared with slices from untreated rats (-48.8 %), suggesting diminished astrocytic glutamate uptake during dehydration. Immunochemical analyses revealed a robust expression of the GLT-1 transporter protein in the SON, which was diminished in SON punches from dehydrated rats compared with untreated controls. Thus, dehydration leads to increased tonic activation of presynaptic mGluRs on glutamate terminals, consistent with a decrease in glutamate buffering capacity. The resulting reduction in glutamate release probability may compensate for the increase in glutamate release sites that occurs during dehydration.

Functional synaptic plasticity in hypothalamic magnocellular neurons.

The hypothalamic-neurohypophysial system undergoes dramatic morphological plasticity in response to physiological activation during parturition/lactation and dehydration, including somatic swelling, decreased glial coverage and increased synaptic innervation of the magnocellular neuroendocrine cells. Recent in-vitro electrophysiological studies in hypothalamic slices have demonstrated that coordinate changes in the synaptic physiology of the magnocellular neurons also occur under these conditions. Thus, the synaptic release of glutamate and GABA onto magnocellular neurons is increased during lactation and with chronic dehydration, and changes in postsynaptic glutamate and GABAA receptor expression lead to alterations of the functional properties of the glutamate and GABAA receptor channels. The presynaptic noradrenergic facilitation of glutamate release and inhibition of GABA release is also markedly enhanced following chronic dehydration. Additionally, both parturition and chronic dehydration are accompanied by an increase in the tonic activation of presynaptic metabotropic glutamate receptors due to the higher ambient glutamate concentration caused by decreased glial coverage and the resultant reduction in glutamate reuptake. Together, these electrophysiological studies reveal profound functional plasticity in the synaptic physiology of magnocellular neurons at parturition and following dehydration. The plastic changes support an increase in the excitability of magnocellular neuroendocrine cells by increasing glutamate inputs, decreasing GABA inputs, enhancing excitatory noradrenergic modulation, and reducing synaptic glutamatergic noise.