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Neurophypophyseal Hormones From Genomics and Physiology to Disease

Activation of adenosine A_2A receptors alters postsynaptic currents and depolarizes neurons of the supraoptic nucleus

Department of Cell Biology and Neuroscience, University of California, Riverside, California

Submitted 20 October 2005 ; accepted in final form 14 April 2006

Supraoptic nucleus (SON) neurons secrete oxytocin or vasopressin in response to various physiological stimuli (e.g., lactation/suckling, dehydration). Released near fenestrated capillaries of the neurohypophysis, these peptides enter the blood and travel to peripheral target organs. The pervasive neuromodulator adenosine, acting at A₁ receptors, is an important inhibitory regulator of magnocellular neuroendocrine cell activity. Another high-affinity adenosine receptor exists in this system, however. We examined the physiological effects of adenosine A_2A receptor activation and determined its localization among various cell types within the SON. In whole cell patch-clamp recordings from rat brain slices, application of the selective adenosine A_2A receptor agonist CGS-21680 caused membrane depolarizations in SON neurons, often leading to increased firing activity. Membrane potential changes were persistent (>10 min) and could be blocked by the selective A_2A receptor antagonist ZM-241385, or GDP--S, the latter suggesting postsynaptic sites of action. However, ±--methyl-(4-carboxyphenyl)glycine or TTX also blocked CGS-21680 effects, indicating secondary actions on postsynaptic neurons. In voltage-clamp mode, application of CGS-21680 caused a slight increase (8%) in high-frequency clusters of excitatory postsynaptic currents. With the use of specific antibodies, adenosine A_2A receptors were immunocytochemically localized to both the magnocellular neurons and astrocytes of the SON. Ecto-5'nucleotidase, an enzyme involved in the metabolism of ATP to adenosine, was also localized to astrocytes of the SON. These results demonstrate that adenosine acting at A_2A receptors can enhance the excitability of SON neurons and modulate transmitter release from glutamatergic afferents projecting to the nucleus. We suggest that adenosine A_2A receptors may function in neuroendocrine regulation through both direct neuronal mechanisms and via actions involving glia.

magnocellular; astrocytes; oxytocin; vasopressin; neuroendocrine; glutamate; immunocytochemistry; 5'nucleotidase

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