Ketamine Inhibits ATP-Evoked Exocytotic Release of Brain-Derived Neurotrophic Factor from Vesicles in Cultured Rat Astrocytes.

Abstract

In the brain, astrocytes signal to neighboring cells via regulated exocytotic release of gliosignaling molecules, such as brain-derived neurotrophic factor (BDNF). Recent studies uncovered a role of ketamine, an anesthetic and antidepressant, in the regulation of BDNF expression and in the disruption of astrocytic Ca2+ signaling, but it is unclear whether it affects astroglial BDNF release. We investigated whether ketamine affects ATP-evoked Ca2+ signaling and exocytotic release of BDNF at the single-vesicle level in cultured rat astrocytes. Cells were transfected with a plasmid encoding preproBDNF tagged with the pH-sensitive fluorescent protein superecliptic pHluorin, (BDNF-pHse) to load vesicles and measure the release of BDNF-pHse when the exocytotic fusion pore opens and alkalinizes the luminal pH. In addition, cell-attached membrane capacitance changes were recorded to monitor unitary vesicle interaction with the plasma membrane. Intracellular Ca2+ activity was monitored with Fluo-4 and confocal microscopy, which was also used to immunocytochemically characterize BDNF-pHse-laden vesicles. As revealed by double-fluorescent micrographs, BDNF-pHse localized to vesicles positive for the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins, vesicle-associated membrane protein 2 (VAMP2), VAMP3, and synaptotagmin IV. Ketamine treatment decreased the number of ATP-evoked BDNF-pHse fusion/secretion events (P < 0.05), the frequency of ATP-evoked transient (P < 0.001) and full-fusion exocytotic (P < 0.05) events, along with a reduction in the ATP-evoked increase in intracellular Ca2+ activity in astrocytes by ~70% (P < 0.001). The results show that ketamine treatment suppresses ATP-triggered vesicle fusion and BDNF secretion by increasing the probability of a narrow fusion pore open state and/or by reducing astrocytic Ca2+ excitability.