The astrocyte Ca²⁺ is crucial in the astrocyte-neuron comunication. Cannabinoid receptors (CBR) play key roles in brain physiology but their expression and function in astrocytes remain unknown. We investigated the expression of cannnabinoid receptors by astrocytes in situ and their involvement in the neuron-astrocyte communication. We used electrophysiological and Ca²⁺ imaging techniques in mice brain slices.

We found:

1) Local application of CBR agonists increased the astrocyte Ca²⁺ levels. The Ca²⁺ elevations were insensitive to glutamatergic, GABAergic, cholinergic, and purinergic antagonist receptors but were abolished by the CB1 receptor antagonist AM251. Furthermore, Ca²⁺ elevations were not observed in slices from CB1R-/- transgenic mice. Therefore, the observed responses were mediated by CB1R activation in astrocytes.

2) Cannabinoid-induced astrocyte Ca²⁺ elevations were unaffected by pertussis toxin but were abolished by PLC inhibitors and thapsigargin.

3) Endocannabinoids released by depolarization of pyramidal neurons modulated Ca²⁺ signal. These effects were abolished by AM251.

4) Endocannabinoid-mediated astrocyte Ca²⁺ elevations stimulate the release of glutamate from astrocytes, evoking NMDA receptor-dependent slow inward currents (SICs) in adjacent neurons.

We conclude:

1. Hippocampal astrocytes in situ express functional CB1Rs.

2. CB1 receptors in astrocytes are activated by endocannabinoids released from pyramidal neurons, increasing the Ca²⁺ levels in astrocytes.

3. Present results show a new form of neuron-astrocyte communication based on endocannabinoid signalling, and suggest a new endocannabinoid-glutamate signalling pathway, where astrocytes serve as a bridge for interneuronal communication.

Supported by: Ministerio de Educación y Ciencia (BFU2004-00448) and CAM (200620M083), Spain. MN is a FPI-MEC predoctoral fellow.