Group 1 metabotropic glutamate receptors (I-mGluRs) are proposed to mediate their effects through phospholipase Cβ dependent formation of the second messenger inositol-1,4,5-trisphosphate (IP3). Subsequent activation of IP3Rs releases intracellular calcium from the ER into the cytosol. Recently however, it has been suggested that I-mGluR - mediated intracellular calcium signals are dependent on activation of ryanodine receptors (RYRs) by cyclic ADP ribose (cADPR) in acutely dissociated rat hippocampal neurons. The current study therefore sought to determine if I-mGluR - mediated calcium signals in cultured rat hippocampal neurons (obtained from Sprague Dawley rats, P3-P5) were also dependent on cADPR activation of RYRs. Intracellular somatic calcium signals were recorded following loading with the calcium sensitive dye, fluo-2 AM (150μM). Experiments were carried out at room temperature with neurons continuously perfused (2ml/min) with a standard saline solution (HBSS). The specific I-mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG; 50μM; 2min) was applied in the absence or presence of antagonists of the cADPR - dependent signalling pathway. Data are expressed as mean ± S.E.M. Application of the cADPR antagonist nicotinamide (5mM) significantly reduced DHPG - evoked [Ca2+]i signals by 35 ± 6% (n= 40; P < 0.05). Similarly, application of the RYR antagonist dantrolene (10μM) significantly reduced DHPG - evoked [Ca2+]i signals by 18 ± 4% (n= 61; P < 0.01). In conclusion these results suggest I-mGluR - evoked [Ca2+]i signals may be mediated, at least in part, via cADPR - dependent activation of RYRs in cultured rat hippocampal neurons.