Developmental regulation of CB1-mediated spike-time dependent depression at immature mossy fiber-CA3 synapses

Early in postnatal life, mossy fibres (MF), the axons of granule cells in the dentate gyrus, release GABA which is depolarizing and excitatory. Synaptic currents undergo spike-time dependent long-term depression (STD-LTD) regardless of the temporal order of stimulation (pre versus post and viceversa). Here we show that at P3 but not at P21, STD-LTD, induced by negative pairing, is mediated by endocannabinoids mobilized from the postsynaptic cell during spiking-induced membrane depolarization. By diffusing backward, endocannabinoids activate cannabinoid type-1 (CB1) receptors probably expressed on MF. Thus, STD-LTD was prevented by CB1 receptor antagonists and was absent in CB1-KO mice. Consistent with these data, in situ hybridization experiments revealed detectable level of CB1 mRNA in the granule cell layer at P3 but not at P21. These results indicate that CB1 receptors are transiently expressed on immature MF terminals where they counteract the enhanced neuronal excitability induced by the excitatory action of GABA

Adenosine down-regulates giant depolarizing potentials in the developing rat hippocampus by exerting a negative control on glutamatergic inputs

Adenosine is a widespread neuromodulator that can be directly released in the extracellular space during sustained network activity or can be generated as the breakdown product of adenosine triphosphate ( ATP). Whole cell patch- clamp recordings were performed from CA3 principal cells and interneurons in hippocampal slices obtained from P2 - P7 neonatal rats to study the modulatory effects of adenosine on giant depolarizing potentials ( GDPs) that constitute the hallmark of developmental networks. We found that GDPs were extremely sensitive to the inhibitory action of adenosine ( IC50 = 0.52 mu M). Adenosine also contributed to the depressant effect of ATP as indicated by DPCPX- sensitive changes of ATP- induced reduction of GDP frequency. Similarly, adenosine exerted a strong inhibitory action on spontaneous glutamatergic synaptic events recorded from GABAergic interneurons and on interictal bursts that developed in CA3 principal cells after blockade of gamma- aminobutyric acid type A ( GABA(A)) receptors with bicuculline. All these effects were prevented by DPCPX, indicating the involvement of inhibitory A1 receptors. In contrast, GABAergic synaptic events were not changed by adenosine. Consistent with the endogenous role of adenosine on network activity, DPCPX per se increased the frequency of GDPs, interictal bursts, and spontaneous glutamatergic synaptic events recorded from GABAergic interneurons. Moreover, the adenosine transport inhibitor NBTI and the adenosine deaminase blocker EHNA decreased the frequency of GDPs, thus providing further evidence that endogenous adenosine exerts a powerful control on GDP generation. We conclude that, in the neonatal rat hippocampus, the inhibitory action of adenosine on GDPs arises from the negative control of glutamatergic, but not GABAergic, inputs

Presynaptic R-type calcium channels contribute to fast excitatory synaptic transmission in the rat hippocampus

The possibility that R-type calcium channels contribute to fast glutamatergic transmission in the hippocampus has been assessed using low concentrations of NiCl2 and the peptide toxin SNX 482, a selective antagonist of the pore-forming alpha (1E) subunit of R-type calcium channel. EPSPs or EPSCs were recorded in the whole-cell configuration of the patch-clamp technique mainly from CA3 hippocampal neurons. Effects of both NiCl2 and SNX 482 were tested on large (composite) EPSCs evoked by mossy and associative-commissural fiber stimulation. NiCl2 effects were also tested on minimal EPSPs-EPSCs. Both substances reduced the amplitude of EPSPs-EPSCs. This effect was associated with an increase in the number of response failures of minimal EPSPs-EPSCs, an enhancement of the paired-pulse facilitation ratios of both minimal and composite EPSCs, and a reduction of the inverse squared coefficient of variation (CV-2). The reduction of CV-2 was positively correlated with the decrease in EPSC amplitude. The inhibitory effect of NiCl2 was occluded by SNX 482 but not by omega -conotoxin-MVIIC, a broad-spectrum antagonist thought to interact with Nand P/Q-type calcium channels, supporting a specific action of low concentrations of NiCl2 on R-type calcium channels. Together, these observations indicate that both NiCl2 and SNX 482 act at presynaptic sites and block R-type calcium channels with pharmacological properties similar to those encoded by the alpha (1E) gene. These channels are involved in fast glutamatergic transmission at hippocampal synapses