Determinants of the G protein-dependent opioid modulation of neuronal calcium channels.

The modulation of a family of cloned neuronal calcium channels by stimulation of a coexpressed mu opioid receptor was studied by transient expression in Xenopus oocytes. Activation of the morphine receptor with the synthetic enkephalin [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAMGO) resulted in a rapid inhibition of alpha1A (by approximately 20%) and alpha1B (by approximately 55%) currents while alpha1C and alpha1E currents were not significantly affected. The opioid-induced effects on alpha1A and alpha1B currents were blocked by pertussis toxin and the GTP analogue guanosine 5'-[beta-thio]diphosphate. Similar to modulation of native calcium currents, DAMGO induced a slowing of the activation kinetics and exhibited a voltage-dependent inhibition that was partially relieved by application of strong depolarizing pulses. alpha1A currents were still inhibited in the absence of coexpressed Ca channel alpha2 and beta subunits, suggesting that the response is mediated by the alpha1 subunit. Furthermore, the sensitivity of alpha1A currents to DAMGO-induced inhibition was increased approximately 3-fold in the absence of a beta subunit. Overall, the results show that the alpha1A (P/Q type) and the alpha1B (N type) calcium channels are selectively modulated by a GTP-binding protein (G protein). The results raise the possibility of competitive interactions between beta subunit and G protein binding to the alpha1 subunit, shifting gating in opposite directions. At presynaptic terminals, the G protein-dependent inhibition may result in decreased synaptic transmission and play a key role in the analgesic effect of opioids and morphine.

Regulation of excitatory transmission at hippocampal synapses by calbindin D28k.

Distinct subpopulations of neurons in the brain contain one or more of the Ca(2+)-binding proteins calbindin D28k, calretinin, and parvalbumin. Although it has been shown that these high-affinity Ca(2+)-binding proteins can increase neuronal Ca2+ buffering capacity, it is not clear which aspects of neuronal physiology they normally regulate. To investigate this problem, we used a recently developed method for expressing calbindin D28k in the somatic and synaptic regions of cultured hippocampal pyramidal neurons. Ninety-six hours after infection with a replication-defective adenovirus containing the calbindin D28k gene, essentially all cultured hippocampal pyramidal neurons robustly expressed calbindin D28k. Our results demonstrate that while calbindin D28k does not alter evoked neurotransmitter release at excitatory pyramidal cell synapses, this protein has a profound effect on synaptic plasticity. In particular, we show that calbindin D28k expression suppresses posttetanic potentiation.