Neurotransmitter release depends on the expression and organization of various molecules, including calcium (Ca2+) channel and neurotransmitter receptor subtypes, in the presynaptic nerve terminal. Group III metabotropic glutamate receptors (mGluRs), which act as autoreceptors on glutamatergic nerve terminals, inhibit transmitter release largely through inhibiting Ca2+ influx. Millán et al. used Ca2+ imaging of rat brain synaptosomes, in combination with pharmacological and immunofluorescent analysis, to investigate the distribution of mGluRs and showed differential localization of two types of mGluRs with different Ca2+ channel subtypes. The authors used the mGluR inhibitor L(+)-2-amino-4-phosphonobutyrate (L-AP4) to implicate two populations of mGluRs in inhibiting glutamate release after depolarization with potassium chloride. Immunocytochemical analysis with antibodies against mGluRs identified the mGluRs as mGlu4 and mGluR7; and labeling for the synaptic vesicle marker synaptophysin, vesicular glutamate transporters, and a γ-aminobutyric acid-synthetic enzyme indicated that most nerve terminals in the synaptosomal preparation were glutamatergic. Individual fura-2 loaded synaptosomes immobilized on coverslips showed differential responses to L-AP4 inhibition of Ca2+ influx, suggesting segregation of the two subtypes of mGluRs in different populations of nerve terminals. Ca2+ imaging experiments with Ca2+ channel blockers indicated a heterogeneous distribution among different subpopulations of nerve terminals, with one population carrying only N-type channels and another carrying both N- and P/Q-type channels. A comparison of the responses of individual synaptosomes to Ca2+ channel blockers and L-AP4 indicated that mGluRs with low affinity for L-AP4 (mGluR7) were largely associated with N-type channels, whereas mGluRs with high affinity for L-AP4 (mGluR4) were mostly found in the synaptosomes containing both N- and P/Q-type channels.
C. Millán, R. Luján, R. Shigemoto, J. Sánchez-Prieto, Subtype-specific expression of group III metabotropic glutamate receptors and Ca2+ channels in single nerve terminals. J. Biol. Chem. 277, 47796-47803 (2002). [Abstract] [Full Text]