Mitral cell presynaptic Ca2+ influx and synaptic transmission in frog amygdala

Dextran-conjugated Ca2+ indicators were injected into the accessory olfactory bulb of frogs in vivo to selectively fill presynaptic terminals of mitral cells at their termination in the ipsilateral amygdala. After one to three days of uptake and transport, the forebrain hemisphere anterior to the rectum was removed and maintained in vitro for simultaneous electrophysiological and optical measurements. Ca2+ influx into these terminals was compared to synaptic transmission between mitral cells and amygdala neurons under conditions of reduced Ca2+ influx resulting from reduced extracellular [Ca2+], blockade of N- and P/Q-type channels, and application of the cholinergic agonist carbachol. Reducing extracellular [Ca2-] had a non-linear effect on release: release was proportional to Ca2+ influx raised to the power of approximate to 3.6, as observed at numerous other synapses. The N-type Ca2+ channel blocker, omega -conotoxin-GVIA (1 muM), blocked 77% of Ca2+ influx and 88% of the postsynaptic held potential. The P/Q-type Ca2+ channel blocker, omega -ogatoxin-IVA (200 nM), blocked 19% of Ca2+ influx and 25% of the postsynaptic field, while the two toxins combined to block 92% of Ca2+ influx and 97% of the postsynaptic field. The relationship between toxin blockade of Ca2+ influx and synaptic transmission was therefore only slightly non-linear; release was proportional to Ca2+ influx raised to the power approximate to 1.4. Carbachol (100 muM) acting via muscarinic receptors had no effect on the afferent volley, but rapidly and reversibly reduced Ca2+ influx through both N- and P/Q-type channels by 51% and postsynaptic responses by 78% i.e. release was proportional to Ca2- raised to the power approximate to2.5. The weak dependence of release on changes in Ca2+ when channel toxins block channels suggests little overlap between Ca2+ microdomains from channels supporting release or substantial segregation of channel subtypes between terminals. The proportionately greater reduction of transmission by muscarinic receptors compared to Ca2- channel toxins suggests that they directly affect the release machinery in addition to reducing Ca2+ influx. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.