Ca2+ permeability and kinetics of glutamate receptors in rat medial habenula neurones: implications for purinergic transmission in this nucleus

1. We have previously investigated P2X receptor-mediated synaptic currents in medial habenula neurones and shown that they can be calcium permeable. W e now investigate the receptor properties of glutamate, the other, more abu ndant excitatory transmitter, to determine its receptor subtypes and their relative calcium permeability. This may have implications for the physiolog ical role of the P2X receptors which mediate synaptic currents. 2. Using fast application of ATP, L-glutamate or kainate to nucleated patch es, glutamate receptors were determined to be of the AMPA subtype but no fu nctional P2X receptors were detected. 3. The deactivation and desensitization rates of the AMPA channel were dete rmined to have time constants of 1.77 +/- 0.21 ms (n = 10) and 4.01 +/- 0.8 5 ms (n = 9) at -60 mV, respectively. AMPA receptors recovered from desensi tization with two exponential components with time constants of 21.08 +/- 2 .95 and 233.60 +/- 51.1. ms (n = 3). None of the deactivation or desensitiz ation properties of the GluR channels depended on membrane potential. 4. The current-voltage relations:hip under different ionic conditions revea led that the GluR channel was equally permeable to Cs+ and Na+ but relative ly impermeable to Ca2+ (P-Ca/P-Cs = 0.13, n = 6). 5. For both synaptic currents and somatic currents activated by fast applic ation of L-glutamate to nucleated patches, decay time constants were simila r at +/-60 mV in the presence of Mg2+ ions. Thus GluR channels appear to be of the AMPA subtype and not the NMDA subtype. 6. Thus, under the conditions of this study, neurones of the medial habenul a lack functional NMDA receptors and possess AMPA receptors that have low p ermeability to Ca2+. We conclude that the P2X receptor-mediated,ted synapti c currents are the only calcium-permeable fast-transmitter gated currents i n these neurones which may be important for their physiological function.