Flash Photolysis Reveals a Diversity of Ionotropic Glutamate Receptors on the Mitral Cell Somatodendritic Membrane

Abstract

Lowe, Graeme. Flash photolysis reveals a diversity of ionotropic glutamate receptors on the mitral cell somatodendritic membrane. J Neurophysiol 90: 1737-1746, 2003. First published April 30, 2003 10.1152/jn.00180.2003. It is widely held that the soma and basal dendrites of olfactory bulb mitral cells receive exclusively inhibitory synaptic input from local interneurons. However, the mitral somatodendritic membrane exhibits immunoreactivity for a variety of glutamate receptors, and blocking GABA receptors unmasks mitral cell self-excitation. This excitation is proposed to be mediated either by diffuse spillover of the mitral cells' own released glutamate, or by punctate transmission from glutamate-releasing granule cells. This study examined the pharmacology and kinetics of glutamate sensitivity of mitral cells by flash photolysis of nitroindoline caged glutamates, which facilitate reliable activation of receptors in the synaptic cleft. Wide-field laser uncaging (3.5-ms flash) of approximately 0.5-1 mM glutamate onto the soma activated large currents with fast (3.4-ms rise, 7.5-ms decay) and slow (64-ms rise, Ͼ10-s decay) components. In 100 M APV, slow currents were reduced to 53% of control (257-ms rise, 2-s decay), displayed outward rectification in 1.3 mM Mg 2ϩ , and blocked by 15 M 5,7-dichlorokynurenate. Responses to Շ100 M glutamate were fully antagonized by 100 M APV, consistent with competitive inhibition at high-affinity NMDA receptors. An APV-resistant NMDA receptor was not observed, refuting the punctate transmission model. Fast currents were blocked by 10 M NBQX, boosted 3.28-fold by 100 M cyclothiazide, and resolved into AMPA (40%) and kainate (60%) receptor components by 100 M SYM2206. The results suggest that self-excitation depends on AMPA, kainite, and conventional NMDA autoreceptors on the mitral cell.

Target cell-specific modulation of neuronal activity by astrocytes

“…The presence of such astrocytic functional domains (42) suggests that it is likely that two different types of astrocytes are responsible for SOCs and SICs. Indeed, SICs were not observed in mitral cells, whereas their dendrites are covered by functional glutamate receptors (43), and synchronous SOCs, but not synchronous SOCs and SICs, were detected in paired recordings of mitral and granule cells. However, we cannot rule out the possibility that a single astrocyte releases GABA and glutamate separately at different and spatially distant release sites.…”

Calcium permeable AMPA receptors and autoreceptors in external tufted cells of rat olfactory bulb

“…The reason for this may be that dendritic tufts are deeply embedded in the glomerular neuropil, perhaps enclosed in glial microcompartments (Kasowski et al, 1999). To overcome diffusion barriers, we employed flash photolysis of caged glutamate, a method capable of rapidly and efficiently activating AMPA receptors in olfactory bulb slices (Lowe, 2003). As in the olfactory nerve shock experiments, either dCK or APV were added to the bath to eliminate NMDA receptor currents and isolate the AMPA/kainate current.…”

“…Thus, fast glutamatergic neurotransmission is critical for ET cell circuit operations in both presynaptic and postsynaptic contexts. Determining the specific physiological properties of glutamate receptors expressed by ET cells is an important step towards a better understanding of their function.Ionotropic glutamate receptors are expressed in a diversity of forms (Dingledine et al, 1999), and recent studies have indicated localization of different subtypes in distinct compartments of olfactory bulb neurons (Trombley and Shepherd, 1993;Montague and Greer, 1999;Blakemore and Trombley, 2003;Lowe, 2003;Horning et al, 2004). In the glomerular layer of the bulb, both NMDA and AMPA/kainate receptors have been characterized by immunolabeling (Giustetto et al, 1997;Montague and Greer, 1999;Sassoe-Pognetto et al, 2003) and physiological studies (Ennis et al, 1996;Aroniadou-Anderjaska et al, 1997;Keller et al, 1998;Edwards and Michel, 2003;Murphy et al, 2004).…”

“…Instead, we directly compared flash lamp and laser evoked currents in the same outside-out patches pulled from the somata of CA1 neurons and concluded that the N 2 laser uncaged approximately 30% of the glutamate released by the flash lamp (0.3% -0.6%; 1.5 -3 μM released glutamate; 300 μJ, 4 ns laser pulse). To estimate the glutamate uncaged by the argon laser, we calculated photolysis of MNI-caged glutamate by a Gaussian beam model developed previously (Lowe, 2003). In a region around the cell, ±10 μm above and below the focal plane, at incident power of 0.25 -0.5 mW (5 ms flash), the uncaging was > 97% within the 1/e 2 beam radius (< 2 μm), so initial uncaged glutamate was ~ 1 mM.…”

Two-photon microscope for multisite microphotolysis of caged neurotransmitters in acute brain slices

“…As the primary excitatory neurotransmitter in the mammalian CNS, glutamate ͑Glu͒ derivatives such as 4-methoxy-7-nitroindolinyl-caged L-glutamate ͑MNI-Glu͒ have proven most useful for studying synaptic integration. 2,9 Bathing an acute brain slice in artificial cerebrospinal fluid ͑aCSF͒ containing MNI-Glu has no observable effect. However, application of near-UV excitation rapidly excitation.…”