Differential effects of NMDA and AMPA glutamate receptors on functional magnetic resonance imaging signals and evoked neuronal activity during forepaw stimulation of the rat

Abstract

Most of the currently used methods for functional brain imaging do not visualize neuronal activity directly but rather rely on the elicited hemodynamic and/or metabolic responses. Glutamate, the major excitatory neurotransmitter, plays an important role in the neurovascular/neurometabolic coupling, but the specific mechanisms are still poorly understood. To investigate the role of the two major ionotropic glutamate receptors [NMDA receptors (NMDA-Rs) and AMPA receptors (AMPA-Rs)] for the generation of functional magnetic resonance imaging (fMRI) signals, we used fMRI [measurements of blood oxygenation level-dependent (BOLD), perfusion-weighted imaging (PWI), and cerebral blood volume (CBV)] together with recordings of somatosensory evoked potentials (SEPs) during electrical forepaw stimulation in the alpha-chloralose anesthetized rat. Intravenous injection of the NMDA-R antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate] (0.06 mg/kg plus 3.6 microg x kg(-1) x h(-1)) significantly decreased BOLD (-51 +/- 19%; n = 5) and PWI (-57 +/- 26%; n = 5) responses but reduced the SEPs only mildly (approximately -10%). Systemic application of the AMPA-R antagonist GYKI-53655 [1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine] significantly decreased both the hemodynamic response (BOLD, -49 +/- 13 and -65 +/- 15%; PWI, -22 +/- 48 and -68 +/- 4% for 5 and 7 mg/kg, i.v., respectively; CBV, -80 +/- 7% for 7 mg/kg; n = 4) and the SEPs (up to -60%). These data indicate that the interaction of glutamate with its postsynaptic and/or glial receptors is necessary for the generation of blood flow and BOLD responses and illustrate the differential role of NMDA-Rs and AMPA-Rs in the signaling chain leading from increased neuronal activity to the hemodynamic response in the somatosensory cortex.

Figure 1.

A, Representative activation maps calculated from BOLD and PWI signals after electrical forepaw stimulation. Forepaw stimulation produced robust and localized hemodynamic changes over primary somatosensory cortex. The color bar indicates the statistical level of significance. B, Time courses of BOLD and PWI signal intensity changes in the control situation without drug application normalized to the baseline signal intensity between the first five stimulation blocks. Stable and robust signal changes were obtained at every stimulus epoch. Stimulation blocks are indicated by the gray bars.

Figure 2.

A, Typical time courses of BOLD (top row) and PWI (bottom row) percentage signal changes after administration of MK-801 (left) and GYKI-53655 (right), normalized to the baseline between the four stimulation blocks preceding the application of glutamate antagonists. Administration of the NMDA-R antagonist (left) after the fifth stimulation epoch (arrows) produced a delayed yet irreversible reduction in the BOLD and PWI responses to stimulation. Conversely, administration of the AMPA-R antagonist (right, arrows) produced immediate and significant reduction in BOLD and PWI signals, but such reduction was reversed within 30 min of administration. Stimulation blocks are marked by gray bars. B, Average time courses of BOLD (top row) and PWI (bottom row) percentage signal changes after administration of MK-801 (left) and GYKI-53655 (right). MK-801 produced a strong reduction of BOLD (−51%) and PWI (−57%) responses. Administration of the AMPA-R antagonist, conversely, produced dose-dependent reductions in BOLD (−49 and −68%) and PWI (−22 and −68%) after 5 and 7 mg/kg, respectively, which completely recovered within 30 min of administration. Error bars indicate 1 SD.

Figure 3.

Time course of T₂*-weighted MRI signal intensity before (first 5 blocks) and after (arrow) intravenous administration of Sinerem, an ultra-small super-paramagnetic iron oxide particles intravascular contrast agent. The blood pool contrast agent causes a general signal decrease in T₂*-weighted images because of increased susceptibility differences. Note that the initially positive signal changes (BOLD) become negative after Sinerem because the susceptibility differences are augmented by the stimulation-induced CBV increases. Five stimulus epochs after Sinerem, GYKI-53655 was administered (2nd arrow), causing an immediate decrease in the CBV-weighted signal that recovered partially after approximately six blocks.

Figure 4.

Representative SEPs in the control phase (black curve) and after MK-801 application (gray curve), identifying the three major components of the SEP: the P1, N1, and P2 peaks.

Figure 5.

Summary of the reduction in amplitude of the individual SEP components and the BOLD and PWI responses attributable to administration of MK-801 and GYKI-53655. Values are expressed in percentage signal decrease (mean ± SD) relative to the control situation (average of 5 stimulation blocks before the injection of drugs). MK-801 produced a small (approximately −10%), uniform yet significant decrease in amplitude of all three SEP components (p < 0.01) but produced robust (approximately −52%) and irreversible decreases in the BOLD and PWI responses (p < 0.01, paired t test); the SEP reduction was significantly smaller than the reduction of the BOLD and PWI response (p < 0.001). GYKI-53655 produced robust, dose-dependent yet reversible decreases in both the SEP amplitudes and the hemodynamic responses to stimulation. Unless stated otherwise, repeated-measures two-way ANOVA with Holm–Sidak post hoc test was used. *p < 0.05, ** p < 0.01, ***p < 0.001 compared with control.