Ikuhiro Kida

Simultaneous activation of mouse main and accessory olfactory bulbs by odors or pheromones

It is generally believed that the main olfactory system processes common odors and the accessory olfactory system is specifically for pheromones. The potential for these two systems to respond simultaneously to the same stimuli has not been fully explored due to methodological limitations. Here we examine this phenomenon using high‐resolution functional magnetic resonance imaging (fMRI) to reveal simultaneously the responses in the main (MOB) and accessory olfactory bulbs (AOB) to odors and pheromones. Common odorants elicited strong signals in the MOB and weak signals in the AOB. 2‐Heptanone, a known mouse pheromone, elicited strong signals in both the MOB and AOB. Urine odor, a complicated mixture of pheromones and odorants, elicited significant signals in limited regions of the MOB and large regions of the AOB. The fMRI results demonstrate that both the main and the accessory olfactory systems may respond to volatile compounds but with different selectivity, suggesting a greater integration of the two olfactory pathways than traditionally believed. J. Comp. Neurol. 489:491–500, 2005.

Odor maps of aldehydes and esters revealed by functional MRI in the glomerular layer of the mouse olfactory bulb

Odorant identity is believed to be encoded in the olfactory bulb (OB) by glomerular activity patterns. It has not yet been possible to visualize and compare entire patterns for different odorants in the same animal because of technical limitations. For this purpose we used high-resolution functional MRI at 7 T, combined with glomerular-layer flat maps, to reveal responses to aliphatic homologues in the mouse OB. These odorants elicited reproducible patterns in the OB, with the medial and lateral regions containing the most intense signals. Unexpectedly, in view of the symmetrical projections of olfactory receptor neurons to medial and lateral glomeruli, the activity patterns in these regions were asymmetrical. The highly activated medial and lateral areas were shared by homologous members, generating a conserved “family signature” for a homologous series. The moderately active areas, including the dorsal region that has been extensively studied by optical imaging, were more sensitive to the length of the carbon chain, producing more subtle features of individual members and different changing trends among homologues. The global mapping with functional MRI not only extended previous studies but also revealed additional rules for representation of homologues in the OB. Insights into possible relations between the functional patterns, molecular projections, and odor perception may now be obtained based on the global from the olfactory epithelium to the OB glomerular activity patterns.

High-Resolution CMRO2 Mapping in Rat Cortex: A Multiparametric Approach to Calibration of BOLD Image Contrast at 7 Tesla:

The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) method, which is sensitive to vascular paramagnetic deoxyhemoglobin, is dependent on regional values of cerebral metabolic rate of oxygen utilization (CMRO2), blood flow (CBF), and volume (CBV). Induced changes in deoxyhemoglobin function as an endogenous contrast agent, which in turn affects the transverse relaxation rates of tissue water that can be measured by gradient-echo and spin-echo sequences in BOLD fMRI. The purpose here was to define the quantitative relation between BOLD signal change and underlying physiologic parameters. To this end, magnetic resonance imaging and spectroscopy methods were used to measure CBF, CMRO2, CBV, and relaxation rates (with gradient-echo and spin-echo sequences) at 7 Tesla in rat sensorimotor cortex, where cerebral activity was altered pharmacologically within the autoregulatory range. The changes in tissue transverse relaxation rates were negatively and linearly correlated with changes in CBF, CMRO2, and CBV. The multiparametric measurements revealed that CBF and CMRO2 are the dominant physiologic parameters that modulate the BOLD fMRI signal, where the ratios of (ΔCMRO2/CMRO2)/(ΔCBF/CBF) and (ΔCBV/CBV)/(ΔCBF/CBF) were 0.86 ± 0.02 and 0.03 ± 0.02, respectively. The calibrated BOLD signals (spatial resolution of 48 μL) from gradient-echo and spin-echo sequences were used to predict changes in CMRO2 using measured changes in CBF, CBV, and transverse relaxation rates. The excellent agreement between measured and predicted values for changes in CMRO2 provides experimental support of the current theory of the BOLD phenomenon. In gradient-echo sequences, BOLD contrast is affected by reversible processes such as static inhomogeneities and slow diffusion, whereas in spin-echo sequences these effects are refocused and are mainly altered by extravascular spin diffusion. This study provides steps by which multiparametric MRI measurements can be used to obtain high-spatial resolution CMRO2 maps.

Dynamics of changes in blood flow, volume, and oxygenation: implications for dynamic functional magnetic resonance imaging calibration

Changes in cerebral blood flow (CBF), volume (CBV), and oxygenation (blood-oxygenation level dependent (BOLD)) during functional activation are important for calculating changes in cerebral metabolic rate of oxygen consumption (CMRo2) from calibrated functional MRI (fMRI). An important part of this process is the CBF/CBV relationship, which is signified by a power-law parameter: γ = ln (1 + ΔCBV/CBV)ln (1 + ΔCBF/CBF). Because of difficulty in measuring CBF and CBV with MRI, the value of γ is therefore assumed to be ~0.4 from a prior primate study under hypercapnia. For dynamic fMRI calibration, it is important to know if the value of γ varies after stimulation onset. We measured transient relationships between ΔCBF, ΔCBV, and ΔBOLD by multimodal MRI with temporal resolution of 500 ms (at 7.0 T) from the rat somatosensory cortex during forepaw stimulation, where the stimulus duration ranged from 4 to 32 secs. Changes in CBF and BOLD were measured before the administration of the contrast agent for CBV measurements in the same subjects. We observed that the relationship between ΔCBF and ΔCBV varied dynamically from stimulation onset for all stimulus durations. Typically after stimulation onset and at the peak or plateau of the ΔCBF, the value of γ ranged between 0.1 and 0.2. However, after stimulation offset, the value of γ increased to 0.4 primarily because of rapid and slow decays in ΔCBF and ΔCBV, respectively. These results suggest caution in using dynamic measurements of ΔCBF and ΔBOLD required for calculating ΔCMRo2 for functional stimulation, when either ΔCBV has not been accurately measured or a fixed value of γ during hypercapnia perturbation is used.

Dependence of Oxygen Delivery on Blood Flow in Rat Brain: A 7 Tesla Nuclear Magnetic Resonance Study

Magnetic resonance imaging (MRI) and spectroscopy (MRS) were used at a magnetic field strength of 7 T to measure CBF and CMRO2 in the sensorimotor cortex of mature rats at different levels of cortical activity. In rats maintained on morphine anesthesia, transitions to lower activity and higher activity states were produced by administration of pentobarbital and nicotine, respectively. Under basal conditions of morphine sulfate anesthesia, CBF was 0.75 ± 0.09 mL · g−1 · min−1 and CMRO2 was 3.15 ± 0.18 μmol · g−1 · min−1. Administration of sodium pentobarbital reduced CBF and CMRO2 by 66% ± 16% and 61% ± 6%, respectively (i.e., “deactivation”). In contrast, administration of nicotine hydrogen tartrate increased CBF and CMRO2 by 41% ± 5% and 30% ± 3%, respectively (i.e., “activation”). The resting values of CBF and CMRO2 for α-chloralose anesthetized rats were 0.40 ± 0.09 mL · g−1 · min−1 and 1.51 ± 0.06 μmol · g−1 · min−1, respectively. Upon forepaw stimulation, CBF and CMRO2 were focally increased by 34% ± 10% and 26% ± 12%, respectively, above the resting nonanesthetized values (i.e., “activation”). Incremental changes in CBF and CMRO2, when expressed as a percentage change for “deactivation” and “activation” from the respective control conditions, were linear (R2 = 0.997) over the entire range examined with the global and local perturbations. This tight correlation for cerebral oxygen delivery in vivo is supported by a recent model where the consequence of a changing effective diffusivity of the capillary bed for oxygen, D, has been hypothetically shown to be linked to alterations in CMRO2 and CBF. This assumed functional characteristic of the capillary bed can be theoretically assessed by the ratio of fractional changes in D with respect to changes in CBF, signified by Ω. A value 0.81 ± 0.23 was calculated for Ω with the in vivo data presented here, which in turn corresponds to a supposition that the effective oxygen diffusivity of the capillary bed is not constant but presumably varies to meet local requirements in oxygen demand in a similar manner with both “deactivation” and “activation.”

Mapping at glomerular resolution: fMRI of rat olfactory bulb.

The rat olfactory bulb contains ∼2000 functional units called glomeruli which are used to recognize specific characteristics of odorants. Activity localization of individual glomerulae (∼0.002 μL) has important consequences for understanding mechanisms in olfactory information encoding. High‐resolution functional MRI (fMRI) data from the rat olfactory bulb are presented using the blood oxygenation level dependent (BOLD) method at 7 T. Either individual or clusters of fMRI voxels suggestive of activity in the olfactory nerve and glomerular layers were reproducibly detected with repeated 2‐min exposures of iso‐amyl acetate at spatial resolution of 0.001–0.003 μL. The importance of glomerular clustering for olfaction and the implications of BOLD mapping with even higher spatial resolution (i.e., ≪0.001 μL voxels) are discussed. High‐resolution in vivo mapping of the rat olfactory bulb with fMRI at high magnetic field promises to provide novel data for understanding olfaction. Magn Reson Med 48:570–576, 2002.

Lamotrigine suppresses neurophysiological responses to somatosensory stimulation in the rodent

Neurotransmitter release and voltage-gated ion channel activity in excitatory neurons are critical for understanding and interpreting neuroimaging signals. Couplings between changes in neural activity and energetic/vascular responses are assumed for interpretation of neuroimaging signals. To investigate involvement of neural events to neuroenergetic/neurovascular responses, we conducted multi-modal magnetic resonance imaging (MRI) measurements (at 7.0 T) and electrophysiological recordings (with high impedance microelectrodes) for local field potential (LFP) and spiking frequency (nu) in alpha-chloralose-anesthetized rats. The rats underwent forepaw stimulation before and after treatment of lamotrigine, a neuronal voltage-gated ion channel blocker and glutamate release inhibitor. Multi-modal MRI measurements of cerebral blood flow (CBF) and blood oxygenation level-dependent (BOLD) signal were combined to estimate changes in cerebral metabolic rate of oxygen consumption (CMRo2). Lamotrigine did not appreciably affect values of nu, CBF, and CMRo2 in the resting state. After lamotrigine treatment, evoked changes in LFP and nu were attenuated, which were consistent with commensurate declines in deltaCBF and deltaCMRo2. While number of evoked BOLD-activated voxels was considerably reduced with lamotrigine, intensities of voxels in middle cortical layers were affected to a lesser degree by lamotrigine. The results suggest that lamotrigine suppresses evoked neurophysiological (i.e., neural/energetic/vascular) responses, both in terms of volume of tissue activated and degree of activation in the foci. Since lamotrigine affects evoked responses but not the basal signals, it can be suggested that glutamate release and activity of voltage-gated ion channels are essential for initiating evoked energetic/vascular responses, and thereby important for interpretation of incremental changes in neuroimaging signal.

Inhibition of Voltage-Dependent Sodium Channels Suppresses the Functional Magnetic Resonance Imaging Response to Forepaw Somatosensory Activation in the Rodent

Results of recent studies suggest that the glutamate–glutamine neurotransmitter cycle between neurons and astrocytes plays a major role in the generation of the functional imaging signal. In the current study, the authors tested the hypothesis that activation of voltage-dependent Na+ channels is involved in the blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) responses during somatosensory activation. The BOLD fMRI and cerebral blood flow (CBF) experiments were performed at 7 Tesla on α-chloralose–anesthetized rats undergoing forepaw stimulation before and for successive times after application of lamotrigine, a neuronal voltage-dependent Na+ channel blocker and glutamate release inhibitor. The BOLD fMRI signal changes in response to forepaw stimulation decreased in a time-dependent manner from 6.7% ± 0.7% before lamotrigine injection to 3.0% ± 2.5% between 60 and 105 minutes after lamotrigine treatment. After lamotrigine treatment, the fractional increase in CBF during forepaw stimulation was an order of magnitude less than that observed before the treatment. Lamotrigine had no effect on baseline CBF in the somatosensory cortex in the absence of stimulation. These results strongly suggest that activation of voltage-dependent Na+ channels is involved in the BOLD fMRI responses during somatosensory activation of the rat cortex.

Adaptation in the rodent olfactory bulb measured by fMRI

Effective evaluation of the odor environment necessitates the ability to attenuate responses to potent background odors in favor of novel and less robust stimuli. Olfactory receptor neuron studies suggest that some of this adaptation takes place in the primary sensory neurons, but the more extensive adaptation seen in higher cortical areas implies the involvement of additional neural mechanisms. At 7.0 T, high‐resolution fMRI was used to assess the response of the rodent olfactory bulb, the most peripheral cortical structure involved in olfactory processing, to a variety of odor stimuli. The results suggest that there are additional regulatory mechanisms in the olfactory bulb that result in greater adaptation in deeper areas than that seen in sensory receptors alone and that the resultant adaptation is positively affected by increasing stimulus duration and concentration and decreasing recovery time. The implications of these findings for the integration of peripheral input with perception are discussed. Magn Reson Med 54:443–448, 2005.

Reproducibility of odor maps by fMRI in rodents.

The interactions of volatile odorants with the approximately 1000 types of olfactory receptor neurons in the olfactory mucosa are represented in the olfactory bulb by glomerular spatial activity maps. If these spatial maps underlie the perceptual identification of odorants then, for a given organism, they must be both specific and reproducible. However, this intra-organism reproducibility need not be present between organisms because genetic and developmental studies of olfactory bulb wiring suggest that there is substantial variation between the glomerular arrangements of closely related organisms and even between the two bulbs in a given animal. The ability of functional MRI (fMRI) to record responses of the entire rodent olfactory bulb repeatedly within the same subject has made it possible to assess the reproducibility of odor-induced spatial activity maps both within and between subjects exposed to equivalent stimuli. For a range of odorants, representing multiple chemical classes, a level of fMRI reproducibility (at 7.0 T and 9.4 T) comparable or superior to other cortical regions was demonstrated. While the responses of different bulbs to the same odorant could be localized within the same broad regions of the glomerular sheet, the precise magnitude and topology of the response within those regions were both often highly variable. These results demonstrate the robustness of high-field fMRI as a tool for assaying olfactory bulb function and provide evidence that equivalent perceptual outcomes may arise from divergent neural substrates.