Effects of Hemodynamic Response Function Selection on Rat fMRI Statistical Analyses

The selection of the appropriate hemodynamic response function (HRF) for signal modeling in functional magnetic resonance imaging (fMRI) is important. Although the use of the boxcar-shaped hemodynamic response function (BHRF) and canonical hemodynamic response (CHRF) has gained increasing popularity in rodent fMRI studies, whether the selected HRF affects the results of rodent fMRI has not been fully elucidated. Here we investigated the signal change and t-statistic sensitivities of BHRF, CHRF, and impulse response function (IRF). The effect of HRF selection on different tasks was analyzed by using data collected from two groups of rats receiving either 3 mA whisker pad or 3 mA forepaw electrical stimulations (n = 10 for each group). Under whisker pad stimulation with large blood-oxygen-level dependent (BOLD) signal change (4.31 ± 0.42%), BHRF significantly underestimated signal changes (P < 0.001) and t-statistics (P < 0.001) compared with CHRF or IRF. CHRF and IRF did not provide significantly different t-statistics (P > 0.05). Under forepaw stimulation with small BOLD signal change (1.71 ± 0.34%), different HRFs provided insignificantly different t-statistics (P > 0.05). Therefore, the selected HRF can influence data analysis in rodent fMRI experiments with large BOLD responses but not in those with small BOLD responses

How Energy Metabolism Supports Cerebral Function: Insights from 13C Magnetic Resonance Studies In vivo

Cerebral function is associated with exceptionally high metabolic activity, and requires continuous supply of oxygen and nutrients from the blood stream. Since the mid-20th century the idea that brain energy metabolism is coupled to neuronal activity has emerged, and a number of studies supported this hypothesis. Moreover, brain energy metabolism was demonstrated to be compartmentalized in neurons and astrocytes, and astrocytic glycolysis was proposed to serve the energetic demands of glutamatergic activity. Shedding light on the role of astrocytes in brain metabolism, the earlier picture of astrocytes being restricted to a scaffold-associated function in the brain is now out of date. With the development and optimization of non-invasive techniques, such as nuclear magnetic resonance spectroscopy (MRS), several groups have worked on assessing cerebral metabolism in vivo. In this context, 1H MRS has allowed the measurements of energy metabolism-related compounds, whose concentrations can vary under different brain activation states. 1H-[13C] MRS, i.e. indirect detection of signals from 13C-coupled 1H, together with infusion of 13C-enriched glucose has provided insights into the coupling between neurotransmission and glucose oxidation. Although these techniques tackle the coupling between neuronal activity and metabolism, they lack chemical specificity and fail in providing information on neuronal and glial metabolic pathways underlying those processes. Currently, the improvement of detection modalities (i.e. direct detection of 13C isotopomers), the progress in building adequate mathematical models along with the increase in magnetic field strength now available, render possible detailed compartmentalized metabolic flux characterization. In particular, direct 13C MRS offers more detailed dataset acquisitions and provide information on metabolic interactions between neurons and astrocytes, and their role in supporting neurotransmission. Here we review state-of-the-art MR methods to study brain function and metabolism in vivo, and their contribution to the current understanding of how astrocytic energy metabolism supports glutamatergic activity and cerebral function. In this context, recent data suggests that astrocytic metabolism has been underestimated. Namely, the rate of oxidative metabolism in astrocytes is about half of that in neurons, and it can increase as much as the rate of neuronal metabolism in response to somatosensory stimulation

Characterization of sustained BOLD activation in the rat barrel cortex and neurochemical consequences

To date, only a couple of functional MR spectroscopy (fMRS) studies were conducted in rats. Due to the low temporal resolution of (1)H MRS techniques, prolonged stimulation paradigms are necessary for investigating the metabolic outcome in the rat brain during functional challenge. However, sustained activation of cortical areas is usually difficult to obtain due to neural adaptation. Anesthesia, habituation, high variability of the basal state metabolite concentrations as well as low concentrations of the metabolites of interest such as lactate (Lac), glucose (Glc) or γ-aminobutyric acid (GABA) and small expected changes of metabolite concentrations need to be addressed. In the present study, the rat barrel cortex was reliably and reproducibly activated through sustained trigeminal nerve (TGN) stimulation. In addition, TGN stimulation induced significant positive changes in lactate (+1.01μmol/g, p<0.008) and glutamate (+0.92μmol/g, p<0.02) and significant negative aspartate changes (-0.63μmol/g, p<0.004) using functional (1)H MRS at 9.4T in agreement with previous changes observed in human fMRS studies. Finally, for the first time, the dynamics of lactate, glucose, aspartate and glutamate concentrations during sustained somatosensory activation in rats using fMRS were assessed. These results allow demonstrating the feasibility of fMRS measurements during prolonged barrel cortex activation in rats

Neuro-behavioural impact of changes in hippocampal neural activity

Hippocampal metabolic hyperactivity and neural disinhibition, i.e. reduced GABAergic inhibition, have been associated with schizophrenia, although a causal link between disinhibition and metabolic hyperactivity remains to be demonstrated. Regional neural disinhibition might also disrupt neural activation in projection sites, such as the prefrontal cortex and striatum, which may contribute to cognitive impairments and positive symptoms characteristic of schizophrenia. To further examine the brain-wide impact of hippocampal disinhibition and the associated behavioural and cognitive changes, we combined ventral hippocampal infusion of the GABA-A antagonist picrotoxin with translational neural imaging and behavioural methods in rats. First, we used a conditioned emotional response paradigm to assess the impact of hippocampal disinhibition on aversive conditioning and salience modulation in the form of latent inhibition (chapter 2), both of which have been reported to be disrupted in schizophrenia. These experiments demonstrated hippocampal disinhibition caused disrupted cue and contextual fear conditioning, whilst we found no evidence that hippocampal disinhibition affects salience modulation as reflected by latent inhibition of fear conditioning. The disruption of fear conditioning resembles aversive conditioning deficits reported in schizophrenia and may reflect disruption of neural processing at hippocampal projection sites. Second, we used SPECT imaging to map changes in brain-wide activation patterns caused by hippocampal GABA dysfunction (chapter 3). SPECT experiments revealed increased neural activation around the infusion site in the ventral hippocampus, resembling metabolic hippocampal hyperactivity consistently reported in schizophrenia. In contrast, activation in the dorsal hippocampus was significantly reduced. This resembles the finding of anterior hippocampal hyperactivity coupled with reduced posterior hippocampal activation in patients with schizophrenia. Hippocampal disinhibition also caused marked extra-hippocampal activation changes in neocortical and subcortical sites, including sites implicated in fear learning and anxiety such as the medial prefrontal cortex (mPFC), septum, lateral hypothalamus and extended amygdala which may contribute to the disruption of fear conditioning demonstrated in chapter 2. Importantly, increased activation in the mPFC corresponds with previously reported prefrontal-dependent attentional deficits caused by hippocampal disinhibition. Third, to complement these findings we used magnetic resonance spectroscopy (MRS) to determine the effects of hippocampal disinhibition on neuro-metabolites within the mPFC (chapter 4). Using MRS, we demonstrated that hippocampal disinhibition causes metabolic changes in the mPFC, reflected by increased myo-inositol and reduced GABA concentrations. Overall, our results demonstrate ventral hippocampal disinhibition causes regional metabolic hyperactivity, supporting a causal role between GABA dysfunction and increased anterior hippocampal activity. In addition, hippocampal disinhibition causes activation and metabolic changes at distal sites, which may contribute to clinically relevant behavioural deficits, including impaired aversive conditioning, as demonstrated in our behavioural studies

Neuro-behavioural impact of changes in hippocampal neural activity

Hippocampal metabolic hyperactivity and neural disinhibition, i.e. reduced GABAergic inhibition, have been associated with schizophrenia, although a causal link between disinhibition and metabolic hyperactivity remains to be demonstrated. Regional neural disinhibition might also disrupt neural activation in projection sites, such as the prefrontal cortex and striatum, which may contribute to cognitive impairments and positive symptoms characteristic of schizophrenia. To further examine the brain-wide impact of hippocampal disinhibition and the associated behavioural and cognitive changes, we combined ventral hippocampal infusion of the GABA-A antagonist picrotoxin with translational neural imaging and behavioural methods in rats. First, we used a conditioned emotional response paradigm to assess the impact of hippocampal disinhibition on aversive conditioning and salience modulation in the form of latent inhibition (chapter 2), both of which have been reported to be disrupted in schizophrenia. These experiments demonstrated hippocampal disinhibition caused disrupted cue and contextual fear conditioning, whilst we found no evidence that hippocampal disinhibition affects salience modulation as reflected by latent inhibition of fear conditioning. The disruption of fear conditioning resembles aversive conditioning deficits reported in schizophrenia and may reflect disruption of neural processing at hippocampal projection sites. Second, we used SPECT imaging to map changes in brain-wide activation patterns caused by hippocampal GABA dysfunction (chapter 3). SPECT experiments revealed increased neural activation around the infusion site in the ventral hippocampus, resembling metabolic hippocampal hyperactivity consistently reported in schizophrenia. In contrast, activation in the dorsal hippocampus was significantly reduced. This resembles the finding of anterior hippocampal hyperactivity coupled with reduced posterior hippocampal activation in patients with schizophrenia. Hippocampal disinhibition also caused marked extra-hippocampal activation changes in neocortical and subcortical sites, including sites implicated in fear learning and anxiety such as the medial prefrontal cortex (mPFC), septum, lateral hypothalamus and extended amygdala which may contribute to the disruption of fear conditioning demonstrated in chapter 2. Importantly, increased activation in the mPFC corresponds with previously reported prefrontal-dependent attentional deficits caused by hippocampal disinhibition. Third, to complement these findings we used magnetic resonance spectroscopy (MRS) to determine the effects of hippocampal disinhibition on neuro-metabolites within the mPFC (chapter 4). Using MRS, we demonstrated that hippocampal disinhibition causes metabolic changes in the mPFC, reflected by increased myo-inositol and reduced GABA concentrations. Overall, our results demonstrate ventral hippocampal disinhibition causes regional metabolic hyperactivity, supporting a causal role between GABA dysfunction and increased anterior hippocampal activity. In addition, hippocampal disinhibition causes activation and metabolic changes at distal sites, which may contribute to clinically relevant behavioural deficits, including impaired aversive conditioning, as demonstrated in our behavioural studies