Investigating neural–hemodynamic coupling and the hemodynamic response function in the awake rat

Abstract

An understanding of the relationship between changes in neural activity and the accompanying hemodynamic response is crucial for accurate interpretation of functional brain imaging data and in particular the blood oxygen level-dependent (BOLD) fMRI signal. Much physiological research investigating this topic uses anesthetized animal preparations, and yet, the effects of anesthesia upon the neural and hemodynamic responses measured in such studies are not well understood. In this study, we electrically stimulated the whisker pad of both awake and urethane anesthetized rats at frequencies of 1–40 Hz. Evoked field potential responses were recorded using electrodes implanted into the contralateral barrel cortex. Changes in hemoglobin oxygenation and concentration were measured using optical imaging spectroscopy, and cerebral blood flow changes were measured using laser Doppler flowmetry. A linear neural–hemodynamic coupling relationship was found in the awake but not the anesthetized animal preparation. Over the range of stimulation conditions studied, hemodynamic response magnitude increased monotonically with summed neural activity in awake, but not in anesthetized, animals. Additionally, the temporal structure of the hemodynamic response function was different in awake compared to anesthetized animals. The responses in each case were well approximated by gamma variates, but these were different in terms of mean latency (approximately 2 s awake; 4 s anesthetized) and width (approximately 0.6 s awake; 2.5 s anesthetized). These findings have important implications for research into the intrinsic signals that underpin BOLD fMRI and for biophysical models of cortical hemodynamics and neural–hemodynamic coupling.

Introduction

The predominant functional brain imaging technique of blood oxygen level-dependent (BOLD) fMRI exploits the hemodynamic response of brain tissue that follows changes in neural activity (Logothetis et al., 2001). A detailed understanding of the relationship between changes in neural activity and the hemodynamic response is therefore important in interpreting the signals acquired in BOLD fMRI. A number of recent studies have addressed this issue directly, reporting measures of both neural and hemodynamic responses obtained in identical stimulation conditions (e.g., Ances et al., 2000, Devor et al., 2003, Hewson-Stoate et al., 2005, Jones et al., 2004, Lindauer et al., 1993, Mathiesen et al., 1998, Nakao et al., 2001, Nemoto et al., 2004, Ngai et al., 1999, Nielsen and Lauritzen, 2001, Sheth et al., 2003, Thompson et al., 2003).

A potential limitation of much of this research is that it uses anesthetized animals, since there is evidence to suggest that cerebral hemodynamics, vascular reactivity, and/or cerebral metabolism are different under different anesthetic regimes (Austin et al., 2005, Bonvento et al., 1994, Jones and Diamond, 1995, Kaisti et al., 2003, Lindauer et al., 1993, Linde et al., 1999, Oz et al., 2004). These issues are further complicated by the possibility that anesthesia disrupts the putative mechanisms that relate neural activity changes to the hemodynamic response function. For example, Nakao et al. (2001) discusses how differing findings regarding the role of neuronal nitric oxide in neurovascular coupling may be attributable to the use of anesthesia. Taken together, these findings have important implications for attempts to construct mathematical models relating neural and hemodynamic responses to the fMRI BOLD signal. The aim of research in this area is to refine the interpretation of functional imaging signals recorded from awake humans in terms of underlying neural processes. Unanesthetized animal preparations are therefore a critical counterpart to the anesthetized animal research models which are already used extensively. As such, we have previously reported an awake animal preparation for studying the physiological basis of functional imaging signals (Martin et al., 2002).

A number of studies have demonstrated that hemodynamic responses to physiological stimulation may differ between anesthetized and unanesthetized animal preparations. For example, fMRI studies in rat (Lahti et al., 1998, Lahti et al., 1999, Peeters et al., 2001, Sicard et al., 2003) and optical imaging spectroscopy studies in monkey (Shtoyerman et al., 2000) or rat (Berwick et al., 2002) report significantly larger responses in unanesthetized animals. Using data from optical imaging spectroscopy studies in awake and anesthetized rats, Berwick et al. (2002) estimated fMRI BOLD signal and oxidative metabolism changes in somatosensory cortex following whisker stimulation and found significant differences between the estimates for awake and anesthetized rats. However, the issue of how neural–hemodynamic coupling is affected by anesthetic state remains unaddressed. The effects of anesthetic state upon other features of the hemodynamic response function are also unknown. This includes the relationship between changes in cerebral blood flow (CBF) and cerebral blood volume (CBV), commonly approximated by the expression 1 + ΔCBV = (1 + ΔCBF)^α, where α is a scaling constant (Grubb et al., 1974). Any differences in the temporal structure of the hemodynamic response function attributable to anesthetic state may have important implications for research in this area.

The rodent whisker-to-barrel system has a well-defined topography and in the cortex a highly concomitant blood supply. As such, it has become a popular model for investigation of neurovascular coupling in animals albeit using a range of anesthetic regimes (e.g., Devor et al., 2003 [urethane]; Gerrits et al., 2000 [urethane]; Hewson-Stoate et al., 2005 [urethane]; Martindale et al., 2003 [urethane]; Nakao et al., 2001 [alpha-chloralose]; Nielsen and Lauritzen, 2001 [alpha-chloralose]; Sheth et al., 2003 [enflurane]; Ureshi et al., 2004 [alpha-chloralose]). The research reported here explores the relationship between experimentally obtained measures of cortical neural and hemodynamic responses to whisker stimulation in a fully conscious rat preparation and compares this to data obtained using a urethane anesthetized rat preparation similar to that used in previous studies. hemodynamic responses were recorded using laser Doppler flowmetry (cerebral blood flow changes) and optical imaging spectroscopy (cerebral blood volume and oxygenation changes). The combination of such techniques provides for complete measurement of the hemodynamic response (Dunn et al., 2003, Jones et al., 2001). Recent research indicates that evoked field potentials (EFPs) are most highly correlated with hemodynamic changes (Caesar et al., 2003, Jones et al., 2004, Logothetis et al., 2001), and in this study, these were recorded using electrodes implanted in somatosensory barrel cortex.