Optogenetic fMRI reveals distinct, frequency-dependent networks recruited by dorsal and intermediate hippocampus stimulations

Highlights

• Stimulation of intermediate, but not dorsal, hippocampus evokes widespread brain activity.

• Dynamic network engagement varies with stimulation frequency.

• Stimulation of dorsal hippocampus evokes negative BOLD signal in dentate gyrus.

• ofMRI responses predict electrographic and behavioral seizures.

Abstract

Although the connectivity of hippocampal circuits has been extensively studied, the way in which these connections give rise to large-scale dynamic network activity remains unknown. Here, we used optogenetic fMRI to visualize the brain network dynamics evoked by different frequencies of stimulation of two distinct neuronal populations within dorsal and intermediate hippocampus. Stimulation of excitatory cells in intermediate hippocampus caused widespread cortical and subcortical recruitment at high frequencies, whereas stimulation in dorsal hippocampus led to activity primarily restricted to hippocampus across all frequencies tested. Sustained hippocampal responses evoked during high-frequency stimulation of either location predicted seizure-like afterdischarges in video-EEG experiments, while the widespread activation evoked by high-frequency stimulation of intermediate hippocampus predicted behavioral seizures. A negative BOLD signal observed in dentate gyrus during dorsal, but not intermediate, hippocampus stimulation is proposed to underlie the mechanism for these differences. Collectively, our results provide insight into the dynamic function of hippocampal networks and their role in seizures.

Introduction

Based on a large volume of anatomical, behavioral, and genetic data, it has been argued that the dorsal and intermediate compartments of the hippocampus are two functionally distinct regions (Fanselow and Dong, 2010). This claim is supported by observations of differences along the dorsoventral axis of the hippocampus, including the distribution of cortical afferents and projections (Cenquizca and Swanson, 2007, de No, 1934, Van Groen and Lopes da Silva, 1985), connections with other subcortical structures (van Groen and Wyss, 1990), commissural and intrinsic connections (van Groen and Wyss, 1990), neurochemical makeup (Garcia Ruiz et al., 1993, Tanaka et al., 2012), and cell morphology (Dong et al., 2009). In addition to these physical differences, physiological properties such as spatial field tuning (Jung et al., 1994), vulnerability to ischemia (Ashton et al., 1989), and evoked field potential responses (Gilbert et al., 1985) also differ along the dorsoventral axis. Furthermore, lesion, stimulation, and pharmacological studies have demonstrated distinct behavioral effects when different areas along this axis are targeted (Hock and Bunsey, 1998, Siegel and Flynn, 1968). Nevertheless, despite the wealth of information on these differences, exactly how they contribute to large-scale functional network activity remains unknown. Furthermore, less is known about the intermediate hippocampus, with most studies focusing on the dichotomy between dorsal and ventral regions. We chose to target the intermediate, rather than ventral, hippocampus to study how network properties differ with small changes in distance along the dorsoventral axis.

While anatomical connectivity provides a foundation for studying a region's functional role in neural circuits, understanding how projections translate to activity is best done by directly manipulating the population of interest and observing the downstream effects. Electrical stimulation was traditionally used for this purpose (Canals et al., 2009, Canals et al., 2008), but the recent development of optogenetics has enabled greater precision in the temporal pattern of excitation and the specific subset of neurons targeted (Boyden et al., 2005, Yizhar et al., 2011, Zhao et al., 2011). Electrophysiology measurements have typically been used to observe downstream effects of stimulation, but are limited by a finite number of recording sites, requiring a priori hypotheses about which regions are affected. As one of the few modalities that can report activity across the entire brain with relatively high spatial resolution, functional magnetic resonance imaging (fMRI) offers a viable alternative to studying the network-level behavior of neural circuits in both humans and small animals (Bullmore and Sporns, 2009, Goloshevsky et al., 2008, Huettel et al., 2004, Yu et al., 2010). By measuring the blood oxygenation-level dependent (BOLD) signal over time, fMRI provides an indirect measure of neuronal activity, including both spatial and temporal dynamics (Kim et al., 2004).

Optogenetic functional magnetic resonance imaging (ofMRI) is a novel technology that combines the precision of optogenetic stimulation with the whole-brain readout capability of fMRI (Desai et al., 2011, Lee, 2011, Lee, 2012, Lee et al., 2010, Vazquez et al., 2013, Weitz and Lee, 2013). Unlike fMRI experiments with cognitive, sensory, or even direct electrical stimuli, ofMRI can investigate the brain's response to a cell type-specific population being driven in a temporally precise manner, revealing important aspects of a network's connectivity and frequency response in vivo. This has enabled investigators, for the first time, to manipulate specific elements of a neural circuit with precision in an intact animal and observe the causal flow of activity within the global brain circuit. By systematically investigating different patterns of input and measuring the brain's response, a region's functional and dynamic connectivity can be characterized. In particular, this technique can be used to investigate how network activity changes with different temporal patterns of input. Several studies employing optogenetics have already demonstrated the distinct frequency responses that neuronal circuits can exhibit (Adamantidis et al., 2007, Carter et al., 2010, Gradinaru et al., 2009). Although several experiments have used ofMRI to examine the functional connectivity of different regions (Abe et al., 2012, Desai et al., 2011, Lee et al., 2010), this aspect of neuronal dynamics has not been explored yet. In addition, several other questions remain unanswered. Can ofMRI detect activity across multiple synapses? Can it dissociate inhibitory and excitatory activities? And can it reveal the spatiotemporal dynamics of large-scale, multi-region networks?

In the present study, we sought to address these issues by using high-resolution, high-field ofMRI to investigate the causal, frequency-dependent network activity driven by CaMKIIa-positive cells in dorsal (DH) and intermediate (IH) hippocampus of rats. CaMKIIa-positive cells in these two regions were selectively stimulated using optogenetic techniques at 6, 10, 20, 40, and 60 Hz during whole-brain fMRI scans. Our results indicate distinct networks recruited by stimulation of either region in a frequency-dependent manner, and point to the potential of ofMRI in uncovering the functional significance of each region in large-scale neuronal networks and behavior.