Electrophysiological Signature Reveals Laminar Structure of the Porcine Hippocampus

Abstract

The hippocampus is involved in aspects of both working and episodic memory, and is a central region of interest in diseases affecting these processes. While the role of the hippocampus in memory is studied predominantly in rodents, rodent neurological disease models have not historically translated well to human treatment. Non-human primate disease models, while highly translational, present both economic and ethical challenges. Therefore, an alternative research model is required to study memory and cognition during development and to follow disease progression. Because pig models are widely used in other translational research, they may provide an excellent compromise between rodents and non-human primates due in part to the gyrencephalic neuroanatomy and significant white matter composition of the porcine brain.In addition, the porcine hippocampal anatomy and cytoarchitecture more closely resembles that of primates than rodents. To advance the study of the neurophysiology of the pig hippocampus and related structures, we utilized stereotaxis combined with single-unit electrophysiological mapping to precisely place laminar silicon depth probes into the dorsal hippocampus of miniature pigs without the need for image guidance. We then implemented an in vivo, electrophysiology-based methodology to identify hippocampal layers and to examine hippocampal neurophysiology using these depth probes. This methodology allowed for the simultaneous recording of laminar field potentials and single-cell activity in multiple layers in the dorsal hippocampus. The laminar structure of dorsal hippocampus and the precise location of the probes were confirmed by electrophysiology and histopathology, eliminating the need to recover electrode tracks in future animals. This methodology allows us to investigate hippocampal electrophysiology under anesthesia, with potential expansion into awake behaving neurophysiology. Accessible neurophysiology of the pig hippocampus may allow for the development of new translational models of neurological disorders, with the possibility of evaluating the neuronal and circuitry disruptions that underlie cognitive and memory dysfunction.peer-reviewed)

Multichannel Silicon Probes for Awake Hippocampal Recordings in Large Animals

“…All animals underwent implantation of multi-channel silicon probe under sterile conditions similar to acute implantation procedure described previously [20]. Briefly, pigs were placed in a stereotactic frame, with the surgical field prepped and draped, and a linear incision was made along the midline.…”

“…The dura was opened in a cruciate manner and the brain was mapped in the sagittal plane with a tungsten electrode (impedance = 0.5 MΩ, measured at 1KHz; Catalog # UEWSEGSEBNNM), utilizing observed spiking activity to generate a two-dimensional map. Based on this map, a silicon probe was inserted so that the spread of electrode sites would span the laminar structure of the dorsal hippocampus at its maximal thickness in the dorsal-ventral plane, perpendicular to the individual hippocampal layers [20]. The multichannel silicon probe was stabilized inside the craniectomy using Tisseel (Catalog # 1504514, Baxter Healthcare, Wayne, PA), creating a semi-floating interface between the silicon probe and the animal's skull.…”

“…Using silicon probes of various designs, the laminar structure of pig dorsal hippocampus was examined electrophysiologically under awake behaving conditions ( Figure 2). Since the hippocampal region of interest (at our standard recording coordinates in medial-lateral (ML) and anteriorposterior (AP) planes) is about 1,600 µm (see [20] for more details), silicon probes with 200 µm site spacing provide good coverage for the laminar structure with enough resolution to identify most layers ( Figure 1A). Oscillatory activity of the porcine hippocampus recorded with NN32/EDGE80 silicon probe is shown in Figure 2 (Site Spacing = 200 µm, Probe Length = 6,200 µm).…”

“…Neurophysiological features of awake porcine hippocampus such as sharp-wave ripple (SPW-R) and the polarity inversion of the LFPs across stratum radiatum are shown ( Figure 2). While this silicon probe allowed us to examine the porcine hippocampus in its entirety (including the deeper layers such as hilus), some of the laminar layers (such as L-M) could have easily been poorly sampled using 200 µm resolution [20].…”

“…Recent advances in high-density linear electrode arrays and wireless recording technology could tremendously enhance translational research studies of large-scale networks in species with large, gyrencephalic brains [20]. Ideally, the probes for electrophysiological recordings from deep brain structures must resolve laminar local field potentials (LFPs) and provide proper sampling densities to resolve single-units across channels for single unit spike-sorting and minimize damage to the neuro-electric interface (for review see [21]).…”