The Chilean Brush-tailed Mouse (Octodon degus) as a New Model for Studying the Visual System

Academic Background

The study of the visual system has always been an important topic in the field of neuroscience. Traditionally, scientists have used nocturnal or crepuscular rodents (such as hamsters, rats, and mice) as models to study the development and function of the visual system. However, the visual systems of these animals are relatively simple and significantly different from those of diurnal mammals like humans. To broaden the scope of research, scientists have begun searching for animal models that more closely resemble the human visual system. The Chilean brush-tailed mouse (Octodon degus), a diurnal and precocial rodent with a cone-rich retina and highly developed retinal structure, has emerged as a potential research model.

The primary goal of this study was to evaluate the visual physiological properties of the Chilean brush-tailed mouse, particularly the visual responses and receptive field (RF) properties of neurons in the superior colliculus (SC). By comparing the visual characteristics of the Chilean brush-tailed mouse with those of other commonly used laboratory rodents, the researchers aimed to reveal the uniqueness of its visual system and explore its potential applications in studying visual system development.

Source of the Paper

This paper was co-authored by Natalia I. Márquez, Alfonso Deichler, Pedro Fernández-Aburto, Ignacio Perales, Juan-Carlos Letelier, Gonzalo J. Marín, Jorge Mpodozis, and Sarah L. Pallas. The research team is affiliated with the University of Chile (Universidad de Chile), Finis Terrae University (Universidad Finis Terrae), and the University of Massachusetts-Amherst in the United States. The paper was first published on December 20, 2024, in the Journal of Neurophysiology.

Research Process

1. Animal Preparation and Experimental Design

The study used 13 Chilean brush-tailed mice, which were housed under laboratory conditions following strict animal welfare standards. The experiments were divided into two main parts: neuroanatomical data collection and electrophysiological recordings.

Neuroanatomical Data Collection

The researchers labeled retinal projections by intraocular injections of neural tracers (e.g., cholera toxin subunit B, CTB). After 5-7 days of survival, the animals were perfused, and brain tissue was sectioned. Using immunohistochemical methods, the researchers observed the distribution of retinal projections and measured the volume of retinal terminals using stereological techniques.

Electrophysiological Recordings

In the electrophysiological experiments, recordings were made from SC neurons in 9 Chilean brush-tailed mice. The animals were anesthetized, and the SC surface was exposed for recording. Tungsten microelectrodes or multi-electrode arrays were used to record neuronal activity. Visual stimuli were presented on an LCD monitor and included a moving white square, sinusoidal gratings, an expanding black disk (simulating an approaching object), and a stationary black disk.

2. Data Processing and Analysis

The recorded neural signals were processed using band-pass filtering and adaptive filtering algorithms to remove background noise. Single neurons were isolated and classified using discrete wavelet transform and k-means clustering algorithms. The researchers fitted Gaussian functions to the receptive fields of neurons and calculated their size and shape. Additionally, the response properties of neurons to spatial frequency and contrast were analyzed.

Key Findings

1. Retinal Inputs and Cytoarchitecture of the Superior Colliculus

The study found that the SC of the Chilean brush-tailed mouse has a highly stratified structure, with all layers clearly distinguishable. Retinal projections primarily originated from the contralateral eye and densely innervated the superficial layers (stratum griseum superficiale, SGS) and the stratum zonale (SZ), while there was almost no retinal input in the deeper layers (stratum griseum intermediale, SGI, and stratum griseum profundus, SGP).

2. Receptive Field Properties

The researchers found that the size of receptive fields in SC neurons increased with depth. Neurons in the superficial layers had smaller receptive fields, while those in the deeper layers had larger ones. Additionally, the shape of the receptive fields changed with depth, transitioning from circular in the superficial layers to elliptical in the deeper layers.

3. Spatial Frequency Tuning

Most SC neurons preferred low/medium spatial frequencies (0.04 and 0.08 cycles/degree, cpd), but some neurons were tuned to high spatial frequencies (0.24 cpd). This suggests that the Chilean brush-tailed mouse has high visual acuity.

4. Contrast Responses

The study found that approximately half of the SC neurons exhibited linear or nearly linear responses to contrast, while the other half showed saturating responses. This contrasts with the nonlinear responses of nocturnal rodents, reflecting the diurnal characteristics of the Chilean brush-tailed mouse’s visual system.

5. Responses to Looming Objects

SC neurons showed a continuous increase in firing rate in response to looming objects, while exhibiting typical on-off responses to stationary objects. This indicates that the SC has a neural mechanism for identifying rapidly approaching objects, which may be related to escape behaviors.

Conclusions and Significance

This study provides the first detailed description of the visual response properties of SC neurons in the Chilean brush-tailed mouse, revealing its high visual acuity and diurnal characteristics. The results suggest that the Chilean brush-tailed mouse, as a diurnal and precocial rodent, has high visual acuity and complex visual processing capabilities, making it an ideal model for studying visual system development and function.

Research Highlights

1. Introduction of a New Model: The Chilean brush-tailed mouse, as a diurnal rodent, fills the research gap between nocturnal rodents and the human visual system.
2. High Visual Acuity: The study found that the Chilean brush-tailed mouse has high visual acuity, particularly in high spatial frequency tuning.
3. Linear Contrast Responses: Unlike nocturnal rodents, SC neurons in the Chilean brush-tailed mouse exhibit linear or nearly linear contrast responses, reflecting the characteristics of its diurnal visual system.