The Turone Sheep Seasonal Brain Dataset

Abstract

To cope with seasonal modifications of the environmental resources, brain anticipates and changes both its structural organisation and functioning. However, the tempo and mode of these central changes have been poorly investigated in mammalians. Here we describe a longitudinal morphometric neuroimaging study in a well know animal model to study seasonality: sheep. Using new magnetic resonance imaging (MRI) resources comprising a high-resolution brain template, its associated tissue priors (500-µm isotropic resolution) and a corresponding sheep brain atlas (202 regions of interest) we investigate the impact of seasonal transitions between winter and summer season on brain microstructure using voxel-based morphometry. We observed significant modifications of grey matter concentration (GMC) in pivotal brain areas involved in circadian rhythms (pineal, hypothalamus) and light processing (suprageniculate nucleus) but also within regions related to sensory processing, learning, memory, behavior control, and social cognition. These findings provide new insights into mammalian brain functioning revealing its flexibility and its adaptability to cope to environmental changes.

Material and methods

Animals

Seventeen adult, sexually mature multiparous ewes (Ovis aries) of 3.4 ± 0.3 years old (weight = 64.5 ± 5.5 kg) have been included in this protocol and have been scanned at the PIXANIM platform (INRAE, French National Research Institute for Agriculture, Food & Environment, Nouzilly, France). Females were ovariectomized and implanted with an oestradiol silastic implant (2 cm) at the end of october, two months before the first scan session. The ewes were kept permanently indoors and fed ad libitum with dehydrated lucerne, maize, straw, and a supplement of vitamins and minerals, and had free access to water. All the procedures were conducted in accordance with the European directive 2010/63/EU on the protection of animals used for scientific purposes and the experimental protocol was approved by the local ethical committee (comité d’éthique en expérimentation animale Val-de-Loire) under reference number 00510.02.

Surgical procedure

First, ewes were fasted 24 h before surgery. Day of surgery, an i.v. injection of thiopental (14 mg/kg body weight, BW; Nesdonal, Merial, Villeurbanne, France) was done to induce analgesia, then animals were intubated and maintained under anesthesia by a mixture of 3 to 4% isoflurane (Vetflurane, Virbac, Carros, France) vaporized in 100% oxygen. Ovariectomy were conducted under sterile surgical conditions. Local anaesthesia with lidocaine (4%, Lurocaïne, Vétoquinol, Luré, France) was given prior laparotomy. Ovaries were surgically extracted, and the tissues were sutured. The entire procedure was performed within 20 min. Postoperative ventilation with oxygen was maintained until the first signs of awakening appeared. Animals were then housed individually for 6 h in a padded stall before being put back with congeners. They received an anti-inflammatory drug for 2 days (2 mg/kg, flunixin meglumine, Finadyne®, Intervet, Beaucouzé, France) to relief pain and an antiedema medication: (1 mg/kg BW of furosemide, Dimazon®, Intervet, Beaucouzé, France) at the end of surgery. Animals were treated with a diuretic medication associating 3 mg/kg of hydrochlorothiazide with 0.03 mg/kg dexamethasone (Diurizone®, Vétoquinol, Luré, France) for 2 days.

Photoperiodic conditions

Two scanning sessions were performed for each animal at opposite moment of the circannual cycle: a first session was performed between mid-January/February (from 13/01/2014 to 17/02/2014), during the sexual season and a second session was performed between mid-June and mid-July (from 16/06/2014 to 22/07/2014) during the season of sexual rest. As 4-5 weeks were necessary to scan the entire group, animals were housed two weeks before and during each scan session (2-3 weeks) in photoperiodic facilities to ensure similar light duration for each subject and to avoid a photoperiodic shift over the scanning sessions (winter scanning session: mean daylight 556.1 min ± 22.26 min; summer scanning session: daylight 943.3 min ± 13.90 min). Therefore, ewes were housed under a 9h light/15h dark photoperiod (light on at 8AM, daylight duration 540 min) during the winter scan session and under a 15h light/9h dark photoperiod (lights on at 6h AM, daylight duration 960 min) during the summer scan session.

Melatonin response to photoperiodic exposure

To ensure that animals were responsive to the photoperiodic treatment, one week before the start of each MRI scan session, serial blood samplings were performed overnight to measure the pattern of blood melatonin secretion. This was done following previous procedures used in our laboratory (Tricoire et al., 2002). Briefly, animals were housed individually 24h hours and implanted with a catheter into the jugular before blood sampling to limit potential stressful response of the animals. Blood sampling was performed under dim red light once per hour from one hour before the lights turn off until 2 hours after they turn on. Blood samples were then centrifugated and plasma stored at -80°c until assay. Melatonin assay was performed as previously documented in our laboratory (Tricoire et al., 2002).

In vivo MRI acquisitions

Animal preparation for MRI data acquisitions were performed as previously described (Ella et al., 2015, 2017). Briefly, animals were anesthetized with an intramuscular injection of ketamine just before the scan, intubated, maintained during the scan on 3% isoflurane vaporized in oxygen and continuously monitored by a MR-compatible Aestiva®/5 systems (Madison, USA). Three MR acquisitions were performed (see Ella et al., 2015 for the details of each acquisition) on each animal secured in a prone position in 3 Tesla VERIO Siemens systems (Erlangen, Germany), front legs apart and bent towards the abdomen, using a flexible coil (Siemens FLEX Large 4 elements) tied around the head. The sequences have been optimized to be perform in time compatible with anaesthesia (≤1h), to reduce artefact (folding, truncation, etc.) and to optimize SNR. For each acquisition parameters have been set as previously describe in Ella and Keller (2015):- Three dimensional SPC-IR acquired in the sagittal plane (Echo Time/Repetition Time = 413 ms/4000 ms, Flip Angle = 120°, Inversion Time = 380 ms, Number of Excitation = 10, Partial Fourier = 1, Slice Thickness = 0.35 mm, Slice Number = 208, Field of View = 179.2x179.2 mm, matrix = 512x512, final resolution 0.35 mm3).- Three dimensional T1 MPRAGE acquired in the sagittal plane (Echo Time/Repetition Time = 3.18 ms/2500 ms, Flip Angle = 12°, Inversion Time = 900 ms, Number of Excitation = 8, Partial Fourier = 1, Slice Thickness = 0.5 mm, Slice Number = 288, Field of View = 192x192 mm, matrix = 384x384, final resolution 0.5mm3).- Three dimensional T2 MEDIC acquired in the sagittal plane (Echo Time/Repetition Time = 2.1 ms/38 ms, Flip Angle = 8°, Number of Excitation = 6, Partial Fourier = 0.75, Slice Thickness = 0.4 mm, Slice Number = 256, Field of View = 179.2x179.2 mm, matrix = 448x448, final resolution 0.4 mm3).

Additional feature: Computed Tomography (CT) sheep template.

Tomographic data of nine ewes’ skulls have been acquired on our CT-scan (Siemens Somatom Definition AS, Siemens Corp., Germany). The X-ray source was set at 100 kV and 120 mA/s. A total of 800 slices were acquired using the following parameters: Thickness = 0.4 mm, Slice Number = 800, Field of View = 204,8x 204,8 mm, matrix = 512x512, final resolution 0.4 mm3) reconstructed using a filter Safire I26. DICOM data were converted to NIFTI format and organized as a standardized data sets that accordingly to the Brain Imaging Data Structure (BIDS) using BIDScoin and are downloadable on Zenodo.