Optogenetics is a fantastic technique, enabling the control of mouse behaviour with a high degree of temporal and neuron-specific precision. However, due to the high levels of light power needed to activate channelrhodopsin and its variants, a typical system will use a high intensity laser or LED connected via fibre-optics.
In my experience of doing opto’s, the fibres have proved to be the biggest technical issue, due to the high level of stress they incur in the mice. This comes as a result of a number of factors, including the requirement for housing the mice in an open cage, with a tether on the head that places some amount of torque on them at all times, and requires them to have their head upright. Also, because of the fragility of the optic fibres, the mice will often be housed alone and in a sterile environment. All this amounts to both unnecessary suffering for the animals, which as researchers we a morally bound to reduce whenever possible, but also to stress that will inevitably impact on any behavioural measure you are investigating.
This brings me to a paper that was published recently by Anpilov et al. in Neuron1, where they developed a wireless optogenetic stimulator to overcome these issues with fibre-connected opto’s and investigate social behaviours in a “semi-natural” setup.
From a technological standpoint, I am very interested in the device they developed, which is my primary interest in this paper. Their device is almost ludicrously simple – it’s just an LED connected to 2 button batteries via a magnetic switch, which results in a total weight of about 1g (Figure 1A). They connect this to the implanted optic fibre and cement the whole lot onto the mouse’s head. Then the LED can be switched on externally by proximity of a magnet (Figure 1B). This means that the mice can be kept in a complex group-housed environment and the opto’s switched on at will remotely (Figure 1C).
In fact, the researchers placed the magnet above the feeder, so they don’t ever need to disturb the animals. This enabled Anpilov et al. to influence aggression, grooming and other social behaviours in response to oxytocin activation, which would otherwise be extremely challenging to investigate using classical fibre-connected opto’s.
Figure 1. A Wireless Device for Prolonged Optogenetic Manipulation in a Semi-natural
Setup
(A) Schematic illustration of the wireless device. Two batteries are connected in series to an LED through a magnetic-field dependent reed-switch. The LED is attached on top of an optic cannula positioned above the dorsal part of the PVN.
(B) A device mounted on a freely behaving mouse activated by a magnet.
(C) Schematic illustration of the semi-ethological arena and software-controlled electromagnet installed on the feeder. The arena consists of an open 70 x 50 cm box containing a nest, feeders, water, elevated areas, and barriers.
(D) Light power emitted at the tip of the optic fiber as a function of the number of 2 s light pulses. Battery capacity is sufficient for over 215 pulses.
(E) Section through a 3D map of blue light intensity along the axis of an illuminating fiber in graymatter. The slice was imaged from below as the fiber was lowered through. The section is superimposed with a contour map of iso-intensity lines corresponding to light intensity levels. Light intensity >= 8microwatt/mm2 is sufficient for effective SSFO photoactivation. Taken from Anpilov et al. 2020 Neuron.
However, I do have some concerns to voice about the device they used, related to the actual light output they can achieve. Essentially, you can’t expect high light power through an optic fibre from a battery-connected LED – the light scatter doesn’t allow it. The upshot being that Anpilov only got 3.2mW/mm2, which is much dimmer than a typical opto system which will deliver 100-300mW/mm2 (our current system delivers around 175mW/mm2). Furthermore, their system has a maximum on-time of around 400s, which is why they cleverly used the SSFO to provide long-term activation after a very short stimulation pulse.
So, while the system developed in this paper clearly works well for their application of oxytocin-mediated social behaviours, I can imagine the further applications being relatively limited due to the following reasons:
- Need to use SSFO or equivalent long-term responsive rhodopsin loses the high temporal precision you need for many optogenetic applications
- Very low light output will likely mean further applications will be limited to those with very strong/sensitive behavioural responses, and probably with very dense neuron populations
- The LED is only ON in close proximity to a magnet, which limits the design of environments that allow for activation without disturbing the animals.
Overall, I was impressed by the paper and am very interested in their approach to negate the drawbacks of fibre-connected optogenetics. But, I think their device has its own drawbacks, which if overcome would produce some very powerful tools with wide-ranging applications for in vivo optogenetics.
1 Anpilov et al. 2020 Neuron Wireless Optogenetic stimulation of Oxytocin neurons in a semi-natural setup dynamically elevates both pro-social and agonistic behaviours.