Optogenetics in psychiatry: The light ahead.

Complexities of the human mind have been beyond the scope of understanding because a intricate neuronal network and difficulty in specific localization and assessment of area responsible for a specific behavior; more so in a freely moving living being. Optogenetics off late has been able to address this issue to great extent and holds promises for future. Relevant literatures in this direction were looked into and the salient aspects of this science is being discussed here with specific relevance to psychiatry.

To fathom the complexity of human behavior it is imperative to find out how various neural networks in the brain assimilate, integrate, and interpret various information; which are brought to its fore. This information could be external sensations or internal memories and might consequently decide the type of feeling, thinking, or actions. Major hindrances in better understanding of these processes at neural networks are huge diversity of brain cells and complexity in the pattern of neuronal firing. To activate or deactivate a specific cell or its connection at a precise time to elicit a specific behavior had so far been beyond the scope and advent of science. Validated functional imaging methods have failed the optimism of fulfilling its promises. Optogenetic molecular tools however soar our expectation higher towards better ability to map the human behavior towards better understanding and management.[1]

Optogenetics is a science which targets at rapid (in milliseconds) control of precise events in a living organism by optical means. It integrates optics and genetics to display gain or loss of function of precise events within specific living cell. This noninvasive use of light to control neurons opens to us a wider opportunity in the field of neuropsychophysiology. Studies have shown more precise stimulation with the optical methods than any other method (like macroelectrode, etc.) because of better temporospatial resolution.[2]

HISTORY

In 1979, Francis Crick felt the unmet need to control one type of neuronal cell leaving another unaffected for better understanding of neurosciences. He speculated light to serve better as control tool, but science at that time was not aware of how to make a cell respond to light. Though in the field of microbiology it was known since 1971 that there are microorganisms that have ion pumps which get activated by visible light (bacteriorhodopsin); it was only in 2005 that it was described that introduction of microbial opsin gene without any other component (single component) made neurons responsive to light. Over the years further tools were developed which could turn neurons on and off on exposure to various colors of light.[2]

Technology

In optogenetics, tools are coded genetically and driven optically. Components of technology includes[1234]:

Building of light responsive resources.

Incorporation of light responsive molecule to target tissues.

Delivery of light to the target tissues.

Analysis of response or behavior.

Building of light responsive resources

These molecules are seven transmembrane proteins called microbial (type I) opsin. Microbial opsin includes a family of ion pumps and channels like halorhodopsin, channelorhodopsin, bacteriorhodopsin, etc. These proteins are product of opsin genes. Opsin proteins are dependent on retinoid factor (retinal) for its function as rhodopsin. As retinoids are in abundance in mammalian brain, microbial opsin gene can be easily introduced into mammalian neurons as a single component. These proteins can lead to excitation (e.g., channelorhodopsin) or inhibition (e.g., halorhodopsin) of specific neurons or cell type. Channelorhodopsin leads to cellular activation by depolarization with blue light, while halorhodopsin causes cellular deactivation by hyperpolarization with blue light. With further molecular engineering and genomic efforts, these tools can be experimentally manipulated to have desired physiological effects or kinetic property or change in wavelength or spatial extent of light (e.g., humanized channelorhodopsin, etc.).

Incorporation of light responsive molecule to target tissues

Opsin protein can be genetically coded provided the opsin gene is delivered to the neurons for expressions. This can be done via various methods:

Transfection method: Viruses whose genomes are capable of encoding opsin genes are made to infect neurons. These neurons are subsequently able to express opsin proteins. Lentiviruses, adeno-associated viruses (AAV), etc., have been used for this. This method is fast and effective.

Transgenic method: Deoxyribonucleic acid (DNA) is incorporated with promoter element to enable selective expression of gene in the destined animal neurons. This method is ideal but time consuming.

Delivery of light to the target tissues

Light is required to be delivered to the site to activate or deactivate opsin expressing neurons. This is achieved by inserting optical fibers or other devices in brain with other end attached to light sources like laser or light-emitting diode (LED). For the structures on surface, small light source can be mounted on the head. Hybrids of fiberoptics and electrodes (optrodes) permitted high speed read out simultaneously and while keeping pace with speed of inputs.

Analysis of response or behaviors

With the help of optogenetic tools, role of specific neurons played in specific type of complex behavior can be understood. This technology is capable of testing large sets of neurons at subcircuit level enabling testing of hypothesis of how neurons encode information and work together. One can deactivate many areas and activate specific area and evaluate the results or functions. Simultaneously it also can be learnt that how specific activation/modulation can lead to changes in intended behavior. With the help of this technology, spatial distribution and information of cell type can be extracted readily from the imaging data. Voltage sensitive dyes or genetically encoded sensors have been used to read out neural activity. Functional magnetic resonance imaging (fMRI) have been used.

Optogenetics in neuropsychiatry

Studies have been carried out in various mammals which suggest its significant application in the field of psychiatry and behavioral sciences. Few are as under.

Understanding of psychophysiology

Reward seeking: Infection of AAV encoding for channelorhodopsin/halorhodopsin into the basolateral amygdala in mice resulted in expression of opsin in excitatory neurons which left amygdale and formed glutaminergic synapses on target neurons in the nucleus accumbens. Delivery of light increased/decreased his behavior towards rewarding stimulus depending on what protein was being expressed.[5]

Fear Conditioning: Brief optical activation of lateral amygdale in rats when coterminated with auditory stimulus of longer duration made mice to fear the sound as evident by increased freezing on the starting of sound.[6]

Understanding of psychopathology

Narcolepsy and sleep wake transition: Application of optogenetic device led to specific activity pattern in hypocretin neurons in lateral hypothalamus of mice. Certain patterns were also found responsible for sleep wake transition.[7]

Schizophrenia: Prior studies have shown that parvalbumin (fast spiking inhibitory neurons) which are responsible for gamma oscillation in brain are altered in schizophrenia. Review of optogenetic studies did show a causal role of these neurons in modulation of gamma oscillations and consequently the flow of information in neocortical circuit.[8910]

Autism: Gamma oscillations are also altered in autism, but in a different manner. Studies do causally implicate changes in excitation-inhibition balance in the social dysfunction, abnormal gamma oscillation, and information processing.[8910]

Posttraumatic stress disorder: Optogenetic studies have found the role of both hippocampus and neocortex in long-term contextual fear memories.[11]

Anxiety disorder: Optogenetic studies have found specific intra-amygdala pathway to be responsible for unconditioned anxiety. Stimulation of terminals of basolateral amygdale in central of central nucleus of amygdale produced antianxiety effect in mice.[12]

Depression: Symptoms of depression affects various domain of brain function like anhedonia, hopelessness, psychomotor function, etc., Studies have suggested afferent axons from globally distributed brain regions and not the local cell bodies to be responsible for this complex illness. In one study, optogenetic stimulation of medial prefrontal cortex led to potent antidepressant effect.[1314]

Future directions

Optogenetic studies have literally shown the light ahead. It would enable integration of psychology, physiology, pharmacology, genetics, and imaging in the paradigms of psychiatry. Effort lies further ahead in:

Understanding of brain wide wiring diagram and molecular phenotype of various optogenetically controlled cells.

Three-dimensional and simultaneous visualization of neural activity pattern in response to optogenetic stimulation in context of various psychiatric illnesses.