eNpHR

eNpHR, short for enhanced halorhodopsin, is a light-activated chloride pump used in optogenetics to suppress neuronal activity. It is an engineered version of halorhodopsin from Natronomonas pharaonis (NpHR) with improved expression, membrane trafficking, and photocurrent strength, enabling more reliable silencing of neurons in vivo.

Mechanism and properties: eNpHR responds to yellow light, typically in the 590 to 630 nanometer range. Upon illumination, it pumps chloride ions into the neuron, hyperpolarizing the cell and increasing the threshold for action potential generation. The enhanced variants are designed to exhibit stronger membrane localization, higher photocurrents, and reduced intracellular aggregation, providing more robust and consistent silencing. Researchers often fuse eNpHR to fluorescent reporters to aid visualization of expression and localization.

Applications and delivery: eNpHR is delivered to neurons via viral vectors (for example, adeno-associated virus) or used in transgenic animals under cell-type–specific promoters. Light is delivered through implanted optical fibers or transparent cranial windows. It is widely used to investigate the causal role of specific neuronal populations in circuits and behavior, frequently in experiments that pair inhibition with concurrent activation of other pathways or with excitatory opsins for bidirectional control.

Limitations and considerations: Prolonged or high-intensity yellow light can produce tissue heating and phototoxic effects. Temporal dynamics may not match the fastest excitatory opsins, and repeated or long-term silencing could perturb intracellular chloride homeostasis. Despite these caveats, eNpHR remains a common and valuable tool for reversible neuronal inhibition in optogenetics.

Variants and related tools: The eNpHR family includes versions such as eNpHR3.0, which incorporate design changes to improve trafficking and expression; these are commonly used in conjunction with fluorescent tags to monitor expression.