The trade-off between pulse duration and power in optical excitation of midbrain dopamine neurons approximates Bloch’s law

• Published in: Behavioural brain research 2022-02, Vol.419, p.113702-113702, Article 113702
• Main Authors: Pallikaras, Vasilios, Carter, Francis, Velazquez-Martinez, David Natanael, Arvanitogiannis, Andreas, Shizgal, Peter
• Format: Article
• Language: English
• Subjects: Animals; Behavior, Animal - physiology; ChannelRhodopsin-2; Channelrhodopsins; Dopamine; Dopaminergic Neurons - physiology; Male; Optogenetics; Rats; Rats, Long-Evans; Reward; Reward seeking; Temporal integration; Time Factors; Ventral Tegmental Area - physiology
• ISSN: 0166-4328; 1872-7549
• DOI: 10.1016/j.bbr.2021.113702

Optogenetic experiments reveal functional roles of specific neurons. However, functional inferences have been limited by widespread adoption of a restricted set of stimulation parameters. Broader exploration of the parameter space can deepen insight into the mapping between selective neural activity and behavior. In this way, characteristics of the activated neural circuit, such as temporal integration, can be inferred. Our objective was to determine whether an equal-energy principle accounts for the interaction of pulse duration and optical power in optogenetic excitation. Six male TH::Cre rats worked for optogenetic (ChannelRhodopsin-2) stimulation of VTA dopamine neurons. We used a within-subject design to describe the trade-off between pulse duration and optical power in determining reward seeking. Parameters were customized for each subject based on behavioral effectiveness. Within a useful range of powers (~12.6–31.6 mW) the product of optical power and pulse duration required to produce a given level of reward seeking was roughly constant. Such reciprocity is consistent with Bloch’s law, which posits an equal-energy principle of temporal summation over short durations in human vision. The trade-off between pulse duration and power broke down at higher powers. Thus, optical power and duration can be adjusted reciprocally for brief durations and lower powers, and power can be substituted for pulse duration to scale the region of excitation in behavioral optogenetic experiments. The findings demonstrate the utility of within-subject and trade-off designs in optogenetics and of parameter adjustment based on functional endpoints instead of physical properties of the stimulation. [Display omitted] •We provide behaviorally derived intensity-duration curves for ChannelRhodopsin-2.•Pulse duration trades off almost perfectly with optical power within useful ranges.•This trade-off breaks down at high optical powers.•Pulse duration and optical power scale the area of neuronal excitation equivalently.•Behaviorally derived trade-offs can reveal optogenetic excitation mechanisms.