A repeating fast radio burst source in a globular cluster

Fast radio bursts (FRBs) are flashes of unknown physical origin¹. The majority of FRBs have been seen only once, although some are known to generate multiple flashes^2,3. Many models invoke magnetically powered neutron stars (magnetars) as the source of the emission^4,5. Recently, the discovery⁶ of another repeater (FRB 20200120E) was announced, in the direction of the nearby galaxy M81, with four potential counterparts at other wavelengths⁶. Here we report observations that localized the FRB to a globular cluster associated with M81, where it is 2 parsecs away from the optical centre of the cluster. Globular clusters host old stellar populations, challenging FRB models that invoke young magnetars formed in a core-collapse supernova. We propose instead that FRB 20200120E originates from a highly magnetized neutron star formed either through the accretion-induced collapse of a white dwarf, or the merger of compact stars in a binary system⁷. Compact binaries are efficiently formed inside globular clusters, so a model invoking them could also be responsible for the observed bursts.

Cortical oscillations and the origin of express saccades

The latencies of visually guided saccadic eye movement can form bimodal distributions. The 'express saccades' associated with the first mode of the distributions are thought to be generated via an anatomical pathway different from that for the second mode, which comprises regular saccades. The following previously published observations are the basis for a new alternative model of these effects: (i) visual stimuli can cause oscillations to appear in the electroencephalogram; (ii) visual stimuli can cause a negative shift in the electroencephalogram that lasts for several hundreds of milliseconds; and (iii) negativity in the electroencephalogram can be associated with reduced thresholds of cortical neurons to stimuli. In the new model both express and regular saccades are generated by the same anatomical structures. The differences in saccadic latency are produced by an oscillatory reduction of a threshold in the saccade-generating pathway that is transiently produced under certain stimulus paradigms. The model has implications regarding the functional significance of spontaneous and stimulus-induced oscillations in the central nervous system.

Spectral tuning of rhodopsin and metarhodopsin in vivo

Color vision is dependent upon the expression of spectrally distinct forms of rhodopsin in different photoreceptor cells. To identify the structural features of rhodopsin that regulate spectral sensitivity and absorption in vivo, we have constructed a series of chimeric Drosophila rhodopsin molecules, derived from a blue- and a violet-sensitive rhodopsin, and used P element-mediated germline transformation to generate transgenic flies that express the modified pigments in the R1-R6 photoreceptor cells of the compound eye. Our analysis of these animals indicates that multiple regions of the opsin protein are involved in regulating rhodopsin spectral sensitivity and that the native and photoactivated forms of rhodopsin can be tuned independently of each other. These results demonstrate the feasibility of designing receptor molecules with specifically modified activated states.

Ectopic expression of ultraviolet-rhodopsins in the blue photoreceptor cells of Drosophila: visual physiology and photochemistry of transgenic animals

We have generated transgenic flies expressing R7 cell-specific opsins in the major class of photoreceptor cells of the Drosophila retina and characterized their spectral properties using high-resolution microspectrophotometry and sensitivity recordings. We show that the Rh3 and Rh4 opsin genes encode UV-sensitive opsins with similar spectral properties (lambda max = 345 nm and 375 nm), and that Rh3 corresponds to the R7p and R7marg class of visual pigments. We have also generated Rh3 and Rh4 isoform-specific antibodies and present an R7 cell map of the Drosophila retina. In a related set of experiments, we show that it is possible to coexpress two different visual pigments functionally in the same cell and produce photoreceptors that display the summed spectral response of the individual pigments. These findings open up the possibility of tuning an animal's visual behavior by targeted expression of combinations of opsin genes to selective types of photoreceptors.

Targeted misexpression of a Drosophila opsin gene leads to altered visual function

Drosophila mutants transformed with a chimaeric gene that expresses the ocellar visual pigment in the major class of photoreceptor cells of the retina were used to investigate the properties of this minor pigment. The photoreceptor cells in which this opsin was misexpressed showed new spectral characteristics and physiology.

The kinetics of formation of metarhodopsin in intact photoreceptors of the fly

The formation of metarhodopsin in fly photoreceptor no. 1--6 occurs at room temperature with a time constant of 125 microseconds (Q10 approximately 2.5). The formation of rhodopsin is faster by factor of 1/10 to 1/100 at least.