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One brain region proposed to mediate these effects is the periaqueductal gray (PAG), an evolutionarily conserved midbrain structure that contains multiple subpopulations of peptidergic neurons. Here we ask which cell types in the larval zebrafish PAG respond to changes in ambient illumination. By registering\n                  <jats:italic>post hoc<\/jats:italic>\n                  gene expression to neurons recorded during exposure to sensory stimuli, we show that GABAergic PAG neurons expressing the neuropeptide Relaxin-3 (\n                  <jats:italic>rln3a<\/jats:italic>\n                  +) are persistently activated by darkness, while a neighboring subpopulation of GABAergic PAG neurons expressing\n                  <jats:italic>penka<\/jats:italic>\n                  is not. Zebrafish lacking\n                  <jats:italic>rln3a<\/jats:italic>\n                  show a reduction in locomotion during darkness. We propose that darkness recruits Relaxin-3-releasing PAG neurons to promote vigilance and potentiate defense circuits. These results identify a specific peptidergic population of PAG neurons that are potentially relevant to behavioral state dysregulation associated with excessive darkness.\n                <\/jats:p>","DOI":"10.64898\/2026.09.26.754586","type":"posted-content","created":{"date-parts":[[2026,10,2]],"date-time":"2026-10-02T03:25:16Z","timestamp":1790911516000},"source":"Crossref","is-referenced-by-count":0,"title":["Relaxin-3 neurons of the zebrafish periaqueductal gray mediate behavioral responses evoked by darkness"],"prefix":"10.64898","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-4765-4813","authenticated-orcid":false,"given":"Joanne Shu Ming","family":"Chia","sequence":"first","affiliation":[{"name":"NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Michael","family":"Martin","sequence":"additional","affiliation":[{"name":"University of Toronto, Scarborough, Canada"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2220-8596","authenticated-orcid":false,"given":"Grace M","family":"Padilla","sequence":"additional","affiliation":[{"name":"Department of Neurobiology, School of Biological Sciences. University of California, San Diego. La Jolla, CA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0003-8369-2523","authenticated-orcid":false,"given":"John","family":"Jefferson","sequence":"additional","affiliation":[{"name":"Department of Neurobiology, School of Biological Sciences. University of California, San Diego. La Jolla, CA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Venkatesh","family":"Krishna","sequence":"additional","affiliation":[{"name":"University of Toronto, Scarborough, Canada"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6188-9582","authenticated-orcid":false,"given":"Gilles","family":"Vanwalleghem","sequence":"additional","affiliation":[{"name":"Department of Molecular Biology and Genetics, Aarhus University, Denmark"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8208-0136","authenticated-orcid":false,"given":"Ruey-Kuang","family":"Cheng","sequence":"additional","affiliation":[{"name":"Institute of Cell and Molecular Biology, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6218-3995","authenticated-orcid":false,"given":"Seetha","family":"Krishnan","sequence":"additional","affiliation":[{"name":"NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1425-5277","authenticated-orcid":false,"given":"Matthew","family":"Lovett-Barron","sequence":"additional","affiliation":[{"name":"Department of Neurobiology, School of Biological Sciences. 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