An immunocytochemical study of encephalic photoreceptors in three species of lamprey.

The extraretinal and extrapineal photoreceptors of three species of adult lamprey, sea lamprey (Petromyzon marinus), river lamprey (Lampetra fluviatilis) and silver lamprey (Ichthyomyzon unicuspis) were studied using antibodies raised against photoreceptor rod and cone opsins, alpha-transducin and arrestin. In all three species cells in the pineal organ (P), parapineal organ (PP), nucleus preopticus (T5), nucleus commissurae postopticae (D8), nucleus ventralis hypothalami (D10) and nucleus dorsalis hypothalami (D11) were labelled by one or more of the anti-opsin antibodies. In addition, anti-arrestin antibodies labelled cells within the D8 and anti-alpha-transducin antibodies labelled cells within the pineal complex and hypothalamus (primarily D8 and/or D10). A more variable and species dependent pattern of opsin, arrestin and alpha-transducin labelling was observed within the nucleus commissurae postinfundibularis (D12) in an area comprising the nucleus dorsalis thalami pars subhabenularis (D4sh) and nucleus dorsalis thalami pars caudalis/nucleus commissurae posterioris (D4c/M1), and in the proximity of the second Müller cells in the ventrocaudal diencephalon (2.MZ/M6). The majority of the neurons labelled within the pineal and parapineal organs and hypothalamus were periventricular with clear cerebrospinal fluid contacts (CSF-contacting neurons). Labelled neurons in the epithalamic (D4sh and D4c/M1) and caudal diencephalon (2.MZ/M6) had no obvious ventricular contacts. We speculate that the "primitive" vertebrate brain of lampreys represents an ancestral condition in which different populations of encephalic photoreceptors are associated with different behavioural and physiological responses. Image-forming vision needs an eye, but irradiance detection does not require a specialised organ. Rather the photoreceptors could be closely associated with their effector systems within the brain.