Neural connections of the pineal organ in the primitive bony fish Acipenser baeri : A carbocyanine dye tract‐tracing study

Abstract

The pineal organ of the sturgeon (Acipenser baeri; Chondrostei) consists of an elongated terminal vesicle and a long stalk that runs to the region of the posterior commissure. In order to investigate the neural connections of the pineal organ of the surgeon, a fluorescent indocarbocyanine (DiI) was applied to the pineal stalk of paraformaldehyde-fixed brains. This application strongly labeled the pineal tracts (right and left), which form an extensive bilateral projection to the brain. The fibers of these tracts could be followed to their different targets in the brain, namely the preoptic region, the rostral habenulae, the medial subhabenular and retrohabenular (post- and subcommissural) region, the medial thalamus, the dorsal hypothalamus, the pretectal area, and medial and dorsolateral tegmental mesencephalic regions (interstitial nucleus of Cajal, oculomotor nucleus, mesencephalic reticular area, nucleus profundus mesencephali, and central gray). A few fibers reach the interpeduncular nucleus and the rostral optic tectum. Pinealofugal fibers branched profusely, and some of them crossed the midline ventrally in the optic chiasma, the postoptic commissure, the conspicuous preinfundibular commissure or, occasionally, the mesencephalic tegmentum. In addition to pinealofugal fibers, this type of DiI application labeled a few cells, here interpreted as pinealopetal neurons, in the posthabenular-dorsomedial thalamic region and the central gray of the mesencephalon. The wide distribution of pinealofugal projections in sturgeon is reminiscent of that in the lamprey. Comparison of the present findings with those of other authors in teleosts, amphibians, and reptiles also suggests that there has been a reduction in the number of pineal projections in these lines.

Distribution of gamma‐aminobutyric acid immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri): Comparison with other fishes

“…Present results show the presence of a GABAergic population of multipolar efferent neurons in the pineal organ of the sturgeon. Pinealofugal projections of the sturgeon pineal were already traced to some centers of the diencephalon and midbrain using DiI labeling (Yáñez & Anadón, 1998), but the actual targets of GABAergic pinealofugal fibers are not known. The presence of GABAergic projection neurons was also reported in the pineal organ of lampreys (Pombal et al., 1999) and sharks (Carrera et al., 2006), where they are multipolar.…”

“…The organization of the sturgeon brain is mainly known from morphological studies with classical neuroanatomical methods (Golgi methods: Johnston, 1898, 1901; Nissl methods: Nieuwenhuys, 1998; Rupp & Northcutt, 1998) and tract‐tracing studies (Albert et al., 1999; Huesa et al., 2003, 2006; New & Bodznick, 1985; New & Northcutt, 1984a; New & Northcutt, 1984b; Northcutt, 2011; N. Yamamoto et al., 1999; Yáñez & Anadón, 1998). Immunohistochemical studies in the brain of sturgeons have reported the organization of the monoaminergic and cholinergic systems (Adrio et al., 1999, 2000, 2002; Piñuela & Northcutt, 2007) as well as that of some peptidergic systems (Adrio et al., 2005, 2008; Baker & Bird, 2002; González et al., 1992; Leprêtre et al., 1993).…”