Terminal morphology of two branches arising from a single stem-axon of pretectal (PSm) neurons in the common carp

Abstract

The induction of postsynaptic structures by presynaptic terminals is suggested in a teleost brain. Neurons in the nucleus pretectalis superficialis pars magnocellularis (PSm) in the common carp are known to send fibers to the corpus mamillare (CM) and the nucleus lateralis valvulae (NLV). Individual axons of PSm neurons bifurcate (or give off an axon collateral), both of which reach the target areas in the CM and NLV. The morphology of horseradish peroxidase-labeled terminals in the CM and NLV appears quite different in light microscopy. Terminals in the CM appear as a fine network of beaded (2-4 microns in diameter) fibers, while those in the NLV are larger (8-12 microns in transverse diameter) and cup-shaped, partially enveloping the soma of individual NLV neurons. In electron microscopy, however, these synapses in the CM and NLV share several ultrastructural similarities. Small (0.2 to 0.4-micron thick, 0.4 to 0.7-micron long) spine-like protrusions arising from dendrites in the CM, and from cell bodies in the NLV, invaginate into the axon terminals, and the synaptic junctions are always formed at the base of the protrusion in both areas. Development of this unusual morphology is inferred to be directed from the presynaptic side. The morphological similarity of the spine-like protrusions to the "spinule," which is thought to be formed in response to synaptic activation, is discussed.

Calretinin immunoreactivity in the brain of the zebrafish, Danio rerio: Distribution and comparison with some neuropeptides and neurotransmitter‐synthesizing enzymes. II. Midbrain, hindbrain, and rostral spinal cord

“…In trout, the NLV and the SGN are separated more topographically than in zebrafish, avoiding confusion between them; in this species, there are cholinergic cells in the SGN, but they are lacking in the NLV (Pérez et al,2000). Moreover, despite the fact that the vast majority of NLV neurons gives rise to an outstanding projection to the cerebellar valvula and corpus in cyprinids and other teleosts (Ito and Yoshimoto,1990; Ito et al,1997; Yang et al,2004), ChAT‐ir fibers were not observed in these cerebellar structures of trout (Pérez et al,2000) or zebrafish (Clemente et al,2004; Mueller et al,2004; present results), suggesting that the rostral cholinergic subgroup of the SGN defined here in zebrafish is not part of the NLV. Because cholinergic fibers probably arising from ChAT‐ir NLV cells have been reported in the cerebellum of a cyprinid, Phoxinus phoxinus (Ekström,1987), whether the absence of this innervation in zebrafish is due to methodological constraints or represents an actual between‐species difference has to be clarified.…”

Afferent and efferent connections of the cerebellum of a salmonid, the rainbow trout (Oncorhynchus mykiss): A tract‐tracing study

“…In the carp, labeled cells from the LV were observed in the diffuse nucleus and nucleus ventromedialis thalami (Ito and Yoshimoto,1990). On the other hand, in trout no labeled cells were observed in the magnocellular superficial pretectal nucleus (PSM), which projects to the LV in the carp and goldfish (see Northcutt and Braford,1984; Ito et al,1997). Although there is the possibility that the nuclei termed PSM in trout and carp were not homologous, available results suggest that diencephalic inputs to LV vary widely among teleost species, perhaps reflecting divergent specialization of forebrain centers.…”

Afferent and efferent connections of the cerebellum of the chondrostean Acipenser baeri: A carbocyanine dye (DiI) tracing study

“…In teleosts, the lateral nucleus of the valvula (NLV) and the related precerebellar dorsal tegmental nucleus are highly specialized, consisting mostly of small adendritic cells and some larger dendrite‐bearing elements (Ito and Yoshimoto, 1990). Moreover, the small adendritic cells of the NLV receive characteristic large, cup‐shaped endings forming spinule‐bearing synapses (Ito and Yoshimoto, 1990; Ito et al, 1997). In the sturgeon, the midbrain precerebellar neurons have a simple morphology, although they show a short nonbranched dendrite, suggesting that they may be similar to but less specialized than the monopolar NLV cells of teleosts.…”