Distribution and morphology of serotonin-immunoreactive axons in the retrohippocampal areas of the New Zealand white rabbit

Abstract

This study provides a detailed light microscopic description of the morphology and distribution of serotonin-immunoreactive axons in the paleocortical retrohippocampal areas, viz. the subiculum, presubiculum, parasubiculum and entorhinal area, and the adjoining neocortical perirhinal and retrosplenial cortices of the New Zealand white rabbit. Serotonergic axons could be segregated into three different fiber types named fine fibers, beaded fibers and stem-axons. Fine fibers were evenly distributed thin axons with small fusiform/granular varicosities. Beaded fibers were thin axons with large varicosities, predominantly located in the retrohippocampal supragranular layers, where they often formed pericellular arrays. Stem-axons were thick straight, nonvaricose axons seen in the white matter of psalterium dorsale, alveus and the plexiform layer. The paleocortical retrohippocampal areas had a dense supragranular innervation with numerous tortuous fine and beaded fibers, intermingled in conglomerates with conspicuous varicosities forming pericellular arrays. In contrast, the neocortical area 17 and the lateral part of the perirhinal cortex (area 36) were innervated by evenly distributed fine fibers with a moderate number of small varicosities and few ramifications, whereas, the retrosplenial cortex (areas 29e, 29ab and 29cd), and the medial part of the perirhinal cortex (area 35) displayed an intermediate innervation pattern, probably reflecting the transitional nature of these areas being located between the paleo- and the neocortex. The described dualistic innervation pattern may functionally enable the serotonergic system to exert a strong influence on the supragranular layers of the retrohippocampal areas and thus on the neural input entering these areas from the perirhinal and neighboring polymodal association neocortices, whereas the innervation pattern in the adjoining neocortical areas points towards a more diffuse and general modulation of neural activity herein.

Brain anatomy of the 4‐day‐old European rabbit

“…Moreover, the differentiation into medial and lateral parts cannot be clearly ascertained (Bjarkam et al. ). The entorhinal cortex is commonly perceived as a major input and output structure of the hippocampal formation.…”

Serotonin fibre sprouting and increase in serotonin transporter immunoreactivity in the CA1 area of hippocampus in a triple transgenic mouse model of Alzheimer’s disease

“…, 2008). SERT‐IRFs were classified as BF, FF and SA according to their morphological features, as already described (Bjarkam et al. , 2005; Keuker et al.…”

“…, 2005; Hensler, 2006). DR axons are fine fibres (FF) with small and regularly spaced varicosities, whilst MR axons are either non‐varicose (SA; stem axons) or thick with large irregular varicosities (beaded fibres – BF; Bjarkam et al. , 2005; Keuker et al.…”

The Retrograde Connections and Anatomical Segregation of the Göttingen Minipig Nucleus Accumbens

“…The anti DARPP-32 (Dopamine and cAMP-regulated neuronal phosphoprotein) immunohistochemical procedures were performed in accordance with the avidin-biotin method (Bjarkam et al, 2004, 2005). Anti-DARPP-32 was applied as primary antibodies, whereas the secondary antibody was biotin-labeled anti-sheep IgG.…”