Distribution and morphology of serotonin‐immunoreactive axons in the hippocampal region of the New Zealand white rabbit. I. Area dentata and hippocampus

Abstract

This study provides a detailed light microscopic description of the morphology and distribution of immunohistochemically stained serotonergic axons in the hippocampal region of the New Zealand white rabbit. The serotonergic axons were segregated morphologically into three types: beaded fibers, fine fibers, and stem-axons, respectively. Beaded fibers were thin serotonergic axons with large varicosities, whereas thin axons with small fusiform or granular varicosities were called fine fibers. Finally, thick straight non-varicose axons were called stem-axons. Beaded fibers often formed large conglomerates with numerous boutons (pericellular arrays) in close apposition to the cell-rich layers in the hippocampal region, e.g., the granular and hilar cell layers of the dentate area and the pyramidal cell layer ventrally in CA3. The pericellular arrays in these layers were often encountered in relation to small calbindin-D2BK-positive cells, as shown by immunohistochemical double staining for serotonin and calbindin-D28K. The beaded and fine serotonergic fibers displayed a specific innervation pattern in the hippocampal region and were encountered predominantly within the terminal field of the perforant path, e.g., the stratum moleculare hippocampi and the outer two-thirds of the dentate molecular layer. These fibers were also frequently seen in the deep part of the stratum oriens and the alveus, forming a dense plexus in relation to large multipolar calbindin-D28K-positive cells and their basal extensions. Stem-axons were primarily seen in the fimbria and alveus. This innervation pattern was present throughout the entire hippocampal formation, but there were considerable septotemporal differences in the density of the serotonergic innervation. A high density of innervation prevailed in the ventral/temporal part of the hippocampal formation, whereas the dorsal/septal part received only a moderate to weak serotonergic innervation. These results suggest that the serotonergic system could modulate the internal hippocampal circuitry by way of its innervation in the terminal field of the perforant path, the hilus fasciae dentatae, and ventrally in the zone closely apposed to the mossy fiber layer and the pyramidal cells of CA3. This modulation could be of a dual nature, mediated directly by single serotonergic fibers traversing the hippocampal layers or indirectly by the pericellular arrays and their close relation to the calbindin-D28K-positive cells. The marked septotemporal differences in innervation density point toward a difference between the ventral and dorsal parts of the hippocampal formation with respect to serotonergic function and need for serotonergic modulation.

The Avian Hippocampal Formation and the Stress Response

“…5-HT 1A receptor labelling was also lower in the caudal than the rostral HF [Herold et al, 2014], which the authors relate to lower 5-HT levels detected in the caudal HF of pigeons in a previous study . The latter is in contrast to the mammalian pattern, which shows more 5-HT innervation in the temporal than the septal pole of the hippocampus [Gage and Thompson, 1980;Bjarkam et al, 2003] and might argue against our hypothesis that the caudal HF is more involved in emotional processing. Finally, choline acetyltransferase staining is denser in the caudal DL than in the more rostral sections , implying heavier cholinergic innervation of the caudal HF.…”

Redox Dysregulation Affects the Ventral But Not Dorsal Hippocampus: Impairment of Parvalbumin Neurons, Gamma Oscillations, and Related Behaviors

“…It thus supports the concept that the oxidative stress in VH results from higher ROS production in VH compared to DH. The VH could be more vulnerable to redox dysregulation because of its richer catecholamine innervations compared to DH (Oleskevich et al, 1989;Gasbarri et al, 1997;Bjarkam et al, 2003). Auto-oxidation and catabolism of catecholamines form oxidative species including H 2 O 2 , superoxide and o-quinones (Cadet and Brannock, 1998).…”

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

“…Serotonergic neurons in the brainstem of the New Zealand white rabbit undergo the same kind of lateralization as seen in higher primates (Bjarkam et al 1997) and display a specific innervation of the hippocampus proper and the dentate area (Bjarkam et al 2003). This enables the serotonergic system to exert a profound influence on the termination zone of the perforant path as well as on the pyramidal cells in CA3 of the temporal hippocampus, and thus on the internal hippocampal circuitry.…”

“…Beaded fibers were seldom encountered in areas 17, 29ab, 29cd and 36, but were often seen in the superficial layers of the retrohippocampal areas and area 29e. They had an irregular, intermingled course, forming conspicuous conglomerates of bouton-rich fibers resembling pericellular arrays/baskets (Mulligan and To¨rk 1988;Hornung et al 1990;Hornung and Celio 1992;Bjarkam et al 2003). Some of these structures embraced calbindin-D 28K -positive cells (Fig.…”