Septohippocampal neurons in the rat septal complex have substantial glial coverage and receive direct contacts from noradrenaline terminals

Abstract

The ultrastructure of septohippocampal neurons in the septal complex and their relations with catecholamine, principally noradrenaline, terminals were examined in single thin sections. Projection neurons were identified by retrograde transport of wheat-germ agglutinated apo-horseradish peroxidase conjugated to colloidal gold particles (WAHG) following an injection into the hippocampal formation of anesthetized adult rats. After a 1 day survival, sections through the septal complex were labeled with antibodies to tyrosine hydroxylase (TH) or dopamine-β-hydroxylase (DBH). By light microscopy, numerous processes with TH- and DBH-immunoreactivity were near neurons containing retrogradely transported WAHG. By electron microscopy, most WAHG was associated with lysosomes, multivesicular and ‘sequestration’ bodies in the cytoplasm of perikarya and large dendrites. WAHG-labeled perikarya ( n = 114) had a large amount of astrocytic coverage (> 60% of surface) and a low amount of terminal coverage (< 25%). WAHG-labeled perikarya and dendrites were either directly contacted by TH- or DBH-labeled terminals or abutted glial processes apposed to TH- or DBH-labeled terminals. Immunoreactivity for TH and DBH was found primarily in axons and axon terminals. The morphology and synaptic associations of TH-labeled terminals was similar to that reported previously. DBH-labeled terminals ( n = 314; 0.5 ± 0.2 μm in diameter) contained numerous small clear vesicles and from 0–4 large, dense-core vesicles. DBH-containing terminals: (1) contacted perikarya and dendrites (58%), 10% of which contained WAHG; (2) were closely apposed to other terminals (7%); or (3) were separated by glial processes (35%). DBH-labeled terminals formed chiefly symmetric synapses on perikarya. However, most DBH-containing terminals formed both asymmetric and symmetric synapses on the shafts of small dendrites, suggesting both excitatory and inhibitory functions for noradrenaline terminals on septal neurons. The results demonstrate that septohippocampal neurons (1) are mostly engulfed by astrocytes and have very little terminal coverage; (2) are both directly contacted (synapses) and indirectly contacted (appositions to opposing astrocytes or axon terminals) by catecholamine, particularly noradrenaline, terminals.