Postnatal development of the cholinergic innervation in the dorsal hippocampus of rat: Quantitative light and electron microscopic immunocytochemical study

Abstract

Choline acetyltransferase (ChAT) immunocytochemistry was used to examine the distribution and ultrastructural features of the acetylcholine (ACh) innervation in the dorsal hippocampus of postnatal rat. The length of ChAT-immunostained axons was measured and the number of ChAT-immunostained varicosities counted, in each layer of CA1, CA3, and dentate gyrus, at postnatal ages P8, P16, and P32. At P8, an elaborate network of varicose ChAT-immunostained axons was already visible. At P16, the laminar distribution of this network resembled that in the adult, but adult densities were reached only by P32. Between P8 and P32, the mean densities for the three regions increased from 8.4 to 14 meters of axons and 2.3 to 5.7 million varicosities per cubic millimeter of tissue. At the three postnatal ages, the ultrastructural features of ChAT-immunostained axon varicosities from the strata pyramidale and radiatum of CA1 were similar between layers and comparable to those in adult, except for an increasing frequency of mitochondria (up to 41% at P32). The proportion of these profiles displaying a synaptic junction was equally low at all ages, indicating an average synaptic incidence of 7% for whole varicosities, as previously found in adult. The observed junctions were small, usually symmetrical, and made mostly with dendritic branches. These results demonstrate the precocious and rapid maturation of the hippocampal cholinergic innervation and reveal its largely asynaptic nature as soon as it is formed. They emphasize the remarkable growth capacities of individual ACh neurons and substantiate a role for diffuse transmission by ACh during hippocampal development.

Cholinergic Control in Developing Prefrontal–Hippocampal Networks

“…The neonatal Hipp receives a strong input from the cholinergic neurons of the MS (Matthews et al, 1974;Aznavour et al, 2005) that, according to our data, modulates the theta bursts in the CA1 by decreasing their amplitude. Because these bursts drive the prelimbic NG, the cholinergic input to Hipp may indirectly control the amplitude of prefrontal theta-alpha-band activity.…”

Nonsynaptic Chemical Transmission Through Nicotinic Acetylcholine Receptors

“…In older rats, cholinergic boutons establish significantly fewer synapses than in young animals (166). In the developing brain (postnatal day 16 -32), the synaptic incidence of varicosities is 6% in the cortex (11) and 17% in the hippocampus, respectively (112). These numbers are quite close to the adult values of 7 and 14% (169,168).…”

“…The position of cholinergic axons can be identified by staining for vesicular ACh transporter immunoreactivity, which is strongest in the stratum oriens of the CA1 region within the hippocampus (164), indicating that the cholinergic input may predominantly target the basal dendrites of pyramidal neurons and local interneurons. Although less evident, ACh synthesis [shown by choline acetyltransferase (ChAT)-immunostained varicosities] is also more pronounced in the strata pyramidale and oriens than in the radiatum or lacunosum moleculare (11). Another important piece of evidence for the nonsynaptic nature of the cholinergic system is the localization of AChE.…”

Cholinergic Circuits and Signaling in the Pathophysiology of Schizophrenia

“…Presumptive forebrain projection cholinergic neurons migrate from the MGE (and possibly from the anterior entopeduncular/preoptic area) radially to take up locations in basal forebrain nuclei (medial septum, magnocellular nucleus, diagonal band of Broca), whereas the striatal cholinergic interneurons migrate tangentially from the MGE, occupying dispersed sites throughout the striatal plate. Subsequent to this early birth and migration from the MGE, 1-2 weeks pass before these neurons undergo maturation into cholinergic neurons (Aznavour et al, 2005;Berger-Sweeney, 2003;Mechawar and Descarries, 2001). This delay allows time for other populations of predominantly GABAergic neurons to emerge from the MGE and LGE and occupy their appropriate positions throughout the striatum and cortex for radial population of the neocortex with pyramidal cells and for the proper targeting of axons from the dorsal thalamus to project through the striatal region to innervate cortical structures (Flames et al, 2004;Lopez-Bendito et al, 2006;van Vulpen and van der Kooy, 1998).…”