Neocortex provides direct synaptic input to interstitial neurons of the intermediate zone of kittens and white matter of cats: A light and electron microscopic study

Abstract

The existence of direct synaptic input from the neocortex to intermediate zone and white matter interstitial neurons was examined in both neonate and adult cats. This projection was studied by injecting the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) into the neocortex and examining whether cortical efferent axons formed synapses in the intermediate zone or white matter. Anterogradely labeled boutons establishing synapses in the intermediate zone and white matter were found at the electron microscopic level after injecting PHA-L into the primary visual, somatosensory, and suprasylvian cortex. Although labeled synapses were found in the intermediate zone of kittens injected at postnatal days 2 and 6, their morphological features appeared immature compared to those found in kittens aged 3 weeks or in adults. Postsynaptic targets of efferent cortical axons were studied in serial sections and shown to be dendritic shafts and spines. This paper shows that cortical efferent axons contribute synapses to interstitial neurons located in the intermediate zone of kittens and white matter of adults. The functional role of the corticointermediate zone/white matter projection remains to be determined.

Structure and projections of white matter neurons in the postnatal rat visual cortex

“…In addition to subcortically projecting neurons, the deep neuronal layers of rat cortex include the projection of an extensive corticocortical fiber system, which travels in deep cortical neuronal layers (Kreig, 1946;Caviness and Frost, 1980;Chapin et al, 1987;Reep and Goodwin, 1988;Vandevelde et al, 1996). The supposition that WM cells receive synaptic input is wellsupported by a previous study that used correlated light and electron microscopy (Shering and Lowenstein, 1994) where anterograde projections from visual cortical neurons were identified presynaptic to shafts and spines in the underlying WM, presumably the dendritic processes of WM cells (see also Smiley et al, 1998). Electrophysiologic results demonstrate that the WM cells receive spontaneous synaptic inputs (Torres-Reveron and Friedlander, 2001).…”

5-HT3AReceptor-Bearing White Matter Interstitial GABAergic Interneurons Are Functionally Integrated into Cortical and Subcortical Networks

“…Basic morphological features of WMICs were indicative of a neuronal phenotype (Neuburger, 1922;Lund et al, 1975;Kostovic and Rakic, 1980;Chun and Shatz, 1989;Muller, 1994;Shering and Lowenstein, 1994;Eastwood and Harrison, 2005;Friedlander and Torres-Reveron, 2009;Loup et al, 2009), a notion that was also supported by electron microscopy data (Valverde and Facal-Valverde, 1988). While there is ample information about the immunohistochemical signature of WMICs, such as the expression of GABA and specific GABAergic interneuron markers (Innis et al, 1979;Adrian et al, 1983;Schmechel et al, 1984;Shults et al, 1984;Somogyi et al, 1984;Chan-Palay et al, 1985;Sandell, 1986;Yan et al, 1996;Bayraktar et al, 1997;Tao et al, 1999;Suárez-Solá et al, 2009), only a few studies focused on electrophysiological properties of WMICs (Clancy et al, 2001;Torres-Reveron and Friedlander, 2007;Friedlander and Torres-Reveron, 2009).…”

Long-Range GABAergic Connections Distributed throughout the Neocortex and their Possible Function

“…It is known that GABAergic projection neurons in the white matter receive neocortical excitatory inputs as a major neuronal input, but these cells do not appear to receive significant thalamic input (Shering and Lowenstein, 1994; Okhotin and Kalinichenko, 2003, Tomioka et al, 2005). …”