Electron microscopy of Golgi-impregnated interneurons: notes on the intrinsic connectivity of the cerebral cortex

Golgi, Cajal, and the fine structure of the nervous system

Towards the middle of the 20th century, neuroanatomy was on the decline. It was revived by the development of two new methods. One was the Nauta-Gygax method, which selectively stained nerve fibers that had been caused to degenerate by experimental lesions. This allowed connections between various parts of the nervous system to be better determined. The second was electron microscopy, which allowed the structure of neurons and the synapses between them to be examined in detail, and eventually this led to a revival of the Golgi impregnation methods. This occurred in the 1970s because of the desire of electron microscopists to determine the origins of the neuronal profiles they encountered in electron micrographs of various parts of the central nervous system. Eventually this led to the development of Golgi/EM techniques, whereby individual impregnated neurons could first be characterized by light microscopy and then thin sectioned for detailed analyses. Examining the axon terminals of such impregnated neurons, especially those in the cerebral cortex, for the first time revealed details of intercellular connections and allowed neuronal circuits to be postulated. However, Golgi/EM had only a brief, but fruitful existence. It was soon superceded by intracellular filling techniques, which allowed the added dimension that the physiological properties of identified neurons could also be determined.

Colocalization of parvalbumin, calretinin and calbindin D-28k in human cortical and subcortical visual structures

Several studies have demonstrated that three calcium-binding proteins parvalbumin (PV), calbindin D-28k (CB) and calretinin (CR) mark distinct subsets of cortical interneurons. This study demonstrates, in cortical and subcortical visual structures, the coexistence of two calcium-binding proteins in some neuronal subpopulations. The human visual cortex (VC), lateral geniculate nucleus (LGN). lateral inferior pulvinar (LIP) and superior colliculus (SC) were examined by a double-labelling immunocytochemical technique. The VC showed mostly separate populations of PV, CB and CR immunoreactive (-ir) interneurons, but also small populations of double-stained PV + CR and CR + CB neurons, while PV + CB neurons were less frequent. An average of 2.5% of the immunoreactive neurons were double-stained for PV + CR and 7.1% for CR + CB in area 17, while this percentage was slightly higher in association area 18 (3.3 and 7.4%, respectively). In the LGN and LIP, double-stained neurons were scarce, but in the fibre capsule of these nuclei, as well as in the optic radiation (OR) and white matter underlying area 17, both double-stained PV + CR or CR + CB and separate populations of PV-ir, CB-ir and CR-ir neurons and fibres were observed. Unlike the thalamic regions, the SC showed some double-stained PV + CR and CR + CB neurons, scattered both in the superficial and deep layers. These findings are discussed in the light of similar observations recently reported from other regions of the human brain.

Differential effects of the Rac GTPase on Purkinje cell axons and dendritic trunks and spines

Neurons contain distinct compartments including dendrites, dendritic spines, axons and synaptic terminals. The molecular mechanisms that generate and distinguish these compartments, although largely unknown, may involve the small GTPases Rac and Cdc42, which appear to regulate actin polymerization. Having shown that perturbations of Rac1 activity block the growth of axons but not dendrites of Drosophila neurons, we investigated whether this also applies to mammals by examining transgenic mice expressing constitutively active human Rac1 in Purkinje cells. We found that these mice were ataxic and had a reduction of Purkinje-cell axon terminals in the deep cerebellar nuclei, whereas the dendritic trees grew to normal height and branched extensively. Unexpectedly, the dendritic spines of Purkinje cells in developing and mature cerebella were much reduced in size but increased in number. These 'mini' spines often form supernumerary synapses. These differential effects of perturbing Rac1 activity indicate that there may be distinct mechanisms for the elaboration of axons, dendrites and dendritic spines.