Perisomatic innervation and neurochemical features of giant pyramidal neurons in both hemispheres of the human primary motor cortex.

Betz cells-the gigantopyramidal neurons found in high amount in the primary motor cortex-are among of the most characteristic neuronal cells. A part of them contains the calcium-binding protein parvalbumin (PV) in primates. However, less is known about these cells in the human motor cortex despite their important role in different neurological disorders. Therefore, the aim of our study was to investigate the neurochemical features and perisomatic input properties of Betz cells in control human samples with short post-mortem interval. We used different microscopic techniques to investigate the primary motor cortex of both hemispheres. The soma size and density, and expression of PV of the Betz cells were investigated. Furthermore, we used confocal fluorescent and electron microscopy to examine their perisomatic input. The soma size and density showed moderate variability among samples and hemispheres. Post-mortem interval and hemispherical localization did not influence these features. Around 70% of Betz cells expressed PV, but in less intensity than the cortical interneurons. Betz neurons receive dense perisomatic input, which are mostly VIAAT- (vesicular inhibitory amino acid transporter) and PV immunopositive. In the electron microscope, we found PV-immunolabelled terminals with asymmetric-like synaptic structure, too. Terminals with morphologically similar synaptic specialisation were also found among vGluT2- (vesicular glutamate transporter type 2) immunostained terminals contacting Betz cells. Our data suggest that Betz cells' morphological properties showed less variability among subjects and hemispheres than the density of them. Their neurochemical and perisomatic input characteristics support their role in execution of fast and precise movements.

Post mortem single-cell labeling with DiI and immunoelectron microscopy unveil the fine structure of kisspeptin neurons in humans.

Kisspeptin (KP) synthesizing neurons of the hypothalamic infundibular region are critically involved in the central regulation of fertility; these cells regulate pulsatile gonadotropin-releasing hormone (GnRH) secretion and mediate sex steroid feedback signals to GnRH neurons. Fine structural analysis of the human KP system is complicated by the use of post mortem tissues. To gain better insight into the neuroanatomy of the somato-dendritic cellular compartment, we introduced the diolistic labeling of immunohistochemically identified KP neurons using a gene gun loaded with the lipophilic dye, DiI. Confocal microscopic studies of primary dendrites in 100-µm-thick tissue sections established that 79.3% of KP cells were bipolar, 14.1% were tripolar, and 6.6% were unipolar. Primary dendrites branched sparsely, contained numerous appendages (9.1 ± 1.1 spines/100 µm dendrite), and received rich innervation from GABAergic, glutamatergic, and KP-containing terminals. KP neuron synaptology was analyzed with immunoelectron microscopy on perfusion-fixed specimens. KP axons established frequent contacts and classical synapses on unlabeled, and on KP-immunoreactive somata, dendrites, and spines. Synapses were asymmetric and the presynaptic structures contained round and regular synaptic vesicles, in addition to dense-core granules. Although immunofluorescent studies failed to detect vesicular glutamate transporter isoforms in KP axons, ultrastructural characteristics of synaptic terminals suggested use of glutamatergic, in addition to peptidergic, neurotransmission. In summary, immunofluorescent and DiI labeling of KP neurons in thick hypothalamic sections and immunoelectron microscopic studies of KP-immunoreactive neurons in brains perfusion-fixed shortly post mortem allowed us to identify previously unexplored fine structural features of KP neurons in the mediobasal hypothalamus of humans.