The post-embedding immunogold (PI) technique for immunolabeling of neuronal tissues utilizing standard thin-section transmission electron microscopy (TEM) continues to be a prime method for understanding the functional localization of key proteins in neuronal function. Its main advantages over other immunolabeling methods for thin-section TEM are (1) fairly accurate and quantifiable localization of proteins in cells; (2) double-labeling of sections using two gold particle sizes; and (3) the ability to perform multiple labeling for different proteins by using adjacent sections. Here we first review in detail a common method for PI of neuronal tissues. This method has two major parts. First, we describe the freeze-substitution embedding method: cryoprotected tissue is frozen in liquid propane via plunge-freezing, and is placed in a freeze-substitution instrument in which the tissue is embedded in Lowicryl at low temperatures. We highlight important aspects of freeze-substitution embedding. Then we outline how thin sections of embedded tissue on grids are labeled with a primary antibody and a secondary gold particle-conjugated antibody, and the particular problems encountered in TEM of PI-labeled sections. In the Discussion, we compare our method both to earlier PI methods and to more recent PI methods used by other laboratories. We also compare TEM immunolabeling using PI vs. various pre-embedding immunolabeling methods, especially relating to neuronal tissue.
Oligodendrocytes are the myelinating cells of the central nervous system. They provide trophic, metabolic, and structural support to neurons. In several pathologies such as multiple sclerosis (MS), these cells are severely affected and fail to remyelinate, thereby leading to neuronal death. The gold standard for studying remyelination is the g-ratio, which is measured by means of transmission electron microscopy (TEM). Therefore, studying the fine structure of the oligodendrocyte population in the human brain at different stages through TEM is a key feature in this field of study. Here we study the ultrastructure of oligodendrocytes, its progenitors, and myelin in 10 samples of human white matter using nine different markers of the oligodendrocyte lineage (NG2, PDGFRα, A2B5, Sox10, Olig2, BCAS1, APC-(CC1), MAG, and MBP). Our findings show that human oligodendrocytes constitute a very heterogeneous population within the human white matter and that its stages of differentiation present characteristic features that can be used to identify them by TEM. This study sheds light on how these cells interact with other cells within the human brain and clarify their fine characteristics from other glial cell types.
The transient receptor potential vanilloid 1 (TRPV1) is a non-selective ligand-gated cation channel involved in synaptic transmission, plasticity, and brain pathology. In the hippocampal dentate gyrus, TRPV1 localizes to dendritic spines and dendrites postsynaptic to excitatory synapses in the molecular layer (ML). At these same synapses, the cannabinoid CB1 receptor (CB1R) activated by exogenous and endogenous cannabinoids localizes to the presynaptic terminals. Hence, as both receptors are activated by endogenous anandamide, co-localize, and mediate long-term depression of the excitatory synaptic transmission at the medial perforant path (MPP) excitatory synapses though by different mechanisms, it is plausible that they might be exerting a reciprocal influence from their opposite synaptic sites. In this anatomical scenario, we tested whether the absence of TRPV1 affects the endocannabinoid system. The results obtained using biochemical techniques and immunoelectron microscopy in a mouse with the genetic deletion of TRPV1 show that the expression and localization of components of the endocannabinoid system, included CB1R, change upon the constitutive absence of TRPV1. Thus, the expression of fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) drastically increased in TRPV1−/− whole homogenates. Furthermore, CB1R and MAGL decreased and the cannabinoid receptor interacting protein 1a (CRIP1a) increased in TRPV1−/− synaptosomes. Also, CB1R positive excitatory terminals increased, the number of excitatory terminals decreased, and CB1R particles dropped significantly in inhibitory terminals in the dentate ML of TRPV1−/− mice. In the outer 2/3 ML of the TRPV1−/− mutants, the proportion of CB1R particles decreased in dendrites, and increased in excitatory terminals and astrocytes. In the inner 1/3 ML, the proportion of labeling increased in excitatory terminals, neuronal mitochondria, and dendrites. Altogether, these observations indicate the existence of compensatory changes in the endocannabinoid system upon TRPV1 removal, and endorse the importance of the potential functional adaptations derived from the lack of TRPV1 in the mouse brain.