Retrograde and Transganglionic Anterograde Transport of an HRP-Wheat Germ Acclutinin Conjugate as Studied with Tetramethyl Benzidine Ultracyto-chemistry

The protein tracer horseradish peroxidase (HRP) is a primary component in the current method of choice for the anatomical demonstration of neuronal connections. Recently the performance of HRP as a neuronal tracer was improved significantly by conjugating the HRP to wheat germ agglutinin (WGA). When compared to HRP alone, the conjugate was observed to label larger numbers of neurons following comparable injections. In addition, the conjugate has been shown to be transported through peripheral ganglia and into the central projections of the ganglion neurons. However, the ability of the conjugate to demonstrate neuronal connections, as with HRP alone, is greatly dependent on the cytochemical method used to localize the HRP activity. It was previously shown by light microscopy, that the tetramethyl benzidine (TMB) method provided the highest degree of sensitivity when tested against eight other commonly used HRP cytochemical methods. Only recently have the conditions necessary to render the TMB reaction product suitable for electron microscopy been described.

Comparison of horseradish peroxidase visualization methods: quantitative results and further technical specifics.

Four methods used for the neurohistochemical demonstration of horseradish peroxidase (HRP) were quantitatively compared by counting retrogradely labeled neurons found after each method was used. HRP used as a retrograde marker is an important neuroanatomical tracing method, and maximum sensitivity in its demonstration of retrogradely, labeled neurons is important if these neuroanatomical studies are to completely demonstrate afferent neurons. The four methods compared were a diaminobenzidine (DAB) procedure, a Hanker-Yates procedure using P-phenylenediamine and pyrocatechol, an o-dianisidine procedure, and a tetramethyl benzidine (TMB) procedure. The TMB procedure resulted in a more complete topography of neurons afferent to the HRP application site, and demonstrated many more neurons in all afferent cell groups that either of the three other procedures. Use of the TMB method was especially critical in the cases of small HRP applications, a size useful for neuroanatomical studies, where the other methods demonstrated very few or no retrogradely labeled neurons. Neurons were judged to be retrogradely HRP labeled if they had small granules of the reaction product (the color varying with the chromogen) describing the somal shape, usually extending into the processes, and a clear nucleus. In addition, after the o-dianisidine or the TMB reaction a small number of retrogradely labeled neurons had soma and processes especially well filled with reaction product, giving the appearance of neurons from Golgi preparations. For a sensitive TMB reaction giving good results, exact H2O2 concentration, freshly prepared solutions, minimal postreaction exposure to alcohol, counterstaining, and clean glassware were each found to be important.

Sensitivity in horseradish peroxidase neurohistochemistry: a comparative and quantitative study of nine methods.

Nine currently available methods for HRP neurohistochemistry have been compared with each other on matching tissue sections from four rats and four rhesus monkeys. The nine methods investigated in this report are the diaminobenzidine (DAB) procedures of LaVail JH and LaVail MM (J Comp Neurol 157:303, 1974), of Adams JC (Neuroscience 2:141, 1977) and of Streit P and Reubi JC (Brain Res 126:530, 1977); the benzidine dihydrochloride (BDHC) procedures of Mesulam M-M (J Histochem Cytochem 24:1273, 1976) and of De Olmos J and Heimer L (Neurosci Lett 6:107, 1977); the o-dianisidine (O-D) procedure of De Olmos J (Exp Brain Res 29:541, 1977); the p-phenylenediamine dihydrochloride and pyrocatechol (PPD-PC) procedure of Hanker JS et al., (Histochem J 9:789, 1977) and the tetramethyl benzidine (TMB) procedures of Mesulam M-M (J Histochem Cytochem 26:106, 1978) and of De Olmos J et al. (J Comp Neurol 181:213, 1978). Quantitative comparisons were based on counts of retrogradely labeled perikarya. The extent of anterograde transport and the size of the injection site were also compared at a more qualitative level. The results indicate that one TMB procedure (Mesulam M-M, J Histochem Cytochem 26:106, 1978) is distinctly superior to each of the other eight procedures in the number of labeled perikarya that it can demonstrate. Furthermore, these differences are statistically significant at better than the 0.05 level of confidence. Differences in sensitivity are most evident when the perikarya contain small quantities of transported HRP. The same TMB method also demonstrates more anterograde transport and a larger injection site than all the other procedures. If less sensitive procedures are employed, afferent or efferent connections that are clearly demonstrated by this TMB procedure are either underestimated or completely overlooked. It is suggested that sensitivity in HRP neurohistochemistry is determined by multiple factors which include the method of fixation, post-fixation storage, the choice of chromogen, the incubation parameters, the type of HRP enzyme that is administered, and the postreaction treatment.