Retrograde axonal transport specificity in the locus coeruleus neurons after [ 3H]noradrenaline injection into the rat olfactory bulb

Abstract

Retrograde axonal transport process was investigated in the afferent systems to the rat olfactory bulb, after [ 3H]noradrenaline ([ 3H]NA) injection into the olfactory bulb, in order to provide evidence regarding its specificity and the biochemical basis supporting this specificity. Radioautographs showed that [ 3H]NA unilaterally injected into the olfactory bulb at a concentration of 10 −3 M, resulted in labeling of the structures afferent to the olfactory bulb, mainly on the injected side: locus coeruleus (LC), dorsal and central raphes, nucleus of the lateral olfactory tract and piriform cortex. Heavy labeling was observed on the noradrenergic LC cell bodies, whereas the radioautographic reaction was less intense on the other structures. After 10 −4 M injection, the labeling intensity of the LC cell bodies was lower while very rare weakly labeled cell bodies persisted in the dorsal raphe, nucleus of the lateral olfactory tract and piriform cortex. The LC cell bodies were exclusively labeled when the concentration of [ 3H]NA injection was 10 −5 M. All the other structures were devoid of labeling. It was still possible to detect labeled cell bodies in the LC for a 10 −6 M concentration. Following bilateral injections of [ 3H]NA (10 −3 M) the total radioactivity retrogradely transported to the LC represented about 4 times the total radioactivity measured in the periaqueductal gray substance (as control tissue of the tracer diffusion). Fractional study by ethanol of LC tissue homogenate and liquid scintillation counting of each fraction showed that 60% of the total radioactivity (about 2.5 times the control value) was in the supernatant and 40% (about 20 times the control value) was associated with the precipitate. In the other regions such as the dorsal and central raphes and periaqueductal gray substance, radioactivity was mainly found in the supernatant, except for the dorsal raphe whose precipitate contained a low amount of radioactivity (about 4 times the control value). Colchicine (an axonal transport inhibitor) bilaterally injected into the medial forebrain bundle and systemic administration of desipramine (a noradrenaline uptake inhibitor) decreased the radioactivity associated with the LC precipitate by 90 and 85% and the LC supernatant radioactivity by 55 and 35%, respectively. These pretreatments did not significantly affect the radioactivity amounts measured in the different fractions of dorsal and central raphes and periaqueductal gray substance. Radioautographic study after colchicine treatment showed a large decrease in the labeling intensity of the LC cell bodies as compared to the non-treated side. Therefore, we suggest that low concentrations (10 −5 M) of [ 3H]NA injected in the olfactory bulb determine specific conditions of noradrenergic pathway labeling. This specific labeling after migration could be supported by the radioactive ethanol precipitate which would appear to contain [ 3H]NA- and/or 3H-derivatives-binding protein. Such a 3H-macromolecular complex, which could represent the specific carrier, may well undergo retrograde transport from the nerve terminals towards the cell bodies.