The localization of histidine decarboxylase-immunoreactive cell bodies in the guinea-pig brain

In this immunocytochemical study the localization of histaminergic neurons in the guinea-pig brain was investigated using an antiserum raised against rat histidine decarboxylase. The total number of histaminergic cells appeared to be between 4200 and 4800, which is considerably higher than the number reported by others for the rat brain. The major groups of histaminergic cells were confined to the nucleus mammillaris medialis and the nucleus caudalis magnocellularis (CM). Three minor groups of histaminergic cells were found, respectively, in the area of the nucleus premammillaris ventralis, along the ventral aspect of the nucleus supramammillaris and in the nucleus caudalis magnocellularis postmammallaris (PCM). Finally, some scattered histaminergic cell bodies were observed in the nucleus mammillaris posterior. The most ventrally situated groups of histaminergic cells, i.e. the CM and PCM groups, extend into a medial direction as a string of cell bodies which is possibly located in the pia and likely bridges these bilaterally situated cell groups. Some of the histaminergic cell groups found in the guinea-pig brain appear to be absent or much smaller in the rat brain. These data indicate that the histaminergic neuronal systems in the guinea-pig and the rat brain are quite similar with regard to their overall organization, but also appear to display some differences, both with regard to the number and, at least partly, the localization of the cell bodies.