Immunoelectron microscopy of β-endorphinergic synaptic innervation of nitric oxide synthase immunoreactive neurons in the dorsal raphe nucleus

Abstract

On the basis of the comparing of the distribution of β-endorphin-like immunoreactive neuronal fibers and nitric oxide synthase-like immunoreactive neurons in the dorsal raphe nucleus, the synapses between the two immunocytochemically identified neurons were studied with a modified DAB-silver-gold intensification double immunostaining technique at the electron microscopic level. Although both of them can be found in the mediodorsal and medioventral parts of the dorsal raphe nucleus, the synapses between them could only be found in the mediodorsal part. The majority of the β-endorphin-like immunoreactive neuronal fibers contained many dense-cored vesicles. The synapses made by β-endorphin-like immunoreactive neuronal axon terminals on nitric oxide synthase-like immunoreactive neurons were both symmetrical and asymmetrical with the former predominant, especially in the axo-dendritic ones. β-Endorphin-like immunoreactive perikarya could only be found in the ventrobasal hypothalamus. These findings suggest the possibility that the β-endorphin- producing neurons in the ventrobasal hypothalamus could influence nitric oxide synthase-containing neurons in the dorsal raphe nucleus by synaptic relations.