Quantitative immunoelectron microscopic colocalization of GABA and enkephalin in the ventrocaudal periaqueductal gray of the rat

In the present ultrastructural study in the ventrocaudal periaqueductal gra y (PAG) of the rat, the relationship and the association between GABAergic and enkephalinergic neuronal elements were investigated using postembedding colocalization immunogold electron microscopic technique in order to estab lish the precise relationship between these two important neurotransmitters in this part of the brain stem. The GABA-lille neuronal elements were immu noreacted with 20 nm gold particles and the enkephalin (ENK)-like immunorea ctive neurons were labeled with 10 nm gold particles. Double labeling of se ctions with ENK and GABA produced colocalization in 23.3% and 1.2% of axon terminals and dendrites, respectively. Most of the double-labeled terminals contained more GABA-like than ENK-like immunolabeling. Approximately 19.4% of the labeled axon terminals and 8.5% of the labeled dendrites contained only GABA-like immunoreactivity, while 24% of the immunolabeled dendrites w ere immunoreactive with only ENK-like immunoreactivity. The synapses betwee n the two kinds of immunolabeled neuronal profiles appear to be both asymme trical and symmetrical. GABA-like immunolabeled terminals contained small, clear, pleomorphic or round vesicles and were found to make synapses with E NK-like immunolabeled and nonimmunolabeled dendrites, whereas most of the E NK-like immunolabeled axon terminals contained dense-cored vesicles. Approx imately half of the axon terminals (51%) and dendrites (56%) in the ventrol ateral FAG were not labeled for either GABA or for ENK immunoreactivity. Th e results are discussed in terms of GABAergic inhibition of antinociceptive mechanisms in the ventrolateral FAG and of the activation of these mechani sms by ENK neurotransmitter. Synapse 31:216-228, 1999. (C) 1999 Wiley-Liss, Inc.