Synaptic contacts between serotonergic and cholinergic neurons in the rat dorsal raphe nucleus and laterodorsal tegmental nucleus

We examined synaptic connectivity between cholinergic and serotonergic neur ons in the dorsal raphe nucleus and the laterodorsal tegmental nucleus of t he rat. To this purpose we employed two variations (the combination of pre- embedding immunogold-silver intensification with avidin-biotin-peroxidase c omplex technique and the combination of avidin-biotin-peroxidase/3,3'-diami nobenzidine/silver- gold intensification with avidin-biotin-peroxidase/3,3' -diaminobenzidine reaction) of a double pre-embedding immunoelectron proced ure, using primary antibodies against vesicular acetylcholine transporter a nd serotonin. At the light-microscopic level, serotonin-like immunoreactive neurons in the dorsal raphe nucleus appeared as reddish black and vesicula r acetylcholine transporter-like immunoreactive axon terminals were brown c olored using a combination of preembedding immunogold-silver technique and avidin-biotin-peroxidase complex technique. Serotonin-like immunoreactive f ibers projected to the laterodorsal tegmental nucleus. At the electron micr oscopy level, with both methods we observed in the dorsal raphe nucleus ves icular acetylcholine transporter-immunopositive axon terminals in synaptic contact with serotonin-like immunoreactive dendrites and, to a lesser degre e, with serotonin-like immunoreactive cell bodies. These synapses usually w ere of the symmetrical type. Occasionally we noted, next to vesicular acety lcholine transporter-immunopositive axon terminals, also immunonegative ter minals synapsing with the serotonin-like immunoreactive dendrites. In the l aterodorsal tegmental nucleus we found serotonin-like immunoreactive axon t erminals and immunonegative terminals forming synapses with vesicular acety lcholine transporter-immunoreactive dendrites. Most synapses formed by the serotonin-like immunopositive terminals were of the asymmetrical type. Our results suggest that serotonergic neurons in the dorsal raphe nucleus a nd cholinergic neurons in the laterodorsal tegmental nucleus may reciprocal ly influence each other by means of synaptic connectivity. Such connectivit y may serve to regulate pain sensation, or be involved in the regulation of the sleeping-waking cycle. (C) 2000 IBRO. Published by Elsevier Science Lt d.