Association of dopaminergic terminals and neurons releasing nitric oxide in the rat striatum: an electron microscopic study using NADPH-diaphorase histochemistry and tyrosine hydroxylase immunohistochemistry.

To examine synaptic input and association of terminals containing dopamine and other transmitters to rat striatal nitric oxide synthase-expressing neurons, an electron microscopic study using tyrosine hydroxylase (TH) immunohistochemistry combined with histochemistry for NADPH-diaphorase (NADPHd) was performed. NADPHd-positive neurons had medium-sized cell bodies containing a highly invaginated nucleus and received relatively sparse synaptic input; 3.6% of boutons apposed to the NADPHd-positive neurons were TH-immunoreactive. Of these TH-immunoreactive boutons, two synaptic contacts showing symmetrical synaptic specializations were found on a cell body and a proximal dendrite of a NADPHd-positive neuron. Other nonsynaptic TH-immunoreactive boutons were occasionally associated with unlabeled terminals adjacent to the NADPHd-positive dendrites and also forming asymmetric synaptic contacts with unlabeled spinous or dendritic profiles. These results suggest that activity of the striatal neurons that release nitric oxide may be regulated by direct synaptic input from dopaminergic neurons and also suggest that the TH-immunoreactive terminals associated with the dendrites of nitric oxide synthase-expressing neurons provide the sites where nitric oxide influences dopamine release from neighboring terminals.