Nitrergic neurons make synapses on dual-input dendritic spines of neurons in the cerebral cortex and the striatum of the rat: Implication for a postsynaptic action of nitric oxide

Pre-embedding electron microscopic immunocytochemistry was used to examine the ultrastructure of neurons containing nitric oxide synthase and to evalu ate their synaptic relationships with target neurons in the striatum and se nsorimotor cerebral sorter. Intense nitric oxide synthase immunoreactivity was found by light and electron microscopy in a type of aspiny neuron scatt ered in these two regions. The intensity of the labeling was uniform in the soma, dendrites and axon terminals of these neurons. In both forebrain reg ions, nitric oxide synthase-immunoreactive neurons received synaptic contac ts from unlabeled terminals, which were mostly apposed to small-caliber den drites. The unlabeled symmetric contacts were generally about four times as abundant as the unlabeled asymmetric contacts on the nitric oxide synthase -immunoreactive neurons. Terminals labeled for nitric oxide synthase were f illed with synaptic vesicles and were observed to contact unlabeled neurons . Only 54% tin the cerebral cortex) and 44.3% tin the striatum) of the nitr ic oxide synthase-immunoreactive terminals making apposition with the targe t structures were observed to form synaptic membrane specializations within the plane of the randomly sampled sections. The most common targets of nit ric oxide synthase-immunoreactive terminals were thin dendritic shafts (54% Of the immunoreactive terminals in the cortex and 75.7% of the immunoreact ive terminals in the striatum), while dendritic spines were a common second ary target (42% of the immunoreactive terminals in the cortex and 20.6% of the immunoreactive terminals in the striatum). The spines contacted by nitr ic oxide synthase-immunoreactive terminals typically also received an asymm etric synaptic contact from an unlabeled axon terminal. These findings suggest that: (i) nitric oxide synthase-immunoreactive neuro ns in the cortex and striatum preponderantly receive inhibitory input; (ii) nitric oxide synthase-containing terminals commonly make synaptic contact with target structures in the cortex and striatum; (iii) spines targeted by nitric oxide synthase-containing terminals in the cortex and striatum comm only receive an asymmetric contact as well, which may provide a basis for a synaptic interaction of nitric oxide with excitatory input to individual s pines. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights reserve d.