Nitric oxide synthase-containing projections to the ventrobasal thalamus in the rat

Microiontophoretic studies of thalamic neurons suggests that nitric oxide ( NO) plays an important role in mediating somatosensory transmission. The th alamus contains few nitric oxide synthase (NOS)-immunoreactive neurons; thu s, the major source of thalamic NO is presumably from NOS-positive axons of extrathalamic origin. The cells of origin of these putative NOS-containing pathways to the ventrobasal thalamus were investigated in rats by combinin g retrograde tracing with immunocytochemistry for NOS. The location and mor phology of double-labeled neurons was compared with that of single-labeled neurons. The most significant sources of NOS-containing afferents to the th alamus were found to be the pedunculopontine (PPN) and laterodorsal tegment al (LDT) nuclei. NOS-immunoreactive neurons in these cholinergic nuclei pro ject bilaterally to the thalamus, most strongly ipsilaterally. The thalamus appears to be a major target of PPN, since even selective thalamic injecti ons result in retrograde labeling of at least one third of its NOS-immunore active neurons. A significant number of NOS-negative neurons in both the PP N and LDT also project to the thalamus. Minor sources of NOS-containing tha lamic afferents include the lateral hypothalamus, the dorsal, median and po ntine raphe nuclei, the parabrachial nuclei, and the pontomedullary reticul ar formation. In all these structures, NOS-negative thalamopetal neurons gr eatly outnumber the NOS-positive ones. Ascending sensory pathways to the th alamus, including those from the sensory trigeminal nuclei, the dorsal colu mn nuclei, and the spinal cord, as well as the auditory and vestibular cent ers, arise exclusively from NOS-negative neurons. The major NOS-positive pr ojections are implicated in affective and alerting systems, supporting that NO may act to modulate attentiveness in thalamic relay nuclei.