Distribution and morphology of tegmental neurons receiving nigral inhibitory inputs in the cat: an intracellular HRP study.

Intracellular recordings and morphological identification of neurons by means of intracellular HRP staining were performed in the midbrain-pontine tegmentum of the cat. Electrical stimulation of the substantia nigra, pars reticulata, induced short-latency inhibitory postsynaptic potentials in tegmental neurons, as previously reported. These tegmental neurons were distributed not only in the pedunculopontine tegmental nucleus (PPTN) (26 cells), but also in the cuneiform nucleus (CNF) (13 cells), the central gray substance (CG) (four cells), the parabrachial nucleus (three cells), and the tegmentum between the inferior colliculus and the CG (two cells). The morphological characteristics of the HRP-stained tegmental neurons were analyzed by camera lucida drawings of 16 well-stained cells, i.e., ten PPTN neurons, three CNF neurons, two neurons in the tegmentum between the IC/CG, and one CG neuron. All of these neurons seem to be classified as "isodendritic" neurons. Regardless of the soma size (15-120 micron in diameter), most of the tegmental neurons showed wide dendritic fields of 1-2 mm, mediolaterally, and 0.5-1 mm, dorsoventrally. These results indicate that the nigrotegmental projections exert an inhibitory influence on the neurons located in a wide variety of nuclei of the midbrain-pontine dorsal tegmentum.