The olivary pretectal nucleus: experimental anatomical studies in the rat

Abstract

The olivary pretectal nucleus (OPN) is one of several pretectal nuclei with direct input from the eye, but is the only one containing neurons whose rate of firing is linearly related to the intensity of light falling on the retina. It is thus the first central station in the pupillary light reflex. We report experimental light (l.m.) and electron microscope (e.m.) studies of this nucleus in adult albino and pigmented rats, the first extensive study of this or of any other mammalian pretectal nucleus. The more cytoarchitecturally distinct rostral part of the nucleus ( pars oralis of F. Scalia ( J. comp. Neurol. 145, 223-253, 1972)), which is approximately 350 μm in diameter in coronal sections was analysed. It displays a peripheral shell of closely packed neurons and a central core in which cell density is lower and in which there are very few myelinated fibres. Cellular and synaptic organization appear to be identical in albino and pigmented animals but the nucleus is more superficially situated in albinos because the brachium of the superior colliculus is thinner than in pigmented animals. Two major cell types were recognized in Golgi preparations. Class I cells were large (somal diameter 15—30 μm) with three to six radiating dendrites which were sparsely branched and sparsely spined, and an axon, arising either from the soma or a proximal dendrite and impregnated for only 20-40 μm from its origin. Class II cells were smaller and commonly bipolar (somal diameter 7-10 x 15-20 μm) with few minimally branched dendrites bearing complex dendritic appendages. The latter were particularly numerous close to the distal extremities of the dendrites and were often also conspicuous close to the origin of the dendrites. One subpopulation of class II cells (class II a cells) were bipolar with dendrites showing a predominantly dorsomedial to ventrolateral orientation; class IIb cells had smaller, often spherical somata and their dendrites tended to curve around the parent cell body to establish a restricted dendritic domain. Few class II cells give rise to unequivocal axons. By e.m., class I cells bodies were characterized by pale, organelle-rich cytoplasm and pale, extensively indented nuclei. Their cell bodies, dendrites and dendritic spines were exclusively postsynaptic. Class II cell bodies were variable in size but generally smaller, less rich in organelles and more irregularly shaped than the cell bodies of class I cells, and their cytoplasm and nuclei more electron dense. Their cell bodies and dendritic shafts contained focal clusters of, and their dendritic appendages and the terminal parts of smaller dendrites, large numbers of, pleomorphic electron lucent synaptic vesicles: these cellular domains were all presynaptic as well as postsynaptic. The neuropil of OPN was characterized by extensive areas of small, closely packed neural profiles engaged in complex synaptic relationships, surrounded by a simpler neuropil in which axodendritic synapses predominated. In addition to the dendrites and appendages of class I neurons (D-profiles) the neuropil contained four principal presynaptic components. RP-boutons (36%) contained spherical synaptic vesicles and large pale mitochondria, made Gray type 1 synaptic contacts and were shown by experimental degeneration after eye enucleation and by labelling after intravitreal injection of HRP (DAB-cobalt chloride method) to be retinal in origin. RD-boutons (3%) contained spherical vesicles and small dark mitochondria, made Gray type 1 synaptic contacts outside the areas of complex neuropil and experimental evidence to be reported elsewhere shows that many and possibly all RD-boutons originate in the superior colliculus. F-boutons (8 %) were varied in size and shape, contained closely packed flattened synaptic vesicles, and made Gray type 2 synaptic contacts. Their origins were not ascertained. P-boutons (18%) irregularly shaped and pale, containing loosely packed pleomorphic synaptic vesicles, and sometimes ribosomes, were both postsynaptic and presynaptic (at small specializations resembling Gray type 2 contacts but with a slightly thicker postsynaptic density) and were identified as the dendrites and dendritic appendages of class II cells. RP-boutons, RD-boutons, F-boutons and P-boutons were all presynaptic to both D-profiles and P-boutons. In addition the cell bodies of class II cells were presynaptic to the dendrites and cell bodies of class I cells and F-boutons made synaptic contact with the cell bodies, somal spines and dendritic shafts of both class I and class II cells and with the axon hillocks and initial axon segments of the former. RP-, RD- and P-boutons were only rarely presynaptic to cell bodies. Analysis of serial sections of the neuropil revealed the presence of serial synapses of variable composition and remarkable complexity, all based on the dual postsynaptic and presynaptic properties of P-boutons. Triplet synapses were characteristic of the complex neuropil and may be concerned with feed-forward inhibition of projection cells, as has been previously suggested for thalamic nuclei, although the functional significance of these and the rather complex synaptic interactions that occur in OPN neuropil remains to be established. The organization of the retinal projections to OPN was analysed by l.m. after unilateral intravitreal injections of HRP or radioactive amino acids. Previous findings that the input is predominantly contralateral were confirmed and evidence obtained that there may be some overlap between the inputs from the two eyes. We have also demonstrated that in parasagittal sections the contralateral retinal terminal fields form a folded sheet resembling a W facing rostrodorsally and have confirmed that the ipsilateral terminal field in the lateral part of the nucleus is larger in pigmented than in albino animals. Radioautographs of transneuronal labelling following injections of [ 3 H] proline and [ 3 H] fucose into one eye revealed a bundle of efferent fibres, which emerged from the ventromedial border of OPN, ran medially and then turned to pass ventrocaudally through the periaqueductal grey to the Edinger-Westphal nucleus. Electron-microscope studies of OPN 1-15 days after contralateral eye enucleation showed the earliest degenerative changes in RP-boutons to consist of vesicle enlargement and an increase in the number of neurofilaments. Massive neurofilamentous hyperplasia associated with loss and clumping of synaptic vesicles was seen in many boutons (and preterminal axons) from 3 to 12 days after enucleation. Moderately electron dense flocculent material also appeared in some affected terminals within two or three days of enucleation, and electron dense degenerating terminals (first seen, but rare, at three days) were common by six days and predominated after about eight days. Glial engulfment of degenerating boutons involved only those showing the dense form of degeneration. ‘Vacated ’ postsynaptic densities and signs of their reoccupation by F- or P-boutons were seen at, and after, 11 days. These findings strengthen the evidence for a central role of the OPN in the pathway for the pupillary light reflex. The surprising complexity of the neuropil of OPN, which resembles the neuropil of the dorsal lateral geniculate nucleus, and the existence of inputs from several sources other than the retina (including superior colliculus and ventral lateral geniculate nucleus) suggest that extensive processing of luminance information may occur within the OPN.