Autoradiographic analysis of ascending projections from the pontine and mesencephalic reticular formation and the median raphe nucleus in the rat

Abstract

Ascending projections from the medial pontine reticular formation, the mesencephalic reticular formation, and the median raphe nucleus were examined using the autoradiographic technique. The majority of the ascending fibers labeled after injections of [³H]-leucine into the nucleus pontis caudalis (RPC) course through the brainstem within the tracts of Forel (tractus fasciculorum tegmenti of Forel) and directly ventral to them. At the caudal diencephalon, Forel's bundle divides into dorsal and ventral components bound primarily for the dorsal thalamus and the subthalamus, respectively. RPC fibers project to several regions involved in oculomotor/visual functions. These include the abducens nucleus, the intermediate gray layer of the superior colliculus (SCi), the anterior pretectal nucleus (APN), the ventral lateral geniculate nucleus (LGNv), and regions of the central gray directly bordering the oculomotor nucleus, the interstitial nucleus of Cajal, and the nucleus of Darkschewitsch. Few, if any, fibers from RPC (or from nucleus pontis oralis--RPO) terminate within the oculomotor nucleus proper. Other sites receiving heavy projections from the RPC include adjacent regions of the pontomesencephalic reticular formation (RF), the parafascicular (PF) and central lateral (CL) nuclei of the thalamus and the fields of Forel/zona incerta (FF-ZI). RPO fibers also ascend predominantly in Forel's bundle. Other ascending tracts for these fibers are the medial longitudinal fasciculus and the central tegmental tract (CTT). RPO fibers distribute significantly to the same structures of the oculomotor/visual system receiving projections from RPC. The RPO projections to the SCi and the APN are particularly pronounced. RPO fibers terminate heavily in several nuclei located ventrally within the rostral midbrain/caudal diencephalon. These include major dopamine-containing cell groups (the retrorubral nucleus, the ventral tegmental area, and the substantia nigra-pars compacta) as well as the interpeduncular nucleus, the lateral mammillary nucleus, and the supramammillary nucleus. Other prominent targets for RPO fibers include the mesencephalic RF, specific regions of the central gray, the PF, the CL, the paracentral and central medial nuclei of the thalamus, and the FF/ZI. The major bundle of the ascending fibers labeled after injections of the mesencephalic reticular formation (MRF) travels within the CTT in a position just lateral to the central gray, but a significant number of labeled axons also course in Forel's bundle. MRF axons project to several nuclei of the thalamus. MRF fibers terminate densely within the PF, the CL, the paracentral, the central medial, the LGNv, and the reticular thalamic nucleus, and less heavily within the paraventricular nucleus, the nucleus reuniens, and the rhomboid nucleus. Other sites receiving pronounced projections from the MRF include the central gray, the SCi, the FF-ZI, and the subthalamic nucleus. After injections of the median raphe nucleus (MR), labeled axons are seen over a widespread region of the ventral midbrain tegmentum; these axons join the medial forebrain bundles at the caudal diencephalon and continue within them throughout the forebrain. MR fibers distribute heavily to several “limbic” structures, including the interpeduncular nucleus, the medial mammillary nucleus, the VTA, the lateral habenula, the medial preoptic area, the nuclei of the vertical and horizontal limbs of the diagonal band, the septum, the nucleus accumbens, and the hippocampal formation. Possible functions for some of the connections described herein are discussed.