DIENCEPHALIC PROJECTION TO RETICULOSPINAL NEURONS INVOLVED IN THE INITIATION OF LOCOMOTION IN ADULT LAMPREYS LAMPETRA-FLUVIATILIS

Morphological and electrophysiological techniques were used to charact erize a diencephalic projection from the ventral thalamus to reticulos pinal neurons and its role in initiating rhythmic locomotor activity i n the spinal cord of adult lampreys (Lampetra fluviatilis). Injection of fluorescein-coupled dextran amine (FDA) into the rhombencephalic re ticular nuclei labeled neurons in the ventral thalamus region on both the ipsilateral side and the contralateral side. Injection of FDA into the ventral thalamus labeled axonal projections in all reticular nucl ei, but no direct projections were found to the spinal cord. Extracell ular stimulation of the ventral thalamus elicited. monosynaptic excita tory postsynaptic potentials (EPSPs), polysynaptic EPSPs, and inhibito ry postsynaptic potentials (IPSPs) in reticulospinal neurons in the po sterior (prrn) and middle (mrrn) rhombencephalic reticular nuclei. The monosynaptic EPSPs were blocked by the glutamate antagonist kynurenic acid and can be considered glutamatergic. The monosynaptic EPSPs were potentiated (up to 12 minutes) following a brief high-frequency stimu lation. Stimulation of the ventral thalamus induced rhythmic firing of reticulospinal neurons and elicited rhythmic burst activity in the sp inal ventral roots. The projections from the ventral thalamus to the r eticulospinal neurons in the prrn and mrrn thus provide excitatory inp uts to the reticulospinal neurons, which, in turn, can activate the sp inal circuits underlying locomotion. Also, the input nuclei to the ven tral thalamus were labeled following injection of FDA into this nucleu s. Labeled cells were found in the olfactory bulb, pallial areas, stri atum, preoptic nucleus, hypothalamus, dorsal thalamus, optic tectum, a nd dorsal isthmic gray. The ventral thalamus, therefore, receives inpu ts from several different regions in the brain and controls the level of excitability in reticulospinal neurons. (C) 1997 Wiley-Liss, Inc.