Diencephalic projection to reticulospinal neurons involved in the initiation of locomotion in adult lampreys Lampetra fluviatilis.

Morphological and electrophysiological techniques were used to characterize a diencephalic projection from the ventral thalamus to reticulospinal neurons and its role in initiating rhythmic locomotor activity in the spinal cord of adult lampreys (Lampetra fluviatilis). Injection of fluorescein-coupled dextran amine (FDA) into the rhombencephalic reticular 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 nuclei, but no direct projections were found to the spinal cord. Extracellular stimulation of the ventral thalamus elicited monosynaptic excitatory postsynaptic potentials (EPSPs), polysynaptic EPSPs, and inhibitory postsynaptic potentials (IPSPs) in reticulospinal neurons in the posterior (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 stimulation. Stimulation of the ventral thalamus induced rhythmic firing of reticulospinal neurons and elicited rhythmic burst activity in the spinal ventral roots. The projections from the ventral thalamus to the reticulospinal neurons in the prrn and mrrn thus provide excitatory inputs to the reticulospinal neurons, which, in turn, can activate the spinal circuits underlying locomotion. Also, the input nuclei to the ventral thalamus were labeled following injection of FDA into this nucleus. Labeled cells were found in the olfactory bulb, pallial areas, striatum, preoptic nucleus, hypothalamus, dorsal thalamus, optic tectum, and dorsal isthmic gray. The ventral thalamus, therefore, receives inputs from several different regions in the brain and controls the level of excitability in reticulospinal neurons.