Diencephalic projection to reticulospinal neurons involved in the initiation of locomotion in adult lampreysLampetra fluviatilis

Abstract

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.

A Novel Neural Substrate for the Transformation of Olfactory Inputs into Motor Output

“…This region also receives inputs from the OB and is located slightly rostral and dorsal to the PT [22],[27]. A local injection of glutamate antagonists in the DLR did not block RS responses ( n  = 4, Figure 7A, D) to 5–30 µA stimulation of the ON, suggesting that DLR is not a relay in the olfactory-locomotor pathway.…”

“…RS cells also receive projections from the DLR [22]. This region also receives inputs from the OB and is located slightly rostral and dorsal to the PT [22],[27].…”

“…As such, the first olfactory relay, the OB, does not project directly to RS cells as in other sensory systems [34]–[36], but its most caudal target is the PT [37]. In turn, RS cells do not receive inputs from structures more rostral than the diencephalon [22],[38].…”

“…The RS cells receive inputs from the periphery, the spinal cord, and locomotor centers in the forebrain and brainstem, including the mesencephalic locomotor region (MLR [19]–[21]), a highly conserved neural center controlling locomotion in all vertebrate species. Another locomotor center located in the diencephalon has been described in lampreys [22]. This region, referred to as the diencephalic locomotor region (DLR), also projects directly to RS cells.…”

Control of Movement Initiation Underlies the Development of Balance

“…In contrast, asymmetric processing of pitch is straightforward to implement given central vestibular architecture. Pitch information is represented in the brain by distinct nose-up and nose-down channels — independent populations of vestibular neurons with direct projections to movement generation centers [27; 28; 29; 10; 30; 31; 32; 33]. Thus, functional asymmetries between the nose-up and nose-down channels could mediate asymmetric sensitivity.…”