Anatomical Location of the Mesencephalic Locomotor Region and Its Possible Role in Locomotion, Posture, Cataplexy, and Parkinsonism

Abstract

The mesencephalic (or midbrain) locomotor region (MLR) was first described in 1966 by Shik and colleagues, who demonstrated that electrical stimulation of this region induced locomotion in decerebrate (intercollicular transection) cats. The pedunculopontine tegmental nucleus (PPT) cholinergic neurons and midbrain extrapyramidal area (MEA) have been suggested to form the neuroanatomical basis for the MLR, but direct evidence for the role of these structures in locomotor behavior has been lacking. Here, we tested the hypothesis that the MLR is composed of non-cholinergic spinally projecting cells in the lateral pontine tegmentum. Our results showed that putative MLR neurons medial to the PPT and MEA in rats were non-cholinergic, glutamatergic, and express the orexin (hypocretin) type 2 receptors. Fos mapping correlated with motor behaviors revealed that the dorsal and ventral MLR are activated, respectively, in association with locomotion and an erect posture. Consistent with these findings, chemical stimulation of the dorsal MLR produced locomotion, whereas stimulation of the ventral MLR caused standing. Lesions of the MLR (dorsal and ventral regions together) resulted in cataplexy and episodic immobility of gait. Finally, trans-neuronal tracing with pseudorabies virus demonstrated disynaptic input to the MLR from the substantia nigra via the MEA. These findings offer a new perspective on the neuroanatomic basis of the MLR, and suggest that MLR dysfunction may contribute to the postural and gait abnormalities in Parkinsonism.

Keywords: basal ganglia; cataplexy; dopamine; pontine motor; sleep.

Figure 1

Defining the neuroanatomic MLR. (A) Retrogradely labeled (FG) neurons from the ventral horn (C8-T1 level) are seen within the dorsal (dMLR) and ventral (vMLR) components of the MLR. Pedunculopontine tegmental (PPT) neurons, labeled brown for ChAT, are seen lateral to the MLR (A,B). The clear space between them represents the midbrain extrapyramidal area (MEA, also see Figure 7) (A, B). Following spinal injections of FG, almost all FG-ir MLR neurons (brown) also express VGLUT2 mRNA (neurons overlaid by silver grains indicated by arrows) (C). Similarly, about 50% of CTB-ir cells from the spinal cord (C8-T1) also contain OX2 mRNA (arrows) (D). Less than 5% of reticulospinal neurons in the ventral MLR also contain D2 mRNA (arrows) (E). (F) shows the proposed pontine motor neural circuitry.

Figure 2

Motor effects of MLR stimulation. (A) Acute injection of 3 nl glutamate agonist (10%) ibotenic acid in the MLR region unilaterally induces contra-lateral rotation and c-Fos expression (black) in the MLR region, but not in PPT cholinergic neurons (brown, at lower left edge of circumscribed area) (B). The same stimulation induces Fos in spinal motor neurons (C). Fos is seen in spinal cord motor neurons ipsilateral to the stimulation site (D). Lesions of the VMM (E) block neither circling behavior nor Fos expression in the motor ChAT-ir neurons induced by unilateral MLR stimulation.

Figure 3

Activation of the MLR neurons in motor behaviors. (A) Locomotion (on a rotarod for 1 h) induces c-Fos expression in the dorsal MLR but not the ventral MLR. (B) Standing posture (2 h) induces c-Fos in the ventral MLR but not the dorsal MLR.

Figure 4

OX-SAP lesions of the MLR produce cataplexy and complete akinesia. (A) shows lesions (circled) including the vMLR but not the dMLR. A cataplexy attack caused by vMLR lesions, characterized by entering directly from wakefulness to a state of desynchronized EEG and atonia, is seen at the arrow in (B). The animal is seen in a series of video images (C) showing an attack that begins while the animal is feeding, and results in him collapsing on the floor of the cage. The EEG resembles waking EEG not REM sleep EEG (high theta). (D) shows the duration and timing of a series of cataplexy attacks that occur during the dark period (7 p.m.–7 a.m.). Attack episodes vary from 20 to over 200 s in length. Lesions of the dMLR (E) produce freezing attacks or complete akinesia, in which the animal maintains the same posture throughout, despite being awake in the early part of the attack [EEG in (F) shows an awake pattern, photo in (H) is at time point indicated by asterisk] but falling asleep while frozen [EEG in (G) shows sleeping EEG, photo in (I) is at the time point indicated by the asterisk]. The bar shows duration of sleep EEG during the attack.

Figure 5

Spatial relationship of the MLR, MEA, and PPT. The MEA marked by black terminals of a biotinylated dextran (BD) injected into the SNr in both (A,B) situates lateral to the MLR marked brown by FG from the spinal cord in (A) and medial to the PPT, marked brown by ChAT-ir neurons in (B). The ventral MLR (FG) neurons (black arrows) and their dendrites (red arrows) are apposed by SNc terminal boutons (C).

Figure 6

Temporal and spatial distribution of PRV-GFP from the tibialis anterior (TA; flexor) muscle. At 168 h after PRV-GFP injections in TA, PRV-GFP preferentially labels the PAG, MLR, and MEA (A) as well as the VMM, but not the SNr (B). After 196 h, the PRV labeling from the TA shows more extensive labeling in the VMM (C) and SNr (D).

Figure 7

Temporal and spatial distribution of PRV-GFP from the soleus (SOL; postural extensor). At 168 h after PRV-GFP injections in the SOL, PRV-GFP labels the PAG and vMLR and SNc (A,B). PRV-GFP-ir neurons are more abundant in the VMM and SNc after 196 h (C,D). These results suggest that for spinal motor control, the SNc- > vMLR projection may be more important for controlling extensor muscles (posture control), whereas the SNr- > MEA- > MLR projection may be more important for controlling flexor muscles, which are necessary for gait.

Figure 8

Temporal and spatial distribution of PRV-GFP from the tongue. At 72 h after PRV-GFP injection, the MLR (A) and SNc (B) show PRV-GFP-ir neurons ipsilateral to the injection site. After 96 h, the labeling is more extensive (C,D), now including the MEA (C) and SNr (D). The temporal sequence of PRV labeling suggests that the SNc- > MLR pathway requires less time for labeling than the SNr- > MEA- > MLR pathway following tongue injections.