Dopamine control of downstream motor centers

The role of dopamine in the control of movement is traditionally associated with ascending projections to the basal ganglia. However, more recently descending dopaminergic pathways projecting to downstream brainstem motor circuits were discovered. In lampreys, salamanders, and rodents, these include projections to the downstream Mesencephalic Loco-motor Region (MLR), a brainstem region controlling locomotion. Such descending dopaminergic projections could prime brainstem networks controlling movement. Other descending dopaminergic projections have been shown to reach retic-ulospinal cells involved in the control of locomotion. In addition, dopamine directly modulates the activity of interneurons and motoneurons. Beyond locomotion, dopaminergic inputs modulate visuomotor transformations within the optic tectum (mammalian superior colliculus). Loss of descending dopaminergic inputs will likely contribute to pathological conditions such as in Parkinson ’ s disease.

Ascending dopaminergic pathways from meso-diencephalic dopaminergic neurons to the basal ganglia A major substrate involving the role of dopamine in motor control is the ascending pathway from mesodiencephalic dopaminergic neurons to the striatum, the entry structure of the basal ganglia (Pathway 1 in Figure 1).The loss of dopaminergic innervation in the striatum has been associated with motor deficits in Parkinson's disease [1,2].This was one of the first demonstrations in the field of neuroscience of the identification of a neural substrate for a specific pathological condition, Parkinson's disease.It was later found that dopamine exerted its effects in the basal ganglia through two pathways with opposite effects on the output stations of the basal ganglia.Striatal neurons of the "direct pathway" express D 1 receptors, and striatal neurons of the "indirect pathway" express D 2 receptors.Dopamine has pro-locomotor effects through the activation of D 1 receptors, which increase the excitability of direct pathway neurons.Optogenetic stimulation of direct pathway neurons increases locomotor activity, by decreasing the tonic inhibition sent by the basal ganglia output stations to brainstem locomotor circuits [3,4].In addition, dopamine has pro-locomotor effects through the activation of D 2 receptors, which decreases the excitability of indirect pathway neurons.Optogenetic stimulation of indirect pathway neurons decreases locomotor activity, by increasing the inhibitory activity sent by the basal ganglia output stations to brainstem locomotor circuits [3,4].These basal ganglia circuits are well known to contribute to the increased locomotor activity evoked by psychostimulants such as amphetamines or cocaine, which increase dopamine release.
A question that is classically raised relates to whether ascending projections from meso-diencephalic neurons separately encode reward through fast signals (hundreds of milliseconds) and future motor actions through slower signals (tens of seconds to minutes).Using two photon recordings of dopaminergic axons in headrestrained mice over a cylindrical treadmill, Howe and Dombeck [5] examined the changes in fluorescence in the ascending dopaminergic axons at rest, during spontaneous locomotion, and during delivery of unexpected rewards.They found that fast motor-related dopaminergic signals were present in neurons that were mostly not associated with reward.During treadmill locomotion, calcium transients shorter than 1 s were recorded in dopaminergic axons in the striatum 100e150 ms before initiation of locomotion [5] (Figure 2b).Bilateral optogenetic stimulation of dopaminergic terminals in the dorsal striatum often (but not systematically) evoked transitions from rest to locomotion; increasing the stimulation increased acceleration [5].Only a minor portion of ventral tegmental area (VTA) axons responding to reward was also associated with locomotion (19%), whereas no substantia nigra pars compacta (SNc) axon responded both to locomotion and reward [5].This study established that two dopaminergic signals differentially impact motor control and reward.
More recently, da Silva and colleagues (2018) showed that a portion of dopaminergic neurons in the SNc (52%) increase their spiking activity before self-paced movement initiation, and that these cells are largely different from those responding to reward [6].Using transgenic mice, these authors performed extracellular electrophysiological recordings and calcium imaging of dopaminergic neurons in the SNc.For electrophysiological recordings, dopaminergic neurons were specifically made to express channelrhodopsin, a lightsensitive cation channel that depolarizes the cell membrane upon light exposure.Brief pulses of light applied through an optic fiber placed close to the electrode allowed the experimenters to identify among recorded neurons those that spike at short latency following a light pulse and can thus be considered dopaminergic.The authors found that an increase in the activity of some dopaminergic neurons preceded the onset of movement, and the increase in firing rate was positively correlated with the vigour (acceleration) of future movements [6] (Figure 2a).Optogenetic activation of dopaminergic neurons increased the probability and vigour of future movements.Descending dopaminergic pathways to motor circuits.Meso-diencephalic dopaminergic neurons are well known to modulate locomotion through ascending projections to the basal ganglia (for the sake of simplicity the details of the basal ganglia are not included) (rodent: [1][2][3][4][5][6]14]).New descending dopaminergic pathways were reported from meso-diencephalic neurons to the Mesencephalic Locomotor Region (lamprey: [8,31,32]; salamander: [10], rodent: [10,34]), reticulospinal neurons (lamprey: [35]), spinal locomotor circuits (mouse: [48], zebrafish: [45]), and superior colliculus (lamprey [51], mouse [57]).
Conversely, optogenetic inhibition applied in immobile mice increased the probability of impairing movement initiation but had no effect if applied after movement onset [6].
In vivo, dopamine can be measured with high temporal and spatial resolution using fast scan voltammetry [7].This technique was used to show that, in lampreys and salamanders, dopamine release in the MLR is associated with the activation of many reticulospinal neurons located in mesencephalic and rhombencephalic nuclei [8,10].In rats, voltammetry recordings in vivo revealed that a psychostimulant (amphetamine) increases dopamine release in the MLR [8,9].More recently, the use of genetically encoded dopamine indicators was developed (dLight, [11]; GRAB sensors, [12,13]).Using these tools, Markowitz and colleagues (2023) examined the relationship between dopamine release in the dorsal striatum (i.e., the main target of SNc dopaminergic neurons) and motor behaviors using MoSeq, a machine learning method which allows researchers to analyze short behavioral bouts called syllables (around 400 ms in duration, e.g., poses, rearing, grooming, exploration, etc.) [14].They found that dopamine peaked near the middle of each motor syllable but did not appear to encode specific information about syllable identity, kinematics (i.e., amplitude weakly correlated with the movement expressed).Optogenetic stimulation of dopaminergic fibers in the dorsolateral striatum applied during a specific syllable did not cause switching between syllables, but increased the subsequent use of the target syllable per unit time [14].In addition, if the optogenetic stimulation was applied during the fastest occurrences of a specific syllable, the target syllable was later expressed at higher velocity [14].Therefore, dopamine release in the dorsal striatum likely constitutes a signal that invigorates future movement [6,14].

Descending dopaminergic pathways to brainstem and spinal motor circuits
It is well established that dopaminergic neurons influence movement though their ascending projections to the basal ganglia.Therefore, it was surprizing to find that dopaminergic neurons can also project downward, bypassing the basal ganglia, to reach the locomotor control circuitry.Recent studies uncovered that mesodiencephalic neurons project down to the three fundamental locomotor control levels: the Mesencephalic Locomotor Region (MLR), the reticulospinal neurons, and the spinal Central Pattern Generator (CPG) for locomotion (Figure 1).The MLR is a locomotor center located at the junction between the mesencephalon and rhombencephalon.This region controls the initiation of locomotion as well as locomotor speed, gait transitions, and locomotor termination (for review [15e18]).The MLR can initiate locomotion through monosynaptic glutamatergic projections to reticulospinal neurons in lamprey [19], zebrafish [20], salamander [9] and mammals [21].The MLR also sends cholinergic input to reticulospinal neurons expressing nicotinic receptors [22].The MLR also sends cholinergic input to a group of reticular interneurons expressing muscarinic receptors.These interneurons provide additional excitation to reticulospinal neurons [23].Reticulospinal neurons send an excitatory drive to start and maintain the activity of spinal cord neurons involved in the generation of locomotion ( [21,24], for review, [25]).They also send other descending commands involved in stopping locomotion and turning [26,27e29].As described below, these three levels of the locomotor circuitry are modulated by dopaminergic inputs.

Descending dopaminergic inputs to the MLR
A first key anatomical study was provided by the group of Chantal Franc ¸ois in Paris.Her team showed in monkeys that dopaminergic terminals are present in the pedunculopontine nucleus, part of the MLR [30].At that time, the origin and the role of these descending projections were unknown.Tracer injections revealed that dopaminergic fibers originated from mesodiencephalic neurons located in the posterior tuberculum in lamprey [8,31,32] and salamander [10] (Pathway 2 in Figure 1).Some of these mesodiencephalic dopaminergic neurons also projected up to the striatum, the input station of the basal ganglia [8, 10,33].Electrical or pharmacological activation of meso-diencephalic dopaminergic region evoked dopamine release in the MLR in lampreys [8].Similar observations were made in salamanders [10].Dopamine release was correlated in time with the activation of brainstem reticulospinal neurons, known to convey the locomotor drive down to the spinal cord [8,10].Increasing the concentration of dopamine in the MLR increased locomotor activity evoked by stimulation of dopaminergic neurons [8].Microinjection of a D 1 antagonist in the MLR decreased reticulospinal activity and locomotor output evoked by stimulation of the meso-diencephalic dopaminergic region [8] (Figure 2c  and d).In rats, electrical stimulation of the SNc/retrorubral field evoked dopamine release in the pedunculopontine nucleus, and such release was potentiated by amphetamine [10].Interestingly, an anatomical study reported that A13, a more diencephalic group, also sends descending dopaminergic projections to the MLR [34].The detailed contribution of A13 dopaminergic cells to locomotor control remains to be defined.
In lampreys, the meso-diencephalic dopaminergic region sends both dopaminergic and glutamatergic projections in parallel to the MLR [31].Blockade of glutamatergic inputs to the MLR abolishes both the activity of downstream reticulospinal neurons and swimming [31].The dopaminergic inputs were found to amplify the glutamatergic drive to the MLR through the activation of D 1 receptors [31].Anatomical tracing revealed that some dopaminergic neurons were also glutamatergic, and that some non-dopaminergic glutamatergic neurons from the dopaminergic region projected to the MLR [31].Whether a similar dual descending drive reaches the MLR in mammals remains to be established.Several mammalian meso-diencephalic dopaminergic nuclei have a glutamatergic phenotype during development (A8 to A11).This glutamatergic phenotype is kept at adult stage, notably in A10, whereas more rostral dopaminergic nuclei (A12-A13) instead use GABA a co-transmitter [62].

Descending dopaminergic inputs to reticulospinal neurons
In addition to the MLR, descending dopaminergic inputs were found to modulate the activity of reticulospinal neurons (Pathway 3 in Figure 1), that carry the locomotor drive to the spinal cord [35].In lampreys, tracing experiments revealed that dopaminergic neurons project to the four reticular nuclei, and stimulation of the meso-diencephalic dopaminergic region induced dopamine release in all four nuclei as shown with fast scan voltammetry [35].Blockade of D 1 receptors in the reticular region decreased reticulospinal spiking and swimming speed (Figure 2e and f).Anatomical tracing showed that the dopaminergic neurons projecting to the MLR and to reticulospinal neurons constitute mostly segregated populations [35].In addition, some of the meso-diencephalic dopaminergic neurons projecting to the reticulospinal level were found to be glutamatergic [35].This innervation of lamprey reticulospinal neurons is consistent with previous observations indicating that dopaminergic fibers also innervate reticulospinal neurons in teleosts [36,37] and that D 1 receptor activation amplifies synaptic responses in reticulospinal neurons evoked by vestibulocochlear stimulation [36,38,39].

Descending dopaminergic inputs to spinal locomotor circuits
The locomotor role of descending projections from meso-diencephalic dopaminergic neurons to the spinal locomotor circuits (Pathway 4 in Figure 1) was recently re-examined by different groups of researchers.Historically, the dopamine precursor L-DOPA has been used extensively to activate the spinal CPG for locomotion in cats [40].More recently, dopaminergic modulation involving the activation of D 1 and D 2 receptors was shown to modulate the reticulospinal transmission to spinal neurons in lampreys [41], and the CPG activity in amphibians [42] and mice [43,44].In zebrafish, dopamine was recently shown to modulate optomotor responses, through which animals respond to a rotating panorama with compensating movements that can comprise eye, head, or full body locomotor movements [45].The latter study showed that D 1 receptor activation increases locomotor speed during the optomotor response, mostly by increasing the tail bend amplitude (Figure 2h).Whole-cell patch-clamp recordings revealed that D 1 receptor activation increases motoneuron excitability and the synaptic drive they receive during an optomotor response [45] (Figure 2i).The zebrafish spinal cord receives innervation from supraspinal dopaminergic neurons, which can be activated by visual stimulation [46] or display bursting activity during swimming [47].In mice, optogenetic stimulation of the dopaminergic A11 nucleus, which projects to the spinal cord, increases spontaneous locomotor activity [48].The effects of A11 stimulation likely involves a direct effect on the spinal CPG for locomotion, which is sensitive to dopaminergic modulation as demonstrated in many vertebrates [40e45,49].Dopaminergic modulation of spinal networks goes beyond locomotor circuits.In vivo patch-clamp recordings in rodents show that electrical stimulation of A11 decreases firing in dorsal horn neurons that carry nociceptive information through the activation of dopaminergic receptors [50].

Descending dopaminergic inputs to superior colliculus
In the superior colliculus (optic tectum) of lampreys, descending projections from meso-diencephalic dopaminergic neurons were reported by the group of Sten Grillner [32,51,52] (Pathway 5 in Figure 1).As in other vertebrates, the superior colliculus of lamprey plays a key role in controlling gaze, orienting movements, or escape movements [53e55], for review [56]) The descending dopaminergic projections to this region increase or decrease the salience of visual stimuli by modulating tectal output neurons through D 1 or D 2 receptors [51].The mammalian superior colliculus was also found to be innervated by dopaminergic inputs in mice that are in a good position to modulate visuomotor transformation [57].

Conclusions
Altogether, recent findings indicate that, in addition to the basal ganglia, dopamine also modulates different levels of the locomotor circuitry, including the MLR, reticulospinal neurons and the spinal locomotor circuits, as well as other motor centers such as the superior colliculus.These projections were found from basal vertebrates to mammals indicating that dopaminergic control of downstream motor targets is conserved ( [10], for review [58]).The role of descending dopaminergic inputs is likely to increase readiness to move if a decision is taken by the basal ganglia.Future studies should establish whether target specific modulation (MLR, reticulospinal, superior colliculus) is achieved by these descending dopaminergic neurons.It is also highly probable that motor symptoms associated with the loss of dopaminergic neurons result from damage to the descending dopaminergic pathways.Rolland et al. [30] showed that dopaminergic terminals in the MLR are lost in monkeys treated with MPTP, a neurotoxin classically used to model Parkinson's disease.It remains to be established to which extent the loss of descending pathways contributes to the motor deficits in Parkinson's disease, as suggested by lamprey data [8,31].The MLR and reticular regions are increasingly recognized as associated with motor functions other than locomotion, including forelimb movements [59], or breathing regulation [60,61].In Parkinson's disease, it is possible that several brainstem motor circuits lose their dopaminergic inputs, contributing to the vast spectrum of motor deficits reported in this pathology (