Locomotor activity

Locomotor activity is a measure of animal behavior which is employed in scientific research.
Hyperlocomotion, also known as locomotor hyperactivity, hyperactivity, or increased locomotor activity, is an effect of certain drugs in animals in which locomotor activity is increased. It is induced by certain drugs like psychostimulants and NMDA receptor antagonists and is reversed by certain other drugs like antipsychotics and certain antidepressants. Stimulation of locomotor activity is thought to be mediated by increased signaling in the nucleus accumbens, a major brain area involved in behavioral activation and motivated behavior.
Hypolocomotion, also known as locomotor hypoactivity, hypoactivity, and decreased locomotor activity, is an effect of certain drugs in animals in which locomotor activity is decreased. It is a characteristic effect of many sedative agents and general anesthetics. Antipsychotics, which are dopamine receptor antagonists, and many serotonergic agents, such as meta-chlorophenylpiperazine, can also produce this effect, often as a side effect.
Although locomotor activity is mainly an animal behavior test, it has also been evaluated in humans. People with attention deficit hyperactivity disorder, in the manic phase of bipolar disorder, on acute amphetamine, and with schizophrenia show increased locomotor activity, while children with autism show decreased locomotor activity. Conversely, reduced locomotor activity is observed in bipolar individuals on mood stabilizers and may be a characteristic symptom of the inattentive type of ADHD and sluggish cognitive tempo.

Drugs affecting locomotor activity

Dopaminergic agents

Dopamine releasing agents

Hyperlocomotion is induced by dopamine releasing agents and psychostimulants like amphetamine and methamphetamine. These drugs likewise induce stereotypies.

Dopamine reuptake inhibitors

The dopamine reuptake inhibitors amineptine, bupropion, and nomifensine increase spontaneous locomotor activity in animals. The DRI cocaine increases locomotor activity similarly to the preceding DRIs and to amphetamines. The atypical DRI modafinil does not produce hyperlocomotion in animals.

Dopamine receptor agonists

Direct dopamine receptor agonists like apomorphine show biphasic effects, decreasing locomotor activity at low doses and increasing locomotor activity at high doses.

Dopamine receptor antagonists

Drug-induced hyperlocomotion can be reversed by various drugs, such as antipsychotics acting as dopamine D₂ receptor antagonists. Reversal of drug-induced hyperlocomotion has been used as an animal test of drug antipsychotic-like activity. Reversal of amphetamine- and NMDA receptor antagonist-induced stereotypies is also employed as a test of drug antipsychotic-like activity.

Adrenergic agents

Norepinephrine releasing agents

Selective norepinephrine releasing agents include ephedrine, pseudoephedrine, phenylpropanolamine, levomethamphetamine, and D-phenylalaninol. However, these drugs also release dopamine to a much lesser extent.
Ephedrine consistently stimulates locomotor activity in rodents. However, the hyperlocomotion induced by ephedrine may be mediated by dopamine release rather than by norepinephrine release. On the other hand, lesioning the brain noradrenergic system with the noradrenergic neurotoxin DSP-4 reduces dextroamphetamine-induced hyperlocomotion. In addition, the selective α₁-adrenergic receptor antagonist prazosin antagonizes amphetamine-induced hyperlocomotion and knockout of the α_1B-adrenergic receptor dramatically reduces dextroamphetamine-induced hyperlocomotion. In contrast to ephedrine and amphetamine, pseudoephedrine and phenylpropanolamine do not stimulate locomotor activity in rodents. However, in another study, pseudoephedrine was able to increase locomotor activity. A potential confounding factor with β-hydroxyamphetamines like phenylpropanolamine, ephedrine, and pseudoephedrine is that they have lower lipophilicity compared to their amphetamine counterparts, with consequent reduced capacity to cross the blood–brain barrier and produce central nervous system effects.
Conversely, the potencies of monoamine releasing agents in producing amphetamine-type subjective effects in humans have been found to correlate with their potency to induce norepinephrine release and not with their potencies to induce dopamine release. In addition, self-administration of methamphetamine appeared to be relatively resistant to blockade by dopamine receptor antagonists. Findings on the modulation of the ventral tegmental area by the noradrenergic locus coeruleus are mixed and suggestive of both excitatory and inhibitory roles. The α₁-adrenergic receptor appears to be facilitatory, whereas the α₂-adrenergic receptor appears to be inhibitory, and the β-adrenergic receptors appear to not be involved. More research is needed to investigate the role of norepinephrine in dopamine modulation and stimulant-like effects.
In contrast to normal mice, psychostimulants like amphetamine, β-phenethylamine, and methylphenidate lose their ability to elevate brain dopamine but not norepinephrine in dopamine transporter knockout mice and have been found to decrease locomotor activity in these mice. Paradoxically however, cocaine retains reinforcing effects in DAT knockout mice and cocaine and amphetamine are still able to elevate dopamine in the medial nucleus accumbens in these mice. It was found that the norepinephrine reuptake inhibitor reboxetine increases dopamine levels in the nucleus accumbens in DAT knockout mice but not in normal mice, suggesting that the effects of norepinephrine elevation change in the brains of DAT knockout mice.
Whereas dextromethamphetamine is a well-balanced norepinephrine–dopamine releasing agent, levomethamphetamine is a selective NRA. Levomethamphetamine has similar potency as an NRA compared to dextromethamphetamine. Conversely, levomethamphetamine is about 15- to 20-fold less potent in inducing dopamine release than dextromethamphetamine. In accordance with the preceding, levomethamphetamine was found to selectively induce brain norepinephrine release with minimal effect on brain dopamine release across an assessed dosage range in rodents. The drug did not increase locomotor activity at the assessed doses, in which brain dopamine release was not affected. In contrast to levomethamphetamine, dextromethamphetamine at the same doses increased brain levels of both norepinephrine and dopamine and induced dose-dependent hyperlocomotion. Relatedly, levomethamphetamine shows similar sympathomimetic effects as dextromethamphetamine but is substantially less potent as a psychostimulant in animals. As in rodents, levomethamphetamine showed reduced reinforcing and stimulant-like effects compared to dextromethamphetamine in rhesus monkeys.
Animal studies of the reinforcing and cocaine-like effects of dopamine releasing agents with varying capacities to release norepinephrine and serotonin in rodents and monkeys have suggested that in contrast to the case of serotonin release, which inhibits the reinforcing and stimulant-like effects of these agents, norepinephrine release has minimal influence on their misuse liability and associated effects.

Norepinephrine reuptake inhibitors

s, like atomoxetine, reboxetine, and desipramine, do not increase locomotor activity in rodents and instead show no effect on locomotor activity or decrease it. In addition, NRIs decrease amphetamine-, cocaine-, methylphenidate-, and phencyclidine -induced hyperlocomotion in rodents. Accordingly, atomoxetine has been reported to attenuate the stimulant and rewarding effects of dextroamphetamine in humans.
A variety of different NRIs were shown to decrease spontaneous locomotor activity in a novel environment when given acutely and to decrease locomotor activity in both novel and familiar environments when given chronically in rodents. Similarly, norepinephrine transporter knockout mice had low basal locomotor activity. However, combination of an NRI with dopamine reuptake inhibition resulted in increased locomotor activity. It was concluded that norepinephrine reuptake inhibition by itself decreases locomotor activity unless it is combined with dopamine reuptake inhibition.

Serotonergic agents

Serotonin releasing agents

Certain serotonin releasing agents, like MDMA and MDAI, though notably not others, like chlorphentermine, fenfluramine, and MMAI, induce locomotor hyperactivity in animals. This is dependent on serotonin release allowed for by the serotonin transporter and serotonin 5-HT_2B receptor. SERT knockout, pretreatment with serotonin reuptake inhibitors , or serotonin 5-HT_2B receptor knockout, all completely block MDMA-induced locomotor hyperactivity. In addition, locomotor hyperactivity produced by MDMA is partially attenuated by serotonin 5-HT_1B receptor antagonism or by serotonin 5-HT_2A receptor antagonism. The locomotor hyperactivity produced by MDMA is fully attenuated by combined serotonin 5-HT_1B and 5-HT_2A receptor antagonism. Conversely, the serotonin 5-HT_1A receptor is not involved in MDMA-induced hyperlocomotion. Serotonin 5-HT_2C receptor activation appears to inhibit MDMA-induced hyperlocomotion, and antagonism of this receptor has been reported to markedly enhance the locomotor hyperactivity induced by MDMA. Activation of the serotonin 5-HT_2C receptor is known to inhibit dopamine release in the mesolimbic pathway as well as to inhibit dopamine release in the nigrostriatal and mesocortical pathways.
Although the serotonin system has been implicated in the hyperlocomotion of SRAs, certain SRAs, such as MDMA, are actually serotonin–norepinephrine–dopamine releasing agents, and catecholaminergic mechanisms are likely to additionally be involved. Relatedly, the α₁-adrenergic receptor antagonist prazosin completely blocks MDMA-induced hyperlocomotion in animals. In addition, the α₁-adrenergic receptor antagonists prazosin and doxazosin reduce the psychostimulant and/or euphoric effects of MDMA in humans. Similarly, the norepinephrine reuptake inhibitor reboxetine, which prevents MDMA from inducing norepinephrine release, likewise reduces the stimulant effects and emotional excitation of MDMA in humans. Dopamine receptors also appear to be involved in MDMA-induced hyperlocomotion, although findings in this area, both in animals and humans, seem to be conflicting.
In contrast to non-selective SRAs like MDMA, the highly selective SRA MMAI induces hypolocomotion in animals. Similarly, the highly selective SRA chlorphentermine is said to weakly stimulate locomotor activity at low doses and to progressively suppress locomotor activity at higher doses.
The reasons for the differences in locomotor activity with different SRAs are not fully clear. In any case, they may be related to factors such as whether the agents are selective SRAs, whether they additionally act as agonists of serotonin 5-HT₂ receptors, and whether they additionally induce the release of norepinephrine and/or dopamine.