The influence of dopaminergic A10 neurons on the motor pattern evoked by substantia nigra (pars compacta) stimulation

Selective bilateral lesion to caudate nucleus modulates the acute and chronic methylphenidate effects

The psychostimulant methylphenidate (MPD) is currently the most prescribed drug therapy for attention deficit hyperactivity disorder (ADHD) and is used by students as a cognitive enhancer. The caudate nucleus (CN) is a structure within the motive circuit where MPD exerts its effects, it is known to contain high levels of dopaminergic cells and directly influence motor activity. The objective of this study was to understand the role of CN in response to acute and chronic administration of MPD. Specific and non-specific bilateral ablations were created in the CN using electrolytic lesion and 6-Hydoxydopamine (6-OHDA). Four groups of rats were used: control (n=4), sham (n=4), CN electrolytic lesion group (n=8) and CN 6-OHDA injected group (n=8). On experimental day one (ED 1) all rats received a saline injection and baseline locomotive activity was recorded. On ED 2 and ED 3 CN sham, electrolytic lesion and/or 6-OHDA injected groups were made followed by four to five days recovery (ED 3-7), followed by six daily 2.5 mg/kg MPD injections (ED 9-14), three days of washout (ED 15-17) and an MPD re-challenge of drug proceeding the washout days (ED 18). Locomotor activity was obtained at ED 1, 8, 9, and 18 using an open field assay. The results show that the CN electrolytic lesion group responded to the acute and chronic MPD administration similar to the control and sham group, while the CN 6-OHDA injected group prevented the acute and the chronic effects of MPD administration. One possible interpretation why nonspecific electroyltic lesioning of the CN failed to prevent acute and chronic effects of MPD administration is due to destruction of both the direct and the indirect CN pathways which act as an inhibitory/excitatory balance, electroylticelectroyltic. The selective dopaminergic lesioning prevented the effects of MPD administration suggesting that dopaminergic pathways in CN play a significant role in the effects of MPD.

Dopamine triggers skeletal muscle tone by activating D1-like receptors on somatic motoneurons

The dopamine system plays an integral role in motor physiology. Dopamine controls movement by modulation of higher-order motor centers (e.g., basal ganglia) but may also regulate movement by directly controlling motoneuron function. Even though dopamine cells synapse onto motoneurons, which themselves express dopamine receptors, it is unknown whether dopamine modulates skeletal muscle activity. Therefore, we aimed to determine whether changes in dopaminergic neurotransmission at a somatic motor pool affect motor outflow to skeletal muscles. We used microinjection, neuropharmacology, electrophysiology, and histology to determine whether manipulation of D(1)- and D(2)-like receptors on trigeminal motoneurons affects masseter and/or tensor palatini muscle tone in anesthetized rats. We found that apomorphine (a dopamine analog) activated trigeminal motoneurons and triggered a potent increase in both masseter and tensor palatini tone. This excitatory effect is mediated by D(1)-like receptors because specific D(1)-like receptor activation strengthened muscle tone and blockade of these receptors prevented dopamine-driven activation of motoneurons. Blockade of D(1)-like receptors alone had no detectable effect on basal masseter/tensor palatini tone, indicating the absence of a functional dopamine drive onto trigeminal motoneurons, at least during isoflurane anesthesia. Finally, we showed that D(2)-like receptors do not affect either trigeminal motoneuron function or masseter/tensor palatini muscle tone. Our results provide the first demonstration that dopamine can directly control movement by manipulating somatic motoneuron behavior and skeletal muscle tone.

Nurr1-null heterozygous mice have reduced mesolimbic and mesocortical dopamine levels and increased stress-induced locomotor activity

Nurr1, an orphan nuclear receptor, is essential for the differentiation of the midbrain dopamine (DA) neurons; however, its function in adult midbrain DA neurons has not been determined. The present study compared regional brain levels of catecholamines and spontaneous and pharmacologically induced locomotor behaviors between mice heterozygous for the Nurr1-null allele (+/-) and wild type (+/+) littermates. The Nurr1 +/- mice had significantly lower levels of DA in whole brain, midbrain, prefrontal cortex and nucleus accumbens, although no significant differences were observed in the striatum, olfactory bulb or hippocampus. Nurr1 +/- mice displayed significantly greater locomotor activity in a novel open field and after saline injection with no significant difference in activity after treatment with amphetamine (2.5 or 5.0 mg/kg) or MK 801 (0.2 or 0.4 mg/kg). A similar elevation in locomotor activity was observed in Nurr1 +/- mice at 35 days old as was found in 70 days old adults. These data demonstrate that the loss of a single Nurr1 allele results in reduced DA levels in mesolimbic and mesocortical pathways and increased locomotor activity in response to mild stress. The involvement of Nurr1 in DA neurotransmission and the implications for schizophrenia are discussed.

Influence of ventral tegmental area (A10 region) on flight behaviour elicited by hypothalamic stimulation in the cat

The influence of the ventral tegmental area (VTA) (dopaminergic A10 group neurons) on flight behaviour, induced by hypothalamic stimulation, was studied in the cat. Co-stimulation of hypothalamus and VTA (ipsi- or contralateral) induced an increase of the flight latency. Slow-motion analysis of flight behaviour showed that this increase was due to the augmentation of the fixation latency (FL) whereas the upright latency (UL) was not modified. Sulpiride injection (50 mg/kg i.p.) provoked the disappearance of VTA effect without affecting the basal behavioural sequence. The results show that DA A10 group neurons increase the attentive component of the flight reaction, suggesting a possible influence of the DA A10 system on the mechanisms modulating focused attention in the animal.

Relation between motor asymmetry and direction of rotational behaviour under amphetamine and apomorphine in rats with unilateral degeneration of the nigrostriatal dopamine system

The study examines whether in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra, the direction of amphetamine- and apomorphine-induced rotations is determined by a bias to step more frequently with one hindleg. Results show that there is a strong asymmetry in usage of hindlegs under apomorphine but a weak one under amphetamine. Moreover, under apomorphine, virtually all steps are backward. Under amphetamine, however, steps are directed both backward and forward, although backward steps are relatively more prevalent when striatal dopamine depletion is nearly total. The different styles of rotation reflect the activation of pure turning under apomorphine, and turning combined with forward progression under amphetamine. It is suggested that presence of forward progression during turning depends on the capacity to modulate striatal activation. Moreover, the different styles of rotation are inconsistent with the hypothesis that a bias in leg usage sets the direction of turning. Instead, it seems that unilateral striatal activation sets the directionality of turning, independent of any hindlimb motor asymmetries.