Stimulation of D1- or D2-receptors in drug-naive rats with different degrees of unilateral nigro-striatal dopamine lesions

Parasitoid wasp venom manipulates host innate behavior via subtype-specific dopamine receptor activation

The subjugation strategy employed by the jewel wasp is unique in that it manipulates the behavior of its host, the American cockroach, rather than inducing outright paralysis. Upon envenomation directly into the central complex (CX), a command center in the brain for motor behavior, the stung cockroach initially engages in intense grooming behavior, then falls into a lethargic sleep-like state referred to as hypokinesia. Behavioral changes evoked by the sting are due at least in part to the presence of the neurotransmitter dopamine in the venom. In insects, dopamine receptors are classified as two families, the D1-like and the D2-like receptors. However, specific roles played by dopamine receptor subtypes in venom-induced behavioral manipulation by the jewel wasp remain largely unknown. In the present study, we used a pharmacological approach to investigate roles of D1-like and D2-like receptors in behaviors exhibited by stung cockroaches, focusing on grooming. Specifically, we assessed behavioral outcomes of focal CX injections of dopamine receptor agonists and antagonists. Both specific and non-specific compounds were used. Our results strongly implicate D1-like dopamine receptors in venom-induced grooming. Regarding induction of hypokinesia, our findings demonstrate that dopamine signaling is necessary for induction of long-lasting hypokinesia caused by brain envenomation.

Highlighted Article: Subtype-specific dopamine receptors are involved in the manipulation of host behavior by the parasitoid jewel wasp.

Dual orexin and MCH neuron-ablated mice display severe sleep attacks and cataplexy

Orexin/hypocretin-producing and melanin-concentrating hormone-producing (MCH) neurons are co-extensive in the hypothalamus and project throughout the brain to regulate sleep/wakefulness. Ablation of orexin neurons decreases wakefulness and results in a narcolepsy-like phenotype, whereas ablation of MCH neurons increases wakefulness. Since it is unclear how orexin and MCH neurons interact to regulate sleep/wakefulness, we generated transgenic mice in which both orexin and MCH neurons could be ablated. Double-ablated mice exhibited increased wakefulness and decreased both rapid eye movement (REM) and non-REM (NREM) sleep. Double-ablated mice showed severe cataplexy compared with orexin neuron-ablated mice, suggesting that MCH neurons normally suppress cataplexy. Double-ablated mice also showed frequent sleep attacks with elevated spectral power in the delta and theta range, a unique state that we call 'delta-theta sleep'. Together, these results indicate a functional interaction between orexin and MCH neurons in vivo that suggests the synergistic involvement of these neuronal populations in the sleep/wakefulness cycle.

Light/dark phase-dependent spontaneous activity is maintained in dopamine-deficient mice

Dopamine is important for motor control and involved in the regulation of circadian rhythm. We previously found that dopamine-deficient (DD) mice became hyperactive in a novel environment 72 h after the last injection of L-3,4-dihydroxyphenylalanine (L-DOPA) when dopamine was almost completely depleted. DD mice did not initially exhibit hyperactivity in their home cages, but the animals exhibited hyperactivity several hours after the last L-DOPA injection. The regulation of motor activity in a novel environment and in home cages may be different. A previous study reported that DD mice became active again approximately 24 h after the last L-DOPA injection. One speculation was that light/dark phase-dependent spontaneous activity might be maintained despite dopamine deficiency. The present study investigated whether spontaneous home cage activity is maintained in DD mice 24–43 h and 72–91 h after the last L-DOPA injection. Spontaneous activity was almost completely suppressed during the light phase of the light/dark cycle in DD mice 24 and 72 h after the last L-DOPA injection. After the dark phase began, DD mice became active 24 and 72 h after the last L-DOPA injection. DD mice exhibited a similar amount of locomotor activity as wildtype mice 24 h after the last L-DOPA injection. Although DD mice presented a decrease in activity 72 h after the last L-DOPA injection, they maintained dark phase-stimulated locomotor activation. Despite low levels of dopamine in DD mice, they exhibited feeding behavior that was similar to wildtype mice. Although grooming and rearing behavior significantly decreased, DD mice retained their ability to perform these activities. Haloperidol treatment significantly suppressed all of these behaviors in wildtype mice but not in DD mice. These results indicate that DD mice maintain some aspects of light/dark phase-dependent spontaneous activity despite dopamine depletion, suggesting that compensatory dopamine-independent mechanisms might play a role in the DD mouse phenotype.

Unilateral Lesion of Dopamine Neurons Induces Grooming Asymmetry in the Mouse

Grooming behaviour is the most common innate behaviour in animals. In rodents, it consists of sequences of movements organized in four phases, executed symmetrically on both sides of the animal and creating a syntactic chain of behavioural events. The grooming syntax can be altered by stress and novelty, as well as by several mutations and brain lesions. Grooming behaviour is known to be affected by alterations of the dopamine system, including dopamine receptor modulation, dopamine alteration in genetically modified animals, and after brain lesion. While a lot is known about the initiation and syntactic modifications of this refined sequence of movements, effects of unilateral lesion of dopamine neurons are unclear particularly regarding the symmetry of syntactic chains. In the present work we studied grooming in mice unilaterally lesioned in the medial forebrain bundle by 6-hydroxydopamine. We found a reduction in completion of grooming bouts, associated with reduction in number of transitions between grooming phases. The data also revealed the development of asymmetry in grooming behaviour, with reduced tendency to groom the contralateral side to the lesion. Symmetry was recovered following treatment with L-DOPA. Thus, the present work shows that unilateral lesion of dopamine neurons reduces self-grooming behaviour by affecting duration and numbers of events. It produces premature discontinuation of grooming chains but the sequence syntax remains correct. This deficient grooming could be considered as an intrinsic symptom of Parkinson’s disease in animal models and could present some similarities with abnormalities of motor movement sequencing seen in patients. Our study also suggests grooming analysis as an additional method to screen parkinsonism in animal models.

Deranged NMDAergic cortico-subthalamic transmission underlies parkinsonian motor deficits

Parkinson's disease (PD) is the most prevalent hypokinetic movement disorder, and symptomatic PD pathogenesis has been ascribed to imbalances between the direct and indirect pathways in the basal ganglia circuitry. Here, we applied glutamate receptor blockers to the subthalamic nucleus (STN) of parkinsonian rats and evaluated locomotor behaviors via single-unit and local-field recordings. Using this model, we found that inhibition of NMDAergic cortico-subthalamic transmission ameliorates parkinsonian motor deficits without eliciting any vivid turning behavior and abolishes electrophysiological abnormalities, including excessive subthalamic bursts, cortico-subthalamic synchronization, and in situ beta synchronization in both the motor cortex and STN. Premotor cortex stimulation revealed that cortico-subthalamic transmission is deranged in PD and directly responsible for the excessive stimulation-dependent bursts and time-locked spikes in the STN, explaining the genesis of PD-associated pathological bursts and synchronization, respectively. Moreover, application of a dopaminergic agent via a microinfusion cannula localized the therapeutic effect to the STN, without correcting striatal dopamine deficiency. Finally, optogenetic overactivation and synchronization of cortico-subthalamic transmission alone sufficiently and instantaneously induced parkinsonian-associated locomotor dysfunction in normal mice. In addition to the classic theory emphasizing the direct-indirect pathways, our data suggest that deranged cortico-subthalamic transmission via the NMDA receptor also plays a central role in the pathophysiology of parkinsonian motor deficits.

Effects of intranasally applied dopamine on behavioral asymmetries in rats with unilateral 6-hydroxydopamine lesions of the nigro-striatal tract

Due to its lipophobic properties, dopamine is unable to cross the blood-brain barrier following systemic application. However, recently it has been demonstrated that, when applied directly via the nasal passages in the rat, dopamine exerts neurochemical and behavioural action, including increases of dopamine in striatal subregions, antidepressive-like action, and increased behavioral activity. These effects could potentially be mediated by exogenous dopamine acting as a direct agonist at postsynaptic dopamine receptors. However, it is also possible that intranasally applied dopamine acts indirectly via the modulation of the activity of dopaminergic cell bodies. To approach this question, the present study used rats with unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal tract, as these lesions lead to pharmacologically stimulated behavioural asymmetries which are specific for direct and indirect dopamine agonists. We found that 7 days of repeated treatment with intranasal dopamine induced a sensitization of the turning response to amphetamine, but not to apomorphine. Furthermore, intranasal dopamine dose-dependently increased the use of the forepaw ipsilateral to the 6-OHDA-lesioned side of the brain. These results suggest that intranasally administered dopamine acts via an indirect mechanism of action, putatively by increasing the release of endogenous dopamine in the brain.

Pharmacological characterization of the effects of dopamine D(1) agonists on eye blinking in rats

Dopamine D(1)-like partial agonists antagonize some abuse-related effects of cocaine and have been proposed as candidate medications for psychostimulant abuse. Earlier studies have showed that D(1)-like agonists increase eye blinking in monkeys and that the magnitude of this effect may be related to agonist efficacy. These studies characterized the effects of D(1)-like agonists on eye blinking in female Sprague-Dawley rats placed in customized restraint tubes. After vehicle injections, eye blink rates averaged 2.1+/-0.25 blinks/min, or 31+/-4 blinks/15 min. The D(1)-like agonists SKF 82958 and R(+)-6Br-APB dose-dependently increased eye blinking to 136 and 124/15 min, respectively. The selective D(1)-like antagonist SCH 23390 decreased eye blinking and the peripherally selective D(1)-like agonist fenoldopam, the D(2)-like agonist (+)PHNO, and the indirect dopamine agonist methamphetamine all failed to alter eye blink rates relative to vehicle levels. Additional studies with unique congeners and isomers of the D(1)-like partial agonist SKF 83959, MCL 202, MCL 204, MCL 206, MCL 207 and MCL 209, resulted in only moderate increases in eye blink rates (27-84 blinks/15 min). These effects were dose-related for one compound, MCL 209 (max 84+/-19 blinks/15 min) and plateaued at the highest doses, suggestive of partial agonist effects. Additionally, the agonist MCL 206, like the D(1)antagonist SCH 23390, antagonized the effects of SKF 82958 on eye blinking. The findings suggest that D(1)-like agonists increase eye blinking in rats and that these effects may provide a simple measure that can be used to distinguish partial D(1)-like ligands. Further studies with novel D(1)-like partial agonists may be useful in the development of pharmacotherapies for cocaine abuse.

Behavioral effects of the R-(+)- and S-(-)-enantiomers of the dopamine D(1)-like partial receptor agonist SKF 83959 in monkeys

Dopamine D(1)-like partial receptor agonists such as SKF 83959 have been proposed as potential candidates for the treatment of cocaine addiction. The present studies were conducted to further characterize SKF 83959 by pharmacologically evaluating effects of its R-(+)- and S-(-)-enantiomers, MCL 202 and MCL 201, respectively, on overt behavior (eye blinking) and schedule-controlled performance in squirrel monkeys. MCL 202, like the D(1) full receptor agonist SKF 82958, produced dose-related increases in eye blinking and decreases in rates of fixed-ratio responding. However, the magnitude of effects of MCL 202 on eye blinking was less than observed with SKF 82958. In contrast to the effects of its R-(+) enantiomer, MCL 201 was relatively devoid of behavioral activity up to doses that were approximately 10-fold greater than MCL 202. Pretreatment with the selective D(1)-like receptor antagonist SCH 39166 dose-dependently antagonized increases in eye blinking produced by MCL 202, confirming the involvement of D(1) mechanisms in its effects. A dose-ratio analysis of the antagonism of effects of MCL 202 by SCH 39166 revealed an apparent pA(2) value of 7.675 with a slope of -0.78+/-0.04. In further studies, pretreatment with MCL 202 antagonized the effects of SKF 82958 on eye blinking and, like SCH 39166, schedule-controlled behavior in a dose-related manner. A dose-ratio analysis of the antagonist effects of MCL 202 on the SKF 82958-induced increases in eye blinking revealed ratios of 2.7, 4.8 and 31.1 for 0.1, 0.3 and 1.0 mg/kg dose of the antagonist, respectively, indicative of a significant change in the potency of SKF 82958. These results suggest that MCL 202, like its parent compound SKF 83959, has both D(1) receptor-mediated agonist and antagonist properties, consistent with its characterization as a partial agonist at the D(1)-like receptor. In addition, the inactivity of MCL 201, the S-(-)-enantiomer, suggests that the behavioral effects of SKF 83959 can be attributed primarily to the activity of its R-(+)-enantiomer.

Comparison of the discriminative-stimulus effects of SKF 38393 with those of other dopamine receptor agonists

The dopamine D(1)-like receptor agonists have traditionally been defined molecularly by their efficacy in stimulating adenylyl cyclase. However, evidence correlating the effectiveness of these drugs in behavioral assays and their effectiveness biochemically has not been forthcoming. The present study compared the discriminative-stimulus effects of the D(1)-like partial agonist SKF 38393 with several other D(1)-like agonists, an indirect agonist, cocaine, and a D(2)-like agonist, quinpirole. Rats were trained under a fixed-ratio 30-response schedule to discriminate SKF 38393 (5.6 mg/kg) from vehicle. Under this schedule, 30 consecutive responses on one of two keys were reinforced with food presentation after a pre-session injection of 5.6 mg/kg SKF 38393, and 30 consecutive responses on the alternative key were reinforced after saline injection. When daily performances were stable, substitution patterns for several compounds were assessed during test sessions in which 30 consecutive responses on either key were reinforced. Quinpirole and cocaine each produced saline-appropriate responding. In contrast, the D(1)-like agonists, SKF 75670 and SKF 77434, fully substituted for SKF 38393. Curiously, SKF 82958, which is considered a full agonist based on adenylyl cyclase assays, was less effective in substituting for SKF 38393 (maximum drug-appropriate responding 66%) than was the partial agonist SKF 75670. The present results suggest that second messenger effects other than stimulation of adenylyl cyclase may play an important role in the behavioral effects of dopamine D(1)-like agonists.

SKF 83959 is an antagonist of dopamine D1-like receptors in the prefrontal cortex and nucleus accumbens: a key to its antiparkinsonian effect in animals?

SKF 83959 that has a unique antiparkinson profile in animal models of Parkinson's disease is an in vitro dopamine D1 antagonist of receptors coupled to adenylyl cyclase. We hypothesized that SKF 83959, among others, interacts with dopamine D1 receptors coupled to adenylyl cyclase in the nucleus accumbens and the prefrontal cortex. Effects of intra-accumbal injections of SKF 83959 on locomotor activity were compared to effects of the dopamine D1 agonist SKF 81297 and the dopamine D1 antagonist SCH 39166. Similarly to SCH 39166, SKF 83959 did not affect locomotor activity, but counteracted SKF 81297-induced locomotor activity. Effects of unilateral intra-prefrontal injections of SKF 83959 on rotational behaviour were compared to the effects of the dopamine D1 agonist SKF 81297 and the dopamine D1 antagonists SCH 23390 and SCH 39166 in rats selected on basis of their high locomotor response to novelty and pretreated with a subcutaneous injection of 0.75 mg/kg dexamphetamine. Like SCH 39166 and SCH 23390, SKF 83959 induced a bias for contralateral rotating and blocked the SKF 81297-induced bias for ipsilateral rotating. In conclusion, SKF 83959 is an in vivo antagonist of dopamine D1 receptors that are coupled to adenylyl cyclase in the nucleus accumbens and the prefrontal cortex. The role of these receptors in the antiparkinson profile of SKF 83959 is discussed.