Yawning and suppression of exploration induced by dopamine agonists: no relation to extracellular striatal levels of dopamine

Yawning as a Rare Side Effect With Increased Escitalopram Dose: A Case Report

Yawning is a normal physiological process that occurs naturally in all human beings in different settings, such as hunger, drowsiness, or stress. It is typically harmless, but abnormal yawning can be seen in many medical conditions. In psychiatry, it frequently occurs in disorders like depression, insomnia, and anxiety due to disturbed sleep. It has also been observed as an adverse reaction of some drugs, like escitalopram, a selective serotonin reuptake inhibitor. Escitalopram is a widely prescribed, well-tolerated antidepressant and antianxiety drug that can induce a range of side effects, one of which is excessive yawning. Its excessive occurrence can be distressing for patients, affecting their socio-occupational functioning. Clinically, differentiating yawning induced by escitalopram treatment from that in depression can be a diagnostic hurdle. Awareness and recognition of this lesser known side effect can improve patient outcomes by allowing for timely adjustments and easing the discomfort.

Levodopa enhances immobility induced by spinal cord electromagnetic stimulation in rats

BACKGROUND

The repetitive ElectroMagnetic Stimulation (rEMS) is an innocuous method applied to modulate neurocircuits in real-time to study the physiology of the central nervous system and treat neuropsychiatric conditions. Preliminary data suggest that spinal rEMS induces behavioral changes in awake rats. However, the mechanisms behind this phenomenon remain largely unknown.

METHODS

Twenty-five male Wistar rats were divided into five subgroups of five animals each: one subgroup was drug-free, two subgroups received Levodopa+Benserazide 250+25mg/kg for two or seven days, and the remaining two subgroups received Haloperidol 0.1 or 0.3mg/kg for two days. The animals were restrained during sham rEMS (day 1) followed by real rEMS of the cervicothoracic region at a different day (day 2 or 7, depending on subgroup). Four behavioral parameters were quantified: Walking, Climbing, Grooming, and Cornering.

RESULTS

rEMS reduced Walking and increased Cornering duration when applied over the cervicothoracic region of drug-free animals. A pretreatment with Levodopa+Benserazide for two or seven days induced an additional decrease in Walking after rEMS. This reduction was maximum after the treatment for seven days and associated with extinction of Climbing and increase in Cornering. A pretreatment with Haloperidol 0.1mg/kg reduced Grooming after rEMS, but did not prevent the reduction in Walking.

CONCLUSIONS

Cervicothoracic rEMS induced complex immobility responses that are in part modulated by dopaminergic pathways in rats. Further studies are necessary to determine the specific mechanisms involved.

Drug-induced yawning: a review of the French pharmacovigilance database

OBJECTIVE

To review the reports with 'yawning' as an adverse drug reaction (ADR) reported into the French Pharmacovigilance Database.

METHODS

All the observations with 'yawning' reported in the French Pharmacovigilance Database until December 2004 were reviewed. We recorded drug(s) involved, characteristics of patients (age, sex and underlying disease) and of ADR (seriousness, delay in occurrence, evolution, imputability).

RESULTS

Twenty-eight reports were recorded between 1985 and December 2004. The sex ratio of the patients included in these reports was 1.5 and the mean age was 46.2 (2-78) years. Thirty-eight drugs were involved, mainly serotoninergic agents (serotonin reuptake inhibitors [12]), dopaminergic agents (levodopa [3], dopamine agonists [3], monoamine oxidase B inhibitor [1]), opioids (morphine [1], methadone [1], buprenorphine [1], dextromethorphan [1]), benzodiazepines (4) and sodium channel inhibitors (lidocaine [2], flecainide [1]). Four ADRs were rated 'serious' (leading to hospitalisation). Patient outcome was usually favourable after drug withdrawal.

CONCLUSION

Despite its necessary methodological drawbacks (mainly under-reporting), this study reveals that several drugs may induce yawning in humans. Our work also indicates that stimulation of central dopamine or serotonin receptors elicits yawning in humans. This study underlines the role of several drugs in yawning and shows that this ADR is not systematically listed in the summary product characteristic even when it can be explained by the pharmacodynamic properties of the drugs.

Differential contribution of storage pools to the extracellular amount of accumbal dopamine in high and low responders to novelty: effects of reserpine

The present study examined the effects of reserpine on the extracellular concentration of accumbal dopamine in high responders (HR) and low responders (LR) to novelty rats. Reserpine reduced the baseline concentration of extracellular accumbal dopamine more in HR than in LR, indicating that the dopamine release is more dependent on reserpine-sensitive storage vesicles in non-challenged HR than in non-challenged LR. In addition, reserpine reduced the novelty-induced increase of the extracellular concentration of accumbal dopamine in LR, but not in HR, indicating that the dopamine release in response to novelty depends on reserpine-sensitive storage vesicles only in LR, not in HR. Our data clearly demonstrate that HR and LR differ in the characteristics of those monoaminergic storage vesicles that mediate accumbal dopamine release.

Enhanced binding of dopamine D1receptors in caudate-putamen subregions in High-Yawning Sprague-Dawley rats

Previous reports have shown that the inbred High-Yawning (HY) and Low-Yawning (LY) rats differ in several behavioral characteristics related to mesolimbic and nigrostriatal dopamine (DA) function. To determine if differential expression of DA receptors or DA transporter may mediate the behavioral differences in these two sublines of the Sprague-Dawley rat, we performed a quantitative autoradiography study of the DA D1-like, D2-like, and DA-transporter binding in the basal ganglia and nucleus accumbens. The results show that levels of the D1 binding in the caudate-putamen of the HY rat were higher than in the LY animals, whereas no significant differences in the DA D2 receptors and DA transporter were noted in these sublines. These data suggest that the differences in DA receptors in D1 binding in HY rats may in part have contributed to the behavioral differences related to DA functions such as grooming and penile erection. Our findings are consistent with previous reports showing a decrease in the behavioral responses after systemic administration of DA agonist in LY compared to HY rats.

Behavioral response profiles following drug challenge with dopamine receptor subtype agonists and antagonists in developing rat

As part of an investigation into the effects of gestational ethanol (ETOH) exposure on the developing dopamine (DA) system, pregnant Sprague-Dawley rats were exposed to one of three conditions: ETOH, pair-fed (PF) to the ETOH group, or ad libitum lab chow controls (LC). In this paper we report behavioral drug challenge effects for offspring of the two control groups (PF and LC). Male and female pups between postnatal days (PNDs) 21 and 23 in age were exposed to one of three intraperitoneal/subcutaneous doses of one of eight drugs chosen to assess the functional status of the DA D(1), D(2), and D(3) receptor subtype, or a saline control. Agonists were SKF 38393, apomorphine (APO), quinpirole (QUIN), and 7-hydroxy-N,N-di-n-propyl-2-amino-tetralin [7-OH-DPAT (DPAT)]; antagonists were spiperone (SPIP), SCH 23390, and two recently developed D(3) antagonists nafadotride (NAF) and PD 152255. Immediately following drug injection, pups were placed in observation cages, where eight behaviors (square entries, grooming, circling, rearing, sniffing, head and oral movements, and yawning) were scored at 3-min intervals for 30 min. Classic behavioral profiles were generally obtained for the high-dose mixed agonists APO, DPAT, and QUIN, which potently increased square entries, rearing, and sniffing, while reducing grooming and head movements. However, low-dose APO had no effect on behavior. The D(1) agonist, SKF 38393, had a strikingly different behavioral profile; it had no effect on square entries at any dose, while increasing grooming and sniffing at the medium dose. The D(1) antagonist, SCH 23390, profoundly decreased all behaviors except oral and head movements, especially at high doses. In contrast, the effects of the D(2) antagonist, SPIP, were limited to increasing sniffing at the medium dose. The two putative D(3) antagonists, NAF and PD 152255, presented strikingly different profiles. NAF induced a pattern of behavioral suppression that resembled the profile of high-dose SCH, while high-dose PD 152255 stimulated behavior. The failure of low-dose APO to have any effect on behavior suggests that the D(2) autoreceptor is not functional in preweanling rats. This hypothesis is further supported by the lack of behavioral suppression seen with low-dose QUIN and DPAT. Failure of NAF to produce behavioral activation at low doses and the stimulatory effects seen with PD 152255 suggests that either the D(3) autoreceptor, the postsynaptic D(3) receptor, or both are not fully functional at this age as well.

Yawning

YAWNING IS A COMMON PHYSIOLOGICAL EVENT THAT CAN BE DIVIDED INTO THREE DISTINCT PHASES: a long inspiratory phase, a brief acme and a rapid expiration. The aim of yawning is not yet well defined. However this semi-voluntary event increases vigilance and aims to alert when drowsiness occurs. Yawning probably has an important role for social communication as well. Yawning can be responsible for pain, luxation or even transient ischaemic attack. Abnormal yawning is present in various pathologies: migraine, Parkinson's disease, tumours, psychiatric diseases, infections or iatrogenic pathologies. The neuro-pharmacology of yawning is complex and knowledge of its mechanisms is incomplete. While under the control of several neurotransmitters, yawning is largely affected by dopamine. Dopamine may activate oxytocin production in the paraventricular nucleus of the hypothalamus. Oxytocin may then activate cholinergic transmission in the hippocampus and, finally, acetylcholine might induce yawning via the muscarinic receptors of the effectors. This is an over-simplification; many other molecules can modulate yawning, such as nitric oxide, glutamate, GABA, serotonin, ACTH, MSH, sexual hormones and opium derivate peptides. Dopamine involvement in yawning could have practical applications in the study of new drugs or the exploration of neurological diseases such as migraine or psychosis. 2001 Harcourt Publishers Ltd

Animal models for the negative symptoms of schizophrenia

Negative or defect symptoms refer to a reduction in normal functioning. In schizophrenia, negative symptoms encompass, among others, anhedonia, flat affect, avolition and social withdrawal. These symptoms have been found to be particularly prominent in the more chronic phase of the illness and seem to be virtually insensitive to current antipsychotic treatment. This review focuses on the possibilities and limitations of animal models for the negative symptoms of schizophrenia. Following a review of the negative symptoms in schizophrenia, attention is focused on the two symptoms most often modelled in animals - anhedonia and social withdrawal. We then look at the important question of how to model schizophrenic pathology in animals. Since the exact pathology is still far from clear, most efforts have in the past concentrated on using psychotomimetic drugs such as amphetamine or phencyclidine. The recently accumulated knowledge that schizophrenia probably results from disturbances in the normal development of the brain has led to a surge of new animal models in which the long-term consequences of early manipulations are investigated. However, so far these models have predominantly concentrated on the positive rather than the negative symptoms of schizophrenia. The last part of this review is dedicated to the question of validation of animal models for anhedonia and social withdrawal. The general conclusion is that very few models have so far been adequately tested. The lack of currently effective treatment further hampers the study of such validation.

Decreased GAP-43/B-50 phosphorylation in striatal synaptic plasma membranes after circling motor behavior during development

We evaluated the in vitro phosphorylation of the presynaptic substrate of protein kinase C (PKC), GAP-43/B-50 and the PKC activity in the striatum of rats submitted to a circling training (CT) test during postnatal development. Motor activity at 30 days of age, but not at other ages, produced a unilateral reduction (-29.5%; p<0.001) in the level of GAP-43/B-50 endogenous phosphorylation in the contralateral striatum with respect to the ipsilateral side, while non-trained control animals did not show asymmetric differences. Compared to controls, the contralateral striatum of trained animals also showed a significant reduction (-29.3%; p<0. 001) in the incorporation of 32P-phosphate into GAP-43. This decreased in vitro GAP-43 phosphorylation was seen at 30 min, but not immediately after circling motor behavior. This contralateral change in GAP-43 phosphorylation correlated with the running speed developed by the animals [(r=0.9443, p=0.0046, n=6, relative to control group) and (r=0.8813, p=0.0203, n=6, with respect to the ipsilateral side of the exercised animals)]. On the contrary, GAP-43/B-50 immunoblots did not show changes in the amount of this phosphoprotein among the different experimental groups. Back phosphorylation assays, performed in the presence of bovine purified PKC, increased the level of GAP-43/B-50 phosphorylation in the striatum contralateral to the sense of turning [(+22%; p<0.05, with respect to ipsilateral side of the same trained group) and (+21%; p<0.05, relative to control group)]. Taken together, these results demonstrate that the activity developed in the CT test induces a reduction in the phosphorylation state of GAP-43/B-50 in the specific site for PKC. We conclude that general markers of activity-dependent neuronal plasticity are also altered in the same period that long-lasting changes in striatal neuroreceptors are triggered by circling motor behavior.

Effects of intrastriatal infusion of D2 receptor antisense oligonucleotide on apomorphine-induced behaviors in the rat

An antisense oligonucleotide strategy was employed to specifically deplete postsynaptic striatal D2 receptors in order to determine the possible role of presynaptic D2 autoreceptors in mediating behavioral responses induced by low doses of apomorphine. A phosphorothioate-modified antisense oligonucleotide complementary to the first 19 bases of the coding region of D2 receptor mRNA, a scrambled sequence comprising the same bases, or saline was infused bilaterally into the striatum of adult rats, twice daily for 2 days via indwelling cannulae. After an interval of 8-12 h, rats were habituated and challenged with high (300 micrograms/kg; subcutaneous) or low (50 micrograms/kg; s.c.) doses of apomorphine or its vehicle (0.1% ascorbic acid). Yawning, vacuous chewing mouth movements, hypoexploration, and penile grooming induced by low-dose apomorphine were unaffected by antisense infusion into the striatum, whereas stereotypic sniffing following high-dose apomorphine was markedly suppressed. Intrastriatal infusion of antisense resulted in significantly diminished [3H]-raclopride binding, while binding of [3H]-SCH 23390 (D1 receptors) and [3H]-WIN 35428 (dopamine transporter) was unchanged. D2 mRNA levels determined by quantitative in situ hybridization were normal in the striatum and the substantia nigra. Our results confirm that stereotypic sniffing is mediated via postsynaptic D2 receptors in the striatum, and favor the notion that behavioral responses induced by low doses of apomorphine are mediated by presynaptic D2 autoreceptors.

Sulpiride antagonizes the biphasic locomotor effects of quinpirole in weanling rats

Low doses of dopamine (DA) agonists such as the D2 receptor subfamily agonist quinpirole are thought to stimulate DA autoreceptors selectively, thereby inhibiting locomotor activity. High doses of quinpirole initially suppress and later activate locomotion during a single test-session; the activation is presumably due to stimulation of postsynaptic receptors. The aim of this study was to investigate whether pretreatment with a selective DA D2 receptor antagonist, sulpiride, could block the putative autoreceptor-mediated inhibition at a lower dose than was required to block the postsynaptically mediated activation. Male and female 30-day-old rats were injected SC with one of eight doses of sulpiride (0.313-40 mg/kg) or the vehicle. Sixty minutes later, rats were injected SC with 0.2 mg/kg quinpirole or the vehicle. Five minutes after the second injection, rats were placed in automated activity monitors which recorded locomotor behavior for 60 min at 5-min intervals. Quinpirole at this dose first suppressed and later increased locomotor activity. Sulpiride pretreatment dose-dependently reversed both the early inhibition and later activation of quinpirole-induced locomotion. However, sulpiride did not block the quinpirole-induced early suppression at a lower dose than was required to block the later activation. Thus, there was no evidence that the locomotor suppression elicited by quinpirole is mediated by a more sensitive subset of DA receptors.

Apomorphine-induced yawning in the rat: influence of fasting and time of day

Yawning behavior is an experimental tool to study physiological responses, to elucidate the mechanisms of action of some drugs and hormones, and it is also a paradigm for some diseases and for dopamine (DA) agonists' clinical use. In this study, the effects of 24- and 48-h fasting as well as the influence of the light-dark cycle on apomorphine (APO)-induced yawning were evaluated. Initially, control and 48-h-fasted adult male rats were tested for yawning induced by APO (50, 100, 150 micrograms/kg, SC). The most effective dose tested was 100 micrograms/kg. Fasting significantly lowered yawning in all doses tested. Comparison between 24- and 48-h-fasted rats for APO (100 micrograms/kg)-induced yawning showed no significant difference between groups. Ad lib-fed groups were tested for APO (100 micrograms/kg)-induced yawning in both the light and in the dark phases of the cycle. Total number of yawnings increased significantly in the dark period. The present data show that fasting reduces and dark period increases APO-induced yawning in rats, suggesting that these conditions modulate the expression of this behavior.

Effect of BCH 325 (Pro-D-Phe-Pro-Gly) on central dopaminergic functions

Three behavioral models were used to characterize the pharmacological action of BCH 325 on central dopaminergic transmission. The effect of acute SC treatment with BCH 325 upon dopaminergic mechanisms affecting motor activity was studied on the climbing behavior of mice. It was shown that the beta-casomorphin analogue evoked a dose-dependent increase in apomorphine (APO)-induced hypoactivity that was reversed by sulpiride (SULP). In in vitro studies on slices of nucleus accumbens of mice it could be demonstrated that 10(-6) M APO caused a reduction of K(+)-stimulated [14C]dopamine (DA) release that was potentiated following simultaneous incubation with 10(-6) M BCH 325. To prove a postsynaptic influence in D1 receptor-mediated behavior pattern, the action of BCH 325 was studied on bromocriptine (BROMO)-evoked yawning behavior of rats after pretreatment with reserpine (RES) or saline. The peptide could not influence the BROMO yawning after saline administration, but it was able to normalize the number of yawns, which were reduced after RES. To investigate the effect of BCH 325 on postsynaptic D2 receptors, jerking behavior on RES-pretreated rats after a high dose of BROMO was used. Following RES pretreatment only, the number of BROMO-induced jerks was decreased by treatment of rats with 0.5 mumol/kg BCH 325. In contrast, the jerking behavior was enhanced by 0.5 mumol/kg BCH 325 in rats that were additionally treated with alpha-methyl-p-tyrosine (MPT). In biochemical studies on slices of the nucleus accumbens of mice, the in vivo pretreatment with RES caused a reduction of K(+)-stimulated [14C]DA release that was blocked by the SC administration of 0.5 mumol/kg BCH 325.(ABSTRACT TRUNCATED AT 250 WORDS)

The ontogeny of apomorphine-induced alterations of neostriatal dopamine release: effects on spontaneous release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on the extracellular levels of dopamine and the dopamine metabolite 3,4-dihydroxyphenylacetic acid were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10-11, 21-22, and 35-36 days old and adults and quantified by HPLC with electrochemical detection. Apomorphine attenuated extracellular levels of dopamine in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine on extracellular dopamine was observed in all age groups except at 10-11 days of age. Extracellular levels of 3,4-dihydroxyphenylacetic acid were also significantly decreased in all age groups, consistent with the hypothesis that synthesis-modulating dopamine autoreceptors in the neostriatum are functional by 5 days of age. Apomorphine had a significantly greater effect on extracellular 3,4-dihydroxyphenylacetic acid levels at the 0.05 and 0.1 mg/kg doses in the 5- and 10-11-day-old age groups compared with the other ages. Absolute levels of extracellular dopamine were significantly attenuated at 5 days of age compared with the other ages, and absolute levels of extracellular 3,4-dihydroxyphenylacetic acid monotonically increased with age.

The role of dopamine in the maintenance and breakdown of D1/D2 synergism

The neurochemical factors involved in the maintenance and breakdown of dopamine D1/D2 receptor synergism were investigated by giving rats various pharmacological treatments that diminish the ability of dopamine to interact with its D1 and/or D2 receptors. Following these treatments, rats were observed for the expression of stereotyped motor behavior in response to independent stimulation of D1 or D2 receptors. Independent D2-mediated responses were observed: (a) 2 h after the last of three daily reserpine (1 mg/kg) injections, (b) 48 h after bilateral 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal pathways, (c) 24 h after a concentrated 48-h regimen (one injection/6 h) of eticlopride (0.5 mg/kg) or eticlopride+SCH 23390 (0.5 mg each), and (d) 2 h after a concentrated 48-h regimen (one injection/6 h) of alpha-methyl-p-tyrosine (alpha MPT; 100 mg/kg), but not after control treatments or a concentrated regimen of SCH 23390 alone. By contrast, independent D1-mediated responses were observed only after three daily reserpine injections or 48 h after bilateral 6-OHDA lesions. Independent D1-mediated stereotypy was not observed under control conditions or following a concentrated 48-h regimen of (a) SCH 23390 or eticlopride (0.5 mg/kg each) alone or in combination, (b) a high dose of SCH 23390 (1.0 mg/kg), (c) alpha MPT (100 mg/kg), or (d) alpha MPT (100 mg/kg)+SCH 23390 (1.0 mg/kg). Reserpine, bilateral 6-OHDA, and alpha MPT treatments produced striatal dopamine depletions of 96%, 92%, and 71%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation by low doses of selected neuroleptics of food-induced conditioned place preference in rats

Numerous data support the hypothesis that dopamine (DA) plays a crucial role in reward-related processes and in incentive learning in animals and man. The possibility that various neuroleptics exhibiting a high affinity for the dopaminergic D2 (and D3) receptors could reinforce DA transmission was studied using the conditioned place preference paradigm (CPP) in rats. This was done by examining the ability of these compounds to potentiate the reinforcing properties of food in hungry rats subjected to a version of the CPP paradigm which consisted of repeated pairings of food with a single environmental cue, the floor texture of an open field. During the test session when food was no longer available in the open field, the increase in the time spent by drug-free rats on the food-paired texture was assumed to indicate the perceived rewarding value of the food. This time was significantly lengthened when the specific D2 (D3)-receptor antagonists sulpiride (4 mg/kg), amisulpride (0.5, 1 mg/kg) or pimozide (0.03, 0.06 mg/kg) were administered before the food conditioning sessions. Larger doses of these compounds as well as haloperidol, metoclopramide and the non-specific D1-D2 antagonist, chlorpromazine, regardless of the doses tested, did not exhibit this effect, but rather reduced the food-induced CPP, an action usually associated with neuroleptics. The positive effects of amisulpride was reversed by a D1 receptor antagonist, SCH 23390 (0.01 mg/kg). These results suggest that, as with amphetamine (0.5 mg/kg), some D2-specific neuroleptics enhance the incentive value of food in a narrow range of low doses, an effect proposed to reflect a "prohedonic" property.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of daily SKF 38393, quinpirole, and SCH 23390 treatments on locomotor activity and subsequent sensitivity to apomorphine

In three experiments, male Wistar rats (250-350 g) were injected (SC) daily with the D1-type dopamine receptor agonist, SKF 38393 (0.0, 4.0, 8.0, or 16.0 mg/kg), the D2-type dopamine receptor agonist, quinpirole (0.0, 0.3, or 3.0 mg/kg), and/or the D1-type dopamine receptor antagonist, SCH 23390 (0.0 or 0.5 mg/kg) for 8-10 days. After each daily injection, the rats were tested for locomotor activity in photocell arenas for 20 min. Following this subchronic pretreatment, all rats were challenged with the mixed dopamine receptor agonist apomorphine (1.0 mg/kg, SC) and tested for locomotor activity. SKF 38393 treatments produced a dose-dependent decrease in locomotor activity which did not significantly change across days. Quinpirole also depressed locomotor activity when first injected, but this quinpirole-induced inhibition of activity progressively decreased across days. When subsequently challenged with apomorphine, rats in both the SKF 38393 and the quinpirole pretreatment groups displayed greater locomotor activity than rats pretreated with only vehicle. Although SCH 23390 pretreatments did not affect subsequent sensitivity to apomorphine, SCH 23390 completely blocked the effect of quinpirole. These results suggest that although repeated D1 receptor stimulation may be sufficient to induce behavioral sensitization to apomorphine, D2 receptor stimulation also contributes to the effect.

Do autoreceptors mediate dopamine agonist--induced yawning and suppression of exploration? A critical review

The hypothesis that stimulation of dopamine autoreceptors is the mechanism by which dopamine agonists induce yawning and suppression of exploration is critically examined. It is shown that the relation between reduced extracellular dopamine levels, assessed by microdialysis, and behavioural effects of dopamine agonists, a dopamine synthesis inhibitor and a granule storage blocker is highly inconsistent. The time-course and duration of the behavioural effects of dopamine agonists differ from the reduction of extracellular dopamine. Amphetamine cotreatment is shown to increase dopamine levels, while yawning and suppression of exploration can still be induced. The data strongly indicate that autoreceptors are not the mediators of these behavioural effects. It is proposed that postsynaptic receptors mediate dopamine agonist induced yawning and suppression of exploration. Evidence is also presented showing that yawning and suppression of exploration are not functionally equivalent.

Yawning produced by dopamine agonists in rhesus monkeys

Yawning was recorded from five rhesus monkeys restrained in a chair after i.m. injection of dopaminergic compounds: apomorphine (0.03 mg/kg), quinpirole (0.01 mg/kg), and (-)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine (1 mg/kg). SKF 38393 or physostigmine produced no yawning. Yawning from apomorphine was blocked by chlorpromazine or SCH 23390 (0.03 mg/kg). Sulpiride (10 mg/kg) was ineffective. The difference between rats and monkeys in their yawning response to dopaminergic compounds is discussed.

Dissociation of autoreceptor activation and behavioral consequences of low-dose apomorphine treatment

1. Low dose dopaminergic agonist effects have been used as a behavioral screen for identifying compounds with selective autoreceptor activity. 2. However, results from several recent investigations suggest that these behaviors may not be generated from an autoreceptor substrate but rather from a subpopulation of postsynaptic dopamine receptors with a high affinity for the agonist. 3. In support of this hypothesis, the present investigation reports that both hypomotility and yawning, induced in the rat with 0.07 mg/kg apomorphine, were not paralleled by autoreceptor-induced reductions in transmitter metabolism from either mesolimbic or neostriatal dopamine regions.

Effects of apomorphine on novelty-induced place preference behavior in rats

Adult male rats were exposed to one of two different stimulus compartments by being placed into the compartment for 30 min on each of eight consecutive days. Following this exposure, each rat was administered 0, 0.05, 0.1, 0.5 or 5.0 mg/kg apomorphine. Thirty min after injection, each animal was given free-choice access to the familiar (exposed) compartment and to the novel (nonexposed) compartment. As expected, saline-injected control animals displayed a preference for the novel compartment. This novelty preference was disrupted in animals given either 0.05 or 0.1 mg/kg apomorphine, but not in animals given either 0.5 or 5.0 mg/kg apomorphine. The disruption in novelty preference by the low doses of apomorphine did not reflect a disruption of locomotor activity, as there was no direct relationship between the preference for novelty and the rate of horizontal or vertical activity among the different treatment groups. Instead, the low doses of apomorphine may have inhibited dopamine function by blocking presynaptic autoreceptors selectively, and thus the reinforcing effect of the novel stimulation may have been attenuated.

The reduction of water intake in rats caused by a low dose of apomorphine is unaltered by alpha-methyl-p-tyrosine: are autoreceptors not involved?

Low doses of the dopamine (DA) agonist apomorphine (APO) induces a behavioural syndrome characterized by reduced spontaneous activity, reduced food and water intake and induction of yawning and penile erections. Traditionally these effects of APO have been considered to be caused by a preferential stimulation of DA autoreceptors, causing a decreased amount of transmitter at the postsynaptic receptors. If this is so, it could be hypothesized that 1) the same behavioural effects should be obtained if DA transmission is decreased by some other means, for example by synthesis inhibition, and that 2) the response to APO should be altered if DA transmission is already lowered. It was found that high doses of alpha-methyl-p-tyrosine (alpha-MPT; 50-200 mg/kg) did not reduce water intake in thirsty rats, which low doses of APO do. It was further found that pretreatment with alpha-MPT did not alter the response to APO. These results are difficult to reconcile with the DA autoreceptor hypothesis claiming that behavioural effects of low doses of APO are caused by a decreased release of DA. An alternative interpretation is that low doses of APO stimulates a certain population of sensitive postsynaptic D-2 receptors.