On the role of serotonin in apomorphine-induced locomotor stimulation in rats

Why can't rodents vomit? A comparative behavioral, anatomical, and physiological study

The vomiting (emetic) reflex is documented in numerous mammalian species, including primates and carnivores, yet laboratory rats and mice appear to lack this response. It is unclear whether these rodents do not vomit because of anatomical constraints (e.g., a relatively long abdominal esophagus) or lack of key neural circuits. Moreover, it is unknown whether laboratory rodents are representative of Rodentia with regards to this reflex. Here we conducted behavioral testing of members of all three major groups of Rodentia; mouse-related (rat, mouse, vole, beaver), Ctenohystrica (guinea pig, nutria), and squirrel-related (mountain beaver) species. Prototypical emetic agents, apomorphine (sc), veratrine (sc), and copper sulfate (ig), failed to produce either retching or vomiting in these species (although other behavioral effects, e.g., locomotion, were noted). These rodents also had anatomical constraints, which could limit the efficiency of vomiting should it be attempted, including reduced muscularity of the diaphragm and stomach geometry that is not well structured for moving contents towards the esophagus compared to species that can vomit (cat, ferret, and musk shrew). Lastly, an in situ brainstem preparation was used to make sensitive measures of mouth, esophagus, and shoulder muscular movements, and phrenic nerve activity–key features of emetic episodes. Laboratory mice and rats failed to display any of the common coordinated actions of these indices after typical emetic stimulation (resiniferatoxin and vagal afferent stimulation) compared to musk shrews. Overall the results suggest that the inability to vomit is a general property of Rodentia and that an absent brainstem neurological component is the most likely cause. The implications of these findings for the utility of rodents as models in the area of emesis research are discussed.

Effects of p-chloroamphetamine on brain serotonin neurons

p-Chloroamphetamine (PCA) is a useful pharmacologic tool for selectively increasing brain serotonin function acutely by release of serotonin into the synaptic cleft. PCA produces behavioral, neurochemical and neuroendocrine effects believed due to serotonin release after doses in the range of 0.5-5 mg/kg. At higher doses and at longer times, PCA causes depletion of brain serotonin. The mechanisms of this depletion are not well understood but require the serotonin uptake carrier. Antagonism of PCA-induced depletion of brain serotonin is a useful means of assessing the ability of a compound to block the serotonin uptake carrier on brain serotonin neurons. PCA can also be used as a neurotoxic agent to deplete brain serotonin in functional studies, apparently by destroying some serotonergic nerve terminals. Used in this way, PCA has an advantage over 5,6- and 5,7-dihydroxytryptamines in being effective by systemic injection, and it affects brain serotonergic projections with a different neuroanatomic specificity than the dihydroxytryptamines.

Striatal depth EEG reveals postsynaptic activity of striatal neurons following dopamine receptor stimulation and blockade

This paper demonstrates a technique for measuring depth electroencephalographic (EEG) recordings from the freely moving mouse. This technique minimizes electrical artifact associated with gross movements by amplifying the current of the EEG signal directly at permanently indwelling electrodes. Stable EEG signals, with high signal-to-noise ratios, can be obtained from these animals while their movement inside the testing cage remains relatively unrestricted. We used this technique to examine the effects of dopamine (DA) receptor agonist and antagonist treatments on depth EEG signals generated within the striatum. Baseline measures of spontaneous striatal EEG activity were obtained prior to drug administration and post-drug measures of striatal activity were subsequently obtained. Apomorphine treatment resulted in desynchronization of striatal EEG signals while haloperidol or sulpiride treatment induced slow wave synchronization. Fast Fourier analysis of EEG signals revealed that DA agonist and antagonist treatment altered spontaneous striatal EEG activity in an opposite manner--relative to baseline signals, apomorphine attenuated low frequency components and augmented higher frequency components of the signal while haloperidol augmented low frequency components and attenuated higher frequency components of the signal. Moreover, mice pretreated with unilateral intracerebral injections of sulpiride and subsequently administered systemic apomorphine simultaneously demonstrated EEG synchronization on the side ipsilateral to the injection of sulpiride and EEG desynchronization on the contralateral side. The population of neurons examined in the medial striatum appear to have the properties of being excitatory to DA agonist stimulation and show decreased activity following DA antagonist treatment. These results suggest that striatal EEG activity may be used as measure of postsynaptic activity of dopaminergic neurons.

Drugs affecting serotonin neurons

Advances in serotonin pharmacology, the development of drugs that intervene at specific sites to modify serotonergic function, have accompanied advances in the understanding of physiologic roles of serotonin present in neurons and elsewhere and of serotonin receptors that are widely distributed in brain and many peripheral tissues. The pharmacologic advances have sometimes been stimulated by developments in serotonin physiology, such as the recognition of multiple serotonin receptor subtypes, and in other cases have been a major factor in providing new insights into physiologic roles of serotonin. Drugs that modify serotonin function have a variety of therapeutic applications currently and many more potential therapeutic uses to be explored in the future. Having drugs that act with high specificity or selectivity on particular enzymes in serotonin biosynthesis, on particular serotonin receptors, or at other sites such as uptake carriers for serotonin not only offers the hope of improved clinical therapy in diseases caused by abnormal serotonergic function or in which alteration of serotonergic function can alleviate symptoms, but also provides valuable pharmacologic tools for learning more about serotonin physiology and probing the functional status of serotonergic systems. The next few years promise to yield important new serotonergic drugs.

Effects of prenatal 5-methoxytryptamine and parachlorophenylalanine on serotonergic uptake and behavior in the neonatal rat

Parachlorophenylalanine (pCPA) or 5-methoxytryptamine (5MT) was administered to pregnant Sprague Dawley rats from day 8 (D8) of gestation till D17 and from D12 until birth respectively. Birth weights of both drug groups of neonates were approximately 20% less than the saline-injected controls. 5MT neonates showed a significant reduction of high affinity 3H-5HT uptake in the brainstem at all three time points: D1, D15, D30, and a slight reduction in the forebrain reaching significance only on D30. The pCPA animals showed a significant reduction in the brainstem and forebrain on D1 and D30, but only a small nonsignificant reduction in both areas on D15. Behaviors measured on day 15 revealed that in general activity, spontaneous alternation, and passive avoidance both drug groups of neonates showed deficits: less activity, less alternation, and less avoidance.

Brain amines and effects of chlordiazepoxide on motor activity in response to stress

The effect of chlordiazepoxide (CDP) on emotional responsiveness to stress was determined in CD-1 male mice. The relationship of the monoamines to the mediation of emotional behavior was examined with drugs having selective actions on serotonin (5-HT), norepinephrine (NE), and dopamine (DA). Emotional behavior as measured by locomotor activity was increased by stress. This activation enhanced the stimulatory effect of low doses of CDP (5 and 10 mg/kg) and attenuated the depressant action of higher doses (20 and 40 mg/kg). Quipazine (0.5 mg/kg) reduced the depressant effect of CDP in stressed animals. Its action failed to support a proposed anti-serotonergic action of CDP and implicated possible dopaminergic involvement. In stressed mice, apomorphine (0.5 mg/kg) and clonidine (0.1 mg/kg) antagonized the stimulatory action of low doses of CDP. Behavioral effects of clonidine provide support for the notion that the stimulatory effects of CDP may be due to enhanced catecholamine (CA) neurotransmission. Whole brain levels of NE and DA were significantly increased when clonidine was combined with CDP. This indicated a possible reduction in CA turnover and activity.

p-Chlorophenylalanine and p-chloroamphetamine pretreatment of apomorphine-challenged rats: effects on solitary and social behavior

Following an initial rise in locomotor activity, apomorphine in large doses causes a concurrent rise in brain serotonin levels, locomotor akinesia, and stereotypic gnawing. However, reports to date have failed to observe any effect of pretreatment with serotonin depletors parachlorophenylalanine (pCPA) or parachloroamphetamine (pCA) on apomorphine-induced stereotypy. In the present study the effects of pCPA (250 or 400 mg/kg i.p., 3 days) and pCA (6.4 or 10.4 mg/kg i.p., 3 days) pretreatment on apomorphine-induced (5.0 mg/kg s.c., 5 min) behavior of male rats in the open-field were compared. For half of the trials in the 78 min session, the rats were alone and for half of the trials they were paired with an untreated male rat. pCA pretreatment increased the frequency of line-crossing and of jumping, whereas pCPA pretreatment increased the duration of bouts of locomotion and gnawing. These behavioral differences may be related to the interaction of pCA and pCPA with dopaminergic subsystems in the brain.

The comparative roles of dopaminergic and serotonergic mechanisms in mediating quipazine induced increases in locomotor activity

The effect of dopaminergic and serotonergic agonists and antagonists on quipazine induced locomotor activity was investigated in rats. Though quipazine is generally considered to be a relatively pure serotonergic agonist, its effects on locomotor activity were inhibited by small doses of a centrally acting DA receptor blocking agent (haloperidol), while three different serotonergic (5-HT) antagonists were without effect on this behavior. Moreover, quipazine induced locomotor activity was markedly inhibited by the 5-HT substrate 5-hydroxytryptophan. The data suggest that quipazine induced locomotor activity primarily involves dopaminergic mechanisms, with 5-HT playing either no role in stimulating this behavior, or a subsidiary one, requiring intact central dopaminergic receptors for its expression.

Supersensitivity to apomorphine in experimentally induced hypokinesia and drug-induced modifications of the apomorphine response

The development and degree of supersensitivity to the locomotor stimulant effect of apomorphine were studied in rats which had been rendered hypokinetic by bilateral injections of 6-hydroxydopamine into the anterolateral hypothalamus. Up to 2 days after surgery the effect of apomorphine was comparable in lesioned and normal rats, indicating that dopaminergic supersensitivity did not develop over this short period. As the duration between the 6-hydroxydopamine injections and time of testing with apomorphine increased, the animals became progressively more sensitive to the stimulant effects of apomorphine. Pretreatment with butaclamol reduced the effect of apomorphine in a dose-dependent manner. A high dose of clozapine also antagonized the effect of apomorphine, but a low dose potentiated it. No inhibition was observed following administration of the alpha-adrenergic antagonist, phenoxybenzamine, or the beta-adrenergic antagonist, propranolol. The 5HT antagonist methysergide and the anticholinergic drug, scopolamine potentiated the effects of apomorphine.l These studies suggest that the apomorphine-induced ambulation in hypokinetic rats is primarily mediated through dopaminergic mechanisms but both serotonergic and cholinergic mechanisms exert modulating influences.

Modulation of intracranial self-stimulation behaviour by local perfusions of dopamine, noradrenaline and serotonin within the caudate nucleus and nucleus accumbens

In order to examine the possible role of dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) in the control of intracranial self-stimulation behaviour (ICSS) a push-pull perfusion system was used to administer different consecutive doses of DA, NA and 5-HT to discrete regions within the caudate nucleus (CN) and nucleus accumbens (NAC) of rats during ICSS. Electrode placements supporting ICSS were in both the medial forebrain bundle (MFB) and the ventral mesencephalic tegmentum (VMT). Animals were allowed to determine the ICSS pulse train duration thereby permitting three measures of ICSS behaviour: (1) mean presses/min; (2) mean duration/press and (3) mean time pressed/min. Eleven electrode/cannula combinations were found to be responsive to both DA and 5-HT. The DA response profile was typified by a significant increase in mean presses/min, a significant decrease in mean duration/press and no significant change in the mean time pressed/min. The response profile for 5-HT was the converse of the DA pattern for the first two measures of ICSS, however, again there was no significant change in the mean time pressed/min. In addition, 8 electrode/cannula combinations were sensitive to NA; for 5 combinations the response pattern was similar to that of DA, however, the changes in ICSS were generally larger and of longer duration. For the remaining 3 combinations sensitive to NA there was no major change in mean presses/min but a dramatic increase in the mean duration/press and consequently, an increase in the mean time pressed/min. It is suggested that the central control of ICSS behaviour might depend, in part, upon a relative balance between DA and 5-HT systems within the CN and NAC. The operational characteristics of this balance may be subject to additional modulation by the activation of an adrenergic receptor.

Lysergic acid diethylamide: evidence for stimulation of pituitary dopamine receptors

Lysergic acid diethylamide (LSD), 0.05 mg/kg and 0.20 mg/kg, significantly decreased plasma prolactin (PRL) levels in male rats. LSD, 0.20 mg/kg, also inhibits the increase in plasma PRL levels produced by chlorpromazine (CPZ), 5 mg/kg, and alpha-methyl-paratyrosine (AMPT), 50 mg/kg, both of which interfere with dopaminergic inhibition of PRL secretion. LSD was more potent than methysergide, a serotonin receptor blocker, in lowering plasma PRL levels and more potent than apomorphine, a known direct acting dopamine agonist, in blocking the increase in plasma PRL produced by quipazine, a 5-HT agonist. These results suggest LSD has potent dopamine agonist properties on the rat pituitary or hypothalamic dopamine receptors which directly or indirectly inhibit PRL secretion.

Differentiaton of neuropharmacological actions of apomorphine and d-amphetamine

The dopaminergic agonists apomorphine and d-amphetamine elicit hyperthermic, hyperkinetic and stereotypic responses in the rabbit. The present investigation compares the influence exerted by various serotonergic antagonists upon these activities. Apomorphine-induced hyperthemia was antagonized by p-chlorophenylalanine (pCPA), cyproheptadine and cinanserin and was restored in pCPA-pretreated rabbits by regeneration of central serotonin levels, d-Amphetamine-induced hyperthermia was reduced by pCPA; restored in pCPA-pretreated animals by regeneration of central serotonin levels; and was uninfluenced by cyproheptadine and cinanserin. Apomorphine-induced locomotor stimulation was unaltered by serotonergic antagonists; however, these same doses of anti-serotonergic agents all markedly reduced d-amphetamine-induced hyperkinesia. Serotonergic antgaonists also failed to affect apomorphine-induced compulsive gnawing but did significantly enhance d-amphetamine-induced compulsive gnawing. It is concluded from these data that the neuropharmacological activities of apopmorphine and d-amphetamine in the rabbit differ in their dependence upon central serotonergic mechanisms.

Apomorphine in the rat nucleus accumbens: effects on the synthesis of 5-hydroxytryptamine and noradrenaline, the motor activity and the body temperature

Apomorphine (10 mug) was injected bilaterally into the nucleus accumbens or the neostriatum of rats. The application of apomorphine to the nucleus accumbens, but not to the neostriatum, enhanced the accumulation of 5-hydroxytryptophan in the central nervous system following inhibition of the aromatic L-amino acid decarboxylase. The effect was greatest in the brain stem particularly in the pons plus medulla oblongata. Similar but smaller rises were observed for the accumulation of DOPA in noradrenalinerich regions. The motor activity was increased and the body temperature was decreased by apomorphine in the nucleus accumbens, whereas smaller or no effects were obtained from the neostriatum.

Effects of drugs acting on cerebral 5-hydroxytryptamine mechanisms on dopamine-dependent turning behaviour in mice

1. The effects of drugs acting on cerebral 5-hydroxytryptaminergic mechanisms on drug-induced turning behaviour in mice with unilateral destruction of nigro-striatal dopaminergic nerve terminals have been studied. 2. Administration of L-tryptophan (400 mg/kg) or 5-hydroxytryptophan (200 mg/kg) increased brain 5-hydroxytryptamine and decreased the turning induced by both apomorphine (2 mg/kg) and amphetamine (5 mg/kg). 3. Parachlorophenylalanine (3 X 500 mg/kg) decreased brain 5-hydroxytryptamine and increased both apomorphine and amphetamine-induced circling behaviour. 4. Varying the protein content of dietary intake significantly altered brain 5-hydroxytryptamine and tryptophan levels, spontaneous locomotor activity and amphetamine-induced circling behaviour in these mice. 5. Systemic administration of methysergide (0.5-4 mg/kg), lysergic acid diethylamide (0.025-0.2 mg/kg), cyproheptadine (2.5-20 mg/kg) or clomipramine (0.6-20 mg/kg) produced no consistent effect on drug-induced turning behaviour. 6. The results suggest that circling behaviour due to striatal dopamine receptor stimulation is depressed by an elevation of brain 5-hydroxytryptamine and enhanced by a reduction in brain 5-hydroxytryptamine. 7. The possible physiological relationship between dopamine and 5-hydroxytryptamine neurones in the basal ganglia is discussed.

Effect of harmine and brain lesions on apomorphine induced motor activity

Application of harmine (10 mg/kg IP) 30 min before apomorphine decreased the motoric effects of the latter. Following harmine an increase in 5-HT and a decrease in 5-HIAA in different brain regions have been found. Injection of 5,6-DHT into nucleus medianus raphe 7 days before the experiment caused a significant increase of the apomorphine effect. Harmine pretreatment reduced this escessive motility as well as additional lesion of the substantia nigra with 6-OH-DA. Lesion induced by 6-OH-DA alone was without significant effect on the hypermotility following apomorphine. Application of PCPA 3 days before testing elicited an increase of apomorphine-induced hypermotolity which could be abolished by preceding harmine application. The experiments demonstrate the inhibitory effect of the central serotoninergic system on the apomorphine syndrome as well as the serotoninergic-dopaminergic interaction in hypermotility.

Influence of apomorphine on brain serotonin turnover rate

Apomorphine (5.0 mg/kg) accelerated the disappearance of 5-HIAA from the brain of pargylinepretreated rats as well as depletion of brain 5-HT caused by inhibition of its synthesis. The latter effect has been abolished by spiroperidol. The results obtained suggest that apomorphine increases the 5-HT turnover rate, secondary to the stimulation of central dopamine receptors.

Regional neurochemical studies on the effect of beta, beta'-iminodipropionitrile (IDPN) in the rat

A permanent hyperkinetic syndrome, characterized by excitation, choreiform head and neck movements and circling, which has led to it being called collectively the "ECC-syndrome," is induced in rats by the daily IP administration of beta, beta'-iminodipropionitrile (IDPN), 300 mg/kg, for 7 days. The levels of the biogenic amines, norepinephrine (NE), dopamine (DA), serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA), were measured in the striatum, midbrain, medulla, cortex, and cerebellum on the day the syndrome appeared (day 7) and one week later (day 14). The biogenic amine most affected by IDPN administration was 5-HT. On day 7, striatal 5-HT levels increased and 5-HIAA levels decreased while in the medulla and midbrain, 5-HIAA levels increased. On day 14, significant reductions in both 5-HT, in the midbrain, striatum, and cortex, and 5-HIAA, in all regions except the cortex, were observed. NE was markedly increased in the medulla, midbrain, and striatum on day 7, whereas on day 14 it was found to be within the normal range in these same regions. With the exception of a slight, but significant, increase in the cortex on day 7, DA levels in all regions were found to be relatively unaffected by IDPN administration on both day 7 and day 14. In an attempt to detect degenerative changes which might be taking place in the brain and which might provide an explanation for the permanency of the behavioral disturbances, the uptake of [3H]-labeled NE, DA, and 5-HT into synaptosomal-rich preparations of striatum and the uptake of NE and 5-HT into the midbrain area were compared between normal and syndromized rats on both day 7 and day 14. Small changes were observed but they were not statistically significant. The alterations of 5-HT and 5-HIAA levels in several regions of the brain under the conditions examined may indicate that IDPN's neurotoxicity primarily affects 5-HT-containing neurones. The active membrane transporting system of the nerve endings studied, however, remained relatively intact. This latter finding eliminates the possibility that neuronal degeneration in these areas is responsible for the decreased 5-HT and 5-HIAA levels or is the pathology underlying the permanency of the syndrome. These results are evaluated in terms of a possible model for hyperkinetic disorders.