Role of globus pallidus GABA and opiate receptors in apomorphine circling in nigro-striatal lesioned rats

Summary The effect of lesioning the ipsilateral globus pallidus (GP) on apomorphine-induced circling in nigrostriatal lesioned rats was investigated. A GP electrolesion almost abolished circling. Drugs that affect GABA and opiate receptors were injected in GP through a cannula. Circling was antagonized by the GABA antagonists picrotoxin and bicuculline, the GABA agonist muscimol and by baclofen. Opiate receptor agonists including morohine, levorphanol, [d-Ala², d-Leu⁵]-enkephalin and -endorphin had no effect on circling in GP. Ethylketazocine caused a pronounced, naloxone-reversible slowing of apomorphine circling. Apomorphine-induced circling behaviour may be modulated by GP GABA receptors and kappa-type opiate receptors.     

Dopamine agonists increase pallidal unit activity: Attenuation by agonist pretreatment and anesthesia

Intravenous administration of a single bolus dose of 1 μ mol/kg of the dopamine agonists, pergolide and lisuride, caused marked increases in the unit activity of globus pallidus neurons in awake, paralyzed, locally anesthesized and artificially respired rats. These agonist effects were similar to those observed after administration of 1 μ mol/kg apomorphine to awake, paralyzed rats; in rats anesthetized with chloral hydrate, however, responses to apomorphine were markedly attenuated. Subsequent administration of haloperidol reversed the effects of pergolide and pretreatment with haloperidol blocked the effects of lisuride. LSD (1 μ mol/kg i.v.) did not effectively stimulate pallidal neuronal activity, suggesting that the ability to stimulate pallidal firing rates correlates better with dopamine, as opposed to serotonin, agonist potency. The ability of a non-excitatory dose of apomorphine to attenuate responses of pallidal neurons to a normally excitatory 1 μ mol/kg dose of this agonist administered subsequently, was reconfirmed. Pretreatment with this ‘priming’ dose of apomorphine also attenuated the rate increases produced by d-amphetamine (8.7 μ mol/kg) and enhanced the rate inhibitory effects of haloperidol. The ‘priming’ effect appears related to the dopamimetic effects of apomorphine; a non-excitatory dose of a second dopamine agonist, lisuride (0.07 μ mol/kg i.v.), similarly blocked the effect of excitatory doses of lisuride (1 μ mol/kg i.v.) on pallidal activity     

A biphasic influence of globus pallidus lesions: spontaneous catalepsy followed by anticataleptic effect

The behavioural and histological effects of unilateral or bilateral lesions induced by kainic acid injections into the globus pallidus were investigated in rats. Both lesions provoked a behavioural syndrome similar to those seen in animals treated systemically with neuroleptics or opiates. Animals displayed akinesia, ptosis, catalepsy, hypothermia and muscular rigidity. Also a marked hypersensitivity to touch, and a sensory neglect to touch and pain limited to hindlegs, adipsia, aphagia and high mortality of lesioned rats were observed. These symptoms were much stronger and lasted longer (catalepsy lasted over 15 days) in bilaterally lesioned animals. Subcutaneous injections of apomorphine in bilaterally lesioned rats abolished akinesia and catalepsy while rigidity and ptosis were unaffected. In unilaterally lesioned rats in which the lesion-induced spontaneous catalepsy already disappeared the spiperone-induced catalepsy was suppressed while in bilaterally lesioned animals which showed still pronounced lesion-induced catalepsy the spiperone-induced catalepsy was unchanged when compared to the sham-operated rats. Our results and the literature data suggest that the lesions of the globus pallidus produce biphasic effects: spontaneous catalepsy and unchanged neuroleptic catalepsy in the first phase and suppression of the neuroleptic catalepsy in the second phase. The role of the globus pallidus as a distal link (for neostriatum and n. accumbens) in neuronal chain forming a matrix of central patterns of catalepsy, akinesia and rigidity is discussed.     

Effets de lésions de l'hypothalamus et du globus pallidus et d'injections d'apomorphine dans le globus pallidus,le noyau caudé, la substantia nigra et le septum sur le 〈comportement d'agressivité induit par l'apomorphine〉 chez le rat

Intraspecific apomorphine-induced aggressive behavior in the rat was not affected following electrolytic lesions of the ventromedial hypothalamus. Some inhibition of the aggressive behavior was found after lateral lesions and an almost total suppression after destruction of globus pallidus.These results, as well as those following localized injections of apomorphine into the septum, substantia nigra, caudate nucleus, and globus pallidus suggested that the latter anatomical region may be the major site of the action of apomorphine in the behavior studied. The role of acetylcholine is discussed.

     

Repeated heroin administration increases extracellular opioid peptide-like immunoreactivity in the globus pallidus/ventral pallidum of freely moving rats

Microdialysis was used to investigate the effects of heroin administration on extracellular opioid peptide levels in the globus pallidus/ventral pallidum of freely moving rats. Two injections of heroin (0.6 mg/ kg IP) were given 3 h apart. The first injection had no significant effect on opioid peptide levels, but the second injection produced a transient yet significant increase (+268%) in opioid peptide-like immunoreactivity in pallidal dialysates, peaking 1 h after injection. This effect was blocked by administration of naloxone (3 mg/kg IP) prior to the second injection. The implications of these data are discussed with regards to the role of the endogenous opioid peptide system in opiate reward. Received: 6 October 1997/Final version: 2 March 1998     

Intravenous GABA agonist administration stimulates firing of A10 dopaminergic neurons

Increasing intravenous doses of two γ-aminobutyric acid (GABA) agonists, muscimol and 4,5,6,7-tetrahydro-isoxazolo-[5,4-c]pyridin-3-ol (THIP), caused dose-related increases in extracellular, single unit activity of A₁₀ dopaminergic neurons of the ventral tegmental area. Muscimol, the more potent of the two compounds, stimulated firing at doses approximately 15–20 times lower than the doses of THIP required to elicit equivalent excitatory effects. The maximum stimulation achieved was approximately 150 and 140% of the baseline firing rate for muscimol and THIP, respectively.     

Cataleptic and anticataleptic effects of muscimol and gabaculine injected into globus pallidus and substantia nigra,and interactions with haloperidol or benzodiazepines

Summary Intranigral injection of muscimol induced hyperactivity in rats and antagonized haloperidol-induced catalepsy. Intranigral injection of gabaculine, an inhibitor of GABA transaminase, induced similar effects 5 h after injection, when the nigral GABA content was increased 7-fold. On the other hand, injections of muscimol (30 ng) into the globus pallidus potentiated the cataleptic effect of haloperidol, and muscimol alone in high doses (100 and 200 ng) induced catalepsy. Gabaculine also induced catalepsy of medium intensity and potentiated the effect of haloperidol 24 h after injection, when GABA was increased in the globus pallidus as well as in the substantia nigra. Injections of muscimol into either the globus pallidus or substantia nigra increased striatal HVA and enhanced haloperidol-induced elevation of HVA.Three benzodiazepines, nitrazepam, diazepam and chlordiazepoxide administered orally, potentiated the effect of muscimol (30 ng) injected into the globus pallidus and induced catalepsy. A similar effect was not obtained with phenobarbital.It is suggested that stimulation of GABA receptor or increase of GABA content in the sustantia nigra antagonize haloperidol-induced catalepsy by activation of nigral dopaminergic system, and that enhancement of pallidal GABA function induces catalepsy by non-dopaminergic mechanisms. Potentiation of haloperidol-induced catalepsy by benzodiazepines may be due to enhancement of GABA-ergic transmission within the globus pallidus.     

Oro-facial dyskinesia and the sub-commissural part of the globus pallidus in the cat: role of acetylcholine and its interaction with GABA

The possible role of cholinergic mechanisms in the sub-commissural part of the globus pallidus (scGP) in the induction of oro-facial dyskinesia (OFD) was studied in cats. Local injections of the cholinergic agonist carbachol into the scGP elicited tongue protrusions in a dose dependent way (100–1000 ng/0.5 l). The effect elicited by 1000 ng carbachol was selectively antagonized by the cholinergic antagonist scopolamine (10 g/0.5 l); this dose of scopolamine was ineffective when injected alone. The tongue protrusions resulted from both normal and abnormal movements: whereas normal movements simply consisted of protruding the flat tongue, abnormal movements implied a variety of movements, especially curling upwards the lateral side(s) or tip of the tongue inside or outside the oral cavity. The abnormal carbachol-induced tongue protrusions formed part of a syndrome marked by dyskinetic movements of the muscles of the eye, ear and cheek, and were identical to those seen previously after local injections of picrotoxin (250–500 ng). Intra-pallidal injections of the abovementioned dose of scopolamine had no effect on the tongue protrusions induced by local injections of 375 ng picrotoxin. However, local injections of 100 ng muscimol, which was previously found to attenuate significantly the effect of 375 ng picrotoxin and which was ineffective when injected alone, significantly attenuated the tongue protrusions induced by local injections of 1000 ng carbachol. These data suggest that the cholinergic effects are mediated via a GABAergic mechanism, but not vice versa. The results are discussed in view of GABAergic and anticholinergic therapies used in oro-facial dyskinesia.     

Spontaneous unit activity in the globus pallidus following cumulative injections of morphine in phenobarbital- or chloral hydrate-anesthetized rats

The effect of systemically administered morphine on spontaneous unit activity, recorded from the globus pallidus, was evaluated in this study. A wide range of doses of morphine was administered in order to characterize the influences of morphine on pallidal activity in a dose-related manner. Two separate studies were conducted. In the first study, a semichronic preparation, lightly anesthetized with phenobarbitol, was used. Acutely prepared, chloral hydrate-anesthetized rats were utilized in the second experiment.With phenobarbital, morphine caused a significant reduction in pallidal activity in 75% of the cells recorded, whereas only 45% of the pallidal cells responded similarly to injection of morphine in chloral hydrate-anesthetized animals. However, the dose of morphine required to decrease unit activity was substantially less with chloral hydrate than with phenobarbital anesthesia.     

Individual and morphological differences in the behavioural response to apomorphine in rats

The topography of stereotyped behaviour produced by apomorphine in rats was studied by using either a scoring system, based on observation in a wire cage, or by quantification of horizontal and vertical activities, and of the total distances run in an open field, using an automatic recording system. The latter design was combined with a classification of the type of stereotyped behaviour observed during recording. In addition, the reproducibility of the nature of the stereotyped behaviour and its dose-dependence in individual animals was evaluated. In rats observed in a wire cage, apomorphine at lower doses (0.25 or 0.50 mg/kg SC) produced stereotyped sniffing. Increasing the doses led to stereotyped licking and the largest dose (5.00 mg/kg SC) produced predominantly stereotyped gnawing, as was demonstrated graphically. The type of behaviour produced by 2 mg/kg apomorphine in the open field was reproduced well in individuals after a second administration 4 days later. The shift from sniffing to gnawing was observed in most, but not all of the individually classified animals after administration of the largest dose (5 mg/kg). The locomotor part of motility was highest in sniffing animals and lower when gnawing occurred. The non-locomotor part of motility was low in sniffing rats and increased when licking and gnawing occurred. In some of the animals a characteristic climbing behaviour was observed in addition after the larger doses, which did not interfere with sniffing, licking or gnawing.A combination of classification by observation and automatic recording seems the most appropriate way to study the topography of stereotyped behaviour produced by apomorphine.     

Adenosine A2A receptor stimulation decreases GAT-1-mediated GABA uptake in the globus pallidus of the rat

We examined modulation of [(3)H]GABA uptake in slices of the rat globus pallidus because stimulation of adenosine A(2A) receptors increases extracellular GABA in this structure. Pharmacological analysis showed that GAT-1 is the main transporter present in these slices. Both adenosine and the A(2A) agonist CGS 21680 reduced GABA uptake. Antagonist ZM 241385 prevented these effects. Agents that increase protein kinase A activity like forskolin and 8-bromo-cAMP also inhibited GABA uptake. The inhibition of uptake produced by these substances and by CGS 21680 was prevented by the protein kinase A blocker H-89. The protein phosphatase blocker okadaic acid reduced uptake; this effect and the response to CGS 21680 were not additive. The effective concentrations of adenosine (EC(50)=15.2microM) are within the range measured in the interstitial fluid under some physiological conditions. Thus, inhibition of uptake may be important in increasing interstitial GABA during endogenous adenosine release.     

Discriminative stimulus effects of a low dose of apomorphine in the rat

The discriminative stimulus (DS) effect of apomorphine was investigated in rats trained in a two-lever, food-reinforcement procedure. Rats were given subcutaneous injections of saline or 0.1 mg/kg apomorphine HCl, 15 min before training sessions. The training dose of apomorphine was chosen to activate dopamine autoreceptors selectively. Stimulus generalization studies demonstrated that the DS effects generalized completely to other directacting dopaminergic agonists such as N-n-propylnorapomorphine (NPNA), pergolide, lergotrile, and bromocriptine. The indirect-acting dopamine agonists, (+)amphetamine, cocaine, and methylphenidate produced predominantly saline-appropriate lever responses. The DS effect of apomorphine at the training dose was incompletely antagonized by haloperidol or metoclopramide. The dopaminergic antagonists tested, however, also partially generalized to apomorphine. Both enantiomers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) produced apomorphine-appropriate lever choice with the (-) enantiomer being slightly more potent. The discriminative property of this (0.1 mg/kg) dose of apomorphine has characteristics consistent with selective dopamine autoreceptor activation.     

Behavioral effects of apomorphine and diisobutyrylapomorphine in the mouse

Stereotyped climbing and clinging responses of the mouse to apomorphine or its ester prodrug, O,O-diisobutyrylapomorphine were evaluated. Acute doses of the ester up to 0.3 mmoles/kg were tolerated without apparent ill effects. Both aporphines produced dose-dependent behavioral responses that were blocked by neuroleptics. The duration of action of the ester was much greater than that of apomorphine. When maximal initial behavior during the first hour was evaluated, low equimolar doses of apomorphine and the ester were similar in potency; in contrast, the total behavioral response to larger doses of the ester was greater than to apomorphine, evidently reflecting the greater duration of action of the ester. Behavioral responses to both agents during the first hour decreased at doses above 0.1 mmoles/kg. Oxidized or O-methylated apomorphine did not antagonize the behavioral effects of apomorphine. Systemic injection of apomorphine or diisobutyrylapomorphine led to detectable levels of free apomorphine as estimated by a sensitive and selective fluorimetric assay. The time-course and magnitude of the behavioral effects of both agents corresponded closely with brain levels of apomorphine. Apomorphine and dopamine (but not diisobutyrylapomorphine) stimulated adenylate cyclase activity in mouse striatal homogenates—an effect antagonized by neuroleptic drugs but not propranolol. Apomorphine exerted a biphasic effect on the cyclase in vitro and increased cyclic AMP levels in the striatum in vivo. The prolonged activity of apomorphine esters as depot prodrug agonists of putative dopaminergic mechanisms in the brain may provide clinically desirable characteristics.     

The effects of intrathecal administration of the dopamine agonist apomorphine on penile reflexes and copulation in the male rat

Relatively high doses of systemically administered apomorphine inhibit penile reflexes. It is possible that these inhibitory effects are due, at least in part, to actions of apomorphine on the lumbosacral spinal cord. The present experiments examined this possibility by injecting apomorphine (10 and 50 g/5.0 l vehicle) into the lumbosacral subarachnoid space through chronic, indwelling cannulae. Such injections impaired ex copula penile reflexes, slowed the rate of copulation, and decreased the number of intromissions preceding ejaculation. These results suggest that lumbosacral cord dopamine receptors may normally regulate male sexual performance.     

The effect of amitriptyline treatment on the growth hormone response to apomorphine

The growth hormone response to subcutaneous administration of the dopamine agonist apomorphine (0.005 mg/kg) was assessed in six normal male subjects before and at the end of a course of amitriptyline. Amitriptyline treatment significantly reduced the growth hormone response to apomorphine, confirming the findings of an ealier study in depressed patients. The way in which amitriptyline attentuates the effect of apomorphine is not clear. Direct blockade of dopamine receptors in the hypothalamus is a possibility, but long-term amitriptyline treatment could result in adaptive changes in the monoamine pathways which control GH release.     

Behavioral and neurochemical effects of apomorphine in the cat

Administration of apomorphine (2–10 mg/kg i.p.) elicited a number of behaviors, such as limb flicking, abortive grooming, investigatory and hallucinatory-like responses, head and body shakes, and excessive grooming, which we have previously proposed as an animal model for studying the actions of LSD and related hallucinogens. Repeated administration of apomorphine resulted in a significant tolerance, which occurred within 2h of the initial injection, and completely dissipated within 24 h. A pronounced LSD-apomorphine cross tolerance was observed; however there was no significant apomorphine-LSD cross tolerance. Apomorphine-induced behavioral changes were blocked by prior treatment with haloperidol, but were unchanged by pretreatment with L-DOPA. Administration of L-DOPA, in combination with a peripheral decarboxylase inhibitor, did not elicit these characteristic behavioral changes. Increasing synaptic serotonin levels by monoamine oxidase inhibition, precursor administration, or reuptake blockade in general did not alter the behavioral response to apomorphine. Similarly, pretreatment with serotonin receptor blockers produced no large changes in apomorphine-induced behaviors. Prior serotonin depletion with chronic p-chlorophenylalanine administration, however, potentiated certain apomorphine-induced behaviors. Neurochemical studies revealed that apomorphine administration increased striatal dopamine, and decreased dopamine metabolites. Norepinephrine levels were generally decreased throughout the CNS by apomorphine treatment. Administration of apomorphine increased CNS serotonin and 5-hydroxyindoleactic acid levels, while tryptophan levels were unchanged. The biological bases of the limb flick model is discussed in the context of these pharmacological and neurochemical studies.     

Opposing effects of apomorphine on pain in rats. Evaluation of the dose-response curve

The effect of apomorphine on a supraspinally mediated response to pain was studied after subcutaneous administration of 10 different doses (25 μg/kg up to 10 mg/kg). Depending on the dose given, apomorphine was found to induce opposing effects on pain, so that low doses, 25–100 μ/kg, dose-dependently increased the sensitivity to pain. This effect then gradually declined in potency with increasing doses and high doses induced antinociception. The data therefore suggest that the net effect recorded involves the sum of responses from at least two functional systems. Using the Hill equation and the digital computer program NONLIN, we have dissociated the observed effect into two components, each having its particular dose-response characteristics: low doses having an ED₅₀ value of 36 μg/kg produced hyperreactivity to pain, and high doses having an ED₅₀ of 465 μg/kg (in the absence of hyperalgesia) induced antinociception.     

An electrophysiological analysis of the actions of the 3-PPP enantiomers on the nigrostriatal dopamine system

Summary Extracellular single unit recording and micro-iontophoretic studies were carried out in chloral hydrate-anesthetized and gallamine-paralyzed rats to investigate the actions of the enantiomers of the dopamine (DA) analogue 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP, on the nigrostriatal DA system. Intravenously administered (+)-or (–)-3-PPP consistently inhibited nigral DA neuronal activity; these actions were readily antagonized by haloperidol but were not affected by a pretreatment of reserpine plus alpha-methyltyrosine. In contrast to (+)-3-PPP, the (–)-enantiomer produced only partial inhibition of the majority of cells studied and was also capable of partially reversing the inhibitory action of apomorphine. A prior hemitransection of the brain did not alter the inhibitory action of either enantiomer. Whereas iontophoretically ejected (+)-3-PPP consistently reduced DA cell firing rate, similarly applied (–)-3-PPP reduced the activity of only some DA cells, while the majority were not influenced. In addition, iontophoresis of (–)-3-PPP could reduce the inhibitory effect of similarly applied DA or (+)-3-PPP. The (+)-enantiomer reduced caudate neuronal activity both after intravenous administration and iontophoresis. Intravenously administered (–)-3-PPP failed to influence or increased the activity of these neurons and reversed the inhibitory action of apomorphine. However, iontophoretically ejected drug reduced caudate cell activity and did not influence the inhibitory action of DA. The activity of non-DA zona reticulata neurons was inconsistently influenced by the 3-PPP enantiomers. It is concluded that (+)-3-PPP is a directly acting DA agonist, stimulating both DA autoreceptors and postsynaptic DA receptors. In contrast, (–)-3-PPP appears to be a partial agonist at nigral DA autoreceptors, whereas the action of the drug at putative postsynaptic DA receptors in the caudate remains to clarified.