Neurochemical evidence for denervation supersensitivity: the effect of unilateral substantia nigra lesions on apomorphine-induced increases in neostriatal acetylcholine levels

Evidence that antipsychotic drugs are inverse agonists at D2 dopamine receptors

1. The effects of a number of D2-like dopamine receptor antagonists have been determined on forskolin-stimulated cyclic AMP accumulation in Chinese hamster ovary (CHO) cells expressing the human D2short dopamine receptor (CHO-D2S cells). 2. Dopamine inhibited the effect of forskolin (as expected for a D2 receptor). However, all of the antagonists tested, apart from UH232 and (-)-butaclamol, were able to increase cyclic AMP accumulation above the forskolin control level. (+)-Butaclamol elicited a similar stimulation of forskolin-stimulated cyclic AMP accumulation in a CHO cell line expressing human D2long dopamine receptors whereas it exhibited no stimulating effect on forskolin-stimulated cyclic AMP accumulation in untransfected CHO-K1 cells. 3. There was a strong correlation between the EC50 values of these compounds for potentiation of cyclic AMP accumulation and their Ki values from radioligand binding experiments in CHO-D2S cells. 4. The effects of both (+)-butaclamol and dopamine in CHO-D2S cells were inhibited by pre-treatment with pertussis toxin indicating a role for Gi/Go proteins. 5. UH232 did not significantly affect forskolin-stimulated cyclic AMP accumulation but this substance was able to inhibit the effects of both dopamine and (+)-butaclamol in a concentration-dependent manner. Thus the effects of (+)-butaclamol on forskolin-stimulated cyclic AMP accumulation are mediated directly via the D2 receptor rather than by reversal of the effects of an endogenous agonist. 6. These data suggest that the D2 dopamine receptor antagonists tested here, many of which are used clinically as antipsychotic drugs, are in fact inverse agonists at human D2 dopamine receptors.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Receptor mechanisms mediating clozapine-induced c-fos expression in the forebrain

The atypical antipsychotic clozapine produces distinctly different regional patterns of c-fos expression in rat forebrain than does the prototypical neuroleptic haloperidol. While haloperidol-induced c-fos expression appears to be mediated by its D2 dopamine receptor antagonist properties, the mechanisms by which clozapine increases c-fos expression remain uncertain. Using a combination of brain lesion, pharmacological and immunohistochemical techniques, the present study sought to determine the receptor mechanisms by which clozapine increases the number of Fos-like immunoreactive neurons in various regions of the forebrain. To test whether serotonergic and/or noradrenergic systems are involved in clozapine-induced c-fos expression, rats received either 5,7-dihydroxytryptamine lesions of the medial forebrain bundle or 6-hydroxydopamine lesions of the dorsal noradrenergic bundle two weeks prior to clozapine (20 mg/kg) injections. Neither type of lesion affected clozapine-induced c-fos expression in the rat forebrain, suggesting that neither serotonergic nor noradrenergic mechanisms are involved in this action of clozapine. In another experiment, the 5-hydroxytryptamine2 receptor antagonist ritanserin (5 mg/kg), either alone or in combination with haloperidol (1 mg/kg), failed to mimic the pattern of c-fos expression produced by clozapine. This suggests that clozapine's antagonist actions at 5-hydroxytryptamine2 receptors cannot explain the unique pattern of regional c-fos expression produced by this compound. To determine whether the blockade of subtypes of the D2 dopamine receptor family may contribute to clozapine's effects, the dopamine receptor agonists quinpirole and 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) were injected 15 min prior to clozapine. Quinpirole produced a small but significant decrease in clozapine-induced c-fos expression in the medial prefrontal cortex, had larger effects in the lateral septum, and blocked clozapine's actions in the nucleus accumbens and major island of Calleja. Pretreatment with 7-OH-DPAT attenuated clozapine-induced c-fos expression in the nucleus accumbens and lateral septum, completely blocked the expression in the major island of Calleja, but was without effect in the medial prefrontal cortex. Given the different affinities of quinpirole and 7-OH-DPAT for D2, D3 and D4 receptors, these data suggest that clozapine-induced increases in c-fos expression in the nucleus accumbens, major island of Cajella and lateral septal nucleus are due to antagonist actions of this antipsychotic at D3 dopamine receptors. They also indicate that while antagonist actions at D4 receptors may contribute, the primary mechanisms by which clozapine increases c-fos expression in the medial prefrontal cortex remain to be determined.

Denervation-like postsynaptic supersensitivity to dopamine agonists induced by microinjection of colchicine into the substantia nigra pars compacta

Circling behavior induced by dopamine (DA) agonists following microinjection of colchicine or 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta (SNC) or electrolytic lesions of the SNC was investigated. Methamphetamine produced a contralateral circling behavior 3, 7 and 14 days following injection of colchicine into the SNC. Apomorphine produced an ipsilateral circling behavior followed by a contralateral rotation 3 and 7 days after the infusion of colchicine, whereas only an ipsilateral circling behavior was produced by apomorphine on day 14. 6-OHDA lesions of the SNC produced a contralateral circling behavior to apomorphine and an ipsilateral circling to methamphetamine, whereas both apomorphine and methamphetamine induced ipsilateral circling behaviors in rats with electrolytic lesions of the SNC. The possible mechanisms of action of colchicine are discussed in relation to the known effects of colchicine on axoplasmic transport.

Time course of the changes in striatal acetylcholine levels induced by pergolide and haloperidol after lesion of the nigrostriatal dopaminergic pathways in the rat

Lesion of the nigrostriatal dopaminergic pathway in the rat by 6-hydroxydopamine enhances the ability of pergolide to increase striatal acetylcholine levels and prevents the haloperidol-induced decrease in acetylcholine concentrations. This supersensitive response of striatal cholinergic cells is already maximal 6 days after lesion but tends to decrease thereafter. As the time course of the development of the supersensitivity of cholinergic cells differs from that of increased dopamine binding site density, the two are probably not causally related, the former reflecting rather a change occurring beyond the dopamine recognition site.

Quantitative analysis of [3H]spiroperidol binding to rat forebrain sections: plasticity of neostriatal dopamine receptors after nigrostriatal injury

The binding of [3H]spiroperidol to rat coronal sections in vitro was investigated using two procedures: swabbing studies, in which the tissue sections are wiped from the microscope slides after incubation in the presence of [3H]spiroperidol, and autoradiographic studies, in which the autoradiographic negatives are analyzed using computer-assisted densitometry. In the swabbing studies, the pharmacological and kinetic properties of butaclamol-displaceable binding were investigated, and the following results suggest that [3H]spiroperidol binds specifically to only a single site within the basal forebrain of tissue sections and that the site is the dopamine D-2 receptor. The pseudo-first order and first order plots for the rate of association to and dissociation from tissue sections appeared to be linear. Dopamine antagonists, such as haloperidol and butaclamol, were much more effective than dopamine agonists or the serotonin S-2 ligand, ketanserin, in inhibiting [3H]spiroperidol binding. The ability of dopamine agonists to inhibit [3H]spiroperidol binding was markedly reduced by the guanine nucleotide, Gpp(NH)p. Saturation analysis of specific [3H]spiroperidol binding revealed a Kd and Bmax of 0.93 nM and 447 fmol/mg protein, and a Hill coefficient of 1.05. The findings are also compatible with the possibility that [3H]spiroperidol binds to several sites that have identical affinities for this ligand. Densitometric studies were used to assess the effect of lesions on [3H]spiroperidol binding in the neostriatum. Intrastriatal injection of kainic acid substantially reduced 1 microM (+)--butaclamol-displaceable binding, indicating that the receptors are in large part on intrinsic striatal neurons. Neostriatal [3H]spiroperidol binding was investigated 7 days after destruction of the mesotelencephalic dopamine system by the ventral tegmental injection of 6-hydroxydopamine. As determined by saturation analysis, the average values for Kd and Bmax were 0.66 nM and 1212 fmol/mg protein in the intact striatum, and 0.82 nM and 1504 fmol/mg in the denervated striatum. The finding of a significant 23.8% increase in receptor density by the end of the first postoperative week, a period during which behavioral supersensitivity to apomorphine increases rapidly, supports the hypothesis that a proliferation of D-2 receptors underlies the behavioral manifestations of denervation supersensitivity.

Plasticity of neostriatal dopamine receptors after nigrostriatal injury: relationship to recovery of sensorimotor functions and behavioral supersensitivity

Rats given unilateral injections of 6-OHDA along the course of the mesotelencephalic dopaminergic projection show impairments in contralateral sensorimotor functions from which they often recover. Such rats also display an enhanced sensitivity to DA receptor stimulants, e.g. apomorphine, as revealed by contralateral turning, and an increased binding of neuroleptic compounds (e.g. [3H]spiroperidol) to the denervated striatum. This research examines the relationship of these receptor changes to both behavioral supersensitivity and recovery of sensorimotor functions by quantifying the time course of each phenomenon after injury. The supersensitivity to apomorphine and the behavioral recovery developed with a similar time course after injury, being evident within 1.5-3 days and reaching nearly maximal levels by 2 weeks postoperatively. A significant increase in in vivo [3H]spiroperidol binding to the denervated striatum occurred by 4 days postoperatively, and the magnitude of this change increased linearly during the first postoperative month. In contrast, the in vitro binding of this ligand to membranes of the denervated striatum was not increased until 3 weeks after the lesion. The results suggest that a proliferation of DA receptors may contribute to the pharmacological supersensitivity and the recovery of function, and that these early receptor changes may be revealed with greater sensitivity using in vivo binding techniques.

Plasticity of [14C]2-deoxy-D-glucose incorporation into neostriatum and related structures in response to dopamine neuron damage and apomorphine replacement

Unilateral injections of 6-hydroxydopamine into the lateral hypothalamus or ventral tegmental area that damage extensively the mesotelencephalic dopaminergic projection result in significant hemispheric asymmetries in autoradiographic labeling after [14C]2-deoxy-D-glucose (2DG) administration. The incorporation of this glucose analog into the ipsilateral neostriatum, nucleus accumbens septi, olfactory tubercle, and central nucleus of amygdala is decreased relative to the contralateral hemisphere. Concurrently, structures receiving projections from the neostriatum (globus pallidus, entopeduncular nucleus, and substantia nigra, pars reticulata) show enhanced labeling in the 6-hydroxydopamine-injected hemisphere, as do diencephalic structures receiving basal ganglia inputs (ventromedial nucleus and ventrolateral portion of the ventral anterolateral complex of thalamus, lateral portion of lateral habenular nucleus). Most of these 6-hydroxydopamine-induced asymmetries of [14C]2DG labelling are abolished or reversed by the injection of apomorphine i.p. (0.25 mg/kg) or directly into the denervated neostriatum (5 microgram) 7--20 days postoperatively. The metabolic effecgs of apomorphine administration are prevented by pretreatment with the dopamine receptor blocking agent, haloperidol (2mg/kg i.p., 2 h before). Thus, this autoradiographic technique can be used to demonstrate neostriatal output circuits that show altered metabolic activity in response to diminished or excessive forebrain dopamine receptor stimulation. The relevance of these findings to: (1) the behavioral abberations resulting from damage to mesotelencephalic dopaminergic systems; and (2) the 'denervation supersensitivity' to DA receptor stimulants is discussed.

Supersensitivity after intraventricular 6-hydroxydopamine: relation to dopamine depletion

6-Hydroxydopamine increased behavioral response to L-DOPA in proportion to the decrease of dopamine (DA) and DA utake in rat striatum. The increase response to apomorphine, however, only occurred after greater than 80% DA loss. Thus, 6-hydroxydopamine may induce postsynaptic changes only following large lesions.

Differential attenuation by atropine and d-amphetamine on hyperactivity: possible clinical implications

Intraperitoneal administration of physostigmine (0.025 to 0.18 mg/kg) to rats resulted in significant increases in motor activity as measured with jiggle platforms. Doses of physostigmine 0.2 mg/kg or more decreased motor activity. The physostigmine-induced hyperactivity was attenuated by atropine (5 mg/kg) given before or after physostigmine (0.05 mg/kg). On the contrary, d-amphetamine (2 mg/kg), given before or after, significantly potentiated the physostigmine-induced increase in motor activity. The relevance of the cholinergic system in mediating hyperactive behaviour in children is discussed.

Supersensitivity to apomorphine following destruction of the ascending dopamine neurons: quantification using the rotational model

A new surgical preparation is described with which it is possible to quantify the degree of supersensitivity to dopamine receptor-stimulating agents using the rotational model. One group of rats received a unilateral injection of 6-hydroxydopamine which destroys the dopamine-containing neurons in one hemisphere, followed by a diencephalic electrocoagulation which interrupts both afferents and efferents of the striato-pallidal complex in the opposite hemisphere. Another series of animals received only the unilateral electrocoagulation. When given appomorphine both groups of animals rotated toward the side of the electrocoagulation. However, the 6-hdroxydopamine-treated animals were 10-40 times more sensitive to the behavioral effect of the drug. These results contrast with previous reports in which behavioral sensitivity to apomorphine was increased 2- to 7-fold following partial degeneration of central dopamine neurons of following the chronic adminstration of dopamine synthesis inhibitors of receptor blocking agents. The extent of loss of dopamine appears to be a critical factor in determining the degree of supersensitivity which will develop.

Retrograde amnesia after electrical stimulation of the substantia nigra: mediation by the dopaminergic nigroneostriatal bundle

Stimulation of the zona compacta of the substantia nigra (SNC) has been reported to impair the long-term retention of a passive avoidance step-down task. In the present experiments it was observed that previous ipsilateral lesions of the dopaminergic nigro-neostriatal bundle (NSB) with 6-hydroxydopamine blocked the stimulation-induced memory deficit. Unilateral or bilateral lesions of the dopaminergic NSB did not by themselves disrupt acquisition or retention of the passive avoidance task. In addition, the memory deficit observed after electrical stimulation of the caudate-putamen was not reduced by ipsilateral NSB lesions. These results indicate that the retention deficit produced by stimulation of the SNC was mediated by the dopaminergic NSB. However, in view of the fact that lesions of this projection did not affect acquisition or retention of the step-down task, it is suggested that this system is not critically involved in learning and memory. Rather, it is hypothesized that stimulation of the SNC results in an excess release of dopamine in the caudateputamen which interferes with neurochemical and neurophysiological events in this structure which are important substrates for long-term memory.

Influence of 6-hydroxydopamine on the behavioral effects induced by apomorphine or clonidine in rats

The aim of this paper is to examine if central chemical sympathectomy induced by two injections of 6-hydroxydopamine (6-OHDA) in a dose of 250 mug intracerebroventricularly (i.c.v.) affects behavioral phenomena elicited by apomorphine (AP) (1 or 1.2 mg/kg i.p.) or clonidine (CL) (0.1 MG/KG, 5 Or 1 mug/kg i.p.). Experiments were carried out on male Wistar rats. The time of duration of several components of behavior and the degree of irritability of rats were measured. Moreover, open field and hole test were performed. The lower dose of AP did not affected behavior of rats. The higher dose increased the locomotor and exploratory activity of animals. 6-OHDA potentiated these effects of AP. CL (0.1 mg/kg) had a depressive effect on the rats' behavior, which was potentiated by 6-OHDA. CL (5 mug/kg) had no effect on the rats' behavior, but in a dose of 1 mug/kg caused excitatory behavior. This type of behavior was abolished by 6-OHDA. In conclusion, central chemical sympathectomy caused increased sensitivity of the central nervous system on AP. Excitatory behavioral effects of CL in low dosage may be connected with stimulation of central adrenergic receptors. Depressive behavioral effect of CL in high dosage is unspecific. Central chemical sympathectomy affects by different methods the reactivity of dopaminergic and noradrenergic neurons.

Postnatal development of acetylcholinesterase in the caudate-putamen nucleus and substantia nigra of rats

The postnatal development of acetylcholinesterase (AChE, EC 3.1.1.7) and NADH-diaphorase was examined in the caudate-putamen nucleus and substantia nigra of rats ranging from 3 to 90 days in age. From 3 to 15 days post partum islands of AChE and NADH-diaphorase activity were observed in the caudate-putamen nucleus. Individual neuronal somata could also be seen in AChE-stained sections up to 15 days. At later ages neuropil staining became increasingly dense, and this presumably accounted for the infrequent visualization of cell bodies in the brains of older animals. During development AChE appeared in the caudate-putamen nucleus in a lateral to medial topographic order; analogously, enzyme staining in the neostriatum reappeared in the same lateral to medial topographic order in adult rats following irreversible AChE inhibition by intramuscularly injected bis-(1-methylethyl)phosphorofluoridate (di-isopropylfluorophosphate: DFP). Furthermore, DFP treatment in mature animals revealed the presence of AChE in striatal neurons having morphologies similar to those observed in newborn rats. A similar time-course of postnatal AChE development was observed in the substantia nigra. In both the pars compacta and pars reticulata individual cell bodies, which were visible at early ages (3-10 days), became increasingly obscured at later times after birth by extra-somata staining. Between the 6th and 15th postnatal days AChE-containing fibers were seen projecting apparently from pars compacta into pars reticulata. Comparison of the present results with histochemical data of other investigators on the postnatal development of monoamines indicated the likelihood of cholinergicmonoaminergic interactions in the neostriatum and substantia nigra.

Dopamine acetylcholine imbalance in Parkinson's disease. Possible regenerative overgrowth of cholinergic axon terminals

Parkinson's disease is characterised by an imbalance between acetylcholine and dopamine which probably results from the degeneration of a dopaminergic nigrostriatal pathway. A new hypothesis is proposed to explain the development of this imbalance. Applying the concept that degeneration of nerve-fibres in the central nervous system can lead to collateral sprouting of uninjured fibres, it is suggested that the death of dopaminergic nigrostriatal neurons results in sprouting of axons of cholinergic interneurons in the caudate nucleus. This overgrowth could result in the cholinergic innervation of neuronal membranes vacated by degenerated dopaminergic terminals. Thus, the apparent changes in the activity of dopaminergic and cholinergic systems can be accounted for by faulty regeneration in the central nervous system.

Acetylcholinesterase-containing neurons in the neostriatum and substantia nigra revealed after punctate intracerebral injection of di-isopropylfluorophosphate

Infusion of 1 mul arachis oil containing 1.5 mug bis-(1 -methylethyl)phosphorofluoridate (di-isopropylfluorophosphate: DFP) into the caudate--putamen nucleus and substantia nigra of rats produced a considerable reduction of histochemical staining for acetylcholinesterase (AChE) in these two brain regions 30--120 min after injection. Thereafter, regeneration of AChE occurred within the zone of DFP effect. These new stores of AChE were associated with discrete neuronal perikarya and their processes. Intracerebral DFP administration had little or no histochemically detectable effect on NADH-diaphorase. Thionin staining was similarly unaffected. The results with punctate intracerebral application of DFP were replicated by intramuscular injection of 1.5 mg/kg DFP. Although the significance of dopaminergic--cholinergic interactions in the neostriatum could not be elucidated on the basis of these histochemical data, the thesis was advanced that dopamine neurons in the pars compacta of the substantia nigra also contained AChE, possibly to inactivate acetylcholine released from cholinergic fibers afferent to this neural structure.

Supersensitivity of central neurons--a brief review of an emerging concept

The concept that "denervation" or "pharmacological disuse" supersensitivity develops in central neuronal systems subsequent to sustained attenuation of normal neurohumoral mechanisms is reviewed. Particular emphasis is placed on biochemical and electrophysiological parameters of supersensitivity in dopaminergic (striatal) neuronal systems. The possible applicability of theories invoking changes in receptor sensitivity to the phenomenon of narcotic tolerance and physical dependence and to psychoactive drug therapy is discussed.

Acetylcholinesterase neurons in dopamine-containing regions of the brain

A pharmaco-histochemical regimen was used to examine the morphology and internal organization of acetylcholinesterase (AChE, EC 3.1.1.7) neurons in brain areas--the caudate-putamen nucleus, nucleus accumbens, olfactory tubercule, and subtantia nigra--monoaminergically characterized in terms of their dopamine content. Intense, homogenous staining is produced in these neural regions by other histochemical protocols for AChE; individual AChE-containing neurons cannot be observed reliably or consistently. With the present technique, based on the differential regeneration of AChE in the separate subcellular compartments of the neuron (i.e., axon, dendrite, soma) after intramuscular injection of bis-(1-methylethyl)-phosphorofluoridate (di-isopropylfluorophosphate: DFP), it was shown that AChE was associated with neurons whose cell bodies lay within the brain areas studied. Although the significance of dopaminergic-cholinergic relationships in the caudate-putamen complex, nucleus accumbens, and olfactory tubercule could not be established on the basis of these new histochemical data, arguments were presented indicating that dopamine neurons in the zona compacta of the substantia nigra also contained AChE.

Supersensitivity of caudate neurones after repeated administration of haloperidol

The microiontophoretic potencies of dopamine, apomorphine and alpha-aminobutyric acid were examined on caudate neurones, using the same electrodes, in control animals and animals that had received 2mg/kg/day of haloperidol for 9 days. In the haloperidol group, a greater percentage of cells were inhibited by dopamine and apomorphine, indicating the development of a post-synaptic supersensitivity.