Neonatal dopamine lesion in the rat results in enhanced adenylate cyclase activity without altering dopamine receptor binding or dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein (DARPP-32) immunoreactivity

Perinatal 6-Hydroxydopamine to Produce a Lifelong Model of Severe Parkinson's Disease

The classic rodent model of Parkinson's disease (PD) is produced by unilateral lesioning of pars compacta substantia nigra (SNpc) in adult rats, producing unilateral motor deficits which can be assessed by dopamine (DA) D₂ receptor (D₂-R) agonist induction of measurable unilateral rotations. Bilateral SNpc lesions in adult rats produce life-threatening aphagia, adipsia, and severe motor disability resembling paralysis-a PD model that is so compromised that it is seldom used. Described in this paper is a PD rodent model in which there is bilateral 99 % loss of striatal dopaminergic innervation, produced by bilateral intracerebroventricular or intracisternal 6-hydroxydopamine (6-OHDA) administration to perinatal rats. This procedure produces no lethality and does not shorten the life span, while rat pups continue to suckle through the pre-weaning period; and eat without impairment post-weaning. There is no obvious motor deficit during or after weaning, except with special testing, so that parkinsonian rats are indistinguishable from control and thus allow for behavioral assessments to be conducted in a blinded manner. L-DOPA (L-3,4-dihydroxyphenylalanine) treatment increases DA content in striatal tissue, also evokes a rise in extraneuronal (i.e., in vivo microdialysate) DA, and is able to evoke dyskinesias. D₂-R agonists produce effects similar to those of L-DOPA. In addition, effects of both D₁- and D₂-R agonist effects on overt or latent receptor supersensitization are amenable to study. Elevated basal levels of reactive oxygen species (ROS), namely hydroxyl radical, occurring in dopaminergic denervated striatum are suppressed by L-DOPA treatment. Striatal serotoninergic hyperinnervation ensuing after perinatal dopaminergic denervation does not appear to interfere with assessments of the dopaminergic system by L-DOPA or D₁- or D₂-R agonist challenge. Partial lesioning of serotonin fibers with a selective neurotoxin either at birth or in adulthood is able to eliminate serotoninergic hyperinnervation and restore the normal level of serotoninergic innervation. Of all the animal models of PD, that produced by perinatal 6-OHDA lesioning provides the most pronounced destruction of nigrostriatal neurons, thus representing a model of severe PD, as the neurochemical outcome resembles the status of severe PD in humans but without obvious motor deficits.

Perinatal 6-Hydroxydopamine Modeling of ADHD

The neonatally 6-hydroxydopamine (n6-OHDA)-lesioned rat has been the standard for 40 years, as an animal model of attention-deficit hyperactivity disorder (ADHD). Rats so lesioned during postnatal ontogeny are characterized by ~99 % destruction of dopaminergic nerves in pars compacta substantia nigra, with comparable destruction of the nigrostriatal tract and lifelong ~99 % dopaminergic denervation of striatum, with lesser destructive effect on the ventral tegmental nucleus and associated lesser dopaminergic denervation of nucleus accumbens and prefrontal cortex. As a consequence of striatal dopaminergic denervation, reactive serotoninergic hyperinnervation of striatum ensues. The striatal extraneuronal milieu of DA and serotonin is markedly altered. Also, a variety of sensitization changes occur for dopaminergic D₁ and D₂ receptors, and for serotoninergic receptors. Behaviorally, these rats in adulthood display spontaneous hyperlocomotor activity, attentional deficits, and cognitive impairment-all of which are acutely attenuated by the psychostimulants amphetamine (AMPH) and methylphenidate (MPH) (i.e., opposite to the acute effects of AMPH and MPH in intact control rats). The acute behavioral effects of AMPH and MPH in intact and lesioned rats are analogous to their respective acute effects in non-ADHD and in ADHD humans. The neurochemical template of brain, and behavioral series of changes in n6-OHDA-lesioned rats, is described in the review. Despite the fact that nigrostriatal damage is not an underlying pathophysiological process of human ADHD (i.e., lacking construct validity), the described animal model has face validity (behavioral profile) and predictive validity (mirror of ADHD/MPH effects, as well as putative and new ADHD treatment effects). Also described in this review is a modification of the n6-OHDA rat, produced by adulthood partial lesioning of the serotoninergic fiber overgrowth. This ADHD model has even more accentuated hyperlocomotor and attentional deficits, counteracted by AMPH-thus providing a more robust means of animal modeling of ADHD. The n6-OHDA rat as a model of ADHD continues to be important in the search for new ADHD treatments.

Rhes: a GTP-binding protein integral to striatal physiology and pathology

Rhes, the Ras Homolog Enriched in Striatum, is a GTP-binding protein whose gene was discovered during a screen for mRNAs preferentially expressed in rodent striatum. This 266 amino acid protein is intermediate in size between small Ras-like GTP-binding proteins and α-subunits of heterotrimeric G proteins. It is most closely related to another Ras-like GTP-binding protein termed Dexras1 or AGS1. Although subsequent studies have shown that the rhes gene is expressed in other brain areas in addition to striatum, the striatal expression level is relatively high, and Rhes protein is likely to play a vital role in striatal physiology and pathology. Indeed, it has recently been shown to interact with the Huntingtin protein and play a pivotal role in the selective vulnerability of striatum in Huntington's disease (HD). Not surprisingly, Rhes can interact with multiple proteins to affect striatal physiology at multiple levels. Functional studies have indicated that Rhes plays a role in signaling by striatal G protein-coupled receptors (GPCR), although the details of the mechanism remain to be determined. Rhes has been shown to bind to both α- and β-subunits of heterotrimeric G proteins and to affect signaling by both Gi/o- and Gs/olf-coupled receptors. In this context, Rhes can be classified as a member of the family of accessory proteins to GPCR signaling. With documented effects in dopamine- and opioid-mediated behaviors, an interaction with thyroid hormone systems and a role in HD pathology, Rhes is emerging as an important protein in striatal physiology and pathology.

Rodent models of ADHD

The neonatal 6-OHDA-lesioned rat, coloboma mouse, DAT-KO mouse, and spontaneously hypertensive rat (SHR) models all bear a phenotypic resemblance to ADHD in that they express hyperactivity, inattention, and/or impulsivity. The models also illustrate the heterogeneity of ADHD: the initial cause (chemical depletion or genetic abnormality) of the ADHD-like behaviors is different for each model. Neurochemical and behavioral studies of the models indicate aberrations in monoaminergic neurotransmission. Hyperdopaminergic neurotransmission is implicated in the abnormal behavior of all models. Norepinephrine has a dual enhancing/inhibitory role in ADHD symptoms, and serotonin acts to inhibit abnormal dopamine and norepinephrine signaling. It is unlikely that symptoms arise from a single neurotransmitter dysfunction. Rather, studies of animal models of ADHD suggest that symptoms develop through the complex interactions of monoaminergic neurotransmitter systems.

Ontogeny and dopaminergic regulation in brain of Ras homolog enriched in striatum (Rhes)

Rhes is one of several signaling molecules preferentially expressed in the striatum. This GTP-binding protein affects dopamine-mediated signaling and behavior. Denervating the striatum of its dopaminergic inputs in adulthood reduces rhes mRNA expression. Here we show that dopamine depletion in adult rats by 6-hydroxydopamine caused a significant decrease in striatal Rhes protein levels as measured by Western blotting. The role of dopamine input on rhes mRNA induction during ontogeny was also examined. Rhes mRNA was measured on postnatal days 4, 6, 8, 10, 15, and 24 with in situ hybridization to determine its normal ontogeny. Signal in striatum was detectable, but very low, on postnatal day 4 and increased gradually to peak levels at days 15 and 24. Outside of the striatum, rhes mRNA was expressed at high levels in hippocampus and cerebellum during the postnatal period. Hippocampal signal was initially highest in CA3 and dentate gyrus, but shifted to higher expression in CA1 by the late postnatal period. Several other nuclei showed low levels of rhes mRNA during ontogeny. Depletion of dopamine by 6-hydroxydopamine injection on postnatal day 4 did not affect the ontogenetic development of rhes mRNA, such that expression did not differ statistically in lesioned versus vehicle-treated animals tested in adulthood. These findings suggest that although dopamine input is not necessary for the ontogenetic development of rhes mRNA expression, changes in both rhes mRNA and Rhes protein are integral components of the response of the adult striatum to dopamine depletion.

L-DOPA-induced activation of striatal p38MAPK and CREB in neonatal dopaminergic denervated rat: Relevance to self-injurious behavior

The destruction of nigrostriatal dopaminergic neurons with 6-hydroxydopamine (6OHDA) during the neonatal period results in dopamine (DA) loss and susceptibility for self-injurious behavior (SIB) when challenged with L-dihydroxyphenylalanine (L-DOPA), via a supersensitive D1 receptor-mediated mechanism. However, there are no changes in D1 receptor binding or mRNA levels, suggesting a potential postreceptor signaling mechanism(s). Here, we examined whether L-DOPA-induced SIB is associated with altered MAPK signaling (p38MAPK, ERK1/2, and JNK) and their nuclear target, CREB. Neonatal dopaminergic lesioned animals were challenged, as adults, with L-DOPA, observed for SIB for 6 hr, and then sacrificed. The data were grouped as follows: control, lesioned rats without SIB (SIB(-)), and lesioned rats that were positive for SIB (SIB(+)). HPLC analysis of striatal extracts revealed a more significant loss of DA and an increase of serotonin in the SIB(+) than in the SIB(-) group. The striatal levels of TH protein were severely decreased, but D1 receptor levels were unaltered in the lesioned groups. These results confirm and extend previous studies indicating that SIB is associated with a near-total loss of DA and TH, an increase in serotonin, and no change in D1 receptor levels. The present studies further revealed that the levels of active phosphorylated forms of p38MAPK and CREB were significantly higher in the SIB(+) group than in the SIB(-) group in the striatum, but not in cortex or olfactory tubercle. The results indicate an induction of striatal p38MAPK and an activation of its nuclear target, CREB, as additional mechanisms in the genesis of L-DOPA-induced SIB.

Pharmacological models of ADHD

For more than 50 years, heavy metal exposure during pre- or post-natal ontogeny has been known to produce long-lived hyperactivity in rodents. Global brain injury produced by neonatal hypoxia also produced hyperactivity, as did (mainly) hippocampal injury produced by ontogenetic exposure to X-rays, and (mainly) cerebellar injury produced by the ontogenetic treatments with the antimitotic agent methylazoxymethanol or with polychlorinated biphenyls (PCBs). More recently, ontogenetic exposure to nicotine has been implicated in childhood hyperactivity. Because attention deficits most often accompany the hyperactivity, all of the above treatments have been used as models of attention deficit hyperactivity disorder (ADHD). However, the causation of childhood hyperactivity remains unknown. Neonatal 6-OHDA-induced dopaminergic denervation of rodent forebrain also produces hyperactivity - and this model, or variations of it, remain the most widely-used animal model of ADHD. In all models, amphetamine (AMPH) and methylphenidate (MPH), standard treatments of childhood ADHD, typically attenuate the hyperactivity and/or attention deficit. On the basis of genetic models and the noted animal models, monoaminergic phenotypes appear to most-closely attend the behavioral dysfunctions, notably dopaminergic, noradrenergic and serotoninergic systems in forebrain (basal ganglia, nucleus accumbens, prefrontal cortex). This paper describes the various pharmacological models of ADHD and attempts to ascribe a neuronal phenotype with specific brain regions that may be associated with ADHD.

The neonate-6-hydroxydopamine-lesioned rat: a model for clinical neuroscience and neurobiological principles

In 1973, a technique of administering 6-hydroxydopamine (2,4,5-trihydroxyphenylethylamine) intracisternally to neonate rats was introduced to selectively reduce brain dopamine (neonate-lesioned rat). This neonate treatment proved unique when compared to rats lesioned as adults with 6-hydroxydopamine--prompting the discovery of differing functional characteristics resulting from the age at which brain dopamine is reduced. A realization was that neonate-lesioned rats modeled the loss of central dopamine and the increased susceptibility for self-injury in Lesch-Nyhan disease, which allowed identification of drugs useful in treating self-injury in mentally retarded patients. The neonate-lesioned rat has also been proposed to model the hyperactivity observed in attention-deficit hyperactivity disorder. Because the neonate-lesioned rat exhibits enhanced sensitization to repeated NMDA receptor antagonist administration and has functional changes characteristic of schizophrenia, the neonate lesioning is believed to emulate the hypothesized NMDA hypofunction in this psychiatric disorder. Besides modeling features of neurological and psychiatric disorders, important neurobiological concepts emerged from pharmacological studies in the neonate-lesioned rats. One was the discovery of coupling of D1/D2-dopamine receptor function. Another was the progressive increase in responsiveness to repeated D1-dopamine agonist administration referred to as "priming" of D1-dopamine receptor function. Additionally, a unique profile of signaling protein expression related to neonate reduction of dopamine has been identified. Thus, from modeling characteristics of disease to defining adaptive mechanisms related to neonatal loss of dopamine, the neonate-lesioned rat has had a persisting influence on neuroscience. Despite an extraordinary legacy from studies of the neurobiology of this treatment, a host of unknowns remain that will inspire future investigations.

Sexual experience alters D1 receptor-mediated cyclic AMP production in the nucleus accumbens of female Syrian hamsters

Drugs of abuse produce long-term changes in dopamine neurotransmission and receptor-effected intracellular signaling. Similar changes in neuronal activity are mediated by motivated behaviors. To explore cellular mechanisms underlying these neuroadaptations following sexual experience, cyclic AMP accumulation following stimulation of D1 dopamine receptors, G-proteins, and adenylate cyclase was compared in the nucleus accumbens and caudate nucleus of sexually naive and experienced female hamsters following sexual behavior. Direct stimulation of adenylate cyclase with forskolin or indirectly by activation of G-proteins with Gpp(NH)p produced dose-dependent increases in the formation of cyclic AMP in the nucleus accumbens and caudate nucleus, with no effects of sexual experience on these measures. Specific D1 receptor stimulation increased Gpp(NH)p-induced adenylate cyclase activity in the nucleus accumbens and caudate nucleus of all animals. Interestingly, this stimulation was further enhanced only in membranes from the nucleus accumbens, but not from the caudate nucleus, of sexually experienced hamsters compared to the response of naive females. These results demonstrate that sexual behavior experience can sensitize mesolimbic dopamine pathways and that this sensitization occurs through an increase in D1 receptor-mediated signaling.

Chronic food restriction increases D-1 dopamine receptor agonist-induced phosphorylation of extracellular signal-regulated kinase 1/2 and cyclic AMP response element-binding protein in caudate-putamen and nucleus accumbens

Results of behavioral and c-fos immunohistochemical studies have suggested that chronic food restriction and maintenance of animals at 75-80% of free-feeding body weight may increase d-1 dopamine (DA) receptor function. The purpose of the present study was to determine whether D-1 DA receptor binding and/or mitogen-activated protein kinase (MAPK) signaling in caudate-putamen (CPu) and nucleus accumbens (NAc) are increased in food-restricted subjects. In the first experiment, saturation binding of the D-1 DA receptor antagonist [3H]SCH-23390 indicated no difference between food-restricted and ad libitum fed rats with regard to density or affinity of d-1 binding sites in CPu or NAc. In the second experiment, activation of extracellular signal-regulated kinases (ERK1/2) and cyclic AMP response element-binding protein (CREB) by i.c.v. injection of the D-1 DA receptor agonist SKF-82958 (20 microg) were markedly greater in food-restricted than ad libitum fed rats. Given a prior finding that SKF-82958 does not differentially stimulate adenylyl cyclase in CPu or NAc of food-restricted versus ad libitum fed subjects, the present results suggest that increased D-1 DA receptor-mediated ERK1/2 MAP kinase signaling may mediate the enhanced downstream activation of CREB, c-fos, and behavioral responses in food-restricted subjects. It is of interest that food restriction also increased the activation of c-Jun N-terminal protein kinase/stress-activated protein kinase, but this effect was no greater in rats injected with SKF-82958 than in those injected with saline vehicle. This represents additional evidence of increased striatal cell signaling in food-restricted subjects, presumably in response to the i.c.v. injection procedure, although the underlying receptor mechanisms remain to be determined. There were no differences between feeding groups in protein levels of the major phosphatases, MKP-2 and PP1. The upregulation of striatal MAP kinase signaling in food-restricted animals may adaptively serve to facilitate associative learning but, at the same time, increase vulnerability to the rewarding and addictive properties of abused drugs.

Prepulse inhibition of acoustic startle in aromatase knock-out mice: effects of age and gender

Estrogen has been suggested to play a neuromodulatory and neuroprotective role on the brain dopamine system. We used aromatase knockout (ArKO) mice that lack a functional aromatase enzyme and are unable to convert testosterone into estrogen, and assessed prepulse inhibition of acoustic startle, locomotor hyperactivity to amphetamine treatment and rotarod performance. Mice were tested at either 1 month, 4-5 months or 12-18 months of age. In male, but not female ArKO mice, there was an age-related reduction of prepulse inhibition. The 12-18 months old male ArKO mice also showed significantly greater amphetamine-induced hyperactivity. Mice heterozygous for the mutation showed no deficits or were in-between wildtype mice and ArKO mice. We postulate that these data indicate a neuroprotective role of estrogen, particularly in male mice, on ageing of brain mechanisms involved in pre-pulse inhibition and locomotor activity regulation. It is likely that these brain mechanisms are or include dopaminergic activity.

Brain sites of movement disorder: genetic and environmental agents in neurodevelopmental perturbations

In assessing and assimilating the neurodevelopmental basis of the so-called movement disorders it is probably useful to establish certain concepts that will modulate both the variation and selection of affliction, mechanisms-processes and diversity of disease states. Both genetic, developmental and degenerative aberrations are to be encompassed within such an approach, as well as all deviations from the necessary components of behaviour that are generally understood to incorporate "normal" functioning. In the present treatise, both conditions of hyperactivity/hypoactivity, akinesia and bradykinesia together with a constellation of other symptoms and syndromes are considered in conjunction with the neuropharmacological and brain morphological alterations that may or may not accompany them, e.g. following neonatal denervation. As a case in point, the neuroanatomical and neurochemical points of interaction in Attention Deficit and Hyperactivity disorder (ADHD) are examined with reference to both the perinatal metallic and organic environment and genetic backgrounds. The role of apoptosis, as opposed to necrosis, in cell death during brain development necessitates careful considerations of the current explosion of evidence for brain nerve growth factors, neurotrophins and cytokines, and the processes regulating their appearance, release and fate. Some of these processes may possess putative inherited characteristics, like alpha-synuclein, others may to greater or lesser extents be endogenous or semi-endogenous (in food), like the tetrahydroisoquinolines, others exogenous until inhaled or injested through environmental accident, like heavy metals, e.g. mercury. Another central concept of neurodevelopment is cellular plasticity, thereby underlining the essential involvement of glutamate systems and N-methyl-D-aspartate receptor configurations. Finally, an essential assimilation of brain development in disease must delineate the relative merits of inherited as opposed to environmental risks not only for the commonly-regarded movement disorders, like Parkinson's disease, Huntington's disease and epilepsy, but also for afflictions bearing strong elements of psychosocial tragedy, like ADHD, autism and Savantism.

Experimental Parkinsonism Modulates Multiple Genes Involved in the Transduction of Dopaminergic Signals in the Striatum

The irreversible loss of the dopamine-mediated control of striatal function is considered the functional substrate of the motor symptoms of Parkinson's disease. This pathological event causes a complex rearrangement of neuronal activity which involves specific dopamine-regulated cellular functions and, secondarily, several other cellular properties and transmitter systems. In the present study, we applied recently developed cDNA microarray technology to investigate the genetic correlates of the alterations produced by 6-hydroxydopamine-induced dopamine denervation in the nucleus striatum. We found that chronic dopamine denervation caused the modulation of 50 different genes involved in several cellular functions. In particular, products of the genes modulated by this experimental manipulation are involved both in the intracellular transduction of dopamine signal and in the regulation of glutamate transmission in striatal neurons, providing some information on the possible neuronal events which lead to the reorganization of glutamate transmission in the striatum of parkinsonian rats.

Neonatal dopamine depletion reveals a synergistic mechanism of mRNA regulation that is mediated by dopamine(D1) and serotonin(2) receptors and is targeted to tachykinin neurons of the dorsomedial striatum

It has been hypothesized that dopamine(D1) and serotonin(2) receptors become sensitized to agonist-mediated regulation of gene expression following loss of dopaminergic innervation to the striatum. We have previously demonstrated that the combined administration of dopamine(D1) and serotonin(2) receptor agonists to dopamine-depleted adult rats induced preprotachykinin mRNA expression within the periventricular rostral striatum to levels which were significantly different than what could be elicited by either agonist alone. In the present study, we have determined that this phenomenon is revealed only after dopamine depletion. In addition, it is targeted primarily to tachykinin producing neurons of the dorsomedial striatum and is dependent on both dopamine(D1) and serotonin(2) receptor activation. Preprotachykinin mRNA levels in the intact striatum were unaltered 4 h following an i.p. injection of either SKF-38393 (1 mg/kg, dopamine(D1) partial agonist) or (+/-)-1-(4-Iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI 1 mg/kg, serotonin(2) agonist). However, the combined application of both agonists increased (+44%) preprotachykinin message levels, but these changes were restricted to the dorsomedial striatum. In adult animals depleted of dopamine as neonates, striatal preprotachykinin mRNA expression was reduced by approximately 50%. From this lowered level of basal expression, DOI or SKF-38393 raised preprotachykinin mRNA levels within the dorsomedial, but not the dorsolateral striatum. Furthermore, co-stimulation of dopamine(D1) and serotonin(2) receptors produced a nearly four-fold induction of preprotachykinin message levels in the dorsomedial striatum that was significantly greater than either agonist alone. Application of both agonists also elevated preprotachykinin mRNA expression within the dorsolateral striatum, but to a lesser extent. All increases in preprotachykinin mRNA resulting from co-application of SKF-38393 and DOI were prevented by pretreatment with either SCH-23390 (1 mg/kg, dopamine(D1) antagonist) or ritanserin (1 mg/kg, serotonin(2) antagonist). Alternately, preproenkephalin mRNA expression was unaffected by dopamine(D1) receptor stimulation, but was slightly elevated by DOI or both agonists together (42-58%) in intact animals. However, neither agonist treatment in this experiment significantly altered preproenkephalin mRNA expression in the dopamine-depleted striatum which was elevated in response to dopamine lesion alone. Dopamine depletion appears to promote a synergistic interaction between dopamine(D1) and serotonin(2) receptors that leads to enhanced expression of striatal preprotachykinin mRNA levels. The localization of this phenomenon to tachykinin neurons of the direct striatonigral pathway specifically within the dorsomedial regions of the rostral striatum may be relevant to the problem of dyskinetic behaviors which arise during the pharmacological treatment of movement disorders.

Striatal preprotachykinin mRNA levels are regulated by stimulatory agents and dopamine D1 receptor manipulation in rodent organotypic slice cultures

We have utilized an organotypic slice culture system to determine factors which directly influence the expression of striatal preprotachykinin (PPT) mRNA. Striatal slices were generated from 3-day-old male rat pups and cultured on Millicell-CM inserts in serum-containing media. Under these conditions, striatal PPT mRNA levels fell significantly (-55.7+/-6.2%) in slices cultured for 2 days in vitro (2DIV) as compared to slices placed in culture for 3 h (0DIV). However, striatal PPT mRNA expression did not decline further in 4DIV cultured slices (-59.6+/-7.1%). When 2DIV slices were exposed to combined high potassium (K(+), 10 mM) and forskolin (10 microM) stimulation for 3 h, PPT mRNA levels were increased within areas of the brain normally associated with tachykinin production. Application of the dopamine (DA) D1 receptor agonist SKF-38393 (10 microM) at 2DIV for 3 h also increased (+162.9+/-28.9%) PPT mRNA expression, but increases were localized within the striatum. SKF-38393-stimulated increases were completely blocked by the D1 antagonist SCH-23390 (10 microM), which alone had no effect on mRNA levels. However, a 3-h incubation with SKF-38393 on 0DIV slice cultures did not affect PPT mRNA expression whereas SCH-23390 decreased PPT message levels (-24.5+/-5.4%). These findings indicate that tachykinin gene expression is inducible within slice culture preparations and that the maintenance of normal striatal PPT mRNA levels depends on DA D1 receptor tone.

Dopamine-deficient mice are hypersensitive to dopamine receptor agonists

Dopamine-deficient (DA-/-) mice were created by targeted inactivation of the tyrosine hydroxylase gene in dopaminergic neurons. The locomotor activity response of these mutants to dopamine D1 or D2 receptor agonists and l-3,4-dihydroxyphenylalanine (l-DOPA) was 3- to 13-fold greater than the response elicited from wild-type mice. The enhanced sensitivity of DA-/- mice to agonists was independent of changes in steady-state levels of dopamine receptors and the presynaptic dopamine transporter as measured by ligand binding. The acute behavioral response of DA-/- mice to a dopamine D1 receptor agonist was correlated with c-fos induction in the striatum, a brain nucleus that receives dense dopaminergic input. Chronic replacement of dopamine to DA-/- mice by repeated l-DOPA administration over 4 d relieved the hypersensitivity of DA-/- mutants in terms of induction of both locomotion and striatal c-fos expression. The results suggest that the chronic presence of dopaminergic neurotransmission is required to dampen the intracellular signaling response of striatal neurons.

The role of dopamine in the expression of morphine withdrawal

1. Both L-dopa and low doses of apomorphine potentiated withdrawal symptoms such as jumping, "wet dog" shakes and burrows. L-dopa reduced hypothermia and potentiated body weight loss, whereas apomorphine produced opposite effects. 2. Higher doses of apomorphine attenuated jumping and burrows but had no effect on "wet dog" shakes. On the other hand, and with the exception of sulpiride, all other dopamine (DA) antagonists produced effects opposite those of the agonists with regard to jumping, "wet dog" shakes and burrows. 3. In addition, DA antagonists reduced hypothermia and body weight loss. The effects of DA agonists and antagonists were investigated in mice injected with 6-hydroxydopamine (6-OHDA) intracerebrally to examine whether DA-mediated effects are somehow linked to noradrenergic pathways. 4. Mice pretreated with 6-OHDA developed a higher degree of naloxone-induced withdrawal jumping than did untreated mice. 6-OHDA reversed the effects of apomorphine on "wet dog" shakes and burrows while abolishing those of L-dopa on all withdrawal symptoms, the only exception being jumping, which remained unchanged. 5. 6-OHDA also reversed the effects of sulpiride on all withdrawal symptoms while reversing the effects of pimozide on jumping, and it abolished its effect on hypothermia. 6. These findings provide evidence suggesting that the effects of DA agonists and antagonists are dependent at least partly on intact noradrenergic pathways.

Differential effects of bilateral dopamine depletion in neonatal and adult rats

Both Lesch-Nyhan syndrome and Parkinson's disease are associated with decreased brain dopamine, yet each disorder is characterized by a different set of motor symptoms. Lesch-Nyhan syndrome is manifested in early childhood, while parkinsonism usually does not appear until adulthood, suggesting that age at the time of dopamine loss is one determinant of the effects of neurotransmitter deficiency. Support for this view is found in studies of animals given dopamine-depleting lesions at different ages and then tested in adulthood. Animals lesioned as neonates show a supersensitivity to dopamine agonists, especially D1-dopamine receptor agonists, and to MK-801, an NMDA receptor antagonist. In addition, neonatally treated animals show a 'priming' effect following repeated exposure to D1-dopamine agonists. Animals depleted of dopamine as adults are more supersensitive to agonists acting on the D2-dopamine receptor, and do not evidence priming to dopamine agonists or an enhanced response to MK-801. These differential pharmacological profiles suggest that the changes in neurotransmitter systems following dopamine depletion are, at least in part, determined by age at the time of the lesion.

Functional and molecular differentiation of the dopamine system induced by neonatal denervation

The administration of the neurotoxin 6-hydroxydopamine (6-OHDA) to damage the mesostriatal dopamine (DA) system in the neonate results in different neurochemical and behavioral consequences as compared to lesions made in adulthood. There have been few direct data to support the conclusion that the behavioral changes following neonatal 6-OHDA lesions reflect plasticity of the DA system. It is our hypothesis that the plasticity of the developing DA system is fundamentally different from that of the adult. Responses to 6-OHDA lesions can only be understood within the context of the status of the mesostriatal DA system at the time of the lesion. There are stages of development in the early postnatal period when certain components of the mesostriatal DA system are differentially sensitive to 6-OHDA lesions. These "windows" of vulnerability can be predicted from an analysis of the developmental expression of DA receptors and the maturation of the subpopulation of the mesostriatal DA system that innervates them. We review the differences in the behavioral plasticity of the adult and neonate sustaining 6-OHDA lesions to the mesostriatal DA system, the mechanisms responsible for the behavioral plasticity in the adult, and our conceptualization of which mechanisms are affected in the neonate.

Neonatal 6-hydroxydopamine lesions lead to opposing changes in the levels of dopamine receptors and their messenger RNAs

Previous studies have established that selective damage to the early-developing components of the mesostriatal dopamine system produces profound changes in dopamine D1 receptor-mediated behaviors, while decreasing D1 receptor density. In order to better understand the effects of early intrastriatal 6-hydroxydopamine lesions, we studied the ontogenetic expression (postnatal days 7, 14, 35 and 90) of D1 and D2 receptors, and their corresponding messenger RNAs, in rats that had received intrastriatal 6-hydroxydopamine or vehicle lesions on postnatal day 1. Using receptor autoradiography, significant (P < 0.05) decreases in [3H]SCH 23390 binding to D1 receptors in the rostral and caudal dorsomedial and ventromedial caudate of 6-hydroxydopamine-lesioned animals were evident by postnatal day 7, and remained depressed at all future time points. A significant decrease in D1 receptor concentration occurred in the dorsolateral caudate at later time points (postnatal days 35 and 90). [3H]Spiperone binding to D2 receptor sites was unchanged throughout the entire study. In situ hybridization for D1 and D2 messenger RNA expression showed contrasting results. 6-Hydroxydopamine induced significant decreases of D1 messenger RNA levels in the dorsolateral and dorsomedial caudate by postnatal day 7. By postnatal day 14, messenger RNA expression was significantly elevated in the dorsomedial and ventromedial caudate of the 6-hydroxydopamine group, and remained elevated thereafter. D1 messenger RNA levels became elevated in the lateral caudate at later time points (postnatal days 35 and 90). The opposing changes in D1 receptor concentrations and the messenger RNA encoding the protein did not occur as a consequence of increased transport of D1 receptors to striatonigral terminals. D2 messenger RNA levels in the dorsal caudate were significantly decreased on postnatal day 7, and became higher than controls at postnatal day 14, but were unchanged from controls at later time points. Together, the D1 receptor and D1 messenger RNA findings suggest that the normal relationship between levels of D1 receptor transcript and D1 receptor protein is permanently altered following the early loss of dopamine. In contrast, the results indicate that dopamine plays a minor role in the early postnatal development of the D2 receptor protein and transcript. These findings suggest that dopamine may be involved in the coordinated expression of some dopamine receptors and their corresponding messenger RNAs during development.

Biochemical and pharmacological characterisation of SR141716A, the first potent and selective brain cannabinoid receptor antagonist

SR141716A is a selective, potent and orally active antagonist of the brain cannabinoid receptor with a long duration of action. This compound shows high affinity for the central cannabinoid receptor (Ki = 2 nM), displays low affinity for the peripheral cannabinoid receptor (Ki > 1000 nM). In vitro, SR141716A antagonizes the inhibitory effects of cannabinoid receptor agonists on both mouse vas deferens contractions and dopamine-stimulated adenylyl cyclase activities in rat brain membranes. After oral administration SR141716A totally inhibited the ex vivo [3H]-CP55,940 binding to cerebral membranes with a ED50 value of 3.5 mg/kg. Furthermore SR141716A antagonizes the classical pharmacological responses elicited by cannabinoid receptor agonists. In addition, SR141716A reverses the inhibitory effect of WIN55212-2 on isoniazid-induced elevation of cGMP in rat cerebellum. This compound will provide a powerful tool for studying the in vivo functions of the anandamide/cannabinoid system.

Dopamine receptor supersensitivity

Dopamine (DA) receptor supersensitivity refers to the phenomenon of an enhanced physiological, behavioral or biochemical response to a DA agonist. Literature related to ontogenetic aspects of this process was reviewed. Neonatal 6-hydroxydopamine (6-OHDA) destruction of rat brain DA neurons produces overt sensitization to D1 agonist-induced oral activity, overt sensitization of some D2 agonist-induced stereotyped behaviors and latent sensitization of D1 agonist-induced locomotor and some stereotyped behaviors. This last process is unmasked by repeated treatments with D1 (homologous "priming") or D2 (heterologous "priming") agonists. A serotonin (5-HT) neurotoxin (5,7-dihydroxytryptamine) and 5-HT2C receptor antagonist (mianserin) attenuate some enhanced behavioral effects of D1 agonists, indicating that 5-HT neurochemical systems influence D1 receptor sensitization. Unlike the relative absence of change in brain D1 receptor number, DA D2 receptor proliferation accompanies D2 sensitization in neonatal 6-OHDA-lesioned rats. Robust D2 receptor supersensitization can also be induced in intact rats by repeated treatments in ontogeny with the D2 agonist quinpirole. In these rats quinpirole treatments produce vertical jumping at 3-5 wk after birth and subsequent enhanced quinpirole-induced antinociception and yawning. The latter is thought to represent D3 receptor sensitization. Except for enhanced D1 agonist-induced expression of c-fos, there are no changes in the receptor or receptor-mediated processes which account for receptor sensitization. Adaptive mechanisms by multiple "in series" neurons with different neurotransmitters may account for the phenomenon known as receptor supersensitivity.

The neonatal lesion of the meso-telencephalic dopaminergic pathway increases intrastriatal D2 receptor levels and synthesis and this effect is reversed by neonatal dopaminergic rich-graft

The ascending dopaminergic pathway of 3-day-old rats has been unilaterally destroyed by the injection of 6-hydroxydopamine into the lateral hypothalamus. Five days later, a suspension containing embryonic dopaminergic neurones was injected in the lesioned neostriatum. Rotational responses to dopaminergic agonists were tested eight months after grafting and animals were killed one month later. Neostriatal dopaminergic D1 and D2 receptors were examined using autoradiography while changes in D2 receptor mRNA levels were studied by in situ hybridization. The lesion induced a behavioural hypersensitivity - as manifested in contralateral rotations - to dopaminergic D1 (SKF 38393) or D2 (LY 171555) agonists which was abolished by the graft. Density of D1 receptors was not affected by the lesion while D2 receptors density was increased by 20-25% in the more rostral part of the neostriatum. Changes in D2 mRNA after the lesion paralleled those observed for D2 receptor density, i.e. D2 mRNA level was increased by 15-19% in the rostral neostriatum. The graft did not influence D1 receptor densities but reversed the post-lesion increase of D2 receptors associated parameters. It is concluded that dopaminergic grafts implanted in neonatal hosts are able to normalise the density of D2 receptors by an action on their synthesis.

Dopaminergic modulation of striatal acetylcholine release in rats depleted of dopamine as neonates

A repeated sessions, in vivo microdialysis design was used to determine the D1- and D2-like receptor modulation of striatal ACh efflux in intact adult rats and those depleted of DA on postnatal Day 3. Systemic administration of the D1-like agonist SKF 38393 (1.0 or 10.0 mg/kg, or the D2-like antagonist clebopride (1.0 or 10.0 mg/kg) increased ACh efflux in both controls and DA-depleted animals. Systemic administration of the D1-like antagonist SCH 23390 (0.05 or 0.2 mg/kg) or D2-like agonist quinpirole (0.5 or 1.0 mg/kg) decreased ACh efflux in both groups of animals. DA-depleted animals exhibited a larger response than did controls to the lower doses of these drugs. Intrastriatal administration of clebopride (10 microM) increased ACh efflux in DA-depleted animals. Finally, basal and clebopride-stimulated ACh efflux were unaffected by the repeated microdialysis sessions. These data demonstrate that the reciprocal modulation of striatal ACh efflux, seen in controls and in rats depleted of DA as adults, is also present in adults depleted of DA as neonates. Because the roles of D1- and D2-receptors in the expression of motor behavior differ between rats depleted of DA as adults vs as neonates, these data suggest that alterations in the dopaminergic modulation of striatal ACh release do not underlie the sparing from motoric deficits seen in animals depleted of DA as neonates.

Unilateral neonatal intracerebroventricular 6-hydroxydopamine administration in rats: I. Effects on spontaneous and drug-induced rotational behaviour and on postmortem monoamine levels

6-Hydroxydopamine (6-OHDA; 100 micrograms in 5 microliters) was injected into the right ventricle (intracerebroventricular, ICV) of 3-day old Sprague-Dawley rats in an attempt to produce a unilateral neonatal dopamine (DA) lesion. At adult stage, the rats were studied for spontaneous, handling- and drug-induced rotational behaviour. The 6-OHDA-treated rats showed hyperreactivity at handling, in the animal facility and in the experimental sets. This behaviour was not observed in vehicle-treated rats, and it did not decrease through the successive experiments. Apomorphine (0.05-1 mg/kg, SC) and caffeine (20 mg/kg SC) produced contralateral rotation in neonatal 6-OHDA, but not in vehicle-injected rats. d-Amphetamine (0.2-2 mg/kg, SC) produced strong, dose-dependent, ipsilateral rotation, while the serotonin (5-HT) releasing agent, p-chloroamphetamine (2 mg/kg, SC) produced a short-lasting and weak ipsilateral rotation in the 6-OHDA-treated rats. On the 6-OHDA-injected side, DA and metabolites levels were reduced by > 70-90% in the striatum, the nucleus accumbens and the tuberculum olfactorium, while in the mesencephalon a 50% decrease was found. On the contralateral side, restricted decreases in DA and metabolites were observed. Noradrenaline (NA) levels were decreased bilaterally in the forebrain. In contrast, 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels were increased in the ipsilateral striatum (> 180%), and tuberculum olfactorium (> 120%). Thus, neonatal unilateral ICV 6-OHDA administration produced a significant unilateral decrease in tissue levels of DA and metabolites, which was most marked in the striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Postsynaptic integration of glutamatergic and dopaminergic signals in the striatum

The aim of this study was to achieve a better understanding of the integration in striatal medium-sized spiny neurons (MSNs) of converging signals from glutamatergic and dopaminergic afferents. The review of the literature in the first section shows that these two types of afferents not only contact the same striatal cell type, but that individual MSNs receive both a corticostriatal and a dopaminergic terminal. The most common sites of convergence are dendritic shafts and spines of MSNs with a distance between the terminals of less than 1-2 microns. The second section focuses on synaptic transmission and second messenger activation. Glutamate, the candidate transmitter of corticostriatal terminals, via different types of glutamate receptors can evoke an increase in intracellular free calcium concentrations. The net effect of dopamine in the striatum is a stimulation of adenylate cyclase activity leading to an increase in cAMP. The subsequent sections present information on calcium- and cAMP-sensitive biochemical pathways and review the regional and subcellular distribution of the components in the striatum. The specific biochemical reaction steps were formalized as simplified equilibrium equations. Parameter values of the model were chosen from published experimental data. Major results of this analysis are: at intracellular free calcium concentrations below 1 microM the stimulation of adenylate cyclase by calcium and dopamine is at least additive in the steady state. Free calcium concentrations exceeding 1 microM inhibit adenylate cyclase, which is not overcome by dopaminergic stimulation. The kinases and phosphatases studied can be divided in those that are almost exclusively calcium-sensitive (PP2B and CaMPK), and others that are modulated by both calcium and dopamine (PKA and PP1). Maximal threonine-phosphorylation of the phosphoprotein DARPP requires optimal concentrations of calcium (about 0.3 microM) and dopamine (above 5 microM). It seems favourable if the glutamate signal precedes phasic dopamine release by approximately 100 msec. The phosphorylation of MAP2 is under essentially calcium-dependent control of at least five kinases and phosphatases, which differentially affect its heterogeneous phosphorylation sites. Therefore, MAP2 could respond specifically to the spatio-temporal characteristics of different intracellular calcium fluxes. The quantitative description of the calcium- and dopamine-dependent regulation of DARPP and MAP2 provides insights into the crosstalk between glutamatergic and dopaminergic signals in striatal MSNs. Such insights constitute an important step towards a better understanding of the links between biochemical pathways, physiological processes, and behavioural consequences connected with striatal function. The relevance to long-term potentiation, reinforcement learning, and Parkinson's disease is discussed.

Changes of D1 and D2 receptors in adult rat neostriatum after neonatal dopamine denervation: quantitative data from ligand binding, in situ hybridization and iontophoresis

The specific binding of [3H]SCH23390 to D1 and of [3H]raclopride to D2 dopamine receptors was measured by autoradiography in the rostral and caudal halves of neostriatum and in the substantia nigra of adult rats subjected to near total destruction of nigrostriatal dopamine neurons by intraventricular 6-hydroxydopamine soon after birth. Three months after this lesion, [3H]SCH23390 binding (D1 receptors) was slightly but significantly decreased in the rostral neostriatum (22%), but unchanged in its caudal half and in the substantia nigra. In contrast, [3H]raclopride binding (D2 receptors) was considerably increased throughout the neostriatum (10-40%), while markedly decreased in the substantia nigra (80%). In the rostral neostriatum, there were no parallel changes in D2 receptor messenger RNA levels, as measured by in situ hybridization on adjacent sections. Caudally, however, slight but significant increases in D2 messenger RNA could be observed (10-20%). As assessed by quantitative iontophoresis, there was a marked enhancement (63%) of the inhibitory responsiveness of spontaneously firing units in the rostral neostriatum to dopamine and the D1 agonist, SKF38393, in neonatally lesioned compared to control rats. On the other hand, responsiveness to PPHT, a potent D2 agonist, appeared to be unchanged. Such opposite changes in the number of D1 and D2 binding sites, dissociated from the expression of D2 receptor messenger RNA and from the sensitivity to dopamine and D1 and D2 agonists, suggested independent adaptations of these various parameters following the neonatal dopamine denervation of neostriatum. They also provided further evidence for mechanisms other than the dopamine innervation in the control of the expression of neostriatal D2 receptor messenger RNA during ontogenesis, and emphasized that the effects of dopamine and its D1 and D2 agonists in neostriatum do not depend strictly on the number of D1 and D2 primary ligand recognition sites.

Ontogenetic SKF 38393 treatments sensitize dopamine D1 receptors in neonatal 6-OHDA-lesioned rats

Neonatal 6-hydroxydopamine (6-OHDA) treatment of rats is associated with supersensitization of the dopamine (DA) D1 agonist induction of stereotyped and locomotor behaviors. The present study was conducted to determine whether ontogenetic treatments of these rats with the DA D1 receptor agonist, SKF 38393, would produce a maximal DA D1 receptor supersensitivity, as measured by locomotor behavior in adulthood. Rat pups were treated daily with SKF 38393-HCl (3.0 mg/kg per day, i.p.) or saline vehicle for 28 consecutive days from birth. These animals were additionally treated at 3 days after birth with 6-OHDA-HBr (100 micrograms, in each lateral ventricle, salt form) or its vehicle. Between 6 and 9 weeks locomotor activity or stereotyped behaviors were observed after weekly challenge doses of SKF 38393-HCl (3.0 mg/kg, i.p.). In the neonatal 6-OHDA group, successive SKF 38393 treatments produced progressively greater locomotor activity. In the group of rats treated during postnatal ontogeny with both 6-OHDA and SKF 38393 daily treatments, the first adult challenge dose of SKF 38393 produced an enhanced locomotor response, greater than that seen in other groups (P < 0.01). Subsequent SKF 38393 treatments of this group produced increasingly greater locomotor responses. SKF 38393-induced stereotyped behavioral effects were greater in the 6-OHDA-lesioned groups, whether or not SKF 38393 was administered ontogenetically. Profound reductions (> 99%) of DA and its metabolites were found in the striatum of neonatal 6-OHDA treated rats, regardless of whether SKF 38393 was co-administered ontogenetically.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo electrochemical measurements and electrophysiological studies of rat striatum following neonatal 6-hydroxydopamine treatment

The effects of neonatal treatment (one day after birth) with the neurotoxin, 6-hydroxydopamine (75 micrograms/10 microliters intracisternal), were studied in the striatum of normal adult and treated Sprague-Dawley rats. Measurements of monoamine levels in the dorsal striatum and nucleus accumbens, by high-performance liquid chromatography coupled with electrochemical detection, showed that neonatal 6-hydroxydopamine treatment produced a permanent and massive destruction of striatal dopamine. The effects were more pronounced in the dorsal striatum than in the nucleus accumbens. In addition, serotonin levels were elevated in the rat striatum as a consequence of the neonatal treatment. Rapid chronoamperometric recordings of K(+)-evoked monoamine overflow using Nafion-coated recording electrodes were investigated in both the dorsal and ventral striatum of control and neonatally lesioned rats. The potassium-evoked responses recorded from the dorsal striatum of the 6-hydroxydopamine-treated rats were significantly reduced in amplitude as compared to controls. In addition, the reduction/oxidation current ratios of the responses were more serotonin-like, in contrast to the dopamine-like current ratios measured in the striatum of untreated animals. In ventral striatum, the amplitudes of the K(+)-evoked responses were not significantly reduced versus control. However, the K(+)-evoked signals were more serotonin-like in their electrochemical characteristics as compared to controls. In addition to the release studies, extracellular single-unit electrophysiological recordings were performed in normal and neonatally 6-hydroxydopamine-treated rats. The spontaneous discharge rate of striatal neurons in the neonatally 6-hydroxydopamine-treated rats was similar to that of control rats. This is in contrast to dopamine lesions in adult animals, where a marked elevation of the discharge rate is observed. Local applications of dopamine and serotonin into the striatum of neonatally 6-hydroxydopamine-treated rats elicited excitations of striatal cells rather than the normal inhibitory effects seen in control animals. Taken together, these data suggest that loss of striatal dopamine terminals at birth leads to both pre- and postsynaptic alterations in monoamine pathways.

Development of monoamine systems after neonatal anoxia in rats

Neurochemical and morphological effects of neonatal anoxia on monoamine systems were studied after 100% N2 exposure for 25 min at 30 h postnatally (postnatal day 2-P2). At 20 min after anoxia, reductions of tissue levels of cerebellar noradrenaline (NA) and striatal dopamine (DA) and metabolites were seen, while 5-hydroxyindoleacetic acid (5-HIAA) was increased in cortex and cerebellum. At P7, NA increased in cerebellum, while serotonin (5-HT) and 5-HIAA decreased in cortex and cerebellum. At P21, increased hippocampal NA and striatal homovanillic acid (HVA) were found, while striatal 5-HT decreased and 5-HIAA increased in striatum and hippocampus. At P60, striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-HIAA levels were found to be enhanced. No effects were seen on 5-HT, tyrosine hydroxylase, or DARPP-32 immunostaining in cortex, hippocampus, and striatum. Thus, the neonatal anoxia induced both acute and persistent neurochemical abnormalities in monoamine systems that were not accompanied by morphological changes detectable with the methods used. The monoamine alterations found could be critically connected to the behavioral disturbances observed in rats after neonatal anoxia. The findings may also be of relevance to dysfunctions seen in humans after perinatal oxygen deficiency, e.g., the attention deficit hyperactivity disorder syndrome.

Developmental regulation of phosphoprotein gene expression in the caudate-putamen of rat: an in situ hybridization study

The regional and cellular ontogeny of the mRNA encoding the dopamine- and cAMP-regulated phosphoprotein, DARPP-32, has been studied in rat striatum by quantitative in situ hybridization histochemistry. The mRNA for DARPP-32 exhibited a characteristic developmental profile. The hybridization signal was first visible on the day of birth, at which time DARPP-32 mRNA was concentrated in patches in the caudate-putamen. By the end of the first postnatal week, the majority of neurons in the caudate-putamen expressed the DARPP-32 message. Levels of mRNA per cell increased markedly during the second postnatal week, and peaked around the beginning of the third week. The adult level of DARPP-32 mRNA was lower than that observed at the apex of mRNA expression, on a per cell basis, while the proportion of neurons expressing detectable levels of message remained relatively constant. In the nucleus accumbens and olfactory tubercle, DARPP-32 mRNA development lagged somewhat behind that observed in the caudate-putamen, but was similar in other respects. A non-quantitative study employing an oligonucleotide probe complementary to the mRNA encoding another cAMP-regulated phosphoprotein, ARPP-21, revealed a similar developmental sequence to DARPP-32. The present results suggest that for DARPP-32 mRNA, genetic and, possibly, environmental factors play a role in determining the developmental patterns observed.

Transient cyclic AMP accumulation mediated by dopamine D1 receptors in the chick embryo optic lobe

[3H]SCH 23390 bound with high affinity (Kd = 0.6 nM) and in a saturable manner (Bmax = 130 fmol/mg protein) to membrane preparations of the chick optic lobe. Pharmacological experiments, using several dopaminergic ligands, revealed that [3H]SCH 23390 bound stereospecifically to dopaminergic receptors of the D1 type in this tissue. Other experiments revealed that dopamine was able to induce cyclic AMP accumulation in the optic lobe (ED50 = 3 microM), an effect that was blocked by fluphenazine, a potent D1 antagonist (IC50 = 1.8 microM). The developmental profile of tissue dopamine-dependent cyclic AMP accumulation, however, was quite different from the differentiation pattern of [3H]SCH 23390 specific binding sites. While [3H]SCH 23390 binding sites increased 4-fold after the 12th embryonic day (E12), dopamine-dependent cyclic AMP accumulation was maximal in earlier stages, decreasing progressively after E10. In tissues from embryos at E16 or older, no difference was observed between basal and dopamine-stimulated levels of cyclic AMP. These data suggest that D1 receptors are coupled to adenylate cyclase in a limited period of the development of the optic lobe and that D1 receptors not coupled to the enzyme can be a common feature in the CNS.

Regional distribution of DARPP-32 (dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein of Mr = 32,000) mRNA in mouse brain

DARPP-32 (dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein of Mr = 32,000) mRNA distribution was examined in adult mouse central nervous system by in situ hybridization. In general, DARPP-32 mRNA was found in regions of brain where cells express the dopamine D1 subtype receptor. Cells of the olfactory tubercle, caudate-putamen, and nucleus accumbens had the highest levels of DARPP-32 mRNA, as did choroid plexus and Purkinje cells. Relatively high levels were found in medial habenula and lateral piriform cortex. Moderate levels were seen in cerebral cortex layer VI, medial piriform cortex, lateral entorhinal cortex, tenia tecta, anterior olfactory nucleus, and lateral bed nucleus of the stria terminalis. Low levels were observed in hippocampus, cerebral cortex layers II and III, olfactory bulb, and the nucleus of the lateral olfactory tract. DARPP-32 mRNA levels in the amygdaloid nuclei varied greatly.

Sensory gating in rats depleted of dopamine as neonates: potential relevance to findings in schizophrenic patients

Based on a recent hypothesis of reduced subcortical dopaminergic tone, evidence of early neurodevelopmental deviation, and acoustic startle abnormalities in schizophrenia, we examined acoustic startle in adult animals depleted of dopamine (DA) as neonates. Male rat pups received intracerebroventricular injections of either 6-hydroxydopamine (6-OHDA, 100 micrograms) or its vehicle on postnatal day 3. At 60 days of age, baseline startle and prepulse inhibition (PPI) of startle were assessed in a no injection condition, with all other animals receiving injections of saline or the DA agonist, apomorphine. Acoustic startle was elicited using 120 db white noise bursts alone or preceded by prepulses of 75, 80, and 85 db. Animals treated with 6-OHDA exhibited a 93% depletion of striatal DA compared to vehicle-treated controls. Whereas DA depleted animals did not differ from controls in the no injection condition, they showed greater baseline startle and reduced PPI compared to controls after saline injections. Depleted animals also showed exaggerated responses to apomorphine, with greater increases in baseline startle, loss of habituation, and decreased PPI compared to controls. Findings indicate that neonatal DA depletions lead to increased baseline startle and impaired sensory gating in adulthood after saline injections and dopamine agonists compared to controls. These findings may be relevant to a subgroup of psychotic patients that exhibit similar startle abnormalities as well as signs of hypodopaminergic function.

Ultrastructural analysis of the serotonin hyperinnervation in adult rat neostriatum following neonatal dopamine denervation with 6-hydroxydopamine

Serotonin (5-HT) immunocytochemistry was used at the electron microscopic level to characterize the ultrastructural features of 5-HT axon terminals (varicosities) hyperinnervating the neostriatum of adult rats, 3 months after a neonatal destruction of the nigrostriatal dopamine system by intraventricular 6-hydroxydopamine. 5-HT-immunostained terminals from the anterior half of the hyperinnervated neostriatum were examined in single thin sections, and compared to their counterparts in vehicle-injected controls with respect to shape, size, organelle content, presence of a synaptic membrane differentiation and composition of the microenvironment. The intrinsic and relational features of the 5-HT-immunostained varicosities were essentially the same in 5-HT-hyperinnervated and control tissue. In particular, the frequency with which these varicosities made synaptic contacts was similarly low in both conditions (6-8% for whole varicosities), as already described in normal adult rat neostriatum. The distributional frequency of elements juxtaposed to the 5-HT-immunostained varicosities was also comparable in control and 5-HT-hyperinnervated tissue. However, in both conditions, there were much fewer dendritic spines in the microenvironment of 5-HT varicosities than around unlabeled terminals randomly selected from the same thin sections. This difference seemed entirely due to the numerous axo-spinous synaptic contacts made by the randomly selected, unlabeled varicosities. Together with recent observations on the 5-HT-hyperinnervation of adult rat hippocampus after grafts of fetal neurons, these data lead to the suggestion that mostly non-junctional neostriatal 5-HT terminals are not committed to a specific intratissular microenvironment. This might in part explain why they grow in excess when reinnervating adult tissue after a lesion or a graft.

Ontogenic homologous supersensitization of dopamine D1 receptors

To determine whether prolonged supersensitization of dopamine D-1 receptors could be produced during ontogeny, rats were treated daily, from birth, for 33 consecutive days with the D-1 receptor agonist, SKF 38393 HCl (3.0 mg/kg per day i.p.). These rats were additionally treated at 3 days after birth with the neurotoxin, 6-hydroxydopamine HBr (6-OHDA; 200 micrograms i.c.v., half in each lateral ventricle) or its vehicle. At 6 to 7 weeks from birth a challenge dose of SKF 38393 HCl (3.0 mg/kg i.p.) increased stereotypy scores for a number of behaviors in 6-OHDA-lesioned rats that were treated repeatedly during ontogeny with SKF 38393. These accentuated behaviors included licking, grooming, taffy pulling, jumping, paw treading and locomotion. Although the findings demonstrate an increased sensitivity of D-1 receptors to an agonist, there was no change in the Bmax or Kd for D-1 receptors in the striatum. In rats that were treated during postnatal development with SKF 38393, but not lesioned with 6-OHDA, SKF 38393-induced stereotyped behaviors were not substantially different from control. The neonatally primed rat model may be useful for probing mechanisms of receptor supersensitivity.

Enhanced adenylate cyclase activity in neonatally dopamine lesioned rats is related to increased Gs-protein coupling

It has previously been shown that neonatal selective lesions of the central dopamine system with 6-hydroxydopamine lead to increased basal and dopamine-stimulated adenylate cyclase activity in striatum without any alterations of dopamine receptor binding characteristics. In the present study, it was shown that adenylate cyclase activity following G-protein stimulation by the GTP analogue 5-guanylimidodiphosphate (Gpp(NH)p) was increased in striatal preparations from neonatally 6-hydroxy-dopamine-lesioned rats, compared with control animals. No difference was seen in forskolin-stimulated enzyme activity between the two groups. These results indicate that neonatal dopamine lesions induce a selective functional supersensitivity at the D1 receptor complex by enhancing the coupling efficiency of the Gs protein to adenylate cyclase, without alterating the catalytic activity of the enzyme.

Ketanserin and mianserin treatment reverses hyperactivity in neonatally dopamine-lesioned rats

Selective brain dopamine (DA) depletions in rats, induced by neonatal intracisternal administration of 6-hydroxydopamine (6-OHDA; 75 μg), caused spontaneous hyperactivity at the adult stage as measured using determinations of locomotion, rearing and total activity. Treatment with ketanserin (0.5 or 1.0 mg/kg, s.c.) reversed the hyperactivity in 6-OHDA-treated animals during a 90-min period following injection, although only the low dose of ketanserin reduced rearings. In control animals ketanserin treatment did not affect the locomotion or total activity counts, while the high dose of ketanserin increased rearings. Following treatment with mianserin (0.5 or 1.0 mg/kg, s.c.), a similar effect was seen; however, it was longer-lasting and mianserin treatment increased activity in controls. Regional analysis of monoamine levels demonstrated a marked reduction of basal forebrain DA levels, while in striatum an increase in serotonin (5-HT) concentration was seen following the 6-OHDA treatment. The results indicate that drugs with a high affinity to 5-HT(2) binding sites can influence the hyperactivity seen in neonatally DA-lesioned rats. This effect might be related to inhibition of 5-HT pathways directly involved in regulation of motor activity or due to alterations in the interaction between the DA and 5-HT systems as a consequence of the early DA lesion.