Dopamine neoinnervation in the substantia nigra and hyperinnervation in the interpeduncular nucleus of adult rat following neonatal cerebroventricular administration of 6-hydroxydopamine

Synaptotagmin-1-dependent phasic axonal dopamine release is dispensable for basic motor behaviors in mice

In Parkinson's disease (PD), motor dysfunctions only become apparent after extensive loss of DA innervation. This resilience has been hypothesized to be due to the ability of many motor behaviors to be sustained through a diffuse basal tone of DA; but experimental evidence for this is limited. Here we show that conditional deletion of the calcium sensor synaptotagmin-1 (Syt1) in DA neurons (Syt1 cKODA mice) abrogates most activity-dependent axonal DA release in the striatum and mesencephalon, leaving somatodendritic (STD) DA release intact. Strikingly, Syt1 cKODA mice showed intact performance in multiple unconditioned DA-dependent motor tasks and even in a task evaluating conditioned motivation for food. Considering that basal extracellular DA levels in the striatum were unchanged, our findings suggest that activity-dependent DA release is dispensable for such tasks and that they can be sustained by a basal tone of extracellular DA. Taken together, our findings reveal the striking resilience of DA-dependent motor functions in the context of a near-abolition of phasic DA release, shedding new light on why extensive loss of DA innervation is required to reveal motor dysfunctions in PD.

Implication of synaptotagmins 4 and 7 in activity-dependent somatodendritic dopamine release in the ventral midbrain

Dopamine (DA) neurons can release DA not just from axon terminals, but also from their somatodendritic (STD) compartment through a mechanism that is still incompletely understood. Using voltammetry in mouse mesencephalic brain slices, we find that STD DA release has low capacity and shows a calcium sensitivity that is comparable to that of axonal release. We find that the molecular mechanism of STD DA release differs from axonal release with regard to the implication of synaptotagmin (Syt) calcium sensors. While individual constitutive knockout of Syt4 or Syt7 is not sufficient to reduce STD DA release, the removal of both isoforms reduces this release by approximately 50%, leaving axonal release unimpaired. Our work unveils clear differences in the mechanisms of STD and axonal DA release.

Neonatal 6-OHDA lesion of the SNc induces striatal compensatory sprouting from surviving SNc dopaminergic neurons without VTA contribution

Dopamine (DA) neurons of the substantia nigra pars compacta (SNc) are uniquely vulnerable to neurodegeneration in Parkinson's disease (PD). We hypothesize that their large axonal arbor is a key factor underlying their vulnerability, due to increased bioenergetic, proteostatic and oxidative stress. In keeping with this model, other DAergic populations with smaller axonal arbors are mostly spared during the course of PD and are more resistant to experimental lesions in animal models. Aiming to improve mouse PD models, we examined if neonatal partial SNc lesions could lead to adult mice with fewer SNc DA neurons that are endowed with larger axonal arbors because of compensatory mechanisms. We injected 6-hydroxydopamine (6-OHDA) unilaterally in the SNc at an early postnatal stage at a dose selected to induce loss of approximately 50% of SNc DA neurons. We find that at 10 and 90 days after the lesion, the axons of SNc DA neurons show massive compensatory sprouting, as revealed by the proportionally smaller decrease in tyrosine hydroxylase (TH) in the striatum compared with the loss of SNc DA neuron cell bodies. The extent and origin of this axonal sprouting was further investigated by AAV-mediated expression of eYFP in SNc or ventral tegmental area (VTA) DA neurons of adult mice. Our results reveal that SNc DA neurons have the capacity to substantially increase their axonal arbor size and suggest that mice designed to have reduced numbers of SNc DA neurons could potentially be used to develop better mouse models of PD, with elevated neuronal vulnerability.

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.

The dual dopamine-glutamate phenotype of growing mesencephalic neurons regresses in mature rat brain

Coexpression of tyrosine hydroxylase (TH) and vesicular glutamate transporter 2 (VGLUT2) mRNAs in the ventral tegmental area (VTA) and colocalization of these proteins in axon terminals of the nucleus accumbens (nAcb) have recently been demonstrated in immature (15-day-old) rat. After neonatal 6-hydroxydopamine (6-OHDA) lesion, the proportion of VTA neurons expressing both mRNAs and of nAcb terminals displaying the two proteins was enhanced. To determine the fate of this dual phenotype in adults, double in situ hybridization and dual immunolabeling for TH and VGLUT2 were performed in 90-day-old rats subjected or not to the neonatal 6-OHDA lesion. Very few neurons expressed both mRNAs in the VTA and substantia nigra (SN) of P90 rats, even after neonatal 6-OHDA. Dually immunolabeled terminals were no longer found in the nAcb of normal P90 rats and were exceedingly rare in the nAcb of 6-OHDA-lesioned rats, although they had represented 28% and 37% of all TH terminals at P15. Similarly, 17% of all TH terminals in normal neostriatum and 46% in the dopamine neoinnervation of SN in 6-OHDA-lesioned rats were also immunoreactive for VGLUT2 at P15, but none at P90. In these three regions, all dually labeled terminals made synapse, in contradistinction to those immunolabeled for only TH or VGLUT2 at P15. These results suggest a regression of the VGLUT2 phenotype of dopamine neurons with age, following normal development, lesion, or sprouting after injury, and a role for glutamate in the establishment of synapses by these neurons.

Enhanced glutamatergic phenotype of mesencephalic dopamine neurons after neonatal 6-hydroxydopamine lesion

There is increasing evidence that a subset of midbrain dopamine (DA) neurons uses glutamate as a co-transmitter and expresses vesicular glutamate transporter (VGLUT) 2, one of the three vesicular glutamate transporters. In the present study, double in situ hybridization was used to examine tyrosine hydroxylase (TH) and VGLUT2 mRNA expression during the embryonic development of these neurons, and postnatally, in normal rats and rats injected with 6-hydroxydopamine (6-OHDA) at P4 to destroy partially DA neurons. At embryonic days 15 and 16, there was a regional overlap in the labeling of TH and VGLUT2 mRNA in the ventral mesencephalon, which was no longer found at late embryonic stages (E18-E21) and postnatally. In normal pups from P5 to P15, only 1-2% of neurons containing TH mRNA in the ventral tegmental area (VTA) and substantia nigra, pars compacta, also displayed VGLUT2 mRNA. In contrast, after the cerebroventricular administration of 6-OHDA at P4, 26% of surviving DA neurons in the VTA of P15 rats expressed VGLUT2. To search for a colocalization of TH and VGLUT2 protein in axon terminals of these neurons, the nucleus accumbens of normal and 6-OHDA-lesioned P15 rats was examined by electron microscopy after dual immunocytochemical labeling. In normal rats, VGLUT2 protein was found in 28% of TH positive axon terminals in the core of nucleus accumbens. In 6-OHDA-lesioned rats, the total number of TH positive terminals was considerably reduced, and yet the proportion also displaying VGLUT2 immunoreactivity was modestly but significantly increased (37%). These results lead to the suggestion that the glutamatergic phenotype of a VTA DA neurons is highly plastic, repressed toward the end of normal embryonic development, and derepressed postnatally following injury. They also support the hypothesis of co-release of glutamate and DA by mesencephalic neurons in vivo, at least in the developing brain.

Modeling tardive dyskinesia: Predictive 5-HT2C receptor antagonist treatment

Tardive dyskinesia (TD), a movement disorder produced by long-term treatment with a classical antipsychotic drug, is generally considered to be a disorder of dopamine (DA) systems, since classical antipsychotics are potent DA D(2) receptor blockers. Also, acute DA D(1) agonist treatment of rats is known to produce vacuous chewing movements (VCMs), a behavioral feature resembling the oral dyskinesia that is so prominent in most instances of TD. In this paper we outline a series of studies in a new animal model of TD in which DA D(1) receptor supersensitivity was produced by neonatal 6-hydroxydopamine (6-OHDA) -induced destruction of nigrostriatal DA fibers. In rats so-lesioned 5-HT receptor supersensitivity is additionally produced, and in fact 5-HT receptor antagonists attenuate enhanced DA D(1) induction of VCMs. Moreover, in 6-OHDA-lesioned rats treated with haloperidol for one year, there a 2-fold increase in numbers of VCMs (vs intact rats treated with haloperidol); and this high frequency of VCMs persists for more than 6 months after discontinuing haloperidol treatment. During this stage, 5-HT(2) receptor antagonists, but not DA D(1) receptor antagonists, attenuate the incidence of VCMs. This series of findings implicates the 5-HT neuronal phenotype in TD, and promotes 5-HT(2) receptor antagonists, more specifically 5-HT(2C) receptor antagonists, as a rational treatment approach for TD in humans.

Dopamine receptor supersensitivity: an outcome and index of neurotoxicity

The characteristic feature of neurotoxicity is a definable lesion which can account for observed deficits, corresponding to loss of nuclei or axonal fibers normally comprising a specific pathway or tract. However, with ontogenetic lesions, the operative definition fails. In rats lesioned as neonates with 6-hydroxydopamine (6-OHDA), near-total destruction of dopamine- (DA-) containing nerves is produced, and this itself is definable. However, the most prominent feature of rats so-lesioned is the DA receptor supersensitivity (DARSS) that develops and then persists throughout the lifespan. DA D(1) receptors show overt supersensitivity to agonists producing vacuous chewing movements (VCMs), while D(1) receptors associated with locomotor activity have a latent supersensitivity that must be unmasked by repeated D(1) or D(2) agonist treatments - a 'priming' phenomenon. This D(1) DARSS is not usually associated in either a change in D(1) receptor number (B(max)) or affinity (K(d)). In contrast to D(1) DARSS, D(2) receptors are not so predictably supersensitized by a lesion of DA neurons. In reality, the permanently exaggerated response to an agonist by supersensitized receptors is per se a manifestation of neurotoxicity. Despite dramatic behavioral responses mediated by supersensitized receptors, DARSS has not been easy to correlate with enhanced production of second messengers or early response genes. Altered signaling (i.e., neuronal cross-talk) in defined pathways may represent the mechanism that produces so-called receptor supersensitization. Long-lived agonist-induced behavioral abnormality, with or without anatomic evidence of a neuronal lesion, is one of the products of DA D(1) receptor supersensitization -- itself an index of neurotoxicity.

Behavioral changes in rats with early ventral hippocampal damage vary with age at damage

Our previous work demonstrated that neonatal (on postnatal day 7, PD7) excitotoxic damage of the ventral hippocampus (VH) results in delayed emergence of behaviors related to dopaminergic (DA) transmission. In this study, the developmental effects of VH lesions induced at two other ages were investigated in the rat. Ibotenic acid or artificial cerebrospinal fluid was infused into the VH of 3- (PD3) or 14- (PD14)-day-old rat pups. Amphetamine-induced (1.5 mg/kg, i.p.) locomotor activity was assessed in the sham and lesioned rats prior to (PD35) and after puberty (PD56 and PD86). Apomorphine-induced (0.75 mg/kg s.c.) stereotypic behaviors were measured on PD56. Similar VH lesions resulted in different profiles of behavioral abnormalities depending upon the age at which they were induced. The PD3 lesioned rats displayed hyperlocomotion to amphetamine only after puberty, while the PD14 lesioned rats manifest hyperlocomotion as early as 3 weeks after surgery (at PD35). Moreover, the PD3 lesioned rats tended to show more stereotypic behaviors in response to apomorphine than the sham-operated controls, while the PD14 rats had a profoundly diminished stereotypic response. The behavioral changes in the PD3 lesioned rats are reminiscent of those previously described in animals lesioned at PD7. In contrast, the deficits in the PD14 lesioned animals resemble those seen before in rats lesioned in adulthood. These results indicate that the pattern of impairments associated with the excitotoxic VH lesion varies with age at lesion, i.e. a similar pattern seems to be associated with lesions up to PD7, but not by PD14. To the extent that the neonatal VH lesion in the rat models certain phenomenological aspects of schizophrenia, including the temporal pattern of symptom onset, these results provide evidence that the model requires an early defect in limbic cortical development.

Morphological and biochemical adaptations to unilateral dopamine denervation of the neostriatum in newborn rats

Basal ganglia of adult rats were examined for morphological and biochemical changes resulting from neonatal unilateral dopamine denervation of the striatum with increasing doses of 6-hydroxydopamine (4, 12 and 20 microg). Rotational behaviour induced by apomorphine (0.1 mg/kg) was observed in all rats injected with the high dose (20 microg) and totally absent in those injected with the low dose (4 microg). As assessed with tyrosine hydroxylase immunocytochemistry, the extent of dopamine denervation within the injected striatum was clearly related to the dose injected. In the mesencephalon, losses of tyrosine hydroxylase-immunoreactive cell bodies were proportional to the dose injected and the extent of neostriatal dopamine denervation. This retrograde cell loss predominated in the ventromedial and lateral parts of the substantia nigra pars compacta, with relative sparing of the ventral tegmental area. After the injection of the intermediate (12 microg) and the high (20 microg) doses, a network of thin tyrosine hydroxylase-immunoreactive fibres was visualized in the ventral part of the pars reticulata ipsilateral to the injected striatum, suggesting a neoinnervation of this structure by dopamine axons. After the high dose, the density of serotonin-immunoreactive fibres was enhanced in the anterior half of the lesioned striatum. Associated changes in dopamine and serotonin content and turnover were also documented on both sides, in the striatum and in two output structures of the basal ganglia, the globus pallidus and the substantia nigra. Dopamine content was decreased only on the injected side. After the low dose, equal reductions (-60%) were observed in the anterior striatum and the substantia nigra, whereas a more marked decrease was measured in the anterior striatum (-93%) than in the substantia nigra (-60% to -74%) after the intermediate and high doses. In the globus pallidus, dopamine tissue content was decreased (-51%) only after the high dose. Dopamine turnover was unchanged after the low dose in all structures examined and was increased in the striatum, on the lesioned side only, after the intermediate and high doses. Serotonin content was increased only on the injected side in the anterior striatum (+50% after the low and +92% after the high dose). Serotonin turnover was unchanged on the injected side but increased by +118% and by +81% in the contralateral anterior striatum after the low and high doses, respectively. It was also increased in both substantia nigra after the high dose. In conclusion, morphological changes similar to those described after a bilateral neonatal lesion were observed on the injected side in the model of the unilateral neonatal nigrostriatal dopamine denervation. Biochemical changes were, however, not restricted to the lesioned side. Notably, changes in serotonin turnover developed on the contralateral side. These morphological and biochemical adaptative changes need to be taken into account in considering the mechanisms implicated in the rotional behaviour measured in these animals.

Persistence of long-lasting serotonin depletion by p-chloroamphetamine in rat brain after 6-hydroxydopamine lesioning of dopamine neurons

In rats that had been treated neonatally with 6-hydroxydopamine (6OHDA) to deplete striatal dopamine more than 95%, a single injection of p-chloroamphetamine (pCA) (5 or 10 mg/kg, i.p.) resulted in depletion of striatal and hippocampal serotonin at 1 week to a similar extent as in control rats. These findings suggest that striatal dopamine is not essential to the long-lasting depletion of brain serotonin by pCA in rats.

Evaluation of three transporter ligands as quantitative markers of serotonin innervation density in rat brain

Direct counting of axon terminals (varicosities) labeled by uptake/storage of a tritiated monoamine provides a means to test radioligands of the corresponding membrane transporter as quantitative markers of regional monoamine innervation density in brain tissue. In autoradiographs from alternate rat brain slices, counts of [3H]5-HT-labeled axon terminals were matched with densitometric measurements of the specific binding of tritiated cyanoimipramine (CYI), citalopram (CITAL), and 6-nitroquipazine (6-NTQ), under conditions of hypo-, normo-, or hyper-5-HT innervation of the neostriatum. A total of 267 pairs of data were subjected to a multilevel analysis (iterative generalized least square procedure). With all three ligands, there was a linear relationship between the density of 5-HT innervation and the density of specific binding and no change in the slope of the regression lines as a function of 5-HT innervation density. Thus, none of these ligands gave any sign of down- or up-regulation of the 5-HT transporter consequent to 5-HT hypo- or hyper-innervation. The regression lines for CYI and CITAL were not significantly different from one another and crossed the ordinate near zero, whereas the regression line for 6-NTQ was less steep and had a higher intercept with the ordinate. In addition, the dispersion of values around the regression line (residuals) was lower with CYI and CITAL than 6-NTQ. It was concluded that both CYI and CITAL may serve as quantitative markers of 5-HT innervation density, at least in vitro, whereas 6-NTQ demonstrates a certain lack of specificity and sensitivity. Further work will be needed to assess the potential of CYI and CITAL for positron emission tomographic studies of living brain. Such empirical testing should also be applicable for screening radioligands of the dopamine or the noradrenaline transporters.

Changes in steady-state levels of tryptophan hydroxylase protein in adult rat brain after neonatal 6-hydroxydopamine lesion

A recently developed technique of immunoautoradiography on nitrocellulose transfers of serial frozen sections was used to determine tryptophan hydroxylase concentration in selected areas of the adult rat brain following neonatal 6-hydroxydopamine destruction of nigrostriatal dopamine neurons. Particular attention was paid to the neostriatum, known to be serotonin-hyperinnervated under these conditions, and to the nucleus raphe dorsalis, containing the cell bodies of origin for these nerve terminals. The hippocampus was also investigated as a territory of structurally intact serotonin innervation arising primarily from the nucleus raphe medianus. Tryptophan hydroxylase protein was measured at successive transverse levels across the entire caudorostral extent of all these regions. Similar measurements of tyrosine hydroxylase protein across the substantia nigra and the neostriatum verified the disappearance of the nigrostriatal dopamine neurons. The average tryptophan hydroxylase tissue concentration in the dorsal third of the serotonin-hyperinnervated neostriatum was up by 36% above control, i.e. significantly less than the number of its serotonin axon terminals or varicosities. This was therefore indicative of a lowering of the tryptophan hydroxylase protein content per serotonin ending. Interestingly, a tight correlation between the respective level-by-level concentrations of tryptophan hydroxylase and tyrosine hydroxylase protein in the control neostriatum allowed the prediction the tryptophan hydroxylase concentration after dopamine denervation with a serotonin hyperinnervation. Tryptophan hydroxylase concentration was also significantly reduced in both the nucleus raphe dorsalis and nucleus raphe medianus, notably at those raphe dorsalis levels known to give rise to the serotonin hyperinnervation of neostriatum. It is hypothesized that the lower steady-state level of tryptophan hydroxylase inside the terminals and cell bodies of hyperinnervating serotonin neurons was the result of a feedback inhibition of the synthesis of the enzyme by its end-product, presumably because of the increased amount of serotonin in these terminals.

Quantification of the serotonin hyperinnervation in adult rat neostriatum after neonatal 6-hydroxydopamine lesion of nigral dopamine neurons

Light microscope autoradiography after uptake and storage of tritiated serotonin (5-HT) in brain slices was used to count 5-HT axon terminals (varicosities) in the 5-HT-hyperinnervated neostriatum of adult rats subjected to neonatal 6-hydroxydopamine treatment and age-matched, normal controls. After correction for incomplete autoradiographic exposure and for section thickness, the results were expressed in millions of varicosities per mm3 of tissue. Control values ranged from 4.8 in the rostral to 6.3 in the caudal neostriatum (5.8 at intermediate level), for an average of 5.6. The corresponding values in 5-HT-hyperinnervated tissue ranged from 9.7 to 7.7 (8.8 at intermediate level), for an average of 8.7 and increases of 102%, 52% and 22% above control in the rostral, intermediate and caudal neostriatum, respectively (average increase of 55%). These data confirmed the predilection of the 5-HT hyperinnervation for the rostral neostriatum and demonstrated its presence in the caudal neostriatum also.

Hypersensitivity to serotonin and its agonists in serotonin-hyperinnervated neostriatum after neonatal dopamine denervation

Neonatal destruction of the nigrostriatal dopamine projection by intraventricular 6-hydroxydopamine leads to a serotonin (5-hydroxytryptamine, 5-HT) hyperinnervation of the adult neostriatum accompanied by increased radioligand binding to 5-HT1B, 5-HT1nonAB and 5-HT2 receptors. The consequences of such 5-HT receptor changes on neuronal responsiveness to 5-HT and corresponding receptor agonists were assessed with a quantitative iontophoretic approach. For comparative purposes, similar data were also obtained from rats 6-hydroxydopamine lesioned as adults, showing severe neostriatal dopamine denervation but no 5-HT hyperinnervation. In controls, 5-HT and its receptor agonists, m-chlorophenylpiperazine (mCPP; 5-HT1B/2C agonist) and dimethoxy-iodophenyl-aminopropane (DOI; 5-HT2A/2C agonist), depressed the firing rate of a majority of the unit tested. Three months after neonatal 6-hydroxydopamine lesion (5-HT-hyperinnervated tissue), inhibitory responses to all three agents were significantly increased and comparable results were obtained for 5-HT and DOI in the rostral versus caudal neostriatum. After 6-hydroxydopamine lesion in adults, neither responsiveness to 5-HT, mCPP or DOI nor the density of 5-HT1B or 5-HT2A binding were significantly different from control. Thus, the up-regulation of 5-HT1B, 5-HT2A and possibly 5-HT2C receptors accompanying the 5-HT hyperinnervation after neonatal but not after adult dopamine denervation was associated with increased responsiveness (IT50) of neostriatal neurons to iontophoresed 5-HT and its receptor agonists. Under these conditions, neostriatal 5-HT transmission might be enhanced in spite of a basal release seemingly comparable to normal (Jackson and Abercrombie, 1992, J. Neurochem. 58, 890).(ABSTRACT TRUNCATED AT 250 WORDS)