Inhibition of striatal acetylcholine release by serotonin and dopamine after the intracerebral administration of 6-hydroxydopamine to neonatal rats

Enhancing the resolution of behavioral measures: Key observations during a forty year career in behavioral neuroscience

This manuscript reviews several key observations from the research program of Professor John P. Bruno that are believed to have significantly advanced our understanding of the brain's mediation of behavior. This review focuses on findings within several important research areas in behavioral neuroscience, including a) age-dependent neurobehavioral plasticity following brain damage; b) the role of the cortical cholinergic system in attentional processing and cognitive flexibility; and c) the design and validation of animal models of cognitive deficits in schizophrenia. In selecting these observations, emphasis was given to examples in which the heuristic potency was increased by maximizing the resolution and microanalysis of behavioral assays in the same fashion as one typically refines neuronal manipulations. Professor Bruno served the International Behavioral Neuroscience Society (IBNS) as an IBNS Fellow (1995-present) and President of the IBNS (2001-02).

It's Tea Time: Interference of Ayahuasca Brew on Discriminative Learning in Zebrafish

Ayahuasca is a psychoactive brew traditionally used in shamanistic and vegetalistic rituals and has recently received lot of attention due to potential cognitive benefits. Ayahuasca effects are caused by the synergistic interaction of β-carbolines (harmine, harmaline and tetrahydroarmine) contained in Banisteriopsis caapi stalks combined with the N,N-dimethyltryptamine (DMT) from Psychotria viridis leaves, a potent agonist to serotonin (5-HT) receptors. The present study approaches the effects of chronic and acute exposure to two Ayahuasca concentrations (0.1 and 0.5 ml/L) on the cognitive ability to discriminate objects in a one-trial learning task in zebrafish. Based on the combination of concentrations and exposure regimens, we divided adult zebrafish in five treatment groups: acute 0.1 and 0.5 ml/L, chronic 0.1 and 0.5 ml/L, and control 0.0 (n = 20 for each group). Then we tested them in a memory task of object discrimination. Acute Ayahuasca exposed groups performed similarly to the control group, however chronically treated fish (13 days) presented both impaired discriminative performance and locomotor alterations. Overall, these results indicate that Ayahuasca is a potent psychoactive drug that, in chronic exposure, negatively affects mnemonic parameters in zebrafish. In single exposure it does not affects cognitive performance, but the higher concentration (0.5) affected locomotion. Moreover, we reinforce the importance of the zebrafish for behavioral pharmacological studies of drug screening, in special to psychedelic drug research.

Behavioral Changes Over Time Following Ayahuasca Exposure in Zebrafish

The combined infusion of Banisteriopsis caapi stem and Psychotria viridis leaves, known as ayahuasca, has been used for centuries by indigenous tribes. The infusion is rich in N, N-dimethyltryptamine (DMT) and monoamine oxidase inhibitors, with properties similar to those of serotonin. Despite substantial progress in the development of new drugs to treat anxiety and depression, current treatments have several limitations. Alternative drugs, such as ayahuasca, may shed light on these disorders. Here, we present time-course behavioral changes induced by ayahuasca in zebrafish, as first step toward establishing an ideal concentration for pre-clinical evaluations. We exposed adult zebrafish to five concentrations of the ayahuasca infusion: 0 (control), 0.1, 0.5, 1, and 3 ml/L (n = 14 each group), and behavior was recorded for 60 min. We evaluated swimming speed, distance traveled, freezing and bottom dwelling every min for 60 min. Swimming speed and distance traveled decreased with an increase in ayahuasca concentration while freezing increased with 1 and 3 ml/L. Bottom dwelling increased with 1 and 3 ml/L, but declined with 0.1 ml/L. Our data suggest that small amounts of ayahuasca do not affect locomotion and reduce anxiety-like behavior in zebrafish, while increased doses of the drug lead to crescent anxiogenic effects. We conclude that the temporal analysis of zebrafish behavior is a sensitive method for the study of ayahuasca-induced functional changes in the vertebrate brain.

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.

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.

Effect of unilateral and bilateral lesions of caudate-putamen on immune response in the rat

In the present study we investigated the effect of selectivity destroyed dopaminergic neurons of the caudate-putamen (CP) on immune reactivity in the rat. Unilateral and bilateral lesioning of CP was performed by one direct stereotaxic injection of 6-hydroxydopamine solution. Sham-lesioned and intact rats served as controls. Two weeks after the operation, the animals were immunized with sheep red blood cells or bovine serum albumin for determination of plaque forming cell-response and Arthus and delayed hypersensitivity skin reactions, respectively. Unilateral and bilateral lesions of CP considerably suppressed PFC-response and Arthus and delayed the hypersensitivity reactions in comparison to control rats, while no differences were observed between unilaterally and bilaterally lesioned animals.

Dopamine depletion abolishes apomorphine- and amphetamine-induced increases in extracellular serotonin levels in the striatum of conscious rats: a microdialysis study

We investigated how serotonergic neurotransmission was affected by 6-hydroxydopamine (6-OHDA) lesioning of the adult rat brain dopamine (DA) system. In this animal model for Parkinson's disease (PD), the effect of destroying ascending DA pathways on extracellular levels of serotonin (5-HT) and 5-HT innervation in rat striatum were examined. Profound unilateral lesions of the nigrostriatal DA pathways were made by infusing 6-OHDA unilaterally into either the right medial forebrain bundle or the right substantia nigra. At 5 weeks after lesioning extracellular levels of DA and 5-HT were determined with microdialysis and high-pressure liquid chromatography under basal conditions and after systemic injections of apomorphine or amphetamine. DA nerve-terminal destruction and 5-HT innervation were determined with quantitative autoradiography. 6-OHDA lesioning reduced extracellular levels of DA below detection limits and led to statistically significant increases in extracellular 5-HT. Apomorphine, and amphetamine, respectively increased extracellular 5-HT to 8.2- and 2.2-fold above baseline levels in intact animals; these effects were absent in 6-OHDA-lesioned animals. Basal levels of [(3)H]paroxetine binding to 5-HT transporters in caudate-putamen increased by 41% in 6-OHDA-lesioned animals. These results suggest that 6-OHDA lesioning led to hyperinnervation of 5-HT nerve terminals and increases in basal extracellular 5-HT levels, but also to an unexplained loss of apomorphine and amphetamine-induced release of 5-HT. Addressing whether this impairment has significance in the onset of PD might lead to development of new strategies to manage parkinsonian symptoms.

The onset of dopaminergic innervation during ontogeny decreases melanotrope proliferation in the intermediate lobe of the rat pituitary

The onset of dopaminergic innervation and its effects on melanotrope proliferation were investigated in the rat pituitary intermediate lobe. Dopamine, and its synthetic rate-limiting enzyme tyrosine hydroxylase, were first detected immunohistochemically on late post-natal day 3 or early postnatal day 4. Axon density was highest at the neural lobe/intermediate lobe border, and decreased toward the pituitary cleft. By postnatal day 10, the adult pattern of tyrosine hydroxylase immunoreactivity was established and remained through post-natal day 14. Neurointermediate lobe dopamine levels, measured by HPLC, correlated well with the increased axon density observed in the immunohistochemical studies. Dopamine could not be measured by our assay (100 fg limit) until post-natal day 3 (439.32 fg/NIL). Dopamine concentration increased to 2.09 +/- 0.425 ng at PN 4, 86.31 +/- 20.42 ng at PN 7, 168.72 +/- 18.37 ng at PN 10. Melanotrope proliferation was determined by [3H]thymidine incorporation before and after innervation. Concomitant with the onset of innervation, the proliferation index dropped from 13.4 +/- 0.01% to 6.5 +/- 0.002% at PN 4, and continued to decrease until a level of 3 +/- 0.003% was established by PN 10. To confirm the inhibitory action of dopaminergic innervation on melanotrope proliferation, rat neonates were injected intracisternally with 150 mg 6-hydroxydopamine to destroy dopaminergic axons within the intermediate lobe. Measurement of dopamine concentrations in neurointermediate lobes of injected animals showed a decrease in dopamine levels as compared to controls. From PN 4 (0.88 +/- 0.165 ng), DA levels gradually increased during development: at PN 5, [DA] = 0.689 +/- 0.104 ng; PN 6 [DA] = 11.60 +/- 2.24 ng; PN 7 [DA] = 20.93 +/- 3.80 ng; and PN 10 [DA] = 27.95 +/- 3.46 ng. Melanotrope proliferation also increased in 6-hydroxydopamine-treated animals. At PN 4, the onset of innervation reduced the pre-innervation proliferation index to 8.75 +/- 0.002%, only a 30% reduction in contrast to the greater than 50% decrease observed in control animals. A stable proliferation level of approximately 7.5% persisted in all subsequent stages with 6-OHDA administration. Our results demonstrated the time of dopamine innervation onset and a characteristic developmental pattern for axons within the rat intermediate lobe. The onset of innervation and increased dopamine concentration suggests increased dopaminergic control of the melanotropes, illustrated specifically by a decrease in their level of proliferation. This is the first presentation of evidence showing that dopaminergic innervation within the intermediate lobe of the rat pituitary regulates melanotrope proliferation.

7-Hydroxy-N,N'-di-n-propyl-2-aminotetraline, a preferential dopamine D3 agonist, induces c-fos mRNA expression in the rat cerebellum

The effects of a preferential dopamine D3 receptor agonist 7-hydroxy-N,N'-di-n-propyl-2-aminotetralin (7-OH-DPAT) on c-fos mRNA expression in the rat cerebellum were studied by Northern blot analysis. 7-OH-DPAT (0.003-10 mg/kg, s.c.) markedly increased c-fos mRNA expression in the cerebellum, while its effects in the striatum, nucleus accumbens, and frontal cortex were negligible. The effect of 7-OH-DPAT on cerebellar c-fos mRNA expression was dose-dependent and statistically significant at doses of 0.3 mg/kg or more. A preferential dopamine D2 agonist, bromocriptine (0.01-3 mg/kg, s.c.), failed to increase c-fos mRNA expression in the cerebellum. The effect of 7-OH-DPAT was blocked by two dopamine D2-type-receptor antagonists, haloperidol and perospirone, but not the D1-type-receptor antagonist SCH23390. Furthermore, dopaminergic denervation by 6-hydroxydopamine did not inhibit but rather potentiated the 7-OH-DPAT-induced c-fos mRNA expression in the cerebellum. These findings suggest that 7-OH-DPAT increases c-fos mRNA expression in the rat cerebellum, probably through postsynaptic dopamine D3 receptor activation.

Differences in behavior and monoamine laterality following neonatal clomipramine treatment

Postnatal treatment between 8 to 21 days of age with clomipramine (15 mg/kg, twice daily) produces an animal model that has many of the behavioral hallmarks of depression. In this study, we investigated the enduring behavioral and neurochemical effects of this early treatment in adult animals. Locomotor activity was increased in clomipramine-treated males, but not females, relative to vehicle-treated subjects. Increases in anxiety-like behavior in the elevated plus maze also were observed in clomipramine-exposed adults, but no sex differences were detected. Clomipramine-treated animals had shifts in the laterality of monoamines in limbic regions with lower serotonin levels on the right side while vehicle-treated animals had lower serotonin on the left side. The lateralization of dopamine content demonstrated the same pattern. This decline in monoaminergic content is consistent with clinical studies demonstrating decrements in serotonin as well as alterations in the lateralization of function in individuals with major depressive order.

Developmental neurobiology: implications for pediatric psychopharmacology

Pediatric psychopharmacology can only be properly understood within the context of developmental neurobiology. Normal maturation of the brain is characterized by the complex anatomic, molecular, and organizational changes required for optimal adaptive functioning. In many areas of the developing nervous system, neurons, cell processes, neurotransmitters, and trophic nerve growth factors are produced in far greater numbers and in different areas than in the mature brain. Psychopathology and stressful environments can lead to enduring changes in brain structure and functioning. A few select psychopharmacological treatments are reviewed to assess their potential impact on CNS development. More clinical and basic science studies are needed to determine whether current treatments have neurotoxic or neuroprotective effects when administered over extended critical early periods of the lifespan.

MDMA ('ecstasy') enhances basal acetylcholine release in brain slices of the rat striatum

The pharmacological basis of acute (+/-)-MDMA (3, 4-methylenedioxymethamphetamine) intoxication still awaits full characterization. According to present knowledge, MDMA enhances the release of serotonin and dopamine in striatal slices and interacts with different types of receptors such as 5-HT2 (5-hydroxytryptamine or serotonin), M1 and M2 muscarinic acetylcholine (ACh), and histamine H1 receptors. Currently, no information is available about the influence of (+/-)-MDMA on striatal cholinergic neurotransmission. In the present study, we used the in vitro perfusion technique to investigate the effect of (+/-)-MDMA on ACh release in rat striatal slices. Perfusions with (+/-)-MDMA (10-300 microM) resulted in a dose-dependent increase of spontaneous ACh release (EC50 approximately 30 microM). The effect was reversible and Ca++- and tetrodotoxin-sensitive. To determine the neurochemical pathways underlying this response, we perfused with (+/-)-MDMA in the presence of various inhibitors of neurotransmitter receptors. Blockade of glutamate or muscarinic ACh receptors as well as 5-HT1, 5-HT2, 5-HT3C or dopamine D2 receptors did not modulate (+/-)-MDMA-induced ACh release. However, the presence of histamine H1 receptor antagonists in the perfusion medium abolished (+/-)-MDMA-induced ACh release. The present data clearly demonstrate that (+/-)-MDMA enhances the activity of striatal cholinergic neurons and suggest an involvement of histamine H1 receptors. The effect is not mediated by glutamate and does not involve the activation of receptors of dopamine D2, 5-HT1, 5-HT2, 5-HT3C or muscarinic ACh. Considering the relatively high affinity of (+/-)-MDMA for the H1 histamine receptor (Ki 6 microM), a direct activation of this type of receptor might represent a plausible mechanism for (+/-)-MDMA-induced ACh release.

Prenatal cocaine exposure alters behavioral and neurochemical sensitization to amphetamine in adult rats

This study examined the neurochemical correlates of amphetamine (AMPH)-induced behavioral effects in prenatally saline (PSAL)-exposed or cocaine (PCOC)-exposed male rats. Pregnant Long-Evans rats received saline or saline containing cocaine hydrochloride (20 mg/kg s.c., b.i.d.) from gestational days 15-21. Animals were left with their biological mothers. Adult offspring were exposed to daily saline or AMPH (0.5, 1.5, or 5 mg/kg, i.p.) injections for 7 days. Behaviors were recorded in an open field during the first hour post-injection. PCOC rats did not exhibit behavioral anomalies during habituation to injection-stress or placement in the open field. PCOC rats displayed significant alterations in stereotyped responses to acute or intermittent exposure to various doses of AMPH. Within 48 h of the final testing day, striatal tissue was obtained from these animals and electrically-evoked [3H]acetylcholine (ACh) release was measured from striatal slices. Superfusion of tissue slices with various concentrations of AMPH (1-1000 nM) produced dose-dependent inhibition of ACh release in both PSAL and PCOC rats repeatedly injected with saline as adults. However, AMPH-induced inhibition of ACh release was decreased in PCOC rats repeatedly injected with AMPH as adults. At 5 mg/kg AMPH, PCOC rats exhibited increased mortality compared to PSAL rats. These data suggest that PCOC exposure produces long-lasting alterations in nigrostriatal transmission and behaviors mediated by this system.

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.

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.

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.

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)

Amphetamine induces excess release of striatal acetylcholine in vivo that is independent of nigrostriatal dopamine

The effect of amphetamine on striatal acetylcholine (ACh) release was studied by an in vivo intrastriatal microdialysis technique. Although we expected systemic amphetamine to inhibit baseline striatal ACh release, the opposite was found. In addition, we found that the amphetamine-induced striatal ACh release did not depend on nigrostriatal DA since 6-hydroxydopamine (6-OHDA) lesions had no effect on amphetamine-induced ACh release. Local intrastriatal injection of amphetamine via the microdialysis probe had no effect on striatal ACh release even when the probe was located more laterally in striatum to take advantage of the medial to lateral gradient of striatal ACh and D2 receptors. The hypothesis that amphetamine increased extracellular striatal ACh by increasing the release of biogenic amines besides dopamine was tested by pharmacological manipulations designed to specifically increase local striatal norepinephrine or serotonin levels. The serotonergic and noradrenergic manipulations had no effect on striatal ACh levels. These results indicate that amphetamine-induced release of ACh in striatum is mediated via distal brain regions that are functionally connected with the striatum.

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

An aberrant network of dopamine axons was found to pervade the rat substantia nigra following neonatal destruction of its dopamine nerve cell bodies and dendrites by cerebroventricular administration of 6-hydroxydopamine. Light-microscopic immunocytochemistry with a primary monoclonal antibody directed against dopamine-glutaraldehyde-protein was used to investigate the time-course of development and the critical period of induction of this ectopic dopamine innervation (neoinnervation). In rats 6-hydroxydopamine-lesioned at postnatal day 3 (P3) and examined at P7, P10, P15, P30 or later, some dopamine fibers were already present in the substantia nigra at P7; their number increased sharply until P15 and only slightly thereafter, assuming a topographic distribution reminiscent of the missing dopamine nerve cell bodies and dendrites. A similar growth of dopamine fibers took place in the substantia nigra after lesions made at P6, P9 and P12, but was less pronounced after lesion at P15 and absent after lesion at P21 or later. Excessive innervation by dopamine axons (hyperinnervation) was concomitantly observed in the nearby interpeduncular nucleus. The sprouting of dopamine axons in both regions was therefore rapid and coincided in time and space with the developmental redistribution of mesencephalic dopamine neurons in normal rat. It is conceivable that these aberrant dopamine innervations play a role in the peculiar behavior and responsiveness to dopaminergic agents manifested by neonatally 6-hydroxydopamine-lesioned rats. It will be of particular interest to investigate the functional consequences of the dopamine neoinnervation in the substantia nigra, where an eventual axonal release might thus be replacing the normal somatodendritic release of this amine.

Heterotypic sprouting of serotonergic forebrain fibers in the brindled mottled mutant mouse

The brindled mottled mouse has a mutation on the X-chromosome which causes alterations in copper metabolism. One role for copper is as a cofactor for dopamine-beta-hydroxylase (DBH), the enzyme that converts dopamine to norepinephrine (NE). This may explain the fact that the hemizygous males have low concentrations of NE, as well as high concentrations of 5-hydroxyindoleacetic acid (5-HIAA) in the brainstem and forebrain. The present study quantified serotonin (5-HT) immunoreactive fibers in the cerebral cortex and striatum of hemizygous males and control littermates on postnatal (P) days 7, 10, 12 and 14. The density of 5-HT immunoreactive fibers was measured using a digitized imaging system in conjunction with darkfield microscopy. Measurements of 5-HT innervation showed an age-dependent increase in density of 5-HT immunoreactive fibers in all layers of the cerebral cortex, with fiber density in brindled mice approximately 70% greater than controls by P14. High performance liquid chromatography confirmed the increased concentrations of 5-HT and 5-HIAA, and the low concentration of NE, in several regions. We believe that these results are an example of heterotypic sprouting of 5-HT neurons, similar to that observed in neonatal rats given 6-hydroxydopamine (6-OHDA). If so, these data provide the first description of 5-HT heterotypic sprouting in mice, and the first description of 5-HT heterotypic sprouting resulting from a natural disease state, rather than an experimentally induced lesion.

Gene expression in central cholinergic neurons in response to axotomy and deafferentation

Although the molecular and cellular responses to injury in the central nervous system (CNS) have been widely investigated, few studies have examined the potential variations between direct and indirect neuronal injury. To differentiate between the response to axotomy and deafferentation, two central cholinergic populations were analyzed: the horizontal limb of the diagonal band of Broca (HLDB) and the interneurons in the corpus striatum (CS). At time points from one hour to eight weeks postinjury the levels of choline acetyltransferase (ChAT) mRNA and protein were assessed by in situ hybridization and immunohistochemistry. Also examined was the expression of the immediate early gene product, c-fos. One week post axotomy, neurons in the HLDB exhibited an increase in the levels of ChAT mRNA without a concomitant increase in ChAT protein, followed by a steady decrease reaching a nadir in both parameters at eight weeks. In contrast, a transient increase occurred at one week postdeafferentation in the levels of both ChAT mRNA and protein in the interneurons of the CS. Axotomized neurons in the HLDB did not exhibit either c-fos mRNA or protein expression, while robust fos induction occurred after one hour in deafferented neurons in the CS. These data demonstrate that the molecular and cellular responses differ following direct and indirect neuronal injury. Furthermore, they suggest that in these central cholinergic populations deafferentation may result in cellular hyperactivity and cell survival while axotomy results in decreased cellular activity and subsequent cellular regression.

Acute versus chronic haloperidol: relationship between tolerance to catalepsy and striatal and accumbens dopamine, GABA and acetylcholine release

Using in vivo microdialysis, changes in extracellular dorsolateral striatum and nucleus accumbens dopamine, GABA and acetylcholine following acute and chronic haloperidol (0.25 mg/kg, s.c.) were evaluated in rats concurrent with the measurement of catalepsy. When administered to drug-naive and chronically treated rats, haloperidol was associated with a consistent and prolonged (> 150 min) increase in dorsolateral striatum and nucleus accumbens DA release and a transient (60 min) increase in dorsolateral striatum GABA release. Haloperidol was also associated with a transient (30 min) increase in dorsolateral striatum acetylcholine release in the chronically treated rats. Basal dopamine and acetylcholine levels were similar in both brain regions; however, basal dorsolateral striatum GABA levels were two-fold higher in the chronically treated rats. Administration of haloperidol was associated with a prolonged (> 150 min) catalepsy in the drug-naive rats which was greatly diminished or absent in chronically treated rats. Additionally, serum haloperidol levels were shown to be similar 120 min following administration of haloperidol in both groups. These results indicate a marked behavioral difference in the effects of haloperidol in drug-naive and chronically treated rats which is not related to an altered bioavailability of the drug and which is dissociated from both basal and haloperidol induced effects on dopamine and acetylcholine release in both brain regions. However, the selective elevation of basal dorsolateral striatum GABA release following chronic administration of haloperidol may contribute to the development of tolerance to catalepsy as well as providing an in vivo neurochemical marker of the long-term effects of haloperidol.

D1 and D2 dopamine receptors do not up-regulate in response to neonatal intrastriatal 6-hydroxydopamine lesions

The extent of dopamine (DA) depletion appears to exert important influences on the plasticity of the DA system following lesions made in adulthood. The aim of this study was to determine if the extent of DA depletion has long-term effects on DA receptor regulation after early neonatal lesions. Early intrastriatal injections of 6-hydroxydopamine (6-OHDA) caused a dose-dependent loss of high-affinity DA uptake sites and mu-opioid receptor patches evident in adulthood. DA receptors did not up-regulate in response to any degree of neonatal DA depletion. A patchy loss of D1 binding was evident following the neonatal lesions, although the loss was somewhat more severe and uniform with the highest dose of 6-OHDA (20 micrograms per striatum). There was also a slight decrease in D2 binding which was not dose-dependent. These results suggest that the consequences of early neonatal DA lesions are not dependent upon the degree of DA depletion, as the effects on DA receptor expression were similar regardless of the extent of the lesions.

Changes of amino acid and monoamine levels after neonatal 6-hydroxydopamine denervation in rat basal ganglia, substantia nigra, and Raphe nuclei

The effects of a neonatal dopaminergic deafferentation with the neurotoxin 6-hydroxydopamine (6-OHDA) on endogenous tissue levels of catecholamines, indoleamines, and amino acids were investigated in discrete rat brain regions. After producing the lesion at postnatal day 3 by intraventricular injections of 6-OHDA, with a desipramine pretreatment to protect noradrenaline neurons, the animals were kept for 3 months. Their brains were dissected to obtain samples of neostriatum, Globus pallidus, Substantia nigra, and Raphe nuclei, which were then analyzed by high-performance liquid chromatography, coupled either to electrochemical detection for aromatic monoamines, or to post-column ninhydrin derivatization with spectrophotometry for amino acids. The neonatal 6-OHDA treatment depleted dopamine (DA) levels in neostriatum, Globus pallidus, and Substantia nigra, but in Raphe nuclei DA was increased. The main metabolites of DA were also decreased in neostriatum, Globus pallidus, and Substantia nigra but remained unchanged in Raphe nuclei. Serotonin (5-HT) and its metabolite 5-hydroxy-indole-3-acetic acid increased in neostriatum and Raphe nuclei; in Substantia nigra there was a slight increase in 5-HT only. The 6-OHDA lesion caused heterogeneous alterations in amino acid contents, which varied according to the region. In the neostriatum there were increases of gamma-aminobutyric acid (GABA), aspartic acid, and glycine. In the Globus pallidus taurine, GABA, glutamic acid, glutamine, aspartic acid, serine, and alanine were elevated. In the Substantia nigra only increases in taurine, GABA, glutamic acid, and glutamine could be documented. This study shows important changes in amino acid levels and in some of their ratios, occurring in different anatomical subdivisions of the basal ganglia and related brainstem nuclei following a neonatal treatment with 6-OHDA. The results thus demonstrate major biochemical modifications in amino acids in the aftermath of a DA denervation and/or a 5-HT hyperinnervation during an early developmental period.

Effect of neurotensin and immunneutralization with anti-neurotensin-serum on dopaminergic-cholinergic interaction in the striatum

The effect of neurotensin (NT) on the release of acetylcholine (ACh) and dopamine (DA) from striatal slices of the rat brain was studied. Neurotensin, 1-150 nM, was able to release ACh from cholinergic interneurons of the striatum. Like the response to electrical stimulation, the ACh-releasing effect of NT was completely inhibited by tetrodotoxin indicating that neuronal firing is involved in its effect. Immunneutralization reduced the stimulation-evoked release of ACh, an effect that was much marked when the inhibitory dopaminergic input was suspended by sulpiride-selective antagonists of D2 receptors. Sulpiride, 0.1 mM, induced a 2-fold increase in the NT- and electrically-induced release of ACh. A quantitatively similar increase was also observed after degeneration of the nigrostriatal DA pathway with 6-hydroxydopamine (6-OHDA) (2 x 250 micrograms/animal, i.c.v.). However, the D2 receptor agonist quinpirole, 0.01 mM, significantly reduced the NT-induced release of ACh by 77%. Neurotensin enhanced the stimulation-evoked release of [3H]DA. These findings indicate that, using field stimulation when dopaminergic, cholinergic and NT-containing neurons are stimulated in concert, NT is capable of releasing both ACh and DA in the striatum, but its effect on ACh release is masked unless the D2 receptor-mediated tonic inhibitory effect of DA released from the nigro-striatal pathway is attenuated. Thus, in Parkinson's disease where the dopaminergic input is impaired, NT may be involved in producing cholinergic dominance.

Serotonin 5-HT1 and 5-HT2 receptors in adult rat brain after neonatal destruction of nigrostriatal dopamine neurons: a quantitative autoradiographic study

Neonatal destruction of nigrostriatal dopamine neurons by cerebroventricular injection of 6-hydroxydopamine (6-OHDA) results in a serotonin (5-HT) hyperinnervation of the rostral neostriatum in adult rat. Quantitative ligand-binding autoradiography was used to compare the density of various 5-HT receptor subtypes in the adult brain of control and neonatally 6-OHDA-lesioned rats. 5-HT1A, 5-HT1B, 5HT1nonAB and 5-HT2 sites were labeled with [3H]8-OH-DPAT, [125I]cyanopindolol, [3H]5-HT and [125I]DOI, respectively, and measured in the rostral and caudal halves of neostriatum and selected forebrain or midbrain regions. 5-HT1A binding, measured after 6 months, was unchanged in all regions examined including the dorsal raphe nucleus. Three months after the lesion, 5-HT1B binding was increased throughout the neostriatum (30%), but also in the substantia nigra (50%) and globus pallidus (30%), suggesting an up-regulation and an increased axonal transport of these receptors in neostriatal projection neurons. 5-HT1nonAB binding was also increased throughout the neostriatum (40%) and in the substantia nigra (50%), but unchanged in the globus pallidus, as if this up-regulation preferentially involved striatonigral as opposed to striatopallidal neurons. 5-HT2 binding showed an even greater increase (60%), which was restricted to the rostral half of neostriatum and also seemed imputable to an up-regulation as heteroreceptors. Even though the exact cause(s) of these receptor increases could not be determined, their anatomical distribution suggested that they were somehow related to the initial dopamine denervation in the case of the 5-HT1B and 5-HT1nonAB receptors, and more tightly linked to the 5-HT hyperinnervation in the case of the 5-HT2 receptors. Such receptor changes could participate in adaptive mechanisms implicating other transmitters and behavioral disturbances observed in this particular experimental model. Interestingly, they could also account for an enhancement of neostriatal 5-HT function even in a condition where extracellular levels of 5-HT apparently remain normal because of increased uptake.

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.

Unilateral dopamine lesions in neonatal, weanling and adult rats: comparison of rotation and reaching deficits

The aim of this study was to compare the functional effects of neonatal, weanling and adult lesions of the dopaminergic (DA) mesencephalic neurones on paw-reaching behaviour. The mesotelencephalic DA pathway was destroyed unilaterally in neonatal (3 and 7 day), weanling (21 day) and adult (2 months) rats by local injection of 6-hydroxydopamine into the medial forebrain bundle at the level of the lateral hypothalamus, followed by behavioural studies conducted 2 months later. Amphetamine and apomorphine induced similar rates of rotation irrespective of the age of the lesion. By contrast skilled reaching with the contralateral paw was profoundly disrupted by lesions made in adult or weanling rats, but a much reduced deficit was observed in neonatally lesioned rats. Tyrosine hydroxylase immunohistochemistry indicated a similar degree of dopamine cell loss from the substantia nigra in all groups. These observations suggest that the host brain undergoes developmental changes 1-3 weeks postnatally that influence the long-term effects of lesions in the nigrostriatal dopamine system.

Amphetamine-induced changes in behavior and caudate extracellular acetylcholine

In vivo microdialysis was used to study the effects of amphetamine on caudate extracellular acetylcholine and to compare these effects to the drug-induced behavioral response profile. Consistent with an inhibitory dopamine/acetylcholine interaction, the dopamine receptor agonist, apomorphine, decreased acetylcholine concentrations, while the dopamine receptor antagonist, haloperidol, increased acetylcholine. In contrast, an intermediate dose of amphetamine (1.75 mg/kg), did not significantly alter acetylcholine levels. Furthermore, a higher dose of amphetamine (5.0 mg/kg) promoted a two-fold increase in acetylcholine levels, and the increase paralleled the appearance of oral stereotypies in these animals. These results suggest that the effects of amphetamine on caudate acetylcholine, which may be implicated in the appearance of stereotyped behaviors, are not strictly dependent on caudate dopamine receptor activation.

Supersensitized oral responses to a serotonin agonist in neonatal 6-OHDA-treated rats

Neonatal 6-hydroxydopamine (6-OHDA) treatment of rats is associated with supersensitization of the dopamine D1 agonist induction of oral activity. The present study was conducted to determine whether induced oral responses to serotonin (5-HT) agonists would be similarly altered in this rat model. At 3 days after birth, rats received desipramine HCl (20 mg/kg, IP) 1 h before 6-OHDA HBr (100 micrograms in each lateral ventricle) or saline-ascorbic acid (0.1%) vehicle. At approximately 9 mo, rats were challenged with the mixed 5-HT1C and 5-HT2 receptor agonist, m-chlorophenylpiperazine diHCl (m-CPP 2HCl; 0.30-6.0 mg/kg, IP) and were then observed for 1 min every 10 min over a 60-min period. m-CPP induced oral activity in both the vehicle and 6-OHDA groups, with the responses of the 6-OHDA group being much greater. An m-CPP dose of 3.0 mg/kg produced a maximal response of 63.6 +/- 3.2 oral movements in the 6-OHDA group. A bell-shaped response curve was obtained, with lower and higher doses of m-CPP producing less of an effect. Attenuation of the m-CPP-induced response by the 5-HT receptor antagonist, mianserin HCl (1.0 mg/kg, IP, 30 min before m-CPP), indicates that the m-CPP effect is receptor mediated. These findings demonstrate that neonatal 6-OHDA treatment produces ontogenic long-lived supersensitization of a 5-HT receptor system in rats.

In Vivo Neurochemical Evaluation of Striatal Serotonergic Hyperinnervation in Rats Depleted of Dopamine at Infancy

Destruction of nigrostriatal dopamine (DA) neurons with 6-hydroxydopamine (6-OHDA) early in development results in hyperinnervation of striatum by the serotonergic afferents deriving from the dorsal raphe nucleus. We have used in vivo microdialysis to investigate the degree to which serotonergic neurotransmission in striatum is altered by this increase in the density of serotonin (5-HT) terminals. The effects of several manipulations known to influence 5-HT function on extracellular 5-HT and 5-hydroxyindoleacetic acid in striatum were compared in adult rats treated neonatally with 6-OHDA and in intact adult rats. Basal levels of 5-HT in extracellular fluid (ECF) of striatum were similar in neonatally DA-depleted rats and in intact rats. Perfusion with the 5-HT reuptake blocker, fluoxetine (100 microM), increased 5-HT in striatal ECF of neonatally DA-depleted rats to levels that were threefold greater than those achieved in intact rats. Likewise, K(+)-depolarization of the 5-HT terminals (100 mM in perfusate) or systemic administration of the 5-HT releaser, (+/-)-fenfluramine (10 mg/kg i.p.), increased the concentration of 5-HT in striatal ECF of neonatally DA-depleted rats to levels approximately threefold greater than those observed in striatum of intact rats. These findings indicate that the 5-HT hyperinnervation of striatum that takes place in rats depleted of DA at infancy is associated with an increased capacity for neurotransmitter release in this system. Concomitant increased in high-affinity 5-HT uptake may prevent the occurrence of any measurable changes in the resting concentration of 5-HT in striatal ECF.

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.

Cholinergic and GABAergic mediations of the effects of apomorphine on serotonin neurons

Apomorphine (APO) has been shown to elevate tryptophan, serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in the dorsal raphe (DR) and its corresponding projection site, the striatum, but not in the median raphe (MR) and its terminal area, the hippocampus. We have previously demonstrated that these effects are indirectly mediated through dopamine (DA) autoreceptors in the substantia nigra and possibly gamma-aminobutyric acid (GABA) neurons in or near the DR. In the present study, we have further found that the effects of APO on 5-HT neurons are also mediated through both nicotinic and M1 muscarinic cholinergic receptors as well as GABAA receptors in the DR. This suggestion is based on the findings that both atropine and mecamylamine antagonized the effects of APO, while carbachol at a high dose exerted an effect opposite to that of APO. Besides, pirenzepine and bicuculline at low doses also antagonized, whereas saclofen did not alter the influence of APO on 5-HT in the striatum. Bicuculline at a higher dose enhanced tryptophan and 5-HT measures by itself. None of the drugs studied had a significant effect on tryptophan, 5-HT, or 5-HIAA in the hippocampus. These results together suggest that DA, ACh, and GABA neurons are all involved in the actions of APO on 5-HT, while the direct synaptic relationships among these neurotransmitters and the precise anatomical locus for these interactions to occur are still unknown. It is possible that APO, by inhibiting DA neuron firing in the substantia nigra and through the GABA disinhibition mechanism, therefore indirectly activates 5-HT neurons in the DR and the striatum. While the above neuronal firing model well explains the elevation of 5-HIAA, the simultaneous increases of tryptophan and 5-HT, especially tryptophan, may be more readily explained by a mechanism of tryptophan uptake upon APO administration. Further anatomical, biochemical, and electrophysiological studies are ongoing to test this hypothesis and to clarify the circuit and the anatomical locus (loci) for these interactions to occur.

Effects of p-chloroamphetamine on release of [3H]gamma-aminobutyric acid from slices of rat caudate-putamen

The effect of endogenous serotonin on the release of [3H]gamma-aminobutyric acid ([3H]GABA) from slices of rat caudate-putamen was studied. p-Chloroamphetamine was used to release endogenous serotonin. p-Chloroamphetamine (100 nM) enhanced the release of [3H]GABA induced by 20 mM K+, while 1000 nM p-chloroamphetamine decreased it. The 5-HT3 receptor antagonists ICS 205-930 (50 nM) and MDL 72222 (100 nM) prevented this facilitation caused by 100 nM p-chloroamphetamine. ICS 205-903 (50 nM), when used alone, reduced the release of [3H]GABA caused by 23 mM K+. This finding confirmed the hypothesis that endogenous serotonin can enhance the release of [3H]GABA via 5-HT3 receptors. In contrast, an effect of 5-HT1 and 5-HT2 receptors could not be clearly established. It is likely that the release of endogenous GABA from striatonigral GABA neurons may also be affected by serotonin via 5-HT3 receptors.

Downregulation of muscarinic receptors in the rat caudate-putamen after lesioning of the ipsilateral nigrostriatal dopamine pathway with 6-hydroxydopamine (6-OHDA): normalization by fetal mesencephalic transplants

The autoradiographic distribution and density of muscarinic receptors was studied in the neostriatum of rats with long-term unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopaminergic pathway and in lesioned rats who had additionally received embryonic substantia nigra grafts in the dopamine denervated striatum. Muscarinic receptors were labeled with [3H]quinuclidinyl benzilate (QNB), M1 receptors were directly labeled with [3H]pirenzepine (PZ) and non-M1 receptors were labeled by the competition of 100 nM PZ with [3H]QNB. The density and distribution of muscarinic receptors were directly compared to the sodium-dependent, high-affinity, choline uptake sites as labeled with [3H]hemicholinium-3 (HC-3). In the 6-OHDA-lesioned animals, there was a 25% reduction in muscarinic receptors labeled with [3H]QNB. Subtype analysis showed that there was a reduction of both M1 (-26%) and non-M1 (-33%) receptors. A normal density of both muscarinic receptor populations was found in animals with successful transplants. Saturation analysis demonstrated that the changes, in muscarinic receptor density, were due to a change in receptor number (Bmax) and not affinity (Kd). There was no significant change in [3H]HC-3 binding in the 6-OHDA-lesioned or transplanted animals, indicating that alterations in muscarinic receptors were not due to transynaptic degeneration of striatal cholinergic interneurons. The findings of downregulation of muscarinic receptors following long-term dopamine denervation and the subsequent normalization of muscarinic receptor density after fetal mesencephalic transplantation suggests that transplanted substantia nigra cells are able to restore inhibitory control on striatal cholinergic interneurons.

Neurotensin regulation of endogenous acetylcholine release from rat striatal slices is independent of dopaminergic tone

The effects of neurotensin (NT) alone or in combination with the dopamine antagonist sulpiride were tested on the release of endogenous acetylcholine (ACh) from striatal slices. NT enhanced potassium (25 mM)-evoked ACh release from striatal slices in a dose-dependent manner. This effect was tetrodotoxin-insensitive, suggesting an action directly on cholinergic elements. The dopamine antagonist sulpiride (5 x 10(-5) M) significantly increased (63%) potassium-evoked ACh release from striatal slices; potassium-evoked ACh release was further increased (90%) in the presence of NT (10(-5) M) and sulpiride (5 x 10(-5) M). The second set of experiments tested the effects of 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra on NT-induced increases of potassium-evoked ACh release. These lesions did not alter the NT regulation of potassium-evoked ACh release from striatal slices, but did significantly increase spontaneous (33%) and potassium-evoked (40%) ACh release from striatal slices. Striatal choline acetyltransferase activity was not affected by 6-OHDA lesions. In addition, following 6-OHDA lesions, sulpiride was ineffective in altering ACh release from striatal slices. Furthermore, evoked ACh release in the presence of the combination of NT and sulpiride was not different from that in the presence of NT alone. These results suggest that in the rat striatum, NT regulates cholinergic interneuron activity by interacting with NT receptors associated with cholinergic elements. Moreover, the NT modulation of cholinergic activity is independent of either an interaction of NT with D2 dopamine receptors or the sustained release of dopamine.

Effects of combined serotonin depletion and lesions of the nucleus basalis magnocellularis on acquisition of a complex spatial discrimination task in the rat

The purpose of the present experiment was to determine the effects of lesions of cholinergic neurons originating from the nucleus basalis magnocellularis (NBM), alone or in combination with central serotonin depletion, on learning and memory in rats trained in the Stone 14-unit T-maze--a complex, positively-reinforced spatial discrimination task. Lesion of cholinergic neurons within the NBM was accomplished by bilateral infusion of ibotenic acid. Serotonin depletion was accomplished by the systemic administration of p-chloroamphetamine (PCA). The results show that PCA-induced serotonin depletion enhanced learning. This effect was completely prevented by NBM lesions, despite the fact that NBM lesions alone did not affect the performance of rats in this task. The results of this study support the view that the cholinergic and serotonergic systems may functionally interact in learning and memory processes. The significance of this interaction in the etiology and treatment of dementia should be further investigated.

Immunohistochemical study of the serotoninergic innervation of the basal ganglia in the squirrel monkey

A specific antibody raised against 5-hydroxytryptamine (5-HT) conjugated to bovine serum albumin was used to study the serotoninergic innervation of the basal ganglia in the squirrel monkey (Saimiri sciureus). At midbrain level, numerous fine 5-HT-immunoreactive axons were seen to arise from the immunopositive neurons of the dorsal raphe nucleus and less abundantly from those of the nucleus centralis superior. The bulk of these axons formed a rather loosely arranged bundle that arched ventrorostrally through the central portion of the midbrain tegmentum and ascended toward the ventral tegmental area. Several fascicles detached themselves from this bundle to reach the substantia nigra where they arborized into a multitude of heterogeneously distributed 5-HT terminals. The 5-HT innervation was particularly dense in the pars reticulata but much less so in the pars compacta of the substantia nigra. More rostrally other 5-HT fibers swept dorsolaterally and formed a remarkably dense network of varicose fibers within the subthalamic nucleus. A multitude of 5-HT axons continued their ascending course within the lateral hypothalamic area, and many of them swept laterally to invade the lenticular nucleus. At pallidal levels, the 5-HT axons arborized much less profusely in the external segment than in the internal segment, which contained numerous 5-HT varicose fibers and terminals arranged in a typical bandlike pattern. At striatal levels, the 5-HT terminals were particularly abundant in the ventral striatum, including the nucleus accumbens and deep layers of the olfactory tubercle. They also abounded in the ventrolateral region of the putamen and the ventromedial aspect of the caudate nucleus. Overall, the number of 5-HT fibers and terminals decreased progressively along the rostrocaudal axis of the striatum and several large and elongated zones rather devoid of 5-HT immunoreactivity were visualized, particularly in the caudate nucleus and the dorsal putamen. These zones of poor 5-HT immunoreactivity were in register with similar areas devoid of tyrosine hydroxylase immunoreactivity as seen on contiguous sections. These findings reveal that all the core structures of the basal ganglia in primates receive a significant serotoninergic input, but that the densities and patterns of innervation vary markedly from one structure to the other.

Behavioral demonstration of a reciprocal interaction between dopamine receptor subtypes in the mouse striatum: possible involvement of the striato-nigral pathway

It is well known that stimulation of the D-2 dopamine receptor in vitro inhibits the increased efflux of cyclic adenosine monophosphate caused by D-1 receptor agonists. Furthermore, behavioral data suggest that the striato-nigral pathway is more involved with the dopamine agonist-induced expression of oral behaviors, which are, in turn, mediated by stimulation of the D-1 receptor. We examined an in vivo model to determine whether this D-1/D-2 reciprocal interaction is detectable at a behavioral level. First, mice were pretreated with wide range of doses of the D-2 antagonist, spiperone, and then injected with a behaviorally active dose of apomorphine (a nonspecific direct dopamine agonist) and were observed for incidence of oral behavior and rated for stereotypic behavior. A biphasic effect of spiperone pretreatment was observed, at some low doses both stereotypy and oral behavior were enhanced, while at high doses, both agonist-induced behaviors were progressively inhibited. To test the specificity of this effect for the striato-nigral pathway, mice were administered discrete electrolytic lesions in the ventral portion of the internal capsule in one hemisphere. The animals that responded to apomorphine by rotating ipsilaterally to the lesion were used in two, five-point apomorphine dose-response curves, one with, and one without, pretreatment with the dose of spiperone which most enhanced stereotypic behavior and incidence of oral behavior. The spiperone pretreatment caused a clear increase in the maximum rotational response to apomorphine without affecting the ED50. These data suggest that behavior associated with the striato-nigral efferent from striatum is marked by the opposition of D-1 and D-2 receptors.

Lesions of the dopaminergic nigrostriatal system in neonatal rats: effects on the electrophysiological activity of striatal neurons recorded during adulthood

The spontaneous activity of single striatal neurons was recorded extracellularly from 3-4-month-old adult rats that had been given dopamine (DA)-depleting brain lesions 3 days after birth. Behavioral observations made prior to recording indicated no gross sensorimotor deficits, yet subsequent biochemical analyses revealed that animals had sustained near-total DA depletions (greater than 99%). Electrophysiological results showed that the firing rates of type II striatal cells were greatly increased relative to control levels. This finding contrasts sharply with the effects of DA-depleting brain lesions given to adult animals, in which similarly high levels of striatal cell activity are invariably associated with akinesia.

Studies on brain monoamine and neuropeptide systems after neonatal intracerebroventricular 6-hydroxydopamine treatment

In order to study the effects of a neonatal dopamine lesion on dopaminergic, serotonergic and peptidergic systems, Sprague-Dawley rats were treated by intracerebroventricular administration of 6-hydroxydopamine (100 micrograms, days 3 and 6) following desipramine pretreatment (25 mg/kg s.c.). At 60-70 days postnatally a profound reduction of dopamine- and 3,4-dihydroxyphenylacetic acid levels was found in striatal and limbic forebrain regions concomitant with an extensive loss of tyrosine hydroxylase-immunoreactive fibers, while no significant alteration in noradrenaline levels was seen. A marked loss of tyrosine hydroxylase-immunoreactive cell profiles was also observed in the substantia nigra and ventral tegmental area in mesencephalon. In striatum, but not in other regions analysed, an almost 100% increase in serotonin levels and serotonin-immunoreactive fiber density was observed following 6-hydroxydopamine treatment. However, the number of serotonin-immunoreactive cell profiles in the median and dorsal raphe nuclei was not altered. The 6-hydroxydopamine treatment also led to reductions in substance P levels in striatum, nucleus accumbens and ventral mesencephalon. The cholecystokinin level in nucleus accumbens and neurotensin level in ventral mesencephalon were also reduced. A neonatal intracerebroventricular 6-hydroxydopamine treatment thus leads to a lesion of dopamine neurons in the mesencephalon with extensive loss of dopamine fibers in several forebrain areas, while localized serotonin fiber sprouting is induced in striatum. Furthermore, concomitant reductions of the levels of peptides related to the dopamine system occur following the 6-hydroxydopamine treatment. Behavioral disturbances such as hyperactivity and cognitive deficiencies occurring after a dopamine lesion early in life might therefore be due to plastic alterations in several different transmitter/neuromodulator systems as a direct or indirect consequence of the lesion.

Dopaminergic inhibition of striatal acetylcholine release after 6-hydroxydopamine

Field stimulation increased tritium efflux from rat striatal slices preincubated with [3H]choline, an index of acetylcholine release. When stimulated at low frequency (1 Hz for 2 min), nomifensine (10 microM) reduced acetylcholine release, while at high frequency (8 Hz for 1 min), sulpiride (1 microM) increased release. After 6-hydroxydopamine-induced damage to dopamine neurons, these effects were reduced but the extent of the depletion necessary to block the drug effects increased with the passage of time. Up-regulation of dopamine D-2 receptors seen at 2 months was not detected unless dopamine depletion was very large (90%). These findings indicate that dopamine, acting at the dopamine D-2 receptor, normally exerts an inhibitory influence over acetylcholine release in striatum, that after partial injury the dopaminergic influence over acetylcholine release can recover with time, and that the recovery involves processes other than changes in the number of D-2 receptors.