Stimulated serotonin release from hyperinnervated terminals subsequent to neonatal dopamine depletion regulates striatal tachykinin, but not enkephalin gene expression

Dopamine (DA) depletion in neonatal rodents results in depressed tachykinin and elevated enkephalin gene expression in the adult striatum (STR). Concurrently, serotonin (5-HT) fibers sprout to hyperinnervate the DA-depleted anterior striatum (A-STR). The present study was designed to determine if increased 5-HT release from sprouted terminals influences dysregulated preprotachykinin (PPT) and preproenkephalin (PPE) mRNA expression in the DA-depleted STR. Three-day-old Sprague-Dawley rat pups received bilateral intracerebroventricular injections of vehicle or the DA neurotoxin 6-hydroxydopamine (6-OHDA, 100 microg). Two months later, rats received a single intraperitoneal injection of vehicle or the acute 5-HT releasing agent p-chloroamphetamine (PCA; 10 mg/kg). Rats were killed 4 h later and striata processed for monoamine content by HPLC-ED and mRNA expression by in situ hybridization within specific subregions of the A-STR and posterior striatum (P-STR). 6-OHDA treatment severely (>98%) reduced striatal DA levels, while 5-HT content in the A-STR was significantly elevated (doubled), indicative of 5-HT hyperinnervation. Following 6-OHDA, PPT mRNA levels were depressed 60-66% across three subregions of the A-STR and 52-59% across two subregions of the P-STR, while PPE mRNA expression was elevated in both the A-STR (50-62%) and P-STR (55-82%). PCA normalized PPT mRNA levels in all regions of the DA-depleted A-STR and P-STR, yet did not alter PPE levels in either dorsal central or medial regions from 6-OHDA alone, but reduced PPE to control levels in the dorsal lateral A-STR. These data indicate that increased 5-HT neurotransmission, following neonatal 6-OHDA treatment, primarily influences PPT-containing neurons of the direct striatal output pathway.     

Effect of repeated administration of dopamine agonists on striatal neuropeptide mRNA expression in rats with a unilateral nigral 6-hydroxydopamine lesion

Summary Striatal mRNA expression for preproenkephalin (PPE) and preprotachykinin (PPT) was studied in unilateral 6-OHDA lesioned rats treated subchronically with a range of selective and non-selective D-1 or D-2 dopamine (DA) agonists. Apomorphine (5mg/kg sc), pergolide (0.5mg/kg sc), SKF 38393 (5mg/kg sc), SKF 80723 (1.5mg/kg sc), and quinpirole (5mg/ kg sc), or 0.9% saline (150l sc) were all given twice daily (except pergolide: once daily) for 7 days. The abundance of PPE mRNA was not altered by any of these DA agonists in the intact striatum contralateral to the 6-OHDA lesion. Only apomorphine and quinpirole increased the abundance of PPT mRNA in the intact striatum. In saline treated 6-OHDA lesioned animals PPE mRNA was elevated (+160%, p < 0.005) and PPT mRNA decreased (–36%, p < 0.005) in the denervated striatum. The up-regulation of striatal PPE mRNA in the lesioned striatum was reversed only by pergolide. The downregulation of striatal PPT mRNA in the lesioned striatum was reversed only by apomorphine. The differential sensitivity of the striatal PPE message to the long-acting DA agonist pergolide, and of the striatal PPT message to the mixed D-1/D-2 DA agonist apomorphine suggests that the striatopallidal enkephalinergic pathways are mainly regulated by prolonged DA receptor stimulation, whereas the striatonigral substance P pathways are mainly regulated by mixed D-1/D-2 DA receptor stimulation.     

D2 dopaminergic regulation of striatal preproenkephalin mRNA levels is mediated at least in part through cholinergic interneurons.

The effect of administration of the muscarinic antagonist scopolamine on the increase in striatal preproenkephalin (PPE) mRNA following a 6-hydroxydopamine (6-OHDA) lesion or chronic D2 dopamine (DA) antagonist treatment was examined by dot-blot hybridization. Administration of scopolamine dose-dependently attenuated the 6-OHDA lesion-induced increase in striatal PPE mRNA. Administration of the D2 DA antagonist eticlopride to naive rats increased striatal PPE mRNA in a dose- and time-dependent fashion. Chronic coadministration of scopolamine attenuated the eticlopride-induced increase in striatal PPE mRNA. Chronic administration of scopolamine alone did not alter striatal PPE mRNA levels. In contrast, chronic administration of eticlopride, scopolamine or the two combined decreased striatal preprotachykinin (PPT) mRNA to the same extent, suggesting that there was no direct interaction between D2 dopaminergic and cholinergic mechanisms in the regulation of striatal PPT mRNA. These data indicate that DA differentially regulates striatal PPE and PPT mRNA and suggest that dopaminergic regulation of striatal PPE mRNA is mediated in part through D2 DA effects on striatal cholinergic neurons.     

Locomotor-activity-induced changes in striatal levels of preprotachykinin and preproenkephalin mRNA. Regulation by the dopaminergic and glutamatergic systems.

The mechanisms by which dopaminergic and glutamatergic inputs interact to regulate striatal neuropeptide expression during physiological motor activity are poorly understood. In this work, striatal expression of preprotachykinin (PPT) and preproenkephalin (PPE) mRNA was studied by in situ hybridization in rats killed 2 h after treadmill running (36 m/min for 20 min). Treadmill running induced a significant increase in the levels of both PPT (60% increase) and PPE (90% increase) mRNA in the striatum of normal rats. The increase in the level of PPT mRNA was blocked in rats previously subjected to nigrostriatal deafferentation (i.e., 6-hydroxydopamine lesion) or pretreated with D1-receptor antagonist SCH-23390 (0.1 mg/kg), the D2-receptor antagonist eticlopride (0.5 mg/kg), or the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 (0.1 mg/kg). The running-induced increase in the level of PPE mRNA was blocked in rats pretreated with SCH-23390 or MK-801. Rats subjected to nigrostriatal deafferentation or pretreated with eticlopride showed an increase in PPE mRNA levels (around 150% and 40% increase, respectively), that was enhanced by running (around 230% and 160% increase, respectively). These results suggest that locomotor activity increases, in a NMDA receptor dependent fashion, the excitatory influence of the corticostriatal glutamatergic system on the two populations of striatal projection neurons, as reflected by increases in the levels of PPT and PPE mRNA. The results obtained after dopamine depletion or injection of dopamine receptor antagonists suggest that a concomitant increase in dopamine release may enhance PPT mRNA level in striatonigral neurons via D1 receptors, and reduce PPE mRNA level in striatopallidal neurons via D2 receptors. Additionally, levels of dopamine and glutamate may be regulated by other complex indirect mechanisms.     

Cortical Regulation of Striatal Neuropeptide Y (NPY)-Containing Neurons in the Rat

This study examined the functional relationships established by nigral, cortical, and thalamic striatal afferent pathways with neuropeptide Y (NPY)-containing neurons in the rat rostral striatum by coupling selective deafferentation procedures and NPY immunohistochemistry. Previous experiments have shown that after unilateral 6-hydroxydopamine (6-OHDA)-induced degeneration of nigrostriatal dopaminergic neurons, the mean number of NPY-immunoreactive (Ir) neurons per frontal section was increased in the striatum ipsilateral to the lesion side and unaltered in the contralateral striatum. The present topographical analysis of the 6-OHDA lesion effects led us to state that the increase in NPY-Ir neuron density occurs in restricted ventral and medial zones of the ipsilateral striatum. Unilateral ablation of the frontoparietal cerebral cortex by thermocoagulation was moreover shown to elicit, 20 - 30 days later, a significant bilateral increase in the number of striatal NPY-Ir cells. The increase was more marked in the striatum ipsilateral to the hemidecortication where it was similar in amplitude to that induced by the 6-OHDA lesion. The topographical analysis of the cortical lesion effects also revealed an uneven striatal response, but, in contrast to that observed for the 6-OHDA lesion, changes were restricted to dorsolateral areas of the striatum in both brain sides, revealing an apparent complementarity of nigral dopaminergic and cortical influences over striatal NPY neuronal system. Combined unilateral nigral and cortical lesions surprisingly counteracted in a survival time dependent manner the effects of each lesion considered separately. In that condition topographical changes related to the 6-OHDA lesion totally disappeared and those related to the cortical lesion were attenuated but still present. These results suggest that expression of striatal dopamine - NPY interaction is dependent on corticostriatal transmission. Interestingly lesion of thalamic areas projecting to the striatum did not significantly modify the mean number of NPY-Ir neurons determined per section from the whole striatal surface, but selectively increased the NPY neuron density in the mediodorsal region of the striatum, suggesting that the striatal NPY-containing neuronal system is also influenced by thalamostriatal projections.     

Preprotachykinin and preproenkephalin mRNA expression within striatal subregions in response to altered serotonin transmission

The effects of lowered serotonin (5-hydroxytryptamine; 5-HT) neurotransmission on preprotachykinin (PPT) and preproenkephalin (PPE) mRNA levels were examined in subregions of the striatum. Adult male rats were treated systemically with para-chlorophenylalanine (pCPA; 350 mg/kg single i.p. injection) which reduced forebrain 5-HT amounts to approximately 20% of saline-injected controls at 24 and 48 h. As measured by Northern analysis, PPT and PPE mRNA levels were elevated 50% and 160% respectively in the anterior ventromedial striatum (region included nucleus accumbens). PPT mRNA levels were raised 90% in posterior striatum (at the level of the globus pallidus) by 48 h post-pCPA injection. To determine if increased PPT and PPE mRNA levels represented a transient response to brief 5-HT inhibition, additional experiments were performed to provide continual suppression of 5-HT within the striatum. First, rats received daily intraperitoneal injections of saline or the 5-HT_1A receptor agonist, 8-OH-DPAT (1 mg/kg), for 7 days to reduce 5-HT release from raphestriatal terminals. In a parallel experiment, the serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT, 5 μg), was stereotaxically injected into the striatum as a means to permanently remove 5-HT terminals. Although levels of each mRNA species were differentially sensitive to 5,7-DHT or 8-OH-DPAT, PPT and PPE mRNAs were lowered between 30–55% within the anterior dorsolateral and ventromedial striatum. Although these results support previous studies suggesting an overall positive regulatory role of serotonin on striatal tachykinin biosynthesis, PPT and PPE gene regulation in certain striatal subregions may be differentially sensitive to lowered 5-HT neurotransmission. This suggestion is supported by observations that acute systemic stimulation of 5-HT_2A/C receptors with DOI (7 mg/kg single i.p. injection) raised PPT and PPE mRNA levels within anterior dorsolateral (30–60%) and posterior (100–200%) striata, but not within the anterior ventromedial striatum.     

Striatal output markers do not alter in response to circling behaviour in 6-OHDA lesioned rats produced by acute or chronic administration of the monoamine uptake inhibitor BTS 74 398

Summary. The monoamine uptake inhibitor BTS 74 398 induces ipsilateral circling in 6-hydroxydopamine (6-OHDA) lesioned rats without induction of abnormal motor behaviours associated with L-dopa administration. We examined whether this was reflected in the expression of peptide mRNA in the direct and indirect striatal output pathways. 6-OHDA lesioning of the nigrostriatal pathway increased striatal expression of PPE-A mRNA and decreased levels of PPT mRNA with PPE-B mRNA expression remaining unchanged. Acute L-dopa administration normalised PPE-A mRNA and elevated PPT mRNA while PPE-B mRNA expression remained unchanged. Acute administration of BTS 74 398 did not alter striatal peptide mRNA levels. Following chronic treatment with L-dopa, PPE-A mRNA expression in the lesioned striatum continued to be normalised and PPT mRNA was increased compared to the intact side. PPE-B mRNA expression was also markedly increased relative to the non-lesioned striatum. Chronic BTS 74 398 administration did not alter mRNA expression in the 6-OHDA lesioned striatum although small increases in PPT mRNA expression in the intact and sham lesioned striatum were observed. The failure of BTS 74 398 to induce changes in striatal neuropeptide mRNA correlated with its failure to induce abnormal motor behaviours or behavioural sensitisation but does not explain how it produces a reversal of motor deficits. An action in another area of the brain appears likely and may explain the subsequent failure of BTS 74 398 and related compounds to exert anti-parkinsonian actions in man. Correspondence: Emma L. Lane, Brain Repair Group, Cardiff University, Museum Avenue, Cardiff CF10 3US, UK     

Anatomical and functional evidence for lesion-specific sprouting of corticostriatal input in the adult rat

Previous studies in our laboratory have shown that cortical lesions induced by thermocoagulation of pial blood vessels, but not by acute aspiration, result in 1) the preservation of control levels of the growth-associated protein (GAP)-43 and 2) a prolonged increase in neurotransmitter gene expression in the denervated dorsolateral striatum. We have examined whether corticostriatal projections from the spared homotypic contralateral cortex contribute to these effects. Adult rats received either a thermocoagulatory or aspiration lesion of the cerebral cortex and, after 30 days, received an injection of the anterograde tracer, Fluoro-Ruby, in the contralateral homotypic cortex. Rats were killed 7 days later, and labeled fibers were examined with fluorescence microscopy in the ipsilateral and contralateral striata. Ipsilateral corticostriatal projections were detected in lesioned and unlesioned rats. Numerous labeled fibers were detected in the contralateral striatum of thermocoagulatory-lesioned but not aspiration-lesioned or control animals, suggesting that contralateral cortical neurons may undergo axonal sprouting in the denervated striatum following a thermocoagulatory lesion of the cortex. To determine whether contralateral corticostriatal fibers play a role in the changes in striatal gene expression induced by the thermocoagulatory lesions, the effects of aspiration lesions, as well as unilateral and bilateral thermocoagulatory lesions of the cortex were compared. Confirming previous results, striatal enkephalin mRNA levels were increased after a unilateral thermocoagulatory lesion. However, they were unchanged after aspiration or bilateral thermocoagulatory lesions, suggesting that sprouting or overactivity of contralateral corticostriatal input contributes to the increase seen after unilateral thermocoagulatory lesions. © 1996 Wiley-Liss, Inc.     

Differential regulation of striatal preproenkephalin mRNA by D1 and D2 dopamine receptors.

The effect of administration of subtype selective dopamine (DA) agonists on the 6-hydroxydopamine (6-OHDA) lesion-induced increase of striatal preproenkephalin (PPE) mRNA was examined by dot-blot hybridization. Eight days following a unilateral 6-OHDA lesion of the substantia nigra pars compacta (SNc), PPE mRNA levels in the ipsilateral striatum were increased approximately two-fold. Administration of the D2 DA agonist, quinpirole, dose-dependently attenuated the 6-OHDA lesion-induced increase in striatal PPE mRNA. The effect of quinpirole was blocked by coadministration of the D2 DA antagonist eticlopride. In contrast, administration of the D1 DA agonist, SKF 38393, either dose-dependently augmented or had no effect on the 6-OHDA lesion-induced increase in striatal PPE mRNA. In the contralateral striatum, administration of quinpirole decreased PPE mRNA, while administration of SKF 38393 increased PPE mRNA compared to sham lesioned control levels. These data suggest the action of DA at D1 and D2 DA receptors differentially regulates striatal PPE mRNA levels and the apparent inhibition of ENK biosynthesis by DA is mediated via an interaction with D2 DA receptors.     

Coadministration of (-)-OSU6162 with l-DOPA normalizes preproenkephalin mRNA expression in the sensorimotor striatum of primates with unilateral 6-OHDA lesions

The substituted phenylpiperidine (-)-OSU6162 is a novel modulator of the dopaminergic systems with low affinity for dopamine D(2) receptors and potent normalizing effects on l-DOPA-induced dyskinesias. We studied the effects of coadministration of (-)-OSU6162 with l-DOPA on the regulation of striatal preproenkephalin (PPE) and prodynorphin (PDyn) mRNA expression in the primate brain by in situ hybridization histochemistry. Common marmoset monkeys sustaining unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway received l-DOPA/carbidopa, l-DOPA/carbidopa plus (-)-OSU6162, or vehicle over 14 days. In vehicle-treated animals, PPE mRNA levels were markedly increased in the sensorimotor territory of the lesioned striatum. By contrast, a rather uniform lesion-induced reduction of PDyn mRNA levels was found in the vehicle group. Subchronic l-DOPA treatment induced a further increase in PPE mRNA expression in a number of sensorimotor and associative subregions of the denervated striatum. Coadministration of (-)-OSU6162 with l-DOPA partially reversed the lesion- and l-DOPA-induced elevation of PPE expression and, by affecting PPE mRNA expression differentially on the intact and lesioned striatum, markedly reduced the side-to-side difference in PPE mRNA expression. The effects on PPE mRNA expression were apparent throughout the rostrocaudal extent of the putamen and the dorsal portions of the caudate nucleus. l-DOPA treatment resulted in an enhancement in PDyn mRNA expression in all functional compartments of the striatum. Coadministration of (-)-OSU6162 had no apparent influence on these l-DOPA-induced changes in PDyn mRNA expression. The present results suggest that (-)-OSU6162 acts primarily by modifying striatal output via the indirect pathway.     

Glucocorticoid regulation of neuropeptide mRNAs in the rat striatum.

Expression of the preprotachykinin (PPT) mRNA and of the preproenkephalin (PPE) mRNA in the rat striatum has been assessed by in situ hybridization. The results demonstrate that the PPT mRNA is regulated by glucocorticoids such that adrenalectomized (ADX) animals replaced with corticosterone for 5 days expressed higher levels of the mRNA than ADX animals. The corticosterone-induced increase in striatal PPT mRNA was evident after 16 h, but not after 2 h, of corticosterone treatment of ADX animals. Elevation of circulating corticosterone levels in intact rats by acute restraint stress, or by corticosterone injection did not change the level of PPT mRNA in the striatum. In intact rats there was a diurnal variation in the level of striatal PPE mRNA expression; adrenalectomy resulted in a decrease in the mRNA level and did not abolish the diurnal variation in expression. The level of PPT mRNA in the striatum was also decreased in response to ADX, but there were no significant diurnal changes in the expression of the PPT mRNA either in the intact or in the ADX animals.     

Activation of striatal neurons by dexamphetamine is antagonized by degeneration of striatal dopaminergic terminals

Summary Unilateral degeneration of nigrostriatal dopaminergic terminals by the intranigral infusion of 6-OHDA produced a decrease in spontaneous multiple unit activity (MUA) in both the ipsilateral and contralateral striata of freely moving rats. Nigral lesions also attenuated the dexamphetamine-induced increase in MUA in the ipsilateral but not in the contralateral striatum. The magnitude of the attenuation in the ipsilateral striatum was directly proportional to the percent depletion of dopamine. Similarly degeneration of dopaminergic terminals produced by a unilateral application of 6-OHDA into the striatum lowered spontaneous MUA and completely antagonized the dexamphetamine-induced increase in MUA in the dopamine-depleted striatum. Although the spontaneous MUA in striata contralateral to a local 6-OHDA treatment was significantly reduced, the response to dexamphetamine was normal. Both striatal and nigral application of 6-OHDA produced dopamine depletion in the ipsilateral striatum and an increase in striatal dopamine levels on the contralateral side. Striatal application of 6-OHDA did not alter dopamine levels in either the olfactory tubercles, piriform cortex or cingulate cortex. It is concluded that the increase in MUA observed in the striatum following dexamphetamine treatment is critically dependent upon the release of dopamine in the striatum. These results support the concept that dopamine may have an excitatory action on some striatal neurons.     

Differential expression of mRNAs of GDNF family in the striatum following 6-OHDA-induced lesion

Changes of mRNA levels of GDNF and its recently discovered congeners persephin, neurturin, artemin in the striatum of lesioned side following 6-hydroxydopamine (6-OHDA) lesion in rodents were investigated with semi-quantitative RT-PCR. Two weeks after the lesions were made, mRNA levels of GDNF family members, except for neurturin, were significantly increased in the striatum on the side ipsilateral to the lesion compared with equivalent tissue of sham control. This increase reached a maximal level 5-7 weeks post-lesion. The marked increase of BDNF mRNA expression was also observed in the ipsilateral striatum with similar time course. These findings suggest that 6-OHDA-induced lesions can change gene expression in denervated target tissue, and that mRNA levels of GDNF family members in striatal cells may be modulated by afferent dopaminergic input in a slow-rising and long-lasting fashion.     

Chronic estrogenic drug treatment increases preproenkephalin mRNA levels in the rat striatum and nucleus accumbens

Estrogens modulate the expression of preproenkephalin (PPE) in the hypothalamus but little is known for other brain regions. The present study investigated the effect of hormonal withdrawal and replacement therapy on PPE expression in the striatum, nucleus accumbens and cortex. Ovariectomized Sprague-Dawley rats were treated for 2 weeks with estradiol, a specific ligand for estrogen receptor alpha (ERalpha), 4,4',4'-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT) and estrogen receptor beta (ERbeta) 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN), or the selective estrogen receptor modulators (SERMs) tamoxifen and raloxifene. Brain PPE mRNA levels, measured by in situ hybridization, were high in the striatum and nucleus accumbens compared to the low expression in the cortex. Ovariectomy decreased uterine weights compared to intact uterus, which was corrected by estradiol and PPT. Tamoxifen and raloxifene partially stimulated uterine weights while DPN left it unchanged. In the anterior, median and posterior striatum and in the core and shell of the nucleus accumbens, ovariectomy decreased PPE mRNA levels compared to intact rats, this was corrected by estradiol treatment except for the posterior striatum. PPT, DPN, tamoxifen and raloxifene reproduced the estradiol effect. In the prefrontal and cingulate cortices, neither ovariectomy nor treatments changed PPE mRNA levels. These results show for the first time that estradiol increases PPE mRNA in the striatum and nucleus accumbens. This effect is observed also with estrogen receptor agonists for the ERalpha and ERbeta as well as with SERMs.     

Cellular localisation of enkephalin gene expression in MPTP-treated cynomolgus monkeys

Cellular sites of enkephalin gene expression were investigated using the technique of in situ hybridization in the normal striatum and in the denervated striatum of monkeys depleted of dopamine by pretreatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Animals received MPTP by either (a) intravenous injection to induce generalized parkinsonism, or (b) infusion into one carotid artery to induce unilateral parkinsonism. The animals which received systemic injections of MPTP were found to have an essentially total loss of nigral dopamine cells whereas the intracarotid MPTP treatment was found to destroy approximately 95% of the dopamine neurons in the ipsilateral substantia nigra. A double-stranded cDNA probe encoding the human preproenkephalin (PPE) gene was isotopically labelled with 35S and used to detect PPE mRNA within striatal tissue sections. Application of this radiolabelled cDNA probe to lightly fixed striatal sections from both groups of animals revealed an increase in expression of PPE mRNA within denervated striatal enkephalinergic neurons relative to control tissue. An increase in the number of detectable enkephalinergic mRNA-positive neurons relative to control tissue was also noted. These results suggest that the nigral dopaminergic neurons tonically inhibit PPE gene expression in the striatum.     

Somatostatin-immunoreactive neurons in the rat striatum: effects of corticostriatal and nigrostriatal dopaminergic lesions

The present study examined the effects of the impairment of corticostriatal and nigrostriatal dopaminergic transmission on the mean number and the topographical distribution of somatostatin-containing neurons in frontal sections of the rat rostral striatum. These neurons, visualized by an immunohistochemical method using a specific anti-somatostatin(28) antibody were shown to be unevenly distributed; the number of immunoreactive perikarya being consistently lower in the dorsolateral and higher in the middle areas of striatal sections than in the remaining parts of the structure. Such a distribution and number were not altered either by unilateral 6-hydroxydopamine (6-OHDA)-induced lesion of the nigrostriatal dopaminergic neurons after 2- to 3-week survival periods, or by alpha-methylparatyrosine-induced dopamine depletion. In animals with similar 6-OHDA-induced lesions, no change in the striatal concentration of somatostatin measured by radioimmunoassay was observed. These results suggest that somatostatin levels in striatal neurons are not under a dopaminergic influence in contrast to that previously described for neuropeptide Y, although both peptides are thought to coexist extensively in the same striatal neuron population. On the contrary, extensive unilateral frontoparietal ablation of the cerebral cortex elicited, 2-3 weeks later, a significant increase in the mean number of somatostatin-immunoreactive cells per section in the ipsilateral striatum preferentially localized to the dorsolateral zone of the structure with no change in the contralateral side. Data from immunohistochemical studies were further discussed in comparison with results obtained by radioimmunoassay showing that similar cortical lesion induced no change in somatostatin endogenous levels in the ipsilateral striatum and a 30% decreased concentration of the peptide in the contralateral striatum. These data suggest that the corticostriatal pathway influences the expression of somatostatin at either a translational, processing or metabolic level in a topographically restricted population of striatal somatostatin-containing neurons.     

Localization of striatal opioid gene expression,and its modulation by the mesostriatal dopamine pathway: An in situ hybridization study

In situ hybridization was used to study the macroscopic distribution and regulatory control of proenkephalin mRNA and prodynorphin mRNA in rat striatum. While proenkephalin mRNA was widely distributed throughout the striatum, levels of prodynorphin mRNA were highest in the medial and ventral portions of the striatum. Furthermore, in contrast to the results for proenkephalin mRNA, the levels of prodynorphin mRNA appeared higher in the nucleus accumbens than in the striatum. The mesostriatal dopaminergic pathway was destroyed by discrete, unilateral injection of 6-hydroxydopamine (6-OHDA) into either the substantia nigra or the neighboring ventral tegmental area (VTA). Lesions of the substantia nigra caused a dramatic ipsilateral increase in the hybridization signal for proenkephalin mRNA, but no change was observed in the hybridization signal for prodynorphin mRNA. Similar effects were seen with VTA lesions. Since destruction of the mesostriatal dopamine system elevates the levels of proenkephalin mRNA, but not of prodynorphin mRNA, in the striatal target neurons, it appears that the mesostriatal pathway exerts a tonic and selective suppression of striatal proenkephalin gene expression at the mRNA level.     

Influence of mesostriatal afferents on the development and transmitter regulation of intrastriatal grafts derived from embryonic striatal primordia.

Embryonic striatal grafts develop a modular organization in which patches of tissue enriched in many transmitter substances characteristic of striatum (P regions) are embedded in surrounds (NP regions) expressing only low levels of these substances. Catecholaminergic fibers from the host brain, identified by their expression of tyrosine hydroxylase (TH), grow into such grafts and selectively terminate in the striatum-like P regions. This terminal pattern suggests that cell-cell affinities between neurons of the substantia nigra and striatum may play a role either in the aggregation of the striatal cells into P regions, or in the targeting of the TH-positive fibers to the cell clusters. In the present study, we tested the first of these possibilities. Striatal grafts derived from embryonic day 15 striatal primordia were implanted into the ibotenate-damaged host striatum of rats previously treated with 6-hydroxydopamine (6-OHDA) to destroy TH-containing dopaminergic nigrostriatal afferents. The 6-OHDA lesions that eliminated nearly all TH-like immunostaining in the host striatum also resulted in disappearance of nearly all TH-positive fibers in the grafts. In this dopamine-depleted environment, the grafts nevertheless developed a clear modular organization. They contained striatum-like patches with neurons expressing many of the neurochemicals characteristic of striatum (ACh, ChAT, calbindin-D28KD, met-enkephalin, and dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein-32,000 or DARPP-32), and these patches were surrounded by graft tissue expressing few of these striatal markers. These observations suggest that the ingrowth of TH-positive fibers from the host is not obligatory for the sorting out of striatal from nonstriatal cells during the formation of P regions in embryonic striatal grafts. Despite the fact that dopaminergic denervation of the host striatum did not disrupt either the aggregation of grafted cells into P regions or the acquisition of striatal neurochemical phenotypes by cells in the P regions, there were clear differences between the staining patterns of these grafts and grafts placed into dopamine-innervated striatum. Most striking was a sharp increase of met-enkephalin-like immunostaining in the P zones of the denervated grafts. Upregulation of met-enkephalin is known to occur in the dopamine-depleted mature striatum, and was observed in the parts of host striatum surrounding the grafts on the side ipsilateral to the 6-OHDA lesions. This result suggests that functional interactions between dopaminergic and enkephalinergic systems can occur in the striatal circuits reconstructed by embryonic striatal grafting. More generally, our results suggest that TH-containing afferents from the host striatum, though not required for induction and maintenance of striatal phenotypy in striatal grafts, can chronically regulate neurotransmitter/neuromodulator expression in neurons of the striatum-like P zones in a manner similar to that found for the normal striatum.