Differential dopaminergic regulation of proenkephalin and prodynorphin mRNAs in the basal ganglia of rats

Neuropathic Pain Dysregulates Gene Expression of the Forebrain Opioid and Dopamine Systems

Disturbances in the function of the mesostriatal dopamine system may contribute to the development and maintenance of chronic pain, including its sensory and emotional/cognitive aspects. In the present study, we assessed the influence of chronic constriction injury (CCI) of the sciatic nerve on the expression of genes coding for dopamine and opioid receptors as well as opioid propeptides in the mouse mesostriatal system, particularly in the nucleus accumbens. We demonstrated bilateral increases in mRNA levels of the dopamine D1 and D2 receptors (the latter accompanied by elevated protein level), opioid propeptides proenkephalin and prodynorphin, as well as delta and kappa (but not mu) opioid receptors in the nucleus accumbens at 7 to 14 days after CCI. These results show that CCI-induced neuropathic pain is accompanied by a major transcriptional dysregulation of molecules involved in dopaminergic and opioidergic signaling in the striatum/nucleus accumbens. Possible functional consequences of these changes include opposite effects of upregulated enkephalin/delta opioid receptor signaling vs. dynorphin/kappa opioid receptor signaling, with the former most likely having an analgesic effect and the latter exacerbating pain and contributing to pain-related negative emotional states.

The influence of group III metabotropic glutamate receptor stimulation by (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid on the parkinsonian-like akinesia and striatal proenkephalin and prodynorphin mRNA expression in rats

Group III metabotropic glutamate receptors (mGluRs) are widely distributed in the basal ganglia, especially on the terminals of pathways which seem to be overactive in Parkinson's disease. The aim of the present study was to determine whether (1S,3R,4S)-1-aminocyclo-pentane-1,3,4-tricarboxylic acid (ACPT-1), an agonist of group III mGluRs, injected bilaterally into the globus pallidus (GP), striatum or substantia nigra pars reticulata (SNr), can attenuate the haloperidol-induced catalepsy in rats, and whether that effect was related to modulation of proenkephalin (PENK) or prodynorphin (PDYN) mRNA expression in the striatum. Administration of ACPT-1 (0.05-1.6 microg/0.5 microl/side) caused a dose-and-structure-dependent decrease in the haloperidol (0.5 mg/kg i.p. or 1.5 mg/kg s.c.)-induced catalepsy whose order was as follows: GP>striatum>SNr. ACPT-1, given alone to any of those structures, induced no catalepsy in rats. Haloperidol (3 x 1.5 mg/kg s.c.) significantly increased PENK mRNA expression in the striatum, while PDYN mRNA levels were not affected by that treatment. ACPT-1 (3 x 1.6 microg/0.5 microl/side) injected into the striatum significantly attenuated the haloperidol-increased PENK mRNA expression, whereas administration of that compound into the GP or SNr did not influence the haloperidol-increased striatal PENK mRNA levels. Our results demonstrate that stimulation of group III mGluRs in the striatum, GP or SNr exerts antiparkinsonian-like effects in rats. The anticataleptic effect of intrastriatally injected ACPT-1 seems to correlate with diminished striatal PENK mRNA expression. However, since the anticataleptic effect produced by intrapallidal and intranigral injection of ACPT-1 is not related to a simultaneous decrease in striatal PENK mRNA levels, it is likely that a decrease in enkephalin biosynthesis is not a necessary condition to obtain an antiparkinsonian effect.

Variations in opioid peptide levels during the estrous cycle in Sprague-Dawley rats

The estrous cycle, with its various hormonal conditions, may provide us with the means of understanding how endocrine states relate to opioid mechanisms. There has been increasing experimental support for interaction between sex steroids and opioid peptides in the central nervous system. Here, we describe fluctuations in endogenous brain immunoreactive (ir) peptide levels during various phases of the estrous cycle in the female Sprague-Dawley rat. Ir levels of dynorphin A, dynorphin B, Leu-enkephalin-Arg(6), Met-enkephalin-Arg(6)Phe(7) and nociceptin/orphanin FQ were measured in the pituitary gland and in 10 areas of the brain during the diestrus, proestrus and estrus phase. In several areas of the brain, basal levels of endogenous opioid peptides showed variation during the course of the estrous cycle. Significant differences were found between the diestrus state and the proestrus and/or estrus conditions, particularly in the nucleus accumbens, caudate putamen and the substantia nigra. The ir levels of the endogenous peptide nociceptin/orphanin FQ became altered in only one of the areas measured, indicating less variance during the estrous cycle. Correlation analyses revealed that significant associations between dynorphin A or dynorphin B and Leu-enkephalin-Arg(6) were found more often during estrus than during the diestrus and proestrus conditions. The ratio between the ir levels of Leu-enkephalin-Arg(6), a cleavage product of the enzymatic conversion of dynorphin peptides into shorter peptides in vivo, and dynorphin peptides was calculated. The significantly lower ratio between Leu-enkephalin-Arg(6) and dynorphin B in diestrus than in proestrus and estrus also indicates cyclic fluctuations in the enzymatic cleavage of dynorphin. These findings are discussed in relation to the possible role of interactions between sex steroids and opioid peptide mechanisms during the normal estrous cycle.

Unilateral dopamine depletion increases expression of the 2A subunit of the N-methyl-D-aspartate receptor in enkephalin-positive and enkephalin-negative neurons

Striatal efferent neurons receive dopamine- and glutamate-utilizing afferents. Previous studies have shown that dopamine depletion increases gene expression in striatopallidal neurons and decreases it in striatonigral neurons. Previous work has also reported increased expression of the 2A subunit of the N-methyl-D-aspartate receptor in the dopamine-depleted striatum. The purpose of this study therefore was to determine whether dopamine depletion differentially alters the expression of the 2A subunit of the N-methyl-D-aspartate receptor in rat striatal neurons. 6-Hydroxydopamine (8microg/2microl) was infused unilaterally into the medial forebrain bundle. Rats were killed three weeks later. Double-label in situ hybridization was performed using an 35S-labeled ribonucleotide probe directed against the messenger RNA of the 2A subunit and a digoxigenin-labeled ribonucleotide probe directed towards preproenkephalin messenger RNA to mark striatopallidal neurons. Analysis of single-labeled film autoradiograms revealed a significant increase in the expression of 2A subunit messenger RNA in the ipsilateral, but not the contralateral, striatum of dopamine-depleted animals, consistent with other studies in the literature. Cellular analysis of 2A subunit expression indicated that as a consequence of dopamine depletion there is a significant increase in the expression of this subunit in both enkephalin-positive and enkephalin-negative neurons. From this study we conclude that dopamine depletion increases messenger RNA expression of the 2A subunit of the N-methyl-D-aspartate receptor in striatopallidal and presumed striatonigral (enkephalin-negative) neurons. Such alterations may affect the pharmacology and function of the resultant receptor, and thus alter glutamate transmission in both populations of medium spiny neurons after dopamine depletion.

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.

Early changes in neuropeptide mRNA expression in the striatum following reserpine treatment

Chronic dopamine depletion produces neurochemical changes within the striatum as well as enhanced behavioral and metabolic responses to dopamine agonists. Changes in striatal neuropeptides have been consistently described, including increased expression of preproenkephalin mRNA and decreased expression of preprotachykinin and prodynorphin mRNA. Acute dopamine depletion following treatment with reserpine also produces enhanced behavioral and metabolic responses to agonist treatment which develop rapidly. In the present study, we used in situ hybridization histochemistry to investigate whether acute neurochemical changes occur following reserpine treatment. We evaluated neuropeptide mRNA expression in the striatum and nucleus accumbens at several time points from 6 to 120 h following single doses of reserpine and AMPT. The aim of these studies was to determine if changes in neuropeptide mRNA expression occur following acute dopamine depletion and whether such changes are specific to the striatum. Changes in striatal neuropeptide mRNA expression developed rapidly. Preproenkephalin mRNA expression by striatopallidal neurons was unchanged at 48 h, but increased by 44% at 120 h. Preprotachykinin mRNA expression in striatonigral neurons was increased at 6 h and then fell, with a maximal decrease of 45% at 48 h and partial recovery by 120 h. Prodynorphin mRNA expression was unchanged. Expression of preproenkephalin and preprotachykinin mRNA was also examined in subregions of the striatum and the nucleus accumbens. Expression of preproenkephalin mRNA was uniform in the striatum and higher in the core than the shell of the nucleus accumbens. Preprotachykinin mRNA expression in the striatum was higher in the lateral quadrants and was higher in the shell than in the core of the nucleus accumbens. The changes in neuropeptide mRNA following treatment with reserpine were only found in the striatum. These data provide further evidence for early alterations in neuronal function in the striatum following acute dopamine depletion and suggest that neuropeptide expression by striatonigral neurons may be more rapidly regulated in response to changes in dopamine levels.

Thalamo-striatal deafferentation affects preproenkephalin but not preprotachykinin gene expression in the rat striatum

This study examined the effects of thalamo-striatal deafferentation on preprotachykinin and preproenkephalin mRNA expression in the rat neostriatum, using quantitative in situ hybridization histochemistry. Unilateral ibotenate-induced intralaminar thalamic lesion produced a significant decrease in preproenkephalin mRNA levels (-27%) restricted to the ipsilateral striatum at 5 days post-lesion. At 12 days post-lesion, significant decreases in striatal preproenkephalin mRNA expression were found on both brain sides. This post-lesional response was more pronounced in the ipsilateral (-32%) than contralateral (-18%) striatum. All these changes were homogeneously distributed between the dorsolateral and ventromedial parts of the striatum. In parallel, no significant change in preprotachykinin mRNA expression was found at either 5 or 12 days after thalamic lesion, when considering the striatum as a whole. However, at 5 days post-lesion, the regional analysis revealed a slight decrease (-17%) in preprotachykinin mRNA expression, confined to the dorsolateral part of the ipsilateral striatum. These results show that thalamic lesion preferentially affects preproenkephalin vs. preprotachykinin gene expression in the striatum, suggesting, at the first site, a predominant influence of thalamo-striatal inputs on the enkephalin-containing striato-pallidal pathway. However, given that the thalamo-striatal projection is strictly ipsilateral, the bilateralization of the down-regulation of preproenkephalin mRNA expression at 12 days post-lesion suggests an involvement of interhemispheric adaptive mechanisms via cortical networks.

Amphetamine-induced preprodynorphin mRNA expression and kappa-opioid receptor binding in basal ganglia of adult rats after prenatal exposure to diazepam

In order to obtain information on the functional state of basal ganglia following prenatal benzodiazepine exposure, preprodynorphin mRNA expression and kappa-opioid receptors were studied in offspring of timed-pregnant Long Evans rats treated with diazepam (1.25 mg/kg/day) on gestational days 14 to 20. Preprodynorphin mRNA was localised by in situ hybridization using a 33P-labeled oligonucleotide. Relative optical density (ROD) was quantified by image analysis in four quadrants of caudate putamen, in nucleus accumbens and olfactory tubercle of adult male rats. Six hours after functional challenge by injection of D-amphetamine (8 mg/kg s.c.), prenatally vehicle-exposed rats exhibited increased preprodynorphin mRNA (ROD) levels in caudate putamen (dorsolateral 187%, dorsomedial 150%, ventrolateral 153%, ventromedial 140% of control), nucleus accumbens (142%) and olfactory tubercle (213%). Prenatal diazepam exposure attenuated the effect of amphetamine in all regions; statistically significant differences between ROD levels of prenatally vehicle/adult amphetamine-treated and prenatally diazepam/adult amphetamine-treated groups were seen in ventrolateral caudate putamen, nucleus accumbens and olfactory tubercle. Baseline levels and topographical distribution of preprodynorphin mRNA remained unchanged. kappa-opioid receptor binding was analyzed in membrane from nucleus accumbens + olfactory tubercle, caudate putamen, and midbrain of male and female offspring using [3h]U69593. Bmax was reduced in nucleus accumbens + olfactory tubercle, but not in caudate putamen or midbrain of adult, prenatally diazepam-exposed male offspring, This effect was not yet seen at earlier postnatal stages (14 and 28 days), and was also absent in females. These data indicate that prenatal exposure to diazepam results in a delayed change in the functional state of dynorphin-containing neurons in several parts of the basal ganglia of adult male offspring. The decreased responsiveness to enhanced dopaminergic transmissions may impair the function of basal ganglia circuitry.

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-HT1A 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 micrograms), 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 by differentially sensitive to lowered 5-HT neurotransmission. This suggestion is supported by observations that acute systemic stimulation of 5-HT2A/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.

Differential messenger RNA expression of prodynorphin and proenkephalin in the human brain

The opiate system is involved in a wide variety of neural functions including pain perception, neuroendocrine regulation, memory, drug reward, and tolerance. Such functions imply that endogenous opioid peptides should have anatomical interactions with limbic brain structures believed to be involved in the experience and expression of emotion. Using in situ hybridization histochemistry, the messenger RNA expression of the opioid precursors, prodynorphin and proenkephalin, was studied in whole hemisphere human brain tissue. Different components of the limbic system were found to be characterized by a high gene expression of either prodynorphin or proenkephalin messenger RNA. Brain regions traditionally included within the limbic system (e.g. amygdala, hippocampus, entorhinal cortex and cingulate cortex) as well as limbic-associated regions including the ventromedial prefrontal cortex and patch compartment of the neostriatum showed high prodynorphin messenger RNA expression. In contrast, high levels of proenkephalin messenger RNA were more widely expressed in the hypothalamus, periaqueductal gray, various mesencephalic nuclei, bed nucleus of the stria terminalis, and ventral pallidum; brain regions associated with endocrine-reticular-motor continuum of the limbic system. The marked anatomical dissociation between the expression of these two opioid peptide genes, seen clearly in whole hemisphere sections, indicates that distinct functions must be subserved by the prodynorphin and proenkephalin systems in the human brain.

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

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

Partial striatal dopamine depletion differentially affects striatal substance P and enkephalin messenger RNA expression

Near total striatal dopamine denervation results in a decrease in substance P and an increase in enkephalin messenger RNA expression in the striatum. It is unknown whether partial depletions of striatal dopamine content produce similar changes in these peptide messenger RNAs. To test whether compensations in dopamine synthesis and release following partial dopamine denervation prevent the lesion-induced alterations in substance P and enkephalin messenger RNAs, varying concentrations of 6-hydroxydopamine were injected unilaterally into the substantia nigra. Seven days after injection of 6-hydroxydopamine (2-16 micrograms) or vehicle, in situ hybridization histochemistry was used to examine tyrosine hydroxylase messenger RNA in the substantia nigra and substance P and enkephalin messenger RNAs in the striatum. The extent of the dopamine depletion was determined by measuring striatal dopamine tissue content. The decrease in tyrosine hydroxylase messenger RNA paralleled the change in striatal tissue dopamine content. Substance P messenger RNA was decreased in all lesioned rats. In contrast, a significant increase in enkephalin messenger RNA was not detected until striatal dopamine was reduced to 10% of control levels. These results suggest that compensations within the residual dopamine system are not sufficient to maintain normal striatal substance P messenger RNA levels in partially denervated animals, but are sufficient to maintain normal striatal enkephalin messenger RNA expression.

The expression of proenkephalin and prodynorphin genes and the induction of c-fos gene by dopaminergic drugs are not altered in the straitum of MPTP-treated mice

The expression of proenkephalin (PENK), prodynorphin (PDYN) and c-fos genes was studied in the striatum of C57B1/6 mice treated with 1-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine (MPTP), which are used as a rodent model of Parkinson's disease (PD). Two weeks after systemic administration of MPTP (2 x 40 mg/kg, s.c. 18h apart), the lesion of the substantia nigra (SN) could be visualised by loss of the nigral tyrosine hydroxylase (TH) mRNA hybridization signal and by a 91% decrease in striatal dopamine levels. The levels of PENK and PDYN mRNAs were not significantly changed in the striatum of the lesioned mice, as compared to non-treated controls. The induction of the immediate early gene c-fos by the dopamine D2 receptor antagonist haloperidol was not altered, while the selective D1 receptor agonist SKF 38393 failed to induce c-fos in the striatum of MPTP-treated mice. These results are in contrast to the data concerning rats with the 6-hydroxydopamine (6-OHDA) lesion of the SN, which serve as another rodent model of PD. In the striata of 6-OHDA-lesioned rats, PENK gene is upregulated, PDYN gene is down-regulated and the induction of c-fos gene by D2 receptor antagonists is abolished, whereas selective D1 receptor agonists induce c-fos gene, which does not occur in non-lesioned rats. We presume that the lack of influence of the MPTP lesion in mice on the striatal gene expression was mainly caused by insufficient dopamine depletion in the striatum, which could not be increased in this model. The importance of the changes observed in 6-OHDA-lesioned rats has been discussed in the context of the mouse and primate MPTP models of PD.

Alterations in mRNA levels of D2 receptors and neuropeptides in striatonigral and striatopallidal neurons of rats with neuroleptic-induced dyskinesias

Chronic neuroleptic treatment in rat produces vacuous chewing movements (VCMs), analogous to TD in humans. We hypothesized that these hyperkinetic movements were due to alterations in striatonigral and striatopallidal GABAergic spiny II neurons. Rats were treated for 36 weeks with haloperidol decanoate and withdrawn for 28 weeks. Striatonigral and striatopallidal neurons were assessed using in situ hybridization histochemistry for mRNA levels of D1 and D2 dopamine receptors, preproenkephalin (ENK), prodynorphin (DYN), protachykinin (substance P), and glutamate decarboxylase (GAD67) in the dorsolateral and ventromedial striatum as well as the nucleus accumbens. Rats that did not develop VCMs (-VCM) had increased D2 receptor and DYN mRNA, and reduced substance P mRNA in the dorsolateral striatum. Rats with persistent VCMs (+VCM) had increased D2 receptor, ENK, and DYN mRNA in both striatal regions, and increased ENK and DYN mRNA in the nucleus accumbens, compared with controls. Relative to -VCM rats, however, +VCM rats only had increased ENK mRNA in the nucleus accumbens. Considering the overall pattern of mRNA changes, the data suggest that alterations in both the D1-mediated striatonigral and the D2-mediated striatopallidal pathways play a role in the expression of the VCM syndrome. To the extent that gene expression parallels changes in neuronal activity, this implies that the VCM syndrome is associated with increased activity in both pathways.

Dopaminergic and muscarinic regulation of striatal enkephalin and substance P messenger RNAs following striatal dopamine denervation: effects of systemic and central administration of quinpirole and scopolamine

Striatal dopamine depletion produces an increase in enkephalin and a decrease in substance P messenger RNAs. Subsequent systemic administration of either the D2 dopamine agonist, quinpirole, or the muscarinic antagonist, scopolamine, results in the reduction of the lesion-induced elevation in striatal enkephalin messenger RNA. These changes in enkephalin messenger RNA levels may be mediated solely within the striatum or through trans-synaptic circuits involving the striatum. To dissociate these possibilities, we have compared the effects of systemic and central administration of quinpirole and scopolamine on striatal enkephalin and substance P messenger RNAs using in situ hybridization histochemistry. Systemic administration of both quinpirole and scopolamine blocked the elevation of striatal enkephalin messenger RNA normally observed in 6-hydroxydopamine-lesioned rats. In addition, high doses of systemic scopolamine (25 and 50 mg/kg per day) prevented the lesion-induced decrease in striatal substance P messenger RNA levels. In order to determine whether the effects of these drugs are mediated directly within the striatum, central administration of quinpirole and scopolamine were compared. In contrast to systemic administration, intraventricular and intrastriatal infusion of quinpirole but not scopolamine prevented the lesion-induced change in striatal enkephalin messenger RNA. However, neither quinpirole nor scopolamine administered centrally affected the level of substance P messenger RNA in the striatum of 6-hydroxydopamine-induced lesioned animals. Together, these data suggest that changes in D2 receptor activation directly in the striatum are responsible for the effects of quinpirole on enkephalin messenger RNA. In contrast, the effect of systemic scopolamine on striatal enkephalin and substance P messenger RNAs may not be mediated within the striatum.

Temporal dissociation between changes in striatal enkephalin and substance P messenger RNAs following striatal dopamine depletion

Changes in the levels of enkephalin and substance P messenger RNA expression were examined in the striatum following dopamine depletion resulting from unilateral injection of 6-hydroxydopamine into the substantia nigra. In response to striatal dopamine depletion, the levels of enkephalin messenger RNA were elevated, whereas substance P messenger RNA was decreased within all regions of the striatum. Examination of the striatal peptide messenger RNAs between one and 21 days after the injection of 6-hydroxydopamine revealed a temporal dissociation between changes in enkephalin and substance P messenger RNAs. Within one day of the 6-hydroxydopamine injection, substance P messenger RNA was significantly decreased by 30% at all levels of the striatum. This decrease was maintained for up to 21 days after the lesion. In contrast, striatal enkephalin messenger RNA was not significantly elevated until three days following the injection of 6-hydroxydopamine, after which there was a gradual increase up to 21 days. In order to correlate alterations in peptide messenger RNA expression with 6-hydroxydopamine-induced changes in striatal dopamine innervation, tissue punches from the striatum were examined for dopamine content at one, two, three and seven days after the lesion. One day after the lesion, striatal dopamine levels were significantly increased by 47%. In contrast, within two days tissue dopamine content was reduced by 77% compared to control levels. A further decrease of 90% or more was observed at three and seven days after the lesion. Taken together, these data demonstrate a temporal dissociation between changes in enkephalin and substance P messenger RNA levels following 6-hydroxydopamine-induced striatal dopamine depletions. This temporal dissociation may reflect a differential response of enkephalin and substance P messenger RNAs to alterations in dopamine release and subsequent receptor activation.

Effect of opioid peptides on dopamine release from nucleus accumbens after repeated treatment with methamphetamine

The effect of opioid peptides on extracellular dopamine levels in the nucleus accumbens was compared between rats treated with methamphetamine and saline repeatedly (for 9 days) by using microdialysis. After the period of repeated treatment, the rats in both groups were kept for an additional 9 days without further treatment. Repeated administration of methamphetamine reduced the decreasing effect of dynorphin (10 microM), applied locally in the perfusate, and enhanced the increasing effect of [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO, 10 microM) on the extracellular dopamine levels in the nucleus accumbens. It is possible that repeated treatment with methamphetamine leads to attenuation of the inhibition and enhancement of the stimulation of dopamine release from the nucleus accumbens via presynaptic dynorphin- and enkephalin-sensitive receptors, respectively.

Effects of intranigral injections of colchicine on the expression of some neuropeptides in the rat forebrain: an immunohistochemical and in situ hybridization study

In the present study, we describe the neurochemical effects of intranigral injections of colchicine in the rat forebrain using immunohistochemistry and in situ hybridization. The observations on the injected side are compared to the contralateral one and to the sham-operated rats. We demonstrate that such injections are able to strongly enhance the immunoreactivity for Met-enkephalin (ME), substance P (SP) and neuropeptide Y (NPY) in numerous nerve cell bodies of the limbic system (injected side), whereas the levels of the corresponding mRNAs are differently modified according to the region examined. A clear correlation between the enhancement of the immunostaining for ME and SP and that of the preproenkephalin (PPA) and preprotachychinin gene transcripts was observed in neuronal perikarya of the medial amygdaloid nucleus (SP), of the dorsolateral hypothalamus (ME) and of the ventromedial hypothalamic nucleus (SP). These observations are interpreted as an induction--or increased expression--of neuropeptide genes in neuronal perikarya postsynaptic to nerve fibers originating in the midbrain and brain stem. In this case, colchicine is thought to block the electrophysiological activity of ascending nerve fibers (anterograde and postsynaptic effect). In the case where the enhancement of the immunoreactivity for the studied neuropeptides was associated with no change or a decreased expression of the corresponding genes in the same brain areas, colchicine may have blocked the axoplasmic transport of peptides in nerve fibers projecting to the midbrain and/or brain stem (6). This may result in a retrograde accumulation of peptides in the nerve cell bodies of origin and, eventually, in a negative feedback regulation of the corresponding encoding genes in these perikarya (retrograde and presynaptic effect of colchicine). The drastic behavioral effects of bilateral intranigral injections of colchicine, on ingestive behavior in particular, have been studied in a following paper.

Molecular aspects of neuropeptide regulation and function in the corpus striatum and nucleus accumbens

In the corpus striatum and nucleus accumbens, neuropeptides participate along with conventional neurotransmitters such as dopamine, gamma-aminobutyric acid (GABA), acetylcholine and glutamate in the regulation of locomotor activity, stereotyped motor behaviors and neural events related to reward and affective state. The present review concerns itself with four major neuropeptide systems--enkephalin, dynorphin, tachykinins and neurotensin--and it summarizes neuroanatomical and functional studies as well as emphasizing regulatory interactions between neurotransmitters and neuropeptides at the level of neuropeptide gene expression. Dopaminergic transmission emanating from midbrain dopaminergic cell bodies of the substantia nigra and the ventral tegmentum regulates striatal and accumbens neuropeptide levels and their mRNAs. Evidence is presented for D1 or D2 receptor involvement as well as D1-D2 interactions that modulate neuropeptide and mRNA levels in striatum and accumbens neurons. Regulatory influences by GABAergic, serotonergic and cortical (glutamatergic) neurotransmission and via sigma receptors and circulating adrenal steroids are also described. The evidence gathered in many laboratories thus far indicates that these major basal ganglia peptidergic systems are modulated dynamically and sometimes in opposing ways by various neurochemical inputs which alter neuropeptide and neuropeptide mRNA levels over both short- and long-term. Neuropeptide systems are involved in the regulation and execution of motor programs and may also be involved in the control of mood and affect as well as self-administration behavior and behavioral sensitization, especially via the nucleus accumbens and its reciprocal connections with the midbrain, hippocampus and frontal cortex. Glucocorticoids modulate mood as well as self-administration behavior and influence locomotor activity and certain forms of stereotypy. The modulation of striatal proenkephalin and protachykinin mRNA levels by adrenal steroids is described along with distribution of adrenal steroid receptor subtypes. Adrenal steroid regulation of neuropeptide gene expression in striatum, accumbens and midbrain suggests that there may be a wider role for glucocorticoids and for other neuropeptide systems in environmental and drug influences on normal and abnormal behaviors involving the nigrostriatal and mesolimic systems.