Cholinergic regulation of tachykinin- and enkephalin-gene expression in the rat striatum

Enkephalins and Pain Modulation: Mechanisms of Action and Therapeutic Perspectives

Enkephalins, a subclass of endogenous opioid peptides, play a pivotal role in pain modulation. Enkephalins primarily exert their effects through opioid receptors located widely throughout both the central and peripheral nervous systems. This review will explore the mechanisms by which enkephalins produce analgesia, emotional regulation, neuroprotection, and other physiological effects. Furthermore, this review will analyze the involvement of enkephalins in the modulation of different pathologies characterized by severe pain. Understanding the complex role of enkephalins in pain processing provides valuable insight into potential therapeutic strategies for managing pain disorders.

Neuroanatomical relationship between the cholinergic and tachykininergic systems in the adult human brainstem: An immunohistochemical study

The cholinergic system plays an important role in brain homeostasis and interacts with the neuropeptidergic systems, and the functional relationships between both systems are well known. However, in the brainstem the possible physiological interactions between neurokinins and acetylcholine are unknown, although both substances have been detected in the same brainstem nuclei and have been implicated in similar functions controlled from brainstem regions such as some cranial motor nuclei. The aim of this work is to determine whether these possible physiological interactions might have a neuroanatomical basis by means of the double immunohistochemical detection of neurokinins (NK) and the enzyme choline acetyl-transferase (ChAT) in the human brainstem. No double-labelled structures were detected, although both NK and ChAT were observed in cell bodies and fibers of the same brainstem nuclei. The distribution of immunoreactive fibers is widespread, and NK and ChAT were observed in several motor cranial nerves as well as in the substantia nigra. The results obtained in the present work provide a neuroanatomical basis for possible physiological interactions between NK and ChAT that may be carried out by volume-transmission mechanisms. These interactions might participate in motor regulation or in limbic pathways as well as influence on other neurotransmitter systems such as the dopaminergic system.

Reversible Pharmacological Induction of Motor Symptoms in MPTP-Treated Mice at the Presymptomatic Stage of Parkinsonism: Potential Use for Early Diagnosis of Parkinson's Disease

A crucial event in the pathogenesis of Parkinson's disease is the death of dopaminergic neurons of the nigrostriatal system, which are responsible for the regulation of motor function. Motor symptoms first appear in patients 20-30 years after the onset of the neurodegeneration, when there has been a loss of an essential number of neurons and depletion of compensatory reserves of the brain, which explains the low efficiency of treatment. Therefore, the development of a technology for the diagnosing of Parkinson's disease at the preclinical stage is of a high priority in neurology. In this study, we have developed at an experimental model a fundamentally novel for neurology approach for diagnosis of Parkinson's disease at the preclinical stage. This methodology, widely used for the diagnosis of chronic diseases in the internal medicine, is based on the application of a challenge test that temporarily increases the latent failure of a specific functional system, thereby inducing the short-term appearance of clinical symptoms. The provocation test was developed by a systemic administration of α-methyl-p-tyrosine (αMpT), a reversible inhibitor of tyrosine hydroxylase to MPTP-treated mice at the presymptomatic stage of parkinsonism. For this, we first selected a minimum dose of αMpT, which caused a decrease of the dopamine level in the striatum of normal mice below the threshold at which motor dysfunctions appear. Then, we found the maximum dose of αMpT at which a loss of dopamine in the striatum of normal mice did not reach the threshold level, and motor behavior was not impaired. We showed that αMpT at this dose induced a decrease of the dopamine concentration in the striatum of MPTP-treated mice at the presymptomatic stage of parkinsonism below a threshold level that results in the impairment of motor behavior. Finally, we proved that αMpT exerts a temporal and reversible influence on the nigrostriatal dopaminergic system of MPTP-treated mice with no long-term side effects on other catecholaminergic systems. Thus, the above experimental data strongly suggest that αMpT-based challenge test might be considered as the provocation test for Parkinson's disease diagnosis at the preclinical stage in the future clinical trials.

Changes in Proenkephalin mRNA expression in forebrain areas after amphetamine-induced behavioural sensitization

Acute and repeated psychostimulant administration induces a long-lasting enhanced behavioural response to a subsequent drug challenge, known as behavioural sensitization. This phenomenon involves persistent neurophysiological adaptations, which may lead to drug addiction. Brain dopaminergic pathways have been implicated as the main neurobiological substrates of behavioural sensitization, although other neurotransmitters and neuromodulators may also participate. In order to investigate a possible involvement of opioid systems in amphetamine (AMPH) behavioural sensitization, we studied the AMPH-induced changes in Proenkephalin (Pro-Enk) mRNA expression in forebrain areas in both drug-naïve and AMPH-sensitized rats. Male Sprague-Dawley rats were sensitized to AMPH by means of a single AMPH (1 mg/kg s.c.) injection and the same dose was injected 7 days later to assess the expression of sensitization. Pro-Enk mRNA levels were evaluated by in situ hybridization in coronal brain sections. AMPH injection induced an increase in Pro-Enk mRNA expression in the nucleus accumbens and the medial-posterior caudate-putamen in drug-naïve rats. Challenge with AMPH to rats injected 1 week earlier with AMPH induced motor sensitization and increased and decreased Pro-Enk mRNA expression in the prefrontal cortex and the anterior medial caudate-putamen, respectively. Our results suggest that alterations in cortical and striatal enkephalinergic systems could contribute to the expression of AMPH behavioural sensitization.

Reliable long-lasting depression interacts with variable short-term facilitation to determine corticostriatal paired-pulse plasticity in young rats

Synaptic plasticity at corticostraital synapses is proposed to fine tune movment and improve motor skills. We found paired-pulse plasticity at corticostriatal synapses reflected variably expressed short-term facilitation blended with a consistent background of longer-lasting depression. Presynaptic modulation via neuotransmitter receptor activation was ruled out as a mechanism for long-lasting paired-pulse depression by examining the effect of selective receptor antagonists. EPSC amplitude and paired-pulse plasticity, however, was influenced by block of D2 dopamine receptors. Block of glutamate transport with l-transdicarboxylic acid (PDC) reduced EPSCs, possibly through a mechanism of AMPA receptor desensitization. Removal of AMPA receptor desensitization with cyclothiazide reduced the paired-pulse depression at long-duration interstimulus intervals (ISIs), indicating that AMPA receptor desensitization participates in corticostriatal paired-pulse plasticity. The low-affinity glutamate receptor antagonist cis-2,3-piperidine dicarboxylic acid (PDA) increased paired-pulse depression, suggesting that a presynaptic component also exists for long-lasting paired-pulse depression. Low Ca(2+)-high Mg(2+) or BAPTA-AM dramatically reduced the amplitude of corticostriatal EPSCs and both manipulations increased the expression of facilitation and, to a lesser extent, they reduced long-lasting paired-pulse depression. EGTA-AM produced a smaller reduction in EPSC amplitude and it did not alter paired-pulse facilitation, but in contrast to low Ca(2+) and BAPTA-AM, EGTA-AM increased long-lasting paired-pulse depression. These experiments suggest that facilitation and depression are sensitive to vesicle depletion, which is dependent upon changes in peak Ca(2+) (i.e. low Ca(2+)-high Mg(2+) or BAPTA-AM). In addition, the action of EGTA-AM suggests that basal Ca(2+) regulates the recovery from long-lasting paired-pulse depression, possibly thourgh a Ca(2+)-sensitive process of vesicle delivery.

A proposed hypothalamic-thalamic-striatal axis for the integration of energy balance, arousal, and food reward

We elaborate herein a novel theory of basal ganglia function that accounts for why palatable, energy-dense foods retain high incentive value even when immediate physiological energy requirements have been met. Basal ganglia function has been studied from the perspective of topographical segregation of processing within parallel circuits, with primary focus on motor control and cognition. Recent findings suggest, however, that the striatum can act as an integrated unit to modulate motivational state. We describe evidence that the striatal enkephalin system, which regulates the hedonic impact of preferred foods, undergoes coordinated gene expression changes that track current motivational state with regard to food intake. Striatal enkephalin gene expression is also downregulated by an intrastriatal infusion of a cholinergic muscarinic antagonist, a manipulation that greatly suppresses food intake. To account for these findings, we propose that signaling through a hypothalamic-midline thalamic-striatal axis impinges on the cholinergic interneurons of the striatum, which via their large, overlapping axonal fields act as a network to modulate enkephalin-containing striatal output neurons. A key relay in this circuit is the paraventricular thalamic nucleus, which receives convergent input from orexin-coded hypothalamic energy-sensing and behavioral state-regulating neurons, as well as from circadian oscillators, and projects to cholinergic interneurons throughout the striatal complex. We hypothesize that this system evolved to coordinate feeding and arousal, and to prolong the feeding central motivational state beyond the fulfillment of acute energy needs, thereby promoting "overeating" and the consequent development of an energy reserve for potential future food shortages.

Neurokinin-1 receptor gene expression in the mouse dorsal horn increases with neuropathic pain

Peripheral nerve injury is associated with hyperesthesia and increased neurokinin-1 receptor (NK-1) expression in the dorsal horn of the spinal cord. To test the hypothesis that NK-1 gene expression underlies these responses, we used solution hybridization-nuclease protection assays to quantify NK-1 mRNA levels in dorsal quadrants of the mouse lumbar dorsal horn. Partial sciatic nerve ligation was associated with mechanical allodynia, thermal hyperalgesia, and an increase in NK-1 mRNA on the ipsilateral, but not contralateral, side. Regression analysis showed that NK-1 mRNA was significantly correlated with thermal paw withdrawal latency but not mechanical threshold. Our results support the idea that substance P is an important mediator of thermal hypersensitivity in the setting of nerve injury and suggest that increased NK-1 receptor transcription precedes increased NK-1 receptor density, ultimately leading to behavioral hypersensitivity to peripheral thermal stimulation.

PERSPECTIVE

The therapeutic efficacy of NK-1 receptor antagonists is unclear. The current data suggest that peripheral nerve injury increases the expression of substance P (NK-1) receptors in the spinal cord dorsal horn; this is correlated with heat hypersensitivity. The analgesic effects of NK-1 antagonists might become apparent if tested against heat-evoked pain in nerve injury patients.

Impaired preprodynorphin, but not preproenkephalin, mRNA induction in the striatum of mGluR1 mutant mice in response to acute administration of the full dopamine D(1) agonist SKF-82958

Metabotropic glutamate receptor 1 (mGluR1) is highly expressed in striatonigral projection neurons of rat striatum. To define the role of mGluR1 in the regulation of striatal gene expression, the responsiveness of the three neuropeptide gene expression to a single injection of the dopamine D(1) agonist SKF-82958 was compared between mGluR1 mutant and wild-type control mice. We found that acute injection of SKF-82958 increased preprodynorphin (PPD), substance P (SP), and preproenkephalin (PPE) mRNAs in the dorsal and ventral striatum of mutant and wild-type mice in a dose-dependent manner (0.125, 0.5, and 2 mg/kg, i.p.) as revealed by quantitative in situ hybridization. However, the induction of PPD mRNA in both the dorsal and ventral striatum of mGluR1 minus sign/minus sign mice was significantly less than that of wild-type +/+ mice in response to the two higher doses of SKF-82958. In contrast to PPD, SP and PPE in the dorsal and ventral striatum of mGluR1 mutant mice were elevated to a similar level as that of wild-type mice. There were no differences in basal levels and distribution patterns of all three mRNAs between the two genotypes of mice treated with saline. These results indicate that mGluR1 selectively participates in striatonigral PPD induction in response to D(1) receptor stimulation.

The role of substance P in depression: therapeutic implications

Substance P (for "powder"), identified as a gut tachykinin in 1931 and involved in the control of multiple other autonomic functions, notably pain transmission, is the focus of intense fundamental and clinical psychiatric research as a central neurotransmitter, neuromodulator, and immunomodulator, along with sister neurokinins A and B (NKA and NKB), discovered in 1984. Substance P is widely distributed throughout the central nervous system, where if is often colocalized with serotonin, norepinephrine, and dopamine. Many neurokinin (NK) receptor antagonists and agonists have been synthesized and some clinically tested. A double-blind study of MK869, a selective NK1 receptor antagonist that blocks the action of substance P, showed significant activity versus placebo and fewer sexual side effects than paroxetine in outpatients with major depression and moderate anxiety. Substance P, which is degraded by the angiotensin-converting enzyme (ACE), may mediate modulation of therapeutic outcome in affective disorders by functional polymorphism within the ACE gene: the D allele is associated with higher ACE levels and increased neuropeptide degradation, with the result that patients with major depression who carry the D allele have lower depression scores and shorter hospitalization. ACE polymorphism genotypinq might thus identify those patients with major depression likely to benefit from NK1 receptor antagonist therapy.

Rescue of locomotor impairment in dopamine D2 receptor-deficient mice by an adenosine A2A receptor antagonist

In Parkinson's disease a degeneration of dopaminergic neurons of the nigrostriatal pathway is observed. Loss of dopaminergic regulation of striatal neuron activity results in altered motor functions. Adenosine A2A (A2AR) and dopamine D2 (D2R) receptors are colocalized in striatal medium spiny neurons. It has been proposed that adenosine binding to A2AR lowers the affinity of dopamine for D2R, thus modulating the function of this receptor. Absence of D2R in knockout mice (D2R-/-) results in impaired locomotion and coordinated movements. This indicates that absence of dopamine in Parkinson's disease might principally affect D2R-mediated effects with regard to locomotor functions. A2AR-selective antagonists have been demonstrated to have anti- parkinsonian activities in various models of Parkinson's disease in rodents and nonhuman primates. In this article, D2R-/- mice were used to explore the possibility that an A2AR antagonist might reestablish their motor impairment. Interestingly, blockade of A2AR rescues the behavioral parameters altered in D2R-/- mice. In addition, the level of expression of enkephalin and substance P, which were altered in D2R-/-, were also reestablished to normal levels after A2AR antagonist treatment. These results show that A2AR and D2R have antagonistic and independent activities in controlling neuronal and motor functions in the basal ganglia. They also provide evidence that selective A2AR antagonists can exhibit their anti-parkinsonian activities through a nondopaminergic mechanism.

Substantia nigra D1 receptors and stimulation of striatal cholinergic interneurons by dopamine: a proposed circuit mechanism

Dopamine release can regulate striatal acetylcholine efflux in vivo through at least two receptor mechanisms: (1) direct inhibition by dopamine D2 receptors on the cholinergic neurons, and (2) excitation initiated by dopamine D1 receptors. The neuroanatomical locus of the latter population of D1 receptors and the pathway(s) involved in the expression of their influence are controversial issues. We have tested the hypothesis that D1 receptors in substantia nigra pars reticulata are involved in the excitatory component of dopaminergic actions on striatal acetylcholine output. In vivo microdialysis was used in awake rats. Infusion of the selective D1 receptor agonist R(+)-1-Phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 38393) hydrochloride into pars reticulata of substantia nigra elicited a significant increase in striatal acetylcholine efflux. Likewise, D-amphetamine applied into pars reticulata of substantia nigra by reverse dialysis produced an elevation in acetylcholine output measured at a second microdialysis probe in the striatum. Application of D-amphetamine in the striatum by reverse dialysis elicited a decrease in striatal acetylcholine efflux that could be reversed subsequently by local application of D-amphetamine in substantia nigra pars reticulata. A 2 mg/kg intraperitoneal dose of D-amphetamine, which has no net effect on striatal acetylcholine output under control conditions, elicited a significant decrease in acetylcholine efflux when the D1 receptor antagonist R(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine (SCH 23390) hydrochloride was applied simultaneously via a second microdialysis probe in substantia nigra pars reticulata. Thus, an excitatory D1-mediated influence on striatal acetylcholine output is initiated in substantia nigra pars reticulata, and this influence contributes to the effects of indirect dopaminergic agonists such as D-amphetamine on striatal acetylcholine efflux. These results indicate an important role of somatodendritic dopamine release, in addition to nerve terminal dopamine release, in the regulation of activity in basal ganglia circuits.

The muscarinic toxin 3 augments neuropeptide mRNA in rat striatum in vivo

The selective M4 muscarinic receptor toxin, MT3, was used in vivo to evaluate the role of M4 receptors in cholinergic inhibition of neuropeptide mRNA expression in striatonigral neurons. Unilateral injection of the muscarinic toxin 3 (0.04-4 nmol) into the dorsal striatum of chronically-cannulated rats elevated basal levels of preprodynorphin, substance P and preproenkephalin mRNAs in the ipsilateral dorsal striatum as revealed by quantitative in situ hybridization. Pretreatment with muscarinic toxin 3 also augmented amphetamine (2.5 mg/kg, i.p.)-stimulated preprodynorphin and substance P expression in the dorsal striatum in a manner similar to that observed after the muscarinic antagonist, scopolamine. Since muscarinic toxin 3 has a much greater affinity for muscarinic M4 receptors than for other subtypes, it is possible that muscarinic toxin 3, by interacting with the muscarinic M4 subtype, regulates basal and/or dopamine-stimulated striatal neuropeptide gene expression.

Intrastriatal injection of a muscarinic receptor agonist and antagonist regulates striatal neuropeptide mRNA expression in normal and amphetamine-treated rats

Systemic administration of the muscarinic receptor antagonist, scopolamine, augments, whereas the muscarinic receptor agonist, oxotremorine, attenuates behaviors (locomotion and stereotypies) and preprodynorphin (PPD) and substance P (SP) gene expression in striatonigral neurons induced by the indirect dopamine receptor agonist, amphetamine (AMPH). In contrast, systemic scopolamine blocks, whereas oxotremorine augments, AMPH-stimulated preproenkephalin (PPE) gene expression in striatopallidal neurons. This study investigated the site of action of these effects by administering scopolamine and oxotremorine directly into the striatum and assessing the expression of neuropeptide mRNAs with quantitative in situ hybridization. Unilateral injection of scopolamine into the dorsal striatum augmented, and oxotremorine attenuated, AMPH (2.5 mg/kg, i.p.)-stimulated behaviors. Intrastriatal scopolamine at a concentration of 62 mM, but not 6.2 mM, increased basal levels of PPD and SP mRNAs in the dorsal striatum. In addition, both 6.2 and 62 mM scopolamine significantly augmented AMPH-stimulated PPD and SP mRNA levels. Intrastriatal infusion of 1.6 or 8.1 mM oxotremorine did not alter basal levels of striatal PPD and SP mRNAs. However, both concentrations of oxotremorine completely blocked AMPH-stimulated SP mRNA and oxotremorine at 8.1 mM blocked AMPH-stimulated PPD mRNA. In contrast, PPE induction by AMPH was blocked by 62, but not 6.2, mM scopolamine. Both concentrations of oxotremorine tended to augment basal and AMPH-stimulated PPE mRNA in the dorsal striatum but the trend was not significant. These data demonstrate an inhibition of striatonigral, and facilitation of striatopallidal, gene expression through activation of local striatal muscarinic receptors, which is consistent with the changes seen after systemic administration of muscarinic agents. Therefore, muscarinic cholinergic regulation of basal and stimulated expression of neuropeptide mRNA is processed within the striatum.

Effects of L-DOPA-therapy on dopamine D2 receptor mRNA expression in the striatum of MPTP-intoxicated parkinsonian monkeys

The cellular expression of dopamine D2 receptor mRNA was examined in striatal (caudate nucleus and putamen) neurones of 9 Macaca fascicularis monkeys rendered parkinsonian by systemic injection of MPTP. Messenger RNA abundance was determined by quantitative in situ hybridization using human-specific 35S-labelled oligonucleotides. Control monkeys were untreated and received neither MPTP nor L-DOPA while the rest were rendered parkinsonian and received chronic levodopa therapy to induce dyskinesia. In the control brains a strong dopamine D2 receptor hybridization signal was detected overlying medium-sized and some large neurons in both the caudate nucleus and putamen. Neurons from the lateral and medial regions of the caudate nucleus, and from the dorsal and ventral regions of the putamen were analysed separately. A significant increase in the cellular abundance of dopamine D2 receptor mRNA was seen in the striatum of MPTP-treated monkeys; this increase being restricted to the population of medium-sized striatal cells. No such increase in dopamine D2 receptor mRNA was observed in (dyskinetic) L-DOPA-treated monkeys suggesting that levodopa-therapy normalises D2 receptor expression in post-synaptic striatal cells. The cellular abundance of dopamine D2 receptor mRNA expressed by large striatal neurons (putative cholinergic cells) was unaffected by either MPTP treatment or levodopa therapy. The implications of these findings for the development of levodopa-induced dyskinesias is discussed.

Muscarinic receptors regulate striatal neuropeptide gene expression in normal and amphetamine-treated rats

This study investigated the effects of pharmacological blockade or stimulation of muscarinic receptors on constitutive and amphetamine-stimulated preprodynorphin, substance P and pre-proenkephalin gene expression in rat striatum. Acute administration of the non-selective muscarinic antagonist, scopolamine (2.5, 5 and 10 mg/kg, s.c.), caused a dose-dependent increase in preprodynorphin and substance P, but not preproenkephalin, messenger RNA expression in the dorsal and ventral striatum as revealed by quantitative in situ hybridization. In contrast, acute injection of the non-selective muscarinic receptor agonist, oxotremorine (0.125, 0.25 and 0.5 mg/kg, s.c.), caused a dose-dependent increase in basal levels of preproenkephalin messenger RNA in the dorsal striatum, without causing a significant effect on constitutive striatal preprodynorphin and substance P expression. Pretreatment with scopolamine (2.5 mg/kg, s.c.) significantly augmented striatal induction of preprodynorphin and substance P messenger RNA induced by acute injection of amphetamine (1.25 and 2.5 mg/kg, i.p.), whereas scopolamine blocked amphetamine-stimulated striatal preproenkephalin expression. Pretreatment with oxotremorine (0.25 mg/kg, s.c.) significantly attenuated amphetamine (1.25 and 2.5 mg/kg, i.p.)-stimulated striatal preprodynorphin and, to a lesser degree, substance P messenger RNA expression. Oxotremorine tended to increase amphetamine-stimulated preproenkephalin messenger RNA expression, but the effect did not reach statistical significance. In addition, scopolamine increased spontaneous, and enhanced amphetamine-stimulated, behavioral activity, whereas oxotremorine attenuated amphetamine-stimulated behaviors. These data support the concept that cholinergic transmission, via interaction with muscarinic receptors, inhibits basal and D1 receptor-stimulated striatonigral dynorphin/substance P gene expression and facilitates striatopallidal enkephalin gene expression.