Cortical regulation of acetylcholine release in rat striatum

Prefrontal Cortex-Driven Dopamine Signals in the Striatum Show Unique Spatial and Pharmacological Properties

Dopamine (DA) signals in the striatum are critical for a variety of vital processes, including motivation, motor learning, and reinforcement learning. Striatal DA signals can be evoked by direct activation of inputs from midbrain DA neurons (DANs) as well as cortical and thalamic inputs to the striatum. In this study, we show that in vivo optogenetic stimulation of prelimbic (PrL) and infralimbic (IL) cortical afferents to the striatum triggers an increase in extracellular DA concentration, which coincides with elevation of striatal acetylcholine (ACh) levels. This increase is blocked by a nicotinic ACh receptor (nAChR) antagonist. Using single or dual optogenetic stimulation in brain slices from male and female mice, we compared the properties of these PrL/IL-evoked DA signals with those evoked by stimulation from midbrain DAN axonal projections. PrL/IL-evoked DA signals are undistinguishable from DAN evoked DA signals in their amplitudes and electrochemical properties. However, PrL/IL-evoked DA signals are spatially restricted and preferentially recorded in the dorsomedial striatum. PrL/IL-evoked DA signals also differ in their pharmacological properties, requiring activation of glutamate and nicotinic ACh receptors. Thus, both in vivo and in vitro results indicate that cortical evoked DA signals rely on recruitment of cholinergic interneurons, which renders DA signals less able to summate during trains of stimulation and more sensitive to both cholinergic drugs and temperature. In conclusion, cortical and midbrain inputs to the striatum evoke DA signals with unique spatial and pharmacological properties that likely shape their functional roles and behavioral relevance.SIGNIFICANCE STATEMENT Dopamine signals in the striatum play a critical role in basal ganglia function, such as reinforcement and motor learning. Different afferents to the striatum can trigger dopamine signals, but their release properties are not well understood. Further, these input-specific dopamine signals have only been studied in separate animals. Here we show that optogenetic stimulation of cortical glutamatergic afferents to the striatum triggers dopamine signals both in vivo and in vitro These afferents engage cholinergic interneurons, which drive dopamine release from dopamine neuron axons by activation of nicotinic acetylcholine receptors. We also show that cortically evoked dopamine signals have other unique properties, including spatial restriction and sensitivity to temperature changes than dopamine signals evoked by stimulation of midbrain dopamine neuron axons.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Histological studies of the effects of chronic implantation of ceramic-based microelectrode arrays and microdialysis probes in rat prefrontal cortex

Chronic implantation of neurotransmitter measuring devices is essential for awake, behavioral studies occurring over multiple days. Little is known regarding the effects of long term implantation on surrounding brain parenchyma and the resulting alterations in the functional properties of this tissue. We examined the extent of tissue damage produced by chronic implantation of either ceramic microelectrode arrays (MEAs) or microdialysis probes. Histological studies were carried out on fixed tissues using stains for neurons (cresyl violet), astrocytes (GFAP), microglia (Iba1), glutamatergic nerve fibers (VGLUT1), and the blood-brain barrier (SMI-71). Nissl staining showed pronounced tissue body loss with microdialysis implants compared to MEAs. The MEAs produced mild gliosis extending 50-100 microm from the tracks, with a significant change in the affected areas starting at 3 days. By contrast, the microdialysis probes produced gliosis extending 200-300 microm from the track, which was significant at 3 and 7 days. Markers for microglia and glutamatergic fibers supported that the MEAs produce minimal damage with significant changes occurring only at 3 and 7 days that return to control levels by 1 month. SMI-71 staining supported the integrity of the blood-brain barrier out to 1 week for both the microdialysis probes and the MEAs. This data support that the ceramic MEA's small size and biocompatibility are necessary to accurately measure neurotransmitter levels in the intact brain. The minimal invasiveness of the MEAs reduce tissue loss, allowing for long term (>6 month) electrochemical and electrophysiological monitoring of brain activity.

Blockade of NMDA receptors in the prefrontal cortex increases dopamine and acetylcholine release in the nucleus accumbens and motor activity

OBJECTIVES

The present study investigates the effects of injections of a specific N-methyl-D-aspartic acid (NMDA) antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phophonic acid (CPP) into the prefrontal cortex (PFC) on the extracellular concentrations of dopamine and acetylcholine in the nucleus accumbens (NAc) and on motor activity in the freely moving rat.

MATERIALS AND METHODS

Sprague-Dawley male rats were implanted with guide cannulas into the medial PFC and NAc to perform bilateral microinjections and microdialysis experiments. Spontaneous motor activity was monitored in the open field.

RESULTS

Injections of CPP (1 microg/0.5 microL) into the PFC produced a significant increase of the baseline extracellular concentrations of dopamine (up to 130%), dihydroxyphenylacetic acid (DOPAC; up to 120%), homovanillic acid (HVA; up to 130%), and acetylcholine (up to 190%) in the NAc as well as motor hyperactivity. In the NAc, perfusion of the NMDA and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate antagonists CPP (50 microM)+6,7-dinitroquinoxaline-2,3-dione (DNQX; 50 microM) through the microdialysis probe blocked acetylcholine release, but not DOPAC and HVA increases produced by CPP injections into the PFC. Also, increases in motor activity produced by prefrontal injections of CPP were significantly reduced by bilateral injections into the NAc of a mixed D1/D2 antagonist, flupenthixol (5 and 25 microg/0.5 microL). Injections into the NAc of the muscarinic antagonist scopolamine (1 and 10 microg/0.5 microL) further increased, and of the nicotinic antagonist mecamylamine (1 and 10 microg/0.5 microL) did not change, the increases in motor activity produced by prefrontal CPP injections.

CONCLUSIONS

These results suggest that the dysfunction of NMDA receptors in the PFC could be a key factor in the neurochemical and motor effects associated with corticolimbic hyperactivity.

Prefrontal cortex-nucleus accumbens interaction: in vivo modulation by dopamine and glutamate in the prefrontal cortex

Previous experimental studies have shown that the prefrontal cortex (PFC) regulates the activity of the nucleus accumbens (NAc), and in particular the release of dopamine in this area of the brain. In the present report we review recent microinjections/microdialysis studies from our laboratory on the effects of stimulation/blockade of dopamine and glutamate receptors in the PFC that modulate dopamine, and also acetylcholine release in the NAc. Stimulation of prefrontal D2 dopamine receptors, but not group I mGlu glutamate receptors, reduces the release of dopamine and acetylcholine in the NAc and spontaneous motor activity. This inhibitory role of prefrontal D2 receptors is not changed by acute systemic injections of the NMDA antagonist phencyclidine. On the other hand, the blockade of NMDA receptors in the PFC increases the release of dopamine and acetylcholine in the NAc as well as motor activity which suggests that the hypofunction of prefrontal NMDA receptors is able to produce the neurochemical and behavioural changes associated with a dysfunction of the corticolimbic circuit. We suggest here that dopamine and glutamate receptors are, in part, segregated in specific cellular circuits in the PFC. Thus, the stimulation/blockade of these receptors would have a different net impact on PFC output projections to regulate dopamine and acetylcholine release in the NAc and in guided behaviour. Finally, it is speculated that environmental enrichment might produce plastic changes that modify the functional interaction between the PFC and the NAc in both physiological and pathological conditions.

Analysis of amino acids and catecholamines, 5-hydroxytryptamine and their metabolites in brain areas in the rat using in vivo microdialysis

In vivo microdialysis, using dialysis probes inserted into discrete brain areas and subsequent analysis of neurotransmitters and related substances in the dialysates (usually with HPLC), has yielded a great deal of important information about the actions of psychotropic drugs and endogenous neurotransmitter systems and about the functional interactions between various brain areas. This paper reviews the principles involved in in vivo microdialysis, its advantages and disadvantages, and recent innovations in methodology and applications. The first section includes brief discussions of principles and applications of dialysis, use of anesthetized versus conscious freely moving animals, and methods used to determine the neural origin of neurotransmitters in the dialysate. The subsequent sections provide detailed descriptions, based largely on our own studies in rats, of stereotaxic surgery, in vivo microdialysis, and dialysate analysis, with an emphasis on amino acids and biogenic amines and their metabolites. A discussion of methodological problems which may be encountered in the analysis of amino acids and biogenic amines is also included.

Cortical and nigral deafferentation and striatal cholinergic markers in the rat dorsal striatum: different effects on the expression of mRNAs encoding choline acetyltransferase and muscarinic m1 and m4 receptors

The regulation of the striatal m1 and m4 muscarinic receptor mRNA as well as the choline acetyltransferase (ChAT) mRNA expression by nigral dopaminergic and cortical glutamatergic afferent fibres was investigated using quantitative in situ hybridization histochemistry. The effects induced by a unilateral lesion of the medial forebrain bundle and a bilateral lesion of the sensorimotor (SM) cortex were analysed in the dorsal striatum 3 weeks after the lesions. Dopaminergic denervation of the striatum resulted in a marked decrease in the levels of m4 mRNA throughout the striatum, while the levels of muscarinic m1 mRNA and ChAT mRNA in cholinergic neurons were unaffected by the lesion. In contrast, following bilateral cortical ablation, the levels of the muscarinic m1 mRNA were significantly increased in the striatal projection area of the SM cortex, whereas the expression of m4 mRNA remained unchanged. Single cholinergic cell analysis by computer-assisted grain counting revealed a decreased labelling for ChAT mRNA per neuron following cortical ablation. However, in contrast to the topographical m1 mRNA changes, the decreased ChAT mRNA expression was evenly distributed within the striatum, suggesting an indirect cortical control upon striatal cholinergic interneurons. Altogether, these data suggest that dopaminergic nigral and glutamatergic cortical afferents modulate differentially cholinergic markers, at the pre- and post-synaptic levels. Beside the fact that nigral and cortical inputs exert an opposite control on cholinergic neurotransmission, our study further shows that this control involved different muscarinic receptor subtypes: the m4 and m1 receptors, respectively.

The parafascicular thalamic nucleus but not the prefrontal cortex facilitates the nitric oxide/cyclic GMP pathway in rat striatum

We investigated whether the parafascicular thalamic nucleus and the prefrontal cortex, the two major excitatory inputs to the striatum, modulate the nitric oxide/cyclic GMP pathway in rat striatum. Electrical stimulation (10 pulses of 0.5 ms, 10 V applied at 10 Hz, 140 microA) delivered bilaterally to the parafascicular thalamic nucleus for a total of 4, 10 and 20 min, time-dependently facilitated cyclic GMP output in the dorsal striatum of freely moving rats, assessed by trans-striatal microdialysis. Electrical stimulation to the prefrontal cortex for a total duration of 20 min did not affect striatal cyclic GMP levels. The facilitatory effect observed after electrical stimulation of the parafascicular thalamic nucleus was blocked by co-perfusion with tetrodotoxin, suggesting that the effect is mediated by neuronal process(es). The non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (30 microM infused into the dorsal striatum), and the competitive one, 3-[(R)-carboxypiperazin-4-yl]-propyl-phosphonic acid (50 microM infused), but not local perfusion of the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid antagonist, 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (15 microM perfused locally), abolished the cyclic GMP response in the striatum. The nitric oxide synthase inhibitor, 7-nitroindazole, applied locally (1 mM), blocked the electrically evoked increase in striatal extracellular cyclic GMP. This increase was also prevented by local application (100 and 300 microM) of 1H-(1,2,4)-oxadiazolo-(4,3a)-quinoxalin-1-one, a selective inhibitor of soluble guanylyl cyclase. The results provide direct functional evidence of selective thalamic facilitation of the nitric oxide/cyclic GMP pathway in the dorsal striatum, through activation of N-methyl-D-aspartate receptors.

Diffuse transmission by acetylcholine in the CNS

Recent immunoelectron microscopic studies have revealed a low frequency of synaptic membrane differentiations on ACh (ChAT-immunostained) axon terminals (boutons or varicosities) in adult rat cerebral cortex, hippocampus and neostriatum, suggesting that, besides synaptic transmission, diffuse transmission by ACh prevails in many regions of the CNS. Cytological analysis of the immediate micro-environment of these ACh terminals, as well as currently available immunocytochemical data on the cellular and subcellular distribution of ACh receptors, is congruent with this view. At least in brain regions densely innervated by ACh neurons, a further aspect of the diffuse transmission paradigm is envisaged: the existence of an ambient level of ACh in the extracellular space, to which all tissue elements would be permanently exposed. Recent experimental data on the various molecular forms of AChE and their presumptive role at the neuromuscular junction support this hypothesis. As in the peripheral nervous system, degradation of ACh by the prevalent G4 form of AChE in the CNS would primarily serve to keep the extrasynaptic, ambient level of ACh within physiological limits, rather than totally eliminate ACh from synaptic clefts. Long-lasting and widespread electrophysiological effects imputable to ACh in the CNS might be explained in this manner. The notions of diffuse transmission and of an ambient level of ACh in the CNS could also be of clinical relevance, in accounting for the production and nature of certain cholinergic deficits and the efficacy of substitution therapies.

The cerebral cortex and parafascicular thalamic nucleus facilitate in vivo acetylcholine release in the rat striatum through distinct glutamate receptor subtypes

Electrical stimulation (ten pulses of 0.5 ms, 10 V applied over 10 s at 10 Hz, 140 microA) delivered bilaterally to the prefrontal cortex or the parafascicular thalamic nucleus of freely moving rats facilitated acetylcholine release in dorsal striata, assessed by trans-striatal microdialysis. The facilitatory effects were blocked by coperfusion with 5 microM tetrodotoxin, suggesting that the release was of neuronal origin. The response of the striatal cholinergic neurons to prefrontal cortical stimulation was short-lived and required a longer period of stimulation (20 min) that the response to thalamic stimulation (4 min) to reach maximal effect. The alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate glutamatergic receptor antagonist 6,7-dinitroquinoxaline-2,3-dione [DNQX; 12 nmol per side, intracerebroventricularly (i.c.v.)] and the AMPA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX; 12 nmol per side, i.c.v. or 12.8 microM infused into the striatum), but not the NMDA-type receptor antagonist MK-801 (0.2 mg/kg, i.p.), abolished the facilitatory effect on striatal acetylcholine release evoked by stimulation of the prefrontal cortex. By contrast, DNQX or NBQX did not prevent the increase in striatal acetylcholine release evoked by parafascicular nucleus stimulation, but MK-801, in accordance with previous results, did so. MK-801 by itself lowered striatal acetylcholine output while DNQX and NBQX did not. The results provide in vivo evidence that the cerebral cortex facilitates cholinergic activity in the dorsal striatum apparently through the non-tonic activation of AMPA-type glutamatergic receptors while the parafascicular nucleus does this through tonic activation of NMDA receptors. Both glutamate receptor types are probably located in the striatum. The overall results suggest that the two pathways operate independently to regulate striatal cholinergic activity through distinct mechanisms.

Amperometric microsensors for monitoring choline in the extracellular fluid of brain

Selective amperometric enzyme microsensors for monitoring low micromolar concentrations of choline in extracellular fluid of rat brain have been developed. Preparation of the choline microsensors involved the modification of carbon fiber microcylinder electrodes (10 microns diameter, 300-400 microns long) with a cross-linked redox-active gel containing horseradish peroxidase and choline oxidase. Rejection of the noise recorded from the choline microsensors implanted in living brain tissue improved the in vivo detection capabilities of the sensors. The microsensors and a differential detection scheme were used to estimate the basal concentration of choline in striatal tissue at 6.6 +/- 2.9 microM and to measure changes in choline concentrations of 6.1 +/- 2.7 microM in vivo. The microsensors were also used to monitor choline produced following the injections of acetylcholine in vivo. Coinjections of neostigmine and acetylcholine significantly lowered the choline response recorded with the microsensors, confirming that the response following the injections of acetylcholine alone was due to the activity of endogenous acetylcholinesterase. Comparison of the maximal rate of decrease in choline concentration following the injections of 1 mM choline and 1 mM acetylcholine was used to estimate the rate of acetylcholine clearance from extracellular fluid through cholinesterase activity at approx. 2.5 microM/min.

Modulatory action of dopamine on acetylcholine-responsive striatal and accumbal neurons in awake, unrestrained rats

In ambulant rats, iontophoresis of low concentrations of dopamine (DA) enhances the response of neurons in striatum and nucleus accumbens to iontophoretic glutamate. In an extension of this line of investigation, we tested the effects of acetylcholine (ACh), a presumed modulator of neuronal function in these same brain regions, and assessed possible DA-ACh interactions. Data were obtained from spontaneously active neurons known to respond to ACh (5-30 nA) when the animals rested quietly with no overt movement. ACh iontophoresis either excited or inhibited striatal and accumbal activity but excitatory effects predominated in both areas. With multiple applications of ACh, especially at the lowest currents tested, either response often was interspersed with instances of no change in firing rate. Responsiveness to ACh also diminished during periods of spontaneous movement when basal firing showed phasic increases in activity. In fact, neurons with the highest rates of basal activity showed the smallest magnitude response to ACh. Prolonged applications (120-180 s) of DA attenuated basal firing as well as the iontophoretic effects of ACh both during the DA application itself and for up to 1 min after DA ejection offset. The result of these inhibitory effects was no net change in the relative magnitude of the ACh response. Thus, although ACh can modulate striatal and accumbal neuronal activity, DA does not regulate this effect in the same way that it regulates the neuronal responsiveness to glutamate.

Role of the parafascicular thalamic nucleus and N-methyl-D-aspartate transmission in the D1-dependent control of in vivo acetylcholine release in rat striatum

We investigated the involvement of glutamatergic neurotransmission in the modulation of D1 receptor-mediated stimulation of acetylcholine outflow in dorsal striatum in freely moving rats, and the relative roles of the thalamostriatal and corticostriatal pathways in this regulation using in vivo microdialysis. The selective N-methyl-D-aspartate non-competitive antagonist dizocilpine maleate (0.1 mg/kg i.p.), but not the kainate/quisqualate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (3 micrograms per side i.c.v.), completely prevented the rise in striatal extracellular acetylcholine elicited by maximal effective doses of the full D1 agonist SKF 82958 (3 mg/kg s.c.) and of the dopamine releaser d-amphetamine (2 mg/kg s.c.). Acute bilateral electrolytic lesions of the parafascicular nucleus of the thalamus prevented the stimulation of striatal acetylcholine output by SKF 82958 and d-amphetamine but only slightly reduced basal acetylcholine release. In contrast acute interruption of the corticostriatal pathway did not alter the effect of the two dopaminergic drugs although it markedly reduced basal striatal acetylcholine release. Lesions of the parafascicular thalamic nucleus, or a low dose of dizocilpine maleate (0.1 mg/kg i.p.), also prevented the acetylcholine-increasing effect of the neuroleptic remoxipride (10 mg/kg s.c.), an effect known to be D1 receptor dependent. The results suggest that striatal projections arising from the parafascicular thalamic nucleus and utilizing N-methyl-D-aspartate receptors play a critical role in the D1-mediated stimulation of acetylcholine release in dorsal striata.

Cortical stimulation induces Fos expression in striatal neurons via NMDA glutamate and dopamine receptors

Cortical electrical stimulation has been shown to induce dense and widespread Fos expression throughout the ipsilateral and contralateral striatum. This raises interest for studying the mechanisms underlying the regulation of striatal neuron activity by cortical afferents, and for elucidating the interactions with other systems. However, the receptors mediating cortical-stimulation-induced expression of Fos in striatal neurons have not been identified. This was studied in the work reported here by stimulating the cortex after administration of glutamate or dopamine receptor antagonists, or after 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal dopaminergic system. Pretreatment with the non-competitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 led to a marked reduction in the stimulation-induced density of Fos-immunoreactive nuclei in both the medial (about 80% reduction) and lateral (about 50-60% reduction) striatum. Preadministration of the D1-selective dopamine antagonist SCH-23390 alone or in combination with the D2-selective dopamine antagonist eticlopride led to a reduction in the stimulation-induced density of Fos-positive nuclei of about 60-65% in the lateral striatum, but no significant change in the medial region. The effects of 6-OHDA lesion were less pronounced, and the stimulation-induced density of Fos-immunoreactive nuclei decreased by only about 25% in the lateral region. These results indicate that both dopamine and NMDA glutamate receptors are involved in the induction of Fos by cortical stimulation, and support the hypothesis that cortex-dopamine interactions in the lateral striatum may be functionally different from those in the medial striatum.