Excitatory neuronal responses to dopamine in the cerebral cortex: involvement of D2 but not D1 dopamine receptors

Chemogenetic Inhibition of Dopamine D1-expressing Neurons in the Dorsal Striatum does not alter Methamphetamine Intake in either Male or Female Long Evans Rats

The biochemical and molecular substrates of methamphetamine (METH) use disorder remain to be elucidated. In rodents, increased METH intake is associated with increased expression of dopamine D1 receptors (D1R) in the dorsal striatum. The present study assessed potential effects of inhibiting striatal D1R activity on METH self-administration (SA) by rats. We microinjected Cre-activated adeno-associated viruses to deliver the inhibitory DREADD construct, hM4D (Gi) - mCherry, into neurons that expressed Cre-recombinase (D1-expressing neurons) in the dorsal striatum of male and female transgenic Long Evans rats (Drd1a-iCre#3). Two weeks later, we trained rats to self-administer METH. Once this behavior was acquired, intraperitoneal injections of clozapine-N-Oxide (CNO) or its vehicle (sterile water) were given to rats before each METH SA session to determine the effect of DREADD-mediated inhibition on METH intake. After the end of the experiments, histology was performed to confirm DREADD delivery into the dorsal striatum. There were no significant effects of the inhibitory DREADD on METH SA by male or female rats. Post-mortem histological assessment revealed DREADD expression in the dorsal striatum. Our results suggest that inhibition of D1R in the dorsal striatum does not suppress METH SA. It remains to be determined if activating D1R-expressing neurons might have differential behavioral effects. Future studies will also assess if impacting D1R activity in other brain regions might influence METH SA.

Sound sequence discrimination learning motivated by reward requires dopaminergic D2 receptor activation in the rat auditory cortex

We have previously reported that sound sequence discrimination learning requires cholinergic inputs to the auditory cortex (AC) in rats. In that study, reward was used for motivating discrimination behavior in rats. Therefore, dopaminergic inputs mediating reward signals may have an important role in the learning. We tested the possibility in the present study. Rats were trained to discriminate sequences of two sound components, and licking behavior in response to one of the two sequences was rewarded with water. To identify the dopaminergic inputs responsible for the learning, dopaminergic afferents to the AC were lesioned with local injection of 6-hydroxydopamine (6-OHDA). The injection attenuated sound sequence discrimination learning, while it had no effect on discrimination between the sound components of the sequence stimuli. Local injection of 6-OHDA into the nucleus accumbens attenuated sound discrimination learning. However, not only discrimination learning of sound sequence but also that of the sound components were impaired. SCH23390 (0.2 mg/kg, i.p.), a D1 receptor antagonist, had no effect on sound sequence discrimination learning, while it attenuated the licking behavior to unfamiliar stimuli. Haloperidol (0.5 mg/kg, i.p.), a D2 family antagonist, attenuated sound sequence discrimination learning, while it had no clear suppressive effect on discrimination of two different sound components and licking. These results suggest that D2 family receptors activated by dopaminergic inputs to the AC are required for sound sequence discrimination learning.

Dopamine Enhances Spatiotemporal Spread of Activity in Rat Prefrontal Cortex

Dopaminergic modulation of prefrontal cortex (PFC) is important for neuronal integration in this brain region known to be involved in cognition and working memory. Because of the complexity and heterogeneity of the effect of dopamine on synaptic transmission across layers of the neocortex, dopamine's net effect on local circuits in PFC is difficult to predict. We have combined whole cell patch-clamp recording and voltage-sensitive dye imaging to examine the effect of dopamine on the excitability of local excitatory circuits in rat PFC in vitro. Whole cell voltage-clamp recording from visually identified layer II/III pyramidal neurons in rat brain slices revealed that, in the presence of bicuculline (10 μM), bath-applied dopamine (30–60 μM) increased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by weak intracortical stimulus. The effect was mimicked by the selective D1 receptor agonist SKF 81297 (1 μM). Increasing stimulation resulted in epileptiform discharges. SKF 81297 (1 μM) significantly lowered the threshold stimulus required for generating epileptiform discharges to 83% of control. In the imaging experiments, bath application of dopamine or SKF 81297 enhanced the spatiotemporal spread of activity in response to weak stimulation and previously subthreshold stimulation resulted in epileptiform activity that spread across the whole cortex. These effects could be blocked by the selective D1 receptor antagonist SCH 23390 (10 μM) but not by the D2 receptor antagonist eticlopride (5 μM). These results indicate that dopamine, by a D1 receptor–mediated mechanism, enhances spatiotemporal spread of synaptic activity and lowers the threshold for epileptiform activity in local excitatory circuits within PFC.

Local application of dopamine inhibits pyramidal tract neuron activity in the rodent motor cortex

Cortical neurons respond in a variety of ways to locally applied dopamine, perhaps because of the activation of different receptors within or among subpopulations of cells. This study was conducted to assess the effects of dopamine and the receptor subtypes that mediate the responses of a specific population of neurons, the pyramidal tract neurons (PTNs) in the rodent motor cortex. The specific subfamilies of dopamine receptors expressed by PTNs also were determined. PTNs were identified by antidromic stimulation in intact animals. Extracellular recordings of their spontaneous activity and glutamate-induced excitation were performed with multi-barrel pipettes to allow simultaneous recording and iontophoresis of several drugs. Prolonged (30 s) application of dopamine caused a progressive, nonlinear decrease in spontaneous firing rates for nearly all PTNs, with significant reductions from baseline spontaneous activity (71% of baseline levels) occurring between 20 and 30 s of iontophoresis. The D1 selective (SCH23390) and the D2 selective (eticlopride) antagonists were both effective in blocking dopamine-induced inhibition in nearly all PTNs. Mean firing levels were maintained within 3% of baseline levels during co-application of the D1 antagonist with dopamine and within 11% of baseline levels during co-application of the D2 antagonist and dopamine. SCH23390 was ineffective however, in 2 of 16 PTNs, and eticlopride was ineffective in 3 PTNs. The dopamine blockade by both antagonists in most neurons, along with the selective blockade by one, but not the other antagonist in a few neurons indicate that the overall population of PTNs exhibits a heterogeneous expression of dopamine receptors. The firing rate of PTNs was significantly enhanced by iontophoresis of glutamate (mean = 141% of baseline levels). These increases were attenuated significantly (mean= 98% of baseline) by co-application with dopamine in all PTNs, indicating dopaminergic interactions with glutamate transmission. The expression of dopamine receptors was studied with dual-labeling techniques. PTNs were identified by retrograde labeling with fast blue and the D1a, D2, or D5 receptor proteins were stained immunohistochemically. Some, but not all PTNs, showed labeling for D1a, D2, or D5 receptors. The D1a and D2 receptor immunoreactivity was observed primarily in the somata of PTNs, whereas D5 immunoreactivity extended well into the apical dendrites of PTNs. In accordance with findings of D1 and D2 receptor antagonism of dopamine's actions, the identification of three DA receptor subtypes on PTNs suggests that dopamine can directly modulate PTN activity through one or more receptor subtypes.

Effect of antipsychotic drugs and selective dopaminergic antagonists on dopamine-induced facilitatory activity in prelimbic cortical pyramidal neurons. An in vitro study

Intracellular recordings were obtained from 119 pyramidal neurons localized in prelimbic cortex, five in the dorsal cingulate cortex, one in the infralimbic cortex, one in the border of prelimbic and cingulate cortex and two in the border of prelimbic and infralimbic cortex. The passive membrane properties of these pyramidal neurons (i.e. resting membrane potential, input membrane resistance, shape of the tetrodotoxin-sensitive action potentials, spike frequency adaptation with a prominent postspike afterhyperpolarization, tetrodotoxin-sensitive inward rectification in the depolarizing direction and the absence of bursting) suggested that they resembled regular spiking or intrinsically bursting pyramidal neurons. Bath application of dopamine (EC50 of 1.8 microM) produced a reversible facilitatory effect on all 119 pyramidal neurons localized in the middle layer of the prelimbic cortex. No consistent change in membrane potential was detected during the application of dopamine. No effect of dopamine was noted on the nine pyramidal neurons that were not localized in the prelimbic cortex. The facilitatory effect of dopamine in prelimbic cortex was concentration dependently antagonized by haloperidol, risperidone, quetiapine, clozapine and by the selective D4 dopaminergic receptor antagonist L-745,870, but not by the selective D2/D3 dopaminergic receptor antagonist (-)-sulpiride. (+)-SCH 23390, which is a selective D1/D5 dopamine receptor antagonist, produced, similarly to dopamine, a facilitatory effect per se, and an additive effect when co-administered with dopamine. These results provide evidence that dopamine has a facilitatory effect specifically on pyramidal neurons localized in the middle layer of prelimbic cortex. Antipsychotic drugs and L-745,870 block this effect of dopamine.

Molecular cloning and tissue distribution of an avian D2 dopamine receptor mRNA from the domestic turkey (Maleagris gallopavo)

The reverse transcriptase-polymerase chain reaction (RT-PCR), in combination with 5' and 3' rapid amplification of cDNA ends (RACE), was used to clone a G protein-coupled receptor from turkey brain mRNA. This cDNA clone has an open reading frame of 1,311 base pairs encoding a 436-residue protein with seven transmembrane-spanning domains and exhibits high homology with previously cloned mammalian D2 dopamine receptors. Northern blot analysis of turkey brain mRNA detected an approximate 2.4-kb transcript. RT-PCR and subsequent nucleotide sequence analysis of turkey brain and peripheral tissue mRNA also demonstrated the presence of an alternatively spliced mRNA corresponding to the predicted D2 short isoform. RT-PCR experiments demonstrated a widespread distribution of alternatively spliced D2 dopamine receptor transcripts throughout the turkey brain and in select peripheral tissues as well. In situ hybridization experiments detected strong autoradiographic signals over much of the turkey telencephalon, diencephalon, mesencephalon, cerebellum, pituitary, and pineal gland. Dopamine has several important functions as a neurotransmitter and hormone in mammals and may have similar actions in avian species. The cloning and tissue distribution of the D2 receptor subtype should enable the investigation of any functional role dopamine and dopamine receptors exert on the physiology and behavior of birds.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Bimodal effects of dopamine D2 receptor agonists on zero Mg(2+)-induced epileptiform activity in the rat cingulate cortex slice

A previous study demonstrated a dopamine D1 receptor-dependent inhibition of zero Mg(2+)-induced epileptiform discharges in the rat cingulate cortex slice suspended in a grease-gap bath. This investigation considers the role of dopamine D2 receptors in the modulation of paroxysmal activity in this in vitro model. Some 123 of 143 slices exhibited spontaneous paroxysmal depolarizations, which in 105 cases were accompanied by secondary depolarizing after-potentials (SDAPs). In 43.5% of slices tested, dopamine preferentially and irreversibly facilitated SDAP production at low bath concentrations (1-100 microM), but at concentrations > 100 microM suppressed all components of the epileptiform responses. Similar dose-related bimodal responses were obtained with the D2 agonists LY 171555, PHNO and 7-OH-DPAT, but not with lisuride or RU 24213, which were exclusively inhibitory. The excitatory response to LY 171555 was attenuated by the D2 antagonist raclopride (2 microM), but not by the D1 antagonist SCH 39166 (0.5 microM). On the other occasions, the sole effect of dopamine (56.5% of slices) and the other D2 agonists, was to preferentially suppress SDAP number at low concentrations (1-100 microM) and to suppress all parameters of the epileptiform response at higher concentrations. The inhibitory effect of the D2 agonist LY 171555 on SDAP formation was paradoxically attenuated by the D1 antagonist SCH 39166, but not by the D2 antagonist raclopride. These results support the notion that dopamine can modulate epileptiform activity differentially, through its actions at D1 and D2 receptors. The possibility that these effects of dopamine may be mediated indirectly is discussed.

Opposing effects of dopamine D2 receptor stimulation on the spontaneous and the electrically evoked release of [3H]GABA on rat prefrontal cortex slices

The spontaneous and the electrically evoked release of [3H]GABA were studied in vitro on slices of rat medial prefrontal cortex. The slices were preincubated with [3H]GABA and then superfused with a Krebs' solution. The superfusion with a Ca(2+)-free medium progressively increased the spontaneous [3H]GABA release and strongly decreased the electrically evoked release of [3H]GABA (-65%). The effects of three dopaminergic D2 receptor agonists (RU24926, lisuride and LY171555) were studied on both the spontaneous and the electrically evoked [3H]GABA release. The spontaneous release of [3H]GABA was increased by exposure to each of these three D2 agonists. RU24926 produced a dose-dependent increase from 10(-9) to 3 x 10(-8) M and the maximal effect was totally abolished by the dopaminergic D2 receptor antagonist sulpiride (10(-5) M). With lisuride a progressive increase of [3H]GABA release was observed and a plateau value was reached with concentrations between 10(-7) and 10(-6) M. These effects were totally reversed by 10(-5) M sulpiride. The dose-response relation for LY171555 was bell-shaped, with a maximal effect being obtained with 10(-9) M) LY171555. This effect decreased with a higher concentration (10(-8) M) and finally was no longer observed for 10(-7) M LY171555. The maximal increase induced by LY171555 was totally abolished by 10(-5) M sulpiride. In contrast, the electrically evoked release of [3H]GABA was inhibited by these three D2 agonists. The IC50 value of the inhibition was 4.1 x 10(-8) M for RU24926 and 2 x 10(-7) M for lisuride. Sulpiride (10(-5) M) totally abolished the effect of 10(-7) M RU24926. In the concentration range of lisuride examined, a 50% reduction of the lisuride inhibition was obtained in the presence of sulpiride (10(-5) M). The dose-response curve obtained with LY171555 had a U-shape, with a maximal inhibition reached with 10(-8) M, whereas no effect was observed with 10(-6) M. The inhibition induced by 10(-8) M LY171555 was completely antagonized by 10(-5) M sulpiride. The D2 agonist-induced inhibition of the electrically evoked release of [3H]GABA was mimicked by dopamine endogenously released by 10(-5) M amphetamine. This effect was reversed by 10(-5) M sulpiride. Our data provide further evidence for a dopaminergic control of GABA interneurons in the prefrontal cortex. This regulation implies the activation of D2 dopaminergic receptors. The possible mechanisms underlying the opposite effects of D2 agonists on the spontaneous and the electrically evoked release of [3H]GABA are discussed.

Quinine potently blocks single K+ channels activated by dopamine D-2 receptors in rat corpus striatum neurons

In single channel recordings from acutely dissociated neurons of the rat corpus striatum, a membrane K+ channel which is activated by dopamine D-2 receptors was blocked by nanomolar concentrations of quinine. An intermittent partial blockade was observed at 4-10 nM quinine, with a voltage dependence consistent with quinine binding to the channel near the extracellular surface of the membrane. A nearly complete blockade of channel current was observed at 100 nM quinine and above. Such concentrations are known to be too low to block various other ion channels, and may be attained in human brain at antimalarial dosages of quinine. Blockade of this channel by quinine may provide a useful experimental probe of dopaminergic function, as an alternative to D-2 receptor binding site blockade by neuroleptics.

Single K+ channels activated by D2 dopamine receptors in acutely dissociated neurons from rat corpus striatum

Corpus striatum neurons acutely dissociated from the brains of young adult rats had membrane surfaces suitable for G omega-seal recording. Whole-cell current-clamp and voltage-clamp recordings indicated that the cells remained electrically excitable after dissociation. Cell-attached recordings frequently revealed single-channel openings in the presence of dopamine or of the D2 dopamine agonist quinpirole. Channel openings were rarely or never observed in the absence of drugs or in the presence of quinpirole plus the dopamine antagonist haloperidol. The D2 antagonist spiperone was more potent at blocking the appearance of the channel than was the D1 antagonist SCH-23390. The channel reversal potential varied with the extracellular K+ concentration as predicted by the Nernst equation. The channel current-voltage relationship was linear, with a conductance of approximately equal to 85 pS in the presence of 140 mM KCl. These results are consistent with the opening of single K+ channels following D2 dopamine receptor activation.

Involvement of M1-muscarinic receptors in the excitation of neocortical neurones by acetylcholine

The technique of microelectrophoresis was used to investigate the cholinoceptor pharmacology of spontaneously active single neurones in the parietal cortex of the rat. Acetylcholine, carbachol and the selective M1-muscarinic receptor agonist, McN-A-343, were each potent excitants (rank order of apparent potency: carbachol greater than acetylcholine greater than McN-A-343). When measured in vitro, the apparent mobilities of carbachol and acetylcholine were similar although significantly less than that of McN-A-343, suggesting that the lower potencies of acetylcholine and McN-A-343 probably reflect a genuine biological phenomenon. In addition to excitation, carbachol also evoked biphasic (excitation/depression) and depressant responses. In contrast to the other cholinoceptor agonists, nicotine produced weak and inconsistent excitations. Excitatory responses to acetylcholine and carbachol were significantly attenuated by the selective M1-muscarinic receptor antagonist, pirenzepine, at a time when the excitatory response to McN-A-343 was also significantly reduced. Responses to phenylephrine were not diminished. On several cells an excitatory response to carbachol was converted to a depression by pirenzepine. These results suggest that the excitatory responses of cortical neurones to cholinoceptor agonists are mediated predominantly by M1-muscarinic receptors. The identity of the receptor mediating the depressant response to carbachol remains uncertain, although nicotinic cholinoceptors do not appear to be involved.

Differential effects of dopamine D-1 and D-2 receptor agonists on EEG activity and behaviour in the rabbit

SKF 38393, a selective agonist for dopamine D-1 receptors, LY 171555, a selective agonist for D-2 receptors and apomorphine, an agonist for both receptor sites, all induced activation of the electrical activity of the brain (EEG) in the rabbit. While SKF 38393 induced EEG changes without concomitant signs of stereotyped behaviour, the injection of both LY 171555 and apomorphine also elicited marked behavioural effects, mostly stereotyped mouth and head movements. The EEG effects of SKF 38393 were prevented by SCH 23390 (0.003 mg/kg i.v.), but not by (-)-sulpiride (6.2-25 mg/kg i.v.). Haloperidol attenuated the effects induced by SKF 38393 only at a dose (1 mg/kg) that induced EEG changes of its own. Similarly, effects of apomorphine on both EEG and behaviour were prevented by SCH 23390 and to a lesser extent by haloperidol, but not influenced by (-)-sulpiride. Different patterns of interactions were observed when D-2 receptors were selectively stimulated by LY 171555. Behavioural effects induced by LY 171555 were fully inhibited by both (-)-sulpiride (6.2-12.5 mg/kg i.v.) and haloperidol (0.1-0.3 mg/kg i.v.). The drug SCH 23390 attenuated some behavioural components at 0.3 mg/kg (i.v.), a dose at least 100-fold that effective on the EEG effects induced by SKF 38393. However, all these antagonists exerted weak or no effects on EEG activation induced by LY 171555 and did not restore the control patterns at any doses examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of both D1 and D2 dopamine receptors appears necessary for full expression of postsynaptic effects of dopamine agonists: a neurophysiological study

The abilities of 4 dopamine agonists to inhibit the tonic single unit activity of substantia nigra dopamine neurons and stimulate tonic activity of globus pallidus neurons were compared to study the agonists' effects on pre- and postsynaptic dopamine receptors, respectively. The agonists studied were apomorphine and pergolide, which interact with both D1 and D2 receptors, and the selective D2 agonists quinpirole and RU 24926. Drugs were administered systemically. The 4 dopamine agonists were equipotent and equiefficacious at inhibiting the firing rates of dopamine neurons. In contrast, their effects on pallidal cells were not identical; apomorphine and pergolide induced significantly greater increases in pallidal cell activity than did quinpirole and RU 24926. In addition, pretreatment with a small dose of quinpirole did not attenuate the excitatory effect of apomorphine on globus pallidus cell activity, as low doses of apomorphine have previously been shown to do. Possible mechanisms underlying the differences in efficacy between the non-selective and D2 selective dopamine agonists in the globus pallidus were investigated. Coadministering quinpirole with apomorphine did not significantly attenuate the effect of apomorphine, suggesting that quinpirole is not a partial agonist at postsynaptic dopamine receptors. In addition, prazosin pretreatment did not attenuate the stimulatory effect of pergolide on firing rates of pallidal cells, indicating that the greater efficacy of the non-selective agonists was not due to concurrent stimulation of alpha 1 adrenergic receptors and dopamine receptors. However, the effect of quinpirole on pallidal cell activity was significantly potentiated by pretreatment with the D1 agonist RS-SKF 38393 but not its inactive enantiomer S-SKF 38393. These results suggest that concurrent D1 and D2 receptor stimulation may be necessary for the full expression of postsynaptic receptor-mediated effects of dopamine and dopamine agonists in the basal ganglia.

Failure of carbachol to influence the release of somatostatin from slices of rat cerebral cortex

A [K+]-related, Ca2+-dependent efflux of immunoreactive somatostatin (IRS) from superfused slices of rat cerebral cortex has been observed; this release paralleled the release of both [14C]noradrenaline and [14C]GABA. However IRS release in this preparation was not stimulated by the muscarinic agonist carbachol at low (10 microM) or high (500 microM) concentrations. Furthermore, 100 or 500 microM carbachol did not affect the IRS efflux from rat cortex slices incubated in the presence of 12, 25 or 53 mM K+.