Bath Application Of Dopamine Research Articles

A cephalic projection neuron involved in locomotion is dye coupled to the dopaminergic neural network in the medicinal leech

It is widely appreciated that the selection and modulation of locomotor circuits are dependent on the actions of higher-order projection neurons. In the leech, Hirudo medicinalis, locomotion is modulated by a number of cephalic projection neurons that descend from the subesophageal ganglion in the head. Specifically, descending brain interneuron Tr2 functions as a command-like neuron that can terminate or sometimes trigger fictive swimming. In this study, we demonstrate that Tr2 is dye coupled to the dopaminergic neural network distributed in the head brain. These findings represent the first anatomical evidence in support of dopamine (DA) playing a role in the modulation of locomotion in the leech. In addition, we have determined that bath application of DA to the brain and entire nerve cord reliably and rapidly terminates swimming in all preparations exhibiting fictive swimming. By contrast, DA application to nerve cords expressing ongoing fictive crawling does not inhibit this motor rhythm. Furthermore, we show that Tr2 receives rhythmic feedback from the crawl central pattern generator. For example, Tr2 receives inhibitory post-synaptic potentials during the elongation phase of each crawl cycle. When crawling is not expressed, spontaneous inhibitory post-synaptic potentials in Tr2 correlate in time with spontaneous excitatory post-synaptic potentials in the CV motor neuron, a circular muscle excitor that bursts during the elongation phase of crawling. Our data are consistent with the idea that DA biases the nervous system to produce locomotion in the form of crawling.

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 microM), bath-applied dopamine (30-60 microM) 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 microM). Increasing stimulation resulted in epileptiform discharges. SKF 81297 (1 microM) 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 microM) but not by the D2 receptor antagonist eticlopride (5 microM). 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.

Action Potential Initiation and Propagation in Layer 5 Pyramidal Neurons of the Rat Prefrontal Cortex: Absence of Dopamine Modulation

Somatic and dendritic whole-cell recording was used to examine action potential (AP) initiation and propagation in layer 5 pyramidal neurons of the rat prelimbic prefrontal cortex. APs generated by somatic current injection, or via antidromic stimulation, were reliably recorded at apical dendritic locations as far as 480 microm from the soma. Although the backpropagation of single APs into the apical dendrite was robust, frequency-dependent attenuation was observed during AP trains delivered at 10-100 Hz. APs were usually initiated close to the soma (presumably in the axon); however, strong depolarizing input to the apical dendrite could generate dendritic spikes that preceded somatic APs. AP backpropagation was dependent solely on activation of dendritic voltage-gated sodium channels and did not require activation of dendritic calcium channels. Despite not playing a role in AP backpropagation, calcium-imaging experiments demonstrated that dendritic calcium channels are activated by backpropagating APs, leading to transient increases in intracellular calcium. In addition, calcium imaging revealed that AP backpropagation into the distal apical tuft was frequency dependent. Finally, we tested whether dopamine, a prominent neuromodulator associated with prefrontal activity, could alter AP initiation or backpropagation. Bath-applied dopamine (10 or 100 microm) did not effect AP backpropagation, frequency-dependent depression, local dendritic spike initiation, or AP-induced calcium signaling. These data indicate that AP backpropagation in prefrontal layer 5 pyramidal neurons is robust but frequency dependent in the distal tuft, requires dendritic sodium rather than calcium channel activation, and, unlike other aspects of neuronal excitability, insensitive to modulation by dopamine.

Dopamine receptor subtypes selectively modulate excitatory afferents from the hippocampus and amygdala to rat nucleus accumbens neurons.

The nucleus accumbens (NAc) receives excitatory afferents from several cortical and limbic regions and dense dopaminergic inputs from the ventral tegmental area. We examined the effects of dopamine (DA) D1 and D2 selective drugs on the responses evoked in the NAc shell neurons recorded in vitro by stimulation of hippocampal and amygdaloid afferents. Activation of hippocampal and amygdaloid afferents induced excitatory postsynaptic responses that were depressed by bath application of DA in most of the cells recorded. The DA effect was substantially blocked by the D1 receptor antagonist SCH 23390, but not by the D2 receptor antagonist eticlopride. Moreover, the D1 receptor agonist SKF 38393, but not the D2 receptor agonist quinpirole, mimicked the effects of DA, although a small population of neurons exhibited a D1-mediated facilitation of the EPSP amplitude following fornix stimulation. These data demonstrate a DA receptor subtype-specific modulation of glutamatergic inputs to the NAc, with D1 agonists attenuating amygdaloid inputs, whereas hippocampal-evoked responses were either depressed or potentiated by D1 stimulation. Such facilitation or attenuation of hippocampal afferents against a background of suppression of other afferents would permit the hippocampus to have a dominant influence over behavior during periods of exploration.

Endogenous and exogenous dopamine presynaptically inhibits glutamatergic reticulospinal transmission via an action of D2-receptors on N-type Ca2+ channels.

In this study, the effects of exogenously applied and endogenously released dopamine (DA), a powerful modulator of the lamprey locomotor network, are examined on excitatory glutamatergic synaptic transmission between reticulospinal axons and spinal neurons. Bath application of DA (1-50 micro m) reduced the amplitude of monosynaptic reticulospinal-evoked glutamatergic excitatory postsynaptic potentials (EPSPs). The effect of DA was blocked by the D2-receptor antagonist eticlopride, and mimicked by the selective D2-receptor agonist 2,10,11 trihydroxy-N-propyl-noraporphine hydrobromide (TNPA). Bath application of the DA reuptake blocker bupropion, which increases the extracellular level of dopamine, also reduced the monosynaptic EPSP amplitude. This effect was also blocked by the D2-receptor antagonist eticlopride. To investigate if the action of DA was exerted at the presynaptic level, the reticulospinal axon action potentials were prolonged by administering K+ channel antagonists while blocking l-type Ca2+ channels. A remaining Ca2+ component, mainly dependent on N and P/Q channels, was depressed by DA. When DA (25-50 micro m) was applied in the presence of omega-conotoxin GVIA, a toxin specific for N-type Ca2+ channels, it failed to affect the monosynaptic EPSP amplitude. DA did not affect the response to extracellularly ejected d-glutamate, the postsynaptic membrane potential, or the electrical component of the EPSPs. DA thus acts at the presynaptic level to modulate reticulospinal transmission.

Dopamine inhibition of evoked IPSCs in rat prefrontal cortex.

Rat prefrontal cortex (PFC) receives substantial dopamine (DA) input. This DA innervation appears critical for modulation of PFC cognitive functions. Clinical and experimental studies have also implicated DA in the pathogenesis of a number of neurological and psychiatric disorders including epilepsy and schizophrenia. However, the actions of DA at the cellular level are incompletely understood. Both inhibitory interneurons and pyramidal cells are targets of DA and may express different DA receptor types. Our recent findings suggest that DA can directly excite cortical interneurons and increase the frequency of spontaneous inhibitory postsynaptic currents (IPSCs). The present study was undertaken to determine the effect of specific DA receptor agonists on evoked (e) IPSCs. Visually identified pyramidal neurons were studied using whole cell voltage-clamp techniques. Bath application of DA 30 microM reduced IPSC amplitude to 80 +/- 4% (mean +/- SE) of control without any significant change in IPSC kinetics or passive membrane properties. The D1-like DA receptor agonist SKF 38393 reduced IPSC amplitude to 71.5 +/- 8%, whereas the D2-like specific agonist quinpirole has no effect on amplitude (94.5 +/- 5%). The D1-like receptor antagonist SCH 23390 prevented DA inhibition of IPSC amplitude (98.2 +/- 4%), whereas IPSCs were still reduced in amplitude (79.7 +/- 4%) by DA in the presence of the D2-like receptor antagonist sulpiride. DA increased significantly paired-pulse inhibition, whereas responses to puff applied GABA were unaffected. Addition of the PKA inhibitor H-8 blocked the effect of DA on IPSCs. These results suggest that DA can decrease IPSCs in layer II-III PFC neocortical pyramidal cells by activating presynaptic D1-like receptors.

Multiple effects of dopamine on layer V pyramidal cell excitability in rat prefrontal cortex.

The mechanisms underlying the inhibitory effects of dopamine (DA) on layer V pyramidal neuron excitability in the prelimbic region of the rat medial prefrontal cortex were investigated. Under control conditions, DA depressed both action potential generation (driven by somatic current injection) and input resistance (R(N)). The presence of GABA(A) receptor antagonists blocked DA-induced depression of action potential generation and revealed a delayed increase in excitability that persisted for the duration of experimental recording, up to 20 min following the washout of DA. In contrast to spike generation, disinhibition did not affect the transient depression of R(N) produced by DA, suggesting independent actions of DA on spike generation and R(N). Consistent with the hypothesis that DA acts to decrease pyramidal cell output via a GABAergic mechanism, DA increased the frequency of spontaneous inhibitory postsynaptic currents in both the absence and presence of TTX. Furthermore focal application of GABA to a perisomatic region mimicked the inhibitory effect of DA on spike production without affecting R(N). Focal application of bicuculline to the same location reversed the inhibitory effect of bath-applied DA on spike generation, while again having no effect on R(N). The depression of R(N) by DA was both occluded and mimicked by the Na(+) channel blocker TTX, suggesting the involvement of a Na(+) conductance in reducing pyramidal cell R(N) during the acute presence of DA. Together these data demonstrate that the acute presence of DA decreases pyramidal neuron excitability by two independent mechanisms. At the same time DA triggers a delayed and longer-lasting increase in excitability that is partially masked by synaptic inhibition.

D2-like dopamine receptor activation excites rat dorsal raphe 5-HT neurons in vitro.

The aim of the present study was to investigate the effect of dopamine (DA) on the excitability of dorsal raphe nucleus (DRN) 5-hydroxytryptamine (5-HT) neurons using the patch-clamp technique in brain slices. Bath application of DA (1-300 microM) produced a concentration-dependent membrane depolarization in all 5-HT neurons examined. This effect persisted in the presence of tetrodotoxin (TTX; 1 microM) and low extracellular calcium. Moreover, blockade of ionotropic glutamate receptors with 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2-amino-5-phosphonopentanoic acid (AP5) did not prevent DA-induced depolarization, indicating that it was mediated by a direct effect of DA on 5-HT neurons. The DA-induced depolarization was not antagonized by selective alpha1-adrenergic receptor antagonists, prazosin and WB 4101, but by a nonselective DA receptor antagonist, haloperidol. In addition, the selective D2-like receptor agonist quinpirole and antagonist sulpiride mimicked and blocked DA-induced depolarization, respectively. These results indicate that DA-induced membrane depolarization in DRN 5-HT neurons is mediated by the activation of D2-like DA receptors. The DA-induced membrane depolarization and inward current were associated with an increase in membrane conductance. Examination of the current-voltage (I-V) relationship for the DA-induced inward current revealed that the amplitude of the current increased with membrane hyperpolarization and reversed polarity at a potential near -15 mV. These data suggest that DA-induced depolarization in DRN 5-HT neurons is not mediated by a decrease in potassium conductance, but most likely by the activation of a nonselective cation current.

Dopamine D(2)-like receptors selectively block N-type Ca(2+) channels to reduce GABA release onto rat striatal cholinergic interneurones.

1. The modulatory roles of dopamine (DA) in inhibitory transmission onto striatal large cholinergic interneurones were investigated in rat brain slices using patch-clamp recording. 2. Pharmacologically isolated GABA(A) receptor-mediated IPSCs were recorded by focal stimulation within the striatum. Bath application of DA reversibly suppressed the amplitude of evoked IPSCs in a concentration-dependent manner (IC(50), 10.0 microM). 3. A D(2)-like receptor agonist, quinpirole (3-30 microM), also suppressed the IPSCs, whereas a D(1)-like receptor agonist, SKF 81297, did not affect IPSCs. Sulpiride, a D(2)-like receptor antagonist, blocked the DA-induced suppression of IPSCs (apparent dissociation constant (K(B)), 0.36 microM), while a D(1)-like receptor antagonist, SCH 23390 (10 microM), had no effect. 4. DA (30 microM) reduced the frequency of spontaneous miniature IPSCs (mIPSCs) without changing their amplitude distribution, suggesting that GABA release was inhibited, whereas the sensitivity of postsynaptic GABA(A) receptors was not affected. The effect of DA on the frequency of mIPSCs was diminished when extracellular Ca(2+) was replaced by Mg(2+) (5 mM), indicating that DA affected the Ca(2+) entry into the presynaptic terminal. 5. An N-type Ca(2+) channel selective blocker, omega-conotoxin GVIA (omega-CgTX, 3 microM), suppressed IPSCs by 65.4 %, whereas a P/Q-type Ca(2+) channel selective blocker, omega-agatoxin IVA (omega-Aga-IVA, 200 nM), suppressed IPSCs by 78.4 %. Simultaneous application of both blockers suppressed IPSCs by 95.9 %. Assuming a 3rd power relationship between Ca(2+) concentration and transmitter release, the contribution of N-, P/Q- and other types of Ca(2+) channels to presynaptic Ca(2+) entry is estimated to be, respectively, 29.8, 40.0 and 34.5 % at this synapse. After the application of omega-CgTX, DA (30 microM) no longer affected IPSCs. In contrast, omega-Aga-IVA did not alter the level of suppression by DA, suggesting that the action of DA was selective for N-type Ca(2+) channels. 6. A G protein alkylating agent, N-ethylmaleimide (NEM), significantly reduced the DA-induced suppression of IPSCs. 7. These results suggest that presynaptic D(2)-like receptors are present on the terminals of GABAergic afferents to striatal cholinergic interneurones, and down-regulate GABA release by selectively blocking N-type Ca(2+) channels through NEM-sensitive G proteins.

Presynaptic dopamine D2-like receptors inhibit excitatory transmission onto rat ventral tegmental dopaminergic neurones.

1. The effects of dopamine (DA) on non-NMDA glutamatergic transmission onto dopaminergic neurones in the ventral tegmental area (VTA) were examined in rat midbrain slices using the whole-cell patch-clamp technique. EPSCs in dopaminergic neurones evoked by focal stimulation within the VTA were reversibly blocked by 5 microM CNQX in the presence of bicuculline (20 microM), strychnine (0.5 microM) and D-amino-5-phosphonopentanoic acid (D-AP5, 25 microM). 2. Bath application of DA reduced the amplitude of EPSCs up to 65.1 +/- 9. 52% in a concentration-dependent manner between 0.3-1000 microM (IC50, 16.0 microM) without affecting the holding current at -60 mV measured using a Cs+-filled electrode. 3. The effect of DA on evoked EPSCs was mimicked by the D2-like receptor agonist quinpirole but not by the D1-like receptor agonist SKF 81297, and was antagonized by the D2-like receptor antagonist sulpiride (KB, 0.96 microM), but not by the D1-like receptor antagonist SCH 23390 (KB, 228.6 microM). 4. Dopamine (30 microM) reduced the mean frequency of spontaneous miniature EPSCs (mEPSCs) without affecting their mean amplitude, and the DA-induced effect on the mEPSCs was dependent on the external Ca2+ concentration. 5. These results suggest that afferent glutamatergic fibres which terminate on VTA dopaminergic neurones possess presynaptic D2-like receptors, activation of which inhibits glutamate release by reducing Ca2+ influx.

Modulation of fictive feeding by dopamine and serotonin in aplysia.

The buccal ganglia of Aplysia contain a central pattern generator (CPG) that mediates rhythmic movements of the buccal apparatus during feeding. Activity in this CPG is believed to be regulated, in part, by extrinsic serotonergic inputs and by an intrinsic and extrinsic system of putative dopaminergic cells. The present study investigated the roles of dopamine (DA) and serotonin (5-HT) in regulating feeding movements of the buccal apparatus and properties of the underlying neural circuitry. Perfusing a semi-intact head preparation with DA (50 microM) or the metabolic precursor of catecholamines (L-3-4-dihydroxyphenylalanine, DOPA, 250 microM) induced feeding-like movements of the jaws and radula/odontophore. These DA-induced movements were similar to bites in intact animals. Perfusing with 5-HT (5 microM) also induced feeding-like movements, but the 5-HT-induced movements were similar to swallows. In preparations of isolated buccal ganglia, buccal motor programs (BMPs) that represented at least two different aspects of fictive feeding (i.e., ingestion and rejection) could be recorded. Bath application of DA (50 microM) increased the frequency of BMPs, in part, by increasing the number of ingestion-like BMPs. Bath application of 5-HT (5 microM) did not significantly increase the frequency of BMPs nor did it significantly increase the proportion of ingestion-like BMPs being expressed. Many of the cells and synaptic connections within the CPG appeared to be modulated by DA or 5-HT. For example, bath application of DA decreased the excitability of cells B4/5 and B34, which in turn may have contributed to the DA-induced increase in ingestion-like BMPs. In summary, bite-like movements were induced by DA in the semi-intact preparation, and neural correlates of these DA-induced effects were manifest as an increase in ingestion-like BMPs in the isolated ganglia. Swallow-like movements were induced by 5-HT in the semi-intact preparation. Neural correlates of these 5-HT-induced effects were not evident in isolated buccal ganglia, however.

Monoamine Control of the Pacemaker Kernel and Cycle Frequency in the Lobster Pyloric Network

The monoamines dopamine (DA), serotonin (5HT), and octopamine (Oct) can each sculpt a unique motor pattern from the pyloric network in the stomatogastric ganglion (STG) of the spiny lobster Panulirus interruptus. In this paper we investigate the contribution of individual network components in determining the specific amine-induced cycle frequency. We used photoinactivation of identified neurons and pharmacological blockade of synapses to isolate the anterior burster (AB) and pyloric dilator (PD) neurons. Bath application of DA, 5HT, or Oct enhanced cycle frequency in an isolated AB neuron, with DA generating the most rapid oscillations and Oct the slowest. When an AB-PD or AB-2xPD subnetworks were tested, DA often reduced the ongoing cycle frequency, whereas 5HT and Oct both evoked similar accelerations in cycle frequency. However, in the intact pyloric network, both DA and Oct either reduced or did not alter the cycle frequency, whereas 5HT continued to enhance the cycle frequency as before. Our results show that the major target of 5HT in altering the pyloric cycle frequency is the AB neuron, whereas DA's effects on the AB-2xPD subnetwork are critical in understanding its modulation of the cycle frequency. Octopamine's effects on cycle frequency require an understanding of its modulation of the feedback inhibition to the AB-PD group from the lateral pyloric neuron, which constrains the pacemaker group to oscillate more slowly than it would alone. We have thus demonstrated that the relative importance of the different network components in determining the final cycle frequency is not fixed but can vary under different modulatory conditions.

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 (EC 50 of 1.8 μM) 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 D 4 dopaminergic receptor antagonist L-745,870, but not by the selective D 2/D 3 dopaminergic receptor antagonist (−)-sulpiride. (+)-SCH 23390, which is a selective D 1/D 5 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.

Dopamine Decreases the Excitability of Layer V Pyramidal Cells in the Rat Prefrontal Cortex

In both primates and rodents, the prefrontal cortex (PFC) is highly innervated by dopaminergic fibers originating from the ventral tegmental area, and activation of this mesocortical dopaminergic system decreases spontaneous and evoked activity in the PFC in vivo. We have examined the effects of dopamine (DA), over a range of concentrations, on the passive and active membrane properties of layer V pyramidal cells from the rat medial PFC (mPFC). Whole-cell and perforated-patch recordings were made from neurons in rat mPFC. As a measure of cell excitability, trains of action potentials were evoked with 1-sec-long depolarizing current steps. Bath application of DA (0.05-30 microM) produced a reversible decrease in the number of action potentials evoked by a given current step. In addition, DA reversibly decreased the input resistance (RN) of these cells. In a subset of experiments, a transient increase in excitability was observed after the washout of DA. Control experiments suggest that these results are not attributable to changes in spontaneous synaptic activity, age-dependent processes, or strain-specific differences in dopaminergic innervation and physiology. Pharmacological analyses, using D1 agonists (SKF 38393 and SKF 81297), a D1 antagonist (SCH 23390), a D2 receptor agonist (quinpirole), and a D2 antagonist (sulpiride) suggest that decreases in spiking and RN are mediated by D2 receptor activation. Together, these results demonstrate that DA, over a range of concentrations, has an inhibitory effect on layer V pyramidal neurons in the rat mPFC, possibly through D2 receptor activation.

Dopamine D1-Like Receptor Activation Excites Rat Striatal Large Aspiny NeuronsIn Vitro

The aim of this study was to elucidate electrophysiologically the actions of dopamine and SKF38393, a D1-like dopamine receptor agonist, on the membrane excitability of striatal large aspiny neurons (cholinergic interneurons). Whole-cell and perforated patch-clamp recordings were made of striatal cholinergic neurons in rat brain slice preparations. Bath application of dopamine (1-100 microM) evoked a depolarization/inward current with an increase, a decrease, or no change in membrane conductance in a dose-dependent manner. This effect was antagonized by SCH23390, a D1-like dopamine receptor antagonist. The current-voltage relationships of the dopamine-induced current determined in 23 cells suggested two conductances. In 10 cells the current reversed at -94 mV, approximately equal to the K+ equilibrium potential (EK); in three cells the I-V curves remained parallel, whereas in 10 cells the current reversed at -42 mV, which suggested an involvement of a cation permeable channel. Change in external K+ concentration shifted the reversal potential as expected for Ek in low Na+ solution. The current observed in 2 mM Ba2+-containing solution reversed at -28 mV. These actions of dopamine were mimicked by application of SKF38393 (1-50 microM) or forskolin (10 microM), an adenylyl cyclase activator, and were blocked by SCH23390 (10 microM) or SQ22536 (300 microM), an inhibitor of adenylyl cyclase. These data indicate, first, that dopamine depolarizes the striatal large aspiny neurons by a D1-mediated suppression of resting K+ conductance and an opening of a nonselective cation channel and, second, that both mechanisms are mediated by an adenylyl cyclase-dependent pathway.

Catecholaminergic modulation of respiratory rhythm in an in vitro turtle brain stem preparation.

An in vitro brain stem preparation from adult turtles was used to determine effects of dopamine (DA) and norepinephrine (NE) on the pattern of respiratory motor output recorded from hypoglossal nerve roots (XII). Bath-applied DA (10-200 microM) increased the frequency of respiratory bursts (peaks) from 0.9 +/- 0.2 to 2.4 +/- 0.3 (SE) peaks/min, resulting in a 99 +/- 9% increase in neural minute activity. R[+]-SCH-23390 (10 microM, D1 antagonist) and eticlopride (20 microM, D2 antagonist) attenuated the DA-mediated increase in peak frequency by 52 and 59%, respectively. On the other hand, the DA-receptor agonists apomorphine (D1, D2), quinelorane (D2), and SKF-38393 (D1) had no effect on peak frequency. Prazosin, an alpha1-adrenergic antagonist (250 nM) abolished the DA-mediated frequency increase. Although NE (10-200 microM) and phenylephrine (10-200 microM, alpha1-adrenergic agonist) increased peak frequency from 0.5 +/- 0.1 to 1.2 +/- 0.3 peaks/min and from 0.6 +/- 0.1 to 1. 0 +/- 0.2 peaks/min, respectively, these effects were not as large as that with DA alone. The data suggest that both dopaminergic and adrenergic receptor activation in the brain stem increase respiratory frequency in turtles, but the DA receptor-mediated increase is dependent on coactivation of alpha1-adrenergic receptors.

Ionotropic and metabotropic activation of a neuronal chloride channel by serotonin and dopamine in the leech Hirudo medicinalis.

1. Cl- channels on the pressure-sensitive (P) neuron in the leech are directly activated by synaptic release of serotonin (5-HT) and are indirectly stimulated by the cAMP second messenger pathway, suggesting an unusual dual regulation of the channels. We have investigated the mode of action of 5-HT and dopamine (DA) on a Cl- channel in adult P cells in culture by recording from cell-attached patches. 2. 5-HT increased Cl- channel activity only when included in the recording pipette and not when applied in the bath. 3. Pipette or, more effectively, bath application of DA led to an increase in Cl- channel activity. This effect was blocked by the potent and specific dopaminergic (DA1) receptor blocker, SCH-23390. 4. The stimulation by DA, but not by 5-HT, was also blocked by the cAMP-dependent protein kinase A (PKA) inhibitor Rp-cAMP and was mimicked by the membrane-permeant cAMP analogue dibutyryl cAMP (db-cAMP). 5. Our results show that 5-HT directly gates a Cl- channel that is also activated by DA via the cAMP pathway. This study demonstrates that a ligand-gated channel can be independently operated by another transmitter acting via a second messenger pathway.

Dopamine modulates graded and spike-evoked synaptic inhibition independently at single synapses in pyloric network of lobster.

Bath application of dopamine (DA) modifies the rhythmic motor pattern generated by the pyloric network in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. Synaptic transmission between network members is an important target of DA action. All pyloric neurons employ both graded transmitter release and action-potential-mediated synaptic inhibition. DA was previously shown to alter the graded synaptic strength of every pyloric synapse. In this study, we compared DA's effects on action-potential-mediated and graded synaptic inhibition at output synapses of the lateral pyloric (LP) neuron. At each synapse the postsynaptic cell tested was isolated from other descending and pyloric synaptic inputs. DA caused a reduction in the size of the LP spike-evoked inhibitory postsynaptic potentials (IPSPs) in the pyloric dilator (PD) neuron. The change in IPSP size was significantly and linearly correlated with DA-induced reduction in the input resistance of the postsynaptic PD neuron. In contrast, graded inhibition, tested in the same preparations after superfusing the stomatogastric ganglion (STG) with tetrodotoxin (TTX), was consistently enhanced by DA. DA shifted the amplitude of spike-evoked IPSPs in the same direction as the alteration of the postsynaptic cell input resistance at two additional synapses tested: DA weakened the LP spike-mediated inhibition of the ventricular dilator (VD) and reduced the VD input resistance, while strengthening the LP --> pyloric constrictor (PY) synapse and increasing PY input resistance. As previously reported, graded inhibition was enhanced at these two LP output synapses. We conclude that DA can differentially modulate the spike-evoked and graded components of synapses between members of a central pattern generator network. At the synapses we studied, actions on the presynaptic cell predominate in the modulation of graded transmission, whereas effects on postsynaptic cells predominate in the regulation of spike-evoked IPSPs.

Dopamine facilitates long-term depression of glutamatergic transmission in rat prefrontal cortex

Using sharp-electrode intracellular recordings, we studied the dopaminergic facilitation of synaptic plasticity in layer I–II afferents—layer V neuron glutamatergic synapses in rat prefrontal cortex in vitro. Tetanic stimulation (100 pulses at 50 Hz, four times at 0.1 Hz) to layer I–II afferents induced N-methyl- d-aspartate receptor-independent long-term depression (>40 min) of the glutamatergic synapses when the stimulation was coupled with a bath-application of dopamine. Tetanic stimulation alone did not induce lasting synaptic changes. Dopamine application alone transiently depressed synaptic responses, which fully recovered within 30 min. Pharmacological analyses with antagonists suggested that dopamine action on either D 1-like or D 2-like receptors can facilitate the induction of long-term depression. However, results with agonists were not fully consistent with the antagonist results: while a D 2 agonist mimicked the facilitatory dopamine effect, D 1 agonists failed to mimic the effect. We also analysed the synaptic responses during tetanus and found that dopamine prolongs membrane depolarization during high-frequency inputs. Postsynaptic membrane depolarization is indeed critical for long-term depression induction in the presence of dopamine, since postsynaptic hyperpolarization during tetanus blocked the dopaminergic facilitation of long-term depression induction. Postsynaptic injection of the Ca 2+ chelator bis-( o-aminophenoxy)- N,N,N′,N′-tetra-acetic acid (100 mM in the electrode) also blocked long-term depression induction. Our results show that dopamine lowers the threshold for long-term depression induction in rat prefrontal glutamatergic transmission. A possible underlying mechanism of this dopaminergic facilitation is the enhancement of postsynaptic depolarization during tetanus by dopamine, which may increase the amount of Ca 2+ entry from voltage-gated channels to the level sufficient for plasticity induction.

Immunohistochemical localization of GABA bR1 in the rat CNS

Intracellular recordings were obtained from rat neostriatal slices. Bath-applied dopamine (1–10 μM) produced a reversible inhibition of the action potentials evoked by direct stimulation and a decrease in the amplitude of the intrastriatally evoked depolarizing postsynaptic potentials. No change in membrane potential was detected during the application of 1–10 μM dopamine. Dopamine application also produced a decrease in anomalous rectification in the depolarizing direction. This subthreshold inward rectification was abolished by tetrodotoxin, but not by calcium-free and cadmium (0.1–1 mM)-containing solutions. The dopamine-induced decrease in excitatory postsynaptic potential amplitude was evident at resting membrane potential or at more positive levels, but was absent at hyperpolarized values of the membrane potential. Addition of bicuculline (50–500 μM) to the medium did not affect the inhibitory action of dopamine. The inhibitory action of dopamina also persisted in calcium-free and cadmium-containing solutions. The adenosine 3′,5′-cyclic monophosphate analogue, 8-bromo-adenosine 3′,5′-cyclic monophosphate (0.1–1 mM), mimicked the effects produced by D1 receptor activation. Bath application of 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF 38393) (1–10 μM), a selective D1 dopaminergic agonist, mimicked the effects of micromolar concentrations of dopamine. The D2 dopaminergic agonists,4,4a,5,6,7,8,8a,9-octahydro-5-n-propyl-2H-pyrazolo-3,4-g-quinoline (LY 171555) and bromocriptine (both at 10 nM-10 μM), had no effects on neostriatal cells. The inhibition induced by micromolar doses of dopamine or SKF 38393 was antagonized by bath applications of R-( + )-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SCH 23390; 0.1–10 μM), a D1-selective antagonist, but not by sulpiride (10nM–10μM), a D2 antagonist.We conclude that the inhibitory effect of dopamine on rat striatal neurons is postsynaptically mediated by the activation of D1 dopaminergic receptors via the reduction of a voltage-dependent tetrodotoxin sensitive inward conductance.