Inhibition from ventral tegmental area of nucleus accumbens neurons in the rat

NMDA receptor contribution to the climbing fiber response in the adult mouse Purkinje cell

Among integrative neurons displaying long-term synaptic plasticity, adult Purkinje cells seemed to be an exception by lacking functional NMDA receptors (NMDA-Rs). Although numerous anatomical studies have shown both NR1 and NR2 NMDA-R subunits in adult Purkinje cells, patch-clamp studies failed to detect any NMDA currents. Using more recent pharmacological and immunodetection tools, we demonstrate here that Purkinje cells from adult mice respond to exogenous NMDA application and that postsynaptic NMDA-Rs carry part of the climbing fiber-mediated EPSC (CF-EPSC), with undetectable contribution from presynaptic or polysynaptic NMDA currents. We also detect NR2-A/B subunits in adult Purkinje cells by immunohistochemistry. The NMDA-mediated CF-EPSC is barely detectable before 3 weeks postnatal. From the end of the third week, the number of cells displaying the NMDA-mediated CF-EPSC rapidly increases. Soon, this EPSC becomes detectable in all the Purkinje cells but is still very small. Its amplitude continues to increase until 12 weeks after birth. In mature Purkinje cells, we show that the NMDA-Rs contribute to the depolarizing plateau of complex spikes and increase their number of spikelets. Together, these observations demonstrate that mature Purkinje cells express functional NMDA receptors that become detectable in CF-EPSCs at approximately 21 d after birth and control the complex spike waveform.

Antagonizing effects of a novel antipsychotic quinolinone derivative (OPC-14597) on dopaminergic inhibition of neuronal activities in the nucleus accumbens

1. The effects of a newly synthesized quinolinone derivative, 7-(4-[4-(2,3-dichlorophenlyl)-1-piperazinyl]butyloxy)-3,4-di hydro-2-(1H)- quinolinone (OPC-14597), an antipsychotic drug, on neuronal activities of the nucleus accumbens (Acc) were investigated in rats anesthetized with chloral hydrate using a microiontophoretic method. 2. Spikes elicited by stimulation of the parafascicular nucleus (Pf) of the thalamus were extracellularly recorded in the Acc neuron of chloral hydrate-anesthetized adult Wistar rats using a glass microelectrode attached along a seven-barreled micropipette, each of which was filled with dopamine, OPC-14597, SKF 38393 (D1 receptor agonist), quinpirole (D2 receptor agonist) and 2M NaCl. The drugs were microiontophoretically applied to the target neurons recorded. 3. Effects of the drugs on the Acc neurons activated monosynaptically by stimulation of the Pf were examined. Spikes elicited by Pf stimulation were inhibited by iontophoretic application of dopamine, SKF 38393 and quinpirole in a dose-dependent manner. 4. Microiontophoretic application of OPC-14597 alone affected the spikes elicited by the Pf stimulation in none of 26 neurons tested. However, the dopamine-, SKF 38393- and quinpirole-induced inhibition of the spike generation in the Acc neurons was antagonized during simultaneous application of OPC-14597. 5. The firing induced by iontophoretically applied glutamate was inhibited by dopamine, SKF 38393 and quinpirole, but not by OPC-14597. However, the dopamine-, SKF 38393- and quinpirole-induced inhibition of the glutamate-induced firing was also antagonized during simultaneous application of OPC-14597 in a dose-dependent manner in all neurons tested. 6. These findings suggest that OPC-14597 blocks dopaminergic inhibition of the Acc neurons receiving input from the Pf by acting on both D1 and D2 receptors located on the neurons.

Inhibition by talipexole, a thiazolo-azepine derivative, of dopaminergic neurons in the ventral tegmental area

A microiontophoretic study using rats anesthetized with chloral hydrate and immobilized with gallamine triethiodide was carried out to compare the effect of talipexole (B-HT 920 CL2:2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine-dihydrochloride), a dopamine autoreceptor agonist, on dopaminergic neurons in the ventral tegmental area (VTA) to non-dopaminergic neurons in the VTA. VTA neurons were classified into two types according to the responses to antidromic stimulation of the nucleus accumbens (Acc): type I neurons with a long spike latency (8.69 +/- 0.24 msec) upon Acc stimulation and low spontaneous firing rate (6.80 +/- 1.34/sec), and type II neurons with a short latency (2.76 +/- 0.20 msec) and high spontaneous firing rate (26.77 +/- 7.05/sec), probably corresponding to dopaminergic and non-dopaminergic neurons, respectively. In type I neurons, microiontophoretic application of talipexole and dopamine inhibited antidromic spike generation elicited by Acc stimulation, and talipexole-induced inhibition was antagonized by domperidone (dopamine D-2 antagonist). In type II neurons, however, the antidromic spikes were not affected by either talipexole or dopamine. Furthermore, spontaneous firing was also inhibited by iontophoretically applied talipexole and dopamine in most type I neurons, but rarely affected by either drug. Inhibitory effects of talipexole were antagonized by domperidone. These results suggest that talipexole acts on dopamine D-2 receptors, thereby inhibiting the dopaminergic neurons in the VTA.

Dopamine D-1 receptor-mediated inhibition of nucleus accumbens neurons from the ventral tegmental area

1. Spike generation by stimulation of the parafascicular nucleus of thalamus was extracellularly recorded in the nucleus accumbens of chloral hydrate-anesthetized adult Wistar rats using a silver-wire microelectrode attached along a seven-barreled micropipette, each of which was filled with dopamine, SKF 38393 (D-1 agonist), bromocriptine (D-2 agonist), haloperidol, SCH 23390 (D-1 antagonist) and domperidone (D-2 antagonist). The drugs were microiontophoretically applied to the target neurons recorded. 2. Effects of dopamine receptor antagonists on the inhibition of the spike generation by conditioning stimuli applied to the ventral tegmental area preceding the test stimulus to the parafascicular nucleus and those of dopamine agonists on the test stimulus-induced spikes were examined. 3. The parafascicular nucleus stimulation-induced spikes were inhibited by dopamine as well as D-1 and D-2 agonists and by the conditioning stimulation of the ventral tegmental area. The conditioning stimulation-induced inhibition was antagonized by haloperidol and SCH 23390, but not by domperidone. 4. Activation of D-1 receptors, which make probably synaptic contact with dopaminergic nerve terminals from the ventral tegmental area, is considered to result in inhibition of the neuronal activity of the nucleus accumbens neurons receiving input from the parafascicular nucleus of the thalamus. In addition, D-2 receptors located extrajunctionally may be involved in the inhibition of the same neurons in the nucleus accumbens.

Responses of units in the mesolimbic system to olfactory and somatosensory stimuli: modulation of sensory input by ventral tegmental stimulation

It is well known that neurons of the nigrostriatal dopamine (DA) system respond to sensory stimuli, and our primary objective here was to ascertain if neurons in the terminal regions of the mesolimbic DA system respond to sensory input also. In addition, the effects of electrical stimulation of the ventral tegmentum, which contains the DA cells of origin of the mesolimbic system, on sensory-evoked responses in mesolimbic neurons was studied. In rats anesthetized with chloral hydrate, responses of single units to olfactory and somatosensory stimuli were recorded in 6 forebrain regions including nucleus accumbens and olfactory tubercle. Both increases and decreases in spontaneous firing rates were evoked in 225 of 336 units by one or more of the 8 types of sensory stimuli employed (5 olfactory, 3 somatosensory). Excitatory responses occurred twice as frequently as inhibitory responses, but a few units responded with excitatory responses to some stimuli and inhibitory responses to others. The proportions of units responsive to olfactory and/or somatosensory stimuli were different in different regions. After electrical stimulation of the ventral tegmentum, sensory-evoked responses were changed in 30 of the 49 units tested (61%). There were increases, decreases or combinations of a decrease followed by an increase in sensory-evoked responses, which persisted for 1-10 min after the application of a single electrical stimulus train. Haloperidol (0.3 mg/kg, i.p.) either blocked the effect of tegmental stimulation or decreased all responses. The present results demonstrated that units in the terminal regions of the mesolimbic DA system are responsive to sensory input and that these responses can be affected by prior electrical stimulation of the ventral tegmentum.

D-2 receptor-mediated inhibition by a substituted quinolinone derivative, 7-[3-(4-(2,3-dimethylphenyl)piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392), of dopaminergic neurons in the ventral tegmental area

A microiontophoretic study was performed to investigate the effects of a newly synthesized quinolinone derivative, 7-[3-(4-(2,3-dimethylphenyl) piperazinyl) propoxy] 2-(1H)-quinolinone (OPC-4392), on neuronal activities of the ventral tegmental area (VTA) of rats anesthetized with chloral hydrate. The VTA neurons, which were identified by antidromic stimulation of the nucleus accumbens (Acc), were classified into type I and type II neurons according to the responses to Acc stimulation: type I neurons had a long spike latency of over 7 msec (9.63 +/- 0.25 msec), and the type II, a short latency of less than 7 msec (2.98 +/- 0.27 msec) upon Acc stimulation. In all of 11 type I neurons, iontophoretically applied OPC-4392 and dopamine inhibited the antidromic spikes elicited by Acc stimulation. This inhibition was antagonized by simultaneous application of domperidone (dopamine D-2 antagonist). However, in 16 out of 19 type II neurons the antidromic spikes were not affected by either OPC-4392 or dopamine. When the effects of iontophoretically applied OPC-4392 and dopamine on spontaneous firings were tested in 32 VTA neurons identified by Acc stimulation (including type I and type II neurons), there was a relationship between the effects of these two drugs. These results suggest that OPC-4392 acts on dopamine D-2 receptors of the dopaminergic neurons in the VTA, thereby inhibiting neuronal activity.

Ventral tegmental area-mediated inhibition of neurons of the nucleus accumbens receiving input from the parafascicular nucleus of the thalamus is mediated by dopamine D1 receptors

Microiontophoretic experiments were performed to determine whether inhibition mediated by the ventral tegmental area neurons of the nucleus accumbens, receiving input from the parafascicular nucleus of thalamus, is mediated by dopamine D1 or D2 receptors, using rats anesthetized with chloral hydrate. Spikes, elicited by test stimuli applied to the parafascicular nucleus were inhibited by conditioning stimuli to the ventral tegmental area, given 30 msec before the test stimuli. This inhibition was antagonized by iontophoretic application of SCH 23390, a D1 antagonist, in 18 of 25 neurons of the nucleus accumbens, but in only 3 of 22 neurons of the nucleus accumbens during application of domperidone, a D2 antagonist. The reduction by conditioning stimulation of the ventral tegmental area of the mean number of spikes of the 25 neurons upon stimulation of the parafascicular nucleus, was abolished by SCH 23390. In contrast, domperidone did not affect the mean number of spikes of the 22 neurons upon stimulation of the parafascicular nucleus in the presence of conditioning stimulation of the ventral tegmental area. In addition, spikes elicited by stimulation of the parafascicular nucleus were dose-dependently inhibited by iontophoretic application of both SKF 38393, a D1 agonist and bromocriptine, a D2 agonist. These results suggest that inhibition by dopamine, derived from the ventral tegmental area of neurons of the nucleus accumbens, receiving input from the parafascicular nucleus, is mediated mainly by dopamine D1 receptors, although both D1 and D2 receptors are expressed on the same neuron of the nucleus accumbens, which is also inhibited by exogenously applied D2 agonists.

Acute effects of methamphetamine applied microiontophoretically to nucleus accumbens neurons in rats

Microiontophoretic studies were performed to elucidate the acute effects of methamphetamine on the nucleus accumbens (Acc) neurons receiving input from the parafascicular nucleus (Pf) of the thalamus using rats anesthetized with chloral hydrate. Spike generation upon Pf stimulation was inhibited by conditioning stimuli applied to the ventral tegmental area (VTA), which is rich in dopamine-containing neurons, and by iontophoretic application of methamphetamine as well as dopamine. The VTA-, methamphetamine- and dopamine-induced inhibition of the spikes elicited by Pf stimulation was antagonized during simultaneous application of haloperidol. Glutamate-induced firing was also inhibited during iontophoretic application of methamphetamine and dopamine in neurons receiving input from the Pf, and the inhibition was blocked by simultaneously applied haloperidol. In the reserpine-treated animals, however, the Pf-induced spikes were not affected by methamphetamine, but inhibited by dopamine. These results indicate that methamphetamine inhibits the Acc neurons receiving input from the Pf, probably by releasing dopamine from dopaminergic nerve terminals from the VTA.

Cholecystokinin-immunoreactive boutons in synaptic contact with hippocampal pyramidal neurons that project to the nucleus accumbens

Neurons in the hippocampal formation of the rat that project to the medial nucleus accumbens were identified following the retrograde transport of a conjugate of horseradish peroxidase with wheat germ agglutinin. The great majority of such projecting neurons were located in the ventral subiculum and were pyramidal in shape; the pyramidal nature of 25 such retrogradely labelled neurons was established by Golgi impregnation. In material processed to reveal both retrogradely labelled cells and cholecystokinin-immunoreactivity, no immunoreactive projecting neurons were found. However, 48 identified projecting neurons, probably pyramidal, were found to receive input from cholecystokinin-immunoreactive boutons that formed symmetrical synaptic contacts with the soma or proximal dendrites. It is suggested that one function of cholecystokinin-immunoreactive neurons in the hippocampal formation might be to influence the output of the pyramidal neurons that project to the nucleus accumbens. Since this pathway is one of the main links between the limbic system and the basal ganglia, it is conceivable that changes in the cholecystokinin levels in the hippocampus, as found in schizophrenia, might influence behaviour through the pathway connecting the hippocampus with the nucleus accumbens.

Effects of ventro-medial mesencephalic tegmentum (VMT) stimulation on the spontaneous activity of nucleus accumbens neurones: influence of the dopamine system

The effects of VMT-stimulation (100-500 microA, 0.6 ms; 1 Hz) on the spontaneous activity of neurones in the nucleus accumbens were analyzed in ketamine-anaesthetized rats. On spontaneously active cells (firing greater than 0.5 spikes/s), 3 types of responses were observed: either inhibition (36%), excitation (5%) or a composite sequence of excitation followed by inhibition (12%). Moreover, 14% of silent nucleus accumbens neurones were excited by single pulse VMT-stimulation. Finally, 3% of nucleus accumbens neurones recorded were driven antidromically by VMT-stimulation. Destruction of dopamine (DA) projections by 6-hydroxydopamine prevented the inhibitory responses to VMT stimulation in the great majority of cells studied, without affecting the excitatory responses. After systemic administration of haloperidol or sulpiride, the inhibitory responses to VMT stimulation were attenuated markedly, whilst the excitatory responses were, however, maintained. These results suggest that the inhibitory, but not the excitatory, effects of VMT-stimulation on nucleus accumbens neurones may be mediated by an activation of the mesolimbic DA system.

A noradrenaline-induced inhibition from locus coeruleus of nucleus accumbens neuron receiving input from hippocampus

Electrophysiological studies using rats anesthetized with chloral hydrate were performed to determine whether or not noradrenaline originating in the locus coeruleus (LC) produces a beta-receptor-mediated inhibition of the nucleus accumbens (Acc) neurons receiving input from the hippocampus (HPC). When conditioning stimuli were applied to the LC preceding a test stimulus to the HPC, an inhibition of spike generation with HPC stimulation was observed during 20-100 msec of the conditioning-test time interval. This inhibition was observed when the stimulating electrode was located in the LC or its immediate vicinity. The spike generation upon HPC stimulation was also inhibited by iontophoretic application of noradrenaline, and the inhibition was antagonized by iontophoretically applied sotalol, a beta-adrenergic blocker, but not by phentolamine, an alpha-adrenergic blocker. These results suggest that noradrenaline derived from the LC produces a beta-receptor-mediated inhibition of the Acc neurons receiving input from the HPC.

Inhibition from locus coeruleus of nucleus accumbens neurons activated by hippocampal stimulation

Electrophysiological studies using rats were performed to examine the influence of locus coeruleus (LC) on nucleus accumbens (Acc) neurons. Spike generation by hippocampal stimulation was inhibited by both LC conditioning stimulation and iontophoretic application of noradrenaline, but spikes elicited by stimulation of parafascicular nucleus of thalamus were rarely affected by LC conditioning stimulation or noradrenaline. The LC-induced inhibition was antagonized by iontophoretic sotalol, but not by phentolamine, suggesting that noradrenaline derived from the LC inhibits the Acc neurons receiving input from the hippocampus, probably acting on a beta-adrenergic receptor.

Effects of haloperidol and sulpiride on dopamine-induced inhibition of nucleus accumbens neurons

Microiontophoretic study was performed to elucidate dopaminergic mechanism in the nucleus accumbens (Acc) of rats anesthetized with chloral hydrate. Iontophoretically applied dopamine produced an inhibition of glutamate-induced firing in 28 (62%) out of 45 Acc neurons tested. The dopamine-induced inhibition of 14 Acc neurons was clearly antagonized by simultaneous application of haloperidol, and a partial antagonism by sulpiride was observed in 3 out of 10 Acc neurons. These results indicate that dopamine produces an inhibition of the Acc neuron and that, compared to haloperidol, sulpiride is a less potent blocker of the postsynaptic dopamine receptor involved in the dopamine-induced inhibition.