Repeated stimulation of dopamine D2-like receptors: reduced responsiveness of nigrostriatal and mesoaccumbens dopamine neurons to quinpirole.

Extracellular recording techniques were used to study antidromically activated nigrostriatal (NSDA) and mesoaccumbens (MADA) dopamine neurons in chloral hydrate-anesthetized rats. Repeated 14-day i.p. treatment with the dopamine D2-like receptor agonists, quinpirole (2 mg/kg/day) or EMD 23448 (2.6 mg/kg/day), resulted in a significant decrease in the average potency and efficacy of i.v. quinpirole (cumulative doses administered on day 15) to inhibit the spontaneous activity of NSDA neurons relative to vehicle controls. Repeated 14-day quinpirole treatment caused a significantly greater decrease in the sensitivity of MADA neurons to i.v. quinpirole challenges than NSDA neurons. When the effects on NSDA neurons were examined after a shorter treatment period, the decrease in the average potency and efficacy of i.v. quinpirole appeared to occur after only 2 days of i.p. quinpirole treatment (2 mg/kg/day). Iontophoretic studies, however, indicated that the average dopamine sensitivity of somatodendritic dopamine autoreceptors on MADA neurons, but not NSDA neurons, was significantly lower relative to controls after 14-day quinpirole treatment (2 mg/kg/day). These results suggest that this quinpirole treatment regimen can differentially affect the average sensitivity of somatodendritic dopamine autoreceptors on MADA and NSDA neurons. The somatodendritic autoreceptors on MADA neurons appear to be more sensitive to the effects of repeated 14-day quinpirole treatment than those on NSDA neurons.

In vivo characterization of two cell types in the rat globus pallidus which have opposite responses to dopamine receptor stimulation: comparison of electrophysiological properties and responses to apomorphine, dizocilpine, and ketamine anesthesia.

Extracellular single-unit recording techniques were used to examine the rat globus pallidus (GP). In both locally anesthetized, paralyzed rats and ketamine-anesthetized rats, we observed two distinct biphasic extracellular waveforms, which we have labeled Type I (negative/positive waveform) and Type II (positive/negative waveform). No significant differences were observed in the firing pattern or number of cells per track between these cell types, although the Type II neurons had a faster mean firing rate in the locally anesthetized animals. A portion of both cell types could be antidromically activated from the subthalamic nucleus, although Type II neurons had significantly slower conduction velocities. The most striking pharmacological difference between the two cell types was that Type I GP neurons were inhibited by systemic administration of the dopamine agonist apomorphine; previous studies have repeatedly shown that Type II GP cells are excited by this treatment. Pretreatment with a subthreshold dose of apomorphine reduced the responsiveness of Type I cells to a subsequent high dose of apomorphine, as has been shown for Type II cells. However, pretreatment with the NMDA antagonist dizocilpine (MK801) produced a significant change in the pattern of response to apomorphine for Type II GP neurons only. Relative to observations in locally anesthetized, paralyzed rats, ketamine anesthesia reduced the firing rate of both cell types, but did not significantly alter their direction of response to apomorphine. Thus, this study has confirmed the existence of two GP cell types with distinct extracellular waveforms and different responses to dopamine receptor stimulation. These data may necessitate a reevaluation of general theoretical models of basal ganglia function in order to account for these opposite effects of dopamine receptor stimulation on pallidal output.

A role for non-NMDA excitatory amino acid receptors in regulating the basal activity of rat globus pallidus neurons and their activation by the subthalamic nucleus.

We have investigated the hypothesis that excitatory amino acid (EAA) receptors in the globus pallidus (GP) play a significant role in maintaining the firing rates of GP neurons under basal conditions and following activation of the subthalamic nucleus (STN). Drugs were infused directly into the GP and/or STN while the extracellular single unit activity of Type II GP neurons was recorded in ketamine-anesthetized rats. Local infusions of the EAA agonists NMDA (30-300 pmol/200 nl) or AMPA (0.1-1 pmol/200 nl) elicited increases in the firing rate of GP neurons in a dose-dependent fashion. Infusion of the GABAA receptor antagonist bicuculline methiodide (1-10 pmol/100 nl) into the STN also elicited dose-related increases in the firing rate of GP neurons. Intrapallidal infusion of the non-NMDA (AMPA/kainate) receptor antagonist NBQX (0.1-1.0 nmol) reduced the basal firing rate of GP neurons by 40%. In contrast, the NMDA antagonist MK-801 (0.01-0.1 nmol) produced no significant effect on basal firing rate. Intrapallidal infusion of the non-selective EAA receptor antagonist kynurenic acid or NBQX reversed or blocked the increase in firing rate of GP neurons following bicuculline-induced activation of the STN. Similar treatment with MK-801, however, had no significant effect on this response. These results indicate that tonic stimulation of non-NMDA receptors plays an important role in maintaining the basal activity of GP neurons and in mediating the effects of increased excitatory input from subthalamic afferent neurons.

Effect of ketamine-anesthesia on N-methyl-D-aspartate-induced activation of type I nucleus accumbens neurons.

Extracellular single-unit recording and microiontophoretic techniques were used to determine the effects of ketamine-anesthesia on N-methyl-D-aspartate (NMDA)-induced excitation of Type I nucleus accumbens neurons. NMDA increased the firing rate of most neurons in this preparation. Thus, it may be concluded that ketamine-anesthesia does not result in blockade of central NMDA receptors. The excitation caused by NMDA was readily reversed in all cases by co-iontophoresis of MK 801, but was generally unaffected by coiontophoresis of ketamine. However, ketamine-anesthesia did significantly increase the current levels necessary for, and limited the magnitude of, NMDA-induced activation of these cells (as compared to urethane-anesthetized rats), suggesting that ketamine is not without effect on NMDA-receptors in vivo.

Behavioral and electrophysiological comparison of ketamine with dizocilpine in the rat.

We have compared the effects of MK 801 and ketamine on a measure of anesthesia (loss of righting reflex) and two measures of basal ganglia dopamine (DA) function: apomorphine (APO)-induced stereotypy and APO-induced excitation of type II globus pallidus (GP) neurons. As expected, ketamine induced anesthesia. High-dose MK 801 administered IP induced ataxia, but not anesthesia. When administered i.v., high-dose MK 801 induced anesthesia in only three of five rats. Using a modified stereotypy scale, it was found that pretreatment with MK 801 blocked APO-induced stereotypic sniffing. Intravenous ketamine also blocked APO-induced stereotypy, but IP ketamine did not. Similar results were observed in neurophysiological studies; MK 801 altered the excitation of type II GP neurons by APO. Intravenous ketamine (5 mg/kg) also altered the responsiveness of these cells to APO, but ketamine anesthesia (150 mg/kg, IP) had no effect. These findings suggest that MK 801 is not an effective anesthetic in rats, and the method of administration of ketamine plays a role in its ability to exert NMDA receptor blockade.

Repeated amphetamine administration: role of forebrain in reduced responsiveness of nigrostriatal dopamine neurons to dopamine agonists.

The effects of repeated amphetamine treatment on single antidromically identified nigrostriatal dopamine-containing (NSDA) neurons were evaluated in rats. The inhibitory potency and efficacy of dopamine (DA) agonists on NSDA neuron spontaneous discharge rate were examined after amphetamine treatment. Repeated amphetamine treatment (14 days, 1 or 6 mg/kg/day i.p.) dose-dependently decreased the sensitivity of NSDA neurons to the inhibitory effects of the i.v. administered quinpirole. The amphetamine-induced alteration in sensitivity to apomorphine and quinpirole was abolished by acute hemitransection of the forebrain/midbrain connections. No change in the responsiveness or sensitivity of NSDA neurons to the inhibitory effects of iontophoretically applied DA was detected after amphetamine treatment for 14 days (4 or 6 mg/kg/day) or 28 days (6 mg/kg/day). These results suggest that these amphetamine regimens alter the sensitivity/responsiveness of forebrain DA receptors but not NSDA cell somatodendritic DA autoreceptors in a dose-dependent manner.

Ascending afferent regulation of rat midbrain dopamine neurons.

Standard, extracellular single-unit recording techniques were used to examine the electrophysiological and pharmacological responsiveness of midbrain dopamine (DA) neurons to selected, ascending afferent inputs. Sciatic nerve stimulation-induced inhibition of nigrostriatal DA (NSDA) neurons was blocked by both PCPA (5-HT synthesis inhibitor) and 5,7-DHT (5-HT neurotoxin), suggesting mediation by a serotonergic (5-HT) system. Direct stimulation of the dorsal raphe (which utilizes 5-HT as a neurotransmitter and inhibits slowly firing NSDA neurons) inhibited all mesoaccumbens DA (MADA) neurons tested. Paradoxically, DPAT, a 5-HT1A agonist which inhibits 5-HT cell firing, enhanced sciatic nerve stimulation-induced inhibition of NSDA neurons. MADA neurons were not inhibited by sciatic nerve stimulation and, therefore, could not be tested in this paradigm. In contrast to the dorsal raphe, electrical stimulation of the pedunculopontine tegmental nucleus preferentially excited slowly firing NSDA and MADA neurons. Thus, both excitatory and inhibitory ascending afferents influence the activity of midbrain DA neurons, and intact 5-HT systems are necessary for sciatic nerve stimulation to alter DA cell activity. However, the role that 5-HT plays in mediating peripheral sensory input remains unclear.

NBQX inhibits AMPA-induced locomotion after injection into the nucleus accumbens.

The role of glutamate receptors in locomotor activity was investigated by examining the ability of 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX), a non-NMDA antagonist, to inhibit the stimulation of locomotion produced by the activation of various excitatory amino acid receptors in the nucleus accumbens. NBQX inhibited the stimulation of locomotor activity produced by intra-accumbens alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) at doses which had no effect on the locomotion produced by kainate or NMDA. Furthermore, this dose of NBQX had no effect on locomotion when injected alone into this brain region. These data suggest that AMPA receptors in the nucleus accumbens may play a very different role in the control of locomotion than NMDA receptors.

Apomorphine-induced changes in striatal and pallidal neuronal activity are modified by NMDA and muscarinic receptor blockade.

Systemic administration of apomorphine decreased the firing rate of caudate Type I neurons and increased the firing rate, presumably via disinhibition (16), of globus pallidus (GP) Type II neurons. In the present study, extracellular single-unit recording techniques were used to demonstrate that systemic administration of the NMDA antagonist dizocilpine (MK801) reduced both the inhibition of caudate neurons by apomorphine as well as the apomorphine-induced excitation of GP neurons. In addition, the muscarinic antagonists atropine and scopolamine had effects similar to dizocilpine. Thus, both glutamate and acetylcholine appear to play a role in dopaminergic modulation of striatal and GP activity.

Dissociative anesthesia and striatal neuronal electrophysiology.

Compared with results obtained in locally anesthetized, paralyzed rats, the dissociative anesthetic ketamine did not alter either the number of spontaneously active striatal neurons or the basal firing rate of striatal neurons; 90% of these cells exhibited the type I striatal neuron waveform. Chloral hydrate anesthesia suppressed both the occurrence and the firing rate of spontaneously active type I cells, but did not alter the activity of type II striatal neurons. Cortical stimulation preferentially activated type II cells in paralyzed rats and in chloral hydrate-anesthetized rats. Thus, under dissociative anesthesia it is possible to study spontaneously active type I striatal neurons. However, a method of activation such as cortical stimulation is necessary to study type II striatal neurons.

Motor effects of globus pallidus stimulation in the rat: lesions to corticofugal fibers block the motor effects.

Unilateral electrical stimulation of the globus pallidus (GP) in anesthetized male rats was used to determine the nature of the activity driven in muscles of the neck and shoulder by GP output. In 6 groups of animals stimulation was coupled with lesions to sites that interrupted corticofugal fibers or GP output. Interruption of corticofugal fibers blocked the driven activity while lesions that compromised GP output left the activity unaffected.

Receptor selectivity of cholecystokinin effects on mesoaccumbens dopamine neurons.

Extracellular recording techniques were combined with antidromic stimulation to examine the effects of C-terminal cholecystokinin (CCK) fragments and CCK antagonists on the activity of identified mesoaccumbens dopamine (MADA) neurons in chloral hydrate-anesthetized rats. These experiments were designed to determine the receptor selectivity of sulfated CCK octapeptide (CCK-8S) effects on MADA cells. Neither CCK tetrapeptide (CCK-4) nor unsulfated CCK octapeptide (CCK-8U) significantly altered MADA cell basal firing rate or responsiveness to the inhibitory effects of the D2 DA agonist quinpirole. As reported previously for ventral tegmental area DA cells, CCK-8S produced increases or decreases in the firing rate of most MADA cells sampled. CCK-8S also enhanced the sensitivity of MADA neurons to quinpirole-induced inhibition. This increase in sensitivity to quinpirole was blocked by pretreatment with the nonselective CCK receptor antagonist proglumide and the preferential CCK-A receptor antagonist CR 1409 but not by the preferential CCK-B receptor antagonist L-365,260. The inactivity of CCK-4 and CCK-8U in these tests and the results with the antagonists suggest that the effects of CCK-8S on MADA neuronal activity are mediated by CCK-A receptors.

Repeated SKF 38393 and nigrostriatal system neuronal responsiveness: functional down-regulation is followed by up-regulation after withdrawal.

The effects of chronic administration of the D1 dopamine (DA) agonist SKF 38393 on the basal activity and electrophysiological and pharmacological responsiveness of nigrostriatal DA neurons were examined by means of extracellular, single-unit recording techniques. Chronic D1 stimulation failed to alter either the basal activity of DA neurons or the potency of quinpirole to induce inhibition of these cells. However, 28-day SKF 38393 treatment (but not 14-day treatment) eliminated the ability of subsequent (24 h later) acute SKF 38393 to alter the rate-dependent nature of quinpirole-induced inhibition. In contrast, one week after a 28-day SKF 38393 treatment we found that quinpirole-induced inhibition by itself was no longer rate-dependent, an effect which was reversed by acute pretreatment with the D1 antagonist SCH 23390. This latter finding is suggestive of enhanced endogenous D1 tone. Similarly, 28-day SKF 38393 treatment eliminated the effect of subsequent acute SKF 38393 on sciatic nerve stimulation-induced inhibition of nigrostriatal DA neurons, whereas one week after the chronic D1 regimen these cells were highly sensitive to acute D1 enhancement of the response to sciatic nerve stimulation. In order to address the postsynaptic effects of chronic D1 stimulation, the influence of iontophoretically administered SKF 38393 was examined on type I caudate neurons. Again, 28-day SKF 38393 treatment resulted in reduced sensitivity of caudate neurons tested 24 h later, and an enhanced sensitivity was observed one week after the completion of chronic SKF 38393 administration. Thus, chronic SKF 38393 induced functional desensitization of D1 receptors, but one-week withdrawal was followed by sensitization.

Effects of (+)-4-propyl-9-hydroxynaphthoxazine on midbrain dopamine neurons: an electrophysiological study.

(+)-4-Propyl-9-hydroxynaphthoxazine (PHNO) is a highly potent, D2-selective dopamine (DA) receptor agonist. In the present study, we have examined the electrophysiological effects of PHNO on identified nigrostriatal DA (NSDA), mesoaccumbens DA (MADA) and Type I caudate neurons. Intravenous PHNO dose-dependently inhibited the firing rate of NSDA and MADA neurons in both chloral hydrate (CH)-anesthetized rats (ED50 values = 21.2 +/- 1.2 and 26.5 +/- 1.4 ng/kg, respectively) and locally anesthetized, paralyzed rats (ED50 values = 105.0 +/- 1.4 and 109.1 +/- 1.4 ng/kg, respectively). PHNO was significantly more potent in the CH-anesthetized rats. There was a significant, positive correlation between basal firing rate and log ED50 for NSDA neurons in both preparations and for MADA cells only in CH-anesthetized animals. Neither pretreatment with the D1 receptor agonist SKF 38393 nor hemitransection of the forebrain altered the rate-dependent nature of PHNO-induced inhibition of NSDA neurons. Likewise, depletions of serotonin with either p-chlorophenylalanine or 5,7-dihydroxytryptamine failed to alter the rate-dependent PHNO-induced inhibition of NSDA neurons. Iontophoretically applied PHNO inhibited both NSDA and MADA neurons to a similar degree as either DA or the D2 agonist quinpirole. In contrast, the similar effects of PHNO and quinpirole on Type I caudate neurons were significantly different than those of DA. These results suggest that PHNO inhibits midbrain DA neurons via stimulation of somatodendritic autoreceptors and that PHNO exhibits an electrophysiological profile characteristic of D2 agonists. In contrast to quinpirole, however, the rate-dependent nature of PHNO-induced inhibition of these cells was resistant to modulation.

Serotonergic afferent regulation of the basic physiology and pharmacological responsiveness of nigrostriatal dopamine neurons.

The electrophysiological responsiveness of nigrostriatal dopamine (DA) neurons to dorsal raphe stimulation and to systemic administration of serotonin (5-HT) selective compounds was examined in chloral hydrate-anesthetized rats. Electrical stimulation of the dorsal raphe selectively inhibited the firing rate of slowly firing (less than 4 spikes/sec) DA neurons. The 5-HT-1A agonists 8-hydroxy-2-(di-n-propylamino)tetralin and 5-methoxy-N,N-dimethyltryptamine preferentially increased the firing rate of slowly firing DA neurons, but did not alter the responsiveness of these cells to quinpirole-induced inhibition of firing rate. This increase in firing rate was not observed following depletion of brain 5-HT by the neurotoxin 5,7-dihydroxytryptamine. The 5-HT-1B agonists trifluoromethylphenylpiperazine and M-chlorophenylpiperazine had only weak inhibitory effects on the firing rates of DA neurons, and also failed to alter the responsiveness of DA neurons to quinpirole-induced inhibition. Depletion of brain 5-HT (greater than 80%) by either para-chlorophenylalanine or 5,7-dihydroxytryptamine eliminated the rate-dependent nature of quinpirole-induced inhibition of nigrostriatal DA neurons, while having limited effects on the basal electrophysiological activity of these cells. These data suggest that 5-HT systems exert subtle influences on the activity and pharmacological responsiveness of nigrostriatal DA neurons.

Statistical analysis of dose-response curves in extracellular electrophysiological studies of single neurons.

The application of polynomial regression and the analysis of covariance (ANCOVA) to dose-response (DR) data derived from extracellular electrophysiological studies of midbrain dopamine neurons and noradrenergic locus coeruleus neurons in vivo is demonstrated and discussed. Third-order polynomial regression was found to be a better method for estimating ED50 values than probit analysis of linear regression. ANCOVA provides a more powerful statistical method than ANOVA for detecting significant differences in ED50 values or DR curves when a confounding variable such as basal discharge rate is present. The methods of analysis presented herein should be useful in the analysis of other types of neurons in electrophysiological studies.

(+/-)-3,4-Methylenedioxymethamphetamine-induced changes in the basal activity and pharmacological responsiveness of nigrostriatal dopamine neurons.

The present study examined the effects of methylenedioxymethamphetamine (MDMA) on the basal activity and pharmacological responsiveness of rat nigrostriatal dopamine (DA) neurons. Under standard in vivo extracellular single-unit recording conditions, acute MDMA administered alone (i.v.) inhibited the firing rate of nigrostriatal DA neurons in a dose-dependent fashion. The potency of MDMA to elicit this inhibition was significantly reduced following depletion of either serotonin or DA. Acute MDMA pretreatments (10 mg/kg i.v., 90 s) also profoundly enhanced the sensitivity of nigrostriatal DA neurons to the rate-inhibitory effects of the D-2 DA receptor agonist quinpirole but not apomorphine. It has previously been demonstrated that the ability of quinpirole and apomorphine to inhibit nigrostriatal DA neuronal activity is dependent on the basal firing rate of the cell. Both acute MDMA and a single dose of MDMA (15 mg/kg i.p.) one week prior eliminated the rate dependency of quinpirole- and apomorphine-induced inhibition of the firing rate of these cells. These data suggest that, although MDMA is known to be a serotonergic neurotoxin, this compound may also exert direct functional effects on the nigrostriatal DA system.

Pedunculopontine tegmental nucleus-induced inhibition of muscle activity in the rat.

The connections of the pedunculopontine tegmental nucleus (PPN) have led us to propose that this structure mediates striatally induced inhibition of muscle activity by directing basal ganglia output to an inhibitory reticulospinal system (nucleus reticularis gigantocellularis and ventralis, nrGi-V). We conducted experiments in order to examine the effects of electrical stimulation of the PPN on the activity of selected neck and shoulder muscles. PPN stimulation at low rates (0.1 Hz) elicited bilateral muscle excitation. As the rate of stimulation was increased (e.g. to 10 Hz), less excitation was observed. Anodal DC current inactivation of the nrGi-V during concurrent 10 Hz PPN stimulation resulted in an augmentation of muscle activity above the levels observed during 10 Hz PPN stimulation alone. PPN stimulation (10 Hz) also profoundly inhibited cortically-induced muscle activity. Further support for our proposal stems from increased baseline activity (0.1 Hz PPN-induced excitation) in animals with ibotenic acid lesions of the PPN as compared to normal animals. Apparently, destruction of the PPN releases the musculature from tonic and/or phasic inhibition. A model is discussed which attempts to account for both the rate-dependent changes in excitation and the inhibition of cortically induced muscle activity.

Electrophysiological characteristics and pharmacological responsiveness of midbrain dopaminergic neurons of the aged rat.

The extracellular electrophysiological activities of A9 and A10 dopaminergic (DA) neurons were examined in chloral hydrate-anesthetized young (3 months) and aged (24-28 months) Fischer 344 male rats. Average firing rate, firing rate distribution and the incidence of spontaneous activity (cells per electrode track) did not change in either region as a function of age. Next, the inhibitory effects of several DA agonists on the firing rates of antidromically identified nigrostriatal DA cells were compared in young and old rats. The inhibitory potencies of i.v. apomorphine (D1/D2 agonist), quinpirole (D2 agonist) and d-amphetamine (indirect agonist) did not change with age. Similarly, the inhibitory effects of microiontophoretically applied dopamine were unaltered in aged rats. Stimulation of the sciatic nerve resulted in brief periods of inhibition of the activity of nigrostriatal DA neurons in both the young and the old age groups. The duration of this inhibitory response was, however, enhanced markedly in the old rats. The D1 agonist SKF 38393 enhanced the duration of the inhibitory effect of sciatic nerve stimulation in both young and old rats. These results suggest that, although no gross differences in the activity of DA cells in young and old rats were evident, subtle changes in the regulation of the activity of these neurons may occur with age.