Evidence for the absence of impulse-regulating somatodendritic and synthesis-modulating nerve terminal autoreceptors on subpopulations of mesocortical dopamine neurons

Footshock stress facilitates self-stimulation of the medial prefrontal cortex but not the lateral hypothalamus in the rat

The effects of stress on self-stimulation were investigated by exposing rats to either controllable, uncontrollable or no footshock. Both controllable and uncontrollable footshock increased medial prefrontal cortex self-stimulation rates immediately as well as 24 h following treatment. Controllable footshock produced a greater enhancement than uncontrollable footshock. In contrast, self-stimulation of the lateral hypothalamus was unaffected by either footshock treatment. These results are interpreted with reference to the neurochemical response of the mesocortical dopaminergic system to acute stress.

Electrophysiological and pharmacological properties of putative A13 incertohypothalamic dopamine neurons in the rat

A13 incertohypothalamic dopamine (DA) neurons were labelled with antibodies raised to tyrosine hydroxylase in the male rat. Electrophysiologically, these neurons could be distinguished from their neighboring non-DA cells by their wide action potentials (greater than 2 ms), slow firing rates (0-3.8 impulses/s) and by the ability of iontophoresed DA and systemically administered apomorphine to inhibit impulse flow. Low doses of the antipsychotic drug haloperidol attenuated DA's response and reversed the apomorphine-inhibition of impulse flow.

Similar effects of daily cocaine and stress on mesocorticolimbic dopamine neurotransmission in the rat

Daily exposure to cocaine or stress has been shown to enhance the motor stimulant effect of a subsequent injection of acute cocaine. Considering that both cocaine and stress enhance dopamine neurotransmission in the central nervous system, it was of interest to determine the effects of daily cocaine and stress on the capacity of acute stress to alter dopamine neurotransmission. Rats were injected with cocaine (15 mg/kg, ip) for 3 days or exposed to daily 20 min of footshock stress (0.3 mA/200 msec/sec) for 10 days. Ten to 14 days later, the rats were exposed to acute footshock or sham shock for 0, 5, 10, or 20 min, and the concentration of dopamine and its metabolites was measured in the A10 and A9 dopamine regions, nucleus accumbens, striatum, and prefrontal cortex. It was found that the daily treatments resulted in an enhancement of dopamine metabolism in the prefrontal cortex and nucleus accumbens in response to acute footshock. In contrast, dopamine metabolism was diminished in the A10 region, and no change was measured in the striatum or A9 region. It is proposed that pretreatment with cocaine or stress alters the response of the mesocorticolimbic dopamine neurons to subsequent stress, so that axonal dopamine neurotransmission is enhanced in the terminal fields and somatodendritic dopamine neurotransmission is diminished. Furthermore, the long-lasting influence of daily cocaine and stress on mesocorticolimbic dopamine responsiveness to subsequent stressful experiences may be relevant in the etiology of psychostimulant-induced psychosis.

Time-dependent changes in the sensitivity of dopamine neurons to low doses of apomorphine following amphetamine infusion

Changes in dopamine autoregulatory mechanisms following a chronic infusion of amphetamine were studied. Rats were infused with D-amphetamine (4.8 mg/day) for 7 days by osmotic minipumps, and were studied at various times after withdrawal. In contrast to no changes in the spontaneous firing rate of single dopamine cells, the potency of apomorphine in the substantia nigra was markedly decreased soon after withdrawal, followed by an increase 7 days later. The ventral tegmental area showed no changes in either spontaneous firing rate or sensitivity to apomorphine following a 7-day withdrawal. Moderate decreases in striatal and tubercle dopamine concentrations were not accompanied by any significant decrease in basal dopamine synthesis. Under intact impulse-flow, the sensitivity of terminal dopamine synthesis to low doses of apomorphine was decreased immediately following withdrawal; by Day 7 supersensitivity was observed. Direct assessment of terminal autoreceptors following a 7-day withdrawal revealed normo- and supersensitivity of these receptors in the striatum and olfactory tubercle, respectively. Possible mediating mechanisms as well as implications of these findings for the development of characteristic behavioral syndromes during and after amphetamine infusion are discussed.

Mesocortical dopaminergic neurons. 1. Electrophysiological properties and evidence for soma-dendritic autoreceptors

Mesencephalic dopaminergic neurons were electrophysiologically identified by a variety of criteria, including antidromic activation from prefrontal or cingulate cortex, neostriatum, or nucleus accumbens in urethane-anesthetized rats. The mean firing rate of 98 mesocortical dopaminergic neurons was 2.9 +/- 0.3 spikes/sec and did not differ from the mean firing rate found for nigrostriatal or nucleus accumbens dopaminergic neurons. Spontaneously active mesocortical dopaminergic neurons were inhibited by intravenous administration of either apomorphine (6 micrograms/kg) or amphetamine (0.25 mg/kg). Whereas most antidromic responses of nigrostriatal and mesoaccumbens neurons consisted of the initial segment spike only, cortically-elicited antidromic responses typically consisted of a full initial segment-soma-dendritic spike. These findings are discussed with regard to the presence of soma-dendritic autoreceptors on mesocortical dopaminergic neurons.

Mesocortical dopaminergic neurons. 2. Electrophysiological consequences of terminal autoreceptor activation

Measurement of drug- and stimulation-induced changes in the electrical excitability of dopaminergic terminals was employed to assess the effects of stimulation of dopamine terminal autoreceptors in the prefrontal cortex in urethane-anesthetized rats. Systemic or local administration of amphetamine decreased, whereas systemic administration of haloperidol increased the excitability of prefrontal cortical dopaminergic terminals of ventral tegmental area dopaminergic neurons. Mesoprefrontal dopaminergic terminal excitability was also responsive to spontaneous and stimulation-induced alterations in the rate of impulses reaching the terminal fields. These results are comparable to those previously reported for nigrostriatal and mesoaccumbens dopaminergic neurons, and are discussed with regard to the operational characteristics of autoinhibition in the mesocortical dopaminergic system.

Differential effect of cocaine on extracellular dopamine levels in rat medial prefrontal cortex and nucleus accumbens: comparison to amphetamine

The technique of in vivo microdialysis was used to measure the extracellular levels of dopamine in the nucleus accumbens septi and medial prefrontal cortex of chloral-hydrate-anaesthetized rats following systemic administration of cocaine and amphetamine. Intravenous injection of cocaine increased the extracellular levels of dopamine in the medial prefrontal cortex and the nucleus accumbens septi in a dose-dependent manner. However, the magnitude of increase was significantly greater in nucleus accumbens than in medial prefrontal cortex. In comparison to cocaine, amphetamine increased the extracellular levels of dopamine in the nucleus accumbens and medial prefrontal cortex to the same degree. Based on the relatively small increase of extracellular dopamine levels in medial prefrontal cortex by cocaine, it is postulated that dopaminergic innervation of other structures besides medial prefrontal cortex may be involved in maintenance of cocaine self-administration.

Chloral hydrate anesthesia alters the responsiveness of identified midbrain dopamine neurons to dopamine agonist administration

Single-unit electrophysiological recording techniques were used to sample the basal activity of antidromically identified nigrostriatal and mesoaccumbens dopamine (NSDA and MADA, respectively) neurons and to examine the responsiveness of these cells to dopamine agonist-induced inhibition of cell firing rate in either chloral hydrate-anesthetized or paralyzed rats. Paralyzed rats exhibited a greater percentage of burst-firing cells (69%) than did anesthetized animals (37%). Furthermore, paralyzed rats were less sensitive to the mixed D1/D2 DA receptor agonist apomorphine and the selective D2 DA receptor agonist quinpirole. However, significantly higher doses of d-amphetamine were required in paralyzed animals only with respect to inhibiting MADA neurons. The abilities of apomorphine and quinpirole to inhibit NSDA cell firing were rate-dependent in both anesthetized and paralyzed rats, whereas d-amphetamine-induced inhibition was rate dependent only in anesthetized animals. In contrast, apomorphine- and quinpirole-induced inhibition of MADA neurons were rate-dependent only in anesthetized rats, whereas d-amphetamine-induced inhibition was rate-dependent only in paralyzed animals. These results suggest that general anesthesia exerts subtle effects on the basal activity and pharmacological responsiveness of midbrain dopamine neurons.

Phencyclidine-induced activation of ventral tegmental A10 dopamine neurons is differentially affected by lesions of the nucleus accumbens and medial prefrontal cortex

The nucleus accumbens and medial prefrontal cortex contain high concentrations of phencyclidine (PCP) binding sites as well as supply inhibitory and excitatory inputs to the ventral tegmental area (VTA). Thus these two regions could be instrumental in mediating the unique bimodal response of A10 neurons to systemically administered PCP. Therefore we evaluated electrophysiologically the effects of lesions of these two areas on this pattern of response. In sham-lesioned controls, i.v. injections of PCP elicited a typical dose-dependent bimodal effect which was characterized by an activation of A10 firing at low dose (reaching a maximum of +44% at 1 mg/kg) followed by a slowing of this response with larger doses. However, in animals with kainic acid or radiofrequency lesions of the nucleus accumbens, PCP produced only a unimodal response resulting in sustained and elevated (+88% in kainate and +55% in radiofrequency lesioned groups) firing rates. Notably, neither basal activity nor the degree of activation of the A10's at doses of PCP less than 1 mg/kg were affected by the lesions. In contrast, excitotoxic destruction of the medial prefrontal cortex had no effect on the response of A10 neurons to PCP even though basal activity was slightly elevated in this group. These results suggest that the inhibitory component of the bimodal response of VTA neurons to systemic PCP is mediated via feedback pathways from the nucleus accumbens, but that the mesocortical prefrontal cortex does not appear to modulate any portion of this bimodal response.

Effects of acute and chronic neuroleptic treatment on the activity of midbrain dopamine neurons

The preceding data suggest that the primary effect of repeated AD administration on most midbrain DA neurons is inactivation. This depolarization-induced cessation of spontaneous activity would appear to have a marked effect on both basal and stimulated DA release from nerve terminals in that several studies, using voltametric techniques, have now demonstrated DA release to be diminished under these conditions. These findings stand in marked contrast to the acute effects of ADs where biochemical techniques have been used to demonstrate a marked increase in the release of DA into projection areas. The combined effects of acute and repeated AD administration on midbrain DA cell activity may explain the delay in onset of both their therapeutic and neurological side effects.

Firing patterns of midbrain dopamine neurons: differences between A9 and A10 cells

Dopamine neurons of the substantia nigra (A9) and the ventral tegmental area (A10), giving rise to the nigrostriatal and mesolimbic dopamine pathways, respectively, are commonly supposed to show similar electrophysiological activity. There are, however, instances where the two systems are differently modulated. To assess possible physiological differences in the neuronal activity of A9 and A10 neurons, randomly sampled single cells were extracellularly recorded in the chloral hydrate-anaesthetized male rat. In addition to firing rate, the degree of burst firing and the regularity of firing were quantitatively analysed. Our results show that although A9 and A10 do not differ in firing rates, A10 neuronal activity is markedly less regular and shows a higher degree of burst firing, as judged from analysis of inter-spike time interval histograms. Mean burst firing values were 3% for the A9, and 23% for the A10 neurons. Regularity was described by variation coefficients of inter-spike time interval histograms. The mean variation coefficient was 38.4% in the A9 group and 63.8% in the A10 group, i.e. the A10 neuronal firing was less regular. The difference in regularity is partly, but not fully, dependent on the difference in burst firing. Previous biochemical and physiological studies strongly support the functional significance of modulatory changes in midbrain dopamine-cell firing patterns. Since the firing pattern of midbrain dopamine cells seems to be controlled by synaptic inputs, our results indicate a higher tonic modulatory influence on the A10 than on the A9 neurons. Thus the present results imply the pharmacological possibility of preferentially affecting A10 versus A9 dopamine cell function.

Preferential effects of caffeine on limbic and cortical dopamine systems

In this study, we investigated the effects of acute caffeine administration on the activity of midbrain dopamine neurons. Caffeine significantly depressed the firing rates of dopamine neurons in the ventral tegmental area (A10 group), but had no significant effect on the firing rates of dopamine neurons in the substantia nigra zona compacta (A9 group). The action of caffeine in A10 was completely blocked by pretreatment with the adenosine agonist L-phenyl-isopropyl-adenosine (L-PIA), confirming numerous lines of evidence that caffeine and other xanthines act as competitive antagonists at adenosine receptors. The dopamine antagonist haloperidol also antagonized the effects of caffeine. This finding is consistent with a mechanism of caffeine-induced depression of dopamine neuron activity involving dopamine release, similar to that observed during amphetamine administration. Finally, the benzodiazepine diazepam also antagonized the dopaminergic effects of caffeine. It appears that, in the rat, caffeine administration inhibits mesolimbic and mesocortical projecting dopamine neurons, but has no effect on dopamine neurons that project to the striatum.

Morphology and electrophysiology of ventral mesencephalon nerve cell cultures

In primary neuron cultures obtained from ventral mesencephalon of mouse fetuses, approximately 10-30% of the neurons were dopaminergic, as demonstrated by a rapid glyoxylic acid histofluorescence procedure, and another 10-30% were GABAergic as demonstrated by autoradiography. Resting membrane potentials averaged -58 mV and input resistances averaged 188 M omega. Action potential (AP) firing patterns were of 3 types: in 49% of cells, depolarizing current elicited bursts of APs of constant amplitude, duration, and interspike interval (Type 1); in 44% of cells, bursts consisted of APs of decreasing amplitude, increasing duration, and increasing interspike interval (Type 2); and in 7% of cells, bursts were initiated by a single high amplitude, short duration AP followed by a series of lower amplitude longer duration APs that progressively increased in amplitude and decreased in duration and interspike interval (Type 3). Calcium APs of two distinct types, differing in duration and rate of rise, were observed when cultures were exposed to tetrodotoxin. Abundant postsynaptic activity was recorded. Simultaneous intracellular recording between pairs of cells demonstrated reciprocal innervation. The neurotransmitter antagonists haloperidol, bicuculline, naloxone, atropine, hexamethonium and pirenzepine affected synaptic activity and/or resting membrane potential of some of the cultured neurons.

Dopamine-containing neurons in the mammalian central nervous system: electrophysiology and pharmacology

A decade of research culminated in the late 1950's with the demonstration that dopamine was a chemical neurotransmitter within the mammalian brain. Since this time, dopaminergic neuronal systems have been extensively studied using numerous techniques. This paper will review the last 14 years of electrophysiological investigation on neurochemically identified dopamine-containing neurons in the central nervous system. This will include an examination of both the electrophysiological and pharmacological characteristics in these cells, as well as the resulting insights into the regulation of dopamine cell electrical activity which is derived from this work.

Anatomical and electrophysiological characterization of presumed dopamine-containing neurons within the supramammillary region of the rat

A combination of immunocytochemical, electrophysiological and pharmacological techniques were employed to study the properties of neurons within the supramammillary (SUM) complex of the rat. The SUM region contains a small, but dense, population of tyrosine hydroxylase immunoreactive neurons. Following injection of the orthograde neuroanatomical tracer, Phaseolus Vulgaris leucoagglutinin, into the SUM region, heavy terminal labeling was observed in the lateral septal nucleus, diagonal band of Broca and bed nucleus of the stria terminalis. The electrophysiological and pharmacological properties of antidromically-activated SUM neurons revealed evidence of two neuronal populations. Both groups of neurons exhibited long duration action potentials (greater than 2 msec) and slow conduction velocities (less than 0.5 m/sec). However, cells in one group were characterized by slow and erratic firing rates and insensitivity to dopamine (DA) autoreceptor agonists. Cells in the other group typically exhibited no spontaneous activity but could be induced to discharge by iontophoretic application of glutamate. These latter cells were sensitive to DA autoreceptor stimulation. Of the two populations of mammilloseptal SUM neurons, the silent population exhibited several properties similar to those of midbrain DA neurons.

Effects of fencamfamine on single unit activity of mesencephalic dopaminergic neurons in rats

Systemic fencamfamine (0.5-16 mg/kg, i.v.) significantly but incompletely inhibited spontaneous activity of nigrostriatal and mesolimbic/mesocortical dopamine (DA) neurons. Inhibition was reversed by haloperidol (0.1 mg/kg, i.v.) and prevented by pretreatment with alpha-methyltyrosine (50 mg/kg, i.v.) plus reserpine (5 mg/kg, i.p.). Pretreatment with alpha-methyltyrosine alone attenuated inhibition at high but not low doses of fencamfamine. Microiontophoresed fencamfamine had little direct effect on DA neurons and did not consistently modulate the effects of co-microiontophoresed DA. In contrast, systemic fencamfamine blocked the inhibitory effects of low doses of apomorphine (10-40 micrograms/kg, i.v.). Fencamfamine appears to be an indirect DA agonist which interacts with both vesicular and newly synthesized DA storage pools. Fencamfamine may also cause a rapid desensitization to the effects of DA autoreceptor stimulation.

Electrophysiological effects of cholecystokinin octapeptide on identified rat nigrostriatal dopaminergic neurons

Cholecystokinin octapeptide (CCK-8) exists in a subpopulation of midbrain dopaminergic (DA) neurons and has been shown to affect the activity of unidentified DA cells. This study describes the effects of the sulfated (CCK-8S) and unsulfated (CCK-8US) peptides (8 micrograms/kg, i.v.) on the ability of apomorphine to inhibit the firing rate of nigrostriatal DA cells identified by antidromic activation from the caudate nucleus of anesthetized rats. CCK-8S excited 9/25 DA cells while CCK-8US was without effect on firing rate (n = 9). CCK-8S pretreatment resulted in complex changes in the sensitivity of nigrostriatal DA cells to apomorphine which were related to whether an initial excitatory response was elicited by CCK-8S. CCK-8US did not alter apomorphine sensitivity. These results suggest that CCK-8S can exert modulatory effects on DA cells independent of a direct excitatory action. The effect of acute CCK-8 injection on the number of spontaneously active DA cells in stereotaxically defined regions of the substantia nigra and ventral tegmental area was also determined. CCK-8S doubled the number of active cells in these areas; CCK-8US did not alter the population activities.

SKF 38393 alters the rate-dependent D2-mediated inhibition of nigrostriatal but not mesoaccumbens dopamine neurons

Previous electrophysiological studies have failed to identify significant effects of the D1 dopamine (DA) agonist SKF 38393, either alone or in combination with the D2 agonist quinpirole (LY 171555), on the spontaneous firing rate of midbrain DA neurons. We have utilized extracellular single-unit recording techniques to examine whether SKF 38393 can alter D2-mediated inhibition of DA cell activity. Quinpirole-induced inhibition of the spontaneous activity of midbrain DA neurons was observed to be positively correlated with the basal firing rate of the neuron being examined (i.e., faster cells required higher doses to achieve 50% and maximal inhibition). Pretreatment with SKF 38393 (1.0 mg/kg, i.v.; 4 minutes) eliminated the rate dependency of quinpirole-induced inhibition of nigrostriatal but not mesoaccumbens DA neurons. This effect of SKF 38393 was blocked both by the D1 antagonist SCH 23390 and by hemitransections of the forebrain. In summary, SKF 38393 appears to exert Dl-specific, feedback pathway-dependent effects on the profile of responsiveness of nigrostriatal DA neurons to D2-mediated inhibition of cell firing rate.

Electrophysiological and pharmacological characterization of identified nigrostriatal and mesoaccumbens dopamine neurons in the rat

Extracellular single-unit recording techniques were used to compare the basal activity and pharmacological responsiveness of identified nigrostriatal and mesoaccumbens dopamine (DA)-containing neurons. The projection area of each DA cell was determined by antidromic activation techniques. The forebrain stimulation used for the cell identification procedure did not alter the pharmacological responsiveness of DA neurons; the inhibitory effect of apomorphine (and d-amphetamine) was identical when stimulation was applied either prior to or following drug administration. Analysis of the spike discharge pattern revealed that a higher proportion of mesoaccumbens DA cells exhibited burst-firing activity. Although the firing pattern of the two populations of burst-firing DA cells was similar in many regards, mesoaccumbens DA cells exhibited a longer postburst inhibition than did nigrostriatal DA cells. Each of the DA agonists, apomorphine, pergolide, B-HT 920, and d-amphetamine, inhibited nigrostriatal and mesoaccumbens DA neuronal activity in a similar fashion. However, there was a marked population difference in the recovery of cell firing in the 10 minutes following apomorphine-induced inhibition; the recovery of mesoaccumbens spike discharges was considerably slower. Although this population difference was apparent to some extent following administration of pergolide or B-HT 920 (but not d-amphetamine), it was considerably less marked. The present findings are discussed with respect to the known regulatory control of midbrain DA neurons.

Cholecystokinin potentiates dopamine inhibition of mesencephalic dopamine neurons in vitro

Cholecystokinin octapeptide sulfate (CCK-S) is a neuropeptide that is co-localized with dopamine (DA) in some neurons of the ventral tegmental area (VTA). A functional role for this peptide/monoamine co-localization has not been firmly established; however, behavioral and in vivo electrophysiological studies indicate that CCK-S modifies the action of DA in some brain areas. A brain slice preparation of the rat VTA was developed in order to examine primary effects of CCK-S on DA-containing neurons, and to determine whether CCK-S modulates the inhibitory action of DA on these neurons. Spontaneously active DA neurons of the VTA were identified on the basis of their characteristic spike waveforms and firing rate as determined with extracellular recording techniques. These cells were inhibited by perfusion with DA in a dose-dependent, sulpiride-reversible manner. CCK-S produced brief excitatory increases in firing rate in 83% of these cells tested. This excitation was dose-dependent, and the excitatory responses frequently diminished even in the continued presence of CCK-S. Prior administration of CCK-S to these cells markedly potentiated DA-induced inhibition of spontaneous firing; the magnitude of this effect ranged from a 24 to 376% increase in the inhibitory response. This CCK-induced potentiation of DA inhibition was not blocked by low calcium, high magnesium superfusion medium, indicating that this effect is a direct consequence of a postsynaptic action on the VTA neurons from which recordings were made. These results suggest that co-localized CCK-S may significantly affect neuronal sensitivity to synaptically released DA.

Dopamine neurons projecting to the medial prefrontal cortex possess release-modulating autoreceptors

The ability of dopamine (DA) agonists and antagonists to modulate the K+-evoked overflow of radioactivity from superfused slices of prefrontal cortex of the rat, preincubated with [3H]DA in the presence of 1 microM desipramine, was examined. Apomorphine and the putative autoreceptor-selective DA agonist EMD 23 448 inhibited the K+-evoked overflow of radioactivity, while the DA antagonist sulpiride enhanced the evoked overflow in a dose-dependent and stereoselective manner. The latter effect was partially reversed by EMD 23 448. More than 95% of the radioactivity retained by the slices chromatographed with DA, while deaminated metabolites represented the majority of both the basal efflux (84% metabolites, 4-5% DA) and evoked overflow (84% metabolites, 14% DA) of radioactivity. These findings indicate that mesoprefrontal DA neurons possess release-modulating nerve terminal autoreceptors. Previous studies have shown that these neurons lack synthesis-modulating autoreceptors. Thus, autoreceptors on prefrontal DA terminals appear to be coupled to regulation of the release but not the synthesis of DA.

Neuronal activity in the ventral tegmental area (VTA) during motivated bar press feeding in the monkey

Neuronal activity of 58 dopaminergic (DA) and 200 non-dopaminergic (non-DA) neurons in the ventral tegmental area (VTA) of female monkeys was recorded, and correlation to bar press feeding, sensory stimulation and change in motivation was investigated. DA neurons, judged by duration of action potentials (more than 2.5 ms) and responsiveness to apomorphine, had lower firing rates (0-8 impulses/s); non-DA neurons had intermediate firing rates (10-30 impulses/s). Two-thirds of the DA and non-DA neurons responded in bar press feeding; the former with mostly tonic and the latter with phasic responses. Fifteen neurons (5%) responded phasically to arm extension toward the bar, 124 (excitation 88, inhibition 36, 45%) during bar press (BP), and 91 (excitation 32, inhibition 59, 33%) during ingestion reward (RW). Most BP responses (84/124, 68%) continued tonically throughout the BP period with no correlation to each BP movement. In 14 neurons (14/124, 11%), firing showed a specific variation: transient early BP responses shifted to tonic steady ones in palatable food trials, and the shifts correlated well with BP speed. In 20 other neurons, firing increased during BP hip lifting, and at specific vocalization to ask for food; it decreased during food ingestion, drinking and inguino-crural stimulation. Apomorphine administration decreased firing for the first 5-15 min, then increased it with frequent lip smacking, nausea, involuntary movement and vocalization. Thus VTA neurons showed mostly steady tonic responses but some specific phasic responses. They responded not only to motor events but also in close relation to changes of motivational aspects. Neuronal responses were excitation during procurement of reward and inhibition during or after perception of reward. This modulation in firing, might be important in the initiation and execution of movement and/or motivated behavior.

Repeated administration of (-)sulpiride and SCH 23390 differentially up-regulate D-1 and D-2 dopamine receptor function in rat mesostriatal areas but not in cortical-limbic brain regions

We studied the possible functional modifications of both D-1 and D-2 dopamine (DA) receptor subtypes following repeated administration of DA antagonists that act selectively on a single class of DA receptors. The functional state of D-1 and D-2 DA receptors in particular was evaluated by measuring SKF 82526-stimulated and bromocriptine-inhibited adenylate cyclase activity in different brain regions of rats treated with saline, SCH 23390, or (-)sulpiride for 21 days. The results indicate that chronic blockade of D-1 DA receptors in striatum, nucleus accumbens, and substantia nigra by SCH 23390 induced up-regulation of the D-1 receptors without changing the functional activity of D-2 receptors. Likewise, chronic blockade of D-2 DA receptors by (-)sulpiride caused up-regulation of D-2 but not D-1 DA receptors in striatum, nucleus accumbens, substantia nigra and pituitary. SCH 23390 or (-)sulpiride did not modify the functional activity of either D-1 or D-2 DA receptors located in frontal cortex and hippocampus. In conclusion, these results indicate that chronic treatment with selective D-1 or D-2 DA receptor blockers induces a receptor-specific up-regulation which involves the DA receptors located in the nigrostriatal system and pituitary but not those in the limbic-cortical areas.

Electrophysiological studies on the specificity of the cholecystokinin antagonist proglumide

Recent evidence suggests that the glutaramic acid derivative proglumide (PROG) is a selective antagonist of cholecystokinin (CCK) in the rat CNS. The extent of this selectivity has now been characterized in more detail. Iontophoretic or intravenous (i.v.) administration of PROG was ineffective against the excitatory effect of iontophoretically applied neurotensin on midbrain dopamine (DA) cells, the excitatory effect of substance P and the inhibitory effect of Met-enkephalin on prefrontal cortical neurons, and the inhibitory effect of histamine on neurons of the sensorimotor cortex. In contrast, PROG blocked the excitatory effect of the C-terminal octapeptide of CCK in all 3 areas. Furthermore, iontophoretic PROG diminished, whereas CCK enhanced the inhibitory effect of similarly applied DA and GABA on DA cells. PROG pretreatment (1 mg/kg, i.v.) reduced the inhibitory potency and maximum effect of i.v. apomorphine (APO) on A9 DA neurons, while the inhibitory potency of APO was enhanced by i.v. CCK. Pretreatment with PROG plus CCK resulted in APO effects which were no different from those after PROG alone. Chronic treatment with PROG (1 mg/kg, p.o., 21 days) resulted in a return to normal of DA cell APO sensitivity. Combined, these findings suggest that PROG may be a relatively selective CCK antagonist, that the functional effect of dendritically released DA may be influenced by endogenously released CCK, and that tolerance may develop to the effects of chronic CCK receptor blockade.

Activity of A9 and A10 dopaminergic neurons in unrestrained rats: further characterization and effects of apomorphine and cholecystokinin

The activity of single dopaminergic (DA) neurons (total n = 77) in the midbrain of awake, unrestrained rats was examined. The firing rates and patterns of ventral tegmental area (A10) cells (n = 39) were similar to those of identified DA neurons in anesthetized and paralyzed rats. Unit activity was briefly stimulated (increased firing rate and burst activity) in response to an auditory stimulus and during manual stimulation of the vibrissae. Similar changes occurred during orienting responses and periods of sniffing. Twenty-six percent of A10 cells recorded appeared to be electronically coupled which matched the prevalence previously observed among A9 neurons. The effects on A9 and A10 DA cell activity of apomorphine and the carboxyterminal octapeptide of cholecystokinin (CCK-8) were then determined. Sequential doses of apomorphine (5-320 micrograms/kg, i.v.) reduced the firing rate of each neuron tested (n = 19). Ten of these cells were classified as 'sensitive' to the drug (ED50 less than 20 micrograms/kg), while the remainder were considerably 'less sensitive' (ED50 greater than 30 micrograms/kg). Cells of either sensitivity were as likely to be found in A9 as in A10. Sulfated CCK-8 (1-16 micrograms/kg, i.v.) excited (firing rate and bursting increased) 73% of the A9 neurons sampled but produced inconsistent effects on A10 cell firing. CCK-8 pretreatment increased the percentage of A9 and A10 neurons which were classified as 'sensitive' to apomorphine (82%). Enhanced sensitivity to apomorphine occurred regardless of the effect of CCK-8 on unit firing. Thus, as has been found in anesthetized rats, CCK-8 appeared to enhance the inhibitory effects of a DA agonist on DA neurons.

D-2 dopamine-receptors regulate the release of [3H]dopamine in rat cortical regions showing dopamine immunoreactive fibers

Using an antibody raised against dopamine the occurrence of dopamine-containing fibers was demonstrated in the prefrontal cortex, anterior cingulate cortex, parietal neocortex, piriform cortex and entorhinal cortex. In extracts of these cortical regions significant amounts of dopamine, although approximately a 100-fold less than in the neostriatum or nucleus accumbens, were detected with high performance liquid chromatography. The release of [3H]dopamine from slices of all these cortical regions was studied in vitro in a superfusion system and desipramine was used to prevent the uptake of [3H]dopamine in noradrenergic nerve terminals. It appeared that the electrically evoked release of radioactivity was inhibited by drugs stimulating D-2 dopamine-receptors in all the regions studied. Cation-exchange column chromatography revealed that the radioactivity released consisted predominantly of [3H]dopamine, indicating that D-2 receptors mediate the inhibition of the release of [3H]dopamine from dopaminergic nerve terminals. Likewise, in the neostriatum as well as in the nucleus accumbens D-2 receptor stimulation inhibits the release of [3H]dopamine. Therefore it is our conclusion that D-2 receptors regulate the release of dopamine from dopaminergic neurons originating in the ventral tegmental area as well as in the substantia nigra.

Anxiolytic-like action of the 3-PPP enantiomers in the Vogel conflict paradigm

The effect of the (+)- and (-)-enantiomers of 3-PPP [conventional and atypical dopamine (DA)-receptor active agent, respectively] were investigated in a commonly used animal model of anxiety: the Vogel licking-conflict test. Low doses (less than or equal to 0.5 mg/kg SC) of both 3-PPP enantiomers resulted in anti-conflict (= anxiolytic-like) actions in this test. (-)-3-PPP proved to be almost as potent as apomorphine in releasing the punished responding (minimum effective doses; (-)-3-PPP: 0.016, and apomorphine: 0.006 mg/kg SC), whereas (+)-3-PPP was about 10 times less effective than apomorphine. In the higher dose range (greater than or equal to 1.0 mg/kg), both 3-PPP enantiomers instead induced an apparent "pro"-conflict effect; i.e. decreased responding to a level significantly below baseline, thus resulting in a biphasic dose-response curve. Simple alterations in the animals' motivation to drink, in shock threshold or in motor capabilities did not seem to be major explanatory factors either for the anti- or for the "pro"-conflict effects. With regard to the latter, the possibility is discussed of an interaction between the experimental test situation and non conflict-related effects of the drugs, thus interfering with the punished drinking. The findings are interpreted within the concept that low doses of the 3-PPP enantiomers, in particular (-)-3-PPP, may attenuate anxiety-elicited increases in the neurotransmission in certain meso-cortical/limbic DA pathways, i.e. consistent with the previously shown preferentially "limbic" net antidopaminergic profile of action of (-)-3-PPP.(ABSTRACT TRUNCATED AT 250 WORDS)

The agranular insular cortex: a site of unusually high dopamine utilisation

Dopamine (DA) utilisation (expressed as homovanillic acid:DA) was compared in the medial prefrontal cortex (FCx), the agranular insular cortex (AgCx), the caudate-putamen (medial, CPM and lateral, CPL divisions) and the nucleus accumbens (NAc). DA utilisation in these regions decreased in the order AgCx greater than FCx greater than CPM, CPL, NAc, whilst the concentration of DA decreased in the reverse order. Thus, although fewer DA neurones appear to innervate AgCx compared with FCx, the rate of DA utilisation/release is much greater in AgCx. It is suggested that this apparently more marked activity in DA neurones innervating AgCx may reflect a relative lack of autoreceptor control.

Properties of D2 dopamine receptor autoradiography: high percentage of high-affinity agonist sites and increased nucleotide sensitivity in tissue sections

[3H]Spiroperidol (spiperone) binding in the presence of mianserin, a serotonin (5-HT2) receptor antagonist, was characterized in rat brain using quantitative autoradiography. All binding parameters were directly determined from film densities. Competition and kinetic studies revealed that [3H]spiroperidol binds to a site having characteristics of the dopamine, D2, receptor in striatum. The general binding parameters were similar to values obtained in homogenate and swabbed section studies except as related to agonist binding and guanine nucleotide sensitivity. Competition studies with dopamine revealed biphasic competition curves with a Kh of 8.23 nM and a Kl of 12.3 microM. The percentage of high-affinity sites was 90%. Guanine nucleotides (1 microM guanylyl-imidodiphosphate) completely converted the high-affinity site to a low-affinity site. Quantitative regional distribution studies revealed high binding in striatum, olfactory tubercle and nucleus accumbens, with lower binding in other dopamine innervated regions including frontal and cingulate cortex. [3H]Spiroperidol was also found to bind to a spirodecanone site with an anatomical localization distinct from the dopamine and serotonin systems and in a region (entorhinal cortex) not previously reported. This report provides a detailed pharmacologic and regional characterization of [3H]spiroperidol binding to D2 receptor in rat brain using quantitative autoradiography to determine all binding parameters. This report also demonstrates an increased percentage of sites in the high-affinity state of the D2 receptor in tissue sections and increased affinity of the guanine regulatory protein for guanine nucleotides.

Alpha 2-receptor control over release of noradrenaline in rat thalamus

Rat brain thalamic tissue was examined for the presence of alpha 2-receptor control over noradrenaline (NA) utilization. Systemically administered clonidine (0.1 mg/kg i.p.) decreased NA turnover, yohimbine (10 mg/kg i.p.) increased NA turnover, and prazosin (0.5 mg/kg i.p.) had no effect. In vitro, yohimbine increased K+-stimulated overflow of endogenous NA in a dose-dependent fashion. It appears that locus coeruleus neurons which project to thalamus exhibit the same type of alpha 2-receptor control as those terminating in other forebrain regions.

Dopamine autoreceptors modulate dopamine release from the prefrontal cortex

Electrical stimulation (at 0.3, 1, or 10 Hz, 120 pulses each) produced a calcium-dependent overflow of radioactivity from slices of the rabbit prefrontal cortex preloaded with [3H]3,4-dihydroxyphenylethylamine ([3H]DA, [3H]dopamine) in the presence of desipramine. Flat frequency-release curves were observed. Apomorphine and LY-171555 inhibited in a concentration-dependent fashion the evoked overflow of DA, an effect antagonized by haloperidol. Stimulation frequencies comparable to normal firing rates of mesocortical neurons (10 Hz) drastically reduced apomorphine-induced inhibition of DA overflow. Haloperidol produced greater facilitation of DA overflow at 10 than at 1 Hz. Nomifensine, a neuronal uptake inhibitor, enhanced DA overflow. These results indicate that mesocortical DA neurons projecting to the prefrontal cortex have release modulatory autoreceptors of the D2 subtype.

Tyrosine hydroxylase activation in mesocortical 3,4-dihydroxyphenylethylamine neurons following footshock

Mild electric footshock resulted in activation of tyrosine hydroxylase (TH) in prefrontal cortex of mice and rats. In mice, the activation was also observed following restraint. Shock-evoked activation of prefrontal cortex TH was characterized by a decrease of apparent Km for the pterin cofactor 6-methyl-5,6,7,8-tetrahydropterin and an increase of Vmax. Activation of prefrontal cortical TH was also demonstrated in vitro following preincubation under conditions that activate cyclic AMP-dependent protein kinase. Treatment of mice with the noradrenergic neurotoxin N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP-4) caused a 70% decrease in prefrontal cortex norepinephrine levels but had no significant effect on the activity of TH in that brain region. Footshock resulted in the activation of prefrontal cortex TH of DSP-4-treated mice, suggesting that shock-evoked activation of the enzyme occurs in terminals of mesocortical 3,4-dihydroxyphenylethylamine neurons.

Modulation of terminal excitability of mesolimbic dopaminergic neurons by D-amphetamine and haloperidol

Electrophysiological techniques were used to study the changes in the terminal excitability of mesolimbic DA and non-DA neurons following the infusion of D-amphetamine (D-AMP) and haloperidol (HAL) into the nucleus accumbens (NAc) of rats. The amount of current needed to evoke antidromic spikes by electrical stimulation of the NAc was used as an index of the excitability of axon terminals of these neurons. The excitability of DA neurons was decreased by D-AMP and increased by HAL. In addition, the effect produced by D-AMP was reversed by HAL. By contrast, these drugs either induced an opposite effect or were ineffective in inducing changes on the excitability of nerve terminals of mesolimbic non-DA neurons. Infusion of the vehicle or saline produced no effect. D-AMP and HAL were still effective in modulating the excitability of mesolimbic DA nerve terminals after the destruction of NAc neurons by ibotenic acid. The results suggest that the effects seen after D-AMP and HAL are mediated primarily by DA autoreceptors. It is likely that the increase in the current needed for evoking antidromic spikes after infusion of D-AMP into the terminal region is the consequence of DA autoreceptor-mediated hyperpolarization of terminal membranes. On the other hand, HAL could exert its actions by blocking autoreceptor-mediated hyperpolarization.

New dopaminergic terminal fields in the motor, visual (area 18b) and retrosplenial cortex in the young and adult rat. Immunocytochemical and catecholamine histochemical analyses

New dopaminergic terminal fields have been visualized in the rat cerebral neocortex, using two morphological methods based on distinct properties of the dopaminergic system: presence of the first synthetic enzyme, tyrosine hydroxylase, and high-affinity uptake of amines. Tyrosine hydroxylase was used as an immunocytochemical marker after destruction of the cortical noradrenergic system, induced either neonatally by 6-hydroxydopamine or later on by DSP4, and controlled by the absence of dopamine beta-hydroxylase immunoreactivity. The uptake and storage of exogenous amines in tissue sections, in the presence of selective high-affinity transport inhibitors, enabled the specific visualization of the dopaminergic system with fluorescence histochemistry. A dopaminergic innervation of low density was observed along a dorsal sagittal strip which extended from the genu of corpus callosum until about 2 mm behind the splenium and encompassed several distinct cytoarchitectonic areas in the sensorimotor and visual cortex (medial and lateral agranular field, area 18b), as well as in discrete zones of the retrosplenial granular 29c,b, and agranular 29d areas. The distribution of these dopaminergic fields suggested a columnar organization. Several characteristics of the dopaminergic innervation were similar to that of the superficial anterior cingulate cortex (area 24): the laminar distribution to the superficial I-III layers, the secondarily developed varicose aspect in catecholamine fluorescence histochemistry and the delayed postnatal ingrowth in contrast with the early prenatal dopaminergic input to the prefrontal cortex. These similarities suggested that the subpopulation of dopaminergic neurons which provides projections to the anterior cingulate cortex could also contribute to the motor and visual cortex and thus play a role in sensorimotor integration. The predictive value of these results in the ascent of the phylogenetic scale are further considered.

Postnatal ontogenesis of the dopaminergic innervation in the rat anterior cingulate cortex (area 24). Immunocytochemical and catecholamine fluorescence histochemical analysis

The postnatal development of the dopaminergic input to the rat anterior cingulate cortex (area 24) was followed using anti-tyrosine hydroxylase immunocytochemistry and catecholamine fluorescence histochemistry in control and noradrenaline-depleted rats. Noradrenaline depletion in the cerebral cortex was obtained by peripheral injections of 6-hydroxydopamine (6-OHDA) at birth or N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) at various postnatal ages and controlled by the absence of dopamine-beta-hydroxylase-labelled axons. The superficial and deep components of the anterior cingulate dopaminergic field developed at a different rate in control as well as lesioned rats. The deep supragenual dopaminergic field was already present at birth like the dopaminergic innervation of the prefrontal cortex area 32. In the superficial field, the molecular layer was reached first from postnatal day 3 (P3) on by positive axons running through the anterior hippocampal continuation and from P5-P6 on by another dopaminergic contingent coming through the deep dopaminergic field and giving off collaterals for layer III. The adult distribution pattern and striking varicose aspect were not reached until P21-P30 and a further increased density was observed until P60. The superficial cingulate dopaminergic field extended into the pregenual part of area 24b. The innervation of the superficial and deep layers of the rat anterior cingulate cortex by two distinct dopaminergic subpopulations, one of them closely related to that of prefrontal cortex area 32, could be compared with other laminar differences. The important functional implications of these data are further discussed.

Footshock and conditioned stress increase 3,4-dihydroxyphenylacetic acid (DOPAC) in the ventral tegmental area but not substantia nigra

The effects of stress on dopamine (DA) metabolism in the mesencephalic DA cell body areas and DA terminal field regions were examined. Both mild footshock stress and exposure to a neutral stimulus previously paired with footshock resulted in a selective increase in the levels of the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the prefrontal cortex as has been previously reported. Footshock stress also resulted in a slight but significant increase in DOPAC levels in the olfactory tubercles. DOPAC levels were selectively increased in the A10 cell body area (ventral tegmental area) but not A9 region (substantia nigra) by both footshock and the conditioned stress paradigm. These data indicate that the cell bodies of origin of the mesocortical dopaminergic system are activated by stress in contrast to those DA neurons innervating the striatum. It appears that mesocortical dopaminergic neurons exhibit different regulatory features than mesolimbic or nigrostriatal neurons.

Electrophysiological evidence for A10 dopamine autoreceptor subsensitivity following chronic D-amphetamine treatment

Extracellular single unit recording techniques were used to determine whether chronic treatment with D-amphetamine (AMP) causes a subsensitivity of dopamine (DA) autoreceptors on A10 DA neurons in the rat ventral tegmental area. Either once daily or twice daily intraperitoneal (i.p.) administration of 5.0 mg/kg AMP for 1 week significantly reduced the ability of intravenous (i.v.) AMP and apomorphine (APO) to suppress the firing of A10 DA neurons when tested 24-32 h after the final administration of i.p. AMP. For both of these treatment regimens, the dose-response curves for AMP and APO induced suppression were shifted approximately 4-fold to the right of control. Following an 8 day abstinence period, only the rats that received twice daily AMP injections exhibited subsensitivity to i.v. AMP and APO; the degree of subsensitivity was reduced by 50% as compared to that observed 24-32 h post-treatment. These results were not due to acute tolerance phenomena since a single i.p. injection of AMP 24-32 h before testing failed to alter sensitivity to i.v. AMP and APO. Rather, the results indicate that chronic AMP treatment reduces the sensitivity of A10 DA neurons to DA agonists. DA autoreceptor subsensitivity was demonstrated further by the finding that the ability of microiontophoretically applied DA to suppress A10 DA neuronal activity was markedly reduced (5.8-fold shift of the dose-response curve) by chronic AMP treatment (2 X 5 mg/kg/day). In contrast, there was no alteration in the ability of iontophoretic gamma-amino butyric acid (GABA) to suppress A10 DA activity in chronic AMP rats. Chronic AMP-treatment also increased the number of spontaneously active A10 DA neurons as well as their basal firing rate. It is suggested that the ability of chronic AMP treatment to decrease the auto-regulatory ability of A10 DA neurons may be related to the phenomena of behavioral sensitization and AMP psychosis.