Dose-dependent biphasic alterations in the spontaneous activity of neurons in the rat neostriatum produced by d-amphetamine and methylphenidate

Neural substrates of trait impulsivity, anhedonia, and irritability: Mechanisms of heterotypic comorbidity between externalizing disorders and unipolar depression

Trait impulsivity, which is often defined as a strong preference for immediate over delayed rewards and results in behaviors that are socially inappropriate, maladaptive, and short-sighted, is a predisposing vulnerability to all externalizing spectrum disorders. In contrast, anhedonia is characterized by chronically low motivation and reduced capacity to experience pleasure, and is common to depressive disorders. Although externalizing and depressive disorders have virtually nonoverlapping diagnostic criteria in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, heterotypic comorbidity between them is common. Here, we review common neural substrates of trait impulsivity, anhedonia, and irritability, which include both low tonic mesolimbic dopamine activity and low phasic mesolimbic dopamine responding to incentives during reward anticipation and associative learning. We also consider how other neural networks, including bottom-up emotion generation systems and top-down emotion regulation systems, interact with mesolimbic dysfunction to result in alternative manifestations of psychiatric illness. Finally, we present a model that emphasizes a translational, transdiagnostic approach to understanding externalizing/depression comorbidity. This model should refine ways in which internalizing and externalizing disorders are studied, classified, and treated.

Heterogeneity of dopamine neuron activity across traits and states

Midbrain dopamine neurons fire irregularly, with interspersed clusters of high-frequency spikes, commonly called 'bursts'. In this review we examine such heterogeneity in activity, and provide insight into how it can participate in psychiatric conditions such as drug addiction. We first describe several techniques used to evaluate dopamine neuron activity, and comment on the different measures that each provides. We next describe the activity of dopamine neurons in 'basal' conditions. Specifically, we discuss how the use of anesthesia and reduced preparations may alter aspects of dopamine cell activity, and how there is heterogeneity across species and regions. We also describe how dopamine cell firing changes throughout the peri-adolescent period and how dopamine neuron activity differs across the population. In the final section, we discuss how dopamine neuron activity changes in response to life events. First, we focus attention on drugs of abuse. Drugs themselves change firing activity through a variety of mechanisms, with effects on firing while drug is present differing from those seen after drug discontinuation. We then review how stimuli that are rewarding, aversive, or salient can evoke changes in firing rate and discharge pattern of dopamine neurons, and provide behavioral relevance of dopamine signaling. Finally, we discuss how stress can modulate dopamine neuron firing and how this may contribute to the role that stressful experiences play in psychiatric disorders such as addiction and depression.

Amphetamine's dose-dependent effects on dorsolateral striatum sensorimotor neuron firing

Amphetamine elicits motoric changes by increasing the activity of central neurotransmitters such as dopamine and serotonin, but how these neurochemical signals are transduced into motor commands is unclear. The dorsolateral striatum (DLS), a component of the cortico-subcortical reentrant motor loop, contains abundant neurotransmitter transporters that amphetamine could affect. It has been hypothesized that DLS medium spiny neurons contribute to amphetamine's motor effects. To study striatal activity contributing to amphetamine-induced movements, activity of DLS neurons related to vertical head movement was recorded while tracking head movements before and after acute amphetamine injection. Relative to saline, all amphetamine doses induced head movements above pre-injection levels, revealing an inverted U-shaped dose-response function. Lower doses (1 mg/kg and 2 mg/kg, intraperitoneal) induced a greater number of long (distance and duration) movements than the high dose (4 mg/kg), which induced stereotypy. Firing rates (FR) of individual head movement neurons were compared before and after injection during similar head movements, defined by direction, distance, duration, and apex. Changes in FR induced by amphetamine were co-determined by dose and pre-injection FR of the neuron. Specifically, all doses increased the FRs of slower firing neurons but decreased the FRs of faster firing neurons. The magnitudes of elevation or reduction were greater at lower doses, but less pronounced at the high dose, forming an inverted U function. Modulation of DLS firing may interfere with sensorimotor processing. Furthermore, pervasive elevation of slow firing neurons' FRs may feed-forward and increase excitability in thalamocortical premotor areas, contributing to the increased movement initiation rate.

Dose- and rate-dependent effects of cocaine on striatal firing related to licking

To examine the role of striatal mechanisms in cocaine-induced stereotyped licking, we investigated the acute effects of cocaine on striatal neurons in awake, freely moving rats before and after cocaine administration (0, 5, 10, or 20 mg/kg). Stereotyped licking was induced only by the high dose. Relative to control (saline), cocaine reduced lick duration and concurrently increased interlick interval, particularly at the high dose, but it did not affect licking rhythm. Firing rates of striatal neurons phasically related to licking movements were compared between matched licks before and after injection, minimizing any influence of sensorimotor variables on changes in firing. Both increases and decreases in average firing rate of striatal neurons were observed after cocaine injection, and these changes exhibited a dose-dependent pattern that strongly depended on predrug firing rate. At the middle and high doses relative to the saline group, the average firing rates of slow firing neurons were increased by cocaine, resulting from a general elevation of movement-related firing rates. In contrast, fast firing neurons showed decreased average firing rates only in the high-dose group, with reduced firing rates across the entire range for these neurons. Our findings suggest that at the high dose, increased phasic activity of slow firing striatal neurons and simultaneously reduced phasic activity of fast firing striatal neurons may contribute, respectively, to the continual initiation of stereotypic movements and the absence of longer movements.

Dose-response characteristics of methylphenidate on locomotor behavior and on sensory evoked potentials recorded from the VTA, NAc, and PFC in freely behaving rats

BACKGROUND

Methylphenidate (MPD) is a psychostimulant commonly prescribed for attention deficit/hyperactivity disorder. The mode of action of the brain circuitry responsible for initiating the animals' behavior in response to psychostimulants is not well understood. There is some evidence that psychostimulants activate the ventral tegmental area (VTA), nucleus accumbens (NAc), and prefrontal cortex (PFC).

METHODS

The present study was designed to investigate the acute dose-response of MPD (0.6, 2.5, and 10.0 mg/kg) on locomotor behavior and sensory evoked potentials recorded from the VTA, NAc, and PFC in freely behaving rats previously implanted with permanent electrodes. For locomotor behavior, adult male Wistar-Kyoto (WKY; n = 39) rats were given saline on experimental day 1 and either saline or an acute injection of MPD (0.6, 2.5, or 10.0 mg/kg, i.p.) on experimental day 2. Locomotor activity was recorded for 2-h post injection on both days using an automated, computerized activity monitoring system. Electrophysiological recordings were also performed in the adult male WKY rats (n = 10). Five to seven days after the rats had recovered from the implantation of electrodes, each rat was placed in a sound-insulated, electrophysiological test chamber where its sensory evoked field potentials were recorded before and after saline and 0.6, 2.5, and 10.0 mg/kg MPD injection. Time interval between injections was 90 min.

RESULTS

Results showed an increase in locomotion with dose-response characteristics, while a dose-response decrease in amplitude of the components of sensory evoked field responses of the VTA, NAc, and PFC neurons. For example, the P3 component of the sensory evoked field response of the VTA decreased by 19.8% +/- 7.4% from baseline after treatment of 0.6 mg/kg MPD, 37.8% +/- 5.9% after 2.5 mg/kg MPD, and 56.5% +/- 3.9% after 10 mg/kg MPD. Greater attenuation from baseline was observed in the NAc and PFC. Differences in the intensity of MPD-induced attenuation were also found among these brain areas.

CONCLUSION

These results suggest that an acute treatment of MPD produces electrophysiologically detectable alterations at the neuronal level, as well as observable, behavioral responses. The present study is the first to investigate the acute dose-response effects of MPD on behavior in terms of locomotor activity and in the brain involving the sensory inputs of VTA, NAc, and PFC neurons in intact, non-anesthetized, freely behaving rats previously implanted with permanent electrodes.

Actions of methylphenidate on dopaminergic neurons of the ventral midbrain

BACKGROUND

Methylphenidate has been suggested to exert its therapeutic effect mainly by blocking the dopamine transporter. In spite of the importance of this interaction, no detailed information is available yet on its actions on single dopaminergic neurons.

METHODS

We examined the effects of methylphenidate on dopaminergic neurons using electrophysiological recordings from rat midbrain slices.

RESULTS

Methylphenidate inhibited spontaneous firing and caused a membrane hyperpolarization in current clamp or an outward current in voltage clamp. These effects were antagonized by the D(2) receptor antagonist sulpiride. An acute dopamine-depleting treatment of the slices with the dopa-decarboxylase inhibitor carbidopa significantly reduced the effects of methylphenidate. This drug potentiated, in a concentration-dependent manner, cellular responses to exogenous dopamine application.

CONCLUSIONS

Our electrophysiological data are consistent with the hypothesis that methylphenidate inhibits dopamine transporter and suggest that the depression of firing is mediated by the release of newly synthesized dopamine which accumulates extracellularly due to inhibition of its reuptake.

Acute effects of 3,4-methylenedioxymethamphetamine on striatal single-unit activity and behavior in freely moving rats: differential involvement of dopamine D1 and D2 receptors

3,4-Methylenedioxymethamphetamine (MDMA) is a widely abused amphetamine derivative that increases dopamine (DA) and serotonin release via a reverse transport mechanism. Changes in the activity of striatal neurons in response to increased DA transmission may shape the behavioral patterns associated with amphetamine-like stimulants. To determine how the striatum participates in MDMA-induced locomotor activation, we recorded the activity of >100 single units in the striatum of freely moving rats in response to a dose that increased motor activation (5.0 mg/kg). MDMA had a predominantly excitatory effect on neuronal activity that was positively correlated with the magnitude of locomotor activation. Categorizing neurons according to baseline locomotor responsiveness revealed that MDMA excited significantly more neurons showing movement-related increases in activity compared to units that were non-movement-related or associated with movement-related decreases in activity. Further analysis revealed that the drug-induced striatal activation was not simply secondary to the behavioral change, indicating a primary action of MDMA on striatal motor circuits. Prior administration of SCH-23390 (0.2 mg/kg), a D(1) antagonist, resulted in a late onset of MDMA-induced locomotion, which correlated positively with delayed neuronal excitations. Conversely, prior administration of eticlopride (0.2 mg/kg), a D(2) antagonist, completely abolished MDMA-induced locomotion, which paralleled its blockade of MDMA-induced excitatory neuronal responses. Our results highlight the importance of striatal neuronal activity in shaping the behavioral response to MDMA, and suggest that DA D(1) and D(2) receptors have distinct functional roles in the expression of MDMA-induced striatal and locomotor activation.

Low-dose amphetamine elevates movement-related firing of rat striatal neurons

To study the striatal role in amphetamine's stimulant effects on motor behavior, single neurons were recorded in the dorsolateral striatum of unrestrained rats before and after amphetamine injection (0.5 or 1.0 mg/kg, i.p.). Comparisons of firing were made between similar motor behaviors before and after injection. Mean locomotor firing rates increased 5% to 276% within 30 min after injection and reversed within 2 h. Firing related to specific head- or forelimb-movements, which were similar in all measured parameters before and after injection, was elevated several hundred percent after injection and then reversed, the time course paralleling that of the stimulant effect on these movements. Elevation of movement-related striatal firing rates by low doses of the psychomotor stimulant is in line with established increases in firing rate normally observed for striatal neurons related to motor behavior.

Protein kinase C and dopamine transport—1. Effects of amphetamine in vivo

Rats, injected with small doses of amphetamine (0.03-0.1 mg/kg, i.p.), showed an increase in the soluble and a decrease in the activity of the particulate protein kinase C (PKC) in the striatum, while large doses of amphetamine (0.3-1.0 mg/kg) had the opposite effect of decreasing the soluble and increasing the particulate activity of PKC. These effects were manifested as a change in the Km for calcium, without an alteration in the Vmax. They were attenuated by pretreatment with benztropine, a dopamine (DA) uptake blocker and by alpha-methyl-p-tyrosine (alpha-MT), a DA synthesis inhibitor. The effects of 0.1 mg/kg amphetamine were insensitive to pretreatment with reserpine but were attenuated by the DA antagonists, SCH 23390 or sulpiride. These results suggest that the changes in activity of PKC induced by a small dose of amphetamine were mediated by an activation of DA autoreceptors, through an increase in the biophase concentration of DA at the synapse. In contrast, the effects of 1.0 mg/kg amphetamine on activity of PKC were attenuated by reserpine and by the DA agonists, LY 171555 or SKF 38393. They were, furthermore, potentiated by simultaneous treatment with sulpiride, which indicates that the two drugs act by different mechanisms. These results suggest that larger doses of amphetamine altered the activity of PKC at the DA transport site.

Heterogeneous responses of neostriatal neurons to amphetamine in freely moving rats

Single-unit activity was recorded from the neostriatum of unrestrained, behaving rats. Neuronal discharges were found to vary with specific motor responses, general changes in motor activity, or the presentation of orienting stimuli. In each case, however, 1.0 mg/kg D-amphetamine produced approximately equal numbers of excitations and inhibitions. A subsequent injection of a higher dose (5.0 mg/kg) either produced a greater change in firing rate in the same direction or reversed the direction of the low-dose response. Amphetamine, therefore, does not produce uniformly excitatory effects in the neostriatum of ambulant animals. In fact, the neuronal response to amphetamine appears to reflect a complex interaction of several factors, including ongoing behavior and drug dose.

Electrophysiological effects of methylphenidate on the coeruleo-cortical noradrenergic system in the rat

The effect of methylphenidate on noradrenergic neurotransmission was investigated in urethane-anesthetized rats. The spontaneous activity of locus coeruleus noradrenergic neurons was the same in rats treated for 7 days with methylphenidate as in the controls. In control rats, i.v. methylphenidate induced a reduction of locus coeruleus neuronal firing whereas in rats treated for 7 days with methylphenidate, the same dose of methylphenidate failed to induce any change in locus coeruleus activity. At this time, clonidine induced a lesser reduction of locus coeruleus neuronal firing than in the controls, indicating that their autoreceptors had become desensitized. Following electrical stimulation of the locus coeruleus, most of the spontaneously firing cortical neurons were inhibited but the percentage of such neurons was reduced and the neurons showed a decreased responsiveness after methylphenidate treatment. The responsiveness of cortical neurons to microiontophoretic applications of NA as assessed by the I.T50 method was reduced after 7 days of treatment with methylphenidate. These findings suggest that the efficacy of cortical NA neurotransmission is markedly reduced following methylphenidate treatment.

Serotonergic dorsal raphe neurons: subsensitivity to amphetamine with long-term treatment

Rats were pretreated twice daily for six consecutive days with either saline or 1.0, 5.0, or 10.0 mg/kg (+)-amphetamine. On the following day, single-unit recording techniques were used to identify serotonin-containing neurons in the dorsal raphe nucleus (DRN). Pretreatment with amphetamine did not alter the mean spontaneous firing rate of these cells, but in some instances it appeared to produce periods of irregular bursting. Moreover, the response to challenge injections of amphetamine was reduced significantly by pretreatment with the large dose. Thus, whereas an intravenous challenge of approximately 3.0 mg/kg produced a greater than 50% inhibition of activity in the dorsal raphe nucleus in rats pretreated with saline, 1.0 or 5.0 mg/kg (+)-amphetamine, more than twice the challenge dose was required to suppress the activity of serotonergic neurons in rats pretreated with 10.0 mg/kg (+)-amphetamine. These results parallel those previously obtained with dopaminergic neurons, suggesting that both types of cells lose their sensitivity to amphetamine with repeated injections.

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.

Unilateral, intranigral infusions of amphetamine produce differential, bilateral changes in unit activity and extracellular levels of ascorbate in the neostriatum of the rat

Simultaneous recordings of single-unit activity and oxidation current were obtained bilaterally from the neostriatum of rats in response to a unilateral infusion of D-amphetamine (2.0 microliters of 10 micrograms/microliter) into the substantia nigra. Whereas neuronal activity increases ipsilaterally and decreases contralaterally, the electrochemical signals, which reflect extracellular ascorbate, increase bilaterally. In each case, changes in unit activity precede the change in ascorbate release. Systemic administration of haloperidol (0.2 mg/kg) blocks the increase in oxidation current bilaterally, but reverses the neuronal activity response only on the ipsilateral side. These results lend further support to the view that intranigral amphetamine facilitates neostriatal ascorbate release, but this effect is not correlated with changes in single-unit activity.

Differential response of amygdaloid neurons to clozapine and haloperidol: effects of repeated administration

Rats were pretreated with saline or with behaviorally equivalent doses of clozapine (10.0 mg/kg) or haloperidol (1.0 mg/kg) twice daily for six consecutive days. On the following day, amygdaloid neurons in clozapine-pretreated rats responded to a challenge injection of this drug with a significantly greater increase in firing rate than saline controls. In contrast, amygdaloid neurons generally remained unresponsive to haloperidol even when pretreatment with this drug was extended to 13 days. Neither clozapine nor haloperidol pretreatment, however, altered the response of amygdaloid neurons to d-amphetamine administered after a four-day washout period. Amphetamine inhibited amygdaloid activity to a comparable extent in all rats. Taken together, these results implicate the amygdaloid complex as an important site of action of clozapine and related antischizophrenic drugs.

Dose response for amphetamine-induced changes in dopamine levels in push-pull perfusates of rat striatum

Levels of dopamine were determined in push-pull perfusates of striatum in chloral hydrate-anesthetized rats as a function of increasing systemic doses of amphetamine over the range 0.5-5.0 mg/kg. In the absence of amphetamine administration, basal dopamine levels remained stable for at least 6 h. Perfusate levels of dopamine responded in a quantitatively predictable fashion to increasing doses of amphetamine: (1) the maximal increase in perfusate levels of dopamine after amphetamine, relative to predrug levels, was directly proportional to the dose of the drug up to 3 mg/kg (fivefold after 0.5 mg/kg to 30-fold after 3 mg/kg); (2) the duration over which perfusate levels of dopamine were significantly elevated, with respect to preamphetamine levels, was proportional to the dose of amphetamine up to 5 mg/kg; and (3) each successively higher dose of amphetamine significantly increased the perfusate level of dopamine over that observed at the next lower dose up to 3 mg/kg amphetamine. However, maximal levels of dopamine in striatal perfusates were achieved following 3 mg/kg amphetamine and were not increased further at higher doses of the drug. The data suggest that, at higher doses of amphetamine, extraneuronal metabolism of dopamine may be of sufficient capacity to limit increases in synaptic levels of dopamine. The absence of further increases in perfusate levels of dopamine as the dose of amphetamine is increased beyond 3 mg/kg is discussed in terms of potential relevance to mechanisms of amphetamine-induced stereotyped behaviors.

Unilateral dopamine depletions attenuate the response of striatal neurons to systemic amphetamine in both hemispheres

Neuronal activity was recorded bilaterally from the striatum of intact control animals and rats pretreated 10-15 days earlier with a unilateral intranigral injection of 6-hydroxydopamine. Following isolation of single unit discharges, each group was challenged with intravenous injections of 0.2 mg kg-1 d-amphetamine administered at 2-min intervals. Striatal neurons from intact control animals responded to amphetamine with equal numbers of inhibitions and excitations. In contrast, the predominant response in the animals with lesions was no response at all even with a total cumulative dose of 2.0 mg kg-1. Approximately 50% of the neurons in each striatum of the rats with unilateral lesions failed to respond to amphetamine despite a greater than 98% difference in dopamine levels between hemispheres. The remaining neurons in these animals responded as in intact controls. These results suggest that some functional change occurs following unilateral dopamine depletion that acts to decrease the response of neurons to amphetamine in both the intact and the dopamine-depleted striatum.

Methylphenidate, but not other CNS stimulants, inhibits red blood cell calcium-activated potassium efflux

The first-order rate constant of net potassium efflux, measured in human red blood cell (RBC) suspensions by means of a K+-sensitive electrode, was increased 26 fold (from 0.0025 min-1 to 0.0656 min-1) by 0.5 microM of the calcium ionophore A 23187. Both the basal or the calcium-stimulated potassium efflux remained unchanged following the addition of different CNS stimulants (nikethamide (1 mM), pentylenetetrazol (1mM), doxapram (1 mM), strychnine (0.1 mM), picrotoxin (0.1 mM), or nomifensine (0.1 mM). Methylphenidate (10-100 microM), however, inhibited in a concentration-dependent manner the calcium-stimulated, but not the basal potassium efflux. An IC50 of 190 microM was estimated for this effect.

Iontophoretic evidence for subsensitivity of postsynaptic dopamine receptors following long-term amphetamine administration

Neuronal activity was recorded from the anteromedial neostriatum of rats pretreated twice daily for 6 consecutive days with saline or 5.0 mg/kg d-amphetamine. Glutamate was applied in both groups of animals to increase spontaneous firing rates. Iontophoretic application of increasing currents of DA (20-120 nA) produced a progressive inhibition of unit activity in control animals that was significantly reduced in rats pretreated with amphetamine. These results support the view that long-term amphetamine treatment reduces the sensitivity of postsynaptic DA receptors in the neostriatum.

Nigral dopaminergic neurons: differential sensitivity to apomorphine following long-term treatment with low and high doses of amphetamine

Twice daily injections of 1.0 or 5.0 mg/kg D-amphetamine for 6 consecutive days differentially affected the response of dopaminergic neurons in the substantia nigra pars compacta to challenge injections of apomorphine on the following day. Thus, whereas treatment with the high amphetamine dose produced a dramatic shift to the right of the apomorphine dose-response curve, rats treated with 1.0 mg/kg D-amphetamine responded to apomorphine in the same way as saline-treated controls. These results support the view that long-term treatment with relatively high amphetamine doses is required to produce autoreceptor subsensitivity in the substantia nigra.

Immobilization of rats modifies the response of striatal neurons to dexamphetamine

The effect of dexamphetamine (DEX) on striatal multi-unit activity was examined in freely moving rats and again 24 or 48 hr later during immobilization. Animals which in the freely moving state responded with striatal activation following DEX, 1 mg/kg IP, did not respond to this dose of DEX after immobilization. Similarly with DEX 2.5 mg/kg IP, the incidence of excitatory responses seen in freely moving animals decreased to 18% after immobilization, and the incidence of inhibition and biphasic responses increased from 0% in freely moving animals to 52% in immobilized preparations. The results suggest that the response of striatal neurons to DEX is dependent upon the behavioural state of the animal. Furthermore, these findings indicate that the central actions of DEX are more complex than previously believed, and raises the speculation that the excitatory effects of DEX on striatal neurons may be mediated through excitatory striatal afferents.

Long-term amphetamine treatment attenuates or reverses the depression of neuronal activity produced by dopamine agonists in the ventral tegmental area

Neuronal activity was recorded from the ventral tegmental area (VTA) of immobilized, locally anesthetized rats on the day immediately following long-term treatment (twice daily for 6 consecutive days) with saline, 1.0 or 5.0 mg/kg d-amphetamine (d-AMPH). Each rat was challenged intravenously with d-AMPH (beginning with 0.0625 mg/kg) or with 0.005 mg/kg apomorphine. Treatment with d-AMPH significantly reduced the ability of this drug to inhibit VTA activity. In fact, nearly half of the neurons in the high-dose treatment group were excited by d-AMPH, whereas only 20% of control neurons showed this response. Moreover, apomorphine routinely accelerated firing rate in the VTA following treatment with 5.0 mg/kg d-AMPH but this response was never observed in control neurons, not even in those that were excited by d-AMPH. Thus, tolerance appears to develop to the ability of dopamine agonists to inhibit VTA activity and this effect may be mediated, at least in part, by a subsensitivity of inhibitory dopamine autoreceptors.

Shift from inhibition to excitation in the neostriatum but not in the nucleus accumbens following long-term amphetamine

Neuronal responses to 1.0 or 5.0 mg/kg D-amphetamine were recorded simultaneously in the neostriatum and nucleus accumbens of rats pretreated twice daily with these doses or with saline for 6 consecutive days. In all groups, the number of neurons responding to a challenge injection of either dose of amphetamine with an overall excitation or inhibition was not significantly different. During the first 30-60 min of the drug response, however, neurons in the neostriatum of amphetamine-pretreated rats responded with a significant increase in firing rate compared to saline controls. In the nucleus accumbens, on the other hand, tolerance developed to the inhibition produced by 1.0 mg/kg D-amphetamine, whereas the responses produced by 5.0 mg/kg were not significantly altered by long-term treatment. Liquid chromatography with electrochemical detection revealed that pretreatment with 5.0 mg/kg D-amphetamine produced a slight, but significant, reduction of dopamine and norepinephrine levels in the neostriatum. Catecholamine levels were not significantly altered in the nucleus accumbens by either dose. These electrophysiological and neurochemical changes are discussed in relation to the known involvement of these sites in the dose-dependent behavioral alterations that accompany repeated amphetamine injections.

Dorsal raphe neurons: self-inhibition by an amphetamine-induced release of endogenous serotonin

A direct infusion of amphetamine into the dorsal raphe nucleus of the rat inhibited the activity of serotonergic neurons in this site. An intravenous injection of 5-methoxy-N,N-dimethyltryptamine, a serotonin autoreceptor agonist, mimicked this effect. The amphetamine-induced depression of firing rate was blocked by a subsequent injection of methiothepin, a putative serotonin autoreceptor antagonist, but not by pretreatment with a-methyl-p-tyrosine which depletes brain catecholamines. Amphetamine infusions into the surrounding periaqueductal gray or brainstem reticular formation failed to change dorsal raphe activity. The results of these studies indicate that endogenous serotonin, which can be released by a direct infusion of amphetamine, suppresses neuronal activity in the dorsal raphe nucleus by a process of self-inhibition.

Behavioral observation and intracerebral electrochemical recording following administration of amphetamine in rats

Intracerebral electrochemistry (chronoamperometry) was performed on rats that were administered 1, 4, and 8 mg/kg doses of amphetamine. Graphpoxy working electrodes were implanted bilaterally in nucleus accumbens (ACC) and ventral anterior stratum (VAS). Following drug injection, locomotor and stereotyped behaviors were observed. Intracerebral electrochemical signals reliably increased following injection of amphetamine. The magnitude of these increases did not change significantly across the dose range tested for VAS electrodes. ACC electrodes had increases similar in magnitude to VAS electrodes at 1 and 4 mg/kg. At 8 mg/kg increases obtained from ACC electrodes were significantly lower than those recorded from VAS. Onset of the change in electrochemical signal paralleled the onset of activity or stereotypy but the subsequent declines in signal and behavior were only loosely correlated. At the 4 mg/kg dose, the magnitude of signal increase from striatum was negatively correlated with indices of stereotypy and positively correlated with locomotor counts.

Postnatal development of a cholinergic influence on neuroleptic-induced catalepsy

The development of cholinergic influence on neuroleptic-induced catalepsy was investigated in 10-, 15- and 20-day-old rat pups. It was found that the antimuscarinic atropine was ineffective in decreasing the catalepsy produced by spiroperidol treatment at 10 and 15 days. By day 20, however, atropine attenuated cataleptic behavior in a dose-dependent manner. Atropine alone was shown paradoxically to elicit mild to moderate cataleptic responses in 10- and 15-day-olds, but not at day 20. Clozapine by itself produced the same age dependent pattern of catalepsy response as the spiroperidol and atropine combination treatment. These results suggest that cholinergic mechanisms which interact antagonistically with the dopamine systems underlying cataleptic behavior are not functionally mature until after day 15 in the rat.

Amphetamine psychosis and schizophrenia: a dual model

Several of the behavioral consequences of acute and chronic amphetamine treatment were evaluated and related to the underlying neurochemical correlates of drug treatment. It was suggested that decreased noradrenergic activity after long-term amphetamine treatment influences stimulus sampling, whereas enhanced dopaminergic activity was responsible for the progressive augmentation of stereotypy and self-stimulation behavior observed after long-term exposure to amphetamine. It was hypothesized that amphetamine-induced psychosis and the symptomatology associated with schizophrenia are related to alterations in both norepinephrine and dopamine activity.

D-Amphetamine at low doses suppresses noradrenergic functions

This study examined the effects of d-amphetamine on the firing rate of hippocampal cells which had been shown to have an inhibitory, noradrenergic input from the nucleus locus coeruleus (LC). d-Amphetamine at low doses of 0.1 mg/kg i.v. or 0.5 mg/kg i.p. increased the firing rates of these cells. With higher doses of d-amphetamine, both increases and decreases in the firing rates of hippocampal cells were observed. These differential effects on the firing rate of hippocampal cells were statistically significant (x2 = 13.32, d.f. = 3, P less than 0.01). The increased firing rate of hippocampal cells produced by the low doses of d-amphetamine was blocked by a prior destruction of the LC indicating that the drug effect was mediated by LC neurons. d-Amphetamine also significantly attenuated the decrement in the firing rates of hippocampal cells produced by LC stimulation (P less than 0.01). These results suggest that low doses of d-amphetamine suppress rather than enhance the actions os norepinephrine.

Apparent serotonergic modulation of the dose-dependent biphasic response of neostriatal neurons produced by D-amphetamine

Previous reports indicate that amphetamine produces a dose-dependent shift to the firing rate of neurons in the anterior neostriatum that parallels the shift in behavior from locomotion to focused stereotypy. To determine if para-chlorophenylalanine (PCPA), which alters the pattern of the behavioral response to amphetamine, also changes the pattern of the neuronal response to the drug, a dose-response analysis was performed on amphetamine-induced changes in unit activity in the anterior neostriatum of rats pretreated 48 h previously with 300 mg/kg PCPA or vehicle. An intraperitoneal injection of 1.0 mg/kg D-amphetamine, which inhibited neostriatal activity in vehicle controls, produced in PCPA-pretreated animals a prolonged excitation. In contrast, the pronounced increase in firing rate produced by 7.5 mg/kg D-amphetamine in control rats was significantly reduced following PCPA pretreatment. Liquid chromatography with electrochemical detection revealed that compared to controls, PCPA produced a significant reduction of serotonin, but not dopamine or norepinephrine, in the telencephalon. The differential effects of PCPA on the action of low and high doses of D-amphetamine in the anterior neostriatum may explain the differential influence of serotonergic systems on amphetamine-induced locomotion add stereotypy.

Differential actions of classical and atypical antipsychotic drugs on spontaneous neuronal activity in the amygdaloid complex

Classical antipsychotic drugs such as haloperidol produce akinesia and catalepsy, whereas clozapine and related atypical antipsychotics fail to elicit these behaviors even at relatively high doses. Despite these behavioral differences, a cataleptic dose of haloperidol (2.0 mg/kg) produces changes in neuronal activity in the neostriatum and nucleus accumbens comparable to those produced by a non-cataleptic dose of clozapine (20.0 mg/kg). To further elucidate the brain mechanisms underlying the differential behavioral response to these drugs, an electrophysiological analysis was extended to neurons in the rat amygdaloid complex. Whereas an intraperitoneal injection of 2.0 mg/kg haloperidol generally failed to alter the firing rate of amygdaloid neurons, 20.0 mg/kg clozapine typically produced a prolonged increase in activity. Similarly, clozapine, but not haloperidol, reversed the depression of firing rate produced by 1.0 mg/kg d-amphetamine. The results suggest that neurons in the amygdaloid complex are more responsive to antipsychotic drugs devoid of extrapyramidal side effects than to antipsychotics which elicit parkinsonian-like motor dysfunctions.

Dopaminergic agonists differentially affect open-field activity of rats with A10 lesions

Dopaminergic systems appear to exert considerable control over locomotor activity. Although dopamine neurons are located in relatively close proximity within the mesencephalon, their axons project to more diffuse areas, perhaps reflecting some underlying heterogeneity in their function. The purpose of this study was to determine whether dopamine agonists differentially affect activity by acting upon distinct dopamine systems. Bilateral radio-frequency lesions of area A10 in rats failed to affect spontaneous open-field behavior over a 1-month postoperative period. When injected with 1 mg/kg of apomorphine, however, experimental rats more than doubled their activity as compared to the response of sham-operated controls. In contrast, no difference between the two groups of animals was observed in terms of increased activity following 3 mg/kg of either d-amphetamine or methylphenidate. These results are consistent with previous work indicating the involvement of ventromedial mesencephalic dopamine somata in the control of locomotor activity. The data suggest, however, that systems in addition to the dopaminergic mesolimbic projection are responsible, in part, for the hyperactivity elicited by d-amphetamine or methylphenidate.

Apparent tolerance to some aspects of amphetamine stereotypy with long-term treatment

Previous reports have demonstrated that long-term amphetamine treatment results in a progressive augmentation of locomotion and focused stereotypy in the rat. A series of experiments were conducted to determine whether an increase in dopamine receptor sensitivity is the primary mechanism underlying the behavioral alterations associated with multiple amphetamine injections. Detailed observations of the focused stereotyped behaviors produced by amphetamine revealed that although some components were enhanced with long-term treatment, others were reduced. Thus, whereas repeated administration of 2.5 mg/kg d-amphetamine produced a progressive increase in repetitive head and limb movements, long-term treatment with 5.5 mg/kg d-amphetamine resulted in a reduction of licking and biting behaviors (oral stereotypies). These results, which suggest that different mechanisms mediate the various components of focused stereotypy, argue against the supersensitivity hypothesis. In fact, the apparent tolerance that develops to oral stereotypies may reflect a decrease in dopamine receptor sensitivity since repeated amphetamine administration also reduces the oral stereotypies produced by 0.5 or 2.0 mg/kg apomorphine, a direct acting dopamine agonist. Thus, the behavioral alterations produced by repeated amphetamine injections cannot be explained solely by an increase in receptor sensitivity.

Response sensitization and depression following long-term amphetamine treatment in a self-stimulation paradigm

The effects of long-term amphetamine treatment were evaluated on responding supported by self-stimulation of the substantia nigra. Rats repeatedly treated with d-amphetamine, and tested with a low dose of the drug that ordinarily has no behavioral effect, showed higher response rates than animals repeatedly treated with saline and tested with the same dose of amphetamine. In contrast, a depression in responding was observed among animals that received long-term amphetamine administration and were tested with saline. The effects of long-term amphetamine treatment on self-stimulation could not be explained by the intrusion of drug-induced competitive behaviors such as locomotor activity and stereotypy. The results were attributed to changes in dopamine neurotransmission following prolonged exposure to amphetamine and were also discussed in terms of an animal model for 'amphetamine psychosis' and 'postamphetamine depression' in man.

"Classical" and "atypical" antipsychotic drugs: differential antagonism of amphetamine- and apomorphine-induced alterations of spontaneous neuronal activity in the neostriatum and nucleus accumbens

The ability of clozapine and haloperidol to antagonize the depression of firing rate produced by d-amphetamine and apomorphine in the neostriatum and nucleus accumbens was tested in immobilized, locally anesthetized rats. In the neostriatum, an intraperitoneal injection of 2.5 mg/kg d-amphetamine or 1.0 mg/kg apomorphine produced a prolonged inhibition of neuronal activity that was reversed by a subjsequent injection of either 20 mg/kg clozapine or 2.0 mg/kg haloperidol. An analysis of the onset and magnitude of the blockade revealed that clozapine was more effective than haloperidol in reversing the amphetamine response but that both antipsychotic drugs produced a comparable blockade of the apomorphine-induced depression. Similar results were obtained in the nucleus accumbens. The data indicate that although clozapine acts equieffectively in the neostriatum and nucleus accumbens, this atypical antipsychotic drug, aside from blocking postsynaptic dopamine receptors, may exert at least some of its effects by preventing dopamine release.

Sites of action of amphetamine intrinsic to catecholaminergic nuclei: catecholaminergic presynaptic dendrites and axons

1. It is now well established that the behavioral actions of amphetamine, especially locomotor and stereotyped behaviors, are dependent in part upon the release of catecholamines in the central nervous system. 2. A variety of empirical evidence has established that the release of catecholamines by amphetamine leads to changes in neuronal activity in regions postsynaptic to catecholaminergic nerve terminals. In addition, release of catecholamines is accompanied by a marked inhibition of neuronal firing in catecholaminergic neurons. 3. At least two conceptions have been advanced to account for this marked decrease in catecholaminergic neuronal activity. (a) In one of these, release of catecholamines from presynaptic terminals is thought to lead to a compensatory decrease in neuronal firing rate by means of a postsynaptic neuronal feedback loop from regions innervated by catecholaminergic neurons. (b) In the other, decreased neuronal activity is hypothesized to result from local release of catecholamines onto or near catecholaminergic neuronal dendrites and somata, a phenomenon that has been characterized as self-inhibition. 4. For dopaminergic neurons, recent biochemical, neuropharmacological and neuroanatomical evidence suggests that the latter process could be subserved by dopamine released from dopaminergic neuronal dendrites, i.e., "presynaptic" dendrites.