Decreased spiroperidol and LSD binding in rat brain after continuous amphetamine

Long-term changes in brain following continuous phencyclidine administration: an autoradiographic study using flunitrazepam, ketanserin, mazindol, quinuclidinyl benzilate, piperidyl-3,4-3H(N)-TCP, and AMPA receptor ligands

Phencyclidine induces a model psychosis which can persist for prolonged periods and presents a strong drug model of schizophrenia. When given continuously for several days to rats, phencyclidine and other N-methyl-D-aspartate (NMDA) antagonists induce neural degeneration in a variety of limbic structures, including retrosplenial cortex, hippocampus, septohippocampal projections, and piriform cortex. In an attempt to further clarify the mechanisms underlying these degeneration patterns, autoradiographic studies using a variety of receptor ligands were conducted in animals 21 days after an identical dosage of the continuous phencyclidine administration employed in the previous degeneration studies. The results indicated enduring alterations in a number of receptors: these included decreased piperidyl-3,4-3H(N)-TCP (TCP), flunitrazepam, and mazindol binding in many of the limbic regions in which degeneration has been reported previously. Quinuclidinyl benzilate and (AMPA) binding were decreased in anterior cingulate and piriform cortex, and in accumbens and striatum. Piperidyl-3,4-3H(N)-TCP binding was decreased in most hippocampal regions. Many of these long-term alterations would not have been predicted by prior studies of the neurotoxic effects of continuous phencyclidine, and these results do not suggest a unitary source for the neurotoxicity. Whereas retrosplenial cortex, the structure which degenerates earliest, showed minimal alterations, some of the most consistent, long term alterations were in structures which evidence no immediate signs of neural degeneration, such as anterior cingulate cortex and caudate nucleus. In these structures, some of the receptor changes appeared to develop gradually (they were not present immediately after cessation of drug administration), and thus were perhaps due to changed input from regions evidencing neurotoxicity. Some of these findings, particularly in anterior cingulate, may have implications for models of schizophrenia.

Stimulant-induced psychosis, the dopamine theory of schizophrenia, and the habenula

While one of the original underpinnings of the dopamine theory of schizophrenia was the paranoid psychosis which often develops during the binges or speed runs of chronic amphetamine addicts (and, more recently, in cocaine addicts), neurochemical studies of such drug abusers or from animals given continuous stimulants in an effort to model stimulant psychoses have not played a major role in the further evolution of this theory. One clear persisting alteration produced by continuous amphetamine is a neurotoxicity to dopaminergic innervations in caudate. Yet continuous cocaine administration apparently does not induce a similar neurotoxicity and this makes this effect a poor candidate for an underpinning of stimulant psychoses. However, it has recently been found that both continuous amphetamine and cocaine induce a strong pattern of degeneration which is highly confined to the lateral habenula and its principal output pathway, fasciculus retroflexus. This finding has led to a reconsideration of the role of these structures in psychoses. The habenula, as the chief relay nucleus of the descending dorsal diencephalic system (consisting of stria medullaris, habenula and fasciculus retroflexus), is an important link between limbic and striatal forebrain and lower diencephalic and mesencephalic centers. Studies of glucose utilization have consistently shown the habenula to be highly sensitive to dopamine agonists and antagonists. Lesions of habenula produce a wide variety of behavioral alterations. The dorsal diencephalic system has major and predominantly inhibitory connections onto dopamine-containing cells and it mediates part of the negative feedback from dopamine receptors onto dopamine cell bodies. It represents one of the major inputs in brain to the raphe nuclei and has anatomical and functional connections to modulate important functions such as sensory gating through thalamus, pain gating through central gray and raphe and motor stereotypies and reward mechanisms through substantia nigra and the ventral tegmental area. It is argued that alterations in these pathways are ideal candidates for producing the behaviors which occur during psychosis and that future considerations of the circuitry underlying psychoses need to include this highly important but relatively neglected system.

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.

Behavior, striatal and nucleus accumbens field potential patterns and dopamine levels in rats given amphetamine continuously

Behavioral, electrophysiological (field potential recordings, analyzed by computer) and biochemical techniques were employed before, during and after 7 days of continuous administration of amphetamine to rats. Significant changes were observed using all three protocols. Behavioral alterations were greatest on the second day of treatment and progressively normalized during the remainder of treatment. Electrophysiological changes in the striatum were significant only on the second day of treatment, while electrophysiological alterations in the nucleus accumbens were significant on all treatment and recording days, and the magnitude of the changes paralleled the pattern of overt behavioral changes. Levels of DA in the striatum progressively decreased from normal throughout the treatment, declining to less than half of the control level by the sixth day of treatment. In contrast, DA levels in the nucleus accumbens were augmented on the second day of treatment and progressively approached the control level as treatment continued, again paralleling behavior and changes in the electrophysiology of the nucleus accumbens. These results indicate that continuous administration of amphetamine in rats differentially affects electrical activity in the striatum and nucleus accumbens as well as concentrations of DA. In addition, these results have implications for the study of paranoid schizophrenia.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.

Cerebral decortication reverses the effect of amphetamine on striatal D2 dopamine binding site density

Unilateral ablation of the cerebral cortex produces a decrease of approximately 38% in striatal D2 dopamine binding site density. Chronic d-amphetamine treatment also produces a decrease of about 22% in D2 binding site density. Paradoxically, when rats were hemidecorticated and treated chronically with d-amphetamine, the density of striatal D2 dopamine binding sites on the decorticated side was increased (by about 50%) rather than decreased. Evidently d-amphetamine-induced down-regulation of D2 dopamine binding sites depends on an intact corticostriatal pathway.

Alteration of striatal dopaminergic functions implicated in methamphetamine-induced reverse tolerance in rats

Rats were injected repeatedly with methamphetamine (6 mg/kg per day, for 14 days). The effects of the repeated treatment on the change of behaviors and striatal DA metabolism induced by challenge with DA agonists were studied 10 days after the last injection. Repeated methamphetamine administration decreased the sedative effect of low dose apomorphine, and enhanced apomorphine-induced stereotyped behavior but reduced in after pretreatment with tetrabenazine. Striatal DA, HVA and DOPAC contents, and striatal gamma-butyrolactone-induced DA or DOPA accumulation were not altered by repeated methamphetamine treatment. The effect of apomorphine to decrease DA metabolite content and DA or DOPA accumulation was not changed either. As evidenced by experiments using alpha-MT, repeated methamphetamine administration increased DA utilization after methamphetamine or low dose apomorphine challenge, although it did not change DA utilization at the steady state (without drug challenge). These findings suggest that methamphetamine-induced reverse tolerance is accompanied by an increase of DA utilization resulting from the subsensitivity of DA utilization-modulating autoreceptors.

Subchronic methamphetamine treatment selectively attenuates apomorphine-induced decrease in 3,4-dihydroxyphenylacetic acid level in mesolimbic dopaminergic regions

To investigate mechanisms of behavioral enhancement produced by repeated doses of amphetamines, the effects of apomorphine on 3,4-dihydroxyphenylacetic acid (DOPAC) and dopamine (DA) levels were examined in various brain regions of the rat on the 4th day of withdrawal after repeated administration of saline or methamphetamine (3 mg/kg, s.c.) twice daily for 14 days. Apomorphine (0.1 and 1.0 mg/kg, i.p.) produced a dose-dependent decrease in DOPAC levels and no effect on DA levels in the olfactory tubercle, nucleus accumbens, striatum, frontal and cingulate cortices of saline-treated animals. A decrease in DOPAC levels produced by a low dose of apomorphine was attenuated selectively in the olfactory tubercle and nucleus accumbens of methamphetamine-treated animals. A high dose of apomorphine produced a significant decrease in DOPAC levels in both regions. No such attenuation was obtained in the striatum and the frontal and cingulate cortices. These results suggest that subchronic methamphetamine may induce development of hyposensitivity of presynaptic DA receptors in the mesolimbic regions, which contribute to the behavioral enhancement produced by the drug.

Continuous low-level apomorphine administration induces motor abnormalities and hallucinogen-like behaviors

Continuous low-level (0.825 mg/kg/h for 20 h) administration of AP through SC in-dwelling silicone reservoirs in the rat induced behavioral and biochemical changes that were similar to those induced by low levels (0.1 mg/kg) of acutely administered AP (decreased behavioral activity and decreased dopamine metabolism in the striatum). With longer periods of continuous AP exposure (40 h or more) the activity-depressing effects of low-level AP diminished. Concurrently a novel behavioral syndrome emerged characterized by limb flicks, body shakes, sudden orienting responses, and motor abnormalities, such as tremors of the jaw muscles, chewing movements, prominent tongue extensions, and body 'tics'. This behavioral syndrome became more apparent following cessation of drug treatment. These novel behavioral changes, which were accompanied by increased behavioral responsiveness to acutely administered AP and amphetamine, were correlated with increased levels of dopamine, homovanillic acid, and 3,4-dihydroxyphenylacetic acid in the striatum but not the nucleus accumbens. This novel behavioral syndrome appears to reflect a rebound increase in dopaminergic mechanisms in striatum following their chronic suppression by low levels of AP.

Antidepressants and serotonergic neurotransmission: an integrative review

The effects of acute and chronic antidepressant treatment on various aspects of 5-HT neurotransmission are reviewed, in order to assess the net effect of antidepressants on transmission across 5-HT synapses. Events considered include presynaptic effects of antidepressants (on autoreceptor function, uptake and turnover) and effects on postsynaptic receptor function (assessed by electrophysiological, neuroendocrine, behavioural, and receptor binding methods). Acute antidepressant treatment has variable effects: transmission may be enhanced, unchanged or reduced, depending mainly upon the relative contributions of 5-HT uptake blockade and 5-HT receptor antagonism. However, on chronic administration, most antidepressants appear to enhance 5-HT transmission. This effect is clearest in the case of ECS, which has little effect on 5-HT turnover, but reduces uptake and increases postsynaptic receptor function. MAOIs may be an exception: there is little evidence that MAOIs enhance 5-HT transmission following chronic treatment. Most other antidepressant drugs, including some which are powerful receptor antagonists on acute administration, reduce 5-HT receptor function briefly, but enhance receptor function if several hours elapse between the final injection and testing. Zimelidine has little effect on postsynaptic receptor function, but enhances 5-HT transmission by its powerful blockade of 5-HT uptake. Chronic treatment with antidepressant drugs has usually been found to reduce binding to 5-HT2 receptors; it is difficult to reconcile these observations with the functional studies. In general, with the possible exception of MAOIs, chronic administration of antidepressants may enhance 5-HT transmission by both pre- and post-synaptic effects, and the relative contributions vary. This conclusion supports the classical "indoleamine hypothesis of depression" rather than the more recent "hypersensitive serotonin receptor" theory.

Dopamine receptor parameters detected by [3H]spiperone depend on tissue concentration: analysis and examples

The binding of lipophilic radioligands to homogenized tissue was investigated with the help of a simple, two-component model: a specific component reflects binding to a single and uniform population of sites; a nonspecific component reflects partitioning into the membrane and the entrapment of some drug present in the aqueous phase prior to separation of the particulate fraction. The results indicate that the capacity and the affinity of the receptor may be underestimated when the data are analyzed in terms of total rather than free radioligand. Errors in capacity arise when for a significant fraction of the radioligand access to the receptor is blocked by an unlabelled drug and this appears as nonspecific binding. This is most likely to occur when the partition coefficient is such that the free radioligand is located predominantly in the particulate phase. Errors in affinity reflect the tendency of the membrane to reduce the free concentration of a lipophilic drug in the aqueous phase. A further complication arises when a significant fraction of the total radioligand binds to the receptor. [3H]Spiperone binds to dopamine D2 receptors with a dissociation constant of about 50 pM and partitions into the particulate phase of brain homogenates with a membrane/buffer partition coefficient of 410. As expected, both capacity and affinity can appear to depend on the concentration of tissue used in the assay. If the partition coefficient is known, corrected estimates of both parameters can be obtained knowing only the total concentration of radioligand; if the partition coefficient is not known, the free concentration of radioligand in the aqueous phase must be measured independently. The former procedure requires that the aqueous and particulate components of the system be separated by centrifugation; with filtration, the removal of an indeterminate amount of radioligand from the membrane during washing precludes any correction based on the partition coefficient. For the specific example of [3H]spiperone in human brain, the artifacts become negligible at concentrations of protein below 0.1 mg/ml of incubate. The capacity per unit of original tissue is best determined using unwashed preparations, since about 30% of the total protein and a comparable percentage of the receptors are lost on washing.

Continuous amphetamine intoxication: an animal model of the acute psychotic episode

When amphetamines are administered to humans every few hours for several days, either during the 'speed runs' of addicts or in controlled laboratory settings, the psychosis which reliably results is similar to paranoid schizophrenia in a number of important aspects. This unique regimen of drug intake, which involves the continuous presence of stimulants over a prolonged period of time, can be simulated in animals using subcutaneously implanted slow-release silicone pellets containing d-amphetamine base. Monkeys and rats implanted with these pellets develop stages of behavioural alterations which are somewhat similar in sequence to those observed in humans who have received frequent doses of amphetamine. An initial period of hyperactivity and exploratory behaviour is followed by the gradual development of motor stereotypies which become virtually incessant. A period of relative inactivity then appears which is followed, at 4-5 days after pellet implantation, by a late stage. This final stage is characterized by 'wet-dog' shakes, parasitotic-like grooming episodes, and a variety of other forms of hallucinatory-like behaviour. At about the same time there are distinctive and partially irreversible alterations in dopaminergic innervations of the caudate nucleus, but not in mesolimbic dopamine innervation of the nucleus accumbens or in several other neurotransmitter systems. Continuous amphetamine administration may reproduce some aspects of the prolonged excitation which accompanies an acute psychotic episode and may be a fruitful model for the clarification of the dopamine theory of schizophrenia.

Effect of continuous systemic infusion of D-amphetamine on the sensitivity of nigral dopamine cells to apomorphine inhibition of firing rates

Using a single-unit recording technique, sensitivity of nigral dopamine neurons to the inhibitory effect of apomorphine was examined on various days after a 7-day pretreatment with osmotic minipumps containing D-amphetamine. Continuous systemic infusion of D-amphetamine produced a short-lasting subsensitivity of nigral neurons to apomorphine (0-2 days after D-amphetamine), followed by a supersensitivity on days 7, 8 and 9, post-amphetamine. The supersensitivity seems to persist at least for one week, suggesting one possible mechanism mediating long-term behavioral and biochemical changes following chronic high-dose amphetamine administration.

Phasic alterations in dopamine metabolites following continuous administration of amphetamine

Rats were treated with slow-release (+)-amphetamine pellets for 7 days and then sacrificed at various times following pellet removal. Pellet-treated animals had decreased dopamine levels in caudate but not nucleus accumbens, with no recovery even 30 days after pellet removal. In both caudate and accumbens the levels of 3,4-dihydroxyphenylacetic acid and homovanillic acid were decreased at the time of pellet removal, then increased shortly after pellet removal, and were again decreased at 10 and 30 days after pellet removal. These fluctuations may reflect cyclical alterations in dopamine receptors following continuous amphetamine.

Chronic D-amphetamine and phencyclidine: effects on dopamine agonist and antagonist binding sites in the extrapyramidal and mesolimbic systems

Dopamine agonist and antagonist binding to rat striatal and mesolimbic region tissue was examined in rats chronically treated with D-amphetamine or phencyclidine. In striatum, D-amphetamine produces a down-regulation of [3H]spiroperidol binding but an increase in [3H]ADTN bound (measured in the absence of ascorbate). This suggests that [3H]spiroperidol and [3H]ADTN bind to distinct, separate sites which can be independently regulated.

Serotonin receptor changes after chronic antidepressant treatments: ligand binding, electrophysiological, and behavioral studies

Early biochemical research on antidepressant treatments provided evidence that the treatments alter catecholaminergic and serotonergic activity. The mechanisms of action proposed by the resulting biogenic amine hypotheses of affective disorders, however, are not consistent with the delayed onset of therapeutic effects of antidepressant treatments nor with the acute effects of more recently developed antidepressant drugs. Recent investigation of chronic antidepressant treatments using ligand binding, electrophysiological, and behavioral techniques have attempted to identify subgroups of receptors that might be affected uniquely and specifically by chronic antidepressant treatments. Such receptor changes have been suggested to form a basis for the mechanism of action of antidepressants. At the present time, however, the data produced by ligand binding experiments and electrophysiological experiments investigating serotonergic functioning do not fit together. In addition, interpretational problems and internal contradictions exist within each of the three bodies of data when straightforward hypotheses regarding a serotonergic role in antidepressant treatment are formulated. In order to clarify the serotonergic role in antidepressant drug and ECS effects the functional significance of observed changes in putative serotonergic receptors must be discovered. Unfortunately, putative receptors identified by ligand binding cannot be directly compared to those identified by electrophysiological techniques, because these two methods require the disassembly of the organism in mutually incompatible ways. In order to prove that either or both techniques do in fact identify functional serotonin receptors, investigators need to proceed both more microscopically and also more globally. Further anatomical and physiological studies are necessary to locate putative receptors and to demonstrate their place in existing serotonergic networks. Further behavioral studies must be done to relate alterations in receptor characteristics to the functioning of the intact organism.

Behavioral sensitization and relative hyperresponsiveness of striatal and limbic dopaminergic neurons after repeated methamphetamine treatment

Rats were used in a study of the effects of repeated methamphetamine treatment on stereotyped behavior and striatal and limbic dopamine metabolism in response to challenge with the drug or other dopamine agonists. Repeated administration of d-methamphetamine (6 mg/kg per day for 3-14 days) produced long-term behavioral sensitization (augmented response to a challenge injection) not only to the compound (at 44-89 days after drug withdrawal) but also to apomorphine and nomifensine. Even a single injection of d-methamphetamine (6 mg/kg) enhanced the behavioral response to the drug. A challenge dose of d-methamphetamine (2 mg/kg) markedly increased dopamine turnover (lower dopamine and higher 3,4-dihydroxyphenylacetic acid levels, higher ratios of 3,4-dihydroxyphenylacetic acid over dopamine) in the striatum and mesolimbic area of the sensitized animals on day 15 of withdrawal from treatment repeated for 14 days with the drug (6 mg/kg per day). These findings demonstrate that behavioral sensitization induced by methamphetamine is accompanied by increased central dopaminergic transmission.

Behavioural tolerance to amphetamine and other psychostimulants: the case for considering behavioural mechanisms

An hypothesis is presented about the nature of behavioural tolerance in animals to stimulant drugs. It is suggested that, in many behavioural procedures, tolerance is due to behavioural adaptation to those drug effects which cause disruption of ongoing rewarded behaviour. This unitary hypothesis accounts for the available data on tolerance and cross-tolerance to stimulants more effectively than all of the other more conventional explanations which are based upon dispositional or functional concepts, the most common of which are described, evaluated, and found to be inadequate. Furthermore, it is suggested that attempts to explain tolerance in terms of changes in synaptic functioning are subject to very considerable problems of interpretation and that an analysis of behavioural mechanisms may be of greater value in understanding the process of behavioural tolerance. Evidence for the basic behavioural hypothesis is outlined in some detail, and a theoretical justification presented for its major assumptions. Operant studies of chronic stimulant effects on behaviour have often produced very complex patterns of data, considerable differences being reported both between subjects and between studies. A speculative model is presented which attempts to account for this pattern of data in tolerance studies.

Reduction of 3H-spiroperidol binding in rat striatum and frontal cortex by chronic amphetamine: dose response, time course and role of sustained dopamine release

After 5 days of continuous treatment with d-amphetamine base in doses greater than 0.5 mg/kg/h maintained by subcutaneously implanted osmotic minipumps, specific binding of 3H-spiroperidol was reduced in rat striatum and frontal cortex as previously reported. These effects were dose-dependent at lower doses of amphetamine, whereas with higher doses an apparent ceiling for the reduction in binding was reached at approximately 70% of control values. Similarly, increasing the exposure time to amphetamine for up to 14 days only slightly augmented the reduction in 3H-spiroperidol binding already present after 5 days of treatment. In rats treated for 5 days with amphetamine, concomitant treatment with the dopamine (DA) synthesis inhibitor alpha-methyl-p-tyrosine prevented the decrease in 3H-binding in corpus striatum, and attenuated the decrease in frontal cortex. Furthermore, in rats with unilateral 6-hydroxydopamine lesions of the nigro-striatal DA tract, 5 days of chronic amphetamine had no significant effect on 3H-spiroperidol binding in the denervated striatal tissue. Since a major effect of amphetamine is to release DA from nerve terminals, these results indicate that the reduction of DA receptors by chronic amphetamine in the striatum is mediated by sustained release of DA.

Interaction of synthetic opioid metenkephalin peptide analogs, Lilly 127623 and FK 33-824 with indole hallucinogens: antagonism of N,N-dimethyltryptamine- and LSD-induced disruption of food-rewarded bar pressing behavior in the rat

The selected opioid metenkephalin synthetic peptide analogs Lilly (LY) 127623 and FK 33-824 were tested for behavioral dose effects and potential interaction with N,N-dimethyltryptamine (DMT) and lysergic acid diethylamide-25 (LSD) in adult male Holtzman rats trained on a positive reinforcement fixed-ratio 4 (FR-4) behavioral bar pressing schedule, i.e., a reward of 0.01 ml sugar-sweetened evaporated milk was earned on every fourth bar press. DMT (3.2 mg/kg) and LSD (0.1 mg/kg), administered IP following a 0.9% NaCl 15-20-min control pretreatment, disrupted established food-rewarded FR-4 bar pressing in a consistent and reproducible manner. Animals pretreated IP with predetermined behaviorally noneffective doses of LY 127623 (0.01-0.32 mg/kg) and FK 33-824 (0.001-0.01 mg/kg) 15-20 min prior to receiving DMT demonstrated significant antagonism to DMT-induced disruption of FR-4 bar pressing, while doses of 0.10-0.32 mg/kg LY 127623 and 0.00032-0.0032 mg/kg FK 33-824 significantly antagonized LSD-induced behavioral effects.

Chronic antidepressant therapy and associated changes in central monoaminergic receptor functioning

Acutely administered antidepressants possess a multiplicity of pharmacological actions. However, the fact that agents possessing similar pharmacological actions are devoid of antidepressant activity, together with the lack of correlation between doses required for acute pharmacological effects and clinical efficacy, suggest that the mechanism(s) of action of antidepressants cannot be directly attributed to the acute pharmacological properties of the drugs. The lag phase in onset of clinical effectiveness emphasizes the importance of adaptive changes following chronic antidepressant administration. A rapidly accelerating trend in attempting to delineate the precise molecular mechanisms of action of antidepressants is the shift in emphasis following chronic antidepressant therapies from alterations in uptake, storage, synthesis and release of neurotransmitters to adaptive changes in receptor functioning. These adaptations occur both pre- and postsynaptically. Examples of the former are alpha 2 and DA presynaptic receptors, both being down-regulated by certain forms of chronic antidepressant therapy. The fact that the NE-coupled adenylate cyclase system in rat brain slices is down-regulated by tricyclics, atypical antidepressants, MAO inhibitors and ECT emphasizes the importance of the system. Electrophysiological and behavioral studies point to the up-regulation of central alpha 1 and 5-HT receptor functioning following long-term antidepressant therapy. In contrast to the beta-adrenoceptor, these findings cannot be correlated with data from radioligand binding studies. In general central alpha 1-adrenoceptor binding remains unaltered. This is also true for 5-HT1 binding whereas cortical 5-HT2 binding is both increased and decreased depending on the type of antidepressant therapy being investigated. The relationship of these adaptive changes to the clinical efficacy of antidepressants in man is not clear since there is generally a lack of good models for studying human central receptor functioning. A review of current data from animal studies would tend to disfavour the view that all forms of antidepressant therapy possess a common mechanism of action. Perhaps multiple intervention sites exist. The introduction and evaluation of agents possessing a specificity of pharmacological action will undoubtedly aid psychotherapeutic research. The knowledge that peptides and 'classical' neurotransmitters can co-exist in the same neurone will undoubtedly generate studies of the significance and importance of the co-transmitter function of peptides in the mechanisms of action of antidepressant therapies.

Effect of chronic phencyclidine or d-amphetamine treatment on [3H]spiroperidol binding in rat caudate-putamen and nucleus accumbens

Chronic treatment (20 days) of rats with d-amphetamine decreased the density, but had no effect on the apparent affinity of [3H]spiroperidol binding sites in caudate-putamen and nucleus accumbens. In contrast, chronic phencyclidine treatment had no effect either on the number or on the affinity of [3H]spiroperidol binding sites in these two brain regions.

Behavioural and biochemical changes following chronic administration of L-dopa to rats

Rats were given powdered diet containing L-DOPA (together with the peripheral decarboxylase inhibitor carbidopa) for a period of 6 months. The estimated daily intake was in the range 20-30 mg/kg. Initially, at 1 week and 1 month, L-DOPA-fed rats exhibited enhanced spontaneous locomotor activity, but this fell to within the control range by 3 and 6 months, although (+)-amphetamine-induced hyperactivity was greater at 6 months in L-DOPA-treated animals than in control rats. Six months after receiving L-DOPA in their diet rats showed enhanced stereotypy scores to a series of dopamine agonists administered acutely including (+)-amphetamine, nomifensine, L-DOPA, apomorphine and piribedil compared with the control animals. In another behaviour test L-DOPA administration reduced the cataleptic potency of both fluphenazine and haloperidol was increased. Biochemically 6 months treatment of rats with L-DOPA was associated with significantly increased plasma concentrations of L-DOPA, enhanced striatal levels of L-DOPA, dopamine and dopamine metabolites, enhanced specific binding (as indicated by increased Bmax values) of [3H] spiroperidol, [3H] ADTN and [3H] 5-HT to striatal membranes, and increased basal and dopamine-stimulated striatal adenylate cyclase activity. The results are discussed in the light of changes of sensitivity of cerebral dopamine receptors, an increase in receptor numbers, and the tolerance to L-DOPA which often develop in the treatment of Parkinson's disease.

Chronic phencyclidine, like amphetamine, produces a decrease in [3H]spiroperidol binding in rat striatum

Rats were treated with d-amphetamine sulfate (5 and 10 mg/kg i.p.) and phencyclidine (PCP) (5 mg/kg i.p.) twice per day. After 21 days, [3H]spiroperidol binding in striatum was reduced by all treatments; receptor number (Bmax) and not affinity (KD) was affected. These results suggest that the psychotic effect of PCP may, like those of amphetamine, involve changes in dopamine receptors.

Opposed stages of continuous amphetamine administration: parallel alterations in motor stereotypies and in vivo spiroperidol accumulation

Rats pretreated with different regimens of chronic d-amphetamine (d-Amp) administration were injected 24 h after cessation of chronic d-Amp with either d-Amp (to test the degree to which it now elicited motor stereotypies) or with [3H]spiroperidol (to study in vivo accumulation in 7 brain regions). Rats pretreated with 28 daily injections of d-Amp subsequently evidenced heightened stereotypies to d-Amp whereas other rats pretreated with the same amount of d-Amp for 28 days using slow-release pellets showed decreased stereotypies to d-Amp. In neither of these chronic groups was spiroperidol accumulation altered. But the administration of d-Amp continuously over briefer periods induced large and phasic alterations in both d-Amp induced stereotypes and in spiroperidol accumulation. Rats given continuous d-Amp for 24 h, allowed a 24 h drug-free period, and then injected evidenced a large potentiation of both stereotypies and in vivo accumulation of spiroperidol in the caudate nucleus, whereas after 5 days of continuous d-Amp administration rats showed attenuated stereotypies and decreased [3H]spiroperidol accumulation in the caudate, accumbens, and substantia nigra. These results imply that brief periods of continuous d-Amp administration produce large and phasic changes in dopamine systems.

Rapid decline of stereotyped behavior in rats during constant one week administration of amphetamine via implanted ALZET osmotic minipumps

Groups of rats were subcutaneously implanted with ALZET 2001 type osmotic minipumps containing d-amphetamine. Three doses of amphetamine (calculated as 0.47, 0.94 and 1.41 mg/kg/hour; measured values were approximately 50% lower), continuously released for 7 days, initially produced locomotor stimulation and stereotyped behavior slightly increasing over the first few days. During the later days, however, locomotor stimulation and stereotyped behavior markedly declined indicating tolerance development. These results contrast the often reported development of sensitization to the behavioral effect of amphetamine observed with repeated injections.

Effect of chronic amphetamine administration on central dopaminergic mechanisms in the vervet

A biochemical study of central dopaminergic mechanisms was carried out in brain tissue from vervet monkeys who had been subjected to chronic amphetamine administration. The brain concentrations of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) were markedly reduced. Significant reductions were also observed in the activities of dopa-decarboxylase (DDC) and tyrosine hydroxylase (TH). A kinetic study of TH revealed a 60% reduction in maximum reaction velocity, consistent with destruction of DA neurones. However, homovanillic acid (HVA) concentrations were only moderately reduced, suggesting a nearly normal production of DA. Hence despite the large depletions in DA concentrations, high affinity binding of 3H-spiperone binding to striatal DA receptors was no different from controls. The results are discussed in relation to the amphetamine psychosis in humans.

Enhancement of haloperidol-induced increase in rat striatal or mesolimbic 3,4-dihydroxyphenylacetic acid and homovanillic acid by pretreatment with chronic methamphetamine

After a drug-free period of 1 week following 2 weeks of haloperidol treatment, the increased response of striatal 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) to a challenge dose of haloperidol was significantly reduced. Tolerance to this effect was not, however, seen in the mesolimbic system. Pretreatment of the rats with methamphetamine (MAP) for 8 days prior to chronic haloperidol significantly enhanced the DOPAC and HVA increase produced by the challenge with haloperidol in both brain areas. The reduced response of striatal DOPAC or HVA after chronic haloperidol was prevented by pretreatment with MAP. The data suggest that the long-term dopamine receptor stimulation induced by MAP may antagonize the tolerance produced by chronic haloperidol treatment.