Positron emission tomography reveals elevated D2 dopamine receptors in drug-naive schizophrenics

Brain dopaminergic mechanisms in Parkinson's disease evaluated by positron emission tomography

The meso-striatal dopamine neurons, essential for the automated control of movements, are primarily affected in patients with P.D. Direct study of the role of this pathway in states of disease has not been possible until recently and the application of PET for the in vivo investigation of dopaminergic mechanisms may serve to demonstrate the potential of the technique. One basic idea has been to work out methods to investigate multiple aspects of dopaminergic function, i.e. presynaptic mechanisms such as re-uptake sites and synthesis of neurotransmitter as well as postsynaptic such as receptor properties. Furthermore, efforts have been made to evaluate dopamine degradating enzymes. Preclinical PET-investigations have regularly been performed in Rhesus monkeys and the hemiparkinsonian model produced by infusing MPTP into one internal carotid artery has been of great value to characterize new 11C-labelled tracers. Today 11C--(+)-nomifensine is used to give a measure of dopamine re-uptake sites, probably reflecting nerve terminals. 11C-labelled L-dopa has now been introduced and can be expected to replace 18F-L-fluorodopa as a physiological tracer for precursor transport and transmitter synthesis. Several ligands are available for the quantitation of dopamine receptors--11C-N-methylspiperone and 11C-raclopride have been used in our studies. 11C-L-selegiline and its "inactive" D-form have been used in clinical PET-studies aimed to evaluate the enzyme MAO-B. A summary of in vivo information of dopaminergic mechanisms in P.D. obtained using the above-mentioned tracers and PET is presented.

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

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

Kinetic analysis of central [11C]raclopride binding to D2-dopamine receptors studied by PET--a comparison to the equilibrium analysis

[11C]Raclopride binding to central D2-dopamine receptors in humans has previously been examined by positron emission tomography (PET). Based on the rapid occurrence of binding equilibrium, a saturation analysis has been developed for the determination of receptor density (Bmax) and affinity (Kd). For analysis of PET measurements obtained with other ligands, a kinetic three-compartment model has been used. In the present study, the brain uptake of [11C]raclopride was analyzed further by applying both a kinetic and an equilibrium analysis to data obtained from four PET experiments in each of three healthy subjects. First regional CBV was determined. In the second and third experiment, [11C]-raclopride with high and low specific activity was used. In a fourth experiment, the [11C]raclopride enantiomer [11C]FLB472 was used to examine the concentration of free radioligand and nonspecific binding in brain. Radio-activity in arterial blood was measured using an automated blood sampling system. Bmax and Kd values for [11C]raclopride binding could be determined also with the kinetic analysis. As expected theoretically, those values were similar to those obtained with the equilibrium analysis. In addition, the kinetic analysis allowed separate determination of the association and dissociation rate constants, kon and koff, respectively. Examination of [11C]raclopride and [11C]FLB472 uptake in brain regions devoid of specific D2-dopamine receptor binding indicated a fourth compartment in which uptake was reversible, nonstereoselective, and nonsaturable in the dose range studied.

Beyond the dopamine hypothesis. The neurochemical pathology of schizophrenia

The dopamine hypothesis still provides a valuable approach to the study of schizophrenia and its treatment by drugs. Although the neuroleptic drugs appear to act via an inhibition of dopamine receptors, measurements of dopamine metabolites in vivo, or of the transmitter and its receptors in post-mortem brain tissue, do not provide unequivocal evidence of a hyperactivity of dopaminergic neurotransmission in the disease. Nevertheless, increased dopamine function might be a consequence of a primary neuronal abnormality in another system. Recent imaging studies and neuropathological reports suggest that, in some patients, there may be a deficit and/or disturbance of neurons in certain temporal limbic regions, and this is supported by some neurochemical investigations, particularly of neuropeptide and amino-acid transmitter systems. A loss of such neurons could conceivably lead to a disinhibition of limbic dopamine neurons, providing the means whereby neuroleptic drug treatment might ameliorate the effects of a neuronal deficit in schizophrenia.

A unified theory of mental illness

I propose that in certain mental illnesses the mechanics for making, releasing, and responding to neurotransmitters are all normal; what is faulty is a portion of the mechanism whereby the brain recognizes abnormal behavior and feeds back on to the neuronal pathways producing the behavior to alter function of that neuronal system and re-establish normal behavior. All the above uses the physical structure of the brain. The existence of the feedback mechanism, and its perturbation, is deduced from data about the mechanism of antidepressants.

The efficacy of the D2 and 5-HT2 antagonist risperidone (R 64,766) in the treatment of chronic psychosis. An open dose-finding study

After a wash-out period of 1 week, 20 patients suffering from schizophrenia were treated for 4 weeks in an open dose-finding study with a new serotonin-dopamine antagonist risperidone. All patients completed the trial. The mean daily dose of risperidone was 4.6 mg (range 2-10 mg) at completion. Risperiodone had a rapid onset of action: a highly significant decrease of the total BPRS score (Brief Psychiatric Rating Scale) was already noticed at the end of the second week. This decrease was found in all BPRS factors after 4 weeks. In spite of the withdrawal of antiparkinson medication at selection, a clear decrease of EPS (extrapyramidal symptoms), assessed on the Simpson and Angus Scale, was observed. The Global Therapeutic Impression agreed to the BPRS scores, showing a highly significant improvement after 2 weeks of treatment. Risperidone was very well tolerated, only mild side effects were reported. Vital signs, electrocardiographic parameters and laboratory values remained normal during the trial. This study indicates that risperidone can be an effective and well-tolerated alternative in the treatment of chronic schizophrenia, combining an antipsychotic activity, a beneficial effect on anergia and anxiety depression and a low EPS-inducing profile.

Norepinephrine stimulates behavioral activation in rats following depletion of nucleus accumbens dopamine

Intraventricular (ICV) infusion of norepinephrine (NE) produces locomotor activation in rats that is greatly potentiated by prior depletion of whole brain catecholamines by ICV injection of 6-hydroxydopamine (6OHDA). In a series of experiments, the neural substrates of this potentiated locomotor response were examined. One group of animals received ICV infusion of 6OHDA to deplete whole brain catecholamines. Other rats were pretreated with desmethylimipramine (DMI) and then received 6OHDA infusions into the nucleus accumbens (NAC) to selectively deplete dopamine (DA) from this region. One week later, all animals were tested for their locomotor response to ICV infusion of NE. Both groups of rats exhibited a greatly potentiated locomotor response to ICV NE compared to corresponding sham-lesioned animals. Both ICV and NAC 6OHDA-injected animals also exhibited a supersensitive locomotor response to the DA receptor agonist apomorphine. These results suggest that NE-induced locomotor activation in ICV 6OHDA-treated rats results from the actions of NE on supersensitive NAC DA receptors.

A mechanism for the involvement of colocalized neuropeptides in the actions of antipsychotic drugs

Evidence has accumulated to implicate neuropeptides localized within midbrain dopamine neurons (cholecystokinin, neurotensin, acetylcholinesterase) in synaptic transmission, mental disease, and pharmacotherapy. We suggest a means by which antipsychotic drugs alter the dynamics between dopamine and colocalized peptides: the intrinsic ability of these agents to stimulate dopamine neuronal activity while blocking dopamine receptors modulates the ratio of catecholaminergic to peptidergic transmission within the mesotelencephalic system. Imbalances of peptide and dopamine cotransmission and their modulation by neuroleptics may be relevant to the pathogenesis and pharmacotherapy of schizophrenia.

Gender differences in the brain: are they relevant to the pathogenesis of schizophrenia?

Gender differences are present in the clinical expression of schizophrenia, age of onset, course of illness, and response to pharmacologic treatment. These differences are not surprising in view of the normal gender differences in brain growth, differentiation, adult brain structure, and neurochemistry. The present review examines what is presently known about brain gender differences, and whether this information is consistent with the published reports of brain functional and morphological abnormalities in schizophrenia. Whether gender differences in the brain can explain the gender differences in clinical aspects of the disorder remains unknown.

Affinity chromatography with the immobilized agonist N-0434 yields an active and highly purified preparation of the dopamine D-2 receptor from bovine striatum

Partial purification of the dopamine D-2 receptor from bovine striatum, solubilized in the presence of 1% digitonin, was obtained by chromatography on wheat germ lectin agarose. The preparation was purified approximately 10-fold. The stability of the receptor preparation was considerably improved and non-specific protein absorption on the affinity gel used later was decreased. Further purification was achieved on a column containing a D-2-selective agonist, N-0434. Approximately 90% of the receptor activity was bound to the gel and 20-40% of the activity could be eluted by pH shock. The total purification factor after one affinity chromatography step was estimated to be at least 1500. An active preparation of at least 20% purity was obtained after a second cycle of affinity chromatography. This corresponds to an enrichment of more than 5000 times compared to the solubilized receptor preparation.

Sight and insight: brain dopamine receptor occupancy by neuroleptics visualised in living schizophrenic patients by positron emission tomography

There can be few more impressive sights in contemporary neuroscience and biological psychiatry than the imaging of neurotransmitter receptors in the brains of living human subjects by positron emission tomography (PET; Sedvall et al, 1986). How has such technology advanced our understanding of the pathophysiology and/or treatment of major psychiatric disorders?

Effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system

1. A review of the effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system is presented. 2. The effects of antidepressants on adenylate cyclase activity and on receptor binding in brain tissue are discussed. Effects on a variety of receptor types are considered. 3. The utilization of electrophysiological, behavioral, and neurochemical studies to assess receptor function after chronic antidepressant administration is discussed, as is the use of peripheral receptor estimations in clinical studies. 4. Animal studies on the actions of chronic administration of neuroleptics on pre- and postsynaptic dopamine receptors are reviewed. Effects of these drugs on dopamine receptors in humans are considered from the following perspectives: postmortem and in vivo binding studies in schizophrenia, tardive dyskinesia, and central versus peripheral receptor estimation.

3H-spiperone binding sites in post-mortem brains from schizophrenic patients: relationship to neuroleptic drug treatment, abnormal movements, and positive symptoms

In post-mortem putamen samples from 27 schizophrenics and 27 controls D2 receptors were measured by Scatchard analysis using 3H-spiperone as a ligand. Maximum number of binding sites (Bmax) and apparent dissociation constant (KD) were significantly increased only in patients in whom neuroleptic medication had been given within a three-month period before death. When the neuroleptic medication had been withdrawn at least 3 month before death, there was a slight, but not significant, reduction in Bmax values and unchanged KD values. Withdrawal of neuroleptic drugs was followed by a normalization of the KD values within 2 weeks and a slower reduction of Bmax values. There were 6 schizophrenic patients with mainly positive schizophrenic symptoms and 17 patients with mainly negative symptoms; positive schizophrenic symptoms were not related to higher Bmax values. There was no difference in 3H-spiperone binding between patients with and without movement disorders (tardive dyskinesia or extrapyramidal symptoms).

HLA antigens in drug-induced parkinsonism

The results of two epidemiological studies suggest a hereditary predisposition to develop drug-induced parkinsonism. We investigated human leukocyte antigen (HLA) antigen prevalence rates in patients with neuroleptic-induced parkinsonism. Fifty-two male, white, neuroleptic-treated, chronic in-patients with DSM-III-diagnosed schizophrenia were examined for the presence of parkinsonism. Subjects were tested for 23 type A, 43 type B, 4 type C, and 10 type DR HLA antigens. The group of schizophrenic patients with parkinsonism (n = 29) was compared with the group of schizophrenic patients without parkinsonism (n = 23). There were no significant differences between the two groups with respect to age, duration of neuroleptic exposure, or anticholinergic drug exposure. One HLA antigen, B44, was significantly more prevalent in the group with parkinsonism than in the group without parkinsonism. We derived a relative risk of 7.16 for drug-induced parkinsonism with HLA-B44 present in this group of schizophrenic patients. These data indicate that HLA-B44 may play a role in genetic or immunologic susceptibility to develop drug-induced parkinsonism in white schizophrenic individuals.

Preparation of [11C]- and [125I]IMB: a dopamine D-2 receptor antagonist

A new dopamine D-2 receptor ligand, N-[2-(N'-4-iodobenzyl-N'-methyl)aminoethyl]-5-chloro-2-methoxy-4- (methylamino)benzamide (IMB) was synthesized and labelled with 11C and 125I at high specific activities. Both radiotracers were prepared by alkylation reactions with [11C]iodomethane and [125I]4-iodobenzyl bromide and appropriate 2 degrees amine precursors. Isolated radiochemical yields of 12-20% for [11C]IMB and 35-50% for [125I]IMB were achieved in 24 and 130-150 min respectively. [11C]IMB and [123I]IMB may prove suitable for a direct comparison of PET and SPECT in the localization and quantification of neuroreceptors.

3H-spiroperidol binding in human temporal cortex (Brodmann areas 41-42) occurs at multiple high affinity states with serotonergic selectivity

3H-Spiroperidol labels multiple high affinity states with serotonergic selectivity in human prefrontal cortex and with dopaminergic selectivity in human caudate and putamen. The characteristics of the binding of this ligand in human temporal cortex have not been previously described. Brodmann areas 41-42 in the temporal cortex are associated with primary auditory sensation and, in epileptics, with auditory hallucinatory experiences. We found that in this region of the human brain, antipsychotic ligands bind at multiple high affinity states, the majority of which exhibit serotonergic, rather than dopaminergic, selectivity. Dose-response data is best resolved by a three-site fit. Results of the co-analysis of dose-response data with saturation data indicates that two of the 3H-spiroperidol affinity states may represent the high and low affinity states of the serotonin (5HT)2 receptor, while a third affinity state may represent the 5-HT1A receptor.

[125]I-spectramide: a novel benzamide displaying potent and selective effects at the D2 dopamine receptor

The new substituted benzamide Spectramide, (N-[2-[4-iodobenzyl-N-methylamino]-2-methoxy-4-ethyl]-5-chloro- methylamine] benzamide) labelled with 125I was used as a potent and highly selective dopamine-D2 receptor antagonist in rat striatal homogenates for in vitro receptor binding. Kinetic experiments demonstrated the reversibility of the binding and the estimated Kd from saturation analysis was 25 pM, with a Bmax of 20 pmol/g of tissue. Competition studies showed that spectramide did not interact potently with the D1 or dopamine-uptake site. Drugs known to interact with other receptor systems were weak competitors of the binding, while binding was potently inhibited by other D2 antagonists, such as spiperone and eticlopride. These data indicate that Spectramide binds selectively and with high affinity to the dopamine D2 receptors, and may prove to be a useful tool for the study of these receptors in vivo using PET or SPECT.

A dopaminergic inhibitory postsynaptic potential mediated by an increased potassium conductance

Intracellular recordings from intact pituitary melanotrophs show that, in the same cell, inhibitory postsynaptic potentials resulting from either pituitary stalk stimulation or exogenous dopamine are abolished by D2 receptor antagonists, display identical conductance changes, are reversed in polarity at the same membrane potential and are sensitive to pertussis toxin pretreatment. The reversal potential of the inhibitory postsynaptic potential shows a 65 mV shift with a 10-fold change in external potassium concentration, which is close to that predicted by the Nernst equation. We conclude that activation of this synapse releases dopamine which acts on a D2 receptor to increase potassium conductance via a G-protein-mediated mechanism. This is the first characterization of an inhibitory dopaminergic synapse in the mammalian nervous system.

CCK-8 injected into the nucleus accumbens attenuates the supersensitive locomotor response to apomorphine in 6-OHDA and chronic-neuroleptic treated rats

Postsynaptic dopamine-cholecystokinin (CCK) interactions in the nucleus accumbens were studied in two behavioral preparations of DA receptor supersensitivity: chronic-neuroleptic treated and 6-hydroxydopamine (6-OHDA) denervated rats. Subcutaneous (SC) injections of apomorphine (APO; 0.15 mg/kg) in experiment 1 produced marked hyperlocomotion in rats following 12 days of pretreatment with cis-[Z]-flupenthixol (2 mg/kg; twice per day). Bilateral intra-accumbens (N.Acc.) microinjections of CCK-8 (2 ng and 2 micrograms) reliably reduced APO-stimulated hyperlocomotion. An intermediate CCK dose (20 ng) was without effect. No change in APO responsivity following chronic vehicle treatment was observed and the baseline APO response was not altered by CCK at any dose. Denervation of mesolimbic dopamine (DA) terminals by intra-N.Acc. injections of 6-hydroxydopamine (6-OHDA; 8 micrograms/side) in experiment 2 similarly resulted in intense locomotor hyperactivity after APO stimulation (0.1 mg/kg; SC). Bilateral intra-N.Acc. injections of CCK-8 (1, 10, 100 ng, and 1 micrograms) significantly attenuated the supersensitive locomotor response to APO. As in experiment 1, CCK produced "biphasic" dose-response effects with strong attenuation that persisted throughout the entire 60-min test at both high (1 microgram) and low (1 ng) doses. Intermediate CCK doses (10 and 100 ng) produced only short-term reductions in activity. Hypomotility induced by APO in SHAM-lesioned rats was not effectively reversed by CCK treatments. CCK had no effect on unstimulated baseline locomotor activity in either 6-OHDA or SHAM-lesioned rats. These results provide further evidence that CCK-8 modulates mesolimbic DA activity by functionally opposing the postsynaptic effects of DA in the region of the nucleus accumbens.

Positron emission tomography of the brain

Positron emission tomography (PET) is a technique of transverse tomographic imaging in which detection of two photons emitted from the annihilation of a positron and an electron is used to reconstruct the distribution of a positron emitting isotope within an object. PET provides the capacity to quantitatively measure the local tissue distribution of a variety of radionuclides that are attached to compounds that distribute according to function. Although this technique has been used to measure multiple functions and receptors within the brain, one of the most widespread uses is the measurement of local cerebral glucose metabolism based on the deoxyglucose method. In this article, the application of PET to clinical disorders such as dementia, brain tumors, psychiatric disease, epilepsy, movement disorders, and stroke as well as to normal states such as aging are examined.

Modulation of second messenger function in rat brain by in vivo alteration of receptor sensitivity: relevance to the mechanism of action of electroconvulsive therapy and antidepressants

1. The second messengers cyclic AMP and inositol triphosphate are the intracellular mediators for a number of neurotransmitters for which receptors exist on brain neurons. 2. Up- or down-regulation of these receptors in general produce corresponding changes in the associated second messenger systems. 3. Chronic administration of antidepressants including electroconvulsive shock to rats produces a number of changes in cerebral receptors, notably down-regulation of beta-adrenergic and serotonin 5-HT2 receptors and up-regulation of alpha-1 adrenergic receptors. 4. The changes in receptor number induced by such antidepressant treatments are in general accompanied by corresponding changes in the associated second messenger reactions. 5. Antidepressant administration has also been shown to induce increased post-receptor mediated adenylate cyclase activity in cortical membranes, and similar effects have also been reported in striatum after chronic administration of neuroleptics. The relevance of these effects to the mechanism of action of the drugs is discussed.

Mesolimbic dopamine receptor increases two weeks following hippocampal kindling

Kindled seizures developed in rats following repeated electrical stimulation of the left CA1 region of the hippocampus. Two weeks after the final kindled seizure, the densities of dopamine (DA) D2 receptors were assayed in the left and right amygdaloid area, nucleus accumbens, and nucleus caudatus. A significant increase (107%) in the density of DA D2-receptors in the ipsilateral nucleus accumbens occurred. This finding may help to define the long-term neurochemical consequences of kindling.

Psychosis and positron tomography

The new technology of positron emission tomography (PET) offers hope in developing objective biological indices and correlates of various psychotic states, including schizophrenia. PET is of the order of a million to a billion times more sensitive than MRI (magnetic resonance imaging) and is now successfully measuring the densities of various brain neurotransmitter receptors in health and disease. PET data in schizophrenia patients confirm that delusions and hallucinations are controlled by neuroleptics when the D2 dopamine receptors are specifically blocked. D1 receptors are not involved. The elevation of D2 receptors as a possible basis for psychotic symptoms needs additional investigation because of technical difficulties which need to be resolved. The rapid development of PET indicates its promise for differential diagnosis as well as its value in treating the psychoses.

Receptor binding and selectivity of three 11C-labelled dopamine receptor antagonists in the brain of rhesus monkeys studied with positron emission tomography

The regional distribution of 3 11C-labelled dopamine receptor antagonists, N-methyl spiperone, raclopride and clozapine, in the brain of Rhesus monkeys was studied by positron emission tomography (PET). The measured radioactivities in the striatal area were similar for the 3 antagonists, although the highest selectivity as compared to cerebellum was found for 11C-raclopride 60 min after administration. The selectivity of the radiotracers for the serotonin and D2-dopamine receptors was evaluated after pretreatment of the monkeys with serotonin and dopamine receptor antagonists. 11C-N-methylspiperone and 11C-clozapine both bound to serotonin receptors in the frontal cortex and to D2-dopamine receptors in the striatal area. Raclopride was selectively bound to the D2-dopamine receptors. The radioactivities measured in the striatal area with cerebellum as reference were fitted to a 3-compartment model which made possible evaluation of receptor binding characteristics. The rate proportional to the association rate constant for the receptor, kon and number of receptors, Bmax, varied from 0.02-0.07 min-1 between the studied radiolabelled drugs, whereas the apparent dissociation rate was highest for clozapine. This means that clozapine had the lowest affinity for the receptors in the striatum, assuming that the Bmax values are identical. The observed difference in selective receptor binding and binding characteristics of the 3 tracers may have an influence both on the clinical efficacy and side effects of the studied dopamine receptor antagonists.

Serial [18F]N-methylspiroperidol PET studies to measure changes in antipsychotic drug D-2 receptor occupancy in schizophrenic patients

An indirect approach to the relationship among drug dose, plasma level, and the competition between a labeled neuroleptic drug [18F]N-methylspiroperidol (18F-NMS) for binding sites in striatal tissue in normal and schizophrenic subjects is described. The slope of the line plotting the ratio of activity in the striatum (As) to activity in the cerebellum (Ac) versus time up to 5 hr postinjection of 18F-NMS is taken as a marker of site occupancy. An inverse relation between labeled competitor uptake and drug plasma level has been demonstrated for the classes of antipsychotic drug studied. Striatal uptake studies showed a progressive increase in all subjects following drug withdrawal up to 156 hr postwithdrawal. Uptake and clearance of 18F-NMS in cerebellar tissue was not appreciably affected by antipsychotic medication or drug withdrawal.

Positron emission tomography and subcortical glucose metabolism in schizophrenia

Our previous observation of a disturbed subcortical-to-cortical gradient of activity in schizophrenia was further elucidated by examining glucose metabolism in three subcortical structures: lenticular nucleus, caudate nucleus, and thalamus. Local cerebral glucose metabolism was determined with 18F-fluorodeoxyglucose using positron emission tomography (PET) in a sample of 20 unmedicated schizophrenics and 18 normal volunteers. Repeated evaluations were performed for 12 schizophrenics following treatment with psychotropic medications and for 11 controls. Unmedicated schizophrenics had lower cortical and caudate absolute metabolic rates. Subcortical-to-cortical ratios for the lenticular nucleus and thalamus were increased in schizophrenics compared with controls, reflecting a preservation of activity in these structures relative to decreased cortical metabolism. When patients were grouped by length of medication-free period before the initial study, there was a trend for patients who had been medication free less than 6 months to have higher subcortical ratios. However, there were no consistent effects of medication in the subsample of patients whose PET studies were repeated following treatment. The results demonstrate relative hypermetabolism in structures implicated in dopamine pathways. An understanding of the physiological significance of this finding awaits the combined measurement of metabolic activity and neuroreceptors in schizophrenics.

Localization of nigrostriatal dopamine receptor subtypes and adenylate cyclase

Quantitative autoradiography using [3H]-SCH 23390, [3H]-sulpiride and [3H]-forskolin was used to assess the effects of single and combined neurotoxin lesions of the nigrostriatal pathway in the rat brain on dopamine (DA) receptor subtypes and adenylate cyclase (AC), respectively. Ibotenic acid (IA) lesions of the caudate-putamen (CPu) resulted in near total loss of both [3H]-SCH 23390 and of [3H]-forskolin binding in the ipsilateral CPu and substantia nigra reticulata (SNR). [3H]-sulpiride binding in the CPu was only partially removed by this same lesion, and nigral [3H]-sulpiride binding was virtually unchanged. 6-Hydroxydopamine (6-OHDA) and IA lesions of the substantia nigra compacta (SNC) did not affect [3H]-SCH 23390 or [3H]-forskolin binding, but largely removed [3H]-sulpiride binding in the SNC. A 6-OHDA lesion of the nigrostriatal pathway followed by an ipsilateral IA injection of the CPu failed to further reduce [3H]-sulpiride binding in the CPu. These results demonstrate that postsynaptic DA receptors in the CPu are of both the D1 and D2 variety; however, a portion of D2 receptors in the CPu may be presynaptic on afferent nerve terminals to this structure. D1 receptors in the SNR are presynaptic on striatonigral terminals, whereas the D2 receptors of the SNC are autoreceptors on nigral DA neurons. The existence of presynaptic D2 receptors on nigrostriatal DA-ergic terminals could not be confirmed by this study. Co-localization of D1 receptors and AC occurs in both the CPu and SNR.

Brain imaging: applications in psychiatry

Various brain imaging techniques have become available in the past decade. These include techniques to evaluate brain structure, such as computed tomography and magnetic resonance imaging, and techniques to assess functional activity, such as measurement of regional cerebral blood flow, single photon emission computed tomography, and positron emission tomography. These techniques can be used to map brain structure and function in normal human beings, and they have enlarged our knowledge of the pathophysiology of mental illnesses by demonstrating structural, metabolic, and neurochemical abnormalities in a wide range of mental disorders.