Studies on the up regulation of alpha-adrenoceptors on rat hippocampal perikarya by chemical lesion of the median raphe nucleus

Neurotoxin-induced lesion of the serotonergic raphe-hippocampal pathway produced about a 50% increase in the density of a nM affinity alpha-adrenergic binding site for (3H)WB-4101 in rat hippocampus 18 days postlesion without altering the specific binding of (3H)5-HT to serotonergic receptors. The chronic i.c.v. infusion of serotonin by minipump started at the appropriate time averted or reverted the effect. The dynamics of noradrenergic neurotransmission in the hippocampus was not impaired by lesion of the median raphe nucleus as determined by the uptake and turnover of noradrenaline as well as its release - as reflected by the normetanephrine concentration. In addition, neurotoxin-induced lesion of the dorsal noradrenergic bundle failed to alter either the Bmax or the Kd of (3H)WB-4101 binding to the nM site. Kainic acid-induced destruction of perikarya depressed the nM (3H)WB-4101 binding sites by 60% and completely prevented the up regulation caused by lesion of the median raphe nucleus. Thus, the supersensitivity-like response of the adrenoceptors to the lack of serotonin appears to be localized on kainate-sensitive cells within the hippocampus.     

[3H]Ketanserin (Serotonin Type 2) Binding Increases in Rat Cortex Following Basal Forebrain Lesions with Ibotenic Acid

The response of the serotonergic system following injury to the basal forebrain cholinergic system was investigated in rats. The density of 5-hydroxytryptamine (serotonin) type 2 (S2) receptor sites in the frontal cortex and hippocampus was determined 1 week and 4 months after production of lesions by injections of ibotenic acid into the medial septum and nucleus basalis magnocellularis. One week later, the number of S2 receptor sites in the frontal neocortex, as defined by [3H]ketanserin binding, was unchanged. Four months later, the number of [3H]ketanserin binding sites (and Bmax) was increased and high-affinity [3H]serotonin uptake was decreased in the frontal neocortex, but not in the hippocampus, relative to unlesioned controls. Choline acetyltransferase (acetyl-CoA:choline O-acetyltransferase; EC 2.3.1.6) activity was decreased significantly in the frontal neocortex and hippocampus 1 week and 4 months after surgery. The change in frontal neocortical S2 receptor site density was inversely related to the level of choline acetyltransferase activity, was specific for cholinergic denervation associated with the cortex but not the hippocampus, and may represent a localized denervation supersensitivity due to degeneration of median raphe cortical afferents.     

Effect of specific serotonergic lesions on cholinergic neurons in the hippocampus,cortex and striatum

Local injection of 5, 7-dihydroxytryptamine into the median raphe nucleus of rats pretreated with desipramine decreases the serotonin content of the hippocampus and cortex. The turnover of acetylcholine, as measured by the rate of decline of acetylcholine content after hemicholinium-3, the rate of decline of acetylcholine content after hemicholinium-3, is not affected in the hippocampus or the striatum, but is increased in the cortex by such treatment. Local injection of 5, 7-dihydroxytryptamine into the dorsal raphe nucleus of desipramine-treated rats decreases the serotonin content of the hippocampus, cortex, and striatum. The turnover of acetylcholine is increased in the hippocampus and cortex, but not affected in the striatum. Thus, serotonergic neurons from the median raphe nucleus appear to tonically inhibit cholinergic neurons in the cortex, and serotonergic neurons from the dorsal raphe nucleus appear to tonically inhibit cholinergic neurons in the hippocampus and cortex. These serotonergic neurons do not appear to act tonically on striatal cholinergic neurons.     

Firing of 5-HT neurones in the dorsal and median raphe nucleus in vitro shows differential alpha1-adrenoceptor and 5-HT1A receptor modulation

The median raphe nucleus and dorsal raphe nucleus together are the major source of ascending 5-HT projections. Here, using in vitro extracellular single unit electrophysiology we examined the responses of individual neurones in the rat median raphe nucleus and dorsal raphe nucleus to alpha(1)-adrenoceptor and 5-HT(1A) receptor activation and made comparisons between the two nuclei. In the presence of the alpha(1)-adrenoceptor agonist phenylephrine (1microM) all spontaneously active neurones recorded in the median and dorsal raphe nuclei fired slowly (<5Hz) and regularly. Most were inhibited by 5-HT (10-50microM), although a few were excited by 5-HT. 5-HT-induced inhibition was attenuated by the 5-HT(1A) receptor antagonist, WAY100635 (100nM). Compared to those in the dorsal raphe nucleus, the neurones in the median raphe nucleus which were inhibited by 5-HT had: (1) lower basal firing rates in the continuous presence of phenylephrine (1microM), (2) smaller excitatory responses to higher concentrations of phenylephrine (3-10microM), (3) smaller excitatory responses to brief application of norepinephrine (10-100microM) and (4) smaller inhibitory responses to 5-HT (10-50microM). The lower sensitivity of median raphe neurones to alpha(1)-adrenoceptor excitation and 5-HT(1A) receptor inhibition will have consequences for 5-HT neurotransmission in forebrain regions innervated by the two nuclei.     

Characterization of the alpha adrenergic receptor population in hippocampus up regulated by serotonergic raphe deafferentiation

Serotonergic raphe deafferentiation elicits an up regulation of a nM (3H)WB-4101 binding site in rat hippocampus for which norepinephrine displays high affinity and prazosin displays low affinity. Guanine nucleotide affects the nM binding to hippocampal alpha-1 adrenergic receptors. Firstly, Gpp(NH)p, a nonhydrolyzable analog of GTP, inhibits (3H)WB-4101 binding at 3 nM concentration of the radioligand, the ligand concentration labelling the lower affinity, nM, binding site. Secondly, the addition of Gpp(NH)p causes recovery of the heterogeneity of binding sites lost upon preincubation of the membranes with 100 microM epinephrine, apparently by decreasing the affinity of the nM (3H)WB-4101 binding site for the adrenergic receptors. The phenomenon was still observed in the presence of saturating concentrations of the alpha-2 antagonist, yohimbine, and the beta antagonist, propranolol. The results imply that Gpp(NH)p regulates ligand binding to hippocampal alpha-1 agonist sites. It is likely that agonist and antagonist binding sites for the alpha-1 receptor exist in hippocampus with the agonist site being modulated by serotonin.     

Preferential Potentiation of the Effects of Serotonin Uptake Inhibitors by 5-HT1A Receptor Antagonists in the Dorsal Raphe Pathway: Role of Somatodendritic Autoreceptors

Abstract: 5-HT_1A autoreceptor antagonists enhance the effects of antidepressants by preventing a negative feedback of serotonin (5-HT) at somatodendritic level. The maximal elevations of extracellular concentration of 5-HT (5-HT_ext) induced by the 5-HT uptake inhibitor paroxetine in forebrain were potentiated by the 5-HT_1A antagonist WAY-100635 (1 mg/kg s.c.) in a regionally dependent manner (striatum > frontal cortex > dorsal hippocampus). Paroxetine (3 mg/kg s.c.) decreased forebrain 5-HT_ext during local blockade of uptake. This reduction was greater in striatum and frontal cortex than in dorsal hippocampus and was counteracted by the local and systemic administration of WAY-100635. The perfusion of 50 µmol/L citalopram in the dorsal or median raphe nucleus reduced 5-HT_ext in frontal cortex or dorsal hippocampus to 40 and 65% of baseline, respectively. The reduction of cortical 5-HT_ext induced by perfusion of citalopram in midbrain raphe was fully reversed by WAY-100635 (1 mg/kg s.c.). Together, these data suggest that dorsal raphe neurons projecting to striatum and frontal cortex are more sensitive to self-inhibition mediated by 5-HT_1A autoreceptors than median raphe neurons projecting to the hippocampus. Therefore, potentiation by 5-HT_1A antagonists occurs preferentially in forebrain areas innervated by serotonergic neurons of the dorsal raphe nucleus.     

Up-regulation of serotonergic binding sites labeled by [3H]WB4101 following fimbrial transection and 5,7-dihydroxytryptamine-induced lesions

Lesions of the serotonergic afferents to the hippocampus, by fimbrial transection or by 5,7-dihydroxytryptamine treatment, produce an increase in the Bmax of [3H]WB4101 to its nanomolar affinity binding site, with no effect on its picomolar affinity binding site or on [3H]prazosin binding. The nanomolar site is serotonergic as the serotonergic agonists, serotonin and 8-hydroxydipropylaminotetraline (8-OH-DPAT) have nanomolar affinity for [3H]WB4101 binding when studied in the presence of a prazosin mask (30 nM) of the alpha-1 component of [3H]WB4101 binding. The serotonin receptor antagonists metergoline, lysergic acid diethylamide and lisuride also have high nanomolar affinities while ketanserin, yohimbine, prazosin and noradrenergic agonists have affinities in the micromolar range. Fimbrial transection or 5,7-dihydroxytryptamine injections produced 32% and 44% increases in the Bmax of [3H]WB4101 binding in the presence of a prazosin mask. Serotonin competition for [3H]WB4101 binding was identical in control and experimental tissue from each lesion experiment. Although specific binding of [3H]WB4101 was increased, there was no change in the affinities or the percentages of the two binding components for serotonin competition with [3H]WB4101. These data suggest that removal of the serotonergic input to the hippocampus produces an increase in the Bmax of serotonin receptor binding sites labeled by [3H]WB4101.     

Independent effects of cholinergic and serotonergic lesions on acetylcholine and serotonin release in the neocortex of the rat

Rats received a unilateral lesion of the nucleus basalis magnocellularis (NBM) by infusion of ibotenic acid. In addition, the dorsal raphe nucleus was lesioned by infusion of 5,7-dihydroxytryptamine (5,7-DHT). The release of acetylcholine (ACh), choline, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was measured in the frontal neocortex by means of microdialysis. Lesions of the NBM, but not the raphe nucleus, reduced the release of ACh significantly (–47%). The release of 5-HT and 5-HIAA was reduced by raphe lesions (–44% and –79%), but not by NBM lesions. In no case did the combined lesion affect neurotransmitter release more than a single lesion. These results suggest that serotonergic projections from the dorsal raphe nucleus are not involved in tonic inhibition of ACh release in the neocortex.     

Role of Dopaminergic Neurons in Denervation-Induced α1-Adrenergic Up-Regulation in the Rat Cerebral Cortex

The density of [3H]prazosin binding to alpha 1-adrenoceptors in the rat cortex was measured after selective and mixed noradrenergic or dopaminergic lesions. DSP-4 produced a selective noradrenergic lesion and increased the density of alpha 1-adrenoceptors. 6-Hydroxydopamine produced a selective dopaminergic lesion (after desipramine protection of noradrenergic neurons) and a mixed noradrenergic and dopaminergic lesion that did not change the cortical alpha 1-adrenoceptor binding. On the basis of the results obtained, a hypothesis is put forward that the central dopaminergic system controls the denervation-induced cortical alpha 1-adrenoceptor up-regulation.     

Interaction of propranolol with central serotonergic neurons

Central monoaminergic mechanisms are believed to be involved in cardiovascular regulation. The present study was designed to evaluate the involvement of central serotonergic pathways in the antihypertensive action of propranolol in pentobarbital anesthetized mongrel dogs. Ventriculocisternal perfusion of propranolol (25 ug/kg/min for 30 min) decreased serotonin turnover as indicated by a significant decrease in cerebrospinal fluid levels of 5-hydroxyindoleacetic acid (5-HIAA). This effect was accompanied by a significant reduction in mean arterial pressure and heart rate. These results indicate that propranolol decreases central serotonergic activity and suggests a possible role for central serotonergic pathways in the antihypertensive action of propranolol. Several studies have indicated that central serotonergic pathways participate in the regulation of blood pressure. Brainstem regions including the nucleus tractus solitarius, the raphe nucleus and the anterior hypothalamic preoptic region are involved in cardiovascular control and contain a dense population of serotonergic neurons. A centrally-mediated hypotensive effect of propranolol has been demonstrated. Centrally administered propranolol increases cerebrospinal fluid (CSF) levels of norepinephrine and reduces blood pressure possibly due to decreased peripheral sympathetic nerve activity. Central serotonergic pathways may also be involved in the antihypertensive action of some beta-adrenoceptor antagonists. Destruction of central serotonergic neurons with 5,7-dihydroxytryptamine and desipramine abolished the antihypertensive effect of intracisternal propranolol in sinoaortic denervated dogs. Acute administrations of (-)-propranolol and (-)-pindolol decreased the synthesis rate of serotonin, while acute administration of salbutamol, a beta 2-adrenoceptor agonist, increased 5-HIAA levels in rat brain structures. The present study was designed to evaluate the involvement of central serotonergic pathways in the antihypertensive action of propranolol.     

Multiple binding sites for [3H]clonidine and [3H]WB-4101 in rat brain

Binding of the alpha-adrenergic agonist [3H]clonidine and the alpha-adrenergic antagonist [3H]WB-4101 exhibited multiple binding site characteristics in both rat frontal cortex and cerebellum. Kinetic analysis of the dissociation of both radioligands in rat frontal cortex suggests two high affinity sites for each ligand. Competition of various noradrenergic agonists and antagonists for [3H]WB-4101 binding yielded shallow competition curves, with Hill coefficients ranging from 0.45 to 0.7. This further suggests multiplicity in [3H]WB-4101 binding. In the rat cerebellum, competition of various noradrenergic drugs for [3H]clonidine binding yielded biphasic competition curves. Furthermore Scatchard analysis of [3H]clonidine binding in rat cerebellum showed two high affinity sites with KD = 0.5 nM and 1.9 nM, respectively. Competition of various noradrenergic drugs for [3H]WB-4101 binding in the rat cerebellum yielded biphasic competition curves. Lesioning of the dorsal bundle with 6-hydroxydopamine did not significantly affect the binding of either [3H]clonidine or [3H]WB-4101. These findings for both [3H]clonidine and [3H]WB-4101 binding in rat frontal cortex and cerebellum can be explained by the existence of postsynaptic binding sites for both 3H ligands.     

Sodium-dependent [3H]imipramine binding in rat hippocampus and its relationship to serotonin uptake

The relationship of [3H]imipramine recognition sites and serotonergic function was investigated by simultaneously determining the desipramine-defined and sodium-dependent components of [3H]imipramine binding and the serotonin levels and uptake in hippocampus of rats without and with selective lesion of serotonergic neurons with 5,7-dihydroxytryptamine. In control rats, the desipramine-defined [3H]imipramine binding to hippocampal membranes showed a high affinity (Kd = 2 nM) and low affinity (Kd = 31 nM) component. In contrast, the Scatchard analysis of sodium-dependent binding revealed a single class of sites of high affinity (Kd = 1.5 nM). Displacement of sodium-dependent [3H]imipramine binding by cold imipramine resulted in a steep curve best fitted to a one-site model. Sodium-dependent binding of [3H]imipramine at 4 nM concentration represented only about 38% of desipramine-defined binding. 5,7-Dihydroxytryptamine treatment resulted in marked reduction of hippocampal serotonin concentration and uptake without any changes in norepinephrine levels. Virtually only the low affinity component of desipramine-defined [3H]imipramine binding was detected by Scatchard analysis in 5,7-dihydroxytryptamine lesioned rats. The desipramine-defined "specific" [3H]imipramine binding in hippocampi of lesioned rats was decreased by 46%, whereas the sodium-dependent binding was only 18% of that seen in controls. Desipramine-defined specific binding in absence of sodium was not altered by lesion to serotonergic neurons. The results suggest that desipramine-defined specific [3H]imipramine binding may not be appropriate for studying the role of imipramine sites in relation to serotonin neuronal uptake and that determination of sodium-dependent binding components of both [3H]imipramine binding and serotonin uptake should be used in future studies.     

Stereotaxic Injection of Tetanus Toxin in Rat Central Nervous System Causes Alteration in Normal Levels of Monoamines

A single intraventricular injection of tetanus toxin produced a time-dependent elevation of serotonin levels in brain and spinal cord of adult rats. This tetanus toxin-induced increase was produced in areas of high density of serotonergic innervation, such as the hypothalamus, hippocampus, and spinal cord. Little or no effect was found in the thalamus, cerebellum, and frontal cortex, areas that are poorly innervated by serotonergic terminals. The responses of catecholamines (no change in dopamine level and generalized decrease in norepinephrine) pointed to a specific action of tetanus toxin on the serotonergic system. Stereotaxic injections of tetanus toxin in dorsal or magnus raphe nuclei did not have an evident effect on biogenic amine levels in the brain and spinal cord, respectively. Because direct stereotaxic injections of the toxin in the hypothalamus or hippocampus produced significant serotonin increases in both areas, it is proposed that tetanus toxin interacts with presynaptic targets to produce serotonin accumulation; this is probably due in part to an activation of tryptophan 5-hydroxylase.     

Relationship between levels and uptake of serotonin and high affinity [3H]imipramine recognition sites in the rat brain

High affinity [3H]imipramine binding, endogenous levels of serotonin and noradrenaline, and serotonin uptake were determined in brain regions of rats with selective destruction of serotonergic neurons by 5,7-dihydroxytryptamine (5,7-DHT), of adrenergic neurons by 6-hydroxydopamine (6-OHDA), and of rats treated with reserpine. Neonatal treatment with 5,7-DHT resulted in a significant decrease of both serotonin levels and density (Bmax) of high affinity [3H]imipramine binding sites in the hippocampus. In contrast, an elevation of serotonin levels and an increase in Bmax of [3H]imipramine binding were noted in the pons--medulla region. No changes were observed in the noradrenaline content in either of these regions. Intracerebral 6-OHDA lesion produced a drastic suppression of noradrenaline levels in cerebral cortex but failed to alter the binding affinity (KD) or density (Bmax) of [3H]imipramine recognition sites. A single injection of reserpine (2.5 mg/kg) resulted in marked depletion of both serotonin (by 57%) and noradrenaline (by 86%) content and serotonin uptake (by 87%) in the cerebral cortex but had no significant influence of the parameters of high affinity [3H]imipramine binding in this brain region. The results suggest that high affinity [3H]imipramine binding in the brain is directly related to the integrity of serotonergic neurons but not to the magnitude of the uptake or the endogenous levels of the transmitter, and is not affected by damage to noradrenergic neurons or by low levels of noradrenaline.     

Effect of Treadmill Exercise on Serotonin Immunoreactivity in Medullary Raphe Nuclei and Spinal Cord Following Sciatic Nerve Transection in Rats

The serotoninergic system modulates nociceptive and locomotor spinal cord circuits. Exercise improves motor function and changes dopaminergic, noradrenergic, and serotonergic central systems. However, the direct relationship between serotonin, peripheral nerve lesion and aerobic treadmill exercise has not been studied. Using immunohistochemistry and optic densitometry, this study showed that the sciatic nerve transection increased the serotoninergic immunoreactivity in neuronal cytoplasm of the magnus raphe nuclei of trained and sedentary rats. In the dorsal raphe nucleus the increase only occurred in sedentary-sham-operated rats. In the spinal cord of trained, transected rats, the ventral horn showed significant changes, while the change in dorsal horn was insignificant. Von Frey’s test indicated analgesia in all exercise-trained rats. The sciatic nerve functional index indicated recovery in the trained group. Thus, both the aerobic treadmill exercise training and the nervous lesion appear to contribute to changes in serotonin immunoreactivity.     

Effects of chronic alpha 2-adrenoceptor blockade on platelet and lymphocyte adrenoceptor binding in normal volunteers.

Platelet and lymphocyte adrenoceptor binding was measured in 12 healthy male volunteers before and after 22 days treatment with the alpha 2-adrenoceptor antagonist idazoxan 40 mg tds. Platelet alpha 2-adrenoceptor number assessed by the agonist 3H-UK 14304 [correction of UK 14303] was significantly increased following idazoxan, with a smaller increase in antagonist binding (3H-rauwolscine). Lymphocyte beta-adrenoceptor number was unaltered by idazoxan, although the variance within the sample was significantly increased. Plasma MHPG levels were significantly reduced by chronic idazoxan. These data indicate upregulation of the platelet alpha 2-adrenoceptor in response to chronic blockade and suggest that this may reflect a similar change in presynaptic alpha 2-adrenoceptors which regulate norepinephrine release.     

Effects of the serotonin agonist, quipazine, on luteinizing hormone and prolactin release: evidence for serotonin-catecholamine interactions

The present study tested whether administration of the serotonin agonist, quipazine maleate, affects the secretion of luteinizing hormone (LH) and prolactin (PRL) and concomitantly, the activity of central noradrenergic and dopaminergic systems. Quipazine (15 mg/kg, ip) significantly reduced LH and increased PRL when administered to ovariectomized rats. Associated with these changes, the depletion of dopamine seen after synthesis inhibition with alpha-methyl tyrosine was reduced by quipazine in the caudate nucleus and median eminence, suggesting a depression of dopaminergic activity. The depletion of norepinephrine in the median eminence was unaffected. In a second experiment, quipazine (1 microM) diminished the potassium-induced release of both norepinephrine and dopamine from fragments of medial basal hypothalamus, in vitro. Release from preoptic area was unaffected. These results suggest that central serotonergic systems may interact with noradrenergic and dopaminergic systems that regulate LH and PRL secretion, respectively.     

Long-term administration of cocaine or serotonin reuptake inhibitors results in anatomical and neurochemical changes in noradrenergic, dopaminergic, and serotonin pathways

The catechol and indole pathways are important components underlying plasticity in the frontal cortex and basal ganglia. This study demonstrates that administering rats either cocaine or a selective serotonin (or 5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) for 16 weeks results in reduced density of dopaminergic and noradrenergic terminals in the striatum and olfactory bulb, respectively, reflecting pruning of the terminal arbor of ventral midbrain dopaminergic and locus coeruleus noradrenergic neurones. In the striatum of cocaine-treated animals, basal dopamine levels, as well as cocaine-induced dopamine release, is diminished compared with controls. In contrast, serotonergic fibers, projecting from the raphe, sprout and have increased terminal density in the lateral septal nucleus and frontal cortex, following long-term cocaine or SSRI treatment. This is associated with elevated basal 5-HT and enhanced cocaine-induced 5-HT release in the frontal cortex. The anatomical and neurochemical changes in serotonergic fibers following cocaine or SSRI treatment may be explained by attenuated 5-HT_1A autoreceptor function in the raphe. This study demonstrates extensive plasticity in the morphology and neurochemistry of the catechol and indole pathways that contribute to drug-induced plasticity of the corticostriatal (and other) projections. Moreover, our data suggest that drug-induced plastic adaptation is anatomically widespread and consequently, likely to have multiple and complex consequences.     

The Distribution of Serotonergic Nerves in Microencephalic Rats Treated Prenatally with Methylazoxymethanol

Prenatal exposure of pregnant rats to methylazoxymethanol acetate (MAM) induces microencephaly in the offspring. In the present study of these microencephalic rats (MAM rats) we used quantitative autoradiography to investigate [³H] paroxetine binding sites, which are a selective marker of serotonin (5-HT) transporters (5-HTT). The binding in the accumbens, cortex, hippocampus, and dorsolateral thalamus was significantly increased in MAM rats, compared to the control rats, while there was a significant decrease in the dorsal raphe nucleus of the MAM rats. The levels of 5-HTT mRNA in the dorsal raphe nuclei were analyzed by in situ hybridization, which revealed a significant decrease in 5-HTT mRNA-positive neurons in the MAM rats compared to the control rats. The results imply serotonergic hyperinnervation in the cerebral hemispheres of MAM rats, while a target-dependent secondary degeneration of 5-HT neurons might be induced in the dorsal raphe nuclei of MAM rats.     

High affinity dopamine D2 receptor radioligands. 2. [125I]epidepride, a potent and specific radioligand for the characterization of striatal and extrastriatal dopamine D2 receptors.

Epidepride, (S)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-iodo-2,3-dimethoxybenzamide+ ++, the iodine analogue of isoremoxipride (FLB 457), was found to be a very potent dopamine D2 receptor antagonist. Optimal in vitro binding required incubation at 25 degrees C for 4 h at pH 7.4 in a buffer containing 120 mM NaCl, 5 mM KCl, 2 mM CaCl2 and 1 mM MgCl2. Scatchard analysis of in vitro binding to striatal, medial frontal cortical, hippocampal and cerebellar membranes revealed a KD of 24 pM in all regions, with Bmax's of 36.7, 1.04, 0.85, and 0.37 pmol/g tissue, respectively. The Hill coefficients ranged from 0.91-1.00 in all four regions. The IC50's for inhibition of [125I]epidepride binding to striatal, medial frontal cortical, and hippocampal membranes for SCH 23390, SKF 83566, serotonin, ketanserin, mianserin, naloxone, QNB, prasozin, clonidine, alprenolol, and norepinephrine ranged from 1 microM to greater than 10 microM. Partial displacement of [125I]epidepride by nanomolar concentrations of clonidine was noted in the frontal cortex and hippocampus, but not in the striatum. Scatchard analysis of epidepride binding to alpha 2 noradrenergic receptors in the frontal cortex and hippocampus revealed an apparent KD of 9 nM. At an epidepride concentration equal to the KD for the D2 receptor, i.e. 25 pM, no striatal alpha 2 binding was seen and only 7% of the specific epidepride binding in the cortex or hippocampus was due to binding at the alpha 2 site. Correlation of inhibition of [3H]spiperone and [125I]epidepride binding to striatal membranes by a variety of D2 ligands revealed a correlation coefficient of 0.99, indicating that epidepride labels a D2 site. In vitro autoradiography revealed high densities of receptor binding in layers V and VI of prefrontal and cingulate cortices as well as in striatum. In vivo rat brain uptake revealed a hippocampal:cerebellar and frontal cortical:cerebellar ratio of 2.2:1 which fell to 1.1:1 following haloperidol pretreatment. These properties suggest that [125I]epidepride is a superior radioligand for the in vitro and in vivo study of striatal and extrastriatal dopamine D2 receptors.