Norepinephrine Uptake Inhibitor Research Articles

Cyclic AMP-dependent modulation of vesicular monoamine transport in pheochromocytoma cells.

Cyclic AMP (cAMP) is well known to enhance tyrosine hydroxylase activity in PC12 cells. We were able to demonstrate, however, that the cellular dopamine level in PC12 was lowered by dibutyryl cAMP. Furthermore, the decrease in the cellular level of dopamine was accompanied by about a 10-fold increase in the medium. The aim of this work was to elucidate the effect of cAMP on catecholamine transport. Dibutyryl cAMP did not induce exocytotic release of norepinephrine but rather inhibited its uptake. As with forskolin and cholera toxin, physiological signaling molecules such as vasoactive intestinal polypeptide (VIP) and AMP, for which PC12 cells are known to have receptors linked to activation of adenylate cyclase, also inhibited norepinephrine uptake. The inhibitory effects of dibutyryl cAMP, VIP, and AMP were dose dependent, and EC50 values were estimated to be 100 microM, 10 nM, and 1.0 microM, respectively. The inhibition profile of dibutyryl cAMP over the time course of norepinephrine uptake was biphasic: inhibition became clearly detectable after the cytosolic pool of norepinephrine had been saturated. This profile is similar to that of reserpine. Nomifensine, however, inhibited uptake at a rather constant rate throughout the entire time course. The ATP-dependent serotonin uptake by digitonin-permeabilized cells was lowered to approximately 50% that of the control by dibutyryl cAMP treatment before permeabilization, indicating inhibition of vesicular monoamine transport. This effect was also dependent on a dibutyryl cAMP concentration with an EC50 of < or = 100 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel halogenated analogs of tomoxetine that are potent and selective inhibitors of norepinephrine uptake in brain

Halogenated analogs of the potent norepinephrine (NE) uptake inhibitor, tomoxetine, were synthesized and their affinities for the serotonin (5HT) and NE uptake sites evaluated. One of the most potent was the 2-iodo substituted analog (289306) that inhibited [ 3H]tomoxetine binding to rat cerebral cortex with a K i of 0.37 nM. The compound also inhibited the uptake of [ 3H]NE into rat hypothalamic synaptosomes with a K i of 3.5 nM. This analog was significantly less potent at the 5HT uptake site, as exhibited by a K i of 25 nM in the inhibition of [ 3H]paroxetine binding and a K i of 121 nM in [ 3H]5HT uptake. The resolved ( R) enantiomer (303926) was 10 times more potent as a [ 3H]NE uptake inhibitor and 29 times more potent as an inhibitor of [ 3H]tomoxetine binding than the ( S) enantiomer (303884). Administration of 289306 to rats prior to an i.c.v. injection of 6-hydroxydopamine prevented the depletion of hypothalamic NE and Epi with ED 50 values of 0.28 and 0.47 mg/kg, respectively. Thus, 289306 was a potent inhibitor of NE uptake in vitro and in vivo. In addition, these compounds provide structures for potential ligands for the study of NE uptake sites by autoradiography, PET or SPECT imaging.

Evidence that activation of the hypothalamo-pituitary-adrenal axis by electrical stimulation of the noradrenergic A1 group is not mediated by noradrenaline.

The paraventricular nucleus (PVN) of the hypothalamus, where the CRF-containing neurosecretory cells controlling the hypothalamo-pituitary-adrenal (HPA) axis are located, receives a dense noradrenergic innervation from the A1 group of the caudal ventrolateral medulla. In the present study we studied the relationship between release of noradrenaline (NA) in the PVN and activation of the HPA axis in response to electrical stimulation of the A1 region. In the urethane-anesthetized male rat, extracellular NA in the PVN was monitored on line by electrochemical recording while the activity of the HPA axis was estimated by measurement of ACTH in blood samples. A 1 min, 10 Hz stimulation evoked a significant increase of extracellular NA in the PVN as well as an ACTH surge in blood. The NA and ACTH response evoked by stimulation in the 3- to 14-Hz range were found to be frequency dependent. However, whilst the NA response increased in an exponential manner with respect to frequency, the ACTH response appeared to plateau between 10 and 14 Hz. Specific lesions of the noradrenergic terminals in the PVN, by bilateral local administration of 6-hydroxydopamine, markedly reduced the ACTH response to stimulation. Intracerebroventricular injection of desmethylimipramine, a NA uptake inhibitor, enhanced the increase in extracellular NA evoked by submaximal stimulation about 2.5-fold but did not modify the corresponding ACTH response. Combined intracerebroventricular injection of alpha- and beta-adrenergic antagonists, phentolamine and propanolol respectively, did not prevent the ACTH response evoked by stimulation. Following stimulation of the caudal ventrolateral medulla, the ACTH response thus appears to result from the stimulation of the A1 noradrenergic group projecting to the PVN. However, the inability of pharmacological manipulations which enhance or block central noradrenergic transmission to influence the ACTH response suggests that the noradrenergic endings in the PVN originating from the A1 group use a transmitter other than NA to activate the HPA axis at the PVN level.

Characterization of the effect of cocaine on catecholamine uptake by pregnant myometrium

To characterize the effect of cocaine on catecholamine uptake by myometrium from pregnant women. Slices of myometrium obtained from nine women during elective cesarean delivery at term were incubated with [3H]-norepinephrine and various uptake inhibitors for 30 minutes. The radiolabeled material was extracted with perchloric acid, expressed as percent control (+/- standard error of the mean), and compared by one-factor analysis of variance and Fisher multiple range test. Myometrial uptake was inhibited by cocaine (42 +/- 9%) by neuronal (type 1) uptake inhibitors (desipramine 41 +/- 14%; N-ethylmaleimide 53 +/- 8%), and by extraneuronal (type 2) inhibitors (normetanephrine 56 +/- 19%; corticosterone 73 +/- 9%). When uptake inhibitors were used in combination with cocaine, uptake was not decreased further in the presence of neuronal inhibitors (desipramine plus cocaine 40 +/- 20%; N-ethylmaleimide plus cocaine 42 +/- 4%). However, the effect of cocaine appeared to be added to that of extraneuronal inhibitors (normetanephrine plus cocaine 25 +/- 14%; corticosterone plus cocaine 32 +/- 1%). Catecholamine uptake by myometrium in pregnant women appears to be both extraneuronal and neuronal in nature, and cocaine inhibits the neuronal portion of this uptake. This mechanism may play a role in the increased rate of premature delivery associated with cocaine abuse.

Rapid and long lasting reduction of the hypothermic effect of a D2 dopamine agonist after an intracerebroventricular injection of 6-hydroxydopamine

In mice pretreated intracerebroventricularly (i.c.v.) with 6-hydroxydopamine (6OHDA) (50 μg per mouse), as soon as the hypothermia elicited by the neurotoxin had vanished (3 hr), the hypothermic effect induced by the direct D2 dopamine receptor agonist RU 24926 (1 mg/kg, s.c.), was almost completely suppressed. This reduction in hypothermic effect was observed more than 1 month after the 6OHDA injection. On the 3rd day after 6OHDA injection, this reduction was observed for all tested doses of RU 24926 (0.25–2 mg/kg). It was prevented when an i.p. administration of the norepinephrine uptake inhibitor desipramine (20 mg/kg) was performed 30 min before the 60HDA i.c.v. injection. It was not modified when an i.p. administration of the dopamine uptake inhibitor GBR 12783 (20 mg/kg) was performed 30 min before the 6OHDA i.c.v. injection. The 6OHDA i.c.v. injection modified significantly neither the dopamine nor the serotonin hypothalamic contents. On the contrary it resulted in a marked decrease (− 73%) of the norepinephrine hypothalamic content, which was unchanged by the administration of GBR 12783 (20 mg/kg, i.p.) 30 min before 6OHDA, but completely prevented by desipramine (20 mg/kg, i.p.) 30 min before 6OHDA i.c.v. injection. It is concluded that the hypothermic effect resulting from the stimulation of D2 dopamine receptors involves a central norepinephrine transmission, which is very rapidly altered after 6OHDA administration.

Relationship Between the Systems Responsible for Uptake of 5–hydroxytryptamine and of Noradrenaline by Human Blood Platelets

Human blood platelets carry a high affinity, but low capacity, saturable system for the uptake of noradrenalhe. The uptake is partially Na(+) dependent but cannot be categorised as uptake. It is distinct from the uptake system responsible for 5-hydroxytryptamine transport into the platelet since the selective inhibitors of the platelet uptake system for 5-hydroxytryptamine (citalopram, paroxetine) Wer from those for the uptake system for noradrenaline (normetanephrine, methylisoprenaline). 5-hydroxytryptamine inhibits noradrenaline uptake but with properties inconsistent with competition for the same uptake system while noradrenaline does not inhibit 5-hydroxytryptamine uptake. Neither noradrenaline nor 5-hydroxytryptamine uptake by human platelets is inhibited by dopamine.

Effect of chronic administration of selective 5-hydroxytryptamine and noradrenaline uptake inhibitors on a putative index of 5-HT [formula omitted] receptor function

Chronic treatment with selective 5-HT reuptake inhibitors (SSRI) are therapeutic in obsessive compulsive disorder, depression, anxiety, bulimia nervosa and migraine. In the present study the possibility that SSRI's act by desensitizing 5-HT 2C/5-HT 2B receptors was assessed using a putative in vivo model of 5-HT 2C/5-HT 2B receptor function, mCPP-induced hypolocomotion. mCPP (2,4 and 6 mg/kg i.p. 20 min pretest) reduced locomotion and rears in rats treated acutely or chronically with saline. Acute oral administration of the SSRI's fluoxetine (10 mg/kg), paroxetine (10 mg/kg), or clomipramine (70 mg/kg) or the noradrenaline reuptake inhibitor, desipramine (10 mg/kg), all 1 hr pretest, did not prevent mCPP-induced hypolocomotion. In contrast, chronic treatment with the SSRI's paroxetine and fluoxetine (both 10 mg/kg p.o. daily × 21 days), significantly attenuated the effect of mCPP (4 and 6 mg/kg i.p.) on locomotion and rears 24 hr after the last pretreatment dose. Chronic clomipramine (70 mg/kg p.o. daily × 21 days) also significantly attenuated the effect of mCPP (4 mg/kg i.p.) on rears and tended to reduce the hypolocomotor response. However, chronic treatment with desipramine, (10 mg/kg p.o.daily × 21) had no effect on any of the parameters measured. As chronic fluoxetine and paroxetine did not reduce brain mCPP levels (determined by HPLC 30 min after 4 mg/kg i.p.) the results suggest that chronic SSRI's, but not desipramine, reduce 5-HT 2C/5-HT/ 2B receptor responsivity. If this occurs in man, it may mediate or contribute to their reported therapeutic efficacy in depression, anxiety, bulimia, migraine and alcoholism. It may also be of particular relevance to their unique efficacy in OCD.

Acute vs. chronic EEG effects in maprotiline- and in clomipramine-treated depressive inpatients and the prediction of therapeutic outcome

Patients treated with the norepinephrine uptake inhibitor maprotiline showed an opposite tendency between acute and chronic effect of EEG. Whereas the acute effect indicated a decrease upon EEG vigilance, the chronic effect indicated an increase. Under clomipramine which acts upon different transmitter systems in a complex, up to now not fully understood way, acute and chronic EEG effects could not be differentiated.

ABT-200, a norepinephrine uptake inhibitor, blocks Ca 2+ signals in chromaffin cells

We previously described inhibition by racemic (±)-(1′R ∗,3R ∗)-3- phenyl-1-[1′,2′,3′,4′- tetrahydro-5′,6′- methyl enedioxy-1′- napthalenyl-methyl]-pyrrolidine methanesulfonate (ABT-200), and its two constituent enantiomers, SS,ABT-200 and RR,ABT-200, of nicotine-stimulated but not histamine-stimulated catecholamine release from bovine adrenal chromaffin cells. To test the hypothesis that this inhibition reflects a blockade of Ca 2+ influx, we used fura-2 loaded chromaffin cells to investigate cytosolic Ca 2+ signals. We found that SS,ABT-200 inhibited nicotine- and K +-stimulated Ca 2+ signals, both of which depend on Ca 2+ influx. However, the early phase of the histamine-stimulated Ca 2+ signals, which depends on Ca 2+ mobilization from intracellular stores, was unaffected. We also examined ion flux through the nicotinic receptor by measuring 86rubidium + ( 86Rb +) efflux from preloaded mouse midbrain synaptosomes. We found that SS,ABT-200 partially inhibited nicotine-stimulated 86Rb + efflux, suggesting that it blocks ion flux through the nicotinic receptor directly. These data support a model in which ABT-200 blocks nicotine-stimulated catecholamine release by inhibiting cation flux through multiple channels.

Evidence that the acute behavioral and electrophysiological effects of bupropion (Wellbutrin) are mediated by a noradrenergic mechanism.

Bupropion (BW 323U66) has been considered a dopaminergic antidepressant based on its ability to inhibit the uptake of dopamine (DA) somewhat more selectively than it inhibits uptake of norepinephrine (NE) or serotonin (5-HT). This report describes new evidence that bupropion selectively inhibits firing rates of NE cells in the locus coeruleus (LC) at doses significantly lower than those that inhibit activity of midbrain DA cells or dorsal raphe 5-HT cells. The IC50 dose (13 mg/kg i.p.) for inhibition of LC firing produced plasma concentrations that were not significantly different from those generated by the ED50 in the Porsolt test (10 mg/kg i.p.). The fourfold higher dose needed to inhibit DA cell firing (IC50 = 42 mg/kg i.p.) was similar to the dose associated with locomotor stimulation in freely moving rats. Bupropion did not change the firing rates of 5-HT cells in the dorsal raphe nucleus at any dose. In both in vitro and in vivo tests, the metabolite 306U73 (hydroxybupropion), a weak inhibitor of NE uptake, was approximately equipotent to bupropion with regard to inhibition of LC cells. Another metabolite, 494U73, had no effect on LC firing rates over a wide range of doses. Because of species variation in metabolism, 306U73 was not detected in plasma of rats after i.v. doses of bupropion that inhibited LC firing. Only trace amounts of 306U73 were detected after bupropion dosing for the Porsolt test. Pretreatment with reserpine markedly depleted catecholamines and reduced (by 30-fold) the potency of bupropion to inhibit LC firing.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonergic properties of cocaine: Effects on a 5-HT 2 receptor-mediated behavior and on extracellular concentrations of serotonin and dopamine

The present study examined the ability of cocaine to produce behavioral and neuropharmacological effects through serotonin (5-HT) systems. Pretreatment with fluoxetine or cocaine potentiated the head-shake response to the 5-HT precursor, 5-hydroxytryptophan (5-HTP; 75 mg/kg), a behavior mediated by the activation of 5-HT 2 receptors. This effect was antagonized by the selective 5-HT 2 receptor antagonist ketanserin (1 mg/kg). In contrast, pretreatment with the selective norepinephrine uptake inhibitor desipramine (10 mg/kg) or the selective dopamine (DA) uptake inhibitor GBR 12909 (32 mg/ kg) failed to potentiate the head-shake response. The effects of cocaine on extracellular concentrations of DA and 5-HT in the nucleus accumbens were examined using in vivo microdialysis in a separate group of anesthetized rats. Cocaine (10 mg/ kg) increased the extracellular concentrations of DA and 5-HT by 300–350% over baseline levels. Cocaine's ability to increase the head-shake response and to increase extracellular concentrations of 5-HT may be due to its ability to block 5-HT uptake.

Inhibition of spinal noradrenaline uptake in rats by the centrally acting analgesic tramadol

Tramadol is a centrally acting analgesic with low affinity to opioid receptors. A further mode of action is inhibition of noradrenaline uptake as measured in standard assays. Since tramadol shows antinociception at the spinal site, it was to be tested whether uptake blockade could be verified in spinal tissue. Therefore, synaptosomes and slices had to be prepared from the dorsal half of the spinal cord and the uptake of [ 3H]noradrenaline into synaptosomes to be characterized. The uptake was linear for at least 3 min. The apparent K m was 0.16μM and V max was 7.9 pmol/min/mg protein. Tramadol inhibited the uptake competitively as analysed with Dixon plots with a K i , of 0.6 μM. Uptake inhibition was effected in order of potency by (+)-oxaprotiline > nisoxetine > (−)-tramadol > (−)-oxaprotiline = tramadol > (+)-tramadol. Slices were preincubated with [ 3H]noradrenaline then superfused and stimulated electrically. Nisoxetine, tramadol and its (−)-enantiomer enhanced mainly the stimulation-evoked overflow indicating uptake inhibition without releasing effects. Experiments with inclusion of the noradrenaline uptake inhibitor desipramine provided evidence that tramadol interfered with the noradrenaline transporter. The results show that spinal synaptosomes and slices are valid preparations to study local noradrenaline uptake and release. Tramadol enhances extraneuronal noradrenaline levels in the spinal cord by competitive interference with the noradrenaline uptake mechanism.

Synthesis and pharmacological characterization of ABT-200: a putative novel antidepressant combining potent α-2 antagonism with moderate NE uptake inhibition

ABT-200, (±)-(1′R*, 3R*)-3-phenyl-1-[(1′,2′,3′,4′-tetrahydro-5′,6′-methylenedioxy-1′-naphthalenyl)methyl]pyrrolidine, ( 1a/b) represents the first example of a new structural class of potent α-2 antagonists which possess the additional property of norepinephrine (NE) uptake inhibition. This profile of combined activities is expected to have utility in the treatment of depression.

Mechanisms for the increase in electrically stimulated [3H]norepinephrine release from rat cortical slices by N‐(n‐propyl)‐N‐(4‐pyridinyl)‐1H‐indol‐1‐amine

AbstractN‐(n‐propyl)‐N‐(4‐pyridinyl)‐1H‐indol‐1‐amine (HP 749) is currently in clinical trials for the treatment of Alzheimer's disease (AD). While HP 749 has many pharmacological properties, the biochemical basis for its efficacy in animal models for AD remains unexplained. To this end, we have investigated some biochemical properties of HP 749 as they relate to its effect on electrically stimulated [3H]norepinephrine (NE) release. HP 749 was found to inhibit both [3H]NE uptake and [3H]yohimbine binding to cortical μ2‐adrenergic receptors. Consistent with this profile, HP 749 (1 and 10 μM) enhanced electrically stimulated release of [3H]NE from rat cortical slices. Both clonidine (1 μM) and nomifensine (10 μM) inhibited the effect of HP 749 (1 μM). The enhancement of [3H]NE release produced by the μ2 adrenergic antagonist, idazoxan (0.1 μM), was completely reversed by the μ2 agonist, clonidine (1 μM), but was not affected by the NE uptake inhibitor, nomifensine (10 μM). These results indicate that the HP 749 enhancement of electrically stimulated [3H]NE release is due, at least in part, to a combination of presynaptic μ2‐adrenergic receptor antagonism and NE reuptake blockade. These mechanisms may contribute to some of the adrenergic effects of HP 749.

The effect of antidepressant drugs on the ultradian rhythms of the locomotor activity in rats

Abstract The effect of two antidepressant drugs (+)‐ and (‐)‐oxaprotiline hydrochloride (Ciba‐Geigy, OXA) on the exploration and the basal activity of rats in the light and dark phases of the diurnal cycle was investigated. (+)‐OXA is a selective noradrenaline (NA) uptake inhibitor, while its (‐)‐enantiomer is devoid of such an activity. Male Wistar rats housed individually on a 12:12 h light‐dark schedule (light on: 07.00 h) were treated with (+)‐ or (‐)‐OXA in a dose of 10 mg/kg twice daily, for 14 days. The rats received the last dose at the beginning of the light or dark phase and their movements were registered using Animex actometers for 12 h. The first 30 min of activity was regarded as the exploratory and the further period as the basal activity. In the light phase both (+)‐ and (‐)‐OXA decreased the exploration (80% and 70% of control, resp., p < 0.05) and potentiated it (180% and 190% of control, resp., p < 0.01) in the dark phase. The effect on the ultradian rhythm of the basal activity was pha...

PET studies of cocaine inhibition of myocardial norepinephrine uptake.

Positron emission tomography (PET), [11C]cocaine, and (-)-6-[18F]fluoronorepinephrine [(-)-6-[18F]NE] were used to determine the extent to which the binding of labeled cocaine in the baboon heart represents binding to the norepinephrine transporter and to characterize the functional consequences of cocaine administration on the norepinephrine transporter. Peak heart binding of [11C]cocaine was high (0.038-0.055%/g) and clearance was rapid (t1/2 from peak: 2.5-9 min) for both tracer doses and a pharmacological dose. The binding of a tracer dose of labeled cocaine could not be inhibited by desipramine, tomoxetine, cocaine, nomifensine, or benztropine. The behavior of a pharmacological dose of [11C]cocaine could not be distinguished from a tracer dose and also could not be inhibited by tomoxetine. However, pretreatment with cocaine profoundly inhibited norepinephrine uptake as assessed by (-)-6-[18F]NE. Recovery was slow with only 48% of the baseline (-)-6-[18F]NE uptake being recovered by 78 minutes after cocaine administration. [11C]Benzoylecgonine, a vasoactive metabolite of cocaine, showed negligible retention in heart. The results of this study (i.e., the rapid clearance of cocaine from the heart, the inability to inhibit cocaine binding with desipramine and tomoxetine, and its relatively long-lasting effects on norepinephrine uptake) reinforce the need to understand the link between cocaine pharmacokinetics and norepinephrine transporter function and its relationship to cardiotoxicity.

α2-adrenoceptor binding sites vary along the length of the male reproductive tract: a possible basis for the regional variation in response to field stimulation

Field stimulation (60V, 1 ms, 10 Hz, 10 s) produced monophasic contractions of cauda epididymides that were enhanced by the noradrenaline uptake inhibitor, nisoxetine, but unaffected by the α 2-adrenoceptorantagonist, idazoxan. Similar stimulation of vas deferens produced biphasic contractions that in the presence of idazoxan were enhanced and became monophasic. Nisoxetineaccentuated the separation of the two phases. Radioligand binding studies yielded dissociation constants ( K D) for [ 3H]prazosin binding that were similar (0.1-0.2 nM) in membrane preparations of both halves of the vas deferens and in the cauda epididymis; maximum binding density ( B max) was slightly lower in cauda epididymis (20 fmol/mg protein) than in vas deferens (≈50 fmol/mg protein). K D values for [ 3H]rauwolscine were similar in the two halves of the vas deferens but B max values were higher in the prostatic half of the tissue (39 fmol/mg protein) than in the epididymal half (22 fmol/mg protein). We were unable to detect specific binding of [ 3H]rauwolscine in cauda epididymis. Absence or masking of α 2-adrenoceptors in this tissue would explain our findings that α 2-adrenoceptor agonists and antagonists do not modulate the responses of the cauda epididymis to trains of field stimulation.

Antidepressant-like behavioral effects of serotonin receptor agonists

The clinical discoveries that drugs that stimulate 5-HT neurotransmission, either by inhibiting 5-HT uptake or by stimulating postsynaptic receptors directly, have antidepressant properties has stimulated interest in defining the role of the 5-HT receptor system in the clinical effects of antidepressant drugs. Two approaches are reviewed in this paper that address the neurochemical mediation of the therapeutic effects of antidepressant drugs from the standpoint of animal behavior. The first approach utilizes a behavioral response in rats, the forced swimming test, that correlates well with predicting antidepressant drugs in humans. Studies are reviewed that examined serotonergic compounds in the forced swimming test, from the standpoint of identifying better serotonergic mechanisms involved in the antidepressant response. Both 5-HT uptake inhibitors and 5-HT 1A receptor agonists produce effects in the forced swimming test that are similar to those of other classes of antidepressant drugs. In contrast, agonists at other 5-HT receptors or 5-HT receptor antagonists do not produce antidepressant-like behavioral effects. Evidence for an important role of 5-HT 1A receptors in the antidepressant response is supported by findings that antagonists of 5-HT 1A receptors prevent the ability of 5-HT 1A receptor agonists to reduce immobility in the forced swimming test. The results of studies interfering with 5-HT neurotransmission, either by inhibition of 5-HT synthesis or by the destruction of 5-HT neurons, favor the idea that the effects of 5-HT 1A receptor agonists are produced by the stimulation of postsynaptic 5-HT 1A receptors. The second approach for studying the behavioral effects of antidepressant drugs employs drug discrimination studies, conducted using a discriminated taste aversion procedure, to provide a method for studying the discriminative stimulus effects of the antidepressant 5-HT uptake inhibitor sertraline. Rats were trained to discriminate the effects of sertraline (10 mg/kg) from saline. Other 5-HT uptake inhibitors, such as fluoxetine, fluvoxamine and paroxetine, substituted for the sertraline stimulus. High doses of norepinephrine uptake inhibitors, such as desipramine or maprotiline, were required to produce similar effects. These two behavioral approaches promise to be useful for defining the important pharmacological effects associated with the behavioral effects of antidepressant drugs.

Desmethylimipramine, a potent inhibitor of synaptosomal norepinephrine uptake, has diverse effects on thyroid hormone processing in rat brain. II. Effect on in vivo 5′-deiodination of [ 125I]thyroxine

We have studied the effects of desmethylimipramine (DMI), a tricyclic antidepressant, on thyroid hormone (TH) handling in rat brain in an effort to discover a pharmacological basis for reported interactions between TH, affective disorders and psychotropic drugs. An acute dose of DMI has been used in order to determine the primary effects of the drug in brain without perturbations from secondary effects. Recently we have reported that a single dose of DMI significantly decreases brain uptake of both [ 125I]thyroxine (T 4) and [ 125I]3,3′,5-triiodothyronine (T 3) across the spectrum of thyroid states from hypothyroid (HYPO) to euthyroid (EU) to T 4-induced hyperthyroid (HYPER). To investigate further the effects of DMI on brain processing of TH, we have measured effects of the drug on vivo rates of T 4 to T 3 conversion in a series of experiments in which DMI (25 mg/kg) was given to HYPO, EU and HYPER male rats in conjunction with i.v. [ 125I]T 4. Decreased in vivo conversion ratios (T 3/T 4 ratios) suggest that acute DMI treatment causes a significant decrease in 5′-deiodinase activity in balance of brain (but not cerebellum) in all DMI treated rats as compared to their saline treated controls (ANOVA, P < 0.0001). For assurance that reduced T 3/T 4 in DMI treated rat brain is not the result of DMI enhancement of 5-deiodination of T 3 or T 4, the effect of DMI on concentrations of labeled I −, rT 3, and T 2 (3,3′- and 3′,5′-) was also observed. In no case was there a significant increase in any metabolite in DMI treated rats for any tissue studied. Moreover, labeled I − was decreased in balance of brain in all experimental groups ( P < 0.0001). Thus it appears that DMI neither accelerates T 3 turnover nor enhances any alternative pathway of T 4 metabolism. Rather, acute DMI treatment demonstrates dual effects in balance of brain of both reducing TH uptake and, as now shown, reducing 5′-deiodinase activity. Together, these effects (resulting in diminished brain T 3) are opposite to what should be predicted from knowledge of TH mechanisms by which uniform concentrations of T 3 in brain are maintained. In further investigations of TH interactions with DMI, it will be of interest to establish whether they are adrenergically mediated and how the sites of diminished TH uptake and 5′-deiodination are related.

Na+, K+-ATPase activity in CNS and noradrenergic neurotransmission: time course of differential desipramine (DMI) effects

ATPase activities in CNS membranes were studied after administration of desipramine (DMI), a noradrenaline (NA) uptake inhibitor. In a previous paper we reported that Na+,K(+)-ATPase activity significantly increased 3 h after DMI administration (10 mg/kg) in hypothalamus and mesencephalus but not in cerebral cortex and pons-medulla oblongata membranes (Viola et al., Cell. molec. Neurobiol. 1989, 9, 263-271). Here it was observed that Na+,K(+)-ATPase increase induced by acute DMI disappeared at 24 h in hypothalamus but remained during 21 days in mesencephalus. Na+,K(+)-ATPase increase by acute DMI was inhibited when endogenous NA was depleted by the noradrenergic neurotoxin DSP-4 or the NA synthesis inhibitor alpha-methyl-p-tyrosine. On the whole, Mg(2+)-ATPase activity was not modified by treatment. 5'-nucleotidase, another membrane-bound enzyme, was unchanged by acute DMI. The addition of DMI in vitro (50 ng/mg tissue) during Na+,K(+)-ATPase assay failed to affect ATPase activities. Acute DMI effects on Na+,K(+)-ATPase are thus attributable to noradrenergic neurotransmission rather than to non-specific drug-CNS membrane interaction. Furthermore, DMI produces differential effects on membrane Na+,K(+)-ATPase, depending on treatment conditions and CNS area studied.