Effect of amitriptyline on adrenergic receptor number and second messenger function in rat brain.

Radioligand binding studies were done to investigate the effect of chronic administration of Amitriptyline on alpha1-adrenoceptor (alpha1-AR) receptor mediated response to inositol triphosphate (IP3) in rat brain. Our studies revealed a significant decrease in the densities of alpha1-ARs in cortex and cerebellum of rat brain after chronic administration of Amitriptyline (10 mg kg-1 b.wt.). However, there was no significant change in the affinity of [3H]prazosin to alpha1-ARs. Displacement studies showed that Amitriptyline has higher affinity for alpha1-AR with a Ki value of 182+/-16 nM. Significant change was observed in basal IP3 activity in cortex and cerebellum after Amitriptyline exposure. In cortex and cerebellum of experimental rats the NE (Norepinephrine) stimulated IP3 activity was significantly decreased (1460+/-102 DPM/g tissue; p<0.0001; 1188+/-112 DPM/g tissue; p<0.0001), when compared to NE stimulated IP3 activity (4152+/-286 and 3952+/-245 DPM/g tissue, respectively) in control rats. The decrease in NE stimulated IP3 activity in both regions may be due to the significant downregulation of alpha1-ARs in cortex after Amitriptyline exposure as these sites are positively coupled to IP3. The observed significant decrease in alphal-ARs with concomitant decrease in NE stimulated IP3 activity, after Amitriptyline treatment, suggests that Amitriptyline which has high affinity for these sites, acts by modulating the alpha1-AR receptor mediated response in brain.

Differential modulation of α-1 adrenoceptor subtypes by antidepressants in the rat brain.

The aim of the present study was to examine the effect of chronic antidepressants treatment on the density of α1-adrenoceptor (AR) subtypes in rat brain. Density of total α1 and α(1A)- and α(1Β)-ARs was measured in cortex and cerebellum of rats treated with amitriptyline (AMI), desipramine (DMI) and fluoxetine (FLX), (10 mg/kg body wt), for 30 days, using [³H]prazosin in presence and absence of WB-4101. The density of cortical total α1-ARs was significantly decreased with AMI (54%) and DMI (25%) treatment, without altering the affinity of the receptor. Fluoxetine did not alter the density of cortical α1-ARs. The density of cortical α(1A)-ARs was also significantly decreased with AMI (85%) and DMI (50%) treatment, without affecting the affinity. The density of cerebellar total α1-ARs was significantly decreased with AMI (37%), DMI (50%) and FLX (70%) treatment, without affecting the affinity for [³H]prazosin. The density of α(1A)-ARs was significantly decreased with AMI (67%), DMI (59%) and FLX (92%) treatment. α(1B)-AR density was decreased only with FLX (47%) and DMI (47%) treatment. Correspondingly the basal IP3 and NE (10 µM) stimulated IP3 levels were significantly decreased in AMI (47%), DMI (22%) and FLX (48%) treated rat cortex. The results suggest that chronic antidepressant (AD) treatment down-regulates the cortical and cerebellar total α1-ARs in rat brain. However, α(1A) subtype is predominantly down-regulated by AMI and DMI, where as FLX affects cerebellar α(1A)-ARs. The region-specific and subtype specific down-regulation of α1-ARs density, which occurs after prolonged AD treatment, may underline the therapeutic mechanism of action.

Synthesis and pharmacological evaluation of novel N-alkyl/aryl substituted thiazolidinone arecoline analogues as muscarinic receptor 1 agonist in Alzheimer's dementia models.

Earlier we have reported the effect of arecoline thiazolidinone and morpholino arecoline analogues as muscarinic receptor 1 agonist in Alzheimer's dementia models. To elucidate further our SAR study on the chemistry and muscarinic receptor binding efficacy, a series of novel N-alkyl/aryl substituted thiazolidinone arecoline analogues 6(a-m) were designed and synthesized from 3-pyridine carboxaldehyde by reacting with different amines in the presence of gamma-ferrite as catalyst and subjected to in vitro muscarinic receptor binding studies using male Wistar rat brain membrane homogenate and extended to in vivo pharmacological evaluation of memory and learning in male Wistar rats. Derivative 6j having diphenylamine moiety attached to nitrogen of thiazolidinone showed significant affinity for the M1 receptor binding.

Muscarinic receptor 1 agonist activity of novel N-aryl carboxamide substituted 3-morpholino arecoline derivatives in Alzheimer's presenile dementia models.

Earlier we have reported the effect of arecoline thiazolidinone and morpholino arecoline derivatives as muscarinic receptor 1 agonists in Alzheimer's presenile dementia models. To elucidate further our Structure-Activity Relationship (SAR) studies on the chemistry and muscarinic receptor 1 binding efficacy, a series of novel carboxamide derivatives of 2-(1-methyl-1,2,5,6-tetrahydropyridin-3-yl)morpholine molecule have been designed and synthesized as a new class of M1 receptor agonists with a low toxicity effect profile that enhances memory function in animal models of Alzheimer's presenile dementia and also modulates the APP secretion from rat brain cerebrocortical slices by activating M1 receptor in vitro. Results suggest that compound 9b having methyl group at the para position of the aryl group attached to the carboxamide of morpholino arecoline could emerge as a potent molecule having antidementia activity.

Study of pyruvate kinase activity in human astrocytomas - alanine-inhibition test revisted.

BACKGROUND: Recent studies have confirmed that alterations in the isoenzyme of pyruvate kinase (PK) provide tumor cells with selective growth advantage. AIMS: Our aim was to establish the mean activity of the enzyme PK in human astrocytomas and to look for any trends in the activity with relation to histological grade. MATERIALS AND METHODS: The PK (EC 2.7.1.40) activity was measured in the tumor homogenate by spectrophotometric rate determination. DeltaAbsorbance at 340 nm (A 340 nm ) per minute was obtained using the maximal linear rate for both the test and the blank. Enzyme activity was estimated in the presence and absence of amino acid alanine. RESULTS: The mean PK level in astrocytomas was 3.5 +/- 2.0 mmol/min/mg protein, which was significantly higher (24%; P < 0.001) when compared to 2.8 +/- 0.3 mmol/min/mg protein in control brain. Highest PK activity was noted in grade 2 astrocytomas. In controls there was no change in PK activity in the presence of alanine. In grade 2 astrocytomas there was 7% decrease in mean PK activity in the presence of alanine, this difference in grade 3 astrocytomas was 33% and in grade 4 astrocytomas it was 61%. As the tumors were becoming malignant there was a graded increase in the levels of PK inhibition. CONCLUSIONS: Mean PK activity was significantly higher in astrocytomas. There was a graded increase in level of PK inhibition as the tumors were becoming more malignant.

Muscarinic receptor 1 agonist activity of novel N-arylthioureas substituted 3-morpholino arecoline derivatives in Alzheimer's presenile dementia models.

As part of our continuing effort aimed at the development of selective, efficacious and centrally active M1 muscarinic agonists for the treatment of Alzheimer's presenile dementia, a series of N-arylthioureas substituted 3-morpholino arecoline derivatives 9(a-j) were synthesized by using N-benzyl amino ethanol coupling with alpha-bromo acetyl pyridine followed by reduction and cyclization to develop a new class of M1 receptor agonists. Subsequently the synthesized compounds were subjected to in vitro radioligand M1 receptor affinity studies, IP3 formation studies and also to in vivo pharmacological evaluation of memory and learning in male Wistar rats. Derivatives with chloro (9f) and methoxy (9c) groups on the para position of the benzene ring attached to the nitrogen of thiourea showed several fold high affinity for the M1 receptor (in vitro) among all the synthesized molecules 9(a-j), and also significantly elevated IP3 levels and as well elicited beneficial effects in vivo in memory and learning models in rats (rodent memory evaluation, plus and Y maze studies).

Effect of novel N-aryl sulfonamide substituted 3-morpholino arecoline derivatives as muscarinic receptor 1 agonists in Alzheimer's dementia models.

A series of novel, potent, and selective muscarinic receptor 1 agonists (M1 receptor agonists) that employ a key N-substituted morpholine Arecoline moiety has been synthesized as part of research effort for the therapy of Alzheimer's diseases. The ester group of arecoline (which is reported as muscarinic agonist) has been replaced by N-substituted morpholine ring. The structure-activity relationship reveals that the electron donating 4-substituted sulfonyl derivatives (9a, 9b, 9c, and 9e) on the nitrogen atom of the morpholine ring increases the affinity of M1 receptor binding 50- to 80-fold greater than the corresponding arecoline. Other derivatives also showed considerable M1 receptor binding affinity.

Effect of novel arecoline thiazolidinones as muscarinic receptor 1 agonist in Alzheimer's dementia models.

The discovery of cholinergic deficit in Alzheimer's disease (AD) patient's brain has triggered research efforts, using cholinomimetic approaches for their efficacy in AD therapy. Various therapies may be of potential clinical use in AD. Among these are cholinergic agents, which include muscarinic agonists, acetylcholinesterase inhibitors, and acetylcholine releasing agents. One of the muscarinic agonists tested in AD is arecoline and its bioisosters, which are widely explored as muscarinic receptor 1 agonist (M1 receptor agonist) in AD research. In this regard, five-membered heterocyclic ring system attached arecoline basic nucleus (N-methyl tetrahydropyridines) at third position has been extensively researched on. The present research involved synthesis of arecoline thiazolidinones 5(a-j) by using dipolar addition of 3-aminopyridine and alkyl/aryl carboxaldehydes in presence of gamma ferrite as catalyst. The resulting products were methylated and reduced to get desired products. Subsequently the synthesized arecoline thiazolidinones were subjected to in vitro muscarinic receptor binding studies using male Wistar rat brain (cerebral cortex) membrane homogenate and extended this in vitro study to in vivo pharmacological evaluation of memory and learning in male Wistar rats. Four derivatives (5a-5c and 5e) showed considerable M1 receptor binding affinity (in vitro) and elicited beneficial effects in vivo memory and learning models (Rodent memory evaluation, plus and Y maze studies).

Evidence of linkage and association on 18p11.2 for psychosis.

The genetic basis of bipolar disorder (BPD) and schizophrenia (SCZ) has been established through numerous clinical and molecular studies. Although often considered separate nosological entities, evidence now suggests that the two syndromes may share some genetic liability. Recent studies have used a composite phenotype (psychosis) that includes BPD, SCZ, psychosis not otherwise specified, and schizoaffective disorder, to identify shared susceptibility loci. Several chromosomal regions are reported to be shared between these syndromes (18p, 6q, 10p, 13q, 22q). As a part of our endeavor to scan these regions, we report a positive linkage and association finding at 18p11.2 for psychosis. Two-point linkage analysis performed on a series of 52 multiplex pedigrees with 23 polymorphic markers yielded a LOD score of 2.02 at D18S37. An independent set of 159 parent offspring trios was used to confirm this suggestive finding. The TDT analysis yielded support for association between the marker D18S453 and the disease allele (chi2 = 4.829, P < 0.028). This region has been implicated by several studies on BPD [Sjoholt et al. (2004); Mol Psychiatry 9(6):621-629; Washizuka et al. (2004); Biol Psychiatry 56(7):483-489; Pickard et al. (2005); Psychiatr Genet 15(1):37-44], SCZ [Kikuchi et al. (2003); J Med Dent Sci 50(3):225-229; Babovic-Vuksanovic et al. (2004); Am J Med Genet 124(3):318-322] and also as a shared region between the two diseases [Ishiguro et al. (2001); J Neural Transm 108(7):849-854; Reyes et al. (2002); Mol Psychiatry 7(4):337-339; Craddock et al. (2005); J Med Genet 42(3):193-204]. Our findings provide an independent validation of the above reports, and suggest the presence of susceptibility loci for psychoses in this region.

Cortical alpha-adrenoceptor downregulation by tricyclic antidepressants in the rat brain.

The aim of the present study was to examine the effect of chronic tricyclic antidepressants (TCAs) treatment on the density of alpha-adrenoceptors in the rat brain. Density of alpha1- and alpha2-adrenoceptors was measured in cortex and hippocampus of rats treated with imipramine (IMI, 5mg/kg body weight), desipramine (DMI, 10mg/kg body weight), clomipramine (CMI, 10mg/kg body weight) and amitriptyline (AMI, 10mg/kg body weight), for 40 days, using [3H]prazosin and [3H]clonidine, respectively. The density of cortical alpha1-adrenoceptors was significantly decreased with IMI (46%), DMI (21%), CMI (50%) and AMI (67%) treatment, without altering the affinity of the receptor. The density of cortical alpha2-adrenoceptors was also significantly decreased with DMI (69%), CMI (81%) and AMI (80%) treatment, without affecting the affinity for [3H]clonidine. The density of hippocampal alpha1-adrenoceptors was significantly decreased only with AMI treatment (47%), without affecting the affinity for [3H]prazosin. However, no change in hippocampal alpha2-adrenoceptor density was observed with any of these TCAs. The results suggest that chronic antidepressant (AD) treatment downregulates the cortical, but not hippocampal, alpha1- and alpha2-adrenoceptors in rat brain. The region-specific downregulation of alpha1- and alpha2-adrenoceptors density, which occur after prolonged AD treatment, may underline the therapeutic mechanism of action.

Alterations in 5-HT(1A) receptors and adenylyl cyclase response by trazodone in regions of rat brain.

The in vivo effect of trazodone on the density of [(3)H]5-HT binding sites and 5-HT(1A) receptors and adenylyl cyclase (AC) response was studied in regions of rat brain. The chronic administration of trazodone (10 mg/Kg body wt, 40 days) resulted in a significant downregulation of [(3)H]5-HT binding sites and 5-HT(1A) receptors in cortex and hippocampus. Trazodone significantly (p < 0.0001) decreased the density of [(3)H]5-HT binding sites in cortex (42.6 +/- 3.6 fmol/mg protein, 65%) and hippocampus (12.6 +/- 1.6 fmol/mg protein, 87%) when compared to control values of 121.9 +/- 5.4 and 99.3 +/- 7.5 fmol/mg protein in these regions, respectively. Similarly there was a significant (p < 0.0001) decrease in the density of 5-HT(1A) receptors in both cortex (7.2 +/- 0.5 fmol/mg protein, 70%) and hippocampus (6.3 +/- 1.2 fmol/mg protein, 79%) when compared to control values of 24.2 +/- 2.1 and 30.6 +/- 3.7 fmol/mg protein, in these regions respectively. However, the affinity of [(3)H]5-HT to 5-HT binding sites (1.83 +/- 0.26 nM, p < 0.0001) and [(3)H]8-OH-DPAT to 5-HT(1A) receptors (0.60 +/- 0.06 nM, p < 0.05) was significantly decreased only in cortex when compared to the control K(d) values of 0.88 +/- 0.04 nM and 0.47 +/- 0.02 nM in these regions, respectively. The basal AC activity did not alter in treated rats, where as, the inhibition of forskolin-stimulated AC activity by 5-HT (10 microM) was significantly (p < 0.0001) decreased both in cortex (43%) and hippocampus (40%) when compared to control levels. In conclusion, chronic treatment with trazodone results in downregulation of 5-HT(1A) receptors in cortex and hippocampus along with concomitant increased AC response, suggesting the involvement of 5-HT(1A) receptor-mediated AC response in the mechanism of action of trazodone.

Lamotrigine induced selective changes in 5-HT(1A) receptor mediated response in rat brain.

A new anticonvulsant drug lamotrigine (LTG) has recently been reported to be effective in treating patients with bipolar affective disorder, depression and schizoaffective disorder, suggesting that it is a mood stabilizer. However, the mechanism of action underlying its efficacy in mood disorders is not understood. This study examined the in vivo effect of LTG on 5-HT(1A) receptor-mediated adenylyl cyclase (AC) response in regions of rat brain, as this pathway has been implicated in the therapeutic action of various classes of mood stabilizers. The density of 5-HT(1A) receptors was measured by radioligand binding assay using [(3)H]8-OH-DPAT (0.05-0.8nM) in frontal cortex and hippocampus of rats treated orally with LTG (5mg/kg) for 7 days. AC activity was assayed using [(3)H]ATP. The oral administration of LTG significantly decreased the density of cortical (50%, P<0.001) but not hippocampal 5-HT(1A) receptors, without significant change in the affinity of [(3)H]8-OH-DPAT to 5-HT(1A) receptor in these regions. There was no significant alteration in basal or forskolin-stimulated AC activity in either of regions. However, a significant decrease (P<0.01) in the inhibition of forskolin-stimulated AC activity by 8-OH-DPAT was observed only in cortical membranes of LTG treated rats when compared to control. These results suggest that one mode of action of LTG may be by the downregulation of cortical 5-HT(1A) receptor-mediated AC response.

Differential protection and recovery of 5-HT1A receptors from N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) inactivation in regions of rat brain.

The effect of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on 5-HT1A receptors was studied in Sprague Dawley rats. A single dose of EEDQ (4 mg/kg body wt., i.p.) significantly inactivated 5-HT1A receptors, as measured by [3H]8-hydroxy-2-[di-n-propylamino]-tetralin ([3H]8-OH-DPAT), in cortex (64%, p < 0.0001) and hippocampus (48%, p < 0.0001). A significant (p < 0.01) increase in the affinity of 5-HT1A receptors for radioligand was observed in both regions. A dose dependent protection of cortical 5-HT1A receptors from EEDQ inactivation with pre-treatment of different doses of 8-OH-DPAT (4-20 mg/kg) was observed, along with recovery of affinity of [3H]8-OH-DPAT for 5-HT1A receptors in both regions. Although, a dose of 4 mg/kg of 8-OH-DPAT failed to attenuate the effect of EEDQ on hippocampal 5-HT1A receptors, a significant protection of these receptors was observed with 10 and 20 mg/kg of 8-OH-DPAT. Displacement studies revealed that EEDQ has more affinity for cortical (Ki = 101.3 +/- 11.8 nM) than hippocampal (Ki = 133.5 +/- 25.8 nM) 5-HT1A receptors. A time dependent natural recovery of 5-HT1A receptors from inactivation by a single dose of EEDQ (4 mg/kg) was observed more in cortex compared to hippocampus over a period from 1 day to 14 days. The results of this study suggest that 8-OH-DPAT inhibited EEDQ inactivation of cortical and hippocampal 5-HT1A receptors in a concentration dependent manner. The synthesis and turnover of 5-HT1A receptors differ in cortex and hippocampus, as evident by earlier recovery in the cortex.

Cortical 5-HT(1A) receptor downregulation by antidepressants in rat brain.

Total 5-HT binding sites and 5-HT(1A) receptor density was measured in brain regions of rats treated with imipramine (5 mg/kg body wt), desipramine (10 mg/kg body wt) and clomipramine (10 mg/kg body wt), for 40 days, using [3H]5-HT and [3H]8-OH-DPAT, respectively. It was observed that chronic exposure to tricyclic antidepressants (TCAs) results in significant downregulation of total [3H]5-HT binding sites in cortex (42-76%) and hippocampus (35-67%). The 5-HT(1A) receptor density was, however, decreased significantly (32-60%) only in cortex with all the three drugs. Interestingly, in hippocampus imipramine treatment increased the 5-HT(1A) receptor density (14%). The affinity of [3H]8-OH-DPAT was increased only with imipramine treatment both in cortex and hippocampus. The affinity of [3H]5-HT to 5-HT binding sites in cortex was increased with imipramine treatment and decreased with desipramine and clomipramine treatment. 5-HT sensitive adenylyl cyclase (AC) activity was significantly increased in cortex with imipramine (72%) and clomipramine (17%) treatment, whereas in hippocampus only imipramine treatment significantly increased AC activity (50%). In conclusion, chronic treatment with TCAs results in downregulation of cortical 5-HT(1A) receptors along with concomitant increase in 5-HT stimulated AC activity suggesting the involvement of cortical 5-HT(1A) receptors in the mechanism of action of TCAs.

Modulation of 5-HT1A receptor mediated response by fluoxetine in rat brain.

Radioligand binding studies were done to investigate the effect of chronic administration of fluoxetine on 5-HT1 receptor mediated response to adenylate cyclase (AC) in rat brain. Our studies revealed a significant decrease in the densities of 5-HT1 and 5-HT1A receptor sites in cortex and hippocampus of rat brain after chronic administration of fluoxetine (10 mg/Kg body wt.). However there was no significant change in the affinity of [3H]5-HT and [3H]DPAT for 5-HT1 and 5-HT1A receptor sites, respectively. However, in striatum, along with a significant (75%) downregulation of 5-HT1 sites, the affinity of [3H]5-HT to these sites was increased, as revealed by decrease in Kd (0.50 +/- 0.08 nM). Displacement studies showed that fluoxetine has higher affinity for 5-HT1A receptors with a Ki value of 14.0 +/- 2.8 nM, than 5-HT1 sites. No significant change was observed in basal AC activity in any region after fluoxetine exposure. However, in cortex of experimental rats the 5-HT stimulated AC activity was significantly increased (16.03 +/- 0.97 pmoles/mg protein; p < 0.01), when compared to 5-HT stimulated AC activity (12.98 +/- 0.78 pmoles/mg protein) in control rats. The increase in 5-HT stimulated AC activity in cortex may be due to the significant downregulation of 5-HT1A sites in cortex after fluoxetine exposure as these sites are negatively coupled to AC. The observed significant decrease in 5-HT1 sites with concomitant increase in 5-HT stimulated AC activity, after fluoxetine treatment, suggests that fluoxetine, which has high affinity for these sites, acts by modulating the 5-HT1A receptor mediated response in brain.

Differential effect of lithium on 5-HT1 receptor-linked system in regions of rat brain.

The effect of chronic administration (0.4% for 30 days) of lithium carbonate (Li2CO3) on 5-HT1 receptor-linked second messenger system was studied in regions of rat brain. We observed that chronic treatment of Li2CO3, significantly decreased the density of [3H]5-HT binding sites in cortex (62%), hippocampus (64%) and striatum (65%), compared to the control levels. The affinity of [3H]5-HT to 5-HT1 binding sites was significantly decreased in all the regions. A significant decrease in the density of high affinity 5-HT1A receptor sites was observed in cortex (81%) and hippocampus (42%), without change in the affinity of [3H]8-OH-DPAT for 5-HT1A sites in these regions. 5-HT-stimulated, but not basal, adenylyl cyclase activity was significantly increased in all the regions after Li treatment. The present study concludes that the increase in the 5-HT-stimulated adenylyl cyclase activity might be attributed to the functional downregulation of 5-HT1 receptors, as these are negatively coupled to adenylyl cyclase, suggesting the involvement of 5-HT1 receptor mediated response in the therapeutic efficacy of lithium.

Inactivation of 5-HT1A and [3H]5-HT binding sites by N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ) in rat brain.

Inactivation of 5-HT1A and [3H]5-HT binding sites by N-Ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ) was studied in regions of rat brain. After exposure to EEDQ (4 mg/kg body wt.) for 7 days, it is observed that the density of 5-HT1 receptor sites was decreased by nearly 20% in both cortex and hippocampus. The decrease, however, in 5-HT1A sites was more significant (70%) in both the regions. The affinity of [3H]5-HT to 5-HT1 sites was decreased significantly in both cortex and hippocampus after exposure to EEDQ, without affecting the Kd of 5-HT1A sites. Displacement studies suggested that EEDQ has high affinity to 5-HT1 sites with a Ki of 42.9+/-2.4 nM. After exposure neither basal nor 5-HT stimulated adenylyl cyclase activity was changed in cortex. The results of this study suggest that EEDQ decreases the density of 5-HT1 and 5-HT1A receptor sites but does not cause functional downregulation of these sites in rat brain.

Effect of antidepressants on intracellular Ca++ mobilization in human frontal cortex.

BACKGROUND: It is well documented that central serotonin (5-HT)2 receptor dysfunction is involved in the biochemistry and pathophysiology of depression and might be corrected by antidepressant drug treatment. METHODS: The effect of imipramine (IMI) and fluoxetine (FLX) on 5-HT2A receptor-mediated intracellular calcium ([Ca++]i) mobilization in synaptosomes of human frontal cortex was studied. [Ca++]i was measured using Fura-2AM. RESULTS: It was observed that in response to 5-HT (10 mumol/L) there was a significant increase (254%; p < .001) in [Ca++]i compared to basal level (140.00 +/- 31.77 nmol/L/mg protein). Both IMI and FLX (10-1000 nmol/L) induce mobilization of basal [Ca++]i in a dose-dependent manner. IMI and FLX antagonize the 5-HT-stimulated [Ca++]i mobilization in a dose-dependent manner. IMI showed higher antagonizing effect at lower concentration (10 nmol/L); however, FLX showed maximum antagonizing effect at higher concentration (1000 nmol/L). CONCLUSIONS: It is observed that imipramine and fluoxetine have different effects on antagonizing the 5-HT response in frontal cortex. One mode of action of these antidepressants might be by decreasing the intracellular calcium.

Imipramine-induced changes in 5-HT2 receptor sites and inositoltrisphosphate levels in rat brain.

The effect of chronic administration of Imipramine on [3H]Spiperone binding to 5-HT2 sites and inositoltrisphosphate (IP3) levels in rat cerebral cortex was studied. Our data shows that treatment with imipramine (5 mg/kg body weight, intraperitoneally) for 30 days significantly down regulates 5-HT2 receptors sites (262 +/- 29 fmol/mg protein) in cerebral cortex (38%), compared to control rats (425 +/- 60 fmol/mg protein., P < 0.001). However there was no significant change in the affinity of [3H]-Spiperone binding (kd) to 5-HT2 sites in cerebral cortex after exposure to imipramine (Kd = 0.84 +/- 0.11 nM). It is also observed that imipramine treatment significantly reduces 5-HT stimulated [3H]IP3 formation in cerebral cortex (6,411 +/- 708 dpm/mg protein), compared to the saline treated rats (12,238 +/- 1,544 dpm/mg protein; P < 0.001), with concomitant decrease in Pdtlns-4-5-P2. This study suggests that the therapeutic action of imipramine in brain might be by reducing hypersensitivity of 5-HT2 receptors by down regulation, which leads to reduced levels of inositolphospholipids. This inturn reduces the levels of IP3. In conclusion, imipramine acts at presynaptic site by blocking the reuptake of serotonin and at post synaptic site it downregulates 5-HT2 sites with decreased IP3 levels after chronic exposure.