Poster
# 17
Poster |
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6th Internet World Congress for Biomedical Sciences |
Karen K. Szumlinski(1)
(1)Albany Medical College - Albany. United States
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[Neuroscience] |
[Pharmacology] |
[Psychiatry] |
18-Methoxycoronaridine (18-MC) fig. 1 is a synthetic iboga alkaloid congener, derived from the putative anti-addictive drug, ibogaine (IBO), developed with the goal of making available a safer IBO-like anti-addictive agent. Similar to IBO (1), 18-MC decreases the self-administration of a variety of drugs [incl., morphine (MOR), cocaine (COC), nicotine and alcohol] and blocks some of the acute signs of MOR withdrawal in rats (2,3). In contrast to IBO (1), 18-MC is non-tremorigenic and does not produce cerebellar toxicity, induce bradycardia or attenuate responding for water (2,3). The binding profile for 18-MC is slightly different from that of IBO; IBO binds to mu and kappa opioid receptors, the NMDA receptor, the 5-HT3 receptor and the serotonin transporter (1,4), where 18-MC shows moderate binding affinity for all three known opioid receptors (mu, delta and kappa) and the 5-HT3 receptor (2,3).
The repeated, intermittent administration of stimulant and opioid drugs produces a progressive increase in the behavioural, and often the neurochemical, effects of these drugs (5,6). Termed sensitization (6), this phenomenon is theorized to lie at the core of drug craving and relapse in drug addiction (7). Thus, the possibility exists that IBO and related iboga agents, such as 18-MC, interrupt the psychological and physiological aspects of addiction by modulating the expression of sensitization. Studies to date demonstrated that pretreatment (19 h earlier) with IBO increases the dopamine (DA) response in the nucleus accumbens (NAC) to an acute injection of COC (1) and decreases the DA response to an acute injection of MOR (1). This latter effect is greater in MOR-experienced rats (8). Consistent with these neurochemical findings, previous IBO studies demonstrated that IBO (19 h earlier) increases the potency of COC to induce locomotor responding (1,9-10) and enhances the expression of COC-induced stereotypy (11) in both acute and chronic COC-treated rats. IBO also decreases the locomotor responses to an acute injection of MOR (1,12) and this effect is greater in MOR-experienced rats (12). Studies of 18-MC to date indicate that 18-MC shares some of these DA and locomotor effects of IBO; 18-MC (19 h earlier) decreases the increase in NAC DA induced by acute MOR (2) and increases the locomotion induced by an acute injection of COC (2,3). Given the similar efficacies of IBO and 18-MC on drug self-administration (3), a series of studies determined the effects of 18-MC on the sensitization of DA treansmission in the NAC and on the expression of sensitization of behaviours putatively mediated by DA sensitization (i.e., locomotion and stereotypy) following chronic treatment with MOR, COC or SAL.
LOCOMOTOR ACTIVITY
MOR STUDIES: The effects of pretreatment with 18-MC on MOR-induced locomotor activation were determined in two separate studies, one for acute MOR, the other for chronic MOR. In the acute MOR study, female Sprague-Dawley rats were pretreated with either 18-MC (40 mg/kg) or vehicle (VEH). Nineteen hours later, rats were randomly addigned to received one of seven test doses of MOR (0, 0.05, 0.125, 5, 10, 20 or 30 mg/kg, IP; n=8-14). In the chronic MOR study, rats were administered five daily injections of 20 mg/kg MOR. Following 3 days of withdrawal, animals were pretreated with either 18-MC (40 mg/kg, IP) or VEH and then, 19 h later, were injected with one of four test doses of MOR (0, 5, 10, and 30 mg/kg, IP; n=5-7). Upon completion of the four test doses of MOR (0, 5, 10, and 30 mg/kg, IP; n=5-7). Upon completion of the experiment, rats were designated "sensitized" or "non-sensitization" by an experiment blind to the pretreatment of the animals, based on whether or not the latency to onset of locomotor activation advanced from injection 1 to injection 5 of chronic treatment. In both studies, rats were immediately placed into automated activity monitors after MOR injection, where their locomotor activaty was recroded for 3 h.
COC STUDY: The effects of pretreatment with 18-MC on COC-locomotor activation were determined. Animals were treated in a similar manner as those in the chronic MOR experiment with the following exceptions: 1) for chronic treatment, rats were administered five daily injections of either 15 mg/kg COC or SAL, followed by 2 weeks withdrawal; 2) the COC test doses employed were 0, 10, 20 and 40 mg/kg, IP; and 3) locomotor activity was monitored for 2 h and all COC injection sessions were preceded by a 30 min SAL habituation session.
STEREOTYPY
The effects of pretreatment (40 mg/kg, IP, 19 h earlier) with 18-MC on COC-induced stereotypy were determined. Identical procedures were followed for chronic COC treatment and 18-Mc pretreatment as descrived for the COC locomotor study above. Stereotypy was monitored for 2 or 3 h (chronic treatment and test days, respectively) in clear, cylindrical cages and rated every 20 min by an experimenter blind to the pretreatment of the animals, according to procedures described in Kalivas et al. (7).
IN VIVO MICRODIALYSIS
MOR STUDY: Female Sprague-Dawley rats were stereotaxically implanted with guide cannulae aimed bilaterally at the NAC (AP, + 1.6 mm and L, +/- 1.3 mm with respect to bregmal; V, -4.6 mm, at an angle of 14 degrees). Four days later, rats were placed in a cylindrical chamber with free access to food and water and a dialysis probe (CMA/Microdialysis probes: 2mm) was inserted unilaterally. Artificial CSF containing 146 mM NaCl, 2.7 mM KCl, 1.2 CaCl2 and 1.0 mM MgCl2 was delivered at a flow rate of 1 ĩl/min. Collection of perfusates began the next day and consisted of a 2-h baseline session, MOR injection (20 mg/kg, IP) and then a 3-h test session. Upon completion of injection 1, the dialysis probe was removed. Rats then received 4 addiction, once daily, MOR injections (20 mg/kg) in their home cages which were placed in the experimental room for 3 h. Following 2 days of withdrawal, animals were randomly addigned to receive either 18-MC (40 mg/kg, IP) or VEH, such that pretreatment occurred exactly 19 h prior to MOR administration the next day. Cannulae were inserted unilaterally on the opposite side of the head than that used for injection 1. The next day, perfusate collection and MOR injection occurred as described for injection 1 above. Analysis of the smaples are described below.
COC STUDY: Female rats were implanted with guide cannulae as described above. For chronic treatment, rats received five, once daily, injections of 15 mg/kg COC (as described for the study of COC locomotion) in their home cages, which were placed in the experimental room for 3 h. Chronic COC treatment began the day following surgery. Following 2 weeks withdrawal, 2 probes were inserted bilaterally into the NAC, and testing occurred as described above with the exception that 20 mg/kg COC was administered as the test dose. For both MOR and COC studies, upon completion of an experiment, the locations of the probe tips were verified according to the altlas of Paxinos and Watson (1986). Dialysate samples were assayed for DA, DOPAC and HVA by HPLC with electrochemical detection (ESA Coulochem detector). Chromatograms were integrated, compared to standards and analyzed using Hewlett Packard ChemStation software.
LOCOMOTOR ACTIVITY
MOR STUDIES: In acute MOR-treated rats, 18-MC (40 mg/kg, 19 h earlier) shifted the dose-response function for locomotion to the left of VEH-pretreated rats - Pretreatment by Dose interaction: F( 6,155 ) = 2.61, P<0.03 - fig. 2. Chronic MOR administration (5 x 20 mg/kg) induced locomotor sensitization in approximately 50% of the rats - Group by Injection Number interaction: F( 4, 368 ) = 5.89, P<0.0005; Group by Injection Number by Time interaction: F( 68,6256 ) = 2.46, P<0.00001 - fig. 3. On test day, this sensitization was apparent in VEH-pretreated rats - Group by Dose interaction: F( 3,39 ) = 3.27, P<0.04 - but not in 18-MC-pretreated rats (40 mg/kg, IP, 19 h earlier) - Group by Dose interaction: P=0.95 - fig. 4.
COC STUDY: Compared to injection 1, sensitization of locomotion was expressed by COC treated animals during chronic treatment - Chronic Treatment by Injection Number interaction: F( 4,368 ) = 4.39, P<0.002 - fig. 5. On test day, compared to chronic SAL rats, a sensitization of locomotion was also observed as indicated by a significant shift to the left in the dose-response function for COC-induced locomotion in chronic COC versus chronic SAL treated rats - Chronic Treatment by Dose interaction: F( 3,38 ) = 2.78, P=0.05 -. In terms of total locomotor responding to COC, 18-MC (40 mg/kg, 19 h earlier) produced a marginal shift to the left in the dose-response curves for COC-induced locomotion - Pretreatment by Dose interaction: F( 3,78 ) = 2.55, P=0.06 - fig. 6. However, 18-MC significantly altered the timecourse of COC-induced locomotion overall - Pretreatment by Dose by Time interaction: F( 33,858 ) = 1.65, P<0.02 - fig.7. This effect depended on the previous COC history of the animal - Chronic Treatment by Pretreatment by Dose by Time interaction: F( 11,858 ) = 1.75, P=0.05 -; 18-MC increased the early locomotor-activating effects of COC in rats with previous COC experience - Pretreatment by Dose by Time interaction: F( 44,1056 ) = 1.35, P=0.06 -, without altering the timecourse of acute COC-induced locomotion - Pretreatment by Dose by Time interaction: F( 44,1056 ) = 1.28, P=0.11 -.
STEREOTYPY
A slight sensitization of stereotypy was observed across injections during chronic COC administration in chronic COC rats - Chronic Treatment by Injection Number interaction: F( 4,184 ) = 8.92, P<0.0001 - fig. 8. In VEH-pretreated rats, repeated COC administration induced a slight sensitization of stereotypy in response to the test injections of COC - Chronic Treatment by Time interaction: F( 9,180 ) = 2.14, P<0.03 - fig. 10. Pretreatment with 18-MC (40 mg/kg, 19 h earlier) potentiated the total amount of stereotypy injected by COC, compared to VEH controls - Pretreatment effect: F( 1,40 ) = 57.00, P<0.0001 - fig. 9. Compared to VEH controls, 18-MC significantly increased the amount of stereotypy expressed in response to 40 mg/kg COC - Pretreatment by Dose interaction: F( 1,40 ) = 5.53, P<0.03 - and marginally prolonged the duration of COCīs stereotypic activating effects - Pretreatment by Time interaction: F( 9,180 ) = 1.86, P=0.06 - fig. 10.
IN VIVO MICRODIALYSIS
MOR STUDY: The average basal concentration of DA or either of its metabolites, DOPAC and HVA, did not differ as a consequence of either chronic MOR treatment or 18-MC pretreatment (data not shown). Compared to injection 1, chronic MOR rats displayed a sensitization of DA release in the NAC on test day. 18-MC (40 mg/kg, 19 h earlier) abolished the DA response to MOR - Group by Time interaction: F( 28,294 ) = 3.4, P<0.0001 - and blocked the MOR-sensitized response of DOPAC - Group by Time interaction: F( 28,294 ) = 1.61, P<0.03 - and HVA - Group by Time interaction: F( 28,294 ) = 1.79, P<0.02 - fig. 11.
MOR STUDIES: Pretreament with 18-MC (40 mg/kg, 19 h earlier) shifted the dose-effect curve for acute MOR-induced locomotion to the left of the VEH-pretreated rats, indicating that 18-MC produces an increase in the potency of MOR to elicit locomotor activation in acute MOR-treated rats. This finding is inconsistent with previous reports for the effects of 18-MC on acute MOR-induced alterations in the DA response to acute MOR (13). Thus, it appears that a dissociation exists between the DA and locomotor effects of 18-MC in acute MOR-treated rats, indicating that the locomotor-enhancing effect of 18-MC is mediated by actions of this agent upstream of DA terminals in the NAC. The finding that 18-MC increases the potency of acute MOR to induce locomotor activation is inconsistent also with previous reports for IBO; IBO decreases the efficacy of acute MOR to induce locomotion (1,12). Thus, it appears that 18-MC and IBO interact differentially with the neural substrate(s) mediating MORīs acute locomotor effects. As this behavioural difference does not appear to be related to differences in their DA effects [both compounds decrease MOR-induced DA transmission (3)], other mechanisms may be involved, possibly related to serotonin transmission (3) or their different receptor binding profiles (3). Pretreatment with 18-MC (40 mg/kg, 19 h earlier) blocked the expression of MOR-induced locomotor sensitization. This finding is consistent with previous locomotor studies for IBO in MOR-experienced rats (12) and implies that both IBO and 18-MC interact similarly with the neural substrate(s) mediating MOR-induced locomotion in MOR-experienced rats. Based on the present results and previous microdialysis studies for IBO in MOR-exprienced rats (8), it is proposed that a likely neural substrate mediating the locomotor sensitization effects of iboga agents is the reversal of DA sensitization in the NAC. 18-MC reduces the reinforcing efficacy of MOR as evidenced by a shift downward in the dose-effect curve for MOR self-administration (13). From the present microdialysis findings, it is proposed that the effects of iboga agents on MOR self-administration might be related to an ability to block the expression of psychomotor sensitization produced by self-administration.
COC STUDIES: 18-MC (40 mg/kg, 19 h earlier) increased the potency of COC to induce locomotor activation and enhanced the expression of COC-induced stereotypy in rats chronically administered either SAL or COC. These findings are consistent with previous reports for IBOīs interaction with the motoric effects of COC (9-11) and indicate that the effects of iboga agents on COC-induced motor behaviour are dissociated from their effects on both COC-induced changes in DA transmission in the NAC (present study) and COC self-administration (1-4). In contrast to the observed interaction between 18-MC and COC with respect to behaviour, 18-MC did not significantly alter the acute DA response in the NAC to acute COC, a finding consistent with previous reports from this laboratory (3). However, consistent with the present microdialysis findings for MOR, 18-MC pretreatment abolished the DA respone in the NAC to COC in COC-sensitized rats. These findings indicate that 18-MC can not only block the DAergic effects of drugs of abuse in rats chronically treated with such drugs, but also can reverse the neuroadapations produced by chronic drug experience. Given the putative role for mesolimbic DA transmission in the mediation of the appetitive, rewarding/reinforcing or incentive motivational effects of stimulant and opiate drugs (7), the ability of iboga agents to consistently block the DAergic consequences of repeated drug treatment raises the possibility that iboga agents attenuate drug self-administration by resetting the neuroadaptations in the mesolimbic DA system produced by repeated drug administration.
Pretreatment with the potential anti-addictive drug, 18-MC, increases the potency of both MOR and COC to induce behaviour hyperactivity in rats acutely treated with these addictive substances. These effects are not related to alterations in accumbal DA transmission. In chronic MOR and COC treated rats, 18-MC exerts opposite effects on the potency of these two drugs to induce sensitized levels of motor responding; 18-MC blocks and potentiates, respectively, the expression of MOR and COC locomotor sensitization. Whereas the interaction between MOR and 18-MC in the sensitized rats may involve a blockade of DA sensitization in the NAC, the interactions between 18-MC and COC appear to be non-DAergic. Given that 18-MC effectively blocks the self-administratoin of both MOR and COC, the neural mechanisms through which 18-MC exerts its anti-addictive effects likely involves the ability to reset or reverse the neuroadaptations in the mesolimbic DA system produced by chronic drug self-administration, implicating a role for DA sensitization in the mediation of drug addiction.
This study was supported by NIDA grant DA 03817.
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[Neuroscience] |
[Pharmacology] |
[Psychiatry] |
