Heshmat Rajabi

Alpha-2 Adrenergic Receptor Agonists Block Stress-Induced Reinstatement of Cocaine Seeking

The alpha-2 adrenergic receptor agonists, clonidine, lofexidine and guanabenz, blocked stress- but not cocaine-induced reinstatement of cocaine seeking at doses that suppressed footshock-induced release of noradrenaline in prefrontal cortex and amygdala. Rats were trained to self-administer cocaine (0.5 mg/kg/infusion, i.v; 10–12 days) and, after a drug-free period (7–13 days), were returned to the self-administration chambers for daily extinction and reinstatement test sessions. Both intermittent footshock (15 min, 0.6 mA) and cocaine priming (20 mg/kg, i.p.) reinstated extinguished drug seeking. Pretreatment with either clonidine (20, or 40 μg/kg, i.p.) or lofexidine (50, 100, 150, or 200 μg/kg, i.p.) attenuated footshock- but not cocaine-induced reinstatement of cocaine seeking. Guanabenz (640 μg/kg, i.p.), an alpha-2 agonist with low affinity for imidazoline type-1 receptors, also attenuated footshock- but not cocaine-induced reinstatement of cocaine seeking. The results point to an important role for NE systems in the effects of footshock on relapse to cocaine seeking.

“Breakthrough” Dopamine Supersensitivity during Ongoing Antipsychotic Treatment Leads to Treatment Failure over Time

Antipsychotics often lose efficacy in patients despite chronic continuous treatment. Why this occurs is not known. It is known, however, that withdrawal from chronic antipsychotic treatment induces behavioral dopaminergic supersensitivity in animals. How this emerging supersensitivity might interact with ongoing treatment has never been assessed. Therefore, we asked whether dopamine supersensitivity could overcome the behavioral and neurochemical effects of antipsychotics while they are still in use. Using two models of antipsychotic-like effects in rats, we show that during ongoing treatment with clinically relevant doses, haloperidol and olanzapine progressively lose their efficacy in suppressing amphetamine-induced locomotion and conditioned avoidance responding. Treatment failure occurred despite high levels of dopamine D2 receptor occupancy by the antipsychotic and was at least temporarily reversible by an additional increase in antipsychotic dose. To explore potential mechanisms, we studied presynaptic and postsynaptic elements of the dopamine system and observed that antipsychotic failure was accompanied by opposing changes across the synapse: tolerance to the ability of haloperidol to increase basal dopamine and dopamine turnover on one side, and 20–40% increases in D2 receptor number and 100–160% increases in the proportion of D2 receptors in the high-affinity state for dopamine (D2High) on the other. Thus, the loss of antipsychotic efficacy is linked to an increase in D2 receptor number and sensitivity. These results are the first to demonstrate that “breakthrough” supersensitivity during ongoing antipsychotic treatment undermines treatment efficacy. These findings provide a model and a mechanism for antipsychotic treatment failure and suggest new directions for the development of more effective antipsychotics.

Prolonged rewarding stimulation of the rat medial forebrain bundle: Neurochemical and behavioral consequences.

Extracellular dopamine levels were measured in the rat nucleus accumbens by means of in vivo microdialysis. Delivery of rewarding medial forebrain bundle stimulation at a low rate (5 trains/min) produced a sustained elevation of dopamine levels, regardless of whether train onset was predictable. When the rate of train delivery was increased to 40 trains/min, dopamine levels rose rapidly during the first 40 min but then declined toward the baseline range. The rewarding impact of the stimulation was reduced following prior delivery of stimulation at the high, but not the low, rate. These results support the idea that dopamine tone plays an enabling role in brain stimulation reward and is elevated similarly by predictable and unpredictable stimulation.

MK-801 increases locomotor activity without elevating extracellular dopamine levels in the nucleus accumbens.

In vivo microdialysis was used in freely moving rats to determine whether the locomotor stimulant effects of dizocilpine maleate (MK‐801) were related to increased dopamine (DA) release within the nucleus accumbens (N. Acc.). Each experiment began with a baseline period of at least 2 h (starting 15–20 h after insertion of concentric, removable dialysis probes), during which activity records and dialysate samples were collected every 20 min. Rats in the first experiment then were injected with MK‐801 (0.125, 0.25, or 0.50 mg/kg, i.p.) or saline, and activity and extracellular levels of DA, dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were measured for a further 160 min post‐injection. In a second experiment, rats were given 1.5 mg/kg d‐amphetamine sulphate 40 min after receiving the same doses of MK‐801, and testing was continued for 120 min. Rats in a third experiment were given low, autoreceptor‐preferring doses of apomorphine hydrochloride (25 or 50 μg/kg, s.c.) or its vehicle 40 min after injection of 0.25 mg/kg MK‐801 and then monitored for 120 min. MK‐801 produced strong and consistent increases in locomotor activity that were augmented by amphetamine and greatly reduced by the low doses of apomorphine. MK‐801 did not increase extracellular DA levels within the N. Acc. when given alone, and it failed to influence the changes in extracellular DA produced by d‐amphetamine and apomorphine. MK‐801 did produce consistent, dose‐related increases in DOPAC and HVA that were probably not related to transmitter release. These results indicate that the increases in locomotor activity seen following MK‐801 do not arise from a drug‐induced increase in DA levels within the N. Acc.

Rats Maintained Chronically on Buprenorphine Show Reduced Heroin and Cocaine Seeking in Tests of Extinction and Drug-Induced Reinstatement

Buprenorphine is being introduced as a maintenance therapy in opioid addiction, but it is not clear how buprenorphine will affect co-use of cocaine in opioid users. We examined the effects of chronic buprenorphine (BUP0: 0.0 mg/kg/day; BUP1.5: 1.5 mg/kg/day; BUP3: 3.0 mg/kg/day) on the locomotor activity effects of acute heroin (0.25 mg/kg, subcutaneously (s.c.)) and cocaine (20 mg/kg, intraperitoneally (i.p.)). Buprenorphine had no effect on the stimulatory effect of heroin, but potentiated the locomotor response to cocaine. To investigate further the interactions between buprenorphine (BUP1.5 and BUP3), heroin (0.125, 0.25 and 0.375 mg/kg, s.c.), and cocaine (10, 20 and 30 mg/kg, i.p.), we used in vivo microdialysis and high-performance liquid chromatography to analyze extracellular levels of dopamine (DA) in the nucleus accumbens (NAc). Buprenorphine attenuated the heroin-induced rise in NAc DA, but greatly potentiated the cocaine-induced rise. Finally, we examined the potential of the highest dose of buprenorphine (BUP3) to reduce heroin and cocaine seeking in the presence of drug-associated cues under extinction conditions and in tests for reinstatement induced by heroin (0.25 mg/kg, s.c.), cocaine (20 mg/kg, i.p.), and 15-min footshock stress (0.8 mA, 0.5 s/shock, 40 s mean OFF time) in rats trained to self-administer both drugs. Buprenorphine reduced heroin and cocaine seeking during extinction and following acute heroin and cocaine priming injections, but had no effect on stress-induced reinstatement. These results indicate that the suppression of responding following priming injections of drugs did not result from reduced motor activity, but possibly from a reduction in the salience of drug-associated cues induced by chronic buprenorphine treatment.

Netrin receptor deficient mice exhibit functional reorganization of dopaminergic systems and do not sensitize to amphetamine.

Netrins are guidance cues that play a fundamental role in organizing the developing brain. The netrin receptor, DCC (deleted in colorectal cancer), is highly expressed by dopaminergic (DA) neurons. DCC may therefore participate in the organization of DA circuitry during development and also influence DA function in the adult. Here we show that adult dcc heterozygous mice exhibit a blunted behavioral response to the indirect DA agonist amphetamine and do not develop sensitization to its effects when treated repeatedly. These behavioral alterations are associated with profound changes in DA function. In the medial prefrontal cortex, dcc heterozygotes exhibit increased tyrosine hydroxylase (TH) protein levels and dramatic increases in basal concentrations of DA and DA metabolites. In contrast, in the nucleus accumbens, dcc heterozygotes show no changes in either TH or DA levels, but exhibit decreased concentrations of DA metabolites, suggesting reduced DA activity. In addition, dcc heterozygous mice exhibit a small, but significant reduction in total number of TH-positive neurons in midbrain DA cell body regions. These results demonstrate for the first time that alterations in dcc expression lead to selective changes in DA function and, in turn, to differences in DA-related behaviors in adulthood. These findings raise the possibility that changes in dcc function early in life are implicated in the development of DA dysregulation observed in certain psychiatric disorders, such as schizophrenia, or following chronic use of drugs of abuse.

Estradiol derived from testosterone in prenatal life affects the development of catecholamine systems in the frontal cortex in the male rat

We reported previously that exposure to testosterone (T), neonatally, slows the time-course of development of catecholaminergic activity in the anterior cortex of rat pups. In the present study we assessed the role of T in prenatal life on this development and explored the role of the estrogen metabolite of T, estradiol, in these actions. Male pups born to dams injected daily with 5 mg/kg, s.c., of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD), or vehicle only, between gestation days 10 and 21 were either gonadectomized (GX) or anesthetized, only, within 6 h of birth. Dopamine, DOPAC, and noradrenaline levels were assessed, using HPLC-EC, in punched samples taken from cingulate (CING), agranular insular (AID), parietal (PAR) and occipital (OC) cortex on postnatal (PN) days 4 and 10. At PN4 there were no effects of treatment on amine levels, although there were higher levels in frontal areas. At PN10, ATD and ATD GX animals had higher levels of dopamine, DOPAC, and noradrenaline in CING and AID than normal males. It would appear that T acts prenatally through its metabolite, estradiol, to modulate the development of catecholamine activity in the frontal cortex in the neonatal period.

Effects of cocaine in rats exposed to heroin.

We investigated whether chronic exposure to heroin alters responses to cocaine in ways that might explain the use of cocaine by opioid addicts. To this end, the effects of cocaine (5 and 20 mg/kg) were assessed on locomotor activity of rats chronically exposed to heroin (0.0, 3.5, 7.0, and 14.0 mg/kg/day, over 14 days, via osmotic mini-pumps), or withdrawn from heroin (1 day, acute withdrawal, and 14 days, protracted withdrawal). Chronic heroin exposure, in itself, dose dependently increased locomotion and acute cocaine administration further elevated locomotor activity in a dose-dependent and additive manner. During acute withdrawal, there was a dose-dependent decrease in locomotion that was reversed by cocaine in a dose-dependent manner. During protracted withdrawal, spontaneous locomotion normalized, but rats previously exposed to heroin displayed cross-sensitization to cocaine as indicated by small, but significant, enhanced locomotor response to 5 mg/kg of cocaine, and enhanced intravenous self-administration of low doses of cocaine (0.13 mg/kg/infusion). In a separate study, we measured extracellular dopamine (DA) in the nucleus accumbens (Acb) using in vivo microdialysis before and after acute withdrawal from heroin. During chronic exposure to heroin, basal extracellular DA was elevated dose dependently, whereas in acute withdrawal, levels were not different from those in vehicle-treated rats. In response to cocaine, however, DA activity in the Acb was significantly lower in rats withdrawn from the highest dose of heroin.

Netrin-1 receptor-deficient mice show enhanced mesocortical dopamine transmission and blunted behavioural responses to amphetamine.

The mesocorticolimbic dopamine (DA) system is implicated in neurodevelopmental psychiatric disorders including schizophrenia but it is unknown how disruptions in brain development modify this system and increase predisposition to cognitive and behavioural abnormalities in adulthood. Netrins are guidance cues involved in the proper organization of neuronal connectivity during development. We have hypothesized that variations in the function of DCC (deleted in colorectal cancer), a netrin‐1 receptor highly expressed by DA neurones, may result in altered development and organization of mesocorticolimbic DA circuitry, and influence DA function in the adult. To test this hypothesis, we assessed the effects of reduced DCC on several indicators of DA function. Using in‐vivo microdialysis, we showed that adult mice that develop with reduced DCC display increased basal DA levels in the medial prefrontal cortex and exaggerated DA release in response to the indirect DA agonist amphetamine. In contrast, these mice exhibit normal levels of DA in the nucleus accumbens but significantly blunted amphetamine‐induced DA release. Concomitantly, using conditioned place preference, locomotor activity and prepulse inhibition paradigms, we found that reduced DCC diminishes the rewarding and behavioural‐activating effects of amphetamine and protects against amphetamine‐induced deficits in sensorimotor gating. Furthermore, we found that adult DCC‐deficient mice exhibit altered dendritic spine density in layer V medial prefrontal cortex pyramidal neurones but not in nucleus accumbens medium spiny neurones. These findings demonstrate that reduced DCC during development results in a behavioural phenotype opposite to that observed in developmental models of schizophrenia and identify DCC as a critical factor in the development of DA function.

Neonatal Exposure to Gonadal Hormones Affects the Development of Monoamine Systems in Rat Cortex

The effects of neonatal gonadectomy of male, and testosterone propionate treatment of female rat pups on levels of monoamines and metabolites in the cerebral cortex were assessed using high‐performance liquid chromatography with electrochemical detection. In Experiment 1, pups were killed at 0, 4, 10 and 21 days of age and the anterior and posterior portions of cortex in each hemisphere were removed. At 21 days of age the levels of dopamine in anterior cortex were higher in males and testosterone propionate‐treated females than in females and gonadectomized males. However, dopaminergic activity developed earlier in females than in males and the gonadal hormone manipulations shifted the pattern of development to that of the other sex. In Experiment 2, the effects of these same gonadal hormone manipulations on the uptake, metabolism and storage capacity of catecholamine neurons in the cingulate, agranular insular, parietal and occipital cortex were estimated at 4 and 10 days of age by considering the difference between measured catecholamines in animals pretreated with vehicle or 2.5 mg/kg reserpine and then given 100 mg/kg L‐DOPA. Again, the data indicated earlier development of catecholamine neurons in females, especially in the agranular insular cortex. Dopamine was found to account for group differences; for when dopamine levels alone were considered it was found that, at 4 days of age, females had the highest levels in every area with the exception of the occipital cortex where gonadectomized males had equally high levels. These data suggest a mechanism that might account for sex differences in the development of specific cortical regions.