François Hogenboom

Impulsive Choice and Impulsive Action Predict Vulnerability to Distinct Stages of Nicotine Seeking in Rats

BACKGROUND Although heavy smoking has been associated with impulsivity in humans, it is not clear whether poor impulse control represents a risk factor in the etiology of nicotine dependence. METHODS To address this issue, rats were selected on the basis of individual differences in impulsivity in the delayed reward task (impulsive choice) and the 5-choice serial reaction time task (impulsive action). Subsequently, rats were subjected to a nicotine self-administration (SA) paradigm tailored to measure the motivational properties of nicotine and nicotine-associated stimuli. In separate groups, differences in electrically evoked dopamine release in slice preparations obtained from several mesolimbic brain regions were determined. RESULTS Impulsive action was associated with an enhanced motivation to initiate and maintain nicotine SA. In contrast, impulsive choice predicted a diminished ability to inhibit nicotine seeking during abstinence and an enhanced vulnerability to relapse upon re-exposure to nicotine cues. Impulsive action was associated with reduced dopamine release in the accumbens core and impulsive choice with reduced dopamine release in accumbens core, shell, and medial prefrontal cortex. CONCLUSIONS The strong association between sub-dimensions of impulsivity and nicotine SA implies that interventions aimed to improve impulse control might help to reduce susceptibility to nicotine dependence and/or lead to successful smoking cessation.

μ-, δ- and κ-opioid receptor-mediated inhibition of neurotransmitter release and adenylate cyclase activity in rat brain slices: studies with fentanyl isothiocyanate

Abstract We investigated the effects of [D-Ala 2 , D-Leu 5 ]enkephalin (DADLE). [D-Ala 2 , MePhe 4 ,Gly-ol 5 ]enkephalin (DAGO), [D-Pen 2 ,D-Pen 5 ]enkephalin (DPDPE) (0.01–1 μM) and bremazocine (0.001–03 μM) on the electrically evoked release of radiolabelled neurotransmitters and on the dopamine (DA)-stimulated cyclic AMP efflux from superfused rat brain slices. The differential inhibitory effects of these agonists on the evoked neurotransmitter release indicate that the opioid receptors mediating presynaptic inhibition of [ 3 H]noradrenaline (NA, cortex), [ 14 C]acethylcholine (ACh, striatum) and [ 3 H]DA (striatum) release represent μ, δ and κ receptors, respectively. In agreement with this classification, preincubation (60 min) of the slices with the δ-opioid receptor-selective irreversibel ligand, fentanyl isothiocyanate (FIT, 0.01–1 μM), antagonized the inhibitory effects of DADLE and DPDPE on striatal [ 14 C]ACh release only. On the other hand, the D-1 DA receptor-stimulated cyclic AMP efflux from striatal slices appeared to be inhibited by activation of μ as well as of δ receptors. In this case, the reversible μ antagonist, naloxone (0.1 μM), fully antagonized the inhibitory effect of the μ agonist, DAGO, without changing the effect of the δ agonist DPDPE but was ineffective as an antagonist in slices pretreated with FIT (1 μM). The inhibitory effect of DAGO on the electrically evoked [ 3 H]NA release was antagonized by naloxone whether the receptors were irreversibly blocked by FIT or not. These data not only further support the existence of independent presynaptic μ-, δ- and κ-opioid receptors in rat brain but also evidence strongly that μ and δ receptors mediating the inhibition of DA-sensitive adenylate cyclase could share a common binding site (for naloxone and FIT) and, therefore, may represent constituents of a functional opioid receptor complex.

Long-lasting cognitive deficits resulting from adolescent nicotine exposure in rats.

Adolescence is a developmental period, during which the brain and particularly medial prefrontal cortical (mPFC) regions thereof have not fully matured. Because epidemiological data have suggested that adolescent nicotine use may result in disturbances in cognitive function in adulthood, we investigated the long-term effects of adolescent nicotine exposure in rats. Male Wistar rats were exposed to either nicotine (three times daily, 0.4 mg/kg s.c.) or saline for 10 days during (postnatal day (PND) 34–43) or following (PND 60–69) adolescence. After 5 weeks during adulthood, separate groups of animals were tested in operant paradigms taxing attention and distinct measures of impulsivity. Visuospatial attention and impulsive action were tested in the five-choice serial reaction time task, whereas impulsive choice was assessed in the delayed reward task. Our data show that adolescent, but not postadolescent, nicotine exposure affects cognitive performance in adulthood and results in diminished attentional performance and increments in impulsive action, while leaving impulsive choice intact. This altered cognitive performance appeared to be associated with enhanced releasability of dopamine in the mPFC. Together, these data suggest that adolescence is a time window during which the brain is vulnerable to long-lasting cognitive disturbances resulting from nicotine exposure.

Ontogeny of μ-, δ- and κ-opioid receptors mediating inhibition of neurotransmitter release and adenylate cyclase activity in rat brain

The ontogeny was examined of functional opioid receptors mediating presynaptic inhibition of neurotransmitter release and inhibition of dopamine (DA)-sensitive adenylate cyclase in the rat brain, using highly selective agonists for μ-, δ- and κ-receptors. On gestational day 17 (E17) strong inhibitory effects of the selective μ-agonist DAGO on the electrically evoked release of [3H]noradrenaline from cortical slices and of the selective κ-agonist U-50,488 on the electrically evoked release of [3H]DA from striatal slices were found. Electrically evoked release of [3H]acetylcholine from striatal slices was not detectable before postnatal day 7 (P7), but on that day it was already strongly inhibited by the selective δ-agonist DPDPE. Although μ- and δ-opioid receptors coupled to DA-sensitive adenylate cyclase in the striatum are likely to be physically associated in an opioid receptor complex in the adult, they were found to develop asynchronously. Whereas selective activation of μ-receptors with DAGO resulted in an inhibition of D1 dopamine receptor-stimulated adenylate cyclase activity on E17, activation of δ-receptors with DPDPE was not effective until P14. This study confirms the early appearance of μ- and κ-opioid receptors and the relatively late development of δ-opioid receptors in the rat brain. Most importantly, it shows that in an early stage of development opioids are already able to mediate modulation of noradrenergic (via activation of μ-receptors) and dopaminergic (via activation of μ- and κ-receptors) neurotransmission processes. Therefore, these opioid receptor types could play a role in brain development and/or developmental disturbances.

Presynaptic noradrenergic α-receptros and modulation of 3H-noradrenaline release from rat brain synaptosomes

The depolarization (15 mM K+)-induced release of 3H-NA from superfused rat brain synaptosomes and the effects of alpha-noradrenergic drugs thereon were studied. Noradrenaline (NA; in the presence of the uptake inhibitor desipramine) reduced synaptosomal 3H-NA release. Reduction of the concentration of calcium ions in the medium during K+ stimulation greatly enhanced the sensitivity of 3H-NA release to alpha-receptor-mediated inhibition. Under these conditions NA dose-dependently inhibited 3H-NA release from synaptosomes obtained from cortex or hypothalamus, but did not affect 3H-NA release from striatal (i.e dopaminergic) synaptosomes. Adrenaline, clonidine and oxymetazoline potently inhibited 3H-NA release from cortex synaptosomes at concentrations in the nanomolar range. Phentolamine by itself did not affect synaptosomal 3H-NA release, but antagonized the inhibitory effects of both noradrenaline and adrenaline. The data obtained further substantiate the hypothesis that the alpha-receptors mediating a local negative feedback control of NA release are localized on the varicosities of central noradrenergic neurons, Furthermore, noradrenergic nerve terminals in the hypothalamus appear to be less senstive to alpha-receptor-mediated presynaptic inhibition than those in the cortex.

Alpha-receptor-mediated modulation of 3H-noradrenaline release from rat brain cortex synaptosomes

SummaryThe effects of oxymetazoline and noradrenaline (in the presence of desipramine) on the release of 3H-noradrenaline from rat brain cortex synaptosomes were studied using a superfusion technique. Both drugs (at 1μM concentrations) were found to reduce the depolarization-induced (15 mM K+) release of 3H-noradrenaline. The release-modulating effect of noradrenaline was antagonized by phentolamine and yohimbine.The data provide direct evidence for the hypothesis that α-receptors modulating the release of noradrenaline are localized on varicosities of central noradrenergic neurones.

Inhibition of dopamine-sensitive adenylate cyclase by opioids: possible involvement of physically associated mu- and delta-opioid receptors.

SummaryD-1 dopamine receptor-stimulated cyclic AMP efflux from rat neostriatal slices (induced by 30 μM dopamine + 10 μM (−)sulpiride) was concentration-dependently reduced by morphine, [abetd-Ala-abetd-Leu]-enkaphalin (DADLE), [d-Pen-abetd-Pen]enkephalin (DPDPE) and bremazocine. Naloxone (0.1 μM) selectively antagonized the inhibitory effect of (a submaximally effective concentration of) morphine, whereas ICI 174864 (0.75 μM) completely blocked the inhibitory effects of DADLE, DPDPE and bremazocine without affecting that of morphine, indicating a role of µ- as well as δ-opioid receptors. Upon simultaneous activation of D-1 dopamine receptors and b-opioid receptors the (μ-receptor-mediated) inhibitory effect of morphine was abolished, while it was not changed following simultaneous activation of D-1 and (inhibitory) D-2 dopamine receptors. Cyclic AMP efflux induced by isoprenaline or adenosine was not affected by the opioids and that induced by vasoactive intestinal peptide (VIP) was inhibited by morphine and DADLE only. In the latter case naloxone, but not ICI 174864, antagonized the inhibitory effects.These data show that D-1 dopamine receptor-stimulated adenylate cyclase activity in rat neostriatum, but not that stimulated through other receptors, is inhibited by two pharmacologically distinct opioid receptor subtypes. It is speculated that these μ- and δ-opioid receptors share a common inhibitory guanine nucleotide binding protein and may represent closely associated recognition sites of a functional opioid receptor complex.

α2-Adrenoceptor mediated inhibition of the release of radiolabelled 5-hydroxytryptamine and noradrenaline from slices of the dorsal region of the rat brain

SummaryPossible local interactions between noradrenergic and serotonergic systems in the dorsal raphe region of the rat were investigated by studying the effects of various drugs on depolarization (20 mmol/l K+)-induced release of [3H]5-hydroxytryptamine (5-HT) and [3H]noradrenaline (NA) in vitro using a superfusion method. Exogenous 5-HT did not influence the release of [3H]NA. However, NA (in the presence of 10 μmol/l desipramine) as well as the selective α2-adrenoceptor agonists clonidine and oxymetazoline strongly inhibited [3H]5-HT release. The selective α1-adrenoceptor agonists phenylephrine and methoxamine did not affect the release of either [3H]5-HT or [5H]NA. The inhibition by NA of both [3H]5-HT and [5H]NA release was not affected by the β-adrenoceptor antagonist sotalol nor by the selective α1-adrenoceptor antagonist prazosin. However, phentolamine and the selective α2-adrenoceptor antagonists yohimbine and rauwolscine competitively antagonized the inhibitory effect of NA on [3H]NA release (respective pA2-values 7.5 and 8.3) and on [3H]5-HT release (respective pA2-values 7.7 and 8.2). Moreover, the release of [3H]NA and also, but to a lesser extent, that of [3H]5-HT were increased by the antagonists. It is concluded that the release of both 5-HT and NA in the dorsal raphe region may be subject to presynaptic inhibition by NA via activation of α2-adrenoceptors.

Effect of chronic prenatal morphine treatment on μ-opioid receptor-regulated adenylate cyclase activity and neurotransmitter release in rat brain slices

Timed-pregnant rats received a semisynthetic diet with or without morphine (0.5-1 mg/g) for 2 weeks. After 21 days of gestation the morphine-dependent dams were decapitated and the foetal brains were dissected. Chronic morphine administration caused a profound increase of adenylate cyclase activity stimulated by postsynaptic D1 dopamine receptors in striatal slices. The relative inhibitory effect of [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) on D1-stimulated cyclic AMP (cAMP) production was unaffected. In contrast, cAMP production induced via direct activation of the catalytic unit of adenylate cyclase with forskolin was not changed upon long-term morphine treatment, although DAGO strongly inhibited the effect of forskolin. The electrically evoked release of [3H]noradrenaline (NA) from superfused neocortical slices was strongly enhanced upon morphine treatment, whereas release induced by the calcium ionophore A23187, bypassing voltage-sensitive calcium channels, was unchanged. Again, the inhibitory effect of the mu receptor agonist DAGO was unaffected in neocortical slices from morphine-treated rats. It is suggested that tolerance to morphine may be caused by the fact that the opiate is acting against up-regulated signal transduction mechanisms, rather than by desensitization of central mu-opioid receptors. The pre- and postsynaptic changes may include an enhanced expression and/or biochemical modification of D1 receptors, Gs proteins and calcium channels in central neurons on which mu-opioid receptors are present. At the same time, these adaptive changes may underlie morphine withdrawal phenomena.

Opioid receptor-mediated inhibition of 3H-dopamine and 14C-acetylcholine release from rat nucleus accumbens slices

SummaryThe release of 14C-ACh from rat nucleus accumbens slices, induced by 15 mM [K+], was inhibited by the µ- and δ-opioid agonists DAMGO and DPDPE, respectively, whereas only the κ agonist U50,488 reduced the release of 3H-DA.The opioid receptors involved appear to be localized on nerve terminals, since blockade of action potential propagation by 1 μM TTX did not diminish the inhibitory effects of DAMGO, DPDPE or U50,488.Enhancement of the potassium concentration in the superfusion medium to 56 mM with simultaneous reduction of the Ca2+ concentration from 1.2 mM to 0.12 mM induced a release similar to that caused by 15 mM K+ and 1.2 mM Ca+. Under this conditions, the inhibitory effects of both DAMGO and DPDPE on stimulated 14C-ACh release were reduced, whereas the inhibition of evoked 3H-DA release caused by U50,488 was not affected. Activation of µ- as well as δ-opioid receptors by DAMGO and DPDPE, respectively, inhibited forskolin-stimulated adenylate cyclase activity. However, increasing the intracellular cAMP levels with 0.3 mM 8-bromo-CAMP affected neither the depolarization-induced release of 14C-ACh or 3H-DA from accumbens slices nor the inhibitory effects of opioid receptor activation thereon.The results indicate that the mechanism by which functional µ- and δ receptors presynaptically inhibit the depolarization-induced 14C-ACh release from nucleus accumbens slices is likely to involve an increase of potassium channel conductance. In contrast, activation of κ-opioid receptors, which inhibits depolarization-evoked 3H-DA release, apparently does not result in a hyperpolarization of (dopaminergic) nerve terminals. In none of these inhibitory effects presynaptic adenylate cyclase appears to be involved.