Christina Spyraki

Effects of methyllycaconitine (MLA), an α 7 nicotinic receptor antagonist, on nicotine- and cocaine-induced potentiation of brain stimulation reward

Abstract It has been shown that nicotine facilitates intracranial self-stimulation (ICSS) reward and that nicotinic acetylcholine receptors (nAChRs) in the ventral tegmental area (VTA) are of primary importance for its reinforcing and dependence-producing actions. Recently, we have shown that α7 nicotinic receptors in the VTA contribute to both the acute effects of nicotine on the mesolimbic dopamine system, as well as to nicotine withdrawal reactions. However, it is not yet known whether the same receptor conformation is directly involved in the reinforcing actions of nicotine. Here, using the curve-shift method we studied the effects of methyllycaconitine (MLA), a selective α7 receptor antagonist, microinjected (graded doses: 1, 3, 9 µg/µl per side) into the VTA on the rewarding efficacy of lateral hypothalamic self-stimulation and on the systemic nicotine-induced potentiation of brain stimulation reward. MLA did not affect baseline self-stimulation. Nicotine produced a significant reduction in ICSS threshold, without altering maximal rates of responding, while MLA attenuated the effect of nicotine at the two lower doses. Given the reported interaction between nicotine and cocaine at both the neuronal and the behavioral level, we also examined whether α7 receptor antagonism within the VTA can affect the reinforcing action of cocaine, as measured with ICSS. Interestingly, MLA attenuated the reinforcing effect of cocaine in all doses tested, without altering the maximal rate of responding, i.e. the performance of the animals. These results suggest that α7 nAChRs in the VTA are involved in mediating the reinforcing actions of drugs of abuse, such as nicotine and cocaine, and provide evidence that α7 nAChR antagonists may be clinically useful in attenuating the rewarding effects of addictive drugs.

VENTRAL PALLIDUM SELF-STIMULATION INDUCES STIMULUS DEPENDENT INCREASE IN C-FOS EXPRESSION IN REWARD-RELATED BRAIN REGIONS

Neuronal expression of Fos, the protein product of the immediate early gene c-fos has been used as a high resolution metabolic marker for mapping polysynaptic pathways in the brain. We used Fos immunohistochemistry to reveal neuronal activation following self-stimulation of the ventral pallidum. Four groups of rats were allowed to self-stimulate for 30 min with 0.4 s trains of cathodal rectangular pulses of constant intensity (0.4 mA) and duration (0.1 ms). Each group was assigned a different pulse frequency, (3, 17, 24 and 50 pulses/stimulation train). Which was preselected from within each animals rate-frequency function. The subjects that were assigned three pulses failed to self-stimulate and were considered as controls. The subjects that were assigned 17 pulses self-stimulated at half-maximal rate, whereas those that were assigned 24 and 50 pulses self-stimulated at maximal rates. The animals were sacrificed 90 min after the self-stimulation session and their brains were processed for Fos-like immunoreactivity. Fos-like immunoreactivity was found to increase as a function of pulse frequency in several brain regions known to be involved in drug and/or brain stimulation reward (medial prefrontal cortex, lateral septum, nucleus accumbens; lateral hypothalamus and ventral tegmental area), whereas it was not affected in structures devoid of such involvement (substantia nigra reticulata and dorsolateral striatum). The level of Fos expression induced by trains of 50 pulses was considerably higher than that produced by 24 pulses although both frequencies supported the same (maximal) self-stimulation rate. This finding indicates that Fos expression correlated with reward magnitude (known to increase between these frequencies), not with bar-pressing rate, thus suggesting the presence of a reward-specific effect. The finding of a frequency-dependent Fos expression in a behavioural paradigm can be considered analogous to a pharmacological dose-response curve and, as such, our results may open new avenues for the use of Fos immunohistochemistry in quantitative neurobehavioural studies.

Ventral pallidum self-stimulation: a moveable electrode mapping study.

The distribution of electrical self-stimulation (ESS) foci within the ventral pallidum (VP) was mapped using moveable electrodes in rats. The function relating ESS bar-pressing rate to the frequency of cathodal rectangular pulses (0.4 mA and 0.1 ms) was obtained for several positions of a moveable electrode in the VP and in the various adjacent to VP nuclei. The rate-frequency functions were fitted to a sigmoid model to obtain the asymptotic rate and threshold frequency. ESS was found in almost all (98%) VP sites tested and to a lesser degree (66%) in the surrounding areas (namely globus pallidus and caudate). Depending on the VP site, maximum rates varied from 14 to 85 bar presses/min, whereas threshold frequencies varied from 10.2 to 36.4 pulses/train; no correlation between these two aspects of ESS was found. Extra-pallidal areas contained less low-frequency threshold sites compared to VP. The lowest threshold found in the VP was slightly higher than that usually obtained for the most rewarding brain areas (VTA, dorsal raphé, LH, amygdala), which suggests that the VP represents an important structure for reward. Furthermore the threshold frequencies were found to decline along the rostrocaudal axis of the VP which supports the view that the VP is heterogeneous in regard to reward related functions.

AMPA‐receptor involvement in c‐fos expression in the medial prefrontal cortex and amygdala dissociates neural substrates of conditioned activity and conditioned reward

Exposure to an environment, previously conditioned to amphetamine (1 mg/kg, i.p.), induced locomotor activity and c‐fos expression (a marker for neuronal activation) in the mouse medial prefrontal cortex (mPFC) and amygdala; acute or repeated amphetamine (1 mg/kg, i.p.) administration induced c‐fos expression additionally in the nucleus accumbens. An α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole propionate (AMPA)‐receptor antagonist, 2,3‐dihydroxy‐6‐nitro‐7‐sulphamoyl‐benzo(f)quinoxaline (NBQX), blocked expression of conditioned activity, and prevented the increase in c‐fos expression in mPFC, implicating mPFC AMPAergic transmission in the conditioned component of behavioural sensitization to amphetamine. NBQX failed to block the expression of amphetamine‐conditioned place preference, a measure of conditioned reward, or conditioned c‐fos expression in the amygdala, an area implicated in the expression of conditioned place preference. These findings indicate that the conditioned components of behavioural sensitization depend on AMPA‐receptor‐mediated activation in mPFC, but that conditioned reward does not.

Neuropharmacological evidence for the role of dopamine in ventral pallidum self.stimulation

The present study examines the role of dopaminergic neurotransmission in modulating the reinforcing effect of ventral pallidum (VP) intracranial self-stimulation (ICSS). Fifty four adult rats were implanted with a monopolar moveable stimulating electrode in the VP. Rate-frequency functions were determined by logarithmically decreasing the number of pulses in a stimulation train from a value that sustained maximal responding to one that did not sustain responding. After the ICSS thresholds stabilized, the animals received treatments with several doses of cocaine and of various selective drugs acting at the level of DA receptor subtypes. Their effects on threshold and asymptotic rate were analyzed. Cocaine produced a significant decrease in ICSS threshold but had no significant effect on the asymptotic rate. A significant decrease in ICSS threshold was also seen with the D3 agonist 7-OH-DPAT. This was associated with a decrease rather than an increase in performance. D1 and D2DA receptor blockers (haloperidol, SCH-23390, raclopride and sulpiride) produced a dose dependent increase in ICSS threshold and a decrease in the maximal rate. The results suggest that DA plays a modulatory role in VP intracranial self-stimulation, and that D1, D2 and D3 receptors are involved in the mediation of this effect, although to different extents.

Chronic desipramine treatment selectively potentiates somatostatin-induced dopamine release in the nucleus accumbens.

Dopamine and somatostatin have been implicated in the pathophysiology of depression. We have employed in vivo microdialysis to investigate the regulation of dopamine release by somatostatin in the nucleus accumbens and the striatum of awake, freely moving rats, and to ascertain how this regulation may be affected by desipramine treatment. Somatostatin‐14 (10−4 m) infusion induced an increase in the release of dopamine and a decrease in the release of its metabolites in both the nucleus accumbens (568% of basal) and the striatum (546% of basal). Chronic desipramine treatment resulted in an exaggerated somatostatin‐induced increase of dopamine levels, specifically in the nucleus accumbens (3542% compared with 564% of basal in the striatum), whereas acute desipramine treatment had no effect (582% of basal) compared with saline treated rats. Basal concentrations of dopamine and metabolites were not influenced by either chronic or acute treatment of desipramine in either brain area. These results demonstrate that somatostatin regulates dopamine release in the nucleus accumbens and the striatum. Chronic antidepressant treatment influences somatostatins actions on dopamine function selectively in the nucleus accumbens.

Dopamine‐somatostatin interactions in the rat striatum: An in vivo microdialysis study

Dopamine‐somatostatin interactions were investigated in the rat striatum using in vivo microdialysis. Somatostatin‐14 and somatostatin‐28 (10−4, 10−5, 10−6 M) were infused, and the levels of dopamine and its metabolites DOPAC and HVA were assessed using high pressure liquid chromatography with electrochemical detection. Somatostatin‐14 was more effective than somatostatin‐28 in producing a dose‐dependent increase in dopamine levels with no significant alterations in the levels of the metabolites. To assess the effect of dopamine on somatostatinergic neurons, dopaminergic agents were administered and somatostatin levels measured using a radioimmunoassay. The nonselective agonist apomorphine was administered subcutaneously (0.00, 0.05, 0.10, 0.50, 1.00 mg/kg) or directly infused (10−4, 10−5 M) in the striatum. The selective D1 and D2 dopamine antagonists SCH23390 and sulpiride, respectively, were also infused at concentrations of 10−4 and 10−5 M. None of these agents elicited any significant changes in the somatostatin release in the striatum, while altering dopamine release. This study provides for the first time evidence regarding dopamine‐somatostatin interactions in the awake and freely moving animal. The results confirm that somatostatin modulates the function of dopaminergic neurons in the striatum and provide new evidence that somatostatin‐14 may differentially regulate dopamine release. Furthermore, our findings suggest that dopamine does not play a major role in the regulation of somatostatin neurons.

Antidepressants Influence Somatostatin Levels and Receptor Pharmacology in Brain

This study investigated how the administration (acute and chronic) of the antidepressants citalopram and desmethylimipramine (DMI) influences somatostatin (somatotropin release inhibitory factor, SRIF) levels and SRIF receptor density (sst1–5) in rat brain. Animals received either of the following treatments: (1) saline for 21 days (control group), (2) saline for 20 days and citalopram or DMI for 1 day (citalopram or DMI acute groups), (3) citalopram or DMI for 21 days (citalopram or DMI chronic groups). Somatostatin levels were determined by radioimmunoassay. [125I]LTT SRIF-28 binding in the absence (labeling of sst1–5) or presence of 3 nM MK678 (labeling of sst1/4) and [125I]Tyr3 octreotide (labeling of sst2/5) binding with subsequent autoradiography was performed in brains of rats treated with both antidepressants. Somatostatin levels were increased after citalopram, but not DMI administration, in the caudate-putamen, hippocampus, nucleus accumbens, and prefrontal cortex. Autoradiography studies illustrated a significant decrease in receptor density in the superficial and deep layers of frontal cortex (sst2), as well as a significant increase in the CA1 (sst1/4) hippocampal field in brains of chronically citalopram-treated animals. DMI administration increased sst1/4 receptors levels in the CA1 hippocampal region. These results suggest that citalopram and to a lesser extent DMI influence the function of the somatostatin system in brain regions involved in the emotional, motivational, and cognitive aspects of behavior.

Effect of morphine applied by intrapallidal microdialysis on the release of dopamine in the nucleus accumbens

The effect of morphine, administered intrapallidally, on extracellular concentrations of DA, DOPAC, and HVA in the nucleus accumbens and striatum was studied in the behaving rat using the in vivo microdialysis technique. Unilateral application of morphine hydrochloride was performed through microdialysis probes into the rat ventral pallidum (10 microliters of 0, 2.6, 4.0, 13.0, and 26.0 mM) or globus pallidus (10 microliters of 0 and 26.0 mM). The levels of DA, DOPAC, and HVA were measured using the HPLC with EC detection in dialysates collected from the nucleus accumbens, anteromedial, and anterolateral striatum. Samples were taken every 45 min over 3 h before and over 5 h after morphine or vehicle administration. Administration of morphine into the ventral pallidum resulted in increased DOPAC and HVA concentrations in the nucleus accumbens. Pretreatment with naloxone (1 mg/kg, SC) abolished this effect of morphine. Administration of morphine into the globus pallidus resulted in increased DA, DOPAC, and HVA concentrations in the nucleus accumbens and DA in the anteromedial striatum. The levels of DA and metabolites in anterolateral striatum remained rather unchanged following morphine administered into the ventral pallidum or the globus pallidus. The changes in DA neurotransmission into the nucleus accumbens induced by morphine application into the ventral pallidum and globus pallidus are reminiscent of a phasic and tonic release of DA respectively. The results show that intrapallidal morphine increases DA neurotransmission in nucleus accumbens and suggest that the effect of morphine is mediated by ventral pallidum/mesolimbic and globus pallidus/thalamocortical pathways, depending on the site of injection.

Pallidal substrate of morphine-induced locomotion

Bilateral microinjections of morphine hydrochloride (5.0; 7.5; 10.0 micrograms/0.5 microliters/side) or saline were infused into 3 different regions (dorsal, medial, ventral) of the rat globus pallidus, to examine their effects on locomotor activity. Locomotor activity of each rat was measured 45 min before and 90 min after saline or morphine pallidal microinjections. Morphine induced a dose-dependent increase in locomotion. This increase in locomotion was also significantly different between the 3 pallidal regions. Pretreatment with naloxone (1 mg/kg, sc) inhibited the morphine (7.5 micrograms) hyperlocomotion elicited from all three pallidal areas. The results suggest that the entire pallidum serves as substrate of morphine hyperlocomotion mediated by opiate receptors.