Andreas Kastellakis

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.

Biphasic effects of Δ9-tetrahydrocannabinol on brain stimulation reward and motor activity

Δ(9)-tetrahydrocannabinol (Δ(9)-THC), the main psychoactive ingredient of marijuana, has led to equivocal results when tested with the intracranial self-stimulation (ICSS) procedure or the open-field test for motor activity, two behavioural models for evaluating the reward-facilitating and locomotor stimulating effects of drugs of abuse, respectively. Therefore, in the present study, the effects of high and low doses of Δ(9)-THC were compared in the ICSS procedure and the open-field test. Moreover, the involvement of CB(1) receptors in tentative Δ(9)-THC-induced effects was investigated by pre-treating the animals with the CB(1) receptor antagonist SR141716A (rimonabant). The results obtained show that low doses of Δ(9)-THC induce opposite effects from high doses of Δ(9)-THC. Specifically, 0.1 mg/kg Δ(9)-THC decreased ICSS thresholds and produced hyperactivity, whereas 1 mg/kg increased ICSS thresholds and produced hypoactivity. Both effects were reversed by pre-treatment with SR141716A, indicating the involvement of CB(1) receptors on these actions. Altogether, our results indicate that Δ(9)-THC can produce acute activating effects in locomotion that coincide with its reward-facilitating effects in the ICSS paradigm. The present findings provide further support that Δ(9)-THC induces behaviours typical of abuse and substantiate the notion that marijuana resembles other drugs of abuse.

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.

The effects of ventral tegmental administration of GABAA, GABAB, NMDA and AMPA receptor agonists on ventral pallidum self-stimulation

The ventral pallidum (VP) is a basal forebrain structure that is interconnected with motor and limbic structures and may be considered as an interface between motivational and effector neural signals. Results from a considerable number of studies suggest that this structure is critically involved in reward-related behavior. The VP shares reciprocal connections with other reward-implicated regions, such as the ventral tegmental area (VTA). This anatomy predicts that drug-induced neuronal alterations in the VTA could profoundly alter the function of the VP. Here, using the curve-shift intracranial self-stimulation method, we studied the effects of muscimol (GABA(A) agonist), baclofen (GABA(B) agonist), NMDA and AMPA, microinjected bilaterally into the VTA on the rewarding efficacy of VP self-stimulation. Central injections of the highest dose of muscimol (0.128 microg) resulted in significant elevations in VP self-stimulation thresholds, indicating a reduction in the rewarding efficacy of the stimulation. Elevations in VP self-stimulation thresholds were also evident after intrategmental injections of higher doses of baclofen (0.12, 0.48 microg). By contrast, intrategmental activation of NMDA and AMPA receptors did not affect reward thresholds. These findings suggest that GABAergic and glutamatergic transmission in the VTA activate different circuits that may mediate different functions. Thus, the VTA--VP projection activated by GABA modulates VP stimulation reward, while the projection activated by glutamate may be involved in reward-unrelated effects, rather than in the processing of reward. The decreased rewarding efficacy of VP self-stimulation following intrategmental injections of muscimol and baclofen may be due to GABAergic modulation of ventral tegmental dopaminergic and nondopaminergic neurons projecting to the VP.

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.

Involvement of the ventral tegmental area opiate receptors in self-stimulation elicited from the ventral pallidum

Abstract Enkephalinergic and dopaminergic mechanisms have been implicated in the electrical self-stimulation (SS) behavior. The present set of experiments investigated the role of opioid receptors within DA-innervated brain regions (nucleus accumbens and ventral tegmental area) in the ventral pallidum self-stimulation (VP-SS). Forty-one rats used in this study were implanted with a monopolar moveable stimulating electrode in the VP. A rate-frequency curve-shift method was applied to determine the reward (threshold) and motor functions (asymptotic rate) of self stimulation elicited from the VP. One group received systemic treatment of graded doses (vehicle; 1.25; 2.50 mg/kg) of morphine injected IP, 60 min before behavioural testing. The results showed a tendency for increased threshold of VP-SS and of the asymptotic rate of responding. Three additional groups were implanted with guide cannulae in the nucleus accumbens (NAC), the ventral tegmental area (VTA) or dorsally to the VTA and received microinjections of morphine (vehicle; 1.25; 2.50; 5.0; 10.0 μg/0.5 μl per side). Central injections of morphine higher than 1.25 μg/side into the VTA were associated with a significant reduction in VP-SS thresholds, indicating a potentiative effect on reward. Microinjections of morphine either into the NAC or into the dorsal tegmentum did not produce significant alterations on thresholds or responding of VP-SS. In order to investigate the extent to which the VTA-NAC dopamine projection was involved in the SS behavior elicited from the ventral pallidum, we tested SS in animals that suffered NAC 6-hydroxydopamine (6-OHDA) lesions. Rats suffering NAC dopamine depletion along with their corresponding controls showed similar levels of thresholds and responding to the ones exhibited prior to the lesion, revealing that NAC dopamine is not necessary to maintain VP-SS. The results suggest that stimulation of opioid receptor in the VTA increases the rewarding efficacy of VP-SS. This effect might be due to the modulation of VTA-DA neurons projecting to the VP rather than to the NAC.

Multiple sclerosis, cannabinoids, and cognition.

There is increasing interest in the therapeutic potential of cannabis-based medicinal extracts in multiple sclerosis. Cognitive deficits that have been attributed to long-term heavy recreational use of cannabis are not necessarily extended to controlled pharmaceutical use of cannabis-based medicinal extracts. Available data indicate that after relatively short-term administration of cannabis-based medicinal extracts no significant cognitive decline occurs. Due to the absence of large scale long-term systematic clinical trials of cannabis-based medicinal extracts in multiple sclerosis therapeutics, however, many issues remain unresolved, including the possible adverse effects of cannabis-based medicinal extracts on cognition. This article critically reviews the current literature and considers the potential for cognitive adverse effects of long-term cannabinoid use in multiple sclerosis therapeutics.

Hippocampal sst1 receptors are autoreceptors and do not affect seizures in rats

Somatostatin-14 (SRIF-14) exerts anticonvulsive effects in several rat seizure models, generally attributed to sst2 receptor activation. Whereas sst1 immunoreactivity has been localized to both polymorphic interneurons and principal cells in the rat hippocampus, its potential role as an inhibitory autoreceptor or as a receptor involved in mediating anticonvulsive actions remains unknown. We showed that intrahippocampal administration of the sst1 antagonist SRA880 (1 μM) induced a robust increase in hippocampal SST-14 levels without affecting gamma-aminobutyric acid levels in conscious rats, indicating that the sst1 receptor acts as an inhibitory autoreceptor. SRA880 did not affect seizure severity and did not reverse the anticonvulsive action of SRIF-14 (1 μM) against pilocarpine-induced seizures, suggesting that hippocampal sst1 receptors are not involved in the anticonvulsive effects of SRIF-14.

Effects of lithium and aripiprazole on brain stimulation reward and neuroplasticity markers in the limbic forebrain

Bipolar disorder (BD) is a severe pathological condition with impaired reward-related processing. The present study was designed to assess the effects of two commonly used BD medications, the mood stabilizer lithium chloride (LiCl) and the atypical antipsychotic and antimanic agent aripiprazole, in an animal model of reward and motivation and on markers of neuroplasticity in the limbic forebrain in rats. We utilized intracranial self-simulation (ICSS) to assess the effects of acute and chronic administration of LiCl and aripiprazole on brain stimulation reward, and phosphorylation studies to determine their effects on specific cellular neuroplasticity markers, i.e., the phosphorylation of CREB and crucial phosphorylation sites on the GluA1 subunit of AMPA receptors and the NA1 and NA2B subunits of NMDA receptors, in the limbic forebrain. Chronic LiCl induced tolerance to the anhedonic effect of the drug observed after acute administration, while chronic aripiprazole induced a sustained anhedonic effect. These distinct behavioral responses might be related to differences in molecular markers of neuroplasticity. Accordingly, we demonstrated that chronic LiCl, but not aripiprazole, decreased phosphorylation of CREB at the Ser133 site and NA1 at the Ser896 site in the prefrontal cortex and GluA1 at the Ser831 site and NA2B at the Ser1303 site in the ventral striatum. The present study provides evidence for BD medication-evoked changes in reward and motivation processes and in specific markers of neuronal plasticity in the limbic forebrain, promoting the notion that these drugs may blunt dysregulated reward processes in BD by counteracting neuronal plasticity deficits.