Interactions of lithium and drugs that affect signal transduction on behaviour in rats

Effects of mood stabilizers on the inhibition of adenylate cyclase via dopamine D(2)-like receptors

OBJECTIVE

The mood stabilizing drugs lithium, carbamazepine and valproate modulate brain adenosine monophosphate (cAMP) levels, which are assumed to be elevated in bipolar disorder patients. The aim of this work was to investigate how these three mood stabilizing agents affect the regulation of cAMP levels by dopamine D(2)-like receptors in vitro in rat cortical neurons in culture and in vivo in the rat prefrontal cortex.

METHODS

The production of cAMP was measured in the cultured cortical neurons or in microdialysis samples collected from the prefrontal cortex of freely moving rats using the [8-(3)H] and [(125)I] radioimmunoassay kits.

RESULTS

In vitro and in vivo data showed that the treatment with the mood stabilizing drugs had no effect on basal cAMP levels in vitro, but had differential effects in vivo. Direct stimulation of adenylate cyclase (AC) with forskolin increased cAMP levels both in vitro and in vivo, and this effect was significantly inhibited by all three mood stabilizers. Activation of dopamine D(2)-like receptors with quinpirole partially inhibited forskolin-induced increase in cAMP in untreated cultures, but no effect was observed in cortical neuron cultures treated with the mood stabilizing drugs. Similar results were obtained by chronic treatment with lithium and valproate in the prefrontal cortex in vivo. However, surprisingly, in carbamazepine-treated rats the activation of dopamine D(2)-like receptors enhanced the responsiveness of AC to subsequent activation by forskolin, possibly as a consequence of chronic inhibition of the activity of the enzyme.

CONCLUSIONS

It was shown that each of these drugs affects basal- and forskolin-evoked cAMP levels in a distinct way, resulting in differential responses to dopamine D(2)-like receptors activation.

Lithium-methomyl induced seizures in rats: a new model of status epilepticus?

Behavioral, electroencephalographic (EEG) and neuropathological effects of methomyl, a carbamate insecticide reversibly inhibiting acetylcholinesterase activity, were studied in naive or lithium chloride (24 h, 3 mEq/kg, s.c.) pretreated male Wistar rats. In naive animals, methomyl with equal potency produced motor limbic seizures and fatal status epilepticus. Thus, the CD50 values (50% convulsant dose) for these seizure endpoints were almost equal to the LD50 (50% lethal dose) of methomyl (13 mg/kg). Lithium pretreated rats were much more susceptible to convulsant, but not lethal effect of methomyl. CD50 values of methomyl for motor limbic seizures and status epilepticus were reduced by lithium pretreatment to 3.7 mg/kg (a 3.5-fold decrease) and 5.2 mg/kg (a 2.5-fold decrease), respectively. In contrast, lithium pretreatment resulted in only 1.3-fold decrease of LD50 value of methomyl (9.9 mg/kg). Moreover, lithium-methomyl treated animals developed a long-lasting status epilepticus, which was not associated with imminent lethality observed in methomyl-only treated rats. Scopolamine (10 mg/kg) or diazepam (10 mg/kg) protected all lithium-methomyl treated rats from convulsions and lethality. Cortical and hippocampal EEG recordings revealed typical epileptic discharges that were consistent with behavioral seizures observed in lithium-methomyl treated rats. In addition, convulsions induced by lithium-methomyl treatment were associated with widespread neurodegeneration of limbic structures. Our observations indicate that lithium pretreatment results in separation between convulsant and lethal effects of methomyl in rats. As such, seizures induced by lithium-methomyl administration may be an alternative to lithium-pilocarpine model of status epilepticus, which is associated with high lethality.

The effect of lithium chloride on morphine-induced tolerance and dependence in isolated guinea pig ileum

The chronic use of opioids is often accompanied by the development of tolerance and/or dependence upon these agents due to the adaptive changes in the response of the subject to the agent. On cellular level, these phases of altered responsiveness have been shown to be the sequelae of a combination of multiple independent components acting in concert. Changes in the number, affinity, or membrane trafficking of opioids receptors, the coupling of receptors to G-proteins or in associated second messenger systems have been implicated in underlying the aforementioned phenomena. Several observations have been shown that lithium is able to contradict the expected response in animals pre-treated with morphine. These facts clearly manifest the involvement of lithium in at least one of the diverse pathways that lead to morphine dependence and/or tolerance. The aim of the present study was to investigate the effect of lithium on acute morphine-induced tolerance and dependence in an in vitro model of isolated guinea pig ileum which has been extensively used for the assessment of these effects of opioids. Morphine inhibited electrically stimulated twitch of ileum in a concentration-dependent manner (pD(2)=7.27+/-0.16). Tolerance to this effect was induced by the incubation of ileum with 2xIC(50) of morphine for 2 h that induced a degree of tolerance of 14.7. The co-incubation of ileum with morphine along lithium chloride (1 mM) reduced the degree of tolerance significantly (P<0.001) and restored the sensitivity of ileum to the morphine inhibitory effect. Lithium chloride can also reduce the expression of tolerance to morphine significantly (P<0.01). Dependence was induced by incubation with 4xIC(50) of morphine for 2 h and was assessed based on naloxone-induced contractions (10(-5 )M). Lithium chloride (1 mM) can attenuate the development but not the expression of dependence to morphine as shown by the significant decrease in naloxone-induced contractions (P<0.05). These results suggest that lithium chloride can reduce the development and expression of acute tolerance to and development of dependence on morphine in the myenteric plexus of guinea pig ileum.

Binding characteristics of high-affinity dopamine D2/D3 receptor agonists, 11C-PPHT and 11C-ZYY-339 in rodents and imaging in non-human primates by PET

We have evaluated the in vitro autoradiographic binding characteristics and in vivo brain distribution of two high-affinity dopamine D2/D3 receptor agonists, (+/-)-2-(N-phenethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-PPHT) and (+/-)-2-(N-cyclohexylethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-ZYY-339) in rodents and in monkeys using positron emission tomography (PET). In vitro autoradiograms in rat brain slices with (11)C-PPHT and 11C-ZYY-339 revealed binding to dopaminergic regions in the striata, which was substantially (>90%) displaced by 10 microM sulpiride. Striatal binding was also removed in the presence of 5-guanylylimidophosphate (Gpp(NH)p), indicative of binding of these radiotracers to the high-affinity (HA) state. The results of in vivo studies in rats exhibited binding of the two radiotracers to the striata (striata/cerebellum approached 2 in 30 min). The regional selectivity to the striata was reduced by preadministration of haloperidol. PET studies in male rhesus monkeys using an ECAT EXACT HR+ scanner indicated localization of 11C-PPHT and 11C-ZYY-339 in the striata and thalamus. Striata to cerebellum and thalamus to cerebellum ratios were low (1.5 and 1.3, respectively, at 30 min postinjection) for both 11C-PPHT and 11C-ZYY-339, apparently due to the slower nonspecific clearance from cerebellum. These findings with 11C-PPHT and 11C-ZYY-339 indicate the possibility of in vivo imaging of high-affinity state of dopamine D2/D3 receptors in both the striata and the thalamus.

Increased extracellular serotonin level in rat hippocampus induced by chronic citalopram is augmented by subchronic lithium: neurochemical and behavioural studies in the rat

RATIONALE

A substantial number of patients do not respond sufficiently to antidepressant drugs and are therefore often co-medicated with lithium as an augmentation strategy. However, the neurochemical rationale behind this strategy needs to be further clarified.

OBJECTIVES

We examined the effect of chronic citalopram and subchronic lithium, alone or in combination, on (a) serum levels of citalopram and lithium, (b) animal behaviour and (c) hippocampal serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels. Furthermore, we examined the serum level of citalopram and hippocampal 5-HT following one acute citalopram injection.

METHODS

Microdialysis in the freely moving animals was used to determine hippocampal 5-HT and 5-HIAA. The animal behaviour was examined in the open field and forced swim test. RESULTS. We found that chronic administration of citalopram (20 mg/kg/24 h s.c.) significantly increased the 5-HT baseline relative to vehicle-treated rats. Addition of subchronic lithium (60 mmol/kg chow pellet p.o.) to chronic citalopram therapy further elevated the 5-HT levels. Moreover, we found acute citalopram (5 mg/kg s.c.) to increase the 5-HT level. The immobility time in the FST and the locomotion in the OF were unaffected by any treatments.

CONCLUSIONS

The present results support the assumption that increases in hippocampal 5-HT neurotransmission may be important in the augmentatory effect of lithium.