Chronic underactivity of medial frontal cortical beta2-containing nicotinic receptors increases clozapine-induced working memory impairment in female rats

Heterogeneity across brain regions and neurotransmitter interactions with nicotinic effects on memory function

Nicotinic acetylcholine receptors have been shown in many studies to be critically involved in memory function. The precise roles these receptors play depend on the receptor subtype, their anatomic localization, their interactions with other parts of the neural systems underlying cognition and the particular domain of cognitive function. Nicotinic agonists can significantly improve learning, memory, and attention. Nicotinic receptors in the hippocampus are innervated by cholinergic projections from the medial septum and diagonal band. Local infusions of either α7 or α4β2 nicotinic antagonists into either the dorsal or ventral hippocampus produce amnestic effects in rats navigating about a radial arm maze. There is cholinergic innervation of nicotinic receptors in other components of the limbic system as well. In the basolateral amygdala and the anterior thalamus, similar amnestic effects of nicotinic α7 and α4β2 antagonists are seen. Interestingly, there are no additive amnestic effects observed in these limbic areas when α7 and α4β2 receptor antagonists are combined. The particular expression patterns of α7 and α4β2 nicotinic receptors in these limbic and cortical areas may explain this nonadditivity, but further research is needed to determine the specific cause of this phenomenon. Nicotinic receptor mechanisms in the limbic system play an important role in cognitive impairment for a variety of neurological disorders, including Alzheimer's disease and schizophrenia. Alzheimer's disease results in a dramatic decrease in hippocampal nicotinic receptor density, affecting α4β2 receptor expression most prominently. In schizophrenia, there are anomalies in α7 nicotinic receptor expression, which seem to be crucial for the cognitive impairment of the disorder. Chronic nicotine exposure, such as seen with tobacco use, results in an increase in nicotinic receptor density in the limbic system. This effect appears to be related to the desensitization of nicotinic receptors seen after agonist application. Open questions remain concerning the role of desensitization versus activation of nicotinic receptors in cognitive improvement.

Complex relationships of nicotinic receptor actions and cognitive functions

Nicotine has been shown in a variety of studies to improve cognitive function including learning, memory and attention. Nicotine both stimulates and desensitizes nicotinic receptors, thus acting both as an agonist and a net antagonist. The relative roles of these two actions for nicotine-induced cognitive improvement have not yet been fully determined. We and others have found that acute nicotinic antagonist treatment can improve learning and attention. Nicotine acts on a variety of nicotinic receptor subtypes. The relative role and interactions of neuronal nicotinic receptor subtypes for cognition also needs to be better characterized. Nicotine acts on nicotinic receptors in a wide variety of brain areas. The role of some of these areas such as the hippocampus has been relatively well studied but other areas like the thalamus, which has the densest nicotinic receptor concentration are still only partially characterized. In a series of studies we characterized nicotinic receptor actions, anatomic localization and circuit interactions, which are critical to nicotine effects on the cognitive functions of learning, memory and attention. The relative role of increases and decreases in nicotinic receptor activation by nicotine were determined in regionally specific studies of the hippocampus, the amygdala, the frontal cortex and the mediodorsal thalamic nucleus with local infusions of antagonists of nicotinic receptor subtypes (α7 and α4β2). The understanding of the functional neural bases of cognitive function is fundamental to the more effective development of nicotinic drugs for treating cognitive dysfunction.

Role of α7- and α4β2-nAChRs in the neuroprotective effect of nicotine in stress-induced impairment of hippocampus-dependent memory

We have previously shown that nicotine prevents stress-induced memory impairment. In this study, we have investigated the role of α7- and α4β2-nicotinic acetylcholine receptors (nAChRs) in the protective effect of nicotine during chronic stress conditions. Chronic psychosocial stress was induced using a form of rat intruder model. During stress, specific antagonist for either α7-nAChRs [methyllycaconitine (MLA)] or α4β2-nAChRs [dihydro-β-erythroidine (DHβE)] was infused into the hippocampus using a 4-wk osmotic pump at a rate of 82 μg/side.d and 41 μg/side.d, respectively. Three weeks after the start of infusion, all rats were subjected to a series of cognitive tests in the radial arm water maze (RAWM) for six consecutive days or until the animal reached days to criterion (DTC) in the fourth acquisition trial and in all memory tests. DTC is defined as the number of days the animal takes to make no more than one error in three consecutive days. In the short-term memory test, MLA-infused stressed/nicotine-treated rats made similar errors to those of stress and significantly more errors compared to those of stress/nicotine, nicotine or control groups. This finding was supported by the DTC values for the short memory tests. Thus, MLA treatment blocked the neuroprotective effect of nicotine during chronic stress. In contrast, DHβE infusion did not affect the RAWM performance of stress/nicotine animals. These results strongly suggest the involvement of α7-nAChRs, but not α4β2-nAChRs, in the neuroprotective effect of chronic nicotine treatment during chronic stress conditions.

PPI deficit induced by amphetamine is attenuated by the histamine H1 antagonist pyrilamine, but is exacerbated by the serotonin 5-HT2 antagonist ketanserin

RATIONALE

Prepulse inhibition (PPI) of the startle response is a classic model of sensorimotor gating. Robust PPI impairments can be induced by dopamine agonists such as the indirect agonist amphetamine. The antipsychotic clozapine can attenuate PPI impairment induced by dopamine agonists. Clozapine is a complex drug with antagonistic effects on a variety of receptors, including serotonin and histamine. The relative contribution of its component actions to its efficacy is still unclear.

OBJECTIVES

To better characterize the role of histamine and serotonin receptors in the modulation of PPI in rats, we studied the effects of the H(1) histamine antagonist pyrilamine (10, 20, and 40 mg/kg) on amphetamine-induced (1 mg/kg) PPI deficits (Experiment 1); and the interaction of pyrilamine (20 mg/kg) with the 5-HT(2) antagonist ketanserin (1 and 2 mg/kg) on the amphetamine-induced PPI disruption (Experiment 2).

METHODS

Tactile startle stimuli consisted of 30 PSI air-puffs. Three acoustic prepulse intensity levels were used: 68, 71, and 77 dB, presented on a 65-dB background noise. In both experiments, all animals received all drug doses and combinations with different counterbalanced orders.

RESULTS

Pyrilamine (20 mg/kg) was effective in counteracting the PPI impairment caused by amphetamine administration, whereas ketanserin exacerbated the amphetamine-induced PPI deficit.

CONCLUSIONS

Based on its ability to reverse amphetamine-induced PPI deficits, blockade of histamine H(1) receptors seems to contribute to the therapeutic effect of the antipsychotic clozapine. Serotonin 5-HT(2)-receptor blockade, though, does not appear to contribute to this effect, and may in fact detract from it.

Atypical antipsychotics as noncompetitive inhibitors of alpha4beta2 and alpha7 neuronal nicotinic receptors

It has been suggested that the interaction of antipsychotic medications with neuronal nicotinic receptors may increase the cognitive dysfunction associated with schizophrenia and may explain why current therapies only partially address this core feature of the illness. In the present studies we compared the effects of the atypical antipsychotics quetiapine, clozapine and N-desmethylclozapine to those of the typical antipsychotics haloperidol and chlorpromazine on the alpha4beta2 and alpha7 nicotinic receptor subtypes. The binding of [(3)H]-nicotine to rat cortical alpha4beta2 receptors and [(3)H]-methyllycaconitine to rat hippocampal alpha7 receptors was not affected by any of the compounds tested. However, Rb(+) efflux evoked either by nicotine or the selective alpha4beta2 agonist TC-1827 from alpha4beta2 receptors expressed in SH-EP1 cells and nicotine-evoked [(3)H]-dopamine release from rat striatal synaptosomes were non-competitively inhibited by all of the antipsychotics. Similarly, alpha-bungarotoxin-sensitive epibatidine-evoked [(3)H]-norepinephrine release from rat hippocampal slices and acetylcholine-activated currents of alpha7 nicotinic receptors expressed in oocytes were inhibited by haloperidol, chlorpromazine, clozapine and N-desmethylclozapine. The inhibitory effects on nicotinic receptor function produced by the antipsychotics tested occurred at concentrations similar to plasma levels achieved in schizophrenia patients, suggesting that they may lead to clinically relevant effects on cognition.

Nicotinic antagonist effects in the mediodorsal thalamic nucleus: regional heterogeneity of nicotinic receptor involvement in cognitive function

Nicotine has been found in many studies to improve cognitive function. However, some studies have not found this effect and others have seen nicotine-induced impairments. Systemic administration bathes the brain with drugs. However, the brain is quite intricately organized with various regions playing very different roles in the bases of cognitive function. We have examined the role of nicotinic receptors in a variety of brain areas for memory. In the hippocampus and amygdala, local infusions of both alpha7 and alpha4beta2 antagonists methyllyaconitine (MLA) and dihydro-beta-erythroidine (DHbetaE) significantly impair memory. In the current studies we locally infused acute and chronic doses of MLA and DHbetaE into the mediodorsal thalamic nucleus and tested memory function on a 16-arm radial maze. The rats also received systemic nicotine to determine the impact of more generalized nicotine effects. Since nicotinic treatments are being developed for cognitive impairment of schizophrenia, interactions were studied with the antipsychotic drug clozapine. In the acute study, the 6.75 microg/side of DHbetaE improved working memory. Co-administration of MLA reversed the DHbetaE-induced improvement. Chronic DHbetaE infusions into the mediodorsal thalamic nucleus also improved working memory. Systemic nicotine reversed this effect. Clozapine had no significant interaction. Nicotinic alpha4beta2 receptors in the mediodorsal thalamic nucleus appear to play an opposite role with regard to working memory than those in the hippocampus and amygdala. Heterogeneity in response to nicotinic drugs given systemically may be due to anatomically distinct nicotinic systems in the brain and their unique roles in the neural bases of cognitive function.