Effects of antipsychotic D2 antagonists on long-term potentiation in animals and implications for human studies

In people with schizophrenia, cognitive abilities - including memory - are strongly associated with functional outcome. Long-term potentiation (LTP) is a form of neuroplasticity that is believed to be the physiological basis for memory. It has been postulated that antipsychotic medication can impair long-term potentiation and cognition by altering dopaminergic transmission. Thus, a systematic review was performed in order to assess the relationship between antipsychotics and D2 antagonists on long-term potentiation. The majority of studies on LTP and antipsychotics have found that acute administration of antipsychotics was associated with impairments in LTP in wild-type animals. In contrast, chronic administration and acute antipsychotics in animal models of schizophrenia were not. Typical and atypical antipsychotics and other D2 antagonists behaved similarly, with the exception of clozapine and olanzapine. Clozapine caused potentiation independent of tetanization, while olanzapine facilitated tetanus-induced potentiation. These studies are limited in their ability to model the effects of antipsychotics in patients with schizophrenia as they were largely performed in wild-type animals as opposed to humans with schizophrenia, and assessed after acute rather than chronic treatment. Further studies using patients with schizophrenia receiving chronic antipsychotic treatment are needed to better understand the effects of these medications in this population.

Nicotine and clozapine cross-prime the locus coeruleus noradrenergic system to induce long-lasting potentiation in the rat hippocampus

A priming-challenge schedule of nicotine treatment causes long-lasting potentiation (LLP), a form of synaptic plasticity closely associated with the norepinephrine (NE) neurotransmitter system, at the medial perforant path (MPP)-dentate gyrus (DG) synapse in the rat hippocampus. Previous reports revealed that nicotine activates the locus coeruleus (LC) noradrenergic (NAergic) system and this mechanism may underlie its beta-adrenoceptor sensitive LLP effects. Clozapine, an atypical antipsychotic, is also known to activate the LC. Interactions between nicotine and clozapine are of interest because of the prevalence of smoking in patients with schizophrenia and increasing interest in the use of nicotinic receptor ligands as cognitive enhancers. Rats were subchronically primed with nicotine, clozapine or saline. Twenty-one to twenty-eight days later, the effects of the nicotine, clozapine or saline challenge on the evoked field excitatory postsynaptic potentials (fEPSP) at the MPP-DG monosynaptic pathway were recorded as a measure of LLP. We confirmed the hypothesis that a challenge dose of either nicotine or clozapine induces LLP exclusively in nicotine- and clozapine-primed rats, and not in saline-primed rats, thus indicating a cross-priming effect. Moreover, unilateral suppression of LC using lidocaine abolished the LLP induced by nicotine in clozapine-primed rats. Furthermore, systemic treatment with clonidine (an α2 adrenoceptor agonist that reduces NAergic activity via autoreceptors) prior to the challenge doses blocked the nicotine/clozapine-induced LLP in nicotine- and clozapine-primed rats. These findings may add to understanding of the cognitive enhancing effects of nicotine.