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Age‐related expression of σ1 receptors and antidepressant efficacy of a selective agonist in the senescence‐accelerated (SAM) mouse

The σ1 receptor is a unique intracellular receptor whose activation results in an efficient modulation of several neurotransmitter responses. Its role as a target for the rapid nongenomic effects of neuro(active)steroids and the age‐related diminutions in steroid levels suggested that targeting the σ1 receptor might allow alleviation of age‐related neuronal dysfunctions. We examined here the expression and behavioral efficacy of σ1 receptors in the senescence‐accelerated (SAM) mouse model. The σ1 receptor mRNA expression was measured by using comparative RT‐PCR in the olfactory bulb, hippocampus, hypothalamus, cortex, or cerebellum of senescence‐prone SAMP/8 and senescence‐resistant SAMR/1 control animals. No difference was observed between substrains in 6‐, 9‐, and 12‐month‐old (m.o.) mice. The σ1 protein expression was analyzed by using immunohistochemical techniques. Labeling was intense in the olfactory bulb, hippocampus, hypothalamus, and midbrain of both SAMR/1 and SAMP/8 mice, and the distribution appeared unchanged in 6‐, 9‐, and 12‐m.o. animals. The receptor's in vivo availability was examined by using in vivo [3H](+)‐SKF‐10,047 binding. No age‐related difference was observed in the olfactory bulb, hippocampus, hypothalamus, cortex, cerebellum, and brainstem of 6‐ or 12‐m.o. SAMR/1 or SAMP/8 mice. The antidepressant efficacy of the selective agonist igmesine was examined in the forced‐swimming test. The compound decreased significantly the immobility duration at 60 mg/kg in 6‐ and 12‐m.o. SAMR/1 and in 6‐m.o. SAMP/8 mice. In 12‐m.o. SAMP/8 mice, the drug efficacy was facilitated; a significant effect was measured at 30 mg/kg. Decreased neurosteroid levels, particularly of progesterone, were seen in 12‐m.o. SAMP/8 mice that might explain the enhanced efficacy of igmesine. Preserved σ1 receptor expression and enhanced behavioral efficacy of σ1 agonists were measured in SAM animals, confirming the therapeutic opportunies for selective ligands against age‐related mood disorders. © 2005 Wiley‐Liss, Inc.