The prevalence of depression increases with aging. We hypothesized that like humans, old animals exhibit anhedonic-like behavior, along with signs of behavioral despair. In rodents, anhedonia, a reduced sensitivity to reward, which is listed as a core feature of major depression in the DSM-IVR, can be measured by a decrease in intake of and preference for sweet solutions. Here, sucrose intake, forced swimming, immobility in the modified tail suspension test, novelty exploration, grooming, anxiety and locomotor activity were compared in naïve 3-and 18-month-old male C57BL/6 mice. The absolute amounts and the ratio of consumed 1% sucrose solution to water intake was significantly smaller in 18-month-old mice than in 3-month-old mice. The consumption of 5%-sucrose solution requiring high levels of drinking effort, novelty exploration in two setups and grooming behavior in the splash test were reduced in older animals. Analysis of other behaviors suggested that the above-mentioned signs of anhedonic-like traits were unlikely to be attributable to the potential effect of aging on metabolic needs for water, taste perception, motor capabilities or the induction of essential anxiety and neophobia. A 4-week treatment with the antidepressant imipramine (7 mg/kg/day) or dimebon, a compound with suggested neuroprotective proneurogenic properties (1 mg/kg/day) restored sucrose intake and preference in 18-month-old mice. Meanwhile, young and old mice showed no differences in the parameters of behavioral despair evaluated in the forced swim and modified tail suspension tests. Thus, the behavioral profile of aged mice parallels that of humans with elderly depression, in whom the symptoms of hedonic deficits typically outweigh affective disturbances. The assessment of anhedonic-like traits with the sucrose preference test in 18-month-old mice will be useful in preclinical studies of elderly depression.

The cloning and expression of DNA for the three major opioid receptor types (mu, delta, and kappa) present new research opportunities for the characterization of opioid drugs and their interactions with these receptors. Genomic and cDNA clones for opioid receptors exist for several animal species including mouse, rat, guinea pig, and human. These include clones for all three human opioid receptor types. The receptor proteins consist of about 400 amino acids and have the characteristic seven transmembrane domain structure of G-protein-coupled receptors. There is about 60% amino acid identity between opioid receptor types and about 90% identity between a receptor type cloned from different animal species. All opioid receptor types mediate the inhibition of adenylyl cyclase in response to agonist binding. Radioligand binding and functional studies using the cloned receptors tend to support current conclusions on opioid drug receptor selectivity and activity. Investigations of opioid receptor chimeras and single amino acid mutants are providing information on the ligand recognition sites of these receptors and essential support for the development of computational opioid receptor models. A molecular model of the human delta opioid receptor is included in this review.

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Randomly paired rats were food deprived overnight and placed in an apparatus compelling them to compete for a food reward. About half of these pairs developed a dominant-submissive relationship measured as a significant difference in time spent on the feeder by each rat. This relationship developed over a 2-week period and remained stable for at least the next 5 weeks. Treatment of the submissive subjects, for at least 2 weeks, with imipramine, desipramine, or fluoxetine (10 mg/kg) significantly reduced submissive behavior. The effect faded after cessation of treatment with desipramine. Fluoxetine was further tested at 2.5- and 5-mg/kg doses and showed a dose-dependent reduction of submissive behavior. Treatment of submissive rats with the anxiolytic diazepam (1 mg/kg) was ineffective. The prevalence of dominant-submissive relationships and the effect of desipramine and imipramine on submissive behavior were gender independent. The predictive, face, and construct validity of the behavioral test is discussed.

In seeking broad-spectrum anticonvulsants to treat epilepsy and other neurological disorders, we synthesized and tested a group of sulfamide derivatives (4a-k, 5), which led to the clinical development of 4a (JNJ-26990990). This compound exhibited excellent anticonvulsant activity in rodents against audiogenic, electrically induced, and chemically induced seizures, with very weak inhibition of human carbonic anhydrase-II (IC(50) = 110 microM). The pharmacological profile for 4a supports its potential in the treatment of multiple forms of epilepsy, including pharmacoresistant variants. Mechanistically, 4a inhibited voltage-gated Na(+) channels and N-type Ca(2+) channels but was not effective as a K(+) channel opener. The pharmacokinetics and metabolic properties of 4a are discussed.

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