Why mice are neither miniature humans nor small rats: a cautionary tale involving 5-hydroxytryptamine-6 serotonin receptor species variants.

Serotonin [5-hydroxytryptamine (5-HT)] is a biogenic amine neurotransmitter that modulates a host of important biological processes, such as mood, cognition, perception, feeding behavior, smooth muscle contractility, and platelet aggregation. The myriad actions of 5-HT are mediated by one or more of at least seven subtypes of receptors, all but one of which (i.e., the 5-HT3 receptor) are members of the G protein-coupled receptor superfamily (Kroeze et al., 2002) (see phylogram, Fig. 1). Receptors that are closer on the phylogram are more likely to share pharmacological properties than receptors that are farther apart (distant relatives). One of the most enigmatic 5-HT receptors, the 5-HT6 receptor, was cloned in 1993 from a rat striatal cDNA library taken from sequence homology with existing 5-HT receptors and found to be expressed in the striatum, olfactory tubercule, cortex, and hippocampus (Monsma et al., 1993; Ruat et al., 1993; Ward et al., 1995). The human (Kohen et al., 1996) and mouse (Kohen et al., 2001) 5-HT6 receptors were subsequently cloned with the human version shown to be pharmacologically similar to the rat (Kohen et al., 1996). Shortly after the discovery of 5-HT6 receptors, Roth et al. (1994) reported that a large number of typical and atypical antipsychotic drugs bound with unexpectedly high affinity (Ki values of 20 nM) to cloned 5-HT6 receptors. The expression of 5-HT6 receptors in limbic areas and the basal ganglia, and the selective labeling of 5-HT6 receptors in rat brain with [H]clozapine (Glatt et al., 1995), strongly implicated 5-HT6 receptors in at least some of the actions of antipsychotic drugs. More recently, 5-HT6 receptors have been demonstrated to regulate central cholinergic neurotransmission. In this regard, the administration of the 5-HT6 receptor-selective antagonist Ro 04-6790 reversed scopolamine-induced rotation in 6-hydroxydopamine–lesioned rats (Bourson et al., 1998). Additionally, Rogers and Hagan (2001) and Woolley et al. (2001) found that either 5-HT6 receptor antisense oligonucleotides or 5-HT6 receptor-selective inhibitors enhanced retention by rats of the learned platform position in the Morris water maze. These data suggest that 5-HT6 receptor antagonists might boost cholinergic neurotransmission and reduce the cognitive impairments experienced by patients with dementia or schizophrenia. Intriguingly, Tsai et al. (1999) determined that the 267C allele of the 5-HT6 receptor is a significant risk factor for Alzheimer’s disease. Taken together, these findings indicate that 5-HT6 antagonists might prove useful in treating a number of common illnesses, including dementia and schizophrenia. The now-classic approach for validating 5-HT6 receptors as molecular targets for therapeutics is to construct a 5-HT6 knockout mouse and to characterize its phenotype. As Hirst et al. (2003) discovered, however, it is unlikely that 5-HT6 knockout mice will be useful for validating the 5-HT6 receptor as a therapeutic target because of pronounced and unexpected species differences in both receptor regional distribution and pharmacology. It is now widely appreciated that slight differences in rodent and human amino acid sequences can lead to unexpectedly large differences in the pharmacology of the receptors, with potentially disastrous effects for drug-discovery efforts. What has not been clearly documented until the Hirst et al. study (2003), however, is that mouse receptors could be significantly different from rat receptors. In the article published in this issue of Molecular Pharmacology, Hirst et al. (2003) elegantly demonstrate that the mouse 5-HT6 receptor is, in nearly every respect, distinct from rat and human 5-HT6 receptors. They investigated these differences because of preliminary studies in which they were unable to quantify mouse 5-HT6 receptors with a highly selective radioligand ([I]SB-258585) that labels both rat and human 5-HT6 receptors. In addition to species differences in the binding of drugs to 5-HT6 receptors, they found differences in the regional expression of 5-HT6 receptors. Thus, quantitative polymerase