Commentary Serotonin receptor knockouts: A moody subject

The neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) is believed to play a significant role in determining one’s emotional state. Indeed, serotonergic synapses are sites of action for a number of mood-altering drugs, including the now-legendary antidepressant Prozac (fluoxetine) (1). As a result, there has been tremendous interest in identifying molecular components of the serotonergic system, including cell surface receptors and transporters, and understanding whether and how these proteins contribute to the regulation of mood and emotion. This quest is driven, in part, by the possibility that behavioral disorders, such as depression or anxiety, may be linked to deficits in one or more components of this signaling system. Such information could, in turn, focus attention on specific targets for the development of novel drugs with which to treat psychiatric disorders. In the case of serotonin, this is a particularly challenging goal because the system is quite complex, consisting of at least 14 distinct receptor subtypes (2). Nevertheless, pharmacological and physiological studies have highlighted a subset of 5-HT receptor subtypes worthy of more immediate genetic analysis. One of these, the 5-HT1A receptor, is the focus of two studies by Ramboz et al. (3) and Heisler et al. (4) in recent issues of the Proceedings. These groups used gene ‘‘knockout’’ methods to generate mouse lines lacking 5-HT1A receptors so that they could assess the effects of receptor ablation on behavior, using models of anxiety and depression. Why is the 5-HT1A subtype considered to be a particularly important and interesting member of the serotonin receptor family? One reason is that the 5-HT1A receptor has for years been synonymous with the classical ‘‘autoreceptor’’ on serotonergic neurons in raphé nuclei of the brain stem (5). These cells synthesize the majority of serotonin in the brain, sending their axonal projections throughout the central nervous system. Activation of 5-HT1A receptors on cell bodies of these neurons inhibits release of serotonin, thereby attenuating serotonergic signaling at large. As such, this receptor represents a potentially important regulatory site for modulating the actions of serotonin in the brain and spinal cord. Interest in this receptor also stems from clinical success with drugs that interact with this site. Most notably, partial agonists of the 5-HT1A receptor, such as buspirone and gepirone, are effective as anxiolytic (antianxiety) agents (6, 7), suggesting that alterations in 5-HT1A receptor activity may be linked to serotoninmediated changes in mood. In addition, antagonists of the 5-HT1A receptor appear to accelerate and enhance the antidepressant action of so-called selective serotonin reuptake inhibitors (SSRIs) such as Prozac (8, 9), which increase levels of serotonin in the synaptic cleft by blocking transporters on the presynaptic membrane. In other words, blockade of inhibitory autoreceptors may augment the ability of SSRIs to elevate synaptic serotonin levels. Related speculation suggests that desensitization of 5-HT1A autoreceptors represents a significant component of the antidepressant action of chronically administered SSRIs (10). Finally, it must be mentioned that not all 5-HT1A receptors are presynaptic; postsynaptic receptors are expressed in a number of brain regions to which serotonergic neurons project, including the hippocampus, cerebral cortex, and amygdala (11, 12). As in the case of presynaptic autoreceptors, activation of postsynaptic 5-HT1A receptors leads to hyperpolarization of the neuron and the consequent inhibition of neurotransmitter release. This effect appears to be mediated through a biochemical signaling pathway in which 5-HT1A receptors activate a G protein (Gi)coupled inwardly rectifying potassium channel (13, 14). In light of the pharmacological evidence that 5-HT1A receptors exert negative ‘‘feedback’’ control on serotonergic neurons, one would predict that mice lacking this receptor should show elevated levels of extraneuronal serotonin, or an increase in the amount of serotonin released after nerve stimulation. However, neither group observed a significant change in the serotonin content of brains from mutant animals compared with wild-type controls. Furthermore, Ramboz et al. (3) measured the amounts of serotonin released after electrical stimulation of slices taken from mesencephalic and hippocampal regions of the brain. In slices from wild-type animals, the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8OH-DPAT) reduced serotonin release by 30–40%, suggesting that at least some component of autoregulation via the 5-HT1A receptor is reconstituted in this assay. Interestingly, knockout animals showed no significant difference from wild-type controls in the amount of electrically evoked serotonin release from these slices. Moreover, in contrast to wild types, the evoked release observed in mutants was unaffected by pretreatment with 8-OH-DPAT, as one would expect in the absence of functional 5-HT1A receptors. If one assumes that the in vitro slice preparation recapitulates regulation by presynaptic autoreceptors in vivo (perhaps a large assumption because somatodendritic receptors are lost in this preparation), then these findings suggest either that the 5-HT1A receptor does not play a significant part in modulating serotonin release or that its role has been subsumed by another subtype. Perhaps the most obvious candidate for such a functional substitution is the 5-HT1B receptor. Like the 5-HT1A receptor, the 5-HT1B subtype is coupled negatively to adenylate cyclase and can be found on serotonergic cells of the raphé nucleus, where it is located at the axon terminal (2, 5). 5-HT1B agonists, such as the antimigraine drug sumatriptan, can inhibit neurotransmitter release from these cells, as Ramboz et al. demonstrate, using slices from wild-type or mutant brains. The authors suggest that 5-HT1B receptors may be up-regulated in the brains of knockout mice to compensate for the loss of somatodendritic 5-HT1A autoreceptors, thereby modulating the release of serotonin (and possibly other transmitters) through increased inhibition at the axon terminal. If so, then the serotonergic feedback circuit shows impressive plasticity in the face of a genetic mutation that might otherwise perturb homeostatic mechanisms controlling transmitter release. These findings also remind us that the effects of any given