5-HT receptor sub-types: aspects of their regulation and function

Retinal projection to the dorsal raphe nucleus in the Chilean degus (Octodon degus)

A substantial projection from the retina to the dorsal raphe nucleus (DRN) has been demonstrated in the Chilean degus, a diurnal/crepuscular hystricomorph rodent. Following intraocular injection of cholera toxin subunit B (CTB), immunocytochemically labeled CTB-positive axons and terminals were observed in all major retinorecipient nuclei as well as in the DRN and periaqueductal gray (PAG) of the mesencephalon. Two streams of optic axons to the DRN were observed: one descending from the optic tract at the level of the pretectum and anterior superior colliculus, the other emerging as a small fascicle at the anterior pole of the inferior colliculus and descending bilaterally through the PAG. Contralateral retinal afferents in the DRN appeared to terminate primarily in the dorsomedial and lateral subdivisions of the DRN, and a less extensive ipsilateral component also was observed. Axonal arborizations were characterized by short branches and multiple varicosities, both in the DRN and in the PAG. The extent and density of DRN retinal afferents were not as extensive as previously observed in Mongolian gerbils using identical techniques, but the retinal-DRN projection is considerably larger in degus than in rats. The functional significance of the retinal-DRN pathway remains to be determined, although a variety of evidence indicates that light may directly affect the activity of neurons and serotonin levels in the DRN.

Endogenous serotonin-2A and -2C receptors in Balb/c-3T3 cells revealed in serotonin-free medium: desensitization and down-regulation by serotonin

We studied the endogenous expression of the serotonin-2A (5-hydroxytryptamine2A, 5-HT2A) 5-HT2C, and a splice-variant of the 5-HT2C receptor in murine Balb/c-3T3 fibroblast cells that is revealed when these cells are maintained in medium containing 5-HT-free serum. RNA editing of the 5-HT2C receptor was exclusively at a single brain-specific site. Addition of 5-HT (EC50 = 23 +/- 2.9 nM) induced an immediate release of calcium from an ionomycin-sensitive intracellular store by coupling to a pertussis toxin-insensitive pathway. The 5-HT-induced calcium mobilization displayed a 5-HT-2-like pharmacology, and ligand binding analyses indicated the presence of specific binding sites (27.5 +/- 2 fmol/mg protein) with a 5-HT2A-like pharmacology. Although the 5-HT2A receptor site was predominant, the smaller component of 5-HT2C receptors alone was sufficient to mediate a maximal calcium response. The 5-HT-induced increase in [Ca2+]i was reversibly inhibited by >75% following a 12-hr pretreatment (T1/2 = 2 hr) with 5-HT (EC50 = 400 nM). Extended treatment (24-96 hr) with 5-HT induced a complete functional desensitization that was associated with a partial (60%) reduction in 5-HT2 receptor number, implicating both receptor down-regulation and post-receptor mechanisms in 5-HT-induced desensitization. Long-term (hours to days) treatment with 5-HT did not modulate DNA synthesis, cell proliferation, or transformation in Balb/c-3T3 cells. These results demonstrate that Balb/c-3T3 cells express endogenous 5-HT2 receptors that are desensitized by the 5-HT present in normal serum, illustrating the importance of growth conditions in the identification of receptor responsiveness. The lack of proliferative response to 5-HT in Balb/c-3T3 suggests a putative role of desensitization as a "safety valve" to prevent abnormal cell growth during sustained 5-HT2 receptor activation.

Multiple serotonin receptors: too many, not enough, or just the right number?

In this manuscript, current knowledge about central nervous system serotonin (5-HT) receptors is discussed with an emphasis toward describing the functional significance of the multiple 5-HT receptors. Five characteristics of 5-HT receptors, which are hypothesized to contribute to this functional significance, are discussed: (a) 5-HT has varying affinity and potency for the different receptor subtypes; (b) multiple transduction pathways are used by the different receptor subtypes; (c) receptor subtypes differ in their susceptibility to agonist-mediated desensitization/downregulation; (d) receptor subtypes interact in mediating cellular responses to the neurotransmitter; and (e) receptor subtypes respond differently to changes in the physiological environment. It is hypothesized that these characteristics of the multiple neurotransmitter receptors provide the nervous system with a capacity for coding and decoding of 5-HT-mediated neuronal transmission that could not take place with a single neurotransmitter receptor. Serotonergic regulation of female reproduction and regulation of glucocorticoid release are used to illustrate the integrative potential deriving from the existence of multiple 5-HT receptors.

A pharmacological analysis of serotonergic receptors: effects of their activation of blockade in learning

1. The authors have tested several 5-HT selective agonists and antagonists (5-HT1A/1B, 5-HT2A/2B/2C, 5-HT3 or 5-HT4), an uptake inhibitor and 5-HT depletors in the autoshaping learning task. 2. The present work deals with the receptors whose stimulation increases or decreases learning. 3. Impaired consolidation of learning was observed after the presynaptic activation of 5-HT1B, 5-HT3 or 5-HT4 or the blockade of postsynaptic 5-HT2C/2B receptors. 4. In contrast, an improvement occurred after the presynaptic activation of 5-HT1A, 5-HT2C, and the blockade of presynaptic 5-HT2A, 5-HT2C and 5-HT3 receptors. 5. The blockade of postsynaptic 5-HT1A, 5-HT1B, 5-HT3 or 5-HT4 receptors and 5-HT inhibition of synthesis and its depletion did no alter learning by themselves. 6. The present data suggest that multiple pre- and postsynaptic serotonergic receptors are involved in the consolidation of learning. 7. Stimulation of most 5-HT receptors increases learning, however, some of 5-HT subtypes seem to limit the data storage. 8. Furthermore, the role of 5-HT receptors in learning seem to require an interaction with glutamatergic, GABAergic and cholinergic neurotransmission systems.