There are at least two basal cell populations in the olfactory epithelium that could give rise to olfactory neurons during development, in the normal adult, and after experimentally induced receptor cell death. These populations have been subdivided as horizontal (HBC) and globose (GBC) basal cells on the basis of morphological criteria and by staining with antibodies against cytokeratin. HBCs are positive for cytokeratin while GBCs are negative. We have studied which cell type is induced to divide during receptor cell regeneration stimulated by olfactory bulbectomy using a combination of immunocytochemistry and autoradiography. By examining which population increases its labeling index with 3H-thymidine (3H- TdR) at various times after bulbectomy, it is shown that there is an increase in 3H-TdR uptake in the cytokeratin-negative GBCs with no change in the cytokeratin-positive HBCs. This suggests that the GBCs are specifically induced to divide in response to cues that accompany receptor cell death, and it is thus concluded that these cells are among the precursors of new olfactory receptor neurons.

ResultsUsing PCR, we constructed chimeric subunits in which the α7 cytoplasmic loop immediately after TM3 and before TM4 was replaced with the homologous region of the α3 or α5 sequence (Fig. 1a). The N-terminus up to TM2 regulates intersubunit assembly into receptor complexes, and α7 subunits do not coassemble with nAChR subunits, as demonstrated for endogenous subunits in CG neurons and for chimeric α7 subunits expressed in Xenopus laevis oocytes 4,5,12,13 . In particular, we have observed that coexpression of chimeric α7 subunits containing the α3 cytoplasmic loop together with wild-type α3 and β4 subunits in Xenopus oocytes results in the formation of two distinct receptor types that have the pharmacological properties of Bgt-nAChRs and nAChRs, respectively (data not shown). Thus, a difference in the distribution of chimeric α7 as compared to wild-type α7 on the infected CG neuron surface would result from the added α3 or α5 cytoplasmic loop. It cannot be explained by assembly with an endogenous nAChR subunit that can target to the synapse. Different types of neurotransmitter receptors coexist within single neurons and must be targeted to discrete synaptic regions for proper function. In chick ciliary ganglion neurons, nicotinic acetylcholine receptors (nAChRs) containing α3 and α5 subunits are concentrated in the postsynaptic membrane, whereas α-bungarotoxin receptors composed of α7 subunits are localized perisynaptically and excluded from the synapse. Using retroviral vector-mediated gene transfer in vivo, we show that the long cytoplasmic loop of α3 targets chimeric α7 subunits to the synapse and reduces endogenous nAChR surface levels, whereas the α5 loop does neither. These results show that a particular domain of one subunit targets specific receptor subtypes to the interneuronal synapse in vivo. Moreover, our findings suggest a difference in the mechanisms that govern assembly of interneuronal synapses as compared to the neuromuscular junction in vertebrates.1998 Nature America Inc.• http://neurosci.nature.com 1998 Nature America Inc.• http://neurosci.nature.com

Neurons engage in two distinct types of cell-cell interactions: they receive innervation and establish synapses on target tissues. Regulatory events that influence synapse formation and function on developing neurons are largely undefined. We show here that nicotinic acetylcholine receptor (AChR) subunit transcript levels are differentially regulated by innervation and target tissue interactions in developing chick ciliary ganglion neurons in situ. Using ganglia that have developed in the absence of pre- or postganglionic tissues and quantitative RT-PCR, we demonstrate that alpha 3 and beta 4 transcript levels are increased by innervation and target tissue interactions. In contrast, alpha 5 transcript levels are increased by innervation, but target tissues have little effect. Whole-cell ACh-induced currents, used to estimate the number of functional AChRs, change in correlation with alpha 3 and beta 4, but not alpha 5, transcript levels. A model is proposed in which the changes in AChR subunit expression regulate levels of synaptic activity, which is a critical determinant of synapse stabilization and elimination, and neuronal cell death.