| There is little information available about cholinergic synaptogenesis in the central nervous system. The expression of nicotinic acetylcholine receptors (nAChRs) was examined in neurons from the cholinoceptive interpeduncular nucleus (IPN), which receives extensive cholinergic innervation from the medial habenula nucleus. Several nAChR subunit transcripts are abundantly expressed in the IPN. A protein that regulates nAChR expression at the neuromuscular junction, ARIA (Acetylcholine Receptor Inducing Activity), is expressed in all cholinergic nuclei in the CNS, including the medial habenula, which provides the major input to the IPN.; ACh-evoked ionic currents were characterized using the patch-clamp technique. In neurons cultured from the IPN of newborn rats, three types of nicotinic whole-cell currents could be distinguished based on their kinetic properties and pharmacology. Type I currents exhibited fast onset and rapid desensitization and were blocked by methyllycaconitine (MLA) and {dollar}alpha{dollar}-bungarotoxin ({dollar}alpha{dollar}BTX). Type II and III currents differed from type I responses regarding activation and desensitization processes, both of which were slower. Neither {dollar}alpha{dollar}BTX nor MLA blocked these nAChRs. Using ACh and cytisine as agonists, we found that nAChRs underlying slow currents exhibited distinct pharmacological profiles. In some cells expressing slow currents, ACh was a potent agonist (EC{dollar}rmsb{lcub}50ACh{rcub}=6.5mu M),{dollar} but was less potent in others (EC{dollar}rmsb{lcub}50ACh{rcub}=157mu M).{dollar} Cytisine acted as a potent (EC{dollar}rmsb{lcub}50Cyt{rcub}=0.6mu M),{dollar} but relatively ineffective (efficacy 10% of ACh) agonist on cells that exhibited high ACh-sensitivity. Cytisine was more potent (EC{dollar}rmsb{lcub}50Cyt{rcub}=37.5mu M){dollar} and effective (efficacy 120% of ACh) compared to ACh in cells that were less sensitive to ACh. Receptor classes also could be distinguished based on sensitivity to dihydro-{dollar}beta{dollar}-erythroidine. Single channel analysis indicated that at least two different types of AChRs were underlying type II and III currents.; Immunocytochemical analysis showed that IPN cells contain several ARIA receptors including erbB2, erbB3 and erbB4. Treatment of IPN cultures with recombinant ARIA induced phosphorylation of p185. Treatment of cultures with ARIA in the continued presence of either forskolin or 8-Bromo-cAMP was found to increase type I current-density. |
