Myelination requires oligodendrocyte-neuron communication, and both neurotransmitters and contact interactions are essential for this process. Oligodendrocytes are endowed with neurotransmitter receptors whose expression levels and properties may change during myelination. However, only scant information is available about the extent and timing of these changes or how they are regulated by oligodendrocyte-neuron interactions. Here, we used electrophysiology to study the expression of ionotropic GABA, glutamate, and ATP receptors in oligodendrocytes derived from the optic nerve and forebrain cultured either alone or in the presence of dorsal root ganglion neurons. We observed that oligodendrocytes from both regions responded to these transmitters at 1 day in culture. After the first day in culture, however, GABA sensitivity diminished drastically to less than 10%, while that of glutamate and ATP remained constant. In contrast, the GABA response amplitude was sustained and remained stable in oligodendrocytes cocultured with dorsal root ganglion neurons. Immunochemistry and pharmacological properties of the responses indicated that they were mediated by distinctive GABA A receptors and that in coculture with neurons, the oligodendrocytes bearing the receptors were those in direct contact with axons. These results reveal that GABA A receptor regulation in oligodendrocytes is driven by axonal cues and that GABA signaling may play a role in myelination and/or during axonglia recognition.

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1. Native Xenopus oocytes were voltage clamped and exposed to Ringer solutions containing low concentrations of divalent cations. Oocytes, held at ‐60 mV, developed a reversible non‐inactivating smooth inward current (Ic) associated with an increase in membrane conductance. 2. Ic was selectively carried by cations (Na+, K+), indicating that the current was not the result of a non‐specific membrane breakdown, but was due instead to removal of a blocking effect of divalent cations on a specific population of endogenous ionic channels located in the oocyte membrane. 3. The blocking effects of Ca2+ and Mg2+ were voltage dependent, implying action at a binding site within the pore of the cationic channel. For example, the half‐maximal inhibition (IC50) of Ic by Ca2+ was 61 microM in oocytes held at ‐60 mV and 212 microM in oocytes held at 0 mV. 4. The Ic channels could be unblocked by depolarization of the membrane even in the presence of physiological concentrations of Ca2+ or Mg2+. The unblocking of the channels was observed as a slowly developing outward current. 5. The novel cationic current was substantially reduced following in vitro maturation of oocytes by treatment with progesterone (10 microM, 4‐5 h). 6. The physiological role of Ic channels remains to be elucidated. Nonetheless, their characteristics explain the ionic basis of the sensitivity of oocytes to reductions in extracellular divalent cations and raise the possibility that the channels play a role in calcium homeostasis.

Voltage-clamp techniques were used to study the membrane currents elicited by follicle stimulating hormone (FSH) and acetylcholine (ACh) in follicleenclosed oocytes of Xenopus laevis (follides). Both agonists caused complex responses that were more evident when the follicles were in hypotonic Ringer solution (HR; 190.4 mosM). In this medium, currents activated by FSH regularly showed three phases whereas currents activated by ACh displayed three to six phases. At a holding potential of -60 mV, FSH, and ACh responses involved combinations of inward and outward currents. Both FSH and ACh responses included a slow smooth inward component that was associated with an increase in membrane conductance, mainly to C1-(Sin)-This current was strongly dependent on the osmolarity of the external solution: an increase in osmolarity of the HR solution of 18-20 mosM caused a 50% decrease in Sin. In contrast, a fast and transient C1-current (Fin) specifically elicited by ACh was not dependent on osmolarity. Both, Fin and Sin currents required the presence of follicular cells, since defolliculation using three different methods abolished all the response to FSH and at least four components of the ACh responses, The membrane channels carrying Fin and oscillatory CIcurrents elicited by stimulation of ACh or serum receptors, were much more permeable to I-and Br-than CI-, whereas Sm channels were equally permeable to these anions. Unlike the oscillatory CI-currents generated in the oocyte itself, Sin and Fin currents in follicle-enclosed oocytes were not abolished by chelation of intracellular Ca 2+, either with EGTA or BAPTA, which suggests that intracellular Ca 2+ does not play a critical role in the activation of these currents. Our experiments show that Si, and Fin currents are quite distinct from the previously characterized oscillatory C1-responses of oocytes. Moreover, the results strongly suggest that the FSH and ACh receptors, the CI-channels mediating the Fin and Sin currents, together with the necessary elements for their activation, are all located in the follicular cells and not in the oocyte.Many aspects of follicular cell physiology in Xenopus laevis, and other species, are regulated by hormones and neurotransmitters, including FSH and ACh. The follicular CI-currents described in this paper may play an important role in the follicular cell-oocyte development.

Ionic currents elicited via purinergic receptors located in the membrane of Xenopus follicles were studied using electrophysiological techniques. Follicles responded to ATP-activating inward currents with a fast time course ( F in). In Ringer solution, reversal potential ( E rev) of F in was −22 mV, which did not change with external substitutions of Na+ or K+, whereas solutions containing 50 or 5% of normal Cl−concentration shifted E rev to about +4 and +60 mV, respectively, and decreased F in amplitude, indicating that F in was carried by Cl−. F in had an onset delay of ∼400 ms, measured by application of a brief jet of ATP from a micropipette positioned near the follicle (50 μm). F in was inhibited by 50% in follicles pretreated with pertussis toxin. This suggests a G protein-mediated receptor channel pathway. F in was mimicked by 2-MeSATP and UTP, the potency order (half-maximal effective concentration) was 2-MeSATP (194 nM) > UTP (454 nM) > ATP (1,086 nM). All agonists generated Cl− currents and displayed cross-inhibition on the others. F in activation by acetylcholine also cross-inhibited F in-ATP responses, suggesting that all act on a common channel-activation pathway.