Serotonin induces inward potassium and calcium currents in rat cortical astrocytes.

Ca2+ imaging and patch-clamp techniques were used to study the effects of serotonin (5-HT) on ionic conductances in rat cortical astrocytes. 1 and 10 microM serotonin caused a transient increase in intracellular calcium (Ca(i)) levels in fura-2AM-loaded cultured astrocytes and in astrocytes acutely isolated and then cultured in horse serum-containing medium for over 24 h. However, the acutely isolated (less than 6 h from isolation) astrocytes, as well as acutely isolated astrocytes cultured in serum-free media, failed to respond to 5-HT by changes in Ca(i). Coinciding with the changes in Ca(i) levels, inward currents were activated by 10 microM 5-HT in cultured, but not in acutely isolated astrocytes. Two separate types of serotonin-induced, small-conductance inward single-channel currents were found. First, in both Ca2+-containing and Ca2+-free media serotonin transiently activated a small-conductance apamin-sensitive channel. Apamin is a specific blocker of the small-conductance Ca2+-activated K+ channel (sK(Ca)) When cells were pre-treated with phospholipase C inhibitor U73122 no 5-HT-induced sK(Ca) channel openings were seen, indicating that this channel was activated by Ca2+ released from intracellular stores via IP3. A second type of small inward channel activated later, but only in the presence of external Ca2+. It was inhibited by the L-type Ca2+ channel blockers, nimodipine and nifedipine. Both types of channel activity were inhibited by ketanserin, indicating activation of the 5-HT2A receptor.

Transmitter-induced calcium responses differ in astrocytes acutely isolated from rat brain and in culture.

Glial fibrillary acid protein (GFAP)-positive astrocytes isolated from the cerebral cortices of 3-10-day-old rats frequently showed increased intracellular Ca2+ concentration responses to L-glutamate and glutamate analogues. However, few of the acutely isolated cells responded to ATP, and no such cells responded to serotonin [5-hydroxytryptamine (5-HT)]. The same cell that failed to respond to ATP or 5-HT often responded to glutamate. Culturing acutely isolated cells in media containing horse serum decreased Ca2+ responses to glutamate but increased the responses to ATP and induced responses to 5-HT. In primary cultures prepared from the cerebral cortices of 1-day-old rats and cultured in horse serum, fewer of the cells responded to glutamate, but almost all cells responded to ATP and 5-HT. The lack of or limited response to, 5-HT or ATP in the acutely isolated cells seems unlikely to be due to selective damage to the respective receptors because acutely isolated GFAP-negative cells showed responses to ATP, several different proteases and mechanical dissociation yielded cells that also responded to glutamate but not to ATP, and exposure of primary cultures to papain did not abolish Ca2+ responses to several transmitters. The responses of the acutely isolated cells to glutamate but limited or lack of responses to ATP and 5-HT also correspond to what has been seen so far for astrocytes in situ. Thus, the present studies provide direct evidence that some of the receptors seen in primary astrocyte cultures may reflect a response to culture conditions and that, in the context of the relevant information so far available, acutely isolated astrocytes seem to reflect better the in vivo state.

beta-Amyloid peptide-induced morphological changes coincide with increased K+ and Cl- channel activity in rat cortical astrocytes.

Alzheimer's disease (AD) is a slowly progressing neurodegenerative disease characterized by the loss of neurons and formation of amyloid plaques, often surrounded by reactive astrocytes. Astrocytes are important regulators of the normal neuronal environment, and changed astrocyte function may lead to increased neuronal vulnerability. The slow onset of the disease with a gradual increase in the beta-amyloid peptide (beta-AP) concentrations may alter astrocyte function long before any visible symptoms of the disease are observed. We, therefore, studied in vitro the effects of small amounts of beta-AP(1-40) and -(25-35) on rat cortical astrocyte function observing changes in cell morphology, intracellular calcium levels (Cai), and ion channel activity. Incubation with 10 and 200 nM beta-APs caused increased process formation and hypertrophy. Stellation was also detected when astrocyte cultures were incubated with 1 microM AlCl3 alone, or together with beta-APs. Fura-2AM-loaded astrocytes were used to test whether the morphological changes were connected to changes in Cai levels. 1 microM beta-AP(1-40) induced transient Cai increase in approximately 17%, and beta-AP(25-35) in approximately 36% of astrocytes. In patch-clamp studies, increased K+ and Cl- channel activity was detected with 10-100 nM beta-AP(1-40). With large amounts (20 microM) of beta-AP(1-40), an additional giant channel activity emerged. These beta-AP-induced changes in astrocyte function may eventually be critical for the neuronal survival in Alzheimer's disease.