Calcium regulation in neurons: transport processes

Responses to Metabotropic Glutamate Receptor Activation in Cerebellar Purkinje Cells: Induction of an Inward Current

The responses to activation of metabotropic glutamate receptors (mGluRs) of Purkinje cells in rat cerebellar slice cultures were investigated using intracellular recordings in single-electrode voltage-clamp mode combined with microfluorometric measurements of cytosolic free calcium using fura-2. Purkinje cells were perfused with saline containing 0.5 microM tetrodotoxin and 10 microM bicuculline and voltage-clamped at -60 mV. Bath-applied trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 50 - 100 microM), a selective agonist of mGluRs, induced a transient inward current that was followed by an outward current. The response induced by t-ACPD was not affected by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, up to 40 microM). In contrast, inward currents caused by (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 1 - 2 microM) were completely abolished, while inward currents caused by quisqualate (0.25 microM) were only partially depressed by CNQX (5 - 40 microM). The inward current induced by t-ACPD was unaffected by external Ba2+ (1 mM), tetraethylammonium (10 mM) and Cs+ (1 mM), and was associated with an increase in apparent input conductance of the cell membrane. The extrapolated reversal potential of inward currents induced by t-ACPD was +18 mV while Cl- currents induced by muscimol reversed at -66 mV. Inward currents induced by t-ACPD, but not those induced by AMPA, were associated with a rise in cytosolic Ca2+ concentration and suppressed by intracellular injection of a calcium chelator. Replacement of external Na+ by choline or Li+ depressed the inward current and resulted in a slower decay of the Ca2+ signal.

Outgrowth morphology and intracellular calcium of crustacean neurons displaying distinct morphologies in primary culture

Peptide-secreting neurons from crustacean X-organ regenerating in defined culture possess different ionic current profiles correlated with two distinct morphological types, veiling and branching; voltage-dependent Ca2+ current is prominent in neurons consistently extending large veils, but is small in neurons that repetitively branch. Intracellular free calcium levels ([Ca2+]i) have been implicated in the regulation of neurite outgrowth underlying the establishment of distinct morphologies. Here, basal [Ca2+]i was measured by fura-2 fluorescence ratio imaging from these morphologically distinct neurons and compared. Both morphological types can extend out processes over a [Ca2+]i range (approximately 50 to 300 nM) that is much greater than that reported for neurons of other phyla. Application of high K+ saline led to increases in [Ca2+]i in soma, neurite, and lamellipodium of veiling neurons. Increases were greater for veiling than branching neurons. These observations were consistent with the previous voltage clamp data for calcium currents. Media altered to perturb [Ca2+]i were used to assess the role of [Ca2+]i in veiling or branching outgrowth programs. Outgrowth of veiling cells was arrested by addition of 100 microM Cd2+, a calcium channel blocker. Outgrowth resumed following brief exposures to Cd2+. Branching neurons were unaffected by Cd2+. Cd2+ at lower levels (10 microM) had no effect on outgrowth of either neuronal type, whereas at higher levels (1 mM), outgrowth of both types was arrested. Reduction of extracellular sodium to 0.001 of normal concentration stopped veiling outgrowth, but branching outgrowth continued, although it was less robust. Addition of tetrodotoxin (1 microM) did not alter outgrowth of either neuronal type relative to controls. Thus, peptidergic neurons of differing intrinsic morphologies maintain similar basal [Ca2+]i levels under identical culture conditions, yet show differing sensitivities to manipulations influencing [Ca2+]i with respect to regenerative outgrowth, but not its form.

Cloning of two isoforms of the rat brain Na(+)-Ca2+ exchanger gene and their functional expression in HeLa cells

Two functional isoforms of the rat brain Na(+)-Ca2+ exchanger were isolated from a lambda ZAP hippocampus cDNA library. The open reading frame of clone RBE-1 codes for a protein 935 amino acids long, and that of clone RBE-2 codes for a protein 958 amino acids long. Expression HeLa cells of Na+ gradient dependent Ca2+ transport activity was determined following transfection of the cells with either RBE-1 or RBE-2. Both clones expressed proteins that exchange Na+ with Ca2+ in an electrogenic manner and none of them exhibited a dependency of the antiport on K, since they transported Ca2+ in an Na+ gradient dependent manner in external choline chloride as well.

Peptidergic neurons of the crab, Cardisoma carnifex, in defined culture maintain characteristic morphologies under a variety of conditions

Peptidergic neurons dissociated from the neurosecretory cell group, the X-organ, of adult crabs (Cardisoma carnifex) show immediate outgrowth on unconditioned plastic dishes in defined medium. Most of the neurons can be categorized as small cells, branchers or veilers. A fourth type, "superlarge," found occasionally, has a soma diameter greater than 40 microns and multipolar outgrowth. We report here the effects on morphology that follow alterations of the standard defined culturing conditions. The three common types of neurons are present when cells are grown in crab saline or saline with L-glutamine and glucose (saline medium). Changes of pH between 7.0 to 7.9 have no effect. Osmolarity changes cause transient varicosities in small cells. In some veilers, pits rapidly appear in the veil and then disappear within 35 min. In cultures at 26 degrees C instead of 22 degrees C, veilers extend processes from the initial veil in a pattern similar to branchers, and the processes of adjacent veilers sometimes form appositions. Culturing in higher [K+]o medium ([K+]o = 15-110 mM; standard = 11 mM) has no long-term effect, but growth is arrested by [K+]o greater than 30 mM. Cultures were also grown in media in which [Ca2+]o ranged from 0.1 microM to 26 mM (standard = 13 mM). Outgrowth occurred from all neuronal types in all [Ca2+]o tested. Thus, the expression of different outgrowth morphologies occurs under a wide variety of culturing conditions.