RAPID COMMUNICATION Spontaneous Apamin-Sensitive Hyperpolarizations in Dopaminergic Neurons of Neonatal Rats

Seutin, Vincent, Laurent Massotte, Jacqueline Scuvée-Moreau, ena observed in our study were due to the anesthetic, some (n Å 16) neonatal rats were killed without prior anesthesia. The other and Albert Dresse. Spontaneous apamin-sensitive hyperpolarizations in dopaminergic neurons of neonatal rats. J. Neurophysiol. animals were anesthetized with halothane. All animals were decapitated. 80: 3361–3364, 1998. Intracellular recordings from substantia nigra slices revealed the existence of spontaneous hyperpolarizations Horizontal slices were prepared as described (Seutin et al. 1997), and the same landmarks were used to localize the substantia (amplitude 2–8 mV, duration 100–400 ms) at 060 mV in most dopaminergic neurons of neonatal (9–15 days) but not adult rats. nigra. Slices were completely submerged in a solution of the following composition (in mM): 126 NaCl, 2.5 KCl, 1.2 NaH2PO4, These events were blocked by apamin (300 nM) and bicuculline methochloride (100–300 mM), which blocks apamin-sensitive 1.2 MgCl2 , 2.4 CaCl2 , 11 glucose, and 18 NaHCO3, saturated with 95% O2-5% CO2 (pH 7.4). Flow rate and temperature were set at currents. They were unaffected by the selective g-aminobutyric acid-A (GABAA) antagonists SR95531 (100 mM) and picrotoxin 2.5 ml/min and 35 { 0.57C, respectively. All drugs were bath applied. (30–50 mM), the GABAB antagonist CGP35348 (300 mM), the D2 antagonist haloperidol (1 mM), and the metabotropic antagonist Intracellular recordings were made with microelectrodes (resistance: 60–150 MV) filled with 2 M KCl. In some cases, 1,2-bis(2MCPG (1 mM). The hyperpolarizations were strongly attenuated or abolished when recording electrodes contained 200 mM 1,2aminophenoxy)ethane-N ,N ,N*,N*-tetraacetic acid (BAPTA) was added to this solution. Most experiments were carried out in the bis(2-aminophenoxy)ethane-N ,N ,N*,N*-tetraacetic acid. They were resistant to tetrodotoxin in the majority of the cells. They bridge balance mode at approximately 060 mV. Because most DA neurons were spontaneously active, a constant negative current had some voltage dependency and were in some cases transiently potentiated when cells were briefly depolarized by current injecinjection (040 to 0200 pA) was applied. The area under the curve for the afterhyperpolarization (AHP) tion. We conclude that dopaminergic neurons have developmentally regulated physiological properties. These spontaneous hyperafter a spike was measured as described previously (Seutin et al. 1997), except that either 300 nM apamin or 300 mM bicuculline polarizations might affect the firing rate of these cells, which was found to be lower in neonates than in adults. methochloride (BMC) were used as blockers. We have recently shown that these concentrations of the drugs fully block the AHP (Seutin et al. 1997). I N T R O D U C T I O N Drugs used and their supplier were as follows: apamin, BMC, picrotoxin, cesium chloride, BAPTA (Sigma); tetrodotoxin (TTX) A large body of literature shows that electrophysiological (ICN Biomedicals, Aurora, OH); MCPG (a-methyl-4-carboxyproperties of mammalian neurons undergo developmental phenylglycine) (Research Biochemicals International) , SR95531 changes. For example, modifications in the kinetics of synap(2-[carboxy-3 * -propyl] -3-amino-6-paramethoxy-phenyl-pyritic currents (Carmignoto and Vicini 1992) and in fundamendazinium bromide) (gift from Sanofi-Winthrop), BHT920 (6tal properties ( i.e., inhibitory or excitatory) of neurotransallyl-2-amino-5,6,7,8,-tetrahydro-4H-thiazolo-[4,5-d]azepin)(gift from Boehringer Ingelheim), haloperidol (gift from Janssen Pharmitter systems (Leinekugel et al. 1997) were demonstrated. maceutica) , and CGP35348 [ P-(3-aminopropyl)-P-diethoxyWe have undertaken an in vitro study to address this issue methyl-phosphinic acid; gift from Ciba-Geigy]. in midbrain dopaminergic (DA) neurons. Here, we report All data are expressed as means { SE. Statistical differences that DA neurons of young rats exhibit spontaneous hyperpowere assessed with the x 2 test or Student’s t-test. Differences were larizations that are mostly absent in adults, and we characterconsidered significant at P õ 0.05. ize their pharmacology.