TYROSINE-HYDROXYLASE EXPRESSION IN PRIMARY CULTURES OF OLFACTORY-BULB- ROLE OF L-TYPE CALCIUM CHANNELS

Sensory activity mediates regulation of tyrosine hydroxylase (TH), the first enzyme in the dopamine biosynthetic pathway, in the rodent olfa ctory bulb. The current studies established for the first time primary cultures of neonatal mouse olfactory bulb expressing TH and tested wh ether L-type calcium channels mediate the activity-dependent regulatio n of the dopamine phenotype. After 1 d in vitro (DIV), a small populat ion of TH-immunostained neurons that lacked extensive processes could be demonstrated. After an additional 2 DIV in serum-free medium? the n umber of TH neurons had doubled, and they exhibited long interdigitati ng processes. Membrane depolarization for 48 hr with 50 mM KCI produce d a further 2.4-fold increase in the number of TH-immunoreactive neuro ns compared with control cultures, Increased TH neuron number required at least 36 hr of exposure to KCI, Forskolin, which increases intrace llular cAMP levels, induced a 1.5- to 1.6-fold increase in the number of TH-immunostained neurons. Combined treatment with KCI and forskolin was not additive. Nifedipine, an L-type calcium channel blocker, comp letely prevented the depolarization-mediated increase in TH expression but did not block the response to forskolin, Treatment with Bay K8644 , an L-type calcium channel agonist, also significantly increased the number of TH-expressing neurons. Depolarization also induced alteratio ns in neuritic outgrowth, resulting in a stellate versus an elongate m orphology that, in contrast, was not prevented by nifedipine. These re sults are the first demonstration that in vitro, as in vivo, depolariz ation increases TH expression in olfactory bulb and that L-type calciu m channels mediate this activity-dependent regulation of the dopamine phenotype.