Regulation of tyrosine hydroxylase in olfactory bulb cultures: selective inhibition of depolarization-induced increase by endogenous opioids

Regulation of tyrosine hydroxylase (TH) by second messenger pathway activators was examined in rat olfactory bulb cell cultures. The number of TH-immunoreactive neurons was increased 2–3-fold by 36 h treatments with forskolin (Fsk, 10⁻⁶ M) or phorbol myristate acetate (PMA, 10⁻⁷ M), but was not significantly increased by a depolarizing concentration of KCl (45 mM). In contrast, KCl increased media [Met⁵]enkephalin (ME) immunoreactivity 2-fold in these cultures, equivalent to stimulation with Fsk or PMA. The possibility was examined that ME or another opioid produced by the cultures selectively inhibited the TH response to KCl. Pretreatment with the opioid receptor antagonist naloxone (10⁻⁶ M) greatly increased the number of TH-immunoreactive neurons observed in response to KCl treatment, but had no effect on basal or Fsk-stimulated TH immunostaining, nor on basal or stimulated ME release. The increase in TH-immunoreactivity observed with combined KCl plus naloxone treatment was prevented by pretreating the cultures with the calcium channel blocker nimodipine (10⁻⁶ M), which had no effect on Fsk stimulation or basal TH immunostaining. These data suggest that endogenous opioids selectively inhibit KCl-stimulated Ca²⁺ entry and thus TH induction in olfactory bulb cell cultures. These cultures offer a simple model system for further study of TH regulation in dopaminergic neurons.