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 ac tivators 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 concentr ation of KCl (45 mM). In contrast, KCl increased media [Met(5)]enkepha lin (ME) immunoreactivity 2-fold in these cultures, equivalent to stim ulation with Fsk or PMA. The possibility was examined that ME or anoth er opioid produced by the cultures selectively inhibited the TH respon se to KCl. Pretreatment with the opioid receptor antagonist naloxone ( 10(-6) M) greatly increased the number of TR-immunoreactive neurons ob served 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 na loxone treatment was prevented by pretreating the cultures with the ca lcium channel blocker nimodipine (10(-6) M), which had no effect on Fs k stimulation or basal TH immunostaining. These data suggest that endo genous opioids selectively inhibit KCl-stimulated Ca2+ entry and thus TH induction in olfactory bulb cell cultures. These cultures offer a s imple model system for further study of TH regulation in dopaminergic neurons.