The effects of reserpine treatment (10 mg/kg, i.p.) on the content of neuropeptide Y-like immunoreactivity and catecholamines were compared with the levels of mRNA coding for neuropeptide Y, tyrosine hydroxylase and phenylethanolamine N-methyltransferase in rat sympathetic neurons and adrenal gland. A reversible depletion of neuropeptide Y-like immunoreactivity was observed in the right atrium of the heart, kidney and masseter muscle, while the immunoreactive neuropeptide Y content in the stellate and lumbar sympathetic ganglia and its axonal transport in the sciatic nerve increased following reserpine. The increase in the stellate ganglion was maximal at 48 h and absent 9 days after reserpine treatment. The expression of neuropeptide Y mRNA and tyrosine hydroxylase mRNA in both the stellate and the superior cervical ganglion increased earlier than the neuropeptide Y content, with a clear cut two-fold elevation at 24 h after reserpine. The increase in both mRNAs in the superior cervical ganglion and the depletion of neuropeptide Y, but not of noradrenaline, in terminal areas was prevented after pretreatment both with a nicotinic receptor antagonist (chlorisondamine) and with surgical preganglionic denervation. A marked (75-90%) depletion of neuropeptide Y-like immunoreactivity and adrenaline in the adrenal gland, concomitant with 3-4-fold increases in neuropeptide Y mRNA and tyrosine hydroxylase mRNA expression, was present at 24 h after reserpine treatment. Also in the adrenal gland, there was a reversal of the reserpine-induced increase in neuropeptide Y mRNA and tyrosine hydroxylase mRNA and depletion of neuropeptide Y and adrenaline following splanchnic denervation. Pharmacological, ganglionic blockade prevented the depletion of neuropeptide Y and the increased expression of neuropeptide Y mRNA, but not fully, the tyrosine hydroxylase mRNA elevation. In addition, a marked decrease in phenylethanolamine N-methyltransferase mRNA levels was noted after reserpine. This decrease was reversed by denervation and by ganglionic blockade. Denervation alone led to a small but significant decrease in all mRNAs examined both in the superior cervical ganglion and the adrenal medulla. The present data suggest that the depletion of neuropeptide Y-like immunoreactivity in sympathetic nerves and in the adrenal gland after reserpine is associated with a compensatory increase in neuropeptide Y synthesis and axonal transport, most likely due to increased nicotinic receptor stimulation. Whereas the reserpine depletion of neuropeptide Y in both sympathetic nerves and adrenal gland is related to neuronal activation, adrenal but not nerve terminal depletion of catecholamines can be prevented by the ganglionic blocker chlorisondamine.4+e difference in effect of pharmacological ganglionic