Pattern of presynaptic nerve activity can determine the type of neurotransmitter regulating a postsynaptic event

The mammalian superior cervical ganglion has been the classical preparation for studying cholinergic transmission between neurones. Recently, however, evidence has been presented showing that, in addition to the postsynaptic changes mediated via nicotinic and muscarinic receptors, there is a non-cholinergic component to transmission in this ganglion, as in frog paravertebral ganglia. In the rabbit superior cervical ganglion, Ashe and Libet recorded a late, slow excitatory postsynaptic potential in response to preganglionic nerve stimulation in the presence of nicotinic and muscarinic antagonists. We have found, in the rat superior cervical ganglion, that a postsynaptic biochemical consequence of preganglionic nerve stimulation, namely the acute activation of tyrosine 3-monooxygenase (tyrosine hydroxylase, TH; EC 1.14.16.2), is mediated in part by acetylcholine and in part by a non-cholinergic neurotransmitter. The regulation of this enzyme activity is of particular interest because it catalyses the rate-limiting step in the biosynthesis of the postganglionic neurotransmitter, noradrenaline. In the present paper, we report that the relative importance of cholinergic and non-cholinergic transmission in the regulation of TH activity varies with the pattern of electrical stimulation of the preganglionic nerve trunk

Long-term regulation of tyrosine hydroxylase activity in the superior cervical ganglion in organ culture: Effects of nerve stimulation and dexamethasone

The rat superior cervical ganglion was stimulated in vitro via its preganglionic trunk and then maintained in organ culture for 2-3 days. Following nerve stimulation, the specific activity of tyrosine hydroxylase (TH) increased with a lag period of at least 12 h, and this increased enzyme activity was maintained throughout the culture period. The magnitude of the increase in TH activity depended on the frequency and duration of preganglionic nerve stimulation. This transsynaptic increase in enzyme activity could be completely blocked by the nicotinic antagonist, hexamethonium. The synthetic glucocorticoid dexamethasone, at concentrations above 1 nM, increased TH activity in unstimulated ganglia with a lag period of at least 24 h. The increase in enzyme activity produced by dexamethasone was not affected by nicotinic antagonists or by prior decentralization of the superior cervical ganglion, in contrast to the findings of previous workers. These data indicate that dexamethasone can stimulate ganglionic TH activity in the absence of cholinergic stimulation. In addition, experiments combining dexamethasone with nerve stimulation demonstrate that the steroid can potentiate the effects on TH activity of brief periods of nerve stimulation

Synergistic effects of muscarinic agonists and secretin or vasoactive intestinal peptide on the regulation of tyrosine hydroxylase activity in sympathetic neurons

Cholinergic agonists and certain peptides of the glucagon-secretin family acutely increase tyrosine hydroxylase activity in the superior cervical ganglion in vitro. The present study was designed to investigate possible interactions between these two classes of agonists in regulating catecholamine biosynthesis. Synergistic effects were found between carbachol and either secretin or vasoactive intestinal peptide in the regulation of DOPA (dihydroxyphenylalanine) synthesis. In addition, synergism was found at the level of the accumulation of cyclic adenosine monophosphate, the likely second messenger in the peptidergic regulation of tyrosine hydroxylase activity. The synergism seen with carbachol was blocked by a muscarinic, but not by a nicotinic, antagonist. Synergism was also found between bethanechol, a muscarinic agonist, and secretin, but not between secretin and dimethylphenylpiperazinium, a nicotinic agonist. Since previous immunohistochemical results suggest that vasoactive intestinal peptide and acetylcholine are colocalized in some preganglionic sympathetic neurons, the present data raise the possibility that the two might act synergistically in vivo in regulating catecholamine biosynthesis. Synergistic postsynaptic actions may be a common feature at synapses where peptides of the secretin- glucagon and acetylcholine are colocalized

Regulation of the concentration of adenosine 3',5'-cyclic monophosphate and the activity of tyrosine hydroxylase in the rat superior cervical ganglion by three neuropeptides of the secretin family

Preganglionic nerve stimulation leads to an acute elevation of tyrosine hydroxylase (TH) activity in the rat superior cervical ganglion. This effect is mediated in part by acetylcholine, acting via nicotinic receptors, and in part by a noncholinergic neurotransmitter. As a first step in an attempt to identify this noncholinergic transmitter, we have examined a number of biogenic amines, purine nucleotides, neuropeptides, and other compounds for their ability to increase TH activity. Secretin, vasoactive intestinal peptide (VIP), and PHI (a 27-amino acid peptide with an NH2-terminal histidine and a COOH-terminal isoleucine amide), all members of the secretin family of peptides, increased TH activity acutely. Human pancreatic growth hormone-releasing factor, glucagon, and gastric inhibitory peptide (three other members of this peptide family) and all other transmitter candidates tested had no effect on this enzyme activity. We have examined the possibility that this peptidergic regulation of TH activity is mediated via changes in adenosine 3',5'-cyclic monophosphate (cAMP) levels. When the six members of the secretin family were tested for their ability to increase cAMP levels in the ganglion, secretin, VIP, and PHI significantly increased this cyclic nucleotide, whereas growth hormone-releasing factor, glucagon, and gastric inhibitory peptide produced no significant effects. The rank orders of potency and of efficacy of secretin, VIP, and PHI in altering TH activity and cAMP levels were identical. Furthermore, a strong correlation was found between the cAMP level and the TH activity in individual ganglia exposed to these peptides. Finally, 8-bromoadenosine 3',5'-cyclic monophosphate and forskolin also increased TH activity. We hypothesize that cAMP is the second messenger mediating the increase in TH activity produced by these peptides. The nicotinic agonist dimethylphenylpiperazinium also increased TH activity but did not alter cAMP levels. In contrast, the ability of this nicotinic agonist to increase TH activity, but not that of secretin or VIP, was highly dependent on the calcium concentration of the medium. Since nicotinic stimulation is known to increase calcium entry into ganglion cells, we hypothesize that calcium is the second messenger mediating the increase in TH activity produced by nicotinic agonists. These results indicate that secretin, VIP, PHI, or a related peptide may play an important role in regulating catecholamine synthesis in sympathetic neurons and perhaps in regulating other cAMP-dependent processes. The data also suggest that TH activity in sympathetic ganglia is acutely regulated by more than one intracellular mechanism

Both nicotinic and muscarinic agonists acutely increase tyrosine 3-monooxygenase activity in the superior cervical ganglion

The activity of tyrosine 3-monooxygenase in rat superior cervical ganglia in vitro was measured by monitoring their rate of dopa production. Cholinergic agonists produce a rapid and reversible increase in dopa synthesis. The action of carbachol is largely inhibited by the nicotinic antagonist hexamethonium (3 mM) and is completely blocked by a combination of hexamethonium and the muscarinic antagonist atropine (6 μM). Dimethylphenylpiperazinium (1 mM), a specific nicotinic agonist, produces a 4-fold increase in dopa synthesis. The action of dimethylphenylpiperazinium is blocked by hexamethonium but not by atropine. Bethanechol (1 mM), a muscarinic agonist, causes a 2-fold increase in dopa synthesis. The action of bethanechol is inhibited by atropine but not by hexamethonium. It is concluded that tyrosine 3-monooxygenase activity in rat superior cervical ganglia can be increased by both nicotinic and muscarinic stimulation, that nicotinic stimulation can produce a greater increase than can muscarinic stimulation and that carbachol increases enzyme activity by a combination of both pathways. These cholinergic mechanisms for the acute regulation of tyrosine 3-monooxygenase may be activated in vivo by acetylcholine released from preganglionic neurons and thus may play a role in the physiological regulation of catecholamine synthesis in sympathetic ganglia