Opiate and peptide inhibition of transmitter release in parasympathetic nerve terminals

Abstract

Somatostatin, morphine, and opioids inhibit transmitter release at intact neuromuscular junctions between ciliary ganglion neurons and the choroidal smooth muscle of the chick eye. Somatostatin and morphine, however, have no effect on release from terminals on the striated muscle target of the ciliary ganglion, the iris. In neuronal terminals of both the choroid and the iris, a high-affinity Na+-dependent choline uptake-mediated ACh synthesis is present at hatching. Both tissues exhibit a basal release of 3H-ACh which is potentiated severalfold during a 5 minute incubation in 55 mM K+ Tyrodes. Fifty percent of the basal release and 100% of the stimulated release are Ca2+ dependent and probably mediated through N-like voltage-dependent Ca2+ channels. Co-incubation of the choroid with 10 microM morphine sulfate blocks approximately 90% of the stimulated release. The same effect is seen with 100 nM somatostatin, 10 microM dynorphin, and 100 microM met-enkephalin arginine phenylalanine. Preincubation of the excised choroid with pertussis toxin (200 ng/ml) reverses the inhibitory effects of both morphine and somatostatin. In contrast, 3H-ACh release from terminals in the striated iris is not affected by either morphine or somatostatin at micromolar levels. These results suggest that both opiate and somatostatin receptors are present in the choroid target and that they may act through a final common pathway to modulate ACh release via G proteins. Second messengers such as cyclic AMP or diacylglycerol do not appear to mediate these effects; neither increasing cAMP levels in terminals nor activation of protein kinase C affects evoked release or its inhibition by morphine or other neuromodulators. It is unclear whether endogenous neuromodulation occurs in this system, although somatostatin-like immunoreactivity can be demonstrated in terminals of choroid neurons.

A Nitric Oxide/Cyclic GMP‐Dependent Protein Kinase Pathway Alters Transmitter Release and Inhibition by Somatostatin at a Site Downstream of Calcium Entry

“…It is difficult to compare these data directly with the modulation of Ca" currents in acutely dissociated embryonic ciliary ganglion cell somata (Meriney et al, 1994;Pilar e t al., 1996). Therefore, w e have extended previous observations from intact nerve terminals of hatchling chickens (Gray et al, 1989(Gray et al, , 1990 to an embryonic age [stage (ST) 401 at which we could perform experiments with both intact nerve terminals and dissociated neuronal somata using freshly dissociated ciliary ganglion neurons in vitro to study somatostatin-mediated modulation of ACh release. By comparing potassiumevoked ACh release with photometric measurements of Ca" influx in acutely isolated neurons, we have hypothesized that a cytoplasmic NO-PKG pathway directly affects Ca2+-dependent secretion downstream of Ca2+ entry.…”

“…The subpopulation of ciliary ganglion neurons that express both ACh and somatostatin have been identified as the choroid neurons that innervate the vascular bed of the choroid coat in the chick eye (Epstein e t al., 1988;Gray et al, 1989Gray et al, , 1990De Stefan0 et al, 1993). Because nitric oxide (NO) is known to be a potent modulator of vascular tone (for review, see Garthwaite and Boulton, 1995), and N O synthase has been localized to ciliary ganglion neurons (Nichol et al, 1995), we were prompted to investigate a hypothesized role for NO and PKG in the somatostatin modulation of evoked ACh release from choroid nerve terminals onto vascular smooth muscle.…”

“…Choroid tissue wedges or dissociated ciliary ganglion neurons were preincubated in oxygenated Tyrode's solution containing [3H]choline (final specific activity, 8.1 Ci/mol; total choline concentration, 1 pkl) for 70 min and washed 10-15 times by centrifugation in zero Ca'+ Tyrode's solution to remove extracellular label (Gray et al, 1989(Gray et al, , 1990). Prestimulation or basal release of [3H]choline was measured in 5-min collection periods in normal Tyrode's solution and then a second basal 5-min collection period with or without drug additions.…”

“…Prestimulation or basal release of [3H]choline was measured in 5-min collection periods in normal Tyrode's solution and then a second basal 5-min collection period with or without drug additions. Drugs or peptides were always added in the last 5-min basal collection period before potassium stimulation to allow sufficient time for equilibration either with presynaptic endings within the intact choroid tissue or with dissociated cells (Gray et al, 1989).…”

“…In many cases, this inhibition appears to be membrane-delimited and not to require the participation of any known protein kinases or other soluble intracellular components (for reviews, see Hille, 1992Hille, , 1994Wu and Saggau, 1997;Dolphin, 1998). Previously, Gray et al (1989Gray et al ( , 1990) studied somatostatin-mediated modulation of acetylcholine (ACh) release at the junction of parasympathetic choroid neurons with vascular smooth muscle in the choroid coat isolated from the hatchling chick eye. These studies led to the hypothesis that somatostatin-mediated G protein activation causes a decrease in presynaptic Ca2+ current resulting in decreased ACh release.…”