Topographic relationship of neurotensin-containing axon terminals with cardiac and noncardiac principal ganglion cells in the stellate ganglia of the cat

Occurrence, co-occurrence and topographic distribution of choline acetyl transferase, Met-enkephalin and neurotensin in the stellate ganglion of the cat

The presence of the classical ganglionic transmitter acetylcholine (ACh), its occurrence and possible co-occurrence with the neuromodulator peptides methionine enkephalin (Met-ENK) and neurotensin (NT), as well as the possible coexistence of these peptides in the preganglionic axon terminals of the cat stellate ganglia were investigated with light and confocal microscopy using immunofluorescence. Choline acetyltransferase (ChAT), Met-ENK, and NT immunoreactivity was detected with light microscopy in axon terminals near tyrosine hydroxylase (TH) immunoreactive (IR) cells. Cell bodies immunopositive for ChAT or Met-ENK were also detected and were TH-negative or TH-positive. Denervation by sectioning preganglionic axons produced two effects: the almost complete elimination of IR fibers and an increase in the number of ChATIR and Met-ENKIR cell bodies, together with the appearance of NTIR cell bodies. Preganglionic ChATIR fibers and boutons form a dense network throughout the entire ganglion, with a homogeneous regional distribution. ChAT, Met-ENK, and NT are essentially stored in different nerve endings, although a low level of co-occurrence was detected. NTIR and Met-ENKIR networks of boutons were observed to have independent and somewhat complementary regional distributions. Further analysis with simultaneous triple labeling for NT, Met-ENK, and TH, and confocal microscopy showed fibers and boutons containing Met-ENK or NT reached distinct neurons separately, or both converge onto the same cells. This finding suggests that modulation (the facilitation-inhibition balance) of ganglionic transmission is achieved mainly by the selective and complementary innervation of boutons containing NT (facilitation) and Met-ENK (inhibition) and only rarely by terminals which coexpress both peptides.

Release-depletion and receptor-mediated neuronal internalization of endogenous neurotensin in the stellate ganglion of the cat

The release and depletion of neurotensin in sympathetic preganglionic axon terminals and internalization in principal ganglion cells were investigated in the cat stellate ganglion by means of combined immunohistochemical staining, image analysis and confocal microscopy. Neurotensin stored in preganglionic boutons was released by 40 or 5 Hz electrical stimulation of preganglionic nerves, being depleted to 7.4 and 19.2% of control levels by continuous stimulation lasting 20 or 160 min (both stimuli delivered 48,000 pulses). Once released, neurotensin was internalized by the principal ganglion cells as evidenced by a ring of bright spot-like granules in the perinuclear region indicating the sites of intracellular neurotensin accumulation. Neurotensin internalization was time-dependent, thus, different content was found when the time between the end of stimulation and start of perfusion was varied. The onset of neurotensin internalization appeared in the first minutes, intracellular accumulation was evident at 20 min, maximal internalization occurred at 120 min and, 24 h later internalized neurotensin content had faded. Internalization was partially blocked by the nonpeptide neurotensin antagonist SR48692. These data provide evidence of presynaptic neurotensin release and depletion by electrical stimulation with varied frequencies. They also provide evidence for in situ receptor-mediated internalization of endogenously released neurotensin, raising the possibility that internalization may represent, in addition to some kind of turnover dynamics, an important part of the mechanisms of neuropeptide signaling.