Synaptic morphology of substance P terminals on catecholamine neurons in the commissural subnucleus of the nucleus tractus solitarii in the rat

Abstract

The ultrastructure of substance P-containing nerve terminals synapsing on catecholamine neurons in the rat commissural subnucleus of the nucleus tractus solitarii (NTScom) was studied using a double immunocytochemical labeling technique. Although there were numerous tyrosine hydroxylase-immunoreactive (TH-I) somata present, substance P immunoreactive (SP-I) cell bodies were only occasionally found in the NTScom. At the light microscopic level, many SP-I terminals were seen closely associated with TH-I dendrites and somata. At the electron microscopic level, SP-I terminals synapsing on TH-I structures were also readily encountered. SP-I terminals contained small, clear, and predominantly spherical vesicles (32 +/- 4 nm diameter), as well as large dense-cored vesicles approximately 100 nm in diameter. Postsynaptic TH-I dendritic profiles of various calibers and somata were encountered. These postsynaptic TH-I structures often showed postsynaptic densities. The morphological features of the SP-TH synapses in the present study, that is, the size of synaptic vesicles and the presence of postsynaptic densities, are quite different from those of central carotid sinus afferent synapses reported in our previous study [Chen et al. (1992), J. Neurocytol., 21:137-147]. Therefore, most of the SP terminals of the SP-TH synapses in the NTScom appear not to originate from the carotid sinus afferents. SP-I second-order neurons of the carotid sinus afferent pathway [Chen et al. (1991), J. Auton. Nerv. Syst., 33:97-98] may be one of the possible sources of such terminals.

Respiratory action of capsaicin microinjected into the nucleus of the solitary tract: involvement of vanilloid and tachykinin receptors

“…There are at least two distinct types of SP‐containing terminals in the cNTS ( Kalia et al ., 1984 ; Kubota et al ., 1985 ; Chen et al ., 1994 ). One source of SP is undoubtedly the central terminals of chemoreceptor, baroreceptor and/or pulmonary afferents, whereas the remainder may represent interneurones or the terminals of projections from other brain stem regions, including the parabrachial nucleus, ventrolateral medulla regions of the raphe ( Helke & Hill, 1988 ; Chen et al ., 1994 ).…”

“…There are at least two distinct types of SP‐containing terminals in the cNTS ( Kalia et al ., 1984 ; Kubota et al ., 1985 ; Chen et al ., 1994 ). One source of SP is undoubtedly the central terminals of chemoreceptor, baroreceptor and/or pulmonary afferents, whereas the remainder may represent interneurones or the terminals of projections from other brain stem regions, including the parabrachial nucleus, ventrolateral medulla regions of the raphe ( Helke & Hill, 1988 ; Chen et al ., 1994 ). In addition, the adult rat NTS possesses a very high density of [ 125 I]‐SP binding sites ( Quirion & Dam, 1986 ; Mazzone et al ., 1997 ) and microinjection of SP into the NTS of anaesthetized rats stimulates ventilation ( Chen et al ., 1990 ; Mazzone et al ., 1998 ).…”

“…The NTS receives input from many sources, including peripheral chemo‐ and baroreceptor, and vagal pulmonary afferents, i.e., slowly adapting stretch receptors (SARs), rapidly adapting (irritant) receptors and J‐receptors ( Kalia & Mesulam, 1980 ; Jordan & Spyer, 1986 ). Many of these afferent fibres contain tachykinins, inlcuding substance P (SP) and neurokinin A (NKA), and excitatory amino acids (EAAs) in their central terminals ( Kalia et al ., 1984 ; Helke & Hill, 1988 ; Chen et al ., 1994 ; Mizusawa et al ., 1994 ). SP‐preferring neurokinin 1 (NK 1 ) and NKB‐preferring NK 3 receptors have been identified in the NTS ( Stoessl & Hill, 1990 ; Nakaya et al ., 1994 ; Ding et al ., 1996 ; Mazzone et al ., 1997 ).…”

Ultrastructure of substance P-immunoreactive terminals and their relation to vascular smooth muscle cells of rat small mesenteric arteries

“…This contrasts with the arrangement of SP-containing terminals around rat cerebral arteries, where several types of peptidergic axons (including SPϩ ones) form junctions with an intervening single basal lamina (Matsuyama et al, 1985) similar to those formed by noradrenergic axons in most other vessels (Luff, 1996). In the central nervous system and peripheral ganglia, SPϩ terminals form synapses with conventional pre-and post-synaptic specialisations with other nerve processes (Kummer, 1992;Chen et al, 1994).…”