Neurokinin-1 receptor immunoreactivity in hypotension sensitive sympathetic preganglionic neurons

Axonal Injury Induces ATF3 in Specific Populations of Sacral Preganglionic Neurons in Male Rats

Compared to other neurons of the central nervous system, autonomic preganglionic neurons are unusual because most of their axon lies in the periphery. These axons are vulnerable to injury during surgical procedures, yet in comparison to peripheral neurons and somatic motor neurons, the impact of injury on preganglionic neurons is poorly understood. Here, we have investigated the impact of axotomy on sacral preganglionic neurons, a functionally diverse group of neurons required for micturition, defecation, and sexual function. We have previously observed that after axotomy, the injury-related transcription factor activating transcription factor-3 (ATF3) is upregulated in only half of these neurons (Peddie and Keast, 2011: PMID: 21283532). In the current study, we have investigated if this response is constrained to particular subclasses of preganglionic neurons that have specific functions or signaling properties. Seven days after unilateral pelvic nerve transection, we quantified sacral preganglionic neurons expressing ATF3, many but not all of which co-expressed c-Jun. This response was independent of soma size. Subclasses of sacral preganglionic neurons expressed combinations of somatostatin, calbindin, and neurokinin-1 receptor, each of which showed a similar response to injury. We also found that in contrast to thoracolumbar preganglionic neurons, the heat shock protein-25 (Hsp25) was not detected in naive sacral preganglionic neurons but was upregulated in many of these neurons after axotomy; the majority of these Hsp25 neurons expressed ATF3. Together, these studies reveal the molecular complexity of sacral preganglionic neurons and their responses to injury. The simultaneous upregulation of Hsp25 and ATF3 may indicate a distinct mechanism of regenerative capacity after injury.

Elimination of rat spinal substance P receptor bearing neurons dissociates cardiovascular and nocifensive responses to nicotinic agonists

Intrathecal (IT) delivery of nicotinic agonists evokes dose dependent nocifensive behavior and cardiovascular responses. Previous studies suggested that these effects may be attenuated by the loss of substance P positive (sP(+)) primary afferents. To further characterize these cell systems, we examined the effect of selectively destroying neurokinin 1 receptor bearing (NK1-r(+)) dorsal horn neurons on IT nicotinic agonist evoked responses. In the dorsal spinal cord, confocal immunohistochemical microscopy revealed that nAChR subunits (alpha3, alpha4, alpha5, beta2 and beta4), NeuN B (neuronal marker) and NK1-r were all co-expressed in the superficial dorsal horn; however alpha3, alpha5, beta2 and beta4 exhibited the highest degree of colocalization with NK1-r expressing neurons. After intrathecal substance P-saporin (sP-SAP), NK1-r(+) cell bodies and dendrites in the superficial dorsal horn were largely abolished. The greatest loss in co-expression of nAChR subunits with NK1-r was observed with alpha3, alpha5, beta2 and beta4 subunits. Following intrathecal sP-SAP, the nocifensive responses to all nicotinic agonists were reduced; however, in contrast, while cardiovascular responses evoked by IT nicotine were unaltered, IT cytisine and epibatidine exhibited enhanced tachycardia and pressor responses. These results indicate subunit-specific relationships between the NK1-r and nicotinic receptor systems. The loss of nocifensive activity after destruction of the NK1-r bearing cells in spite of the persistence of nicotinic subunits on other cells, emphasizes the importance of the superficial marginal neuron in mediating these nicotinic effects. Further, the exaggerated cardiovascular responses to cytisine following loss of NK1-r bearing cells suggest the presence of a nicotinic receptor-mediated stimulation of inhibitory circuits at the spinal level.

Immunoreactivity for cocaine- and amphetamine-regulated transcript in rat sympathetic preganglionic neurons projecting to sympathetic ganglia and the adrenal medulla

Many sympathetic preganglionic neurons (SPN) in the intermediolateral cell column (IML) contain cocaine- and amphetamine-regulated transcript (CART), but the function of these CART-immunoreactive (IR) neurons is unknown. To test the possibility that CART might mark SPN involved in cardiovascular regulation, we first established whether all CART neurons in the spinal cord were SPN by double-immunofluorescent labelling for CART and choline acetyltransferase (ChAT). All autonomic subnuclei contained SPN immunoreactive for ChAT plus CART. Occasional ChAT-negative, CART-positive neurons occurred adjacent to the IML, indicating the existence of CART-IR interneurons. We then retrogradely labelled SPN with cholera toxin subunit B (CTB) from a variety of targets and used double immunofluorescence to detect CTB and CART. Among SPN in the IML, 43% projecting to the coeliac ganglion, 34% projecting to the major pelvic ganglion, and about 15% projecting to the superior cervical ganglion or adrenal medulla contained CART. CART also occurred in most SPN projecting to the major pelvic ganglion from either the central autonomic area (63%) or the intercalated nucleus (58%). Finally, we used drug-induced hypotension in conscious rats to evoke Fos immunoreactivity in barosensitive SPN and immunostained to reveal Fos and CART. CART immunoreactivity was present in 41% of the Fos-IR barosensitive neurons, which were concentrated in the IML of segments T5-T13. CART-positive, Fos-negative neurons also occurred in the same segments. These results indicate that CART occurs in barosensitive SPN, nonbarosensitive SPN, and interneurons. Thus, CART is not an exclusive marker for cardiovascular SPN but is likely to influence many autonomic activities.

Upregulation of tachykinin NK-1 and NK-3 receptor binding sites in the spinal cord of spontaneously hypertensive rat: impact on the autonomic control of blood pressure

1 Effects of intrathecally (i.t.) injected tachykinin NK-1 and -3 receptor agonists and antagonists were measured on mean arterial blood pressure (MAP) and heart rate (HR) in awake unrestrained spontaneously hypertensive rats (SHR,15-week-old) and age-matched Wistar Kyoto rats (WKY). Quantitative in vitro autoradiography was also performed on the lower thoracic spinal cord of both strains and Wistar rats using specific radioligands for NK-1 receptor ([(125)I]HPP[Arg(3),Sar(9),Met(O(2))(11)]SP (3-11)) and NK-3 receptor ([(125)I]HPP-Asp-Asp-Phe-N-MePhe-Gly-Leu-Met-NH(2)). 2 The NK-1 agonist [Sar(9),Met(O(2))(11)]SP (650 and 6500 pmol) decreased MAP and increased HR in WKY. The fall in MAP was blunted in SHR and substituted by increases in MAP (65-6500 pmol) and more sustained tachycardia. The NK-3 agonist senktide (6.5-65 pmol) evoked marked increases in MAP and HR (SHR>>>WKY), yet this response was rapidly desensitized. Cardiovascular effects of [Sar(9),Met(O(2))(11)]SP (650 pmol) and senktide (6.5 pmol) were selectively blocked by the prior i.t. injection of LY303870 (NK-1 antagonist, 65 nmol) and SB235375 (NK-3 antagonist, 6.5 nmol), respectively. Antagonists had no direct effect on MAP and HR in both strains. 3 Densities of NK-1 and -3 receptor binding sites were significantly increased in all laminae of the spinal cord in SHR when compared to control WKY and Wistar rats. The dissociation constant was however not affected in SHR for both NK-1 (K(d)=2.5 nM) and NK-3 (K(d)=5 nM) receptors. 4 Data highlight an upregulation of NK-1 and -3 receptor binding sites in the thoracic spinal cord of SHR that may contribute to the hypersensitivity of the pressor response to agonists and to the greater sympathetic activity seen in this model of arterial hypertension.

Preprotachykinin A mRNA is colocalized with tyrosine hydroxylase-immunoreactivity in bulbospinal neurons

Previous studies have generated controversy about the extent of co-localization between substance P- and catecholamine-containing neurons that project to the spinal cord. In earlier studies, estimates using immunofluorescence after colchicine have ranged from almost all, to almost none. We sought to resolve this issue by combining in situ hybridization and immunofluorescence. Catecholamine (A1 to A7, C1 to C3; tyrosine hydroxylase immunoreactive) neurons in the rat brainstem were examined to determine their content of mRNA for the preprotachykinin-A gene. In the A1 to A7 and the C1 to C3 cell groups, preprotachykinin-A mRNA was found only in substantial amounts in the C1-C3 cell groups. On average 20.9+/-0.9% (234/1120, n=3) of rostral C1 neurons contained preprotachykinin-A mRNA. Co-localization was also observed in C2 and C3 neurons to a similar extent. Retrograde tract-tracing with cholera toxin B subunit was used to identify bulbospinal neurons and 17.9+/-2.7% (96/529 cells) of the bulbospinal tyrosine hydroxylase-containing neurons of the rostral C1 cell group were found to contain preprotachykinin-A mRNA. In addition a new population of non-catecholaminergic bulbospinal preprotachykinin-A neurons is described in an area corresponding to the recently described caudal pressor area. To confirm that the preprotachykinin-A mRNA observed in cells in the medulla was converted to protein, dual immunofluorescence for fiber labeling at the confocal level was carried out. This confirmed colocalization of substance P and tyrosine hydroxylase in the intermediolateral cell column, but nowhere else, in a small number of cases. The results provide evidence for a much larger population of substance P/neurokinin A containing neurons in the brainstem than was previously suspected. Furthermore, many of these neurons are catecholaminergic and spinally projecting. The specific sympathetic outflow that these neurons influence remains to be determined.

Control of postganglionic sympathetic efferent fibers by neurokinin 1 receptors in rats

Substance P (SP) is released peripherally from nociceptive terminals and has a direct effect on vascular cells. The present study suggests an indirect effect as well. In normal rats and rats with one hindpaw inflamed for 48 h following intraplantar injection of complete Freund's adjuvant, neurokinin 1 (NK1) receptors were immunohistochemically localized on postganglionic sympathetic axons. The percentage of NK1-labeled axons in the gray rami from normal rats was 35.0+/-5.0; in inflamed rats this percentage increased significantly to 49.1+/-2.3. These data suggest a sensory-sympathetic reflex, where SP activates these peripheral sympathetic receptors with subsequent release of noradrenaline and other compounds to affect vascular cells indirectly. This control is enhanced after inflammation.

Evidence for a GABA(B) receptor component in the spinal action of Substance P (SP) on arterial blood pressure in the awake rat

1 The activation of tachykinin NK(1) receptors in the rat spinal cord produced a transient drop in arterial blood pressure followed by a more prolonged pressor effect which is mediated by the stimulation of the sympatho-adrenal system. This study aims at characterizing the spinal mechanism of that initial hypotension occurring in awake unrestrained rats. 2 The initial hypotension (-18+/-2.0 mmHg at 1 min) and the tachycardia (110+/-10 b.p.m.) produced by the intrathecal (i.t.) injection of the stable NK(1) receptor agonist [Sar(9), Met(O(2))(11)]-SP (Sar9, 0.65 nmol) at T-9 spinal cord level was inhibited by the prior injection of 65 nmol LY306740 or LY303870 (NK(1) receptor antagonists). No inhibition was seen when a similar dose of antagonists was given intravenously. 3 The prior i.t. injection of the GABA(B) receptor antagonist CGP52432 (100 nmol) reduced the hypotension evoked by Sar9 (0.65 nmol) and by the GABA(B) receptor agonist baclofen (100 nmol). The GABA(A) receptor antagonist bicuculline (25 nmol, i.t.) was without effect against Sar9, and the GABA(A) agonist muscimol (100 nmol, i.t.) had no cardiovascular effect. 4 The putative involvement of other mediators (dopamine, serotonine, glycine and glutamate) in Sar9-induced hypotension was made unlikely on the basis of various pharmacological treatments. Thus data, suggest that the transient hypotension which occurs upon the activation of NK(1) receptors in the spinal cord is due to the release of GABA which in turn activates GABA(B) receptors to inhibit sympathetic pre-ganglionic fibres. This mechanism may have a physiological significance in the spinal reflex autonomic control of arterial blood pressure.