The central pressor actions of substance P are inhibited by GABA

Role of limbic peptidergic circuits in regulation of arterial pressure, relevant to development of essential hypertension

It is generally accepted that the essential hypertension (EH) is caused by interactions among congenital gene, multiple pathogenetic pressor factors, and disorder of physiologic depressor factors. The central nervous system may play a key role in the development of EH. The underlying mechanisms, however, are not well understood. Studies show that peptidergic transmitters in the limbic forebrain are involved in long-term regulation of arterial pressure and in the pathogenesis of EH. In the limbic forebrain there are peptidergic pressor and depressor circuits. The former includes corticotropin releasing factor-, substance P-, and angiotensin II-circuits; and the latter includes beta-endorphin- and atrial natriuretic peptide-circuits. These circuits extensively interconnect and interact with each other. The altered functions of them may be the pathogenesis of EH. In this review, we focus on the roles of limbic peptidergic circuits in regulation of arterial pressure, relevant to the neurogenetic mechanisms in developing EH.

Corticotropin releasing factor and substance P mediate the nucleus amygdaloideus centralis-nucleus ventromedialis-nucleus dorsomedialis pressor system

Prolonged emotional stress is an important factor in the development of neurogenic hypertension, but its mechanism is still unclear. The purpose of the present study is to analyze the possible neural basis of hypertension induced by prolonged emotional stress. In the brain many nuclei are involved in emotional reaction, stress or defense response; among them the nucleus amygdaloideus centralis (AC) is the most important one which widely connects with other nuclei controlling emotion and stress, such as nucleus ventromedialis (NVM), nucleus dorsomedialis (NDM), nucleus paraventricularis (NPV) etc. These nuclei contain corticotropin releasing factor (CRF)- and substance P (SP)-immunoreactive cell bodies, nerve terminals and corresponding receptors. Our previous and present studies showed that microinjection of CRF or SP into these nuclei induced pressor responses. These data imply that excitation of the AC can activate many nuclei controlling emotion and stress via CRF and SP, and excessive activities of these nuclei may be the neural basis of hypertension induced by prolonged emotional stress. The present study revealed that (1) the AC pressor response to glutamate (Glu) could be reduced by preinjection of CRF antagonist (alpha-Helical CRF[9-41] or SP antagonist ([D-Pro(2), D-Phe(7), D-Trp(9)]-substance P) into bilateral NVM, (2) the NVM pressor response to Glu were decreased by pretreatment of the NDM with CRF- or SP-antagonist, (3) the AC-, NVM- or NDM-pressor responses were all attenuated by preinjection of CRF- or SP-antagonist into bilateral NPV or rostral ventrolateral medulla (RVL). The results indicate that excitation of the AC can indirectly activate the NPV and RVL to evoke pressor response via the NVM-NDM, CRF and SP are transmitters in each connection of this pathway; this is one component of the mechanism underlying the AC pressor response. Taken together with the findings of our previous studies, it provides neurophysiological basis for the above-mentioned implications.

Role of corticotropin-releasing factor and substance P in pressor responses of nuclei controlling emotion and stress

The wide distribution of corticotropin-releasing factor (CRF) and substance P (SP)-immunoreactive cell bodies, nerve terminals and corresponding receptors in pressor nuclei controlling emotion and stress implies that CRF and SP may play important roles in pressor responses of these nuclei; hence CRF or SP was microinjected into these nuclei respectively in Wistar male rats anesthetized with urethane to test this possibility. Microinjection of CRF into nucleus amygdaloideus centralis, nucleus paraventricularis, nucleus ventromedialis, lateral hypothalamus-perifornical region, periaqueductal gray matter, nucleus parabrachialis, locus coeruleus or rostral ventrolateral medulla respectively could evoke pressor responses (but CRF injection into nucleus dorsomedialis could not elicit significant pressor responses). Injection of substance P into all the above nuclei could also elicit hypertensive responses of different magnitudes, whereas normal saline injection into these nuclei had no effect. These results indicate that both CRF and SP in the above mentioned nuclei may play important roles in hypertension induced by prolonged emotional stress.

Further localization of cardiovascular and behavioral actions of substance P in the rat brain

Cardiovascular and behavioral actions of substance P (SP) were examined after microinjection into the medial preoptic area (MPO), anterior hypothalamic area (AH), and ventral tegmental area (VTA) in conscious unrestrained rats. SP elicited marked increases in mean arterial pressure and heart rate as well as stereotyped behaviors of excessive grooming and exploring when injected into the MPO or AH. In the MPO, the latencies to the cardiovascular responses were observed after SP injection into the VTA. These results, together with our previous results, suggest that SP acts as transmitter or modulator in the rostral hypothalamic areas to elicit cardiovascular defense responses. In contrast, SP may not be involved in causing a defense reaction in the more caudal areas of the defense center.

Glycine elicits release of acetylcholine from the nucleus tractus solitarii in rat

Previous studies have suggested that cardiovascular responses elicited by injection of glycine into the nucleus tractus solitarii (NTS) depend upon interactions between glycinergic and cholinergic neuronal elements in NTS. Release of acetylcholine in response to glycine is one such interaction that has been shown in slices of hippocampus and striatum. In this study we sought to test the hypothesis that glycine causes release of acetylcholine from neurotransmitter stores in NTS. We compared release from NTS with that from adjacent hypoglossal nucleus and from caudate nucleus. Release of radiolabeled acetylcholine was determined in vitro after incubating NTS with [3H]choline. Exposure of NTS and caudate nucleus, but not hypoglossal nucleus, to glycine caused release of acetylcholine in a calcium-dependent manner that varied with concentration of glycine in the incubation medium. The maximally effective concentration (1 mM) of glycine elicited 136% increases over basal levels. Glycine did not elicit release of [3H]acetylcholine from tissue when calcium ion had been removed from the bath. Acetylcholine also was not released if tissue was incubated with either strychnine (10 microM) or hemicholinium-3 (1 mM) prior to exposure to glycine (1 mM). Thus, glycine, acting at strychnine-sensitive receptors in NTS, elicits release of acetylcholine from a portion of locally synthesized neurotransmitter stores.

Inhibition by morphine of the cardiovascular and behavioral responses evoked by centrally administered substance P in conscious rats

The effect of endogenous opioid receptor stimulation on the central cardiovascular and behavioral actions of substance P (SP) was examined in conscious rats. SP (55 pmol) injected intracerebroventricularly (i.c.v.) elicited increases in mean arterial pressure, heart rate, and stereotyped behavioral activation such as exploring and grooming, which were considered to be parts of the cardiovascular defense reaction. Intravenous (i.v.) pretreatment with morphine (2.5 and 5.0 mg/kg) attenuated the cardiovascular and behavioral responses produced by SP i.c.v. dose-dependently. The i.v. pretreatment with naloxone (10 mg/kg) had no effect on the central SP-induced response. Pressor responses elicited by i.c.v. injection of corticotropin-releasing factor or angiotensin II were also attenuated by pretreatment with i.v. morphine (5.0 mg/kg). Our results showed that endogenous opioid receptor stimulation antagonizes the central cardiovascular and behavioral actions of SP. Morphine may not influence the primary site of action of SP but does influence the central neural pathway which conveys the SP-induced sympathetic activation signal.

Identification of the central tachykinin receptor subclass involved in substance P-induced cardiovascular and behavioral responses in conscious rats

To identify the tachykinin receptor subclass involved in the central cardiovascular and behavioral actions of substance P (SP), we compared the central actions of SP with those of neurokinin A (NKA) and senktide in conscious chronically instrumented rats. Intracerebroventricular (i.c.v.) injection of SP (an NK1 agonist) and NKA (an NK2 agonist) increased mean arterial pressure (MAP) and heart rate (HR) dose dependently and these cardiovascular responses were associated with the behavioral responses, comprising excessive grooming and exploring. Both peptides were equipotent to produce the cardiovascular and the behavioral responses. Senktide (a highly selective NK-3 agonist), injected i.c.v. increased the HR markedly. The behavioral response, 'wet dog shakes', was observed most frequently after senktide and was dissociated from the HR response. Pretreatment with a peripheral NK-1-selective antagonist, L-668,169, attenuated the NKA-induced cardiovascular and behavioral responses but not the SP-induced responses. However, pretreatment with a peripheral NK-2-selective antagonists, L-659,877, attenuated the SP-induced responses but not the NKA-induced responses. These results suggest that the central cardiovascular and behavioral actions of SP and NKA are mediated by different subclasses of receptors and that the receptor subclasses which are specific for the central nervous system differ from those which mediate the peripheral actions of the two tachykinins.

Localization of the substance P-induced cardiovascular responses in the rat hypothalamus

Intracerebroventricular injection of substance P (SP) has been reported to induce a typical cardiovascular defense response characterized by an increase in blood pressure, heart rate, sympathetic efferent activity, hindlimb vasodilatation and mesenteric vasoconstriction. In this study we employed microinjections of SP to localize the hypothalamic areas in which SP elicits the activation of the cardiovascular system. SP (550 pmol) injected into the anterior hypothalamus (AH) produced, after a short latency, a marked increase in mean arterial pressure and heart rate. In the ventromedial hypothalamus, the magnitude of the cardiovascular response to SP was identical to that in the AH, but the response was delayed. SP injected into the posterior hypothalamus failed to induce any cardiovascular response. These results suggest that the anterior and ventromedial parts of the hypothalamus are responsible for eliciting the central cardiovascular effects of SP in conscious rats.

Neurochemical mediators of the behavioural effects of receptor-selective substance P agonists administered intrathecally in the rat

Recently, two compounds have been developed, designated septide and senktide, which are highly selective agonists for the substance P receptor, types NK-1 and NK-3, respectively. Each of these, when injected intrathecally in awake rats, produced a distinct and non-overlapping constellation of sensory and behavioural effects which were subsets of the symptoms evoked by intrathecal administration of substance P. Prior systemic administration of 5-hydroxytryptamine (5-HT), alpha-adrenergic and opiate receptor antagonists, at doses sufficient to block the behavioural effects of the corresponding receptor agonists, did not alter responses to intrathecally injected septide or senktide. This was so, even for symptoms which suggested inhibitory mediation, hypoalgesia and (transient) motor flaccidity. Septide and senktide, administered by lumbar puncture and by indwelling catheter, produced identical results. Finally, in contrast to some other peptides, flaccid paralysis induced by senktide was not accompanied by spinal necrosis.

Brain neuropeptides: actions on central cardiovascular control mechanisms

The many peptides we have not considered (e.g. gastrin, motilin, FMRFamide, carnosine, litorin, dermorphin, casomorphin, eledoisin, prolactin, growth hormone, neuromedin U, proctolin, etc.) were omitted due to lack of information as far as any putative central cardiovascular effects are concerned. However, even for some of these peptide pariahs intriguing snippets of information are available now (e.g. ref. 85), although as we write, the list of possible candidates for investigation grows longer. On an optimistic note, it is becoming clear that many brain neuropeptides may have important effects on cardiovascular regulation. It seems feasible that 'chemically coded' pathways in the brain might be the neuroanatomical correlate of a 'viscerotopic' organization of cardiovascular control mechanisms, whereby the activity of the heart and flows through vascular beds are individually controlled, but in an integrated fashion, utilizing particular combinations of neurotransmitters and neuropeptides within the brain. Such possibilities can only be investigated, properly, by measurement of changes in cardiac output and regional haemodynamics in response to appropriate interventions, in conscious, unrestrained animals.

Preganglionic sympathetic neurones innervating the rat adrenal medulla: immunocytochemical evidence of synaptic input from nerve terminals containing substance P, GABA or 5-hydroxytryptamine

Sympathetic preganglionic neurones that innervate the adrenal medulla were identified for subsequent light and electron microscopic study by the retrograde transport of horseradish peroxidase (HRP) or a conjugate of HRP and cholera B-chain. Most labelled neurones were found in the intermediolateral column, but some occurred in the intercalated nucleus and in the lateral funiculus of the thoracic spinal cord. Three morphologically distinct types of neurone were retrogradely labelled, two of which had dendrites that extended medially towards the central canal and laterally across the entire lateral funiculus. A combination of retrograde labelling with pre-embedding immunocytochemistry allowed us to demonstrate synaptic contacts between boutons immunoreactive for substance P or 5-hydroxytryptamine (5-HT) and the cell bodies or proximal dendrites of sympathoadrenal neurones. The 5-HT-immunoreactive boutons appeared to be of two morphologically distinct types. Postembedding immunocytochemistry enabled us to show that sympathoadrenal neurones receive a heavy synaptic innervation from GABA-immunoreactive boutons: 32% of a random series of boutons in synaptic contact with cell bodies were GABA-immunoreactive. Proximal dendrites and also distal dendrites within the white matter were ensheathed in synaptic boutons, 37% of which were GABA-immunoreactive. It is concluded that sympathoadrenal neurones receive at least 4 distinct types of afferent synaptic input: from neurones containing substance P, or GABA and from two types of neurones containing 5-HT. The presence of synaptic inputs on distal dendrites that extend across the white matter adds further complexities to the control of the activity of sympathetic preganglionic neurones.