Inhibition of isotonic sodium chloride intake in the rat by selective tachykinin agonists

Expression of tachykinin3 and related reproductive markers in the brain of the African cichlid fish Astatotilapia burtoni

Neurokinin B, encoded by the tachykinin3 gene, plays a crucial role in regulating reproduction in mammals via KNDy neurons and interaction with GnRH. Previous work in teleost fishes has focused on hypothalamic tac3 expression for its role in reproduction, but detailed studies on extra-hypothalamic tac3 expression are limited. Here, we identified two tac3 genes in the social African cichlid fish Astatotilapia burtoni, only one of which produces a functional protein containing the signature tachykinin motif. In situ hybridization for tac3a mRNA identified cell populations throughout the brain. Numerous tac3a cells lie in several thalamic and hypothalamic nuclei, including periventricular nucleus of posterior tuberculum, lateral tuberal nucleus (NLT), and nucleus of the lateral recess (NRL). Scattered tac3-expressing cells are also present in telencephalic parts, such as ventral (Vv) and supracomissural (Vs) part of ventral telencephalon. In contrast to other teleosts, tac3 expression was absent from the pituitary. Using double-fluorescent staining, we localized tac3a-expressing cells in relation to GnRH and kisspeptin cells. Although no GnRH-tac3a colabeled cells were observed, dense GnRH fibers surround and potentially synapse with tac3a cells in the preoptic area. Only minimal (<5%) colabeling of tac3a was observed in kiss2 cells. Despite tac3a expression in many nodes of the mesolimbic reward system, it was absent from tyrosine hydroxylase (TH)-expressing cells, but tac3a cells were located in areas with dense TH fibers. The presence of tac3a-expressing cells throughout the brain, including in socially relevant brain regions, suggest more diverse functions beyond regulation of reproductive physiology that may be conserved across vertebrates.

Intraventricular injections of tachykinin NK3 receptor agonists suppress the intake of “salty” tastes by sodium deficient rats

Intraventricular injections of the tachykinin NK3 receptor (NK3-R) agonist, senktide, suppress the ingestion of hypertonic (0.5 M) NaCl by decreasing the initial lick rate and accelerating the decay in lick rate in sodium deficient rats. The present experiment examined whether the effects of intraventricular injections of senktide on lick rate were selective for NaCl solution, or if the ability of NK3-R agonists to inhibit intake generalizes other sodium-containing solutions. The effects of lateral ventricular injections of isotonic saline or senktide (200 ng) on intake and lick rate of 0.5 M solutions of sodium chloride (NaCl), sodium acetate (Na acetate), sodium bicarbonate (Na bicarbonate), and monosodium glutamate (MSG) were measured in sodium deficient rats. Compared to saline injection, senktide injection had no effect on the lick rate or intake of Na bicarbonate. In contrast, intraventricular injection of senktide suppressed the intake of NaCl, Na acetate, and MSG compared to saline injection. Senktide injection accelerated the decay in lick rate for NaCl, Na acetate and MSG, but only suppressed the initial lick rate for NaCl and Na acetate. The results show that activation of NK3-R in sodium deficient rats suppresses the intake of tastes that are classified as "salty" tasting and that the decrease in intake reflects effects on the initial lick rate, the decay in lick rate, or both.

Distribution of Fos-like immunoreactivity within the rat brain following intraventricular injection of the selective NK(3) receptor agonist senktide

Neurokinin B (NKB) is one member of an evolutionarily conserved family of neuropeptides, the tachykinins. Preferential binding of NKB to endogenous NK(3) receptors affects a variety of biological and physiological processes, including endocrine secretions, sensory transmission, and fluid and electrolyte homeostasis. In light of its widespread biological actions, immunohistochemical detection of the c-Fos protein product was used to study the distribution of neuronal activation in the rat brain caused by intraventricular (icv) injections of the selective NK(3) receptor agonist (succinyl-[Asp(6), N-Me-Phe(8)] substance P [6-11]), senktide. Quantitative analysis revealed that treatment with isotonic saline or 200 ng senktide resulted in the differential expression of Fos-like immunoreactivity (FLI) throughout the brain. Senktide induced the highest number of FLI neurons in the lateral septum, bed nucleus of the stria terminalis, amygdala, paraventricular nucleus of the hypothalamus, median preoptic nucleus, organum vasculosum of the lamina terminalis, supraoptic nucleus, periaqueductal gray, and medial nucleus of the solitary tract compared to isotonic saline controls. Additional regions that contained elevated FLI following icv injection of senktide, relative to saline injection, included the cerebral cortex, lateral hypothalamic nucleus, suprachiasmatic nucleus, ventral tegmental area, substantia nigra, inferior colliculus, locus coeruleus, zona incerta, and arcuate nucleus. Our data indicate that activation of NK(3) receptors induces the expression of FLI within circumscribed regions of the rat brain. This pattern of neuronal activation overlaps with nuclei known to regulate homeostatic processes, such as endocrine secretion, cardiovascular function, salt intake, and nociception.

Brain tachykinins and the regulation of salt intake

The regulation of salt intake is achieved through the coordination of behavioral and physiological responses. Brain neuropeptides, such as the tachykinins, play an important role in orchestrating both of these responses. Intraventricular injections of NK3 receptor agonists, such as senktide, are potent in suppressing salt intake. Experimental results show that intraventricular injections of senktide that suppress salt intake have no effect on the ingestion of other tastes, such as sucrose. The means by which senktide suppresses salt intake was investigated in a series of experiments. Taste reactivity and lick rate analyses suggest that the activation of NK3 receptors reduces salt intake by modulating the oral-stimulating property of salt taste.

Lateral ventricular injections of the NK3 agonist senktide affect salt taste-elicited responses

Central injections of the selective tachykinin NK3 receptor agonist senktide (SENK) suppresses salt appetite. Also, following SENK, intraoral infusions of hypertonic NaCl elicit fewer ingestive taste reactivity responses and more aversive responses than following intraventricular injections of isotonic saline. This pattern of taste reactivity results suggest that SENK affects the oral stimulating properties of salt taste. Before accepting this interpretation, however, alternative explanations need to be examined. The following experiments evaluated whether the effects of intraventricular SENK injection on taste reactivity could be due to: 1) a general oral motor impairment that reduces ingestive responding to tastes (Experiment 1) or; 2) SENK having aversive consequences (Experiment 2). In Experiment 1, the effects of intraventricular injections of SENK (200 ng) on taste reactivity responses to 0.5 M NaCl and 0.1 M sucrose were measured in sodium deficient rats. Intraoral infusions of 0.5 M NaCl elicited fewer ingestive taste reactivity responses following SENK than injections of isotonic saline in sodium deficient rats. Sucrose continued to elicit the same high number of ingestive taste reactivity responses following intraventricular injections of isotonic saline and SENK. Thus, SENK did not cause a general decrease in ingestive responding. A conditioned taste aversion test was employed in Experiment 2 to determine if SENK had aversive consequences. Rats were given 30 min access to alanine (0.3 M) and were then administered either lithium chloride (LiCl) or intraventricular injections of SENK (200 ng) on three consecutive days. Rats avoided alanine that was paired with LiCl, but those rats that had alanine paired with SENK showed no avoidance of the taste even after three pairings. These results replicate findings that intraventricular injections of the NK3 agonist SENK decreases the palatability of NaCl (as measured by taste reactivity) and suggest that its effect on NaCl-elicited taste reactivity is not due to the treatment causing a motor impairment or malaise.

Effects of the selective tachykinin NK3 receptor agonist NH2-senktide on intraoral intake and taste reactivity responses elicited by NaCl in sodium-replete rats

Our previous studies have shown that i.c.v. injections of the selective tachykinin NK3 receptor agonist [Asp5,6,MePhe8]substance P(5,11), also referred to as NH2-senktide (NH2-SENK), inhibit salt intake in rats in a two-bottle intake test. The present study evaluated the effect of i.c.v. injections of NH2-SENK on intraoral intake and taste reactivity responses elicited by intraoral infusions of water or NaCl solutions (0.03, 0.15, 0.25 and 0.5 M) in sodium-replete rats. The effect of NH2-SENK on the intake of 0.03, 0.15 and 0.25 M NaCl solutions and of water was also evaluated in a two-bottle intake test. In this test, 31 ng/rat of NH2-SENK significantly reduced the intake of 0.15 and 0.25 M NaCl, but not that of water or of 0.03 M NaCl. The dose of 31 ng/rat of NH2-SENK reduced the intraoral intake of 0.25 and 0.5 M NaCl, while 125 ng/rat reduced the intraoral intake of 0.15, 0.25 and 0.5 M NaCl; neither dose reduced the intraoral intake of water or of 0.03 M NaCl. Taken together, these findings indicate that the effect of NH2-SENK on salt intake is dependent on the concentration of the NaCl solution offered; moreover, the intraoral intake data suggest that the effect of NH2-SENK on salt intake may be exerted to a large extent on the consummatory processes of salt ingestion. NH2-SENK, 31 or 125 ng/rat, altered taste reactivity responses shortly after the beginning of the intraoral infusion. The most peculiar effect was an increase in passive dripping during the first min of intraoral infusion of 0.15, 0.25 and 0.5 M, but not of water or of 0.03 M NaCl. This finding suggests that NH2-SENK elicits a prompt alteration of taste mechanisms for salt, which may account for its antinatriorexic action.