Moxonidine into the lateral parabrachial nucleus modifies postingestive signals involved in sodium intake control

N-methyl-D-aspartate glutamate receptor blockade inhibits the sensitization of sodium appetite independent of alterations in salt palatability

The intraperitoneal injection (ip) of N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine, MK-801, blocks sodium appetite sensitization in rats. Therefore, NMDA receptors seem important for ingestive behavioral adaptations to episodic or chronic periods of dehydration and salt depletion. Orofacial somatic motor responses to intraoral infusion of salt, in a salt taste reactivity test, can be an index of salt palatability. The objective of the present study was to investigate whether ip injection of MK-801 (0.15 mg/kg) alters orofacial responses to intraoral salt (0.3 M NaCl). In Experiment 1, tests were performed 24 h after subcutaneous injection of the natriuretic/diuretic furosemide and removal of ambient sodium. MK-801 inhibited salt intake by 60% average but did not alter salt palatability, compared with vehicle. In Experiment 2, MK-801 was paired with episodes of rapid-onset (1-h latency) sodium appetite, that is, in response to a combination of furosemide with low-dose captopril, or Furo/Cap. A pairing of MK-801, or its vehicle, with Furo/Cap was performed every other three days, three times. In a post-pairing trial, conducted 3 days after the last pairing, MK-801 history did not alter salt palatability in response to Furo/Cap, although it blocked, as expected, salt intake sensitization. In Experiment 3, MK-801 inhibited food intake in animals deprived of food for 24 h, and the respective meal-associated water intake by 60% average. The results suggest that the inhibitory effect of MK-801 on salt intake sensitization is not dependent on increased salt aversion, but they cannot rule out the participation of a general behavioral inhibition on such an effect.

Control of fluid intake in dehydrated rats and evolution of sodium appetite

The objective of the present work is to examine from a new perspective the existence of causal factors not predicted by the classical theory that thirst and sodium appetite are two distinct motivations. For example, we ask why water deprivation induces sodium appetite, thirst is not "water appetite", and intracellular dehydration potentially causes sodium appetite. Contrary to the classical theory, we suggest that thirst first, and sodium appetite second, designate a temporal sequence underlying the same motivation. The single motivation becomes an "intervenient variable" a concept borrowed from the literature, fully explained in the text, between causes of dehydration (extracellular, intracellular, or both together), and respective behavioral responses subserved by hindbrain-dependent inhibition (e.g., lateral parabrachial nucleus) and forebrain facilitation (e.g., angiotensin II). A corollary is homology between rat sodium appetite and marine teleost thirst-like motivation that we name "protodipsia". The homology argument rests on similarities between behavior (salty water intake) and respective neuroanatomical as well as functional mechanisms. Tetrapod origin in a marine environment provides additional support for the homology. The single motivation hypothesis is also consistent with ingestive behaviors in nature given similarities (e.g., thirst producing brackish water intake) between the behavior of the laboratory rat and wild animals, rodents included. The hypotheses of single motivation and homology might explain why hyperosmotic rats, or eventually any other hyperosmotic tetrapod, shows paradoxical signs of sodium appetite. They might also explain how ingestive behaviors determined by dehydration and subserved by hindbrain inhibitory mechanisms contributed to tetrapod transition from sea to land.

Mineral preference in rats treated with muscimol into the lateral parabrachial nucleus

Injection of muscimol, a GABA_A receptor agonist, into the lateral parabrachial nucleus (LPBN) induces 0.3 M NaCl intake in rats. In the present work, we investigated whether such an effect applies to hypertonic (0.3 M) mineral solutions in general or is selective to sodium solutions in a 240 min intake test. Muscimol injection (0.5 nmol/0.2 μL) compared to vehicle injection into the LPBN of adult hydrated rats produced a preferential ingestion of 0.3 M NaCl (25.3 ± 10.2 mL) followed by a 0.3 M NaHCO₃ intake (11.7 ± 5.6 mL), with no significant effect on water, KCl and CaCl₂ intake. Only the effect of muscimol on NaCl intake (19.0 ± 10.4 mL) persisted in cell-dehydrated rats, with hardly any effect on water or other mineral solutions. The results suggest that the LPBN controls the ingestion of hypertonic NaCl and NaHCO₃. They also suggest a selective mechanisms involving the LPBN to check hypertonic sodium intake.