Short-term cross-sensitizion of need-free sugar intake by combining sodium depletion and hypertonic NaCl intake

Reciprocal interactions between sodium appetite and need-free sugar intake

Behavioral sensitization occurs during sodium appetite (expressed as sodium intake to compensate for depleted sodium) and need-free sodium intake (expressed as daily overnight sodium intake in excess of dietary sodium need). Previously, we found that a slow-onset sodium appetite protocol cross-sensitized need-free sucrose intake in sucrose-naïve adult rats. That is, a history of sodium depletion elevated later sucrose intake. The objective of the present work was, first, to investigate whether a protocol that evokes a rapid-onset (within 2 h) sodium appetite using furosemide along with a low dose captopril (Furo/Cap), also cross-sensitizes sucrose intake. Then, we investigated whether 1) sensitization of need-free 0.3 M NaCl intake interacts with need-free sucrose intake, and 2) MK-801, a glutamate NMDA receptor antagonist, inhibits cross-sensitization of sucrose intake. Groups received 3-4 Furo/Cap or vehicle treatments with 48/72-h intervals. We investigated sucrose intake in hydrated and fed conditions for 2 h/day for 5 days, starting 6-10 days after the last Furo/Cap treatment. Episodes of Furo/Cap sensitized need-free sodium intake, as expected. Similar to our prior work, the rapid-onset Furo/Cap protocol cross-sensitized sucrose intake in sucrose-naïve rats and had no persistent effect on blood biochemistry. MK-801 treatment along with Furo/Cap injections appeared to prevent cross-sensitization of sucrose consumption. Sucrose intake tests unexpectedly reduced sensitized need-free sodium intake. However, MK-801 treatment allowed a rebound in need-free sodium intake subsequent to the last sucrose intake test. The results suggest that plasticity in glutamatergic mechanisms mediate inverse and reciprocal interactions between the production of sodium appetite and sucrose intake.

The influence of opioid dependence on salt consumption and related psychological parameters in mice and humans

BACKGROUND

The consumption of dietary salt (NaCl) is controlled by neuronal pathways that are modulated by endogenous opioid signalling. The latter is disrupted by chronic use of exogenous opioid receptor agonists, such as morphine. Therefore, opioid dependence may influence salt consumption, which we investigated in two complimentary studies in humans and mice.

METHODS

Human study: three groups were recruited: i. Individuals who are currently opioid dependent and receiving opioid substitution treatment (OST); ii. Previously opioid dependent individuals, who are currently abstinent, and; iii. Healthy controls with no history of opioid dependence. Participants tasted solutions containing different salt concentrations and indicated levels of salt 'desire', salt 'liking', and perceptions of 'saltiness'. Mouse study: preference for 0.1 M versus 0.2 M NaCl and overall levels of salt consumption were recorded during and after chronic escalating morphine treatment.

RESULTS

Human study: Abstinent participants' 'desire' for and 'liking' of salt was shifted towards more highly concentrated salt solutions relative to control and OST individuals. Mouse study: Mice increased their total salt consumption during morphine treatment relative to vehicle controls, which persisted for 3 days after cessation of treatment. Preference for 'low' versus 'high' concentrations of salt were unchanged.

CONCLUSION

These findings suggest a possible common mechanistic cross-sensitization to salt that is present in both mice and humans and builds our understanding of how opioid dependence can influence dietary salt consumption. This research may help inform better strategies to improve the diet and overall wellbeing of the growing number of individuals who develop opioid dependence.