Gabaergic and opioid receptors mediate the facilitation of NaCl intake induced by α2-adrenergic activation in the lateral parabrachial nucleus

Abstract

Alpha2-adrenergic, gabaergic or opioidergic activation in the lateral parabrachial nucleus (LPBN) increases sodium intake. In the present study, we investigated the effects of single or combined blockade of opioidergic and gabaergic receptors in the LPBN on the increase of 0.3M NaCl intake induced by α2-adrenoceptor activation in the LPBN. Male Holtzman rats (n=5-9/group) with cannulas implanted bilaterally in the LPBN were treated with the diuretic furosemide (10 mg/kg b wt.) combined with low dose of the angiotensin converting enzyme inhibitor captopril (5 mg/kg b wt.) subcutaneously. Bilateral injections of moxonidine (alpha2-adrenergic/imidazoline receptor agonist, 0.5 nmol) into the LPBN increased furosemide+captopril-induced 0.3M NaCl intake (25.8±1.4, vs. vehicle: 3.8±1.1 ml/60 min). The opioidergic receptor antagonist naloxone (100 nmol) or the GABAA receptor antagonist bicuculline (5 nmol) injected into the LPBN partially reduced the increase of 0.3M NaCl intake produced by LPBN moxonidine (11.8±4.0 and 22.8±4.5, respectively, vs. vehicle+moxonidine: 31.6±4.0 ml/60 min, respectively). Similar to the treatment with each antagonist alone, the combined injections of naloxone (100 nmol) and bicuculline (5 nmol) into the LPBN also partially reduced moxonidine effects on 0.3M NaCl intake (15.5±6.5 ml/60 min). The GABAB receptor antagonist saclofen (5 nmol) injected into the LPBN did not change the effects of moxonidine on 0.3M NaCl intake (24.3±7.8 ml/120 min). These results suggest that the increase of 0.3M NaCl intake by α2-adrenergic receptor activation in the LPBN is partially dependent on GABAA and opioid receptor activation in this area.

Importance of the lateral parabrachial nucleus to sodium balance in fluid-depleted rats

“…Injections centered in the ventral lateral and external lateral portions, as well as in the Kölliker-Fuse nucleus observed in some rats were also considered as correctly placed in the LPBN. The sites of the injections in the present study were similar to those of previous studies that showed the effects of moxonidine injected into the LPBN on NaCl and water intake (Andrade et al, 2011(Andrade et al, , 2012(Andrade et al, , 2015 3.2. Experiment 1: Moxonidine injection into the LPBN: urinary sodium excretion and urinary volume in fluid-depleted rats Moxonidine injected bilaterally into the LPBN did not change urinary sodium excretion or urinary volume when compared to the LPBN vehicle injections in FURO + CAP treated rats that had no access to fluids.…”

“…Rats were anesthetized with ketamine (80 mg/kg of body weight) combined with xylazine (7 mg/kg of body weight) and placed in a stereotaxic instrument (Insight EFF 331, Insight Instruments, Ribeirão Preto, Brazil). The surgery for bilateral LPBN cannula placements is described in earlies studies from our lab (Andrade et al, 2004(Andrade et al, ., 2012(Andrade et al, , 2015.…”

Rapid stimulation of sodium intake combining aldosterone into the 4th ventricle and the blockade of the lateral parabrachial nucleus

“…It is not clear the reason for the different effects of methysergide or moxonidine into the LPBN on water intake in rats treated with aldosterone into the 4th V. Different studies have shown that the increase in water intake with methysergide injected into the LPBN when rats simultaneously ingest hypertonic NaCl is not consistent and only in part of the tests with methysergide into the LPBN water intake significantly increased in this condition (Menani et al, 1996(Menani et al, , 2000(Menani et al, , 2002De Luca et al, 2003). Although water intake increased in rats treated with moxonidine combined with high dose of aldosterone into the 4th V, the increase in water intake with moxonidine injected into the LPBN is also not consistent (Andrade et al, 2004(Andrade et al, , 2006(Andrade et al, , 2007(Andrade et al, , 2011(Andrade et al, , 2015, which suggests that rats are driven to ingest sodium in detriment to water when they receive methysergide or moxonidine into the LPBN, probably as a consequence of the strong preference for sodium in this condition. Chronic infusion of aldosterone into the 4th V also strongly increases hypertonic NaCl intake, without significant changes in water intake (Formenti et al, 2013), which confirms that the ingestion of significant amounts of hypertonic NaCl not necessarily causes increases in water intake if rats are motivated to ingest sodium.…”

“…Lesions of the CeA reduce daily sodium intake or that induced by ANG II or mineralocorticoids, suggesting that important facilitatory mechanisms for sodium intake are present in the CeA (Galaverna et al, 1992;Zardetto-Smith et al, 1994). Recent studies have also shown that the blockade of neuronal activity of the CeA with the GABAA agonist muscimol or the antagonism of the opioid receptors with naloxone abolished the increase of sodium intake in rats treated with FURO + CAP combined with the blockade of the LPBN inhibitory mechanisms, suggesting that the LPBN may inhibit CeA mechanisms that facilitate sodium intake (Andrade et al, 2015). It was also suggested that the CeA receives projections from the NTS and particularly from the HSD2 neurons of the NTS .…”

Opioid and α2 adrenergic mechanisms are activated by GABA agonists in the lateral parabrachial nucleus to induce sodium intake

“…Different neurotransmitters and receptors present in the LPBN are involved in the control of water and NaCl intake. Some neurotransmitters/receptors like serotonin, cholecystokinin, corticotrophin release factor and glutamate in the LPBN activates the inhibitory mechanisms, whereas α 2 -adrenoceptors, opioids and GABA deactivates the inhibitory mechanisms (Edwards and Johnson, 1991;Callera et al, 1993;Menani et al, 1996;Menani et al, 1998a;Menani et al, 1998b;Johnson, 1998, Andrade et al, 2004;Callera et al, 2005b;De Castro e Silva et al, 2006;de Oliveira et al, 2007;De Oliveira et al, 2008;De Oliveira et al, 2011;Menani et al, 2014;Andrade et al, 2015;Gasparini et al, 2015b;Pavan et al, 2015).…”