Purinergic and Adrenergic Agonists Synergize in Stimulating Vasopressin and Oxytocin Release

Abstract

The A1 catecholamine neurons of the caudal ventrolateral medulla transmit hemodynamic information to the vasopressin (VP) neurons in the hypothalamus. These neurons corelease ATP with norepinephrine. Perifused explants of the hypothalamoneurohypophyseal system were used to investigate the role of these substances on VP release. ATP (100 M) increased VP release 1.5-fold ( p ϭ 0.027). The response was rapid but unsustained. It was blocked by the P 2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2Ј,4Ј-disulphonic acid (PPADS). The ␣ 1 -adrenergic agonist phenylephrine (PE; 100 M) also increased VP release by 1.5-fold ( p ϭ 0.014). Again, the response was rapid and unsustained. However, simultaneous perifusion of explants with ATP (100 M) and PE (100 M) resulted in a threefold to fourfold increase in VP release, which was sustained for as long as 4 hr. There was a similar synergistic effect of ATP and PE on oxytocin release. Interestingly, the synergistic response was delayed ϳ40 min relative to the response to either agent alone. Several experiments were performed to elucidate the cellular mechanisms of this synergism. The effect was blocked by PPADS, a protein kinase C inhibitor (bisindolylmaleimide I HCl), and actinomycin, an inhibitor of gene transcription. These data suggest that P 2X receptor activation, PKC-mediated phosphorylation, and gene transcription are required for the synergistic response. The marked synergism of these coreleased agents is probably important to achieve sustained increases in plasma VP in response to prolonged hypotension. These observations may also have broad applications to CNS function, because ATP may be coreleased at noradrenergic synapses throughout the CNS. Key words: norepinephrine; ATP; vasopressin; supraoptic nucleus; catecholamine; neurohypophysis; purinergic transmission; blood pressure regulationVasopressin (VP)-synthesizing neurons of the supraoptic and paraventricular nuclei (SON and PVN) receive direct, excitatory input from the A1 noradrenergic neurons of the caudal ventrolateral medulla (Alonso and Assenmacher, 1984;Day et al., , 1992Shioda et al., 1992;Ginsberg et al., 1994;Smith et al., 1995). In addition to norepinephrine (NE), various substances are colocalized or coreleased from the A1 neurons, including neuropeptide Y, substance P, and ATP (Everitt et al., 1984;Sawchenko et al., 1985;Blessing et al., 1986;Beroukas et al., 1989;Lundberg et al., 1989;Bittencourt et al., 1991;Sperlagh et al., 1998). Although abundant evidence supports the importance of the A1 pathway for stimulation of VP release in response to moderate decreases in blood pressure (Raby and Renaud, 1989;Smith et al., 1995), studies directed at demonstrating that NE is the primary transmitter mediating this response were not successful (Day et al., 1990). Adrenoceptor antagonists did not block A1 activation of VP cells, prompting the suggestion that these neurons use a substance other than NE as their principal transmitter (Day et al., 1990). In addition, injections of the broad-spectrum exci...

Pathophysiology and Therapeutic Potential of Purinergic Signaling

“…Purinergic and adrenergic synergism for vasopressin and oxytocin release is consistent with ATP cotransmission in the hypothalamus (Kapoor and Sladek, 2000). Purinergic signaling appears to play a significant role in the regulation of body temperature during fever by central hypothalamic and brainstem nuclei (Gourine et al, 2002(Gourine et al, , 2004.…”

Facilitation of Neurotransmitter and Hormone Release by P2X Purinergic Receptors

“…These involve an increase in neuronal activity (Khakh et al, 2003), potentiation of neurotransmitter release (Sperlagh et al, 2007) and stimulation of hormone secretion (luteinizing hormone, prolactin, oxytocin and vasopressin) (Kapoor and Sladek, 2000;Stojilkovic, 2009;Stojilkovic et al, 2010b).…”

“…Purinergic P2X receptors are expressed throughout the hypothalamus where ATP, coreleased with neuropeptides, appears to be involved in the regulation of hormone secretion (Troadec et al, 1998;Kapoor and Sladek, 2000) and control of specific autonomic functions including the central mechanism of body temperature regulation (Gourine et al, 2002), for example. P2X2, P2X4 and P2X6, but not P2X5 receptor mRNAs have been identified in the rat supraoptic nucleus (SON), ventromedial nucleus, paraventricular nucleus, arcuate nucleus, suprachiasmatic nucleus, and several other hypothalamic areas using in situ hybridization; the P2X1, P2X3 and P2X7 receptors were not tested in this study (Collo et al, 1996).…”

“…These involve stimulation of luteinizing hormone, prolactin, oxytocin and vasopressin hormone secretion by pituitary gland (Kapoor and Sladek, 2000;Stojilkovic, 2009;Stojilkovic et al, 2010b). The anterior pituitary is a heterogeneous gland with multiple cell types that secrete six major peptide hormones necessary for reproduction, lactation, growth, development, metabolic homeostasis, and the response to stress: FSH and LHproducing gonadotrophs, prolactin (PRL)-producing lactotrophs, GH-producing somatotrophs, TSH-producing thyrotrophs, and ACTH-producing corticotrophs.…”

“…In excitable cells, P2XR activation causes an increase in the cytosolic Ca 2+ concentration via two distinct mechanisms: by membrane depolarization resulting in voltage-dependent Ca 2+ entry and by Ca 2+ entry through the P2XR itself. The role of P2XR involves fast synaptic transmission mediated by ATP in both the peripheral (Evans et al, 1992) and central nervous systems (Edwards et al, 1992), modulation of neuronal excitability (Khakh and Henderson, 1998), long-term potentiation (Sim et al, 2006), and stimulation of hormone secretion (luteinizing hormone, prolactin, oxytocin and vasopressin) (Kapoor and Sladek, 2000;Stojilkovic, 2009; amino]-5-quinolinyl]-2-tricyclo[3.3.1.13,7]dec-1-ylacetamide dihydrochloride (AZ 10606120) (Michel et al, 2007) and 3-[[5-(2,3-dichlorophenyl)-1H-tetrazol-1-yl]methyl] pyridine hydrochloride (A 438079) (Nelson et al, 2006). The P2XR subtypes are functionally differentiated also by comparisons in their calcium permeability which is relatively high, in the range from 2.7 % (P2X3R) to 12.4 % (P2X1R) of total ATP-induced current (Egan and Khakh, 2004).…”