Neurotransmitter regulation of somatostatin secretion by fetal rat cerebral cortical cells in culture

Cardiac involvement in the long-term implications of COVID-19

Throughout 2021, the medical and scientific communities have focused on managing the acute morbidity and mortality caused by the coronavirus disease 2019 (COVID-19) pandemic. With the approval of multiple vaccines, there is a light at the end of this dark tunnel and an opportunity to focus on the future, including managing the long-term sequelae in patients who have survived acute COVID-19. In this Perspectives article, we highlight what is known about the cardiovascular sequelae in survivors of COVID-19 and discuss important questions that need to be addressed in prospective studies to understand and mitigate these lasting cardiovascular consequences, including in post-acute COVID-19 syndrome. To provide the greatest benefit to these survivors, prospective studies should begin now, with resources made available to monitor and study this population in the coming years.

In this Perspectives article, the authors highlight what is known about cardiovascular sequelae in survivors of COVID-19 and discuss important questions that need to be addressed in prospective studies to understand and mitigate these lasting cardiovascular consequences, including in post-acute COVID-19 syndrome.

Effects of subchronic and chronic melatonin treatment on somatostatin binding and its effects on adenylyl cyclase activity in the rat frontoparietal cortex

Melatonin and somatostatin are known to exert similar effects on locomotor activity. We have previously demonstrated that acute melatonin treatment regulates somatostatin receptor function in the rat frontoparietal cortex. However, the effects of subchronic and chronic melatonin treatment on the somatostatin receptor-G protein-adenylyl cyclase system in the rat frontoparietal cortex are unknown. Melatonin was administered subcutaneously at a daily dose of 25 microg/kg for 4 days, 1 wk or 2 wk. Twenty-four hours after the last injection, the animals were sacrificed. Melatonin did not alter the somatostatin-like immunoreactivity content in the frontoparietal cortex from control and melatonin-treated rats during any of the previously indicated periods. Four days of melatonin administration induced both an increase in the number of [(125)I]-Tyr11-somatostatin receptors and a decrease in the affinity of somatostatin for its receptors in frontoparietal cortical membranes. The increased number of somatostatin receptors in the melatonin-treated rats was associated with an increased capacity of somatostatin to inhibit basal and forskolin-stimulated adenylyl cyclase activity. Melatonin administration for 4 days induced a higher adenylyl cyclase activity both under basal conditions and after direct stimulation of the enzyme with forskolin. No significant differences were observed in the function of Gi proteins in the 4-day melatonin-treated rats. Western blot analyses showed that the 4-day melatonin treatment reduced Gialpha(2) levels, without altering the amount of Gialpha(1). These melatonin-induced changes reverted to control values after 7 or 14 days of treatment. Altogether, the present findings suggest that subchronic melatonin treatment modulates the somatostatin receptor/effector system in the rat frontoparietal cortex.

Activation of D1 and D2 dopamine receptors increases the activity of the somatostatin receptor-effector system in the rat frontoparietal cortex

The role of dopamine D1 and D2 receptor subtypes in the regulation, in vivo, of the somatostatin (SRIF) receptor-effector system in rat frontoparietal cortex was investigated. The D1-receptor agonist SKF 38393 (4 mg/kg) or the D2-receptor agonist bromocriptine (2 mg/kg), administered intraperitoneally to rats, increased the number of SRIF receptors without altering the affinity constant, an effect antagonized by both SCH 23390 (0.25 mg/kg) and raclopride (5 mg/kg), D1 and D2 receptor antagonists, respectively. These antagonists alone had no effect on [(125)I]Tyr(3) octreotide binding to its receptors. No change in binding was detected when the dopamine agonists were added in vitro. Basal adenylyl cyclase (AC) activity was increased by SKF 38393 treatment and decreased by bromocriptine. Octreotide (SMS 201-995)-mediated inhibition of basal and forskolin-stimulated AC was increased by SKF 38393 or bromocriptine treatment. In frontoparietal cortical slices, basal inositol-1,4, 5-triphosphate (IP(3)) levels were decreased by bromocriptine treatment but were unaffected by SKF 38393. SMS 201-995 increased the IP(3) accumulation in control, SKF 38393-, and bromocriptine-treated rats. Insofar as SRIF and dopamine appear to be involved in motor regulation and could well modulate somatosensory functions in frontal and parietal cortex, respectively, heterologous receptor regulation may have important repercussions regarding the control exerted by these neurotransmitters on frontal and parietal cortical function in the intact animal.

Influence of fluoxetine and p-chloroamphetamine on the somatostatin receptor-adenylyl cyclase system in the rat frontoparietal cortex

There is evidence that suggests a reciprocal functional link between the serotonergic and the somatostatinergic system in the rat frontoparietal cortex. However, to date, the role of endogenous 5-hydroxytryptamine (serotonin) on the regulation of the somatostatin (SS) receptor-adenylyl cyclase (AC) system remains unclear. In the present study, the administration of fluoxetine (10 mg/kg i.p.), a 5-hydroxytryptamine uptake inhibitor in a single dose or administered daily for 14 days increased the number of specific [125I]Tyr11-SS receptors, with no change in the receptor affinity, in rat frontoparietal cortical membranes. However, the capacity of SS to inhibit forskolin (FK)-stimulated AC activity in these membranes was lower than in the control groups. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate (Gpp(NH)p) to inhibit FK-stimulated AC activity in frontoparietal cortical membranes was also decreased in rats acutely and chronically treated with fluoxetine. p-Chloroamphetamine (5 mg/kg i.p.), which leads to a lasting reduction of 5-hydroxytryptamine innervation, administered on days 1, 3 and 5 and the rats sacrificed 1 or 3 weeks after the first injection, decreased the number of SS receptors without changing the receptor affinity. In this experimental group, SS also caused a significantly lower inhibition of FK-stimulated AC activity. p-Chloroamphetamine had no effect on the ability of Gpp(NH)p to inhibit FK-stimulated AC activity in frontoparietal cortical membranes at all the time periods studied. The present results suggest that under normal circumstances some SS receptors are under a tonic stimulatory control through the serotonergic system.

Dopamine enhances somatostatin receptor-mediated inhibition of adenylate cyclase in rat striatum and hippocampus

Although there is evidence that suggests that dopamine (DA) has stimulatory effects on somatostatinergic transmission, it is unknown to date if DA increases the activity of the somatostatin (SS) receptor-effector system in the rat brain. In this study, we evaluated the effects of the administration of DA and the DA D1-like (D1, D5) receptor antagonist SCH 23390 and the D2-like (D2, D3, D4) receptor antagonist spiperone on the SS receptor-adenylate cyclase (AC) system in the Sprague-Dawley rat striatum and hippocampus. An intracerebroventricular injection of DA (0.5 microgram/rat) increased the number of SS receptors and decreased their apparent affinity in the striatum and hippocampus 15 hr after its administration. The simultaneous administration of the DA receptor antagonists SCH 23390 (0.25 mg/kg, ip) and spiperone (0.1 mg/kg, ip) before DA injection partially prevented the DA-induced increase in SS binding. The administration of SCH 23390 plus spiperone alone produced a significant decrease in the number of SS receptors in both brain areas studied at 15 hr after injection, an effect that disappeared at 24 hr. The increased number of SS receptors in the DA-treated rats was associated with an increased capacity of SS to inhibit basal and forskolin (FK)-stimulated (AC) activity in the striatum and hippocampus at 15 hr after injection. This effect had disappeared at 24 hr. By contrast, basal and FK-stimulated enzyme activities were unaltered after DA injection. No significant changes in the levels of the alpha i (alpha i1 + alpha i2) subunits were found in DA-treated rats as compared with control rats. In addition, the immunodetection of the alpha i1 or alpha i2 subunits showed no significant changes in their levels in DA-treated rats when compared with controls. DA injection also induced an increase in SS-like immunoreactive content in the rat striatum but not hippocampus at 15 hr after administration and returned to control values at 24 hr. These results provide direct evidence of a functional linkage between the dopaminergic and somatostatinergic systems at the molecular level.

Histamine H1-receptors modulate somatostatin receptors coupled to the inhibition of adenylyl cyclase in the rat frontoparietal cortex

Since exogenous histamine has been previously shown to increase the somatostatin (SS) receptor-effector system in the rat frontoparietal cortex and both histamine H1-receptor agonists and SS modulate higher nervous activity and have anticonvulsive properties, it was of interest to determine the participation of the H1-histaminergic system in this response. The intracerebroventricular (i.c.v.) administration of the specific histamine H1-receptor agonist 2-pyridylethylamine (PEA) (10 micrograms) to rats 2 h before decapitation increased the number of SS receptors (599 +/- 40 vs 401 +/- 31 femtomoles/mg protein, p < 0.01) and decreased their apparent affinity for SS (0.41 +/- 0.03 vs 0.26 +/- 0.02 nM, p < 0.01) in rat frontoparietal cortical membranes. No significant differences were seen for the basal and forskolin (FK)-stimulated adenylyl cyclase (AC) activities in the frontoparietal cortex of PEA-treated rats when compared to the control group. In the PEA group, however, the capacity of SS (10(-4) M) to inhibit basal and FK (10(-5) M)-stimulated AC activity in frontoparietal cortical membranes was significantly higher than in the control group (34 +/- 1% vs 20 +/- 2%, p < 0.001). The ability of low concentrations of the stable GTP analogue 5'-guanylylimidodiphosphate [Gpp(NH)p] to inhibit FK-stimulated AC activity in frontoparietal cortical membranes was similar in the PEA-treated and control animals. These results suggest that the increased SS-mediated inhibition of AC activity in the frontoparietal cortex of PEA-treated rats may be due to the increase of the number of SS receptors induced by PEA. Pretreatment with the H1-receptor antagonist mepyramine (30 mg/kg, intraperitoneally (IP) prevented the PEA-induced changes in SS binding and SS-mediated inhibition of AC activity. Mepyramine (30 mg/kg, IP) alone had no observable effect on the somatostatinergic system. The in vitro addition of PEA or mepyramine to frontoparietal cortical membranes obtained from untreated rats did not affect the SS binding parameters. Altogether, these results suggest that the H1-histaminergic system modulates the somatostatinergic system in the rat frontoparietal cortex.

Modulation by 5-hydroxytryptamine of the somatostatin receptor-effector system and somatostatin levels in rat brain

The role of 5-hydroxytryptamine (5-HT) in the acute regulation of the rat brain somatostatin (SS) receptor-effector system and somatostatin-like immunoreactivity (SSLI) content was examined. 5-HT administered i.c.v. in a volume of 10 microliters at a dose of 0.5 microgram (pH 3.4) increased the SSLI concentration at 60 min in the Wistar rat frontoparietal cortex and hippocampus (60%, P < 0.05; 72%, P < 0.01; respectively). These changes were associated with a significant increase in the total number of specific SS receptors in the frontoparietal cortex (24%, P < 0.05) and hippocampus (20%, P < 0.05), without changes in the affinity constant as compared with the control group. No significant differences were seen in the basal and forskolin (FK)-stimulated adenylate cyclase (AC) activities in both brain areas of 5-HT-treated rats when compared to the control group. The capacity of SS to inhibit the FK-stimulated AC activity in the frontoparietal cortex and hippocampus of 5-HT-treated rats was lower than in the control groups. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate (Gpp(NH)p) to inhibit FK-stimulated AC activity in frontoparietal cortical and hippocampal membranes was markedly decreased in 5-HT-treated rats. To determine if the above-mentioned changes were related to the 5-HT activation of central 5-HT1 and 5-HT2 receptors, a non-selective 5-HT1 and 5-HT2 receptor antagonist, methysergide, was administered 60 min before the 5-HT injection. Pretreatment with methysergide (5 mg/kg i.p. in a volume of 400 microliters) prevented the 5-HT-induced changes in the SS receptor-effector system and in SSLI levels in both brain areas. Methysergide alone had no observable effect on the somatostatinergic system. These results suggest that the frontoparietal cortical and hippocampal somatostatinergic system can be regulated by 5-HT receptors.

alpha-Fluoromethylhistidine influences somatostatin content, binding and inhibition of adenylyl cyclase activity in the rat frontoparietal cortex

Slow-wave sleep, wakefulness, locomotor activity and learning and memory are regulated in similar ways by somatostatin (SS) and histamine. To clarify the possible role of endogenous histamine on the somatostatinergic system of the rat frontoparietal cortex, we studied the effect of 50 micrograms of alpha-fluoromethylhistidine (alpha-FMH), a specific inhibitor of histidine decarboxylase, administered intracerebroventricularly (i.c.v.) at 1, 4 and 6 h, on somatostatin-like immunoreactivity (SSLI) content and the SS receptor/effector system. The histamine content in the frontoparietal cortex decreased to about 67, 60 and 72% of control values at 1, 4 and 6 h after alpha-FMH administration, respectively. At 6 h after alpha-FMH injection, there was an increase in SSLI content and a decrease in the number of SS receptors, with no change in the apparent affinity. No significant differences were seen for the basal and forskolin (FK)-stimulated adenylyl cyclase (AC) activities in the frontoparietal cortex of alpha-FMH-treated rats when compared to the control group at all times studied. At 6 h after alpha-FMH administration, however, the capacity of SS to inhibit basal and FK-stimulated AC activity in the frontoparietal cortex was significantly lower than in the control group. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate (Gpp(NH)p) to inhibit FK-stimulated AC activity in frontoparietal cortex membranes was the same in the alpha-FMH-treated (6 h) and control animals. Therefore, the decreased SS-mediated inhibition of AC activity observed in the alpha-FMH-treated rats is not due to an alteration at the guanine nucleotide-binding inhibitory protein (Gi) level but rather may be due to the decrease in the number of SS receptors. Taken together, these data suggest that alpha-FMH influences the sensitivity to SS in the rat frontoparietal cortex.