Background: In normoglycaemic pithed rats, cardiac sympathetic control is modulated by serotonin (5-hydroxytryptamine; 5‑HT), which inhibits the cardioaccelerator sympathetic outflow via the activation of 5-HT 1B , 5-HT 1D and 5-HT 5A receptors. Notwithstanding, type 1 diabetes impairs the functionality of the cardiac sympathetic innervation and leads to cardiovascular complications including cardiac autonomic neuropathy. On this basis, the present study investigated whether the influence of 5-HT on cardiac noradrenergic neurotransmission is altered in type 1 diabetic rats, by analysing the profile of the 5-HT receptors involved and their peripheral expression.
Methods: Type 1 diabetes was induced in male Wistar rats with a single injection of streptozotocin (50 mg/kg, i.p.). Four weeks later, the rats were anaesthetized, pithed and prepared for producing tachycardic responses by either electrical preganglionic stimulation (C 7 ‑T 1 ) of the cardioaccelerator sympathetic outflow or i.v. bolus injections of exogenous noradrenaline. Immunohistochemistry analyses were performed to study the expression of 5‑HT 1B , 5-HT 1D and 5-HT 5A receptors in the stellate (sympathetic) ganglion obtained from normoglycaemic and diabetic rats.
Results: The increases in heart rate evoked by both cardiac sympathetic stimulation and exogenous noradrenaline were not modified after saline in diabetic rats. Moreover, i.v. continuous infusions of 5‑HT induced a cardiac sympatho-inhibition that was mimicked by the 5‑HT 1/5A receptor agonist 5‑carboxamidotryptamine, but not by the agonists indorenate (5-HT 1A ), CP 93,129 (5‑HT 1B ), PNU 142633 (5-HT 1D ), or LY344864 (5‑HT 1F ) in the diabetic group. In contrast, the above agonists at 5-HT 1B , 5-HT 1D and 5-HT 1/5A receptors mimicked 5-HT-induced sympatho-inhibition in normoglycaemic rats. In diabetic animals, i.v. administration of SB 699551 (1 mg/kg; 5‑HT 5A receptor antagonist) abolished 5‑CT-induced cardiac sympatho-inhibition. Finally, the immunohistochemistry analysis in the stellate ganglion showed that, as compared to normoglycaemic rats, in diabetic rats (P<0.05): (i) the expression of 5-HT 1B receptors was slightly higher, whereas that of 5-HT 1D receptors was slightly lower; and (ii) there was a clear overexpression of 5-HT 5A receptors.
Conclusions: Taken together, these results show the prominent role of the peripheral overexpression of prejunctional 5-HT 5A receptors in the inhibition of the cardiac sympathetic drive in type 1 diabetic rats. These findings may represent a new pharmacological strategy for the treatment of diabetes-related cardiac abnormalities.