DOPAMINERGIC NEUROTRANSMISSION AT THE PARAVENTRICULAR NUCLEUS OF HYPOTHALAMUS IN CENTRAL REGULATION OF PENILE ERECTION IN THE RAT

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Abstract
Purpose: To investigate whether the paraventricular nucleus of hypothalamus (PVN) is involved in the central regulation of apomorphine-induced penile erection in the rat, and to decipher dopamine receptor subtypes in the PVN that are involved in apomorphine-induced penile erection. Materials and Methods: Male adult Sprague-Dawley rats (200 to 300 gm.) anesthetized with pentobarbital sodium were used. The intracavernous pressure (ICP), recorded along with systemic and mean arterial pressure (SAP, MAP) as well as heart rate (HR), was measured via a 26-gauge needle inserted into one corpus cavernosum. The PVN was activated by stereotaxically delivered apomorphine hydrochloride (0.1 nmol/.100 nl.). Injection of saline into PVN served as a vehicle control. To investigate the participation of dopamine receptor subtypes in the PVN on apomorphine-induced penile erection, D1 or D2 receptor antagonist, SCH-23390 (100 pmol./100 nl.) or sulpiride (100 pmol./100 nl.) respectively, was administered into the PVN prior to subcutaneous application of apomorphine (80 micro g./kg.). The effects on ICP of microinjection of D1, D2 or D3 receptor agonist, SKF-38393 (200 pmol./100 nl.), lisuride (200 pmol./100 nl.) or 7-hydroxy-DPAT (200 pmol./100 nl.) respectively, into the PVN were also evaluated. Results: The mean resting ICP was 5.2 +/- 0.4 mm. Hg. Upon administration of apomorphine into the PVN, there was a significant increase in ICP that peaked at 50.7 +/- 5.3 mm. Hg and persisted for 45.2 +/- 18.0 minutes after an onset latency of 677.7 +/- 311.6 seconds. Yawning and teeth gnashing were also observed in most of animals during the period of ICP increase. There was no significant change in SAP, MAP or HR. In addition, there was no elevation in ICP after administration of saline to the PVN or direct injection of apomorphine into the cavernous tissue. Microinjection of D1 or D2 receptor antagonist into the PVN blocked the increase in ICP after subcutaneous administration of apomorphine. Direct application of D2, but not D1 or D3 receptor agonist into the PVN, on the other hand, increased the ICP. Conclusions: Our results demonstrate that application of apomorphine to the paraventricular nucleus of hypothalamus elicited penile erection in the rat. Such an increase in ICP to apomorphine was due mainly to activation of the D2 receptor subtype in the PVN. These observations. indicate that PVN may be involved in the central regulation of apomorphine-induced penile erection in the rat.