Functional cardiovascular action of L-cysteine microinjected into pressor sites of the rostral ventrolateral medulla of the rat

The endogenous sulfur-containing amino acid l-cysteine injected into the cerebrospinal fluid space of the cisterna magna increases arterial blood pressure (ABP) and heart rate (HR) in the freely moving rat. The present study examined (1) cardiovascular responses to l-cysteine microinjected into the rostral ventrolateral medulla (RVLM), where a group of neurons regulate activities of cardiovascular sympathetic neurons and (2) involvement of ionotropic excitatory amino acid (iEAA) receptors in response. In the RVLM of urethane-anesthetized rats accessed ventrally and identified with pressor responses to l-glutamate (10 mM, 34 nl), microinjections of l-cysteine increased ABP and HR dose dependently (3-100 mM, 34 nl). The cardiovascular responses to l-cysteine (30 mM) were not attenuated by a prior injection of either antagonist alone, MK801 (20 mM, 68 nl) for the NMDA type of iEAA receptors, or CNQX (2 mM) for the non-NMDA type. However, inhibition of both NMDA and non-NMDA receptors with additional prior injection of either antagonist completely blocked those responses to l-cysteine. The results indicate that l-cysteine has functional cardiovascular action in the RVLM of the anesthetized rat, and the responses to l-cysteine involve both NMDA and non-NMDA receptors albeit in a mutually exclusive parallel fashion. The findings may suggest endogenous roles of l-cysteine indirectly via iEAA receptors in the neuronal network of the RVLM for cardiovascular regulation in physiological and pathological situations.