ELECTROPHYSIOLOGICAL EVENTS DURING NEUROEFFECTOR TRANSMISSION IN THE SPLEEN OF GUINEA-PIGS AND RATS

1. Intracellular recordings were made from smooth muscle cells of arte rioles and the capsule of the spleen of guinea-pig and rat, and the re sponses to periarterial or subcapsular nerve stimulation were recorded . The innervation of the spleen was studied using fluorescence and imm unohistochemical techniques. 2. Catecholamine-containing axons were as sociated with smooth muscle of the splenic capsule, trabeculae, arteri oles and amongst cells of the periarteriolar lymphoid sheath. Axons im munoreactive for neuropeptide Y (NPY) and tyrosine hydroxylase were di stributed in an identical manner to catecholamine-containing axons, wh ereas axons immunoreactive for substance P or calcitonin gene-related peptide were present at a very low density in spleens from both specie s. 3. In segments of arterioles, single transmural stimuli evoked exci tatory junction potentials (EJPs) of 1-10 mV amplitude. EJPs facilitat ed during, short trains of stimuli (1-10 Hz) and summated at 10 Hz, of ten initiating a muscle action potential. EJPs persisted in the presen ce of prazosin 1 mu M) and idazoxan (1 mu M), but were abolished by th e P-2x-purinoceptor antagonist suramin (1 mM). 4. Spontaneous depolari zations were observed in smooth muscle cells of arterioles and capsule . Some events in arterioles were observed in the presence of suramin a nd so may originate postjunctionally independently of transmitter rele ase. As single transmural stimuli failed to evoke a depolarization in capsular smooth muscle; spontaneous depolarizations in this tissue pro bably also arise postjunctionally. 5. Short trains of high frequency s timuli (10-35 Hz) evoked biphasic depolarizations of capsular smooth m uscle cells. The initial component peaked 2.5 s following the onset of stimulation; the second component peaked 15 s following the onset and decayed exponentially with a time constant of 15 s. By fitting a prod uct of exponentials to the second component, it was possible to define the initial component, which decayed with a time constant of around 1 .5 s. 6. Neurally evoked depolarizations of capsular smooth muscle wer e abolished by 1. mu M TTX. Blockade of alpha(1)-adrenoceptors with pr azosin reduced the initial component of the depolarization, whereas al pha 2-adrenoceptor blockade with idazoxan virtually abolished the seco nd component. In some cells a small, faster depolarization persisted a fter alpha-adrenoceptor blockade. The slow alpha(2)-adrenoceptor-media ted depolarization was identical to that recorded in the rat tail arte ry and in the guinea-pig mesenteric vein. 7. The data indicate that sy mpathetic neuroeffector transmission from noradrenergic axons containi ng NPY to splenic arterial and capsular smooth muscle occur by differe nt mechanisms.