EFFECTS OF ANGIOTENSIN-II ON VISUAL NEURONS IN THE SUPERFICIAL LAMINAE OF THE HAMSTERS SUPERIOR COLLICULUS

Superficial layer superior colliculus (SC) neurons were recorded extra cellularly with multibarreled recording/ejecting micropipettes. Angiot ensin II was delivered via micropressure ejection during visual stimul ation (n = 215 cells), or during electrical stimulation of either the optic chiasm (OX; n = 150 cells) or visual cortex (CTX; n = 42 cells). Application of angiotensin II decreased visual responses of SC cells to 43.8% +/- 30.7% (mean +/- S.D.) and reduced responses to electrical stimulation of the OX and CTX to 58.6% +/- 34.1% and 43.8% +/- 30.7% of control values, respectively. Angiotensin II enhanced responses by at least 30% in only 6 cells (1.5%). Of the 35 neurons tested with bot h OX and CTX stimulation, the correlation of evoked response suppressi on by angiotensin II was highly significant (r = 0.69; P < 0.001). Thi s suggests that the suppressive effects of angiotensin II were common to both pathways. To test whether the inhibitory effects of angiotensi n II were presynaptic or postsynaptic, Mg2+ ions were ejected iontopho retically to abolish synaptic responses, and the neurons were activate d by iontophoresis of glutamate and then tested with angiotensin II. A ngiotensin II reduced the glutamate-evoked responses to an average 29. 1% +/- 21.1% of control values (n = 9 cells). This suggests that the s ite of action of angiotensin II is most likely postsynaptic. To identi fy which receptors were involved in these effects, angiotensin II was ejected concurrently with the AT(1) antagonist Losartan (DUP753) or wi th either of two AT(2) antagonists, CGP42112A or PD123177. Losartan an tagonized the action of angiotensin II in 65.6% of the cells tested (n = 99) and CGP42112A and PD123177 had antagonistic effects in 58% (n = 65) and 60% (n = 5), respectively. Both classes of antagonists were t ested in 29 cells; and there was no significant correlation between th eir effectiveness. These results suggest that both AT(1) and AT(2) rec eptors may independently mediate the suppressive effects of angiotensi n II, and that collicular neurons may have either or both receptor sub types.