EFFECTS OF NEUROTENSIN ON VISUAL NEURONS IN THE SUPERFICIAL LAMINAE OF THE HAMSTERS SUPERIOR COLLICULUS

Autoradiography with I-125-neurotensin in normal and enucleated hamste rs was used to define the distribution of receptors for this peptide i n the superficial layers of the superior colliculus (SC). Neurotensin binding sites were densely distributed in the stratum griseum superfic iale (SGS), and results from the enucleated animals indicated that the y were not located on retinal axons. The effects of neurotensin on ind ividual superficial layer cells were tested in single-unit recording e xperiments. Neurotensin was delivered via micropressure ejection durin g visual stimulation (n = 75 cells), or during electrical stimulation of either the optic chiasm (OX; n = 47 cells) or visual cortex (CTX; n = 29 cells). In comparison with control values, application of neurot ensin decreased visual responses of all SC cells tested to 54.1 +/- 34 .9% (mean +/- standard deviation; range of decrement 7.5 to 100%; nine cells showed no effect or an increase in visual activity, which for f our of these was greater than or equal to 30%). Neurotensin applicatio n also reduced responses to electrical stimulation of either OX or CTX , respectively, to 65.8 +/- 36.5% of control values (range of decremen t 2.6 to 97.4%; 12 neurons showed a weak increment less than or equal to 30%) and 68.0 +/- 38.5% (range of decrement 3.3 to 100%; five cells showed no effect or an increment, in one case greater than or equal t o 30%). Of the 25 neurons tested with both OX and CTX stimulation, the correlation of evoked response suppression by neurotensin was highly significant (r = 0.70; P < 0.001). This suggests that the suppressive effects of neurotensin were common to both pathways. To test whether t he inhibitory effects of neurotensin were presynaptic or postsynaptic, Mg2+ ions were ejected iontophoretically to abolish synaptic response s, and the neurons (n = 16) were activated by iontophoresis of glutama te and then tested with neurotensin. Neurotensin reduced the glutamate -evoked responses to an average 59.3 +/- 37.9% of control values (rang e 2.3 to 92.5%; one cell showed an increment > 30%). This result sugge sts that the site of action of neurotensin is most likely postsynaptic .