Neonatal subcutaneous administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to hamsters results in a marked depletion of serotonin (5-HT) in cortex and an increase in the concentration of this amine in the superior colliculus (SC). To determine whether this increase was associated with an alteration in the synaptic organization of 5-HT-containing axons in the superficial gray layer of the SC, immunocytochemistry was combined with electron microscopy. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles make synaptic contacts in the superficial gray layer of the hamster's SC. In 5,7-DHT-treated animals, examination of 400 individual profiles indicated that 25.5% of 5-HT-positive profiles made synaptic contacts (P < 0.05). Given the recently demonstrated effect of 5-HT on retinotectal transmission in this species, the present results suggest that the functional organization of the SC may also be markedly altered in animals that sustain neonatal 5,7-DHT administration.

Immunocytochemistry with an antibody‐directed tyrosine hydroxylase (TH) was combined with electron microscopy and serial‐section analysis to examine the synaptic organization of the catecholaminergic projection to the stratum griseum superfciale (SGS) of the hamster's superior colliculus (SC). A total of 250 TH‐immunoreactive profiles within SGS were examined. Of these, 114 (45.6%) made synaptic contacts; 81 (71.1%) were axodendritic, and the remainder (33, 28.9%) were axo‐axonic. Serial‐section analysis was employed to evaluate the presence or absence of synaptic contacts for 26 profiles. Overall, 19 (73.1%) of the profiles followed through serial sections exhibited synaptic contacts. Double staining of single sections with antibodies directed against TH and dopamine‐ß‐hydroxylase (DßH) and examination in the light microscope indicated that virtually all TH‐positive fibers also contained DßH. This indicated that the fibers examined at the electron microscopic level were noradrenergic rather than dopaminergic. These results suggest that norepinephrine may have both pre‐ and postsynaptic actions in the hamster's SC and that at least some of these effects are mediated by conventional synapses.

Immunocytochemistry with an antibody directed against a serotonin (5-HT)-bovine serum albumin conjugate was combined with electron microscopy to assess the synaptic organization of the serotoninergic projection to the stratum griseum superficiale (SGS) of the superior colliculus of hamsters killed on postnatal days (P) 0, 3, 7, 12, 15, 20 and > 60. At least 200 5-HT-immunoreactive profiles were examined at each of these ages. In the newborn (PO) animals, 36.6% of the 5-HT-positive profiles made conventional synapses. This percentage remained fairly constant until P15 when only 17.3% of the profiles made conventional synaptic contacts. On P20, this value decreased to 10.5% and in the adult animals, it fell to 4.4% These results thus indicate a dramatic age-related change in the synaptic organization of the 5-HT input to the hamster's superior colliculus.

Immunocytochemistry with an antibody directed against a serotonin (5-HT)-bovine serum albumin (BSA) conjugate was combined with electron microscopy and serial section analysis to examine the synaptic organization of the serotoninergic projection to the stratum griseum superficiale (SGS) of the hamster's superior colliculus (SC). Most of the 5-HT-immunoreactive swellings in the hamster's SC did not make conventional synaptic contacts. Examination of 250 such swellings in single thin sections revealed only 11 synapses. Assessment of an additional 50 swellings followed through serial sections showed that only 6% (N = 3) of these varicosities made conventional synaptic contacts. All the synapses made by serotoninergic axons were with the dendrites of SC neurons. These results demonstrate that the inhibitory effects of 5-HT on SC neurons in hamster are mainly mediated by the nonsynaptic release of this indoleamine.