Visual Neuroscience (1989), 2, 147-152. Printed in the USA. Copyright © 1989 Cambridge University Press 0952-5238/89 $5.00 + .00 /7-Chloroamphetamine treatment modifies evoked responses to sinusoidal gratings in the pigeon optic tectum R. ALESCI, 1 V. PORCIATTI, 2 L. SEBASTIANI, 1 AND P. BAGNOLI 1 1 Department of Physiology and Biochemistry, University of Pisa, Italy institute of Neurophysiology, C.N.R., Pisa, Italy (RECEIVED November 20, 1987; ACCEPTED May 12, 1988) Abstract This study was performed in order to establish whether selective depletion of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the pigeon optic tectum (TeO) induced by p-chloroamphetamine (p-CA) modified tectal evoked potentials (TEPs). TEPs in response to sinusoidal gratings of different contrast, spatial and temporal frequency were recorded in control pigeons and in pigeons intraperitoneally injected with p-CA (10 mg/kg; two administrations in consecutive days). TEPs of p-CA treated pigeons, as compared to those of control pigeons, were reduced in amplitude as a function of contrast, spatial and temporal frequency. In addition, TEPs of p-CA treated pigeons differed from those recorded in controls in their transfer characteristics of contrast and spatial frequency. In particular, TEPs of p-CA treated pigeons did not saturate at moderate contrast, unlike those of controls. Furthermore, the TEP spatial tuning in p-CA treated pigeons is broader than that in controls; it thus suggests a reduction of spatial-frequency selectivity. These findings indicate that a selective neurotoxin for serotonergic systems, such as p-CA, can serve as a useful denervation tool for the study of the serotonergic function in the pigeon TeO. In addition, selective changes of TEP properties suggest the possibility that serotonergic afferents play a modulatory role on the receptive-field characteristics of tectal neurons. Keywords: Pattern evoked potentials, Optic tectum, p-chloroamphetamine, Serotonin depletion, Pigeon Introduction In a previous paper (Porciatti et al., in this issue), both tran- sient reversal and onset-offset of sinusoidal gratings have been shown to generate evoked potentials on the pigeon tectal sur- face (TEPs). The amplitude of both the transient TEP slow component and steady-state TEP second-harmonic component varies as a function of stimulus contrast, spatial and temporal frequency (Porciatti et al., in this issue). Comparable proper- ties have been reported for single tectal cell responses by a great number of electrophysiological investigations (for review, see Jassik-Gerschenfeld & Hardy, 1984). These findings bring to the main conclusion that TEPs, like single unit responses, can be characterized by spatial-frequency selectivity and contrast sensitivity (Porciatti et al., in this issue). According to Leresche et al. (1984) and Hardy et al. (1982), tectal cell properties differ from those of retinal ganglion cells whose axons represent the major input to the pigeon optic tec- tum (TeO) (for review see Webster, 1974). Tectal cell proper- Reprint requests to: Paola Bagnoli, Department of Physiology and Biochemistry, University of Pisa, Via S. Zeno 31, 56100 Pisa, Italy. ties can be determined at least in part by the interaction of retinal inputs with extraretinal inputs, categorized into visual and nonvisual afferents (Brecha et al., 1976). Among the visual afferents, the TeO receives indirect retinal inputs through var- ious visual structures (Bagnoli et al., 1980; Brecha, 1978; Hodos, 1976; Hunt et al., 1977; Webster, 1974). The nonvisual afferents include those originating from brain stem regions (reticular formation, locus coeruleus, and raphe nuclei) (Brecha, 1978; Hunt & Kunzle, 1976; Voneida & Mello, 1975). Histochemical and immunohistochemical studies have shown a laminar distribution of catecholaminergic (Bagnoli & Casini, 1985) and serotonergic (Yamada & Sano, 1985) fibers and terminals, differentially concentrated within the tectal layers. Biochemical studies have also demonstrated in the pigeon TeO high endogenous levels of serotonin (5-HT) (Alesci & Bagnoli, 1988; Juorio & Vogt, 1967) and norepinephrine (NE) (Bagnoli et al., 1983; Juorio & Vogt, 1967), high-affin- ity uptake systems for these neurotransmitters (Beart, 1976; Cuenod & Henke, 1978), and high density of 5-HT and di- hydroalprenolol binding sites (Vischer et al., 1982). With respect to dopamine (DA), an extremely low DA content 147