Auditory cortical efferent actions upon inferior colliculus unitary activity in the guinea pig Pablo Torterolo a,b , Patricio Zurita a , Marisa Pedemonte a , Ricardo A. Velluti a, * a Neurofisiologia, Departamento de Fisiologia, Facultad de Medicina, Universidad de la Republica, Av. Gral. Flores 2125, Montevideo 11800, Uruguay b Neurofisiologia, Facultad de Ciencias, Universidad de la Republica, Av. Gral. Flores 2125, Montevideo 11800, Uruguay Received 27 January 1998; accepted 22 April 1998 Abstract Differential actions on inferior colliculus central nucleus (ICc) single cells spontaneous activity were observed with both ipsilateral and contralateral auditory cortical electrical stimulation (ACx stimulation). Following ACx stimulation, a firing depres- sion of the spontaneous activity was obtained using contralateral or ipsilateral cortical stimulation, although a greater effect was elicited by the contralateral cortex. In contrast, ipsilateral ACx stimulation elicited more excitation with a shorter latency than contralateral stimulation. In units that failed to show spontaneous firing, the sound-evoked responses and ACx stimulation were studied; ~50% of them demonstrated firing depression to ACx stimulation on either side with either clicks or tone-bursts. Thirty percent of the units failed to show changes in response to any cortical stimulation. A temporary disruption of ICc-evoked neuronal discharge was elicited during contralateral cortex stimulation, as previously reported to occur during sleep. The demonstration that auditory cortices may differentially affect the same ICc unit activity, i.e. spontaneous and evoked, suggests that auditory processing may depend on the ongoing spontaneous activity plus the effects exerted from each auditory cortex activation.  1998 Elsevier Science Ireland Ltd. All rights reserved Keywords: Auditory cortex; Inferior colliculus; Efferent system; Cortical stimulation Single unit recordings from the central nucleus of the inferior colliculus (ICc) have revealed how spiking of such neurons is affected by both its sensory input and the internal state of the brain, e.g. sleep and wakefulness, probably throughout the efferent system [3,7,10,20,22]. Although little is known about the physiology of the rostral- most part of the auditory efferent system, a wide distribution of direct cortical efferent fibers into the ICc nucleus in rats [15] and monkeys [6] has been described. Moreover, Mitani et al. [9] using intracellular recordings, observed excitatory and inhibitory synaptic potentials in ICc neurons in response to ipsilateral-auditory-cortical electrical stimula- tion (ACx stimulation) in cats, although no records of sound-responding neurons were shown. Inhibitory as well as excitatory actions were described in the IC dorsal and central nuclei, extracellularly recorded after ipsilateral ACx stimulation in rats [18] and bats [19,23]. However, there is no report on the effect of either contralateral ACx stimula- tion or of cortical, ipsilateral or contralateral stimulation effects on the same IC unit. The aim of this study was to investigate the effect exerted upon physiologically-charac- terized auditory neurons in the ICc nucleus by ipsilateral and contralateral electrical cortical stimulation in anesthe- tized guinea-pigs. Adult guinea-pigs (Cavia porcellus, n = 12), weighing 400–500 g, were anesthetized with sodium pentobarbital (35 mg/kg, i.p.). Heart rate and body temperature were mon- itored. The head was stereotaxically supported and a hole was opened in the skull over the IC region for micropipette penetrations according to selected coordinates (A, 0.5; L, 2; H, 3–6) [13]. Glass micropipettes (impedance 20–80 MQ), filled with 3 M sodium acetate and Skyblue, were used and their final position was determined histologically, first by Neuroscience Letters 249 (1998) 172–176 0304-3940/98/$19.00  1998 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(98)00367-X * Corresponding author. Tel.: +598 2 9248784; fax: +598 2 9248784; e-mail: rvelluti@fmed.edu.uy