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Inferior colliculus in the rat: Neuronal responses to stimulation of the auditory cortex
Neuroscience Letters, 51 (1984) 235-240
235
Elsevier Scientific Publishers Ireland Ltd.
NSL 02989
I N F E R I O R C O L L I C U L U S IN T H E RAT: N E U R O N A L R E S P O N S E S T O S T I M U L A T I O N OF T H E A U D I T O R Y C O R T E X
JOSEF SYKA and Jll~[ POPEL,~P,
Institute o f Experimental Medicine, Czechoslovak Academy o f Sciences, Lidovf, ch milict" 61, 120 O0 Prague 2 (Czechoslovakia) (Received April 9th, 1984; Revised version received August 3rd, 1984; Accepted August 7th, 1984)
Key words: inferior colliculus - neuronal responses - cortical stimulation - rat
Responses to electrical stimulation of the auditory cortex (silver ball bipolar electrodes, single pulses, duration 0.2 ms, current 0.2-1.5 mA) were recorded in neurones in the inferior colliculus of rats anaesthetized with pentobarbital. Excitatory or inhibitory effects were obtained in 84 out of 162 recorded neurones. The majority of neurones responded with a short excitatory burst (with a latency from 3 to 15 ms); in some of them the initial excitation was followed by inhibition lasting from 30 to 150 ms. Few neurones only reacted to electrical stimulation by inhibition, which occurred 3-10 ms after the stimulus and lasted up to 300 ms. The inhibition either suppressed the spontaneous activity or the acoustically evoked response. Neurones reacting to stimulation of the auditory cortex were found mainly in the caudal and dorsal parts of the inferior colliculus.
Pathways descending from the auditory cortex (AC) to the inferior colliculus (IC) were described in detail in the cat [1, 2, 4] but recently this knowledge was also extended to the rat [3, 5-71. In both species, ipsilateral projections originate in pyramidal cells of layer Vb and terminate mainly in the dorsomedial part of the central IC nucleus, in the pericentral and external nuclei of the IC. As a first step in the investigation of the function o f the corticotectal auditory pathway we investigated the effects of electrical stimulation of the auditory cortex upon the neuronal activity in the IC. It will be shown that the corticotectal auditory fibres m a y influence the activity of m a n y IC neurones, the result usually leading to a brief burst of excitation followed by a longer period of inhibition. Experiments were performed in 17 Sprague-Dawley rats, anaesthetized with pentobarbital (40 mg/kg). The auditory cortex was exposed and two silver-ball electrodes (tip separation less than 1 mm) were placed on the dura mater at the point where the maximal evoked response to acoustic clicks was observed. In some experiments the dura was cut and the cortex (with the exception of the place of stimulation) was covered with agar. Single 0,2-ms electrical pulses, repe:ition rate 1 Hz and intensity ranging f r o m 0.2 to 1.5 m A were used as stimuli. Glass 0304-3940/84/$ 03.00 © 1984 Elsevier Scientific Publishers Ireland Ltd.
236 micropipettes filled with 3 M KCI were inserted into the IC stereotaxically through the cerebellum, i.e. perpendicularly to the frontal plane. An electronically driven microdrive at 2-#m steps was used for insertion of the electrodes into the brain. Extracellular unit activity was recorded with glass microelectrodes; gross evoked responses to electrical and auditory stimuli were also recorded in some experiments. The auditory stimuli were clicks (duration of the electrical pulse 50 ~s) and tone pips (duration 100-200 ms) delivered through a closed piezoelectric sound system, with the possibility of checking the actual sound pressure before the ear drum. The neuronal responses were monitored on a Tektronix 5103 N oscilloscope with a memory screen and photographed with a camera. The temperature of the animal was maintained at 38°C with a heating-pad. Localization of the electrode tracks in the IC was histologically checked. After the experiment the electrode was broken and left in the brain, which was perfused with formalin. Frontal sections were made on a freezing microtome to identify the last electrode track. Other electrode tracks (usually two or three) were reconstructed from stereotaxic and microdrive coordinates. Eighty-four neurones out of the 162 tested (i.e. 52.4°70) were influenced by the electrical stimulation (ES) of the auditory cortex. In the majority of cases, i.e. in 61 neurones, the ES resulted in a brief burst of excitation which occurred after a latency of 3-15 ms (Fig. 1). The burst consisted of 1-10 spikes and the number of spikes was dependent upon the ES intensity; with increasing intensity the number of spikes increased and the latency of the first spike also slightly decreased. The excitation threshold in some neurones was around 0.2 mA, but in some units it was necessary to increase the intensity up to 0.8-1.0 mA until the response was obtained. Essentially, the same results were obtained in animals with the intact dura mater and with the removed dura. Fig. 1 (left side) shows the excitatory response of an IC neurone (73D) to electrical stimulation of the auditory cortex. In this neurone, similarly as in many other IC neurones, the initial excitation was followed by a period of inhibition which lasted from 30 to 150 ms. In neurone 73D the postexcitatory inhibition of spontaneous activity evoked by auditory cortex stimulation is approximately as long as the inhibition of spontaneous activity evoked by a tone at the characteristic frequency (compare panels B and C, left). Inhibition resulting from AC stimulation may also efficiently suppress the excitatory response evoked by the auditory stimulus (panel D, left). In Fig. 1 (right side) another example of the IC neurone reaction to AC stimulation is demonstrated. This consists of an excitatory burst, which is followed by a 70 ms lasting inhibition. The lowest panel (D, right) demonstrates the excitatory burst evoked by AC stimulation cannot be inhibited by stimulation of the ipsilateral ear. This neurone, similar to the majority of other IC neurones, is excited from the contralateral ear and inhibited from the ipsilateral ear (EI type of binaural interaction). While the binaural stimulation (ipsilateral stimulus is 5 dB more intensive than the contralateral one) suppresses the tone-evoked activity, the AC-evoked burst is still present.
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Fig. i. Responses of two inferior colliculus neurones (RCI 73D, left side, and RCI 77B, right side) to electrical stimulation of the auditory cortex and to acoustical stimulation, Intensity of the electrical pulse, tone frequency and intensity are indicated to the left of the neuronal response traces, time in milliseconds is indicated under the neuronal response. All responses represent 5 consecutive summated individual responses, photographed from the screen of a memory oscilloscope.
In 21 out of 84 IC neurones (25o7o) which reacted to auditory cortex stimulation only an inhibitory effect was found. The inhibition, which suppressed the spontaneous activity or the sound-evoked activity, occurred after a 3-15 ms latency and lasted 30-300 ms. The duration of the inhibitory effect was dependent upon the intensity o f the electrical pulse. Fig. 2 (left side), shows an example of the net inhibitory effect of AC stimulation. Panel A (left) displays excitation evoked by the tone at the characteristic frequency (CF = 15 kHz), panel B (left) shows ineffective stimulation at subthreshold intensity (0.4 mA). The increase of intensity to 0.5 m A results in 200 ms lasting inhibition, whereas the 1.0 m A electrical pulse suppresses the activity for about 350 ms. In this neurone, as well as in other IC neurones, the AC-evoked inhibition suppressed unequivocally all auditory-evoked activity and the spontaneous activity. In some IC neurones, however, the inhibitory effects were different with respect to different parts of the sound evoked response. Fig. 2 (right side) in the upper two panels (A and B, right) shows the response of an IC neurone (76D) to tones at the CF (1.5 kHz) which consists of an initial excitatory burst,
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Fig. 2. Responses o f two inferior colliculus neurones (RCI 73K, left side, a n d RCI 76D, right side) to electrical s t i m u l a t i o n o f the a u d i t o r y cortex a n d to a c o u s t i c a l s t i m u l a t i o n . For further details see text to Fig. 1.
followed by inhibition (which is dependent on the tone intensity) and late excitation. The AC stimulation suppresses the late excitation (panel C, right) effectively but does not suppress the initial excitation (panel D, right). The localization of 65 neurones in the IC was estimated from the microdrive checking of the electrode tip and from the histological controls of the electrode track. The majority of neurones reacting to AC stimulation was found in the dorsomedial part of the central IC nucleus and in the caudal and dorsal parts of the IC, i.e. soon after penetration into the IC from the cerebellar side. Few tracks penetrated the external IC nucleus; neurones in the external IC were mostly inhibited by the AC stimulation. Neurones not reacting to the AC stimulation were distributed homogeneously in the caudorostral direction. In agreement with this finding in 5 rats, where the IC was penetrated in the caudorostral direction with macroelectrodes, a pronounced evoked response to AC stimulation was always observed near the caudal and dorsal surface of the IC, whereas the evoked response to the auditory stimuli (clicks) dominated more centrally and rostrally. The results unambiguously demonstrate that the auditory cortex in the rat im-
239
poses a strong influence upon the activity in the inferior colliculus. Electrical stimulation of the cortical auditory field excites the pyramidal cells in layer Vb, the axons o f which descend to the medial geniculate b o d y and to the inferior colliculus [1-7]. The targets of these fibres in the IC of the rat are mainly the ipsilateral dorsomedial part of the central IC nucleus and the external and pericentral nuclei of the IC. In correspondence with the anatomical distribution of the terminals of descending fibres, the cells reacting to electrical stimulation of the AC were found mainly in the dorsomedial part of the central IC nucleus and in the caudal and dorsal parts of the IC, during horizontal penetration of the IC through the cerebellum. A similar distribution was observed in evoked potentials to AC stimulation recorded with macroelectrodes. The antidromic stimulation of cells in the IC m a y be excluded since the IC does not send axons directly to the auditory cortex and the latency of excitatory responses in the IC is too long (3-15 ms) for antidromic stimulation. The thin dura mater in the rat apparently does not present any obstacle to the current spread, since the results were similar in animals with the dura either cut or intact. We suppose that the majority o f neurones of the auditory cortex in the rat, which send their axons to the IC, was stimulated by the current, because the auditory cortical area comprises approximately 3 x 2 mm. The functional role of the activation of the auditory descending system is unknown. The most frequently observed effect of AC stimulation was the inhibition of IC activity. In m a n y IC neurones in the rat the response to tone pips consists of an initial excitation which is followed by a period of inhibition. In some neurones the initial excitation represents the entire response, in other neurones the inhibitory pause is followed by late excitation. It may be assumed that the inhibitory period which occurs after the initial excitation is caused by activation of the auditory cortex. Similar inhibitory effects upon IC neurones were, however, also observed after electrical stimulation of the contralateral IC (Syka and Popelgt~, unpublished data). The cortically evoked inhibition, which lasted f r o m 20 to 300 ms for different IC neurones in our experiments m a y be the decisive factor, which limits the highest repetition rate for clicks in the IC. The repetition rate for clicks in the neurones of the rat IC was found to be in the range of 5-50 clicks per second, with the majority o f neurones lying near the lower border of this range [8]. 1 Adams, J.C., Crossed and descending projections to the inferior colliculus, Neurosci. Lett., 19 (1980) 1-5. 2 Andersen, R.A., Snyder, R.L. and Merzenich, M.M., The topographic organization of corticocollicular projections from physiologically identified loci in the A l, A II and anterior auditory cortical fields of the cat, J. comp. Neurol., 191 (1980) 479-494. 3 Beyerl, B.D., Afferent projections to the central nucleus of the inferior colliculus in the rat, Brain Res., 145 (1978) 209-223. 4 Diamond, I.T., Jones, E.G. and Powel, T.P.S., The projection of the auditory cortex upon the diencephalon and brain stem in the cat, Brain Res., 15 (1969) 305-340. 5 Druga, R. and Syka, J., Ascending and descending projections to the inferior colliculus in the rat, Physiol. Bohemosl., 33 (1984) 31-42.
240 6 Druga, R. and Syka, J., Neocortical projection to the inferior colliculus in the rat, Physiol. Bohemos|., 33 (1984) 251-253. 7 Syka, J., Druga, R., Popel~if, J. and Kalinovfi, B., Functional organization of the inferior colliculus. In J. Syka and L. Aitkin (Eds.), Neuronal Mechanisms of Hearing, Plenum Press, New York, 1981, pp. 137-153. 8 Syka, J., Popel;if, J. and Druga, R., Structure and function of crossed and uncrossed pathways to the inferior colliculus in the rat. In R. Klinke and R. H a r t m a n n (Eds.), Hearing-Physiological Bases and Psychophysics, Springer Verlag, 1983, pp. 224-229.
235
Elsevier Scientific Publishers Ireland Ltd.
NSL 02989
I N F E R I O R C O L L I C U L U S IN T H E RAT: N E U R O N A L R E S P O N S E S T O S T I M U L A T I O N OF T H E A U D I T O R Y C O R T E X
JOSEF SYKA and Jll~[ POPEL,~P,
Institute o f Experimental Medicine, Czechoslovak Academy o f Sciences, Lidovf, ch milict" 61, 120 O0 Prague 2 (Czechoslovakia) (Received April 9th, 1984; Revised version received August 3rd, 1984; Accepted August 7th, 1984)
Key words: inferior colliculus - neuronal responses - cortical stimulation - rat
Responses to electrical stimulation of the auditory cortex (silver ball bipolar electrodes, single pulses, duration 0.2 ms, current 0.2-1.5 mA) were recorded in neurones in the inferior colliculus of rats anaesthetized with pentobarbital. Excitatory or inhibitory effects were obtained in 84 out of 162 recorded neurones. The majority of neurones responded with a short excitatory burst (with a latency from 3 to 15 ms); in some of them the initial excitation was followed by inhibition lasting from 30 to 150 ms. Few neurones only reacted to electrical stimulation by inhibition, which occurred 3-10 ms after the stimulus and lasted up to 300 ms. The inhibition either suppressed the spontaneous activity or the acoustically evoked response. Neurones reacting to stimulation of the auditory cortex were found mainly in the caudal and dorsal parts of the inferior colliculus.
Pathways descending from the auditory cortex (AC) to the inferior colliculus (IC) were described in detail in the cat [1, 2, 4] but recently this knowledge was also extended to the rat [3, 5-71. In both species, ipsilateral projections originate in pyramidal cells of layer Vb and terminate mainly in the dorsomedial part of the central IC nucleus, in the pericentral and external nuclei of the IC. As a first step in the investigation of the function o f the corticotectal auditory pathway we investigated the effects of electrical stimulation of the auditory cortex upon the neuronal activity in the IC. It will be shown that the corticotectal auditory fibres m a y influence the activity of m a n y IC neurones, the result usually leading to a brief burst of excitation followed by a longer period of inhibition. Experiments were performed in 17 Sprague-Dawley rats, anaesthetized with pentobarbital (40 mg/kg). The auditory cortex was exposed and two silver-ball electrodes (tip separation less than 1 mm) were placed on the dura mater at the point where the maximal evoked response to acoustic clicks was observed. In some experiments the dura was cut and the cortex (with the exception of the place of stimulation) was covered with agar. Single 0,2-ms electrical pulses, repe:ition rate 1 Hz and intensity ranging f r o m 0.2 to 1.5 m A were used as stimuli. Glass 0304-3940/84/$ 03.00 © 1984 Elsevier Scientific Publishers Ireland Ltd.
236 micropipettes filled with 3 M KCI were inserted into the IC stereotaxically through the cerebellum, i.e. perpendicularly to the frontal plane. An electronically driven microdrive at 2-#m steps was used for insertion of the electrodes into the brain. Extracellular unit activity was recorded with glass microelectrodes; gross evoked responses to electrical and auditory stimuli were also recorded in some experiments. The auditory stimuli were clicks (duration of the electrical pulse 50 ~s) and tone pips (duration 100-200 ms) delivered through a closed piezoelectric sound system, with the possibility of checking the actual sound pressure before the ear drum. The neuronal responses were monitored on a Tektronix 5103 N oscilloscope with a memory screen and photographed with a camera. The temperature of the animal was maintained at 38°C with a heating-pad. Localization of the electrode tracks in the IC was histologically checked. After the experiment the electrode was broken and left in the brain, which was perfused with formalin. Frontal sections were made on a freezing microtome to identify the last electrode track. Other electrode tracks (usually two or three) were reconstructed from stereotaxic and microdrive coordinates. Eighty-four neurones out of the 162 tested (i.e. 52.4°70) were influenced by the electrical stimulation (ES) of the auditory cortex. In the majority of cases, i.e. in 61 neurones, the ES resulted in a brief burst of excitation which occurred after a latency of 3-15 ms (Fig. 1). The burst consisted of 1-10 spikes and the number of spikes was dependent upon the ES intensity; with increasing intensity the number of spikes increased and the latency of the first spike also slightly decreased. The excitation threshold in some neurones was around 0.2 mA, but in some units it was necessary to increase the intensity up to 0.8-1.0 mA until the response was obtained. Essentially, the same results were obtained in animals with the intact dura mater and with the removed dura. Fig. 1 (left side) shows the excitatory response of an IC neurone (73D) to electrical stimulation of the auditory cortex. In this neurone, similarly as in many other IC neurones, the initial excitation was followed by a period of inhibition which lasted from 30 to 150 ms. In neurone 73D the postexcitatory inhibition of spontaneous activity evoked by auditory cortex stimulation is approximately as long as the inhibition of spontaneous activity evoked by a tone at the characteristic frequency (compare panels B and C, left). Inhibition resulting from AC stimulation may also efficiently suppress the excitatory response evoked by the auditory stimulus (panel D, left). In Fig. 1 (right side) another example of the IC neurone reaction to AC stimulation is demonstrated. This consists of an excitatory burst, which is followed by a 70 ms lasting inhibition. The lowest panel (D, right) demonstrates the excitatory burst evoked by AC stimulation cannot be inhibited by stimulation of the ipsilateral ear. This neurone, similar to the majority of other IC neurones, is excited from the contralateral ear and inhibited from the ipsilateral ear (EI type of binaural interaction). While the binaural stimulation (ipsilateral stimulus is 5 dB more intensive than the contralateral one) suppresses the tone-evoked activity, the AC-evoked burst is still present.
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Fig. i. Responses of two inferior colliculus neurones (RCI 73D, left side, and RCI 77B, right side) to electrical stimulation of the auditory cortex and to acoustical stimulation, Intensity of the electrical pulse, tone frequency and intensity are indicated to the left of the neuronal response traces, time in milliseconds is indicated under the neuronal response. All responses represent 5 consecutive summated individual responses, photographed from the screen of a memory oscilloscope.
In 21 out of 84 IC neurones (25o7o) which reacted to auditory cortex stimulation only an inhibitory effect was found. The inhibition, which suppressed the spontaneous activity or the sound-evoked activity, occurred after a 3-15 ms latency and lasted 30-300 ms. The duration of the inhibitory effect was dependent upon the intensity o f the electrical pulse. Fig. 2 (left side), shows an example of the net inhibitory effect of AC stimulation. Panel A (left) displays excitation evoked by the tone at the characteristic frequency (CF = 15 kHz), panel B (left) shows ineffective stimulation at subthreshold intensity (0.4 mA). The increase of intensity to 0.5 m A results in 200 ms lasting inhibition, whereas the 1.0 m A electrical pulse suppresses the activity for about 350 ms. In this neurone, as well as in other IC neurones, the AC-evoked inhibition suppressed unequivocally all auditory-evoked activity and the spontaneous activity. In some IC neurones, however, the inhibitory effects were different with respect to different parts of the sound evoked response. Fig. 2 (right side) in the upper two panels (A and B, right) shows the response of an IC neurone (76D) to tones at the CF (1.5 kHz) which consists of an initial excitatory burst,
238
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Fig. 2. Responses o f two inferior colliculus neurones (RCI 73K, left side, a n d RCI 76D, right side) to electrical s t i m u l a t i o n o f the a u d i t o r y cortex a n d to a c o u s t i c a l s t i m u l a t i o n . For further details see text to Fig. 1.
followed by inhibition (which is dependent on the tone intensity) and late excitation. The AC stimulation suppresses the late excitation (panel C, right) effectively but does not suppress the initial excitation (panel D, right). The localization of 65 neurones in the IC was estimated from the microdrive checking of the electrode tip and from the histological controls of the electrode track. The majority of neurones reacting to AC stimulation was found in the dorsomedial part of the central IC nucleus and in the caudal and dorsal parts of the IC, i.e. soon after penetration into the IC from the cerebellar side. Few tracks penetrated the external IC nucleus; neurones in the external IC were mostly inhibited by the AC stimulation. Neurones not reacting to the AC stimulation were distributed homogeneously in the caudorostral direction. In agreement with this finding in 5 rats, where the IC was penetrated in the caudorostral direction with macroelectrodes, a pronounced evoked response to AC stimulation was always observed near the caudal and dorsal surface of the IC, whereas the evoked response to the auditory stimuli (clicks) dominated more centrally and rostrally. The results unambiguously demonstrate that the auditory cortex in the rat im-
239
poses a strong influence upon the activity in the inferior colliculus. Electrical stimulation of the cortical auditory field excites the pyramidal cells in layer Vb, the axons o f which descend to the medial geniculate b o d y and to the inferior colliculus [1-7]. The targets of these fibres in the IC of the rat are mainly the ipsilateral dorsomedial part of the central IC nucleus and the external and pericentral nuclei of the IC. In correspondence with the anatomical distribution of the terminals of descending fibres, the cells reacting to electrical stimulation of the AC were found mainly in the dorsomedial part of the central IC nucleus and in the caudal and dorsal parts of the IC, during horizontal penetration of the IC through the cerebellum. A similar distribution was observed in evoked potentials to AC stimulation recorded with macroelectrodes. The antidromic stimulation of cells in the IC m a y be excluded since the IC does not send axons directly to the auditory cortex and the latency of excitatory responses in the IC is too long (3-15 ms) for antidromic stimulation. The thin dura mater in the rat apparently does not present any obstacle to the current spread, since the results were similar in animals with the dura either cut or intact. We suppose that the majority o f neurones of the auditory cortex in the rat, which send their axons to the IC, was stimulated by the current, because the auditory cortical area comprises approximately 3 x 2 mm. The functional role of the activation of the auditory descending system is unknown. The most frequently observed effect of AC stimulation was the inhibition of IC activity. In m a n y IC neurones in the rat the response to tone pips consists of an initial excitation which is followed by a period of inhibition. In some neurones the initial excitation represents the entire response, in other neurones the inhibitory pause is followed by late excitation. It may be assumed that the inhibitory period which occurs after the initial excitation is caused by activation of the auditory cortex. Similar inhibitory effects upon IC neurones were, however, also observed after electrical stimulation of the contralateral IC (Syka and Popelgt~, unpublished data). The cortically evoked inhibition, which lasted f r o m 20 to 300 ms for different IC neurones in our experiments m a y be the decisive factor, which limits the highest repetition rate for clicks in the IC. The repetition rate for clicks in the neurones of the rat IC was found to be in the range of 5-50 clicks per second, with the majority o f neurones lying near the lower border of this range [8]. 1 Adams, J.C., Crossed and descending projections to the inferior colliculus, Neurosci. Lett., 19 (1980) 1-5. 2 Andersen, R.A., Snyder, R.L. and Merzenich, M.M., The topographic organization of corticocollicular projections from physiologically identified loci in the A l, A II and anterior auditory cortical fields of the cat, J. comp. Neurol., 191 (1980) 479-494. 3 Beyerl, B.D., Afferent projections to the central nucleus of the inferior colliculus in the rat, Brain Res., 145 (1978) 209-223. 4 Diamond, I.T., Jones, E.G. and Powel, T.P.S., The projection of the auditory cortex upon the diencephalon and brain stem in the cat, Brain Res., 15 (1969) 305-340. 5 Druga, R. and Syka, J., Ascending and descending projections to the inferior colliculus in the rat, Physiol. Bohemosl., 33 (1984) 31-42.
240 6 Druga, R. and Syka, J., Neocortical projection to the inferior colliculus in the rat, Physiol. Bohemos|., 33 (1984) 251-253. 7 Syka, J., Druga, R., Popel~if, J. and Kalinovfi, B., Functional organization of the inferior colliculus. In J. Syka and L. Aitkin (Eds.), Neuronal Mechanisms of Hearing, Plenum Press, New York, 1981, pp. 137-153. 8 Syka, J., Popel;if, J. and Druga, R., Structure and function of crossed and uncrossed pathways to the inferior colliculus in the rat. In R. Klinke and R. H a r t m a n n (Eds.), Hearing-Physiological Bases and Psychophysics, Springer Verlag, 1983, pp. 224-229.