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Some dorsal raphe axons of the cat bifurcate to project into bilateral medial forebrain bundles
Neuroscience Letters, 42 (1983) 119-124
!19
Elsevier Scientific Pubfishers Ireland Ltd.
SOME DORSAL RAPHE AXONS-OF THE CAT BIFURCATE TO PROJECT INTO BILATERAL MEDIAL FOREBRAIN BUNDLES
KAZUSHIGE WATABE a n d TOYOHIKO SATOH
Department o f Physiology, Aichi-Gakuin University Dental School, 2-11, Suemori, Chikusa-ku, Nagoya 464 (Japan) (Received July 22nd, 1983; Revised version received September 6th, 1983; Accepted September 8th, 1983)
Key words: bifurcated axon - bilateral projection - dorsal raphe neurons - cat
Seventy-one dorsal raphe (DR) neurons of the cat were antidromically activated by electrical stimulation of the medial forebrain bundle (MFB). It was found that not a small number of DR neurons send their axons contralaterally. Seven out of 67 DR neurons (10.4%) responded to stimulation of both sides of MFB. Positive collision test for the bilaterally evoked responses indicates that single axons of these DR neurons bifurcate to innervate bilateral forebrain structures.
The ascending serotonergic fibers in the cat originate mainly from the dorsal raphe (DR) and the central superior raphe, and join to the medial forebrain bundle (MFB) at the level of the lateral hypothalamus [4l. With the aid of the retrograde double labeling technique, it has been reported by two research groups [2, 11] that the rat DR neurons project uniquely unilaterally (i.e. solely ipsilaterally or solely contralaterally) and never bilaterally when examined at homonymous forebrain structures. However, one of these two groups [2| has described that a small number of neurons in the median raphe (MR)and the interfascicular nucleus (IF), which is interposed between the DR and the MRin the rat, has bilateral innervation. In the cat it has not yet been examined whether a single raphe neuron can have bilateral innervation. The present electrophysiological study demonstrates that bilaterally projecting neurons do exist in the cat DR. Six adult cats of either sex, weighing 2.4-3.6 kg, were used. The anesthesia was initiated with pentobarbital sodium (40 mg/kg, i.p.) and maintained with continuous infusion of a mixture of urethane and ,-chloralose (15 and 3 mg/kg/h, respectively) through the brachial vein. The skull overlying the stimulation and recording sites was trepanned. Parallel, bipolar stimulation electrodes (0.2 mm in diameter and with vertical tip separation of 0.5-1.0 mm) were implanted into the unilateral (n = 1) or bilateral (n = 5) MFBs (AP, + 9.0; L, 2,5 - 3.0; subcortical depth D, 22.5-24.0 mm) [3, 4, 8, 9] for antidromic activation of the DR neurons. The 0304-3940/83/$ 03.00 © 1983 Elsevier Scientific Publishers Ireland Ltd.
120
responses were regarded as antidromic when they could follow 100 Hz stimulation with double rectangular pulses which were 0.8-1.5 mA in intensity and 0.1-2.0 ms in duration. The recording from the DR (AP, + 0.6 to - 0 . 9 ; L, 0; H, 0 to - 1.5 mm) was performed with glass pipettes of tip diameter smaller than 2 ttm, which were filled with 2 M NaC! solution saturated with Fast Green F C F (impedance, 2-10 Mfl), and were advanced through the cerebellum at an angle of 45 ° on the sagittal plane. The responses were stored on FM magnetic tapes at frequency response flat up to 5 kHz. The recording sites were marked by a dye injection ( - 10 #A, 20 min). The electrolytic lesions were placed in the stimulated sites ( + 5#A, 10 s). After i.v. injection of a lethal dose of pentobarbital sodium the animal was perfused with 0.9~0 saline followed by 10070 formalin. The recording and stimulation sites were verified on Nissl-stained sections of 30 #m thick. Electrical stimulation of the MFB of either side evoked a monophasic, negative field potential in the DR region (Fig. IA, B). This potential had a latency of 10-12 ms and a duration of 10-15 ms. The amplitude was maximum (0.7 mV) when the electrode was positioned inside the DR nucleus. Seventy-one DR neurons were antidromically activated by the MFB stimulation. Most of the antidromic spike discharges occurred during the midst of or in close association with the negative field potential (Fig. IC). The spike responses were of negative or positive-negative deflection and were presumably due to activation of serotonin-containing neurons because the spikes had a long duration (2-4 ms) (Fig. ID) and a slow spontaneous discharge rate (0-4 Hz) 11, 7, 10, 12]. The criteria for the antidromic responses were their fixed latency and their capability to follow double shocks of short interval. In most of the antidromic spikes evoked by the second stimulus of double shock with less than 10 ms interval, a notch began to appear on the positive phase of the spike. 1his was more prominent with sl~orter intervals, resulting in a marked prolongation t~l" thc spike duration (Fig. I D). A possible mechanism for this deformation of the second spike might be the refractoriness of the soma a n d / o r the recurrent inhibition between serotonin-containing DR neurons [12]. The collision test could not be done in most cases because the majority of neurons had no or very low spontaneous discharge. The mean antidromic latency was 19.6 ms (range, 2.8-54 ms) (Fig. IC). The estimated mean conduction velocity was 0.5 m/s (range, 0.2-3.6 m/s). The refractory period of the axon of antidromically activated DR neurons was about 2 ms, because the antidromic unit response often failed to follow 500 Hz double shocks in the tested neurons. In 67 DR neurons the stimulation was performed in bilateral MFBs to test possible bifurcation of an axon. Seven neurons (10.4°70) could be antidromically activated from the bilateral MFBs (Fig. 2 and Table I). The difference in latency of the tx~o kinds (i.e. left MFB- and right MFB-evoked) of antidromic response in 5 neurons was small (0-5 ms). In these neurons the latency of each antidromic response was between 17 and 40 ms. The other 2 neurons had a very great latency difference (12 and 37 ms). In order to exclude the possibility that the bilateral an-
121 ~ii!~ i~ii~iiiiiili~iili ~~~i~,i,,;ii!;~i ~ ~ii!~il/i¸ilii~ii~ii i/! ~ ,~iiiill ¸
|
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I 20 30 Latency (ms)
40
<
D
~
~!i~
~!
/
i/?~
i ¸¸¸¸¸~
~/~/ iii~ ¸!¸~¸¸¸¸¸¸
Fig. 1. A: negative field potentials evoked antidromically at the same site in the DR after the left (top) and right (bottom) MFB stimulation. Low frequency elimination, 1 Hz. Negativity upward. Calibrations: 0.2 mV, 20 ms. B: the recordifJg site, marked with dye (arrow), of the field potentials shown in A. C: distribution of the latencies of 71 DR neurons activated antidromically after the MFB stimulation. For bilaterally projecting neurons which gave two different latencies, the smaller value was adopted. 62°7o of the antidromic spikes occurred during the negative field potentials. D: antidromic spikes of a DR neuron with typical characteristics of a serotonin-containing neuron. Double shock stimulation of the MFB at 100 and 250 Hz (white dots). The antidromic spikes were expanded in bottom traces to show the notch and long duration of the second spike (about 4 ms). Calibrations; 0.5 mV, and 5 ms for the top traces and 2 ms for the bottom. Abbreviations: AQ, aqueduct; MLF, medial longitudinal fasciculus.
122
tidromic responses were elicited by stimulating the same axon which i~ running through the two stimulated sites by crossing the midline, the collision of the bilaterally evoked responses was tested. When the following condition was fulfilled, the tested neuron was considered to have a bilaterally projecting bifurcated axon: C > !L - R t + r, where C is the time interval between the paired shocks which can produce a collision of the responses evoked by bilateral MFB stimulation, L and R are the antidromic latencies after the left and right MFB stimulation, respectively, and r is the refractory period (about 2 ms) of the axon in the left or right MFB stimulated by the later pulse of the paired shocks [6]. Six neurons were positive in this test (Fig. 2D). In the remaining one neuron which was short-lived the test could not be done. The iaterality of projection of a given DR neuron could be determined only after histological verification of its somatic position which was marked by injecting the dye. Of 29 DR neurons thus identified, 14 responded to the contralateral MFB stimulation. Five of these 14 were bilaterally projecting; in 3 neurons the latency of the contralaterally evoked response was shorter than that of the ipsilaterally evoked one, and in 2 the latencies of both responses were equal (Table I).
Fig. 2. Antidromic responses from a bilaterally projecting DR neuron. The antidromic spike latency after the ip,,i- and contralateral MFB stimulations was 40 (A) and 35 ms ~B), respectively. The contralateral MFB stimulation done 80 ms alter the ipsilaterai MFB stimulation could elicit the antidromic spike IC), but could not at 50 ms interval; note absence of the spike at the white triangle in D. Five s~,'eeps ~'ere ,,tlpcrimposed in A and [.~, single s,,~,'eeps in (" and D. Calibratio~-~s: 0.2 mV, 20 ms for A and B, and 40 ms for C and D.
123 TABLE I LATENCIES OF A N T I D R O M I C UNIT RESPONSES BILATERALLY P R O J E C T I N G NEURONS
TO THE
MFB S T I M U L A T I O N
IN 7
lpsi. and Contra., and Left and Right, are sides of the stimulated MFBs in the histologically identified and unidentified (*) neurons; respectively, lpsi stands for the side ipsilateral to the somatic position of a given neuron. Latencies e:~.pressed in ms. Neuron
1
2 3 4 5 6* 7*
Sides of the stimulated MFBs lpsi.
Contra.
32 33 32 40 54
32 33 31 35 42
Left
Right
17 65
17 28
In contrast to the conclusion by two research groups [2, 11] that in the rat DR there is no bilaterally 9rojecting neuron, the present study in the cat has revealed the existence of bilateral projection. The percentage of the bilaterally projecting neurons in the total population of the cat DR neurons surveyed (10.4°/0) was surprisingly similar to that in the rat MR and IF (10%) [2], suggesting a species difference in distribution in the raphe system of the neurons with comparable anatomofunctional features. The bilaterally projecting neurons in the cat DR seem to branch off a collateral of relatively large diameter, because the majority of the neurons responded antidromically with almost equal latency following MFB stimulation of either side. However, the longer latency response tended to be obtained after ipsilateral stimulation, indicating that the contralaterally projecting fiber can be ~he parent axon. Another point of the present results is that a not small number of DR neurons seem to project contralaterally. This is in accordance with the report by Van der Kooy and Hattori [111 that 15-30°/0 of rat DR neurons project axons to the contralateral caudate-putamen complex. The conduction velocity of 69 DR neurons (97% of recorded neurons) was within the range of unmyelinated C-fibers (slower than 2.4 m/s) [5]. In this respect they are similar to the rat DR neurons (0.3-1.5 m/s) [121. The results of the present experiment point out that a substantial amount of output signals from the DR is directed toward the contralateral brain, and an appreciable part of them is involved in the regulation of correlated functioning of various structures of localization mutually contralateral. This study was partly supported by Grant 58770122 to K.W. from the Ministry of Education, Science and Culture of Japan.
124 I Aghajanian, O.K. and Vandermaelen, C.P., lntracellular recordings from serotonergic dorsal raphe neurons: pacemaker potentials and the effect of LSD, Brain Res., 238 (1982) 463-469. 2 Azmitia, E.C., Bilateral serotonergic projections to the dorsal hippocampus of the rat: simultaneous localization of 'H-5HT and HRP after retrograde transport, J. comp. Neurol., 203 (1981) 737-743. 3 Berman, A.L., The Brain Stem of the Cat, University of Wisconsin Press, Madison, 1968, 175 pp. 4 Bobilfier, P., Seguin, S., Petitjean, F., Salvert, D., Touret, M. and Jouvet, M., The raphe nuclei of the cat brain stem: a topographical atlas of their efferent projections as revealed by autoradiography, Brain Res., 113 (1976) 449-486. 5 Brazier, M.A.B., The Electrical Activity of the Nervous System, Williams and Wilkins, Baltimore, 1968, 317 pp. 6 Deaiau, J.M., Hammond, C., Riszk, A. and Feger, J., Electrophysiological properties of identified output neurons of the rat substantia nigra (pars compacta and pars reticulata): evidences for the existence of branched neurons, Exp. Brain Res., 32 (1978) 409-422. 7 Park, M.R., imai, H. and Kitai, S.T., Morphology and intracellular responses of an identified dorsal raphe projection neuron, Brain Res., 240 (1982) 321-326. 8 Reinoso-Suarez, F., Topografischer Hirnatlas der Katze, Merck, Darmstadt, 1961. 9 Taber-Pierce, E., Foote, W.E. and Hobson, J.A., The efferent connection of the nucleus raphe dorsalis, Brain Res., 107 (1976) 137-144. 10 Trulson, M.E. and Trulson, V.M. Chloral hydrate anesthesia blocks the excitatory responses of dorsal raphe neurons to phasic auditory and visual stimuli in cats, Brain Res., 265 (1983) 129-133. I I Van der Kooy, D. and Hattori, T., Bilaterally situated dorsal raphe cell bodies have only unilateral forebrain projection in rat, Brain Res., 192 (1980) 550-554. 12 Wang, R.Y. and Aghajanian, G.K., Antidromically identified serotonergic neurons in the rat midbrain raphe: evidence for collateral inhibition, Brain Res., 132 (1977) 186-193.
!19
Elsevier Scientific Pubfishers Ireland Ltd.
SOME DORSAL RAPHE AXONS-OF THE CAT BIFURCATE TO PROJECT INTO BILATERAL MEDIAL FOREBRAIN BUNDLES
KAZUSHIGE WATABE a n d TOYOHIKO SATOH
Department o f Physiology, Aichi-Gakuin University Dental School, 2-11, Suemori, Chikusa-ku, Nagoya 464 (Japan) (Received July 22nd, 1983; Revised version received September 6th, 1983; Accepted September 8th, 1983)
Key words: bifurcated axon - bilateral projection - dorsal raphe neurons - cat
Seventy-one dorsal raphe (DR) neurons of the cat were antidromically activated by electrical stimulation of the medial forebrain bundle (MFB). It was found that not a small number of DR neurons send their axons contralaterally. Seven out of 67 DR neurons (10.4%) responded to stimulation of both sides of MFB. Positive collision test for the bilaterally evoked responses indicates that single axons of these DR neurons bifurcate to innervate bilateral forebrain structures.
The ascending serotonergic fibers in the cat originate mainly from the dorsal raphe (DR) and the central superior raphe, and join to the medial forebrain bundle (MFB) at the level of the lateral hypothalamus [4l. With the aid of the retrograde double labeling technique, it has been reported by two research groups [2, 11] that the rat DR neurons project uniquely unilaterally (i.e. solely ipsilaterally or solely contralaterally) and never bilaterally when examined at homonymous forebrain structures. However, one of these two groups [2| has described that a small number of neurons in the median raphe (MR)and the interfascicular nucleus (IF), which is interposed between the DR and the MRin the rat, has bilateral innervation. In the cat it has not yet been examined whether a single raphe neuron can have bilateral innervation. The present electrophysiological study demonstrates that bilaterally projecting neurons do exist in the cat DR. Six adult cats of either sex, weighing 2.4-3.6 kg, were used. The anesthesia was initiated with pentobarbital sodium (40 mg/kg, i.p.) and maintained with continuous infusion of a mixture of urethane and ,-chloralose (15 and 3 mg/kg/h, respectively) through the brachial vein. The skull overlying the stimulation and recording sites was trepanned. Parallel, bipolar stimulation electrodes (0.2 mm in diameter and with vertical tip separation of 0.5-1.0 mm) were implanted into the unilateral (n = 1) or bilateral (n = 5) MFBs (AP, + 9.0; L, 2,5 - 3.0; subcortical depth D, 22.5-24.0 mm) [3, 4, 8, 9] for antidromic activation of the DR neurons. The 0304-3940/83/$ 03.00 © 1983 Elsevier Scientific Publishers Ireland Ltd.
120
responses were regarded as antidromic when they could follow 100 Hz stimulation with double rectangular pulses which were 0.8-1.5 mA in intensity and 0.1-2.0 ms in duration. The recording from the DR (AP, + 0.6 to - 0 . 9 ; L, 0; H, 0 to - 1.5 mm) was performed with glass pipettes of tip diameter smaller than 2 ttm, which were filled with 2 M NaC! solution saturated with Fast Green F C F (impedance, 2-10 Mfl), and were advanced through the cerebellum at an angle of 45 ° on the sagittal plane. The responses were stored on FM magnetic tapes at frequency response flat up to 5 kHz. The recording sites were marked by a dye injection ( - 10 #A, 20 min). The electrolytic lesions were placed in the stimulated sites ( + 5#A, 10 s). After i.v. injection of a lethal dose of pentobarbital sodium the animal was perfused with 0.9~0 saline followed by 10070 formalin. The recording and stimulation sites were verified on Nissl-stained sections of 30 #m thick. Electrical stimulation of the MFB of either side evoked a monophasic, negative field potential in the DR region (Fig. IA, B). This potential had a latency of 10-12 ms and a duration of 10-15 ms. The amplitude was maximum (0.7 mV) when the electrode was positioned inside the DR nucleus. Seventy-one DR neurons were antidromically activated by the MFB stimulation. Most of the antidromic spike discharges occurred during the midst of or in close association with the negative field potential (Fig. IC). The spike responses were of negative or positive-negative deflection and were presumably due to activation of serotonin-containing neurons because the spikes had a long duration (2-4 ms) (Fig. ID) and a slow spontaneous discharge rate (0-4 Hz) 11, 7, 10, 12]. The criteria for the antidromic responses were their fixed latency and their capability to follow double shocks of short interval. In most of the antidromic spikes evoked by the second stimulus of double shock with less than 10 ms interval, a notch began to appear on the positive phase of the spike. 1his was more prominent with sl~orter intervals, resulting in a marked prolongation t~l" thc spike duration (Fig. I D). A possible mechanism for this deformation of the second spike might be the refractoriness of the soma a n d / o r the recurrent inhibition between serotonin-containing DR neurons [12]. The collision test could not be done in most cases because the majority of neurons had no or very low spontaneous discharge. The mean antidromic latency was 19.6 ms (range, 2.8-54 ms) (Fig. IC). The estimated mean conduction velocity was 0.5 m/s (range, 0.2-3.6 m/s). The refractory period of the axon of antidromically activated DR neurons was about 2 ms, because the antidromic unit response often failed to follow 500 Hz double shocks in the tested neurons. In 67 DR neurons the stimulation was performed in bilateral MFBs to test possible bifurcation of an axon. Seven neurons (10.4°70) could be antidromically activated from the bilateral MFBs (Fig. 2 and Table I). The difference in latency of the tx~o kinds (i.e. left MFB- and right MFB-evoked) of antidromic response in 5 neurons was small (0-5 ms). In these neurons the latency of each antidromic response was between 17 and 40 ms. The other 2 neurons had a very great latency difference (12 and 37 ms). In order to exclude the possibility that the bilateral an-
121 ~ii!~ i~ii~iiiiiili~iili ~~~i~,i,,;ii!;~i ~ ~ii!~il/i¸ilii~ii~ii i/! ~ ,~iiiill ¸
|
•
I 20 30 Latency (ms)
40
<
D
~
~!i~
~!
/
i/?~
i ¸¸¸¸¸~
~/~/ iii~ ¸!¸~¸¸¸¸¸¸
Fig. 1. A: negative field potentials evoked antidromically at the same site in the DR after the left (top) and right (bottom) MFB stimulation. Low frequency elimination, 1 Hz. Negativity upward. Calibrations: 0.2 mV, 20 ms. B: the recordifJg site, marked with dye (arrow), of the field potentials shown in A. C: distribution of the latencies of 71 DR neurons activated antidromically after the MFB stimulation. For bilaterally projecting neurons which gave two different latencies, the smaller value was adopted. 62°7o of the antidromic spikes occurred during the negative field potentials. D: antidromic spikes of a DR neuron with typical characteristics of a serotonin-containing neuron. Double shock stimulation of the MFB at 100 and 250 Hz (white dots). The antidromic spikes were expanded in bottom traces to show the notch and long duration of the second spike (about 4 ms). Calibrations; 0.5 mV, and 5 ms for the top traces and 2 ms for the bottom. Abbreviations: AQ, aqueduct; MLF, medial longitudinal fasciculus.
122
tidromic responses were elicited by stimulating the same axon which i~ running through the two stimulated sites by crossing the midline, the collision of the bilaterally evoked responses was tested. When the following condition was fulfilled, the tested neuron was considered to have a bilaterally projecting bifurcated axon: C > !L - R t + r, where C is the time interval between the paired shocks which can produce a collision of the responses evoked by bilateral MFB stimulation, L and R are the antidromic latencies after the left and right MFB stimulation, respectively, and r is the refractory period (about 2 ms) of the axon in the left or right MFB stimulated by the later pulse of the paired shocks [6]. Six neurons were positive in this test (Fig. 2D). In the remaining one neuron which was short-lived the test could not be done. The iaterality of projection of a given DR neuron could be determined only after histological verification of its somatic position which was marked by injecting the dye. Of 29 DR neurons thus identified, 14 responded to the contralateral MFB stimulation. Five of these 14 were bilaterally projecting; in 3 neurons the latency of the contralaterally evoked response was shorter than that of the ipsilaterally evoked one, and in 2 the latencies of both responses were equal (Table I).
Fig. 2. Antidromic responses from a bilaterally projecting DR neuron. The antidromic spike latency after the ip,,i- and contralateral MFB stimulations was 40 (A) and 35 ms ~B), respectively. The contralateral MFB stimulation done 80 ms alter the ipsilaterai MFB stimulation could elicit the antidromic spike IC), but could not at 50 ms interval; note absence of the spike at the white triangle in D. Five s~,'eeps ~'ere ,,tlpcrimposed in A and [.~, single s,,~,'eeps in (" and D. Calibratio~-~s: 0.2 mV, 20 ms for A and B, and 40 ms for C and D.
123 TABLE I LATENCIES OF A N T I D R O M I C UNIT RESPONSES BILATERALLY P R O J E C T I N G NEURONS
TO THE
MFB S T I M U L A T I O N
IN 7
lpsi. and Contra., and Left and Right, are sides of the stimulated MFBs in the histologically identified and unidentified (*) neurons; respectively, lpsi stands for the side ipsilateral to the somatic position of a given neuron. Latencies e:~.pressed in ms. Neuron
1
2 3 4 5 6* 7*
Sides of the stimulated MFBs lpsi.
Contra.
32 33 32 40 54
32 33 31 35 42
Left
Right
17 65
17 28
In contrast to the conclusion by two research groups [2, 11] that in the rat DR there is no bilaterally 9rojecting neuron, the present study in the cat has revealed the existence of bilateral projection. The percentage of the bilaterally projecting neurons in the total population of the cat DR neurons surveyed (10.4°/0) was surprisingly similar to that in the rat MR and IF (10%) [2], suggesting a species difference in distribution in the raphe system of the neurons with comparable anatomofunctional features. The bilaterally projecting neurons in the cat DR seem to branch off a collateral of relatively large diameter, because the majority of the neurons responded antidromically with almost equal latency following MFB stimulation of either side. However, the longer latency response tended to be obtained after ipsilateral stimulation, indicating that the contralaterally projecting fiber can be ~he parent axon. Another point of the present results is that a not small number of DR neurons seem to project contralaterally. This is in accordance with the report by Van der Kooy and Hattori [111 that 15-30°/0 of rat DR neurons project axons to the contralateral caudate-putamen complex. The conduction velocity of 69 DR neurons (97% of recorded neurons) was within the range of unmyelinated C-fibers (slower than 2.4 m/s) [5]. In this respect they are similar to the rat DR neurons (0.3-1.5 m/s) [121. The results of the present experiment point out that a substantial amount of output signals from the DR is directed toward the contralateral brain, and an appreciable part of them is involved in the regulation of correlated functioning of various structures of localization mutually contralateral. This study was partly supported by Grant 58770122 to K.W. from the Ministry of Education, Science and Culture of Japan.
124 I Aghajanian, O.K. and Vandermaelen, C.P., lntracellular recordings from serotonergic dorsal raphe neurons: pacemaker potentials and the effect of LSD, Brain Res., 238 (1982) 463-469. 2 Azmitia, E.C., Bilateral serotonergic projections to the dorsal hippocampus of the rat: simultaneous localization of 'H-5HT and HRP after retrograde transport, J. comp. Neurol., 203 (1981) 737-743. 3 Berman, A.L., The Brain Stem of the Cat, University of Wisconsin Press, Madison, 1968, 175 pp. 4 Bobilfier, P., Seguin, S., Petitjean, F., Salvert, D., Touret, M. and Jouvet, M., The raphe nuclei of the cat brain stem: a topographical atlas of their efferent projections as revealed by autoradiography, Brain Res., 113 (1976) 449-486. 5 Brazier, M.A.B., The Electrical Activity of the Nervous System, Williams and Wilkins, Baltimore, 1968, 317 pp. 6 Deaiau, J.M., Hammond, C., Riszk, A. and Feger, J., Electrophysiological properties of identified output neurons of the rat substantia nigra (pars compacta and pars reticulata): evidences for the existence of branched neurons, Exp. Brain Res., 32 (1978) 409-422. 7 Park, M.R., imai, H. and Kitai, S.T., Morphology and intracellular responses of an identified dorsal raphe projection neuron, Brain Res., 240 (1982) 321-326. 8 Reinoso-Suarez, F., Topografischer Hirnatlas der Katze, Merck, Darmstadt, 1961. 9 Taber-Pierce, E., Foote, W.E. and Hobson, J.A., The efferent connection of the nucleus raphe dorsalis, Brain Res., 107 (1976) 137-144. 10 Trulson, M.E. and Trulson, V.M. Chloral hydrate anesthesia blocks the excitatory responses of dorsal raphe neurons to phasic auditory and visual stimuli in cats, Brain Res., 265 (1983) 129-133. I I Van der Kooy, D. and Hattori, T., Bilaterally situated dorsal raphe cell bodies have only unilateral forebrain projection in rat, Brain Res., 192 (1980) 550-554. 12 Wang, R.Y. and Aghajanian, G.K., Antidromically identified serotonergic neurons in the rat midbrain raphe: evidence for collateral inhibition, Brain Res., 132 (1977) 186-193.