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Both fast and slow relay cells in lateral geniculate nucleus of rabbits receive recurrent inhibition
Brain Research, 271 (1983)335-338 Elsevier
335
Both fast and slow relay cells in lateral geniculate nucleus of rabbits receive recurrent inhibition FU-SUN LO Shanghai Brain Research Institute, Academia Sinica, 319 Yo-Yang Road, Shanghai (China)
(Accepted March 15th, 1983) Key words: albino rabbit - - dorsal lateral geniculate nucleus- - slow relay ceils- - recurrent inhibition
IntraceUular recordings from slow relay cells in the rabbit lateral geniculate nucleus demonstrated that the slow cells, just like the fast ones, receive a monosynapticEPSP and a trisynaptic IPSP from the optic nerve. The IPSP is most likely mediated by interneurones activated by recurrent collaterals of the relay cell axon. In slow relay cells, both the EPSP and the IPSP from the optic nerve are brought about via the same group of slowly-conductingfibers. No mutual inhibition between the fast and the slow conductingsystem was observed in the rabbit lateral geniculate nucleus.
It was described by Fuster, Creutzfeldt and Straschill in 19652 that stimulation of the optic tract (OT) of the rabbit could produce, in the relay cells of the lateral geniculate nucleus (LGN), a response with an EPSP-IPSP sequence as shown in intraceUular recordings. From the fact that the latency of the IPSP was 1.5-3 ms longer than the latency of the EPSP, the authors assumed that 'inhibition of geniculate cells by afferent stimuli is probably mediated mainly by collateral connection within the geniculate. Histologically, there is a basis for this form of inhibition since recurrent collaterals as well as short axon cells, perhaps serving as interneurones, have been found in the geniculate'2. This assumption had not been confirmed, until 1981 when we demonstrated that the IPSP from OT in relay cells of the rabbit LGN was indeed mediated by a recurrent circuit rather than by a feed-forward circuit 5. The evidence on which this belief was based was as follows: (1) the IPSP from OT and the recurrent IPSP from the visual cortex (Cx) had similar configuration; (2) the latency of the IPSP from OT was about 1.6 ms longer than that of the preceding monosynaptic EPSP, indicating that two additional synapses were involved in the inhibitory circuit; (3) 'local' latency, as estimated by an extrapolation procedure, demonstrated that the IPSP from OT was brought about via a neuronal circuit with 3 synapses; and (4) a facilitatory interaction between 0006-8993/83/$03.00© 1983 Elsevier Science Publishers B.V.
IPSPs from Cx and OT was considered suggestive of the possibility that they were mediated by common interneurones in the recurrent circuit. Since this conclusion was drawn from the results of intracellular recordings, it should be taken into account that the impalement of neurones may be preferential to the large ones. The problem whether smaller neurones receive recurrent inhibition is not settled yet. Relay cells in the rabbit LGN, like those in the cat LGN, can be classified into fast (Y-like) and slow (X-like) cells, according to the difference in latency of the spikes evoked by stimulation of the optic nerve (ON). The latency boundary separating the fast from the slow relay cells was taken at 4.3 ms 6. By analogy with the cat1,4, the fast relay cells in the rabbit are presumably also of large size, while the slow cells are of medium size. In the present study, attention was paid particularly to the slow relay cells, in order to determine whether or not recurrent inhibition acts on both fast and slow cells. Albino rabbits weighing 2-3 kg were used in the experiments. Surgical operations were performed under sodium pentobarbital anaesthesia (Nembutal; Abbott; 30-35 mg/kg). During investigation, the rabbits were immobilized by continuous infusion of gallamine triethiodide (Flaxedil; May and Baker) and artificially respirated. ECG, end-tidal CO 2 and body temperature were monitored throughout the experi-
336 ments. The front of the contralateral eye was removed and a stimulating electrode was placed on the optic nerve just behind the eye. Concentric electrodes were stereotaxiacally inserted in the vicinity of the optic chiasm (OX) 7. In order to evoke antidromic spike in the relay cell of LGN, the ipsilateral primary visual cortex was stimulated according to the topographical map of the visual cortex 3. Micropipettes filled with 3 M potassium acetate were used for recording from the relay cells in LGN. The intracellular as well as extracellular response was conventionally displayed and recorded. The records from a typical slow cell are exemplified in Fig. 1. This slow relay cell identified by the antidromic activation from Cx (Fig. 1A) gave rise to a single spike in response to ON stimulation, at a latency of 7.4 ms (Fig. 1C). Stimulation of OX evoked a spike at a latency of 3.6 ms (Fig. 1B). Intracellular recordings from this cell showed the same results as described in the previous paperS, namely, an EPSPIPSP sequence could be evoked by OX stimulation and the IPSPs from either OX or Cx (Fig. 1D and E), despite the difference in amplitude, had rather long duration in contrast with the short duration of the feed-forward IPSP in the cat LGN 5. In addition,
A
Cx
B
the interval between the onset of the EPSP and the onset of the IPSP, as indicated by arrows in Fig. 1F, was 1.8 ms, suggesting that the [PSP was most likely mediated by a trisynaptic circuit. It is therefore reasonable to take the IPSP in slow cells as being brought about through recurrent collaterals, just like the IPSP in fast cells. All the 36 relay cells studied showed an EPSPIPSP sequence in response to stimulation of ON. The latencies of the EPSPs ranged from 1.8 to 5.5 ms with an average of 3.6 ms. The frequency histogram of the latencies for the EPSPs (Fig. 2A, unshaded columns) had a peak at 3.0-3.5 ms and a slow decay from the peak to 5.5 ms, which resulted in an asymmetric contour of the histogram. It was previously described that there were two peaks formed by fast (at 3.0-4.0 ms) and slow (at 5.0-5.5 ms) cells, respectively, in the latency distribution for the spikes evoked by stimulation of ON 6. However, only one peak could be found in the histogram of iatencies for EPSPs from ON, which may be due mainly to the in'sufficient sample (n = 36) in plotting the histogram of EPSP latency. As mentioned above, it is more difficult to impale slow relay cells than fast ones, so that the second peak formed by the slow cells in the histo-
OX
C
ON
!
l
Fig. 1. Records from a slow relay cell in the rabbit LGN. A-C show extracellular recordings and D-F intracellular ones. 4 sweeps superimposed in each record. A: antidromic activation from stimulation of the visual cortex (Cx) at near threshold intensity. B: a spike evoked by stimulation of the optic chiasm (OX). C: a spike evoked by stimulation of the optic nerve (ON). D: an IPSP after Cx stimulation. E and F: EPSP-IPSP sequences followingOX stimulation at different sweep speeds. Arrows in F indicate the onsets of PSPs. Time scale: 5 ms for A-C and F, 50 ms for D and E. Voltagecalibration: 2 mV. Positivedeflexionsupward.
337 B 8.
7-
oo
A 12-
Latenc~s from ON
I0-
88"6
s:
6'
I-'3 EPSP IPSP --
4,
5' n=36
z 4.
09:3
2.
22o
30
4o
5o
so
Latency
70ms
i
~
~, ,;,
.~ ~
-~,~
Lot El:aSP
Fig. 2. A: frequency histograms of latencies for EPSPs and IPSPs from ON. Crossed part represents the overlap of the histograms. B: correlation diagram between EPSP and IPSP latencies of 36 relay cells. Regression line is y = 1.07 x + 1.61. Correlation coefficient: 0.93. Further details in the text.
gram appears less prominent. In order to ascertain whether there was any slow cell included in the sample, the EPSP latency boundary had to be set to separate the fast and the slow cells. Since the latency of the EPSP from ON is shorter than that of the spikes evoked by ON stimulation, the boundary in EPSP latency distribution should be at a value less than 4.3 ms. Even if the boundary is set at 4.3 ms, there are still 7 cells eligible for slow relay cells. In fact, there were more than 7 slow cells included in the sample. It was these slow cells that formed the 'long trail' of the histogram for EPSP latency. The latencies of the IPSPs varied from 3.2 to 6.8 ms with an average of 5.2 ms (Fig. 2A, hatched columns). The frequency histogram showed a peak at 4.5-4.8 ms and a slow decay as well. Generally speaking, the histogram for IPSPs had the same contour as that for EPSPs, except the discordant position on the abscissa, suggesting that the latency difference between EPSP and IPSP in every cell was almost constant. Shown in Fig. 2B is a correlation diagram in which the latencies of the EPSPs from ON were plotted against the latencies of the IPSPs. It can be seen that all points representing either fast or slow relay cells are along a straight line. The slope of the regression line is about 1, indicating that the EPSP and the IPSP in each cell are mediated by the same group of fibers.
The interception is at 1.6 ms, which represents the mean value of difference between the latencies of EPSP and IPSP. The similarity between fast and slow relay cells implies that they are in the same position of the neuronal circuits in LGN. In other words, they all receive recurrent inhibition. An inhibitory convergence of the X- and Y-systems was previously proposed to exist in the cat LGN8. Since X-cells, which received excitatory input via slowly conducting X-system, could also receive inhibitory input via fast conducting Y-system, the IPSP from OX in these cells frequently started at the same time as the EPSP. However, it is not the case in the rabbit L G N , in which the IPSPs from OX in slow (X-like) cells as well as in fast (Y-like) ones always appeared much later than the EPSPs as shown in Figs. 1 and 2. No evidence for mutual inhibition between fast and slow relay cells was observed in the rabbit LGN. It can be asserted that there is no excitatory convergence of the fast and slow systems on the interneurones in the recurrent circuit for the slow relay cells. This work was supported by an Exchange Fellowship of Royal Academy of Engineering Sciences, Sweden. I am grateful to Drs. S. LindstrOm and G. Ahls6n for their advice in this work.
338 1 Friedlander, N. J., Lin, C. S. and Sherman, S. M., Structure of physiologically identified X and Y cells in the cat's lateral geniculate nucleus, Science, 204 (1979) 1114-1116. 2 Fuster, J. M., Creutzfeldt, O. D. and Straschill, M., Intracellular recording of neuronal activity in the visual system, Z. vergl. Physiol., 49 (1965) 605-622. 3 Hughes, A., Topographical relationships between the anatomy and physiology of the rabbit visual system, Docum. Ophthal., 30 (1971) 33-159. 4 LeVay, S. and Ferster, D., Relay cell classes in the lateral geniculate nucleus of the cat and the effects of visual deprivation, J. cornp. Neurol., 172 (1977) 563-584.
5 Lo, F.-S., Synaptic organization of the lateral geniculate nucleus of the rabbit: lack of feed-forward inhibition, Brain Research, 221 (1981) 387-392. 6 Molotchnikoff, S. and Lachapelle, P., Lateral geniculate cell responses to electrical stimulation of the retina, Brain Research, 152 (1978) 81-95. 7 Sawyer, C. H. Everett, J. W. and Green, J. D., The rabbit diencephalon in stereotaxic coordinates, J. comp. Neurol., 101 (1954) 801-824. 8 Singer, W. and Bedworth, N., Inhibitory interaction between X and Y units in the cat lateral geniculate nucleus, Brain Research, 49 (1973) 291-307.
335
Both fast and slow relay cells in lateral geniculate nucleus of rabbits receive recurrent inhibition FU-SUN LO Shanghai Brain Research Institute, Academia Sinica, 319 Yo-Yang Road, Shanghai (China)
(Accepted March 15th, 1983) Key words: albino rabbit - - dorsal lateral geniculate nucleus- - slow relay ceils- - recurrent inhibition
IntraceUular recordings from slow relay cells in the rabbit lateral geniculate nucleus demonstrated that the slow cells, just like the fast ones, receive a monosynapticEPSP and a trisynaptic IPSP from the optic nerve. The IPSP is most likely mediated by interneurones activated by recurrent collaterals of the relay cell axon. In slow relay cells, both the EPSP and the IPSP from the optic nerve are brought about via the same group of slowly-conductingfibers. No mutual inhibition between the fast and the slow conductingsystem was observed in the rabbit lateral geniculate nucleus.
It was described by Fuster, Creutzfeldt and Straschill in 19652 that stimulation of the optic tract (OT) of the rabbit could produce, in the relay cells of the lateral geniculate nucleus (LGN), a response with an EPSP-IPSP sequence as shown in intraceUular recordings. From the fact that the latency of the IPSP was 1.5-3 ms longer than the latency of the EPSP, the authors assumed that 'inhibition of geniculate cells by afferent stimuli is probably mediated mainly by collateral connection within the geniculate. Histologically, there is a basis for this form of inhibition since recurrent collaterals as well as short axon cells, perhaps serving as interneurones, have been found in the geniculate'2. This assumption had not been confirmed, until 1981 when we demonstrated that the IPSP from OT in relay cells of the rabbit LGN was indeed mediated by a recurrent circuit rather than by a feed-forward circuit 5. The evidence on which this belief was based was as follows: (1) the IPSP from OT and the recurrent IPSP from the visual cortex (Cx) had similar configuration; (2) the latency of the IPSP from OT was about 1.6 ms longer than that of the preceding monosynaptic EPSP, indicating that two additional synapses were involved in the inhibitory circuit; (3) 'local' latency, as estimated by an extrapolation procedure, demonstrated that the IPSP from OT was brought about via a neuronal circuit with 3 synapses; and (4) a facilitatory interaction between 0006-8993/83/$03.00© 1983 Elsevier Science Publishers B.V.
IPSPs from Cx and OT was considered suggestive of the possibility that they were mediated by common interneurones in the recurrent circuit. Since this conclusion was drawn from the results of intracellular recordings, it should be taken into account that the impalement of neurones may be preferential to the large ones. The problem whether smaller neurones receive recurrent inhibition is not settled yet. Relay cells in the rabbit LGN, like those in the cat LGN, can be classified into fast (Y-like) and slow (X-like) cells, according to the difference in latency of the spikes evoked by stimulation of the optic nerve (ON). The latency boundary separating the fast from the slow relay cells was taken at 4.3 ms 6. By analogy with the cat1,4, the fast relay cells in the rabbit are presumably also of large size, while the slow cells are of medium size. In the present study, attention was paid particularly to the slow relay cells, in order to determine whether or not recurrent inhibition acts on both fast and slow cells. Albino rabbits weighing 2-3 kg were used in the experiments. Surgical operations were performed under sodium pentobarbital anaesthesia (Nembutal; Abbott; 30-35 mg/kg). During investigation, the rabbits were immobilized by continuous infusion of gallamine triethiodide (Flaxedil; May and Baker) and artificially respirated. ECG, end-tidal CO 2 and body temperature were monitored throughout the experi-
336 ments. The front of the contralateral eye was removed and a stimulating electrode was placed on the optic nerve just behind the eye. Concentric electrodes were stereotaxiacally inserted in the vicinity of the optic chiasm (OX) 7. In order to evoke antidromic spike in the relay cell of LGN, the ipsilateral primary visual cortex was stimulated according to the topographical map of the visual cortex 3. Micropipettes filled with 3 M potassium acetate were used for recording from the relay cells in LGN. The intracellular as well as extracellular response was conventionally displayed and recorded. The records from a typical slow cell are exemplified in Fig. 1. This slow relay cell identified by the antidromic activation from Cx (Fig. 1A) gave rise to a single spike in response to ON stimulation, at a latency of 7.4 ms (Fig. 1C). Stimulation of OX evoked a spike at a latency of 3.6 ms (Fig. 1B). Intracellular recordings from this cell showed the same results as described in the previous paperS, namely, an EPSPIPSP sequence could be evoked by OX stimulation and the IPSPs from either OX or Cx (Fig. 1D and E), despite the difference in amplitude, had rather long duration in contrast with the short duration of the feed-forward IPSP in the cat LGN 5. In addition,
A
Cx
B
the interval between the onset of the EPSP and the onset of the IPSP, as indicated by arrows in Fig. 1F, was 1.8 ms, suggesting that the [PSP was most likely mediated by a trisynaptic circuit. It is therefore reasonable to take the IPSP in slow cells as being brought about through recurrent collaterals, just like the IPSP in fast cells. All the 36 relay cells studied showed an EPSPIPSP sequence in response to stimulation of ON. The latencies of the EPSPs ranged from 1.8 to 5.5 ms with an average of 3.6 ms. The frequency histogram of the latencies for the EPSPs (Fig. 2A, unshaded columns) had a peak at 3.0-3.5 ms and a slow decay from the peak to 5.5 ms, which resulted in an asymmetric contour of the histogram. It was previously described that there were two peaks formed by fast (at 3.0-4.0 ms) and slow (at 5.0-5.5 ms) cells, respectively, in the latency distribution for the spikes evoked by stimulation of ON 6. However, only one peak could be found in the histogram of iatencies for EPSPs from ON, which may be due mainly to the in'sufficient sample (n = 36) in plotting the histogram of EPSP latency. As mentioned above, it is more difficult to impale slow relay cells than fast ones, so that the second peak formed by the slow cells in the histo-
OX
C
ON
!
l
Fig. 1. Records from a slow relay cell in the rabbit LGN. A-C show extracellular recordings and D-F intracellular ones. 4 sweeps superimposed in each record. A: antidromic activation from stimulation of the visual cortex (Cx) at near threshold intensity. B: a spike evoked by stimulation of the optic chiasm (OX). C: a spike evoked by stimulation of the optic nerve (ON). D: an IPSP after Cx stimulation. E and F: EPSP-IPSP sequences followingOX stimulation at different sweep speeds. Arrows in F indicate the onsets of PSPs. Time scale: 5 ms for A-C and F, 50 ms for D and E. Voltagecalibration: 2 mV. Positivedeflexionsupward.
337 B 8.
7-
oo
A 12-
Latenc~s from ON
I0-
88"6
s:
6'
I-'3 EPSP IPSP --
4,
5' n=36
z 4.
09:3
2.
22o
30
4o
5o
so
Latency
70ms
i
~
~, ,;,
.~ ~
-~,~
Lot El:aSP
Fig. 2. A: frequency histograms of latencies for EPSPs and IPSPs from ON. Crossed part represents the overlap of the histograms. B: correlation diagram between EPSP and IPSP latencies of 36 relay cells. Regression line is y = 1.07 x + 1.61. Correlation coefficient: 0.93. Further details in the text.
gram appears less prominent. In order to ascertain whether there was any slow cell included in the sample, the EPSP latency boundary had to be set to separate the fast and the slow cells. Since the latency of the EPSP from ON is shorter than that of the spikes evoked by ON stimulation, the boundary in EPSP latency distribution should be at a value less than 4.3 ms. Even if the boundary is set at 4.3 ms, there are still 7 cells eligible for slow relay cells. In fact, there were more than 7 slow cells included in the sample. It was these slow cells that formed the 'long trail' of the histogram for EPSP latency. The latencies of the IPSPs varied from 3.2 to 6.8 ms with an average of 5.2 ms (Fig. 2A, hatched columns). The frequency histogram showed a peak at 4.5-4.8 ms and a slow decay as well. Generally speaking, the histogram for IPSPs had the same contour as that for EPSPs, except the discordant position on the abscissa, suggesting that the latency difference between EPSP and IPSP in every cell was almost constant. Shown in Fig. 2B is a correlation diagram in which the latencies of the EPSPs from ON were plotted against the latencies of the IPSPs. It can be seen that all points representing either fast or slow relay cells are along a straight line. The slope of the regression line is about 1, indicating that the EPSP and the IPSP in each cell are mediated by the same group of fibers.
The interception is at 1.6 ms, which represents the mean value of difference between the latencies of EPSP and IPSP. The similarity between fast and slow relay cells implies that they are in the same position of the neuronal circuits in LGN. In other words, they all receive recurrent inhibition. An inhibitory convergence of the X- and Y-systems was previously proposed to exist in the cat LGN8. Since X-cells, which received excitatory input via slowly conducting X-system, could also receive inhibitory input via fast conducting Y-system, the IPSP from OX in these cells frequently started at the same time as the EPSP. However, it is not the case in the rabbit L G N , in which the IPSPs from OX in slow (X-like) cells as well as in fast (Y-like) ones always appeared much later than the EPSPs as shown in Figs. 1 and 2. No evidence for mutual inhibition between fast and slow relay cells was observed in the rabbit LGN. It can be asserted that there is no excitatory convergence of the fast and slow systems on the interneurones in the recurrent circuit for the slow relay cells. This work was supported by an Exchange Fellowship of Royal Academy of Engineering Sciences, Sweden. I am grateful to Drs. S. LindstrOm and G. Ahls6n for their advice in this work.
338 1 Friedlander, N. J., Lin, C. S. and Sherman, S. M., Structure of physiologically identified X and Y cells in the cat's lateral geniculate nucleus, Science, 204 (1979) 1114-1116. 2 Fuster, J. M., Creutzfeldt, O. D. and Straschill, M., Intracellular recording of neuronal activity in the visual system, Z. vergl. Physiol., 49 (1965) 605-622. 3 Hughes, A., Topographical relationships between the anatomy and physiology of the rabbit visual system, Docum. Ophthal., 30 (1971) 33-159. 4 LeVay, S. and Ferster, D., Relay cell classes in the lateral geniculate nucleus of the cat and the effects of visual deprivation, J. cornp. Neurol., 172 (1977) 563-584.
5 Lo, F.-S., Synaptic organization of the lateral geniculate nucleus of the rabbit: lack of feed-forward inhibition, Brain Research, 221 (1981) 387-392. 6 Molotchnikoff, S. and Lachapelle, P., Lateral geniculate cell responses to electrical stimulation of the retina, Brain Research, 152 (1978) 81-95. 7 Sawyer, C. H. Everett, J. W. and Green, J. D., The rabbit diencephalon in stereotaxic coordinates, J. comp. Neurol., 101 (1954) 801-824. 8 Singer, W. and Bedworth, N., Inhibitory interaction between X and Y units in the cat lateral geniculate nucleus, Brain Research, 49 (1973) 291-307.