Brain Research, 523 (1990) 171-174 Elsevier
171
BRES 24180
Spinules in axospinous synapses of the rat dentate gyrus: changes in density following long-term potentiation Thomas
S c h u s t e r 1, M a n f r e d
Krug 2 and Jiirgen
Wenzel 1
1Institute of Anatomy, Humboldt University, Medical School, Berlin (G.D.R.) and 21nstiluteof Pharmacology and Toxicology, Medical Academy, Magdeburg (G.D.R.) (Accepted 3 April 1990)
Key words: Dentate gyrus; Axospinous synapse; Synaptic spinule; Long-term potentiation
Following high frequency stimulation of the perforant path the density of axospinous synapses from the middle third of the molecular layer of the rat dentate gyrus did not change significantly. By contrast, within this synaptic population the density of axospinous synapses containing synaptic spinules increased markedly. We interpret these results in terms of a structural modification related to enhanced synaptic efficacy.
Axospinous synapses form about 80-90% of the synaptic contacts in the cerebral cortex 14. The electron microscopic study of these synapses in the molecular layer of the rat dentate gyrus relatively often reveals invaginations of the presynaptic membrane into the presynaptic axon terminal accompanied by a parallel protrusion of the postsynaptic membrane into the presynaptic invagination called synaptic spinule by Tarrant and Routtenberg 19772°. This synaptic spinule, the postsynaptic membrane of which is devoid of postsynaptic density, occurs within the active zone where it forms part of the so-called perforated synapse 1'2'7'13'18, or occurs in the near vicinity of the active zone (Fig. la,b). Examining the axospinous synapses in the middle third of the molecular layer of the rat dentate gyrus following high frequency stimulation of the perforant path we found a significant increase in the number of axospinous synapses containing synaptic spinules. Because of these results synaptic spinules are thought to be incorporated in a process of synaptic turnover enhancing synaptic efficacy. The material analyzed in this study was obtained in the frame of our foregoing work described earlier 15'21. Briefly, adult male Wistar rats of 200-300 g were used for 3 experimental groups of 5 animals per group: passive control, active control and long-term potentiated animals (LTP group). A stimulation electrode was implanted stereotacticaily into the right perforant path. Passive
controls were only implanted. Active controls were stimulated unspecifically with 1200 single pulses of 0.2 Hz frequency. Long-term potentiated animals were stimulated with 4 trains of 300 square wave pulses with a distance of 30 min between the trains, each train consisting of 20 groups of pulses with 0.25 ms duration and 100-400/~A intensity. The frequency within each group was 200 Hz, the distance between the groups was 5 s. In a separate group of animals not used for ultrastructural study the time course of LTP was controlled by an implanted recording electrode in the dentate molecular layer (2 min until 7 days after tetanization). After 8 and 48 h following stimulation animals were administered 70 mg/kg sodium pentobarbital and intracardial perfusion was begun after 5 min in the anesthetized state. The right hippocampi were dissected free and their medial parts were divided perpendicular to the septotemporal axis into 1-mm-thick slices. Within these slices blocks of about 1 mm 2 area were taken from the dorsal dentate gyrus, i.e. the same region where recording electrodes for the electrophysiological control of the stimulation effect had been implanted in the parallel experimental group (see above). Blocks of 4-5 animals per group were available for electron microscopic investigation. Following postfixation, dehydration and embedding in micropal, blocks were trimmed down so as to include the middle third or inner third, respectively, of the molecular layer. Two series of
Correspondence: Th. Schuster, Institut fOr Anatomic, Bereich Medizin (CharitY) der Humboldt-Universit~it, Philippstrasse 12, Berlin, G.D.R. - 1040. 0006-8993/90/$03.50 ~) 1990 Elsevier Science Publishers B.V. (Biomedical Division)
172
Fig. 1. Spine (S) of an axospinous synapse in the rat dentate molecular layer. D, dendrite; sa, spine apparatus, a: synaptic spinule (arrow) within the transmission zone: perforated synapse, x65.300, b: synaptic spinule (arrow) in the neighbourhood of the transmission zone. x44.650.
ultrathin sections from each block were m o u n t e d on grids, stained with uranyle acetate and lead citrate and used for electron microscopy.
Fascia dentata, Stratum molaculare Middle third Effect of LTP 8 hrs following stimulltion
The m o r p h o m e t r i c a l e x a m i n a t i o n included the estimation of the total density of axospinous synapses (number/ 20 p m 2) and the density of axospinous synapses contain-
Fascia dentMa, Stratum molecularl Mk~e ~,d Effect of LTP 48 hrs ~ w i n g stimulation
10-
Fascia dentata, Stratum moler.uiare Effect of LTP 8 hrs following stimulation
10v
9-
8642-
1
PC
AC-
LTP
oto
.~ 40 e,
o)
30
',_:10 Q Q. :(cO
A
PC
II tt
! l
AC
LT P
PC
ii
J! i 0
AC
LTP
C PC
AC
inner third
~ third
inner
middle
third
third
LT P
B
Fig. 2. Density of axospinous synapses and synaptic spinules in the middle third of the rat dentate molecular layer 8 h following stimulation (A) and 48 h following stimulation (B). PC, passive control; AC, active control; LTP, stimulated animals. C: density of axospinous synapses and synaptic spinules 8 h following stimulation in the inner third and in the middle third of the molecular layer.
173 TABLE I Total density of axospinous synapses (number~20 I~ra2) and density of axospinous synapses containing synaptic spinules (number~20 Itm2) in the middle third of the dentate molecular layer 8 h and 48 h following stimulation
Mean values and standard errors. Density of axospinoussynapseswith spinules also in percent of total axospinoussynapses. 8h
48h
Total
Passive control Active control LTP group
7.25 6.57 6.33
With spin.
%
S.E.
~
S.E.
1.58 1.45 1.59
1.00 1.07 2.34
0.22 0.46 0.67
ing synaptic spinules in the middle third of the molecular layer because more than 80% of the entorhinal afferents passing the medial perforant path are terminating here and are forming axospinous synapses 3'1°'12'19. As a further control the density of axospinous synapses with or without synaptic spinules in the inner third of the molecular layer (the closest to the granule cell layer) in the LTP group 8 h following stimulation was also estimated because the afferents primarily terminating here are commissural and associational axons from the hippocampal areas CA3 and CA4 (refs. 3, 11). The total number of axospinous synaptic profiles in the middle third of the dentate molecular layer was about 6 per 20/~m 2 and does not change significantly between the different experimental groups. About 1 axospinous synapse per 20/~m 2, i.e. 15% of all axospinous synaptic profiles of both control groups (passive control, active control, 8 h and 48 h) revealed synaptic spinules (Table I, Fig. 2A,B). This is in agreement with the data published by Spacek (1985) 17 who found in adult mouse visual cortex that 17% of the 'complex' axospinous synapses, i.e. perforated synapses, possess synaptic spinules. By contrast, following high frequency stimulation the density of axospinous synapses containing synaptic
TABLE II Total density of axospinous synapses (number~20l~m2) and density of axospinous synapses containing synaptic spinules (number/2Oltra2) in the LTP group 8 h following stimulation
Middle third and inner third of the dentate molecular layer. Mean values and standard errors. Density of axospinous synapses with synaptic spinules also in percent of the total number of axospinous synapses. Total
Middle third Inner third
With spin.
%
£c
S.E.
~
S.E.
6.33 5.63
1.59 1.51
2.34 0.90
0.67 0.26
37.6 16.0
Total
14.0 16.3 37.6
With spin.
%
~
S.E.
~
S.E.
5.81 6.00 5.84
1.57 1.03 1.54
0.92 1.00 1.78
0.32 0.37 0.68
15.8 17.0 30.5
spinules is found to be significantly increased up to 30% or more 8 h and 48 h following stimulation (P < 0.05, Kruskal-Wallis test): about 2 of 6 axospinous synapses per 20/~m 2 are showing synaptic spinules (Table I, Fig. 2A,B). Comparing on the other hand the densities of axospinous synapses with spinules between the middle third and the inner third of the LTP group 8 h following stimulation (Table II, Fig. 2C) these differences are in the range of those between the control groups and the LTP group 8 h or 48 h following tetanization (Table I, Fig. 2A,B). This means that the density of synaptic spinules in the inner third of the dentate molecular layer is not increased following LTP. Perforated synapses involving both single- and doubleheaded spines have been shown to increase in number following different stimulation, e.g. hippocampal 'kindling '8'9. Long-term potentiation in the rat dentate gyrus produces substantial and significant changes in the entorhinal axospinous synapses, from 'simple' and 'ellipsoid' synaptic profiles to 'U-shaped' and 'spinule shaped' profile categories 4-6. At least the 'spinule shaped' synapses are perforated ones which are thought to represent more effective synapses than non-perforated ones 13,16. The increase in the density of axospinous synapses containing synaptic spinules in the middle third of the dentate molecular layer following high frequency stimulation of the perforant path is in accordance with these results but it focusses attention on a synaptic structure which seems to reflect morphologically, at least to a certain degree, the combined activity of the pre- and the postsynaptic neuron associated with long-lasting facilitation of synaptic transmission. Further research will be necessary to clarify if the 'synaptic spinule' structure is involved in processes of membrane transport or transport of other material from the post- to the presynaptic compartment for the recycling of material or information transfer or both. Nevertheless, it is supposed that spine synapses forming synaptic spinules are conditioned if not the really potentiated synapses.
174
1 Calverley, R.K.S. and Jones, D.G., A serial section study of perforated synapses in rat neocortex, Cell Tiss. Res., 247 (1987) 399-407. 2 Cohen, R.S. and Siekevitz, P., Form of the postsynaptic density. Serial section study, J. Cell Biol., 78 (1987) 36-48. 3 Cotman, C.W. and Nadler, J.V., Reactive synaptogenesis in the hippocampus. In C.W. Cotman (Ed.), Neural Plasticity, Raven Press, New York, 1978, pp. 227-271. 4 Desmond, N.L. and Levy, W.B., Synaptic correlates of associative potentiation/depression: an ultrastructural study in the hippocampus, Brain Research, 265 (1983) 21-30. 5 Desmond, N.L. and Levy, W.B., Changes in the numerical density of synaptic contacts with long-term potentiation in the hippocampal dentate gyrus, J. Comp. NeuroL, 253 (1986) 466-475. 6 Desmond, N.L. and Levy, W.B., Anatomy of associative long-term synaptic modification. In Long-Term Potentiation: From Biophysics to Behavior, Alan R. Liss, New York, 1988, pp. 265-305. 7 Geinisman, Y., Morreii, E and de Toledo-Morrell, L., Synapses on dendritic shafts exhibit a perforated postsynaptic density, Brain Research, 422 (1987) 352-356. 8 Geinisman, Y., Morrell, E and de Toledo-Morrell, L., Remodeling of synaptic architecture during hippocampal 'kindling', Proc. Natl. Acad. Sci. U.S.A., 85 (1988) 3260-3264. 9 Geinisman, Y., Morrell, E and de Toledo-Morrell, L., Perforated synapses on double-headed dendritic spines: a possible structural substrate of synaptic plasticity, Brain Research, 480 (1989) 326-329. 10 Hjort-Simonsen, A. and Jeune, B., Origin and termination of the hippocampal perforant path in the rat studied by silver impregnation, J. Comp. Neurol., 144 (1972) 215-232. 11 Laatsch, R.H. and Cowan, W.M., Electron microscopic studies of the rat. 1. Normal structure with special reference to synaptic organization, J. Comp. NeuroL, 128 (1966) 359-395.
12 Matthews, D.A., Cotman, C. and Lynch, G., An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. 1. Magnitude and time course of degeneration, Brain Research, 115 (1976) 1-21. 13 Peters, A. and Kaiserman-Abramof, J.R., The small pyramidal neuron of the rat cerebral cortex. The synapses upon dendritic spines, Z. Zellforsch., 100 (1969) 487-506. 14 Peters, A., Palay, S.L. and de Webster, H.F., The Fine Structure of the Nervous System: The Neurons and Supporting Cells, Saunders, Philadelphia, 1976. 15 Schuster, T., Krug, M., Plaschke, M., Schulz, E., Wagner, M. and Wenzel, J., Ultrastructural changes of hippocampal neurons and synapses following long-term potentiation - - a morphometric study, Adv. Biosci., 59 (1986) 115-118. 16 Sirevaag, A.M. and Greenough, W.T., Differential rearing effects on rat visual cortex synapses. II. Synaptic morphometry, Dev. Brain Res., 19 (1985) 215-226. 17 Spacek, J., Relationship between synaptic junctions, puncta adhaerentia and the spine apparatus at neocortical axo-spinous synapses. A serial section study, Anat. Embryol., 173 (1985) 129-135. 18 Spacek, J. and Hartmann, M., Three-dimensional analysis of dendritic spines. I. Quantitative observations related to dendritic spines and synaptic morphology in cerebral and cerebellar cortices, Anat. Embryol., 167 (1983) 289-310. 19 Steward, O., Topographic organization of the projections from the entorhinal area to the hippocampal formation of the rat, J. Comp. Neurol., 167 (1976) 285-314. 20 Tarrant, S.B. and Routtenberg, A., The synaptic spinule in the dendritic spine: electron microscopic study of the hippocampal dentate gyrus, Tissue Cell, 9 (1977) 461-473. 21 Wenzel, J., Krug, M., Ott, T. and Schuster, T., Ultrastructural changes in neurons and synapses of the dentate area accompanying long-term potentiation induced by perforant path stimulation, Adv. Biosci., 59 (1986) 109-114.
171
BRES 24180
Spinules in axospinous synapses of the rat dentate gyrus: changes in density following long-term potentiation Thomas
S c h u s t e r 1, M a n f r e d
Krug 2 and Jiirgen
Wenzel 1
1Institute of Anatomy, Humboldt University, Medical School, Berlin (G.D.R.) and 21nstiluteof Pharmacology and Toxicology, Medical Academy, Magdeburg (G.D.R.) (Accepted 3 April 1990)
Key words: Dentate gyrus; Axospinous synapse; Synaptic spinule; Long-term potentiation
Following high frequency stimulation of the perforant path the density of axospinous synapses from the middle third of the molecular layer of the rat dentate gyrus did not change significantly. By contrast, within this synaptic population the density of axospinous synapses containing synaptic spinules increased markedly. We interpret these results in terms of a structural modification related to enhanced synaptic efficacy.
Axospinous synapses form about 80-90% of the synaptic contacts in the cerebral cortex 14. The electron microscopic study of these synapses in the molecular layer of the rat dentate gyrus relatively often reveals invaginations of the presynaptic membrane into the presynaptic axon terminal accompanied by a parallel protrusion of the postsynaptic membrane into the presynaptic invagination called synaptic spinule by Tarrant and Routtenberg 19772°. This synaptic spinule, the postsynaptic membrane of which is devoid of postsynaptic density, occurs within the active zone where it forms part of the so-called perforated synapse 1'2'7'13'18, or occurs in the near vicinity of the active zone (Fig. la,b). Examining the axospinous synapses in the middle third of the molecular layer of the rat dentate gyrus following high frequency stimulation of the perforant path we found a significant increase in the number of axospinous synapses containing synaptic spinules. Because of these results synaptic spinules are thought to be incorporated in a process of synaptic turnover enhancing synaptic efficacy. The material analyzed in this study was obtained in the frame of our foregoing work described earlier 15'21. Briefly, adult male Wistar rats of 200-300 g were used for 3 experimental groups of 5 animals per group: passive control, active control and long-term potentiated animals (LTP group). A stimulation electrode was implanted stereotacticaily into the right perforant path. Passive
controls were only implanted. Active controls were stimulated unspecifically with 1200 single pulses of 0.2 Hz frequency. Long-term potentiated animals were stimulated with 4 trains of 300 square wave pulses with a distance of 30 min between the trains, each train consisting of 20 groups of pulses with 0.25 ms duration and 100-400/~A intensity. The frequency within each group was 200 Hz, the distance between the groups was 5 s. In a separate group of animals not used for ultrastructural study the time course of LTP was controlled by an implanted recording electrode in the dentate molecular layer (2 min until 7 days after tetanization). After 8 and 48 h following stimulation animals were administered 70 mg/kg sodium pentobarbital and intracardial perfusion was begun after 5 min in the anesthetized state. The right hippocampi were dissected free and their medial parts were divided perpendicular to the septotemporal axis into 1-mm-thick slices. Within these slices blocks of about 1 mm 2 area were taken from the dorsal dentate gyrus, i.e. the same region where recording electrodes for the electrophysiological control of the stimulation effect had been implanted in the parallel experimental group (see above). Blocks of 4-5 animals per group were available for electron microscopic investigation. Following postfixation, dehydration and embedding in micropal, blocks were trimmed down so as to include the middle third or inner third, respectively, of the molecular layer. Two series of
Correspondence: Th. Schuster, Institut fOr Anatomic, Bereich Medizin (CharitY) der Humboldt-Universit~it, Philippstrasse 12, Berlin, G.D.R. - 1040. 0006-8993/90/$03.50 ~) 1990 Elsevier Science Publishers B.V. (Biomedical Division)
172
Fig. 1. Spine (S) of an axospinous synapse in the rat dentate molecular layer. D, dendrite; sa, spine apparatus, a: synaptic spinule (arrow) within the transmission zone: perforated synapse, x65.300, b: synaptic spinule (arrow) in the neighbourhood of the transmission zone. x44.650.
ultrathin sections from each block were m o u n t e d on grids, stained with uranyle acetate and lead citrate and used for electron microscopy.
Fascia dentata, Stratum molaculare Middle third Effect of LTP 8 hrs following stimulltion
The m o r p h o m e t r i c a l e x a m i n a t i o n included the estimation of the total density of axospinous synapses (number/ 20 p m 2) and the density of axospinous synapses contain-
Fascia dentMa, Stratum molecularl Mk~e ~,d Effect of LTP 48 hrs ~ w i n g stimulation
10-
Fascia dentata, Stratum moler.uiare Effect of LTP 8 hrs following stimulation
10v
9-
8642-
1
PC
AC-
LTP
oto
.~ 40 e,
o)
30
',_:10 Q Q. :(cO
A
PC
II tt
! l
AC
LT P
PC
ii
J! i 0
AC
LTP
C PC
AC
inner third
~ third
inner
middle
third
third
LT P
B
Fig. 2. Density of axospinous synapses and synaptic spinules in the middle third of the rat dentate molecular layer 8 h following stimulation (A) and 48 h following stimulation (B). PC, passive control; AC, active control; LTP, stimulated animals. C: density of axospinous synapses and synaptic spinules 8 h following stimulation in the inner third and in the middle third of the molecular layer.
173 TABLE I Total density of axospinous synapses (number~20 I~ra2) and density of axospinous synapses containing synaptic spinules (number~20 Itm2) in the middle third of the dentate molecular layer 8 h and 48 h following stimulation
Mean values and standard errors. Density of axospinoussynapseswith spinules also in percent of total axospinoussynapses. 8h
48h
Total
Passive control Active control LTP group
7.25 6.57 6.33
With spin.
%
S.E.
~
S.E.
1.58 1.45 1.59
1.00 1.07 2.34
0.22 0.46 0.67
ing synaptic spinules in the middle third of the molecular layer because more than 80% of the entorhinal afferents passing the medial perforant path are terminating here and are forming axospinous synapses 3'1°'12'19. As a further control the density of axospinous synapses with or without synaptic spinules in the inner third of the molecular layer (the closest to the granule cell layer) in the LTP group 8 h following stimulation was also estimated because the afferents primarily terminating here are commissural and associational axons from the hippocampal areas CA3 and CA4 (refs. 3, 11). The total number of axospinous synaptic profiles in the middle third of the dentate molecular layer was about 6 per 20/~m 2 and does not change significantly between the different experimental groups. About 1 axospinous synapse per 20/~m 2, i.e. 15% of all axospinous synaptic profiles of both control groups (passive control, active control, 8 h and 48 h) revealed synaptic spinules (Table I, Fig. 2A,B). This is in agreement with the data published by Spacek (1985) 17 who found in adult mouse visual cortex that 17% of the 'complex' axospinous synapses, i.e. perforated synapses, possess synaptic spinules. By contrast, following high frequency stimulation the density of axospinous synapses containing synaptic
TABLE II Total density of axospinous synapses (number~20l~m2) and density of axospinous synapses containing synaptic spinules (number/2Oltra2) in the LTP group 8 h following stimulation
Middle third and inner third of the dentate molecular layer. Mean values and standard errors. Density of axospinous synapses with synaptic spinules also in percent of the total number of axospinous synapses. Total
Middle third Inner third
With spin.
%
£c
S.E.
~
S.E.
6.33 5.63
1.59 1.51
2.34 0.90
0.67 0.26
37.6 16.0
Total
14.0 16.3 37.6
With spin.
%
~
S.E.
~
S.E.
5.81 6.00 5.84
1.57 1.03 1.54
0.92 1.00 1.78
0.32 0.37 0.68
15.8 17.0 30.5
spinules is found to be significantly increased up to 30% or more 8 h and 48 h following stimulation (P < 0.05, Kruskal-Wallis test): about 2 of 6 axospinous synapses per 20/~m 2 are showing synaptic spinules (Table I, Fig. 2A,B). Comparing on the other hand the densities of axospinous synapses with spinules between the middle third and the inner third of the LTP group 8 h following stimulation (Table II, Fig. 2C) these differences are in the range of those between the control groups and the LTP group 8 h or 48 h following tetanization (Table I, Fig. 2A,B). This means that the density of synaptic spinules in the inner third of the dentate molecular layer is not increased following LTP. Perforated synapses involving both single- and doubleheaded spines have been shown to increase in number following different stimulation, e.g. hippocampal 'kindling '8'9. Long-term potentiation in the rat dentate gyrus produces substantial and significant changes in the entorhinal axospinous synapses, from 'simple' and 'ellipsoid' synaptic profiles to 'U-shaped' and 'spinule shaped' profile categories 4-6. At least the 'spinule shaped' synapses are perforated ones which are thought to represent more effective synapses than non-perforated ones 13,16. The increase in the density of axospinous synapses containing synaptic spinules in the middle third of the dentate molecular layer following high frequency stimulation of the perforant path is in accordance with these results but it focusses attention on a synaptic structure which seems to reflect morphologically, at least to a certain degree, the combined activity of the pre- and the postsynaptic neuron associated with long-lasting facilitation of synaptic transmission. Further research will be necessary to clarify if the 'synaptic spinule' structure is involved in processes of membrane transport or transport of other material from the post- to the presynaptic compartment for the recycling of material or information transfer or both. Nevertheless, it is supposed that spine synapses forming synaptic spinules are conditioned if not the really potentiated synapses.
174
1 Calverley, R.K.S. and Jones, D.G., A serial section study of perforated synapses in rat neocortex, Cell Tiss. Res., 247 (1987) 399-407. 2 Cohen, R.S. and Siekevitz, P., Form of the postsynaptic density. Serial section study, J. Cell Biol., 78 (1987) 36-48. 3 Cotman, C.W. and Nadler, J.V., Reactive synaptogenesis in the hippocampus. In C.W. Cotman (Ed.), Neural Plasticity, Raven Press, New York, 1978, pp. 227-271. 4 Desmond, N.L. and Levy, W.B., Synaptic correlates of associative potentiation/depression: an ultrastructural study in the hippocampus, Brain Research, 265 (1983) 21-30. 5 Desmond, N.L. and Levy, W.B., Changes in the numerical density of synaptic contacts with long-term potentiation in the hippocampal dentate gyrus, J. Comp. NeuroL, 253 (1986) 466-475. 6 Desmond, N.L. and Levy, W.B., Anatomy of associative long-term synaptic modification. In Long-Term Potentiation: From Biophysics to Behavior, Alan R. Liss, New York, 1988, pp. 265-305. 7 Geinisman, Y., Morreii, E and de Toledo-Morrell, L., Synapses on dendritic shafts exhibit a perforated postsynaptic density, Brain Research, 422 (1987) 352-356. 8 Geinisman, Y., Morrell, E and de Toledo-Morrell, L., Remodeling of synaptic architecture during hippocampal 'kindling', Proc. Natl. Acad. Sci. U.S.A., 85 (1988) 3260-3264. 9 Geinisman, Y., Morrell, E and de Toledo-Morrell, L., Perforated synapses on double-headed dendritic spines: a possible structural substrate of synaptic plasticity, Brain Research, 480 (1989) 326-329. 10 Hjort-Simonsen, A. and Jeune, B., Origin and termination of the hippocampal perforant path in the rat studied by silver impregnation, J. Comp. Neurol., 144 (1972) 215-232. 11 Laatsch, R.H. and Cowan, W.M., Electron microscopic studies of the rat. 1. Normal structure with special reference to synaptic organization, J. Comp. NeuroL, 128 (1966) 359-395.
12 Matthews, D.A., Cotman, C. and Lynch, G., An electron microscopic study of lesion-induced synaptogenesis in the dentate gyrus of the adult rat. 1. Magnitude and time course of degeneration, Brain Research, 115 (1976) 1-21. 13 Peters, A. and Kaiserman-Abramof, J.R., The small pyramidal neuron of the rat cerebral cortex. The synapses upon dendritic spines, Z. Zellforsch., 100 (1969) 487-506. 14 Peters, A., Palay, S.L. and de Webster, H.F., The Fine Structure of the Nervous System: The Neurons and Supporting Cells, Saunders, Philadelphia, 1976. 15 Schuster, T., Krug, M., Plaschke, M., Schulz, E., Wagner, M. and Wenzel, J., Ultrastructural changes of hippocampal neurons and synapses following long-term potentiation - - a morphometric study, Adv. Biosci., 59 (1986) 115-118. 16 Sirevaag, A.M. and Greenough, W.T., Differential rearing effects on rat visual cortex synapses. II. Synaptic morphometry, Dev. Brain Res., 19 (1985) 215-226. 17 Spacek, J., Relationship between synaptic junctions, puncta adhaerentia and the spine apparatus at neocortical axo-spinous synapses. A serial section study, Anat. Embryol., 173 (1985) 129-135. 18 Spacek, J. and Hartmann, M., Three-dimensional analysis of dendritic spines. I. Quantitative observations related to dendritic spines and synaptic morphology in cerebral and cerebellar cortices, Anat. Embryol., 167 (1983) 289-310. 19 Steward, O., Topographic organization of the projections from the entorhinal area to the hippocampal formation of the rat, J. Comp. Neurol., 167 (1976) 285-314. 20 Tarrant, S.B. and Routtenberg, A., The synaptic spinule in the dendritic spine: electron microscopic study of the hippocampal dentate gyrus, Tissue Cell, 9 (1977) 461-473. 21 Wenzel, J., Krug, M., Ott, T. and Schuster, T., Ultrastructural changes in neurons and synapses of the dentate area accompanying long-term potentiation induced by perforant path stimulation, Adv. Biosci., 59 (1986) 109-114.
