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Induction of LTP in rat primary visual cortex: tetanus parameters
Brain Research, 481 (1989)221-227 Elsevier
221
BRE 14284
Induction of LTP in rat primary visual cortex: tetanus parameters Richard L. Berry 1, Timothy J. Teyler I and Han Taizhen 2 1Neurobiology Department, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272 (U.S.A.) and 2Xian Medical University, Department of Physiology, Xian (China) (Accepted 9 August 1988) Key words: Rat; LTP; Visual cortex; Tetanus; Brain slice; Plasticity
Long-term potentiation (LTP), intensively studied in the hippocampus as a possible mnemonic device, has begun to be studied in the neocortex. In this study the effects of varying tetanic stimulus parameters on LTP of field potentials recorded from layer II/III induced by white matter stimulation in the in vitro rat visual cortical slice were examined. Low intensity tetanus was more effective in producing LTP than high-intensity tetanus, although single pulses of very high intensity reliably resulted in LTP. LTP consistently occurred following 2 Hz-60 min, or 100 Hz-10 min tetanus; whereas, 10 min of 7 and 25 Hz tetanus usually resulted in long-lasting depression. Although no obvious rule related tetanus frequency and duration to the incidence of LTP, an inverted-U relationship was found between tetanus frequency and LTP magnitude.
INTRODUCTION Long-term potentiation (LTP) is a leading candimate for the neurophysiological substrate of learning and~,r/aemory s'13'19. Tetanic electrical stimulation of any o f several systems afferent to the hippocampus results in a long-lasting elevation of the hippocampal response to further stimulation of the tetanized pathway 2-4. Whereas numerous studies have investigated hippocampal LTP 17, relatively few studies have investigated LTP in non-hippocampal areas even though there is evidence of LTP in non-hippocampal tissue 11,15 and it is clear that some types of learning and memory do not require intact hippocampi 14 for expression, suggesting that the plastic neural change must be mediated by other brain areas. The present study examines LTP in visual cortex of rat. Several lines of evidence suggest that neocortex displays forms of neuronal plasticity, including LTP. The well-known studies of Hubel and Wiesel 9,22 demonstrating deprivation-induced neural plasticity in visual cortex marked the beginnings of a large body of work devoted to studying plasticity in the visual
cortex of the cat. The hippocampal system and all of neocortex are reciprocally connected and may well work together in subserving learning and memory functions 18. Recent evidence indicates a role for neocortex in forms of learning and memory that appear not to depend on functional hippocampi 5,1°. Recently Toyama and coworkers 11,122°,21 have demonstrated LTP in the in vitro visual cortical slice of the cat and Perkins and Teyler 15 found LTP in the rat visual cortical slice. Both groups of investigators stimulated the white matter underlying primary visual cortex from which they recorded field potentials. In the cat tetanus consisted of a 2 Hz train of stimulus pulses lasting 1 h. This resulted in increases in the magnitude of field potentials of up to 600% which persisted throughout the duration of the preparation (as long as 15 h). Perkins and Teyler administered 10 min long 2 Hz trains of stimulus pulses to rat slices. Increases in field potential amplitudes lasting several hours and reaching up to 215% of baseline resulted from those tetanus parameters. As there have been few studies of cortical LTP, some fundamental aspects of the phenomenon have
Correspondence: R.L. Berry, Neurobiology Department, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272, U.S.A. 0006-8993/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)
222 not been previously characterized. It is particularly important to determine the effects of varying stimulation parameters on LTP. In the present study we report the effects of applying a wide range of tetanic stimulation conditions on LTP in the in vitro rat visual cortical slice. Results show that there are no simple rules relating tetanus frequency and duration to magnitude or incidence of cortical LTP. Stimulus conditions that consistently give rise to cortical LTP are identified as well as conditions which consistently yield a long-lasting depression. MATERIALS AND METHODS
Ninety-four slices were obtained from the neocortex of 63 male and 31 female Long Evans hooded rats aged 14-21 days. As LTP was found to be unrelated to the sex of the animal, data from the two sexes were pooled. Animals were initially anesthetized with methoxyflurane and then partially submerged in ice water until all locomotor activity ceased and breathing was clearly depressed (17-20 °C core temperature). Rats were then removed from the ice bath, a midline incision exposed the skull and neck muscle, and the skull was cut off-midline and retracted to expose the brain. Cold (1-2 °C) artificial cerebral spinal fluid (ACF) (in raM: NaC1, 134; KC1, 5; KH2PO4, 1.25; MgSO4, 2; CaCl2, 2; NaHCO 3, 16; dextrose, 10) gently bathed the brain at this time, and throughout the dissection, to lower metabolism and thereby minimize the effects of hypoxia. A coronal cut was made well anterior to the visual cortex and the brain removed to a chilled dissection platform where saggital and horizontal cuts were administered in making a block of tissue containing the visual cortex of the left hemisphere. The block of tissue was then moved to a tissue chopper and 450-500/~m thick slices were obtained and placed in a dish of cold ACF. Slices were transferred to static pools of 33 °C ACF where they were supported on ACF-soaked filter paper at the liquid/gas interface in a slice chamber with a warmed and moist 95% 02/5% CO2 atmosphere. Slices were incubated under these conditions for 2 h prior to electrophysiological recording. To allow recordings to be targeted in O C l M (monocular segment of primary visual cortex) or O C I B (binocular segment of primary visual cortex), only slices closely matching sections pictured in Zilles 24
were used. Field potentials (FP) were recorded using glass micropipettes filled with 2.0 M NaCI solution with tip diameters yielding impedances of 2.0-3.5 M~2. A monopolar recording configuration was employed with the slice pool grounded. The signal was amplified using a WPI Model DAM-6A preamplifier in series with a 10× booster amplifier providing net gains of 5000-10,000. Data were collected, digitized, and analyzed with a LabMan data acquisition system 6. Electrical stimulation was administered via a 75 /~m microbipolar concentric stimulating electrode 7, driven by a constant voltage Grass SD9 stimulator. A typical slice is depicted in Fig, 1. With the aid of a binocular microscope, the recording electrode was placed 400-500 pm below the cortical surface (layer I I - l l I ) . The stimulating electrode was placed on the gray/white matter border about 250-500 p m lateral to a line orthogonal to the cortical surface and passing through the recording site. This ensures the stimulation of geniculocortical fibers which constitute a major pathway demonstrated to traverse this location 23. Once the electrodes were placed a recording session began with a determination of the threshold stimulation level. The procedure for determining thresholds involved delivering stimulation pulses that were clearly below threshold and increasing stimulus intensity in increments of 0.1 V until a level was
RECORDING
ELECTRODE
__
9TIM:ILTATIO
N
/ Fig. l. Drawing of a typical slice with recording and stimulation sites indicated. In this example the recording electrode is placed 500/~m below the cortical surface and the stimulating electrode 250urn offline.
223 reached that yielded a typical FP. A typical FP (Fig. 2) consisted of an initial small positive peak followed by a negative trough. When considered separately, the amplitudes of the peak and trough were expressed as the absolute value of the difference between the most positive (or negative) value attained and the prestimulus baseline. Otherwise, field potential amplitudes (FPA) were calculated as the difference between the maximum value of the peak and the minimum value of the trough (as in Fig. 2). Slices that exhibited thresholds greater than 2.5 V were considered 'unhealthy' and were not used for recording. For low-voltage conditions, stimulation levels approximately 1 V above the threshold value were used for all further stimulation. For high-voltage conditions, stimulus intensity was increased until a maximum FPA was achieved; a stimulus level yielding approximately 2/3 of this maximum was chosen for all further stimulation. Once the stimulus intensity was determined one stimulus pulse was delivered every 5 min for at least 30 rain providing FP pretetanus baseline data to compare with posttetanus responses. Seventeen slices demonstrating seizure-like afterdischarges or exhibiting unstable baseline responses (FPA's varying by more than + or - 1 5 % ) were considered unhealthy and were not studied further. Following the collection of baseline data experimental slices received tetanic stimulation and FP's were recorded once every 5 min for at least 90 min. At the end of data collection the threshold stimulus intensity was again determined. Slices in which posttetanus FPA's
reached and maintained a level at least 20% higher than the average baseline FPA were said to have demonstrated LTP. A posttetanus decrease in FPA of greater than 20% indicated long-term depression of the FP. FP's from control (no tetanus) slices were obtained once every 5 min for at least 2 h. All stimulus pulses were 0.1 ms in duration. Three levels of stimulus intensities were used: low (1-4 V), high (6-8 V) and very high (> 35 V). Most of the study was comprised of conditions employing lowstimulus intensities. Using low-voltage stimulation, tetanus frequencies ranging from 2 to 200 Hz and total pulses delivered ranging from 120 to 7200, were employed as listed in Table I. All high-voltage conditions employed 2 Hz stimulation and involved either 600 or 1200 total pulses. Very high-voltage conditions only involved administration of single-pulse stimulation. RESULTS An example of LTP of the FPA (176%) is shown in Fig. 2. FP's demonstrated two main components: a small, short-latency (2.2-6.0 ms) positive response possibly reflecting mono- and disynaptic activity 15,21 and a much larger, longer latency (8.8-22.4 ms) response possibly reflecting multisynaptic corticocortical activity. As in Fig. 2, FP's typically returned to baseline within 40 ms following stimulation. Potentiation of the FPA was usually associated with potentiation of both the early, positive and late, negative
PRE-TETANUS
FP AMPLITUDE
r,
POST-TETANUS 100
UV r
|
5 MSEC
Fig. 2. An example of pre- and posttetanus FP's. Positivity is up. In this case tetanus consisted of 60 min of continuous 2 Hz stimulation. A robust 176% maximum potentiation was observed.
224 TABLE
I
Summary of main experimental results Frequency (Hz ) ")
Duration* (rain)
.
.
.
.
.
Incidence of L TP
.
.
.
1
.
.
Proportion unchanged .
.
.
2 2 7 25 50
5 10 10 I0 1
2/4 2/5 6/10 1/4 0/4 0/4
2/4 2/5 4/10 0/4 1/4 4/4
50 50 100 100 100 100 200 200 200
5 10 1 2 5 10 1 5 10
3/5 0/4 1/5 1/4 2/4 5/5 2/4 1/4 2/4
2/5 4/4 4/5 2/4 2/4 3/4 -
.
Proportion dep res'sed .
.
.
Peak LTP (%)
.
1/5 3/4 3/4
1/4 2/4 2/4
Low
High
Mean
136
155
146
138 123 168 -
149 202 168 -
144 156 168 __
181
297 158 255 198 191 189 175 163
241
158 255 175 152 142 175 143
158 255 187 171 166 175 153
* All stimuli, e x c e p t 2 H z which was delivered continuously and 25 H z delivered in trains of 5 pulses, one each 2.5 s, were delivered in trains of 10 pulses, one each 5 s. T h e r e f o r e , 1 min of tetanus consisted of 120 pulses, 5 min = 600 pulses, and 10 min = 1200 pulses.
component. The degree of LTP found for the two components in slices exhibiting potentiation of both is plotted in Fig. 3. The magnitude of LTP of the early, positive component was positively correlated (r = +0.51) with that of the late, negative component. The mean amount of time (posttetanus) required to reach peak LTP was 83.7 min for the early component and 91.7 min for the late component. The latencies of the two components were not significantly different from baseline values following tetanus. In Fig. 4 the FPA's from a potentiated, a non-tetanized control slice and a depressed slice are plotted
as a function of time. FPA's from the 6 control slices examined did not differ from their mean stable baseline FPA's by more than 15% for at least 1Ve h following the baseline period. For both control and experimental slices baseline threshold stimulation intensities usually matched final thresholds and in no case did the two differ by more than 0.2 V. The data shown in Fig. 4 for the potentiated slice typify the FPA data for slices demonstrating LTP. Immediately following tetanus the FPA's of potentiated slices were either unchanged or slightly depressed. Subse-
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L A T E N C Y IN M S E C Fig. 3. Plot comparing the values of m a x i m u m L T P observed for the early, positive and late, negative component of the FP. Latency values represent the time between stimulation and m a x i m u m p o s i t i v e a m p l i t u d e (early component) or most negative a m p l i t u d e (late component).
MINUTES
POST-TETANUS
Fig. 4. P l o t of the FPA's observed, over time, for a potentiated slice (receiving 5 min of 10 pulse, 50 H z trains, one train each 5 s at 2.4 V), a non-tetanized control slice, and a depressed slice (which received 5 min of continuous 2 H z s t i m u l a t i o n at 7 V). The average of the first 7 F P A v a l u e s s e r v e d as a baseline (dashed line) in each case.
225 quent FPA's progressively increased until 1 to 11/2 h posttetanus when maximum potentiation was achieved. Like that shown in Fig. 4, depressed slices were usually markedly depressed immediately following tetanus, often showed some recovery towards baseline, but never fully recovered to pretetanus levels. The effects of low- vs high-stimulus intensity on LTP incidence are shown in Fig. 5 for continuous 2 Hz tetanus delivered for 5 or 10 min. Clearly, at these tetanus frequencies and durations low-stimulus intensity is much more effective than high-stimulus intensity in inducing LTP. Not shown is the fact that 4/10 slices receiving high-intensity stimulation exhibited depressed FPA's following tetanus, whereas only 1/15 slices receiving low intensity stimulation were depressed following tetanus. Due to the apparent advantage of low-stimulation intensity most of the tetanus conditions involved lowvoltage stimulation. Table I summarizes FPA data obtained for low-intensity stimulation at a variety of frequency and duration combinations. Examination of this table reveals that there are no simple rules relating tetanus duration and frequency to the induction of LTP. Instead, it appears that each frequency-duration combination fits into one of 4 classes: those that consistently induce depression (7 Hz-10 min; 25 Hz-10 min), those that rarely or never yield LTP (50 Hz-1, 10 min; 100 Hz-1, 2 min; 200 H z - 5 min), those that consistently give rise to LTP (2 Hz-60 min; 100 Hz-10 min), and those that result in
LTP about 50% of the time (the remaining combinations). Potentiation of FPA's ranged from 123 to 297% of baseline levels. Overall 43% (32/74) of the slices exhibited LTP, 41% demonstrated no change in FPA, and 16% demonstrated depressed FPA's following tetanic stimulation. It is interesting to note that the 7 Hz-10 min and 25 Hz-10 min groups accounted for 50% of the depressed slices. Taking into account only slices exhibiting potentiation, the mean LTP achieved was 176% of baseline. A summary of LTP data collapsed across durations is pictured in Fig. 6. Examination of Fig. 6 reveals no obvious rule relating tetanus frequency to the incidence of LTP. The incidence of posttetanic depression is, however, markedly increased at the highest frequency tested (200 Hz), amounting to about 1/3 of the slices tetanized. There is an inverted-U relationship between tetanus frequency and the magnitude of LTP with peak LTP magnitude occurring at 50 Hz and progressively lower magnitudes as frequency is decreased or increased. To test the possibility that single, high-voltage stimulus pulses could induce LTP in the cortical slice as has recently been reported for the hippocampus 1, 4 slices each received a single high intensity (> 30 V) stimulus pulse. Each of these slices, which received no tetanus and were otherwise tested with low-voltage pulses, demonstrated marked potentiation of the FPA.
300 - RAN6E
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TETANUS DURATION Fig. 5. Comparison of the effects of low- vs high-voltage continuous 2 Hz stimulation on the incidence of LTP. Low-voltage stimulation averaged 2.6 V and yielded a mean FPA of 302/~V. High-voltage stimulation averaged 6.5 V, yielding a mean FPA of 905 pV.
7 HZ
25~Hz
50 HZ
i0O HZ
200 HZ
TETANUS FREQUENCY Fig. 6. Summary of effects of frequency on mean peak LTP observed and incidence of LTP and depression. Bar heights represent mean LTP for slices that exhibited LTP. Data for each frequency are collapsed across duration. The degree of LTP induced by optimum parameters (100% LTP incidence) are marked by asterisks. These parameters are (left to right) as follows: single pulse (very high voltage), 2 Hz-60 min, and 100 Hz - 10 min.
226 DISCUSSION The major findings of this study are as follows: (1) LTP of the layer II/III FPA involved potentiation of both early and later occurring FP components, (2) low-intensity (1-4 V) tetanic stimulation was much more effective than high-intensity (6-8 V) tetanus in inducing LTP, (3) the degree of LTP ranged from 123 to 297%, (4) seven and 25 Hz tetanic stimulation distributed over 10 min usually resulted in depression of the FPA, (5) very high intensity single-pulse stimulation, 1 h of continuous low-intensity 2 Hz stimulation, and 10 min of 100 Hz low-intensity stimulation delivered in trains of 10 pulses (one train each 5 s) each consistently give rise to LTP, (6) no simple rule relates tetanus frequency and duration to the incidence of LTP, and (7) there appears to be an inverted-U relationship between tetanus frequency and LTP magnitude. Our finding that layer II/III LTP involves both short-latency neural activity and later occurring corticocortical activity replicates Perkins and Teyler is and Komatsu et a1.11, who discuss its possible implications for visual cortical plasticity. While LTP is greatest in layer II/III of 11- to 20-day-old (similar to ages in present study) rat visual cortex, in adult rat (60-90 day) visual cortex LTP is greater in infragranular layers is. Therefore, it is possible that the present results might not apply to older animals. LTP in the rat visual cortex differs from that in kitten visual cortex in several ways. Slices from rat are rendered unresponsive 15 when subjected to the tetanus intensities and durations administered to kitten slices (2 Hz for 60 min at an intensity yielding maximum FP) by Toyama and coworkers 11'12'2°'21. Toyama's group reports potentiation of layer II/III FP's in excess of 600%, whereas the maximum LTP magnitude achieved in the present study was 297%. Toyama et al. 2° were able to maintain kitten slices for up to 15 h as opposed to a maximum survival of 8 h for rat slices in this study. These differences might be due to species differences in the hardiness and plasticity of cortical tissue. Another possibility is that the continuous perfusion of incubation media in the studies of Toyama et al. afforded kitten slices an advantage over the static pool employed in the present study. Unfortunately, Toyama's group does not give a detailed justification for the selection of their stimula-
tion parameters (2 Hz for 1 h) nor do they report the effects of other tetanic stimulation parameters. Extrapolations of cortical LTP phenomena across species must be done with caution until further data are" available. The present finding that high-voltage 2 Hz tetanus applied continuously for 10 min yielded only a 20% incidence of LTP contrasts the earlier finding of Perkins and Teyler 15 who found a 55% incidence of LTP using the same tetanus parameters in similarly aged animals. The difference may be due to the fact that in the earlier study 15 a transcortical depth profile involving numerous single-pulse stimulations preceded the tetanic stimulation. Recent experiments in our laboratory (unpublished observations) indicate that the numerous single-pulse stimulations required to obtain a depth profile produce LTP when coupled with tetanic stimulation parameters that produce depression when presented alone. Thus it is possible that the different incidence in LTP found in the two studies is due to differences in the degree of pretetanic stimulation. This possibility and the finding that single pulses of very high-voltage stimulation can induce LTP, indicate the necessity of minimizing the amount of electrical stimulation delivered to cortical afferents if one wishes to avoid confounding results. In the absence of 'excessive' pretetanus stimulation we found low stimulation intensity to be much more effective in inducing LTP. Under the same conditions, high-stimulation intensities often produce depression of the FPA and rarely result in LTP. Whether this reflects an adverse effect of the high-intensity stimulation or a possible enhancement of inhibitory synapses under conditions of high-intensity stimulation remains to be seen. The observation that 7 and 25 Hz low-intensity tetanus gives rise to a depression of the FPA adds strength to the proposal that some tetanus conditions may actually potentiate inhibitory synapses or depotentiate excitatory synapses. Current experiments in our laboratory are further characterizing this long-term depression. The inverted-U relationship between tetanus frequency and the magnitude of LTP are consistent with the proposal that there is an optimum set of stimulation conditions for the induction of cortical LTP. Inconsistent with this idea is the finding that the magnitude of LTP is unrelated to the incidence of LTP. Indeed, the lack of any apparent systematic rules relat-
227 ing tetanus frequency and duration to the incidence of L T P is hard to reconcile with any simple notion of the nature of neocortical LTP. Several alternative possibilities may account for the fact that there are several quite disparate sets of stimulus conditions that consistently give rise to LTP in the visual cortex: (1) Perhaps different subpopulations of cortical cells exhibit differential plastic responses to tetanic stimulation. If so, LTP (or depression) observed under different sets of stimulus conditions may reflect plastic changes in different subpopulations of cells. (2) Different tetanic stimulus conditions may selectively activate different afferents with disparate effects on LTP. (3) Tetanic stimulation of the visual cortex (a heterogenous field) may result in complex plastic interactions that are unclear at present. F u r t h e r examination of the nature of the LTP (for example laminar
REFERENCES 1 Abraham, W.C., Gustafsson, B. and Wigstr6m, H., Single high strength volleys can produce longterm potentiation in the hippocampus in vitro, Neurosci. Lett., 70 (1986) 217-222. 2 Alger, B.E. and Teyler, T.J., Long-term and short-term plasticity in the CA1, CA3 and dentate region of the rat hippocampal slice, Brain Research, 110 (1976) 463-480. 3 Bliss, T.V.P. and Gardner-Medwin, A.R., Long-lasting potentiation of synaptic transmission in the dentate area of the unanaesthetized rabbit following stimulation of the perforant path, J. Physiol. (Lond.), 232 (1973) 357-374. 4 Bliss, T.V.P. and Lamo. T., Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path, J. Physiol. (Lond.), 232 (1973) 331-356. 5 Brown, M.W., Wilson, F.A.W. and Riches, J.P., Neuronal evidence that inferomedial temporal cortex is more important than hippocampus in certain processes underlying recognition memory, Brain Research, 409 (1987) 158-162. 6 Cauller, L.G., Mayhew, W.T. and Teyler, T.J., LabMan Program and Description Available from Central Nervous Systems, 727, Woodgate no. 201, Ravenna, OH 44266, 1983. 7 Chiaia, N. and Teyler, T.J., A simple method for fashioning small diameter concentric bipolar electrodes for stimulation of nervous tissue, J. Neurosci. Methods, 7 (1983) 269-273. 8 Eccles, J.C., Calcium in long-term potentiation as a model for memory, Neuroscience, 4 (1983) 1071-1081. 9 Hubel, D.H. and Wiesel, T.N., The period of susceptibility to the physiological effects of unilateral eye closure in kittens, J. Physiol. (Lond.), 206 (1970) 419-436. 10 Kesner, R.P., DiMattia, B.V. and Crutcher, K.A., Evidence for neocortical involvement in reference memory, Behav. Neural Biol., 47 (1987) 40-53. 11 Komatsu, Y., Fujii, K., Maeda, J., Sakaguchi, H. and Toyama, K., Long-term potentiation of synaptic transmission in kitten visual cortex, J. Neurophysiol., 59 (1988)
analysis of the expression of LTP) p r o d u c e d under each set of stimulus conditions may well reveal differences not found by simply monitoring the field potential in layer II/III. The a p p a r e n t lack of a systematic relationship between tetanus frequency and duration and the incidence of LTP in the visual cortex is perhaps not surprising in light of the enormous complexity of the neocortex. This study has revealed a small part of that complexity and has provided data that should prove valuable in selecting stimulation p a r a m e t e r s for use in further studies of neocortical LTP. ACKNOWLEDGEMENTS W e thank T h o m a s Perkins for advice and technical assistance. This study was s u p p o r t e d in part by grants from N I H (DA03755), O N R (86K0664) and E P A (CR813394).
124-141. 12 Komatsu, Y., Toyama, K., Maeda, J. and Sakaguchi, H., Long-term potentiation investigated in a slice preparation of striate cortex of young kittens, Neurosci. Lett., 26 (1981) 269-274. 13 Lynch, G. and Baudry, M., The biochemistry of memory: a new and specific hypothesis, Science, 224 (1984) 1057-1063. 14 Olton, D.S., Becker, J.T. and Handelmann, G.E., Hippocampus, space and memory, Behav. Br. Sci., 2 (1979) 313-365. 15 Perkins, A.T. IV and Teyler, T.J., A critical period for long-term potentiation in the developing rat visual cortex, Brain Research, 439 (1988) 222-229. 16 Shaw, C, and Teyler, T.J., The neural circuitry of the neocortex examined in the in vitro brain slice preparation, Brain Research, 243 (1982) 35-47. 17 Teyler, T.J. and DiScenna, P., Long-term potentiation, Annu. Rev. of Neurosci., 10 (1987) 131-161. 18 Teyler, T.J. and DiScenna, P., The hippocampal memory indexing theory, Behav. Neurosci., 100 (1986) 147-154. 19 Teyler, T.J. and DiScenna, P., Long-term potentiation as a candidate mnemonic device, Brain Res. Rev., 7 (1984) 15-28. 20 Toyama, K., Komatsu, Y., Maeda, J. and Sakaguchi, H., Differential localization of plastic synapses in the visual cortex of the young kitten, Biomed. Res., 3 (1982) 117-124. 21 Toyama, K., Komatsu, Y. and Nakajima, S., Development and synaptic plasticity of visual cortical circuitry, Jpn. J. Pharmacol., 43 (1987) 46P. 22 Wiesel, T.N. and Hubel, D.H., Single cell responses in striate cortex of kittens deprived of vision in one eye, J. Neurophysiol., 26 (1963) 1003-1007. 23 Woodward, W.R., Chiaia, N., Teyler, T.J., Leong, L. and Coull, B.M., Organization of cortical afferent and efferent pathways in the white matter of the rat visual system, in press. 24 Zilles, K.J., The Cortex of the Rat: a Stereotaxic Atlas, Springer, Berlin-New York (1985).
221
BRE 14284
Induction of LTP in rat primary visual cortex: tetanus parameters Richard L. Berry 1, Timothy J. Teyler I and Han Taizhen 2 1Neurobiology Department, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272 (U.S.A.) and 2Xian Medical University, Department of Physiology, Xian (China) (Accepted 9 August 1988) Key words: Rat; LTP; Visual cortex; Tetanus; Brain slice; Plasticity
Long-term potentiation (LTP), intensively studied in the hippocampus as a possible mnemonic device, has begun to be studied in the neocortex. In this study the effects of varying tetanic stimulus parameters on LTP of field potentials recorded from layer II/III induced by white matter stimulation in the in vitro rat visual cortical slice were examined. Low intensity tetanus was more effective in producing LTP than high-intensity tetanus, although single pulses of very high intensity reliably resulted in LTP. LTP consistently occurred following 2 Hz-60 min, or 100 Hz-10 min tetanus; whereas, 10 min of 7 and 25 Hz tetanus usually resulted in long-lasting depression. Although no obvious rule related tetanus frequency and duration to the incidence of LTP, an inverted-U relationship was found between tetanus frequency and LTP magnitude.
INTRODUCTION Long-term potentiation (LTP) is a leading candimate for the neurophysiological substrate of learning and~,r/aemory s'13'19. Tetanic electrical stimulation of any o f several systems afferent to the hippocampus results in a long-lasting elevation of the hippocampal response to further stimulation of the tetanized pathway 2-4. Whereas numerous studies have investigated hippocampal LTP 17, relatively few studies have investigated LTP in non-hippocampal areas even though there is evidence of LTP in non-hippocampal tissue 11,15 and it is clear that some types of learning and memory do not require intact hippocampi 14 for expression, suggesting that the plastic neural change must be mediated by other brain areas. The present study examines LTP in visual cortex of rat. Several lines of evidence suggest that neocortex displays forms of neuronal plasticity, including LTP. The well-known studies of Hubel and Wiesel 9,22 demonstrating deprivation-induced neural plasticity in visual cortex marked the beginnings of a large body of work devoted to studying plasticity in the visual
cortex of the cat. The hippocampal system and all of neocortex are reciprocally connected and may well work together in subserving learning and memory functions 18. Recent evidence indicates a role for neocortex in forms of learning and memory that appear not to depend on functional hippocampi 5,1°. Recently Toyama and coworkers 11,122°,21 have demonstrated LTP in the in vitro visual cortical slice of the cat and Perkins and Teyler 15 found LTP in the rat visual cortical slice. Both groups of investigators stimulated the white matter underlying primary visual cortex from which they recorded field potentials. In the cat tetanus consisted of a 2 Hz train of stimulus pulses lasting 1 h. This resulted in increases in the magnitude of field potentials of up to 600% which persisted throughout the duration of the preparation (as long as 15 h). Perkins and Teyler administered 10 min long 2 Hz trains of stimulus pulses to rat slices. Increases in field potential amplitudes lasting several hours and reaching up to 215% of baseline resulted from those tetanus parameters. As there have been few studies of cortical LTP, some fundamental aspects of the phenomenon have
Correspondence: R.L. Berry, Neurobiology Department, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272, U.S.A. 0006-8993/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)
222 not been previously characterized. It is particularly important to determine the effects of varying stimulation parameters on LTP. In the present study we report the effects of applying a wide range of tetanic stimulation conditions on LTP in the in vitro rat visual cortical slice. Results show that there are no simple rules relating tetanus frequency and duration to magnitude or incidence of cortical LTP. Stimulus conditions that consistently give rise to cortical LTP are identified as well as conditions which consistently yield a long-lasting depression. MATERIALS AND METHODS
Ninety-four slices were obtained from the neocortex of 63 male and 31 female Long Evans hooded rats aged 14-21 days. As LTP was found to be unrelated to the sex of the animal, data from the two sexes were pooled. Animals were initially anesthetized with methoxyflurane and then partially submerged in ice water until all locomotor activity ceased and breathing was clearly depressed (17-20 °C core temperature). Rats were then removed from the ice bath, a midline incision exposed the skull and neck muscle, and the skull was cut off-midline and retracted to expose the brain. Cold (1-2 °C) artificial cerebral spinal fluid (ACF) (in raM: NaC1, 134; KC1, 5; KH2PO4, 1.25; MgSO4, 2; CaCl2, 2; NaHCO 3, 16; dextrose, 10) gently bathed the brain at this time, and throughout the dissection, to lower metabolism and thereby minimize the effects of hypoxia. A coronal cut was made well anterior to the visual cortex and the brain removed to a chilled dissection platform where saggital and horizontal cuts were administered in making a block of tissue containing the visual cortex of the left hemisphere. The block of tissue was then moved to a tissue chopper and 450-500/~m thick slices were obtained and placed in a dish of cold ACF. Slices were transferred to static pools of 33 °C ACF where they were supported on ACF-soaked filter paper at the liquid/gas interface in a slice chamber with a warmed and moist 95% 02/5% CO2 atmosphere. Slices were incubated under these conditions for 2 h prior to electrophysiological recording. To allow recordings to be targeted in O C l M (monocular segment of primary visual cortex) or O C I B (binocular segment of primary visual cortex), only slices closely matching sections pictured in Zilles 24
were used. Field potentials (FP) were recorded using glass micropipettes filled with 2.0 M NaCI solution with tip diameters yielding impedances of 2.0-3.5 M~2. A monopolar recording configuration was employed with the slice pool grounded. The signal was amplified using a WPI Model DAM-6A preamplifier in series with a 10× booster amplifier providing net gains of 5000-10,000. Data were collected, digitized, and analyzed with a LabMan data acquisition system 6. Electrical stimulation was administered via a 75 /~m microbipolar concentric stimulating electrode 7, driven by a constant voltage Grass SD9 stimulator. A typical slice is depicted in Fig, 1. With the aid of a binocular microscope, the recording electrode was placed 400-500 pm below the cortical surface (layer I I - l l I ) . The stimulating electrode was placed on the gray/white matter border about 250-500 p m lateral to a line orthogonal to the cortical surface and passing through the recording site. This ensures the stimulation of geniculocortical fibers which constitute a major pathway demonstrated to traverse this location 23. Once the electrodes were placed a recording session began with a determination of the threshold stimulation level. The procedure for determining thresholds involved delivering stimulation pulses that were clearly below threshold and increasing stimulus intensity in increments of 0.1 V until a level was
RECORDING
ELECTRODE
__
9TIM:ILTATIO
N
/ Fig. l. Drawing of a typical slice with recording and stimulation sites indicated. In this example the recording electrode is placed 500/~m below the cortical surface and the stimulating electrode 250urn offline.
223 reached that yielded a typical FP. A typical FP (Fig. 2) consisted of an initial small positive peak followed by a negative trough. When considered separately, the amplitudes of the peak and trough were expressed as the absolute value of the difference between the most positive (or negative) value attained and the prestimulus baseline. Otherwise, field potential amplitudes (FPA) were calculated as the difference between the maximum value of the peak and the minimum value of the trough (as in Fig. 2). Slices that exhibited thresholds greater than 2.5 V were considered 'unhealthy' and were not used for recording. For low-voltage conditions, stimulation levels approximately 1 V above the threshold value were used for all further stimulation. For high-voltage conditions, stimulus intensity was increased until a maximum FPA was achieved; a stimulus level yielding approximately 2/3 of this maximum was chosen for all further stimulation. Once the stimulus intensity was determined one stimulus pulse was delivered every 5 min for at least 30 rain providing FP pretetanus baseline data to compare with posttetanus responses. Seventeen slices demonstrating seizure-like afterdischarges or exhibiting unstable baseline responses (FPA's varying by more than + or - 1 5 % ) were considered unhealthy and were not studied further. Following the collection of baseline data experimental slices received tetanic stimulation and FP's were recorded once every 5 min for at least 90 min. At the end of data collection the threshold stimulus intensity was again determined. Slices in which posttetanus FPA's
reached and maintained a level at least 20% higher than the average baseline FPA were said to have demonstrated LTP. A posttetanus decrease in FPA of greater than 20% indicated long-term depression of the FP. FP's from control (no tetanus) slices were obtained once every 5 min for at least 2 h. All stimulus pulses were 0.1 ms in duration. Three levels of stimulus intensities were used: low (1-4 V), high (6-8 V) and very high (> 35 V). Most of the study was comprised of conditions employing lowstimulus intensities. Using low-voltage stimulation, tetanus frequencies ranging from 2 to 200 Hz and total pulses delivered ranging from 120 to 7200, were employed as listed in Table I. All high-voltage conditions employed 2 Hz stimulation and involved either 600 or 1200 total pulses. Very high-voltage conditions only involved administration of single-pulse stimulation. RESULTS An example of LTP of the FPA (176%) is shown in Fig. 2. FP's demonstrated two main components: a small, short-latency (2.2-6.0 ms) positive response possibly reflecting mono- and disynaptic activity 15,21 and a much larger, longer latency (8.8-22.4 ms) response possibly reflecting multisynaptic corticocortical activity. As in Fig. 2, FP's typically returned to baseline within 40 ms following stimulation. Potentiation of the FPA was usually associated with potentiation of both the early, positive and late, negative
PRE-TETANUS
FP AMPLITUDE
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POST-TETANUS 100
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5 MSEC
Fig. 2. An example of pre- and posttetanus FP's. Positivity is up. In this case tetanus consisted of 60 min of continuous 2 Hz stimulation. A robust 176% maximum potentiation was observed.
224 TABLE
I
Summary of main experimental results Frequency (Hz ) ")
Duration* (rain)
.
.
.
.
.
Incidence of L TP
.
.
.
1
.
.
Proportion unchanged .
.
.
2 2 7 25 50
5 10 10 I0 1
2/4 2/5 6/10 1/4 0/4 0/4
2/4 2/5 4/10 0/4 1/4 4/4
50 50 100 100 100 100 200 200 200
5 10 1 2 5 10 1 5 10
3/5 0/4 1/5 1/4 2/4 5/5 2/4 1/4 2/4
2/5 4/4 4/5 2/4 2/4 3/4 -
.
Proportion dep res'sed .
.
.
Peak LTP (%)
.
1/5 3/4 3/4
1/4 2/4 2/4
Low
High
Mean
136
155
146
138 123 168 -
149 202 168 -
144 156 168 __
181
297 158 255 198 191 189 175 163
241
158 255 175 152 142 175 143
158 255 187 171 166 175 153
* All stimuli, e x c e p t 2 H z which was delivered continuously and 25 H z delivered in trains of 5 pulses, one each 2.5 s, were delivered in trains of 10 pulses, one each 5 s. T h e r e f o r e , 1 min of tetanus consisted of 120 pulses, 5 min = 600 pulses, and 10 min = 1200 pulses.
component. The degree of LTP found for the two components in slices exhibiting potentiation of both is plotted in Fig. 3. The magnitude of LTP of the early, positive component was positively correlated (r = +0.51) with that of the late, negative component. The mean amount of time (posttetanus) required to reach peak LTP was 83.7 min for the early component and 91.7 min for the late component. The latencies of the two components were not significantly different from baseline values following tetanus. In Fig. 4 the FPA's from a potentiated, a non-tetanized control slice and a depressed slice are plotted
as a function of time. FPA's from the 6 control slices examined did not differ from their mean stable baseline FPA's by more than 15% for at least 1Ve h following the baseline period. For both control and experimental slices baseline threshold stimulation intensities usually matched final thresholds and in no case did the two differ by more than 0.2 V. The data shown in Fig. 4 for the potentiated slice typify the FPA data for slices demonstrating LTP. Immediately following tetanus the FPA's of potentiated slices were either unchanged or slightly depressed. Subse-
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L A T E N C Y IN M S E C Fig. 3. Plot comparing the values of m a x i m u m L T P observed for the early, positive and late, negative component of the FP. Latency values represent the time between stimulation and m a x i m u m p o s i t i v e a m p l i t u d e (early component) or most negative a m p l i t u d e (late component).
MINUTES
POST-TETANUS
Fig. 4. P l o t of the FPA's observed, over time, for a potentiated slice (receiving 5 min of 10 pulse, 50 H z trains, one train each 5 s at 2.4 V), a non-tetanized control slice, and a depressed slice (which received 5 min of continuous 2 H z s t i m u l a t i o n at 7 V). The average of the first 7 F P A v a l u e s s e r v e d as a baseline (dashed line) in each case.
225 quent FPA's progressively increased until 1 to 11/2 h posttetanus when maximum potentiation was achieved. Like that shown in Fig. 4, depressed slices were usually markedly depressed immediately following tetanus, often showed some recovery towards baseline, but never fully recovered to pretetanus levels. The effects of low- vs high-stimulus intensity on LTP incidence are shown in Fig. 5 for continuous 2 Hz tetanus delivered for 5 or 10 min. Clearly, at these tetanus frequencies and durations low-stimulus intensity is much more effective than high-stimulus intensity in inducing LTP. Not shown is the fact that 4/10 slices receiving high-intensity stimulation exhibited depressed FPA's following tetanus, whereas only 1/15 slices receiving low intensity stimulation were depressed following tetanus. Due to the apparent advantage of low-stimulation intensity most of the tetanus conditions involved lowvoltage stimulation. Table I summarizes FPA data obtained for low-intensity stimulation at a variety of frequency and duration combinations. Examination of this table reveals that there are no simple rules relating tetanus duration and frequency to the induction of LTP. Instead, it appears that each frequency-duration combination fits into one of 4 classes: those that consistently induce depression (7 Hz-10 min; 25 Hz-10 min), those that rarely or never yield LTP (50 Hz-1, 10 min; 100 Hz-1, 2 min; 200 H z - 5 min), those that consistently give rise to LTP (2 Hz-60 min; 100 Hz-10 min), and those that result in
LTP about 50% of the time (the remaining combinations). Potentiation of FPA's ranged from 123 to 297% of baseline levels. Overall 43% (32/74) of the slices exhibited LTP, 41% demonstrated no change in FPA, and 16% demonstrated depressed FPA's following tetanic stimulation. It is interesting to note that the 7 Hz-10 min and 25 Hz-10 min groups accounted for 50% of the depressed slices. Taking into account only slices exhibiting potentiation, the mean LTP achieved was 176% of baseline. A summary of LTP data collapsed across durations is pictured in Fig. 6. Examination of Fig. 6 reveals no obvious rule relating tetanus frequency to the incidence of LTP. The incidence of posttetanic depression is, however, markedly increased at the highest frequency tested (200 Hz), amounting to about 1/3 of the slices tetanized. There is an inverted-U relationship between tetanus frequency and the magnitude of LTP with peak LTP magnitude occurring at 50 Hz and progressively lower magnitudes as frequency is decreased or increased. To test the possibility that single, high-voltage stimulus pulses could induce LTP in the cortical slice as has recently been reported for the hippocampus 1, 4 slices each received a single high intensity (> 30 V) stimulus pulse. Each of these slices, which received no tetanus and were otherwise tested with low-voltage pulses, demonstrated marked potentiation of the FPA.
300 - RAN6E
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TETANUS DURATION Fig. 5. Comparison of the effects of low- vs high-voltage continuous 2 Hz stimulation on the incidence of LTP. Low-voltage stimulation averaged 2.6 V and yielded a mean FPA of 302/~V. High-voltage stimulation averaged 6.5 V, yielding a mean FPA of 905 pV.
7 HZ
25~Hz
50 HZ
i0O HZ
200 HZ
TETANUS FREQUENCY Fig. 6. Summary of effects of frequency on mean peak LTP observed and incidence of LTP and depression. Bar heights represent mean LTP for slices that exhibited LTP. Data for each frequency are collapsed across duration. The degree of LTP induced by optimum parameters (100% LTP incidence) are marked by asterisks. These parameters are (left to right) as follows: single pulse (very high voltage), 2 Hz-60 min, and 100 Hz - 10 min.
226 DISCUSSION The major findings of this study are as follows: (1) LTP of the layer II/III FPA involved potentiation of both early and later occurring FP components, (2) low-intensity (1-4 V) tetanic stimulation was much more effective than high-intensity (6-8 V) tetanus in inducing LTP, (3) the degree of LTP ranged from 123 to 297%, (4) seven and 25 Hz tetanic stimulation distributed over 10 min usually resulted in depression of the FPA, (5) very high intensity single-pulse stimulation, 1 h of continuous low-intensity 2 Hz stimulation, and 10 min of 100 Hz low-intensity stimulation delivered in trains of 10 pulses (one train each 5 s) each consistently give rise to LTP, (6) no simple rule relates tetanus frequency and duration to the incidence of LTP, and (7) there appears to be an inverted-U relationship between tetanus frequency and LTP magnitude. Our finding that layer II/III LTP involves both short-latency neural activity and later occurring corticocortical activity replicates Perkins and Teyler is and Komatsu et a1.11, who discuss its possible implications for visual cortical plasticity. While LTP is greatest in layer II/III of 11- to 20-day-old (similar to ages in present study) rat visual cortex, in adult rat (60-90 day) visual cortex LTP is greater in infragranular layers is. Therefore, it is possible that the present results might not apply to older animals. LTP in the rat visual cortex differs from that in kitten visual cortex in several ways. Slices from rat are rendered unresponsive 15 when subjected to the tetanus intensities and durations administered to kitten slices (2 Hz for 60 min at an intensity yielding maximum FP) by Toyama and coworkers 11'12'2°'21. Toyama's group reports potentiation of layer II/III FP's in excess of 600%, whereas the maximum LTP magnitude achieved in the present study was 297%. Toyama et al. 2° were able to maintain kitten slices for up to 15 h as opposed to a maximum survival of 8 h for rat slices in this study. These differences might be due to species differences in the hardiness and plasticity of cortical tissue. Another possibility is that the continuous perfusion of incubation media in the studies of Toyama et al. afforded kitten slices an advantage over the static pool employed in the present study. Unfortunately, Toyama's group does not give a detailed justification for the selection of their stimula-
tion parameters (2 Hz for 1 h) nor do they report the effects of other tetanic stimulation parameters. Extrapolations of cortical LTP phenomena across species must be done with caution until further data are" available. The present finding that high-voltage 2 Hz tetanus applied continuously for 10 min yielded only a 20% incidence of LTP contrasts the earlier finding of Perkins and Teyler 15 who found a 55% incidence of LTP using the same tetanus parameters in similarly aged animals. The difference may be due to the fact that in the earlier study 15 a transcortical depth profile involving numerous single-pulse stimulations preceded the tetanic stimulation. Recent experiments in our laboratory (unpublished observations) indicate that the numerous single-pulse stimulations required to obtain a depth profile produce LTP when coupled with tetanic stimulation parameters that produce depression when presented alone. Thus it is possible that the different incidence in LTP found in the two studies is due to differences in the degree of pretetanic stimulation. This possibility and the finding that single pulses of very high-voltage stimulation can induce LTP, indicate the necessity of minimizing the amount of electrical stimulation delivered to cortical afferents if one wishes to avoid confounding results. In the absence of 'excessive' pretetanus stimulation we found low stimulation intensity to be much more effective in inducing LTP. Under the same conditions, high-stimulation intensities often produce depression of the FPA and rarely result in LTP. Whether this reflects an adverse effect of the high-intensity stimulation or a possible enhancement of inhibitory synapses under conditions of high-intensity stimulation remains to be seen. The observation that 7 and 25 Hz low-intensity tetanus gives rise to a depression of the FPA adds strength to the proposal that some tetanus conditions may actually potentiate inhibitory synapses or depotentiate excitatory synapses. Current experiments in our laboratory are further characterizing this long-term depression. The inverted-U relationship between tetanus frequency and the magnitude of LTP are consistent with the proposal that there is an optimum set of stimulation conditions for the induction of cortical LTP. Inconsistent with this idea is the finding that the magnitude of LTP is unrelated to the incidence of LTP. Indeed, the lack of any apparent systematic rules relat-
227 ing tetanus frequency and duration to the incidence of L T P is hard to reconcile with any simple notion of the nature of neocortical LTP. Several alternative possibilities may account for the fact that there are several quite disparate sets of stimulus conditions that consistently give rise to LTP in the visual cortex: (1) Perhaps different subpopulations of cortical cells exhibit differential plastic responses to tetanic stimulation. If so, LTP (or depression) observed under different sets of stimulus conditions may reflect plastic changes in different subpopulations of cells. (2) Different tetanic stimulus conditions may selectively activate different afferents with disparate effects on LTP. (3) Tetanic stimulation of the visual cortex (a heterogenous field) may result in complex plastic interactions that are unclear at present. F u r t h e r examination of the nature of the LTP (for example laminar
REFERENCES 1 Abraham, W.C., Gustafsson, B. and Wigstr6m, H., Single high strength volleys can produce longterm potentiation in the hippocampus in vitro, Neurosci. Lett., 70 (1986) 217-222. 2 Alger, B.E. and Teyler, T.J., Long-term and short-term plasticity in the CA1, CA3 and dentate region of the rat hippocampal slice, Brain Research, 110 (1976) 463-480. 3 Bliss, T.V.P. and Gardner-Medwin, A.R., Long-lasting potentiation of synaptic transmission in the dentate area of the unanaesthetized rabbit following stimulation of the perforant path, J. Physiol. (Lond.), 232 (1973) 357-374. 4 Bliss, T.V.P. and Lamo. T., Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path, J. Physiol. (Lond.), 232 (1973) 331-356. 5 Brown, M.W., Wilson, F.A.W. and Riches, J.P., Neuronal evidence that inferomedial temporal cortex is more important than hippocampus in certain processes underlying recognition memory, Brain Research, 409 (1987) 158-162. 6 Cauller, L.G., Mayhew, W.T. and Teyler, T.J., LabMan Program and Description Available from Central Nervous Systems, 727, Woodgate no. 201, Ravenna, OH 44266, 1983. 7 Chiaia, N. and Teyler, T.J., A simple method for fashioning small diameter concentric bipolar electrodes for stimulation of nervous tissue, J. Neurosci. Methods, 7 (1983) 269-273. 8 Eccles, J.C., Calcium in long-term potentiation as a model for memory, Neuroscience, 4 (1983) 1071-1081. 9 Hubel, D.H. and Wiesel, T.N., The period of susceptibility to the physiological effects of unilateral eye closure in kittens, J. Physiol. (Lond.), 206 (1970) 419-436. 10 Kesner, R.P., DiMattia, B.V. and Crutcher, K.A., Evidence for neocortical involvement in reference memory, Behav. Neural Biol., 47 (1987) 40-53. 11 Komatsu, Y., Fujii, K., Maeda, J., Sakaguchi, H. and Toyama, K., Long-term potentiation of synaptic transmission in kitten visual cortex, J. Neurophysiol., 59 (1988)
analysis of the expression of LTP) p r o d u c e d under each set of stimulus conditions may well reveal differences not found by simply monitoring the field potential in layer II/III. The a p p a r e n t lack of a systematic relationship between tetanus frequency and duration and the incidence of LTP in the visual cortex is perhaps not surprising in light of the enormous complexity of the neocortex. This study has revealed a small part of that complexity and has provided data that should prove valuable in selecting stimulation p a r a m e t e r s for use in further studies of neocortical LTP. ACKNOWLEDGEMENTS W e thank T h o m a s Perkins for advice and technical assistance. This study was s u p p o r t e d in part by grants from N I H (DA03755), O N R (86K0664) and E P A (CR813394).
124-141. 12 Komatsu, Y., Toyama, K., Maeda, J. and Sakaguchi, H., Long-term potentiation investigated in a slice preparation of striate cortex of young kittens, Neurosci. Lett., 26 (1981) 269-274. 13 Lynch, G. and Baudry, M., The biochemistry of memory: a new and specific hypothesis, Science, 224 (1984) 1057-1063. 14 Olton, D.S., Becker, J.T. and Handelmann, G.E., Hippocampus, space and memory, Behav. Br. Sci., 2 (1979) 313-365. 15 Perkins, A.T. IV and Teyler, T.J., A critical period for long-term potentiation in the developing rat visual cortex, Brain Research, 439 (1988) 222-229. 16 Shaw, C, and Teyler, T.J., The neural circuitry of the neocortex examined in the in vitro brain slice preparation, Brain Research, 243 (1982) 35-47. 17 Teyler, T.J. and DiScenna, P., Long-term potentiation, Annu. Rev. of Neurosci., 10 (1987) 131-161. 18 Teyler, T.J. and DiScenna, P., The hippocampal memory indexing theory, Behav. Neurosci., 100 (1986) 147-154. 19 Teyler, T.J. and DiScenna, P., Long-term potentiation as a candidate mnemonic device, Brain Res. Rev., 7 (1984) 15-28. 20 Toyama, K., Komatsu, Y., Maeda, J. and Sakaguchi, H., Differential localization of plastic synapses in the visual cortex of the young kitten, Biomed. Res., 3 (1982) 117-124. 21 Toyama, K., Komatsu, Y. and Nakajima, S., Development and synaptic plasticity of visual cortical circuitry, Jpn. J. Pharmacol., 43 (1987) 46P. 22 Wiesel, T.N. and Hubel, D.H., Single cell responses in striate cortex of kittens deprived of vision in one eye, J. Neurophysiol., 26 (1963) 1003-1007. 23 Woodward, W.R., Chiaia, N., Teyler, T.J., Leong, L. and Coull, B.M., Organization of cortical afferent and efferent pathways in the white matter of the rat visual system, in press. 24 Zilles, K.J., The Cortex of the Rat: a Stereotaxic Atlas, Springer, Berlin-New York (1985).