Morphine excitation: Effects on field potentials recorded in the in vitro hippocampal slice

Nruropharmocoloy~. Vol. 19. pp. 507 to 514 Pergamon Press Ltd 1980. Pnnfed m Great Britam

MORPHINE EXCITATION: EFFECTS ON FIELD POTENTIALS RECORDED IN THE IN VITRO HIPPOCAMPAL SLICE J. H. ROBINSON and S. A. DEADWYLER Department

of Physiology and Pharmacology, Bowman Gray School of Medicine, 300 S. Hawthorne Avenue, Winston-Salem. NC 27103, U.S.A. (Acceptrd

17 December

1979)

Summary-The hippocampal slice preparation was used to study the effects of morphine on CA1 field potentials recorded in vitro. Morphine sulfate (10-j M) produced two distinct excitatory effects when added to the bathing media or applied directly to the slice via a pressure pipette injection system. First, morphine produced an increase in amplitude and a reduction in’latency of the CAI population spike elicited by orthodromic stimulation of stratum radiatum, without any change in the amplitude of the synaptic field potential. This was accompanied by a pronounced shift to the left of the population spike input-output curve indicating an increase in excitability of the CA1 neurons. At higher stimulus intensities. orthodromic stimulation produced a series of multiple peaks following the original population spike where only a single spike had been present before morphine application. The increase in population spike amplitude was partially reversed by application of the morphine antagonist naloxone, but naloxone did not affect the number of peaks elicited in morphine-treated slices. Pentobarbital was effective in reducing the number of stimulus-elicited peaks but did not reduce the increase in the population spike. The possibility that these effects are mediated by morphine-induced inhibition of GABAergic synapses in the hippocampus is discussed.

Since the discovery of endogenous opioid peptides by Hughes, Smith, Kosterlitz, Fothorgill, Morgan and Morris (1975) opiates and opioid peptides have been shown to modulate directly or indirectly many of the physiological responses of mammalian nerve cells. The effects of these substances vary depending upon the region of the nervous system in which the opioid agents are applied (Bradley, Gayton and Lambert, 1978) the type of opiate receptor within a given region (Chang, Cooper, Hazum and Cuatrecasas, 1979; Della Bella, Casacci and Sassi, 1978; Martin, Eades, Thompson, Huppler and Gilbert, 1976). and the interaction between the opiates and other neurotransmitters (Algeri, Brunello, Calderini and Cosolazione, 1978; Brase, 1979; Iwamoto and Way, 1979; Llorens, Martres, Baudry and Schwartz, 1978). In addition, it has been shown that these agents exert powerful influences over various other nerve cell processes (Biggio, Casu, Corda, DiBello and Gessa, 1978; Tsang, Tan, Henry and Lal, 1978; Guerrero-Munoz. Guerrero-Munoz and Way, 1979; Guerrero-Munoz, Cerrata, Guerrero-Munoz and Way, 1979) which suggests that the presence of opiate receptors even in small quantities can significantly influence nerve cell activity. It is therefore, of interest to examine some of the characteristics of opiate-mediated neurophysiological changes which occur in brain systems which do not have high opiate receptor density. The possibility that such small opiate receptor populations mediate key events in some systems via strategic locations on Key words: potentials.

hippocampal

slices,

morphine.

CAI

field

specialized neurons is another means whereby opiates could influence large changes in central nervous system activity. The hippocampus has recently been shown to be sensitive to the application of opiates and opioid peptides (Nicoll, Siggins, Ling, Bloom and Guilleman, 1977; Segal, 1977; Deadwyler and Robinson, 1979; Zieglgansberger, French, Siggins and Bloom, 1979). The excitatory effects of opiates on hippocampal pyramidal cells appears to be mediated via opiate specific receptors (Nicoll YTul., 1977). These effects are unlike those exhibited by most other central neurons which are inhibited by opiate application (Bradley et al., 1978). Hippocampal pyramidal cells increase their discharge frequency following opiate and opioid peptide administration. However, these excitatory influences are often erratic and difficult to demonstrate (Fry, Zieglgansberger and Herz, 1979). Recently it has been proposed that these excitatory effects may result from the release of tonic inhibition by inhibitory interneurons in the hippocampus (Zieglgansberger cr al., 1979). In a previous publication it was shown that morphine application to the in tiitro hippocampal slice produced an increase in the excitability of CA1 pyramidal cells (Deadwyler and Robinson, 1979). It was also demonstrated that the morphine-induced excitability changes were synergistic with depolarizing synaptic stimulation which produced intracellularly recorded after-discharges similar to those reported using other types of epileptic agents in this preparation (Schwartzkroin and Prince, 1978). The following report is a more detailed analysis of the excit507

J. II. ROBINSON and S. A. DEARWYLER

Fig. 1. Schematic diagram of the hippocampal slice preparation” Simultaneous recordings from the CA1 pyramidal cell layer (A) and the apical dendritic region (B) of stratum radiatum obtained from stimulation of the Schaffer collateral-commissural afferent fibers (S-C stimulus electrode). Recording from the apical dendrite region gives rise to the monopkasic negative “synaptic” potential (B): “population spike” is recorded at the CA1 pyramidal cell body layer (A). Antidromic activation of CA1 pyramidal cells is via stimulating electrodes positioned in the alveus (Anti). (Abbreviatjons: DGdentate gyrus; SC Sckaffer collateral; Anti-.ant~drom~c stimulator; S-C-Sckaffer collateral; commissural fiber stimulator.)

ability changes produced in the hippocampal slice by application of morphine to the bathing media. The results confirm previous findings and suggest that opiates change CA1 pyramidal cell excitability via mechanisms that are (1) independent of excitatory synaptic inputs, and (2) consistent with the notion of a decrease in tonic inhibitory influence on the CA1 pyramidal cells.

METHODS Male Sprague-Dawley rats (25s35Og) were killed by decapitation, the brain rapidly removed and placed in chilled (l-2°C) glucose-Ringers media (Yamamoto, 1972). Both hippocampi were dissected and placed on a McIlwain tissue chopper. Slices of hippocampus 4.5~6OO~m thick were cut at right angles to its longitudinal axis and immediately transferred to a specially designed incubation chamber for the duration of the experiment. In the chamber, the slices were positioned on a nylon net and partially submerged in 34-%X oxygenated medium, main-

tained at a constant temperature by means of a heated water bath. Warmed 957; O,-5”/, CO* gas was bubbled through the water bath and over the top of the slices. Tissue prepared and maintained in this manner remained viable for 6-8 hr. Further details on the preparation of hippocdmpal slices for electrophysiological study can be found in a number of prior publications (Lynch, Smith, Browning and Deadwyler, 1975; Deadwyler, Dunwiddie and Lynch, 197X; Spencer, Gribkoff, Cotman and Lynch, 1976; Deadwyler and Robinson, 1979). Slices were allowed to equilibrate in the chamber for at least 1 hr prior to any experimental manipulations. After this time, the slices were tested for hyperexcitability in response to orthodromic synaptic stimuIation in order to assure an appropriate baseline from which to test the effects of opioid substances applied to the bathing medium. This was necessitated by the observation that a small number of slices (l&l 5%) exhibited an inherent h~perexci~~bility prior to any experimental manipulation. Only those slices which disptayed normal synaptic excitability (as com-

Morphine excitation in hippocampal slices pared to in uieo recordings) were used in the following experiments. Figure 1 is a schematic representation of the hippocampal slice preparation. Extracellular field potentials were recorded simultaneously from the apical dendritic region (stratum radiatum) of CA1 pyramidal cells and from the CA1 cell layer (stratum pyr~midaIe) when the Schaffer collateral and hipp~ampai commissural aRerents were activated electrically (Fig. I, S-C). Activation of these afferent fibers produced the extracellular negative *‘synaptic” or “dendritic” field potential (Fig. I. B) in the apical dendritic region. Recordings from the CA1 cell layer (Fig. I, A) reflect the electrotonically conducted dipolar positivity of the synaptic potential along with a superimposed negative *‘population spike” or compound action potential from a number of synchronously discharging pyramidal cells (Andersen, Bliss and Skrede, 1971; Deadwyler, West, Cotman and Lynch, 1975a, b; Lynch et al.,1975). Single unit (intra and extracellular) recordings were aiso made from the CA1 cell layer (Deadwyler et al., 1978; Deadwyler and Robinson, 1979). Single unit discharges occurred spontaneously in some cells but were driven in every cell by orthodromic electrical stimulation of the Schaffer collateral afferents. When these cells were driven with antidromic stimulation (Fig. 1, Anti), they responded with short latency discharges and followed high frequency (10%200 Hz) stimulation, providing physiological evidence that they were CA1 pyramidal cells. In all the experiments reported here, extracellular recordings were obtained with glass pipettes filled with 2.0 M NaCl and the intracellular recordings were made from glass pipettes filled with either 2.0M

SYNAPTIC

so9

K-citrate or 4.0 M K-acetate. The intracellular recordings provided a means of verifying changes which occurred in extracellular events, and supplied corroborative evidence that the changes were reflective of postsynaptic events (Deadwyler and Robinson, 1979). Opiates and opioid peptides were applied to the slice by one of two methods: (I) addition to the bathing medium (in various concentrations), or (2) by localized application directly to the slice via a large (broken tip, 20/l) micropipette. Both methods produced similar results with the same drug and drug concentrations (Deadwyler and Robinson, 1979). Drugs employed in this study were: morphine sulfate (Merck Co.); naloxone hydrochloride (Endo Labs); and pentobarbital sodium (Sigma). All drugs were applied in molar concentrations calculated from salt weights, RESL’LTS

Morphine sulfate applied in moderate doses (50jiM-l.OmM) to the bathing medium of the hippocampa1 slice had two major influences on orthodromic CA1 field potentials. The initial effect at low (5&2OOpM) or high (200 PM-1.0 mM) concentrations of the drug was to increase the amplitude and decrease the latency of the CA1 cell layer population spike. These population spike amplitude increases were most frequently observed at or near threshold stimulus intensities. The second effect of morphine on the hippoc~pal slice was the induction of muhiple discharges (extra spikes or muttiple peaks) following sustained low frequency synaptic activation. These changes are shown in Figs 2 and 4. Figure 2 shows

POTENTIAL

POPULATION

SPIKE

6.0

STIMULUS

VOLTS

Fig. 2. Effect of morphine (f.OmM) on inp~t~utput curve for both the synaptic potential and population spike recorded simultaneously from the same hipp~ampal slice. Pm- (solid circles) and postmorpdine (open circles) values represent the average of 8 stirnut& presentations at each stimulus voltage Bars over dots represent largest and smallest standard errors. Calibration: 10 mV and IO msec.

J. H. ROBINSON and S. A. DEADWYLER

510

4.0 -

0”

zi P a IX

3.0.

20. 0 Morphine 1.0 -

Fig. 3. Simultaneous intracellular and extracellular recordings from the CA1 cell layer after addition of morphine sulfate (1.0 mM) to the bathing medium show the peaks in the extracellular potential correspond to small fluctuations in membrane potential during the stimulus elicited depolarizing after potentials (arrows). Calibration: 3 mV and 2 msec.

the effect of l.OmM morphine sulfate on simultaneously recorded population spikes from the CA1 cell layer, and synaptic potentials from the apical dendrites, elicited by stimulation of the Schaffer collateral-commissural afferents. It is quite evident that concentrations of morphine as high as 1.O mM had little or no effect on the input-output (I/O) curve (a function which relates the amplitude of the field potential to the degree of stimulus intensity) for the dendritic synaptic potential. However, there was a pronounced shift to the left in the population spike I/O curve following morphine exposure, indicating a

.

01

l

1.0

0

500

0

ZOOUM

.

so-100uM (12)

A

CONTROL

mM

Fig. 4. different posure limited nificant

l

Control

II0 (Pre

mM) MorphIneI

Mean amplitude of the population spike at five stimulus intensities (voltage) before and after exto 1.O mM morphine sulfate. Effect of morphine is to lower stimulus intensities. Asterisks indicate sigdifferences at the P < 0.01 level using Student’s r-test. Error bars indicate SEM.

marked increase in hippocampal cell excitability. Figure 4 demonstrates the fact that the amplitude increase in the population spike recorded from the cell layer was significant only at threshold and intermediate stimulus intensities. At higher stimulus voltages the amplitude of the population spike was not significantly increased across all slices tested (Fig. 4). The second major effect ofmorphine on hippocampal field potentials was the induction of multiple peaks (Figs 2 and 5) which continued to develop in a linear

(n=IO)

UM

(2) (61 (33)

TV

I

4 &l”L”S2

5

“cll-*GE3

Fig. 5. Dose-response input-output curves relating the number of “peaks” in the CA1 cell layer field potential to stimulus voltage at different concentrations of morphine in the bathing medium. The number of slices tested at each concentration is shown in parentheses. Error bars indicate SEM where appropriate. No SEM is shown for 500 PM concentrations since only 2 slices were used.

Morphine

8

CL IX

2.0- PRE ) _

0 0

*

0 : 12

excitation

in hippocampal

MORPHINE (1.0 mM) 1 I I c 3 11 24

j : aa*

48

36

511

slices

NALOXONE (0.1 mM) 10 2 1 I 60

72

Minutes Fig. 6. Time-course ofdevelopmentof the increase in population spike amplitude produced by morphine in CA1 cell layer. Each point represents the mean amplitude of IS individual population spikes elicited once every 5 set and averaged every 2 min. Morphine (1.0 mM) was infused into the bathing medium at I2 min, removed at 46 min and replaced by naloxone (0.1 mM) which effectively reduced the population spike amplitude increase and eliminated the single “extra peak” produced by morphine. Largest and smallest SEM for all points is indicated by each of the two vertical bars.

fashion, over time, after the first population spike reached maximum amplitude. Figure 5 shows the dose dependency of the multiple peak development as a function of stimulus intensity. Bath concentrations of morphine of less than 1OOpM did not produce an increase in the number of peaks even at high stimulus voltages. However, with 200 PM concentrations, morphine produced multiple peaks at both the medium and high voltage ranges on individual I/O curves. At concentrations above 200 ,uM, additional peaks became noticeable at low as well as high stimulus intensities. Simultaneous intracellular and extracellular recordings showed that these multiple peaks were related to small depolarizing oscillations on the stimulus elicited depolarizing after potentials (DAP) of pyramidal cells exposed to morphine (Fig. 3). These oscillations are the underlying membrane changes that lead to secondary and tertiary discharges in hyperexcitable CA1 pyramidal cells (Schwartzkroin and Prince, 1978). These data indicate that for a given population of CAI cells activated synaptically over a wide range of stimulus values the field potential recorded exhibited (1) an increase in the amplitude of the population spike at lower stimulus intensities, and (2) multiple peaks at higher stimulus intensities. The fact that both phenomena were independent, however, was demonstrated by the finding that the higher morphine concentrations produced an increase in the number of peaks for a given stimulus intensity independent of changes in amplitude of the first population spike (Figs 4 and 5). The specificity of the morphine-induced changes was tested by the exchange of naloxone (1OOpM) for morphine in the bathing medium. The timecourse of the reversal of the morphine effect is illustrated in Figure 6. The population spike elicited by near threshold stimulation increased gradually over the

first

15 min

(1.0 mM).

following

the

addition

of

morphine

The population spike increased to maximum over the next 40 min. Naloxone (1OOpM) was then infused (morphine removed) into the bathing medium and the response amplitude monitored for another 3WlOmin. Naloxone decreased the amplitude of the population spike from its maximum level in morphine by 60-75% (Fig. 6). However, a complete reversal of the morphine effect was not demonstrated with any slice tested. This agrees with earlier findings in which naloxone M:ISshown to antagonize only partially morphine-induced increases in CA1 cell discharges recorded intracellularly (Deadwyler and Robinson, 1979). Naloxone was ineffective in reducing the number of additional peaks elicited in moderate concentrations (0.2-I .OmM) of morphine although, at threshold stimulus intensities, a small extra peak could be eliminated concomitant with the reduction in initial population spike amplitude (see Fig. 6). Although not decreased by naloxone, the number of “morphine” peaks in the CA 1 cell layer field potential was substantially reduced by the addition of pentobarbital sodium to the bathing medium. The number of peaks at all stimulus voltages was reduced significantly by this treatment. Figure 7 shows the change in number of peaks following the application of 200,nM pentobarbital to the bathing medium of 6 slices previously made hyperexcitable by exposure to l.OmM morphine. The reduction in the number of peaks was independent of any change in the amplitude or latency of the initial population spike, suggesting that pentobarbital did not produce its suppressive influence by (1) reducing membrane permeability to sodium, (2) decreasing synaptic current, or (3) decreasing the number of afferent fibers activated by a given stimulus voltage. However, while much more effective than naloxone, pentobarbital also did not completely suppress the morphine peaks since there remained a significant increase in the number of field potential

MORPHINE

t LOW STIMULUS

PENTOBARB

1

MEDIUM

I

timi

YOLTAGE

F1g 7 Elfaxive JWCJSd by pentohilrhitai aod1unt \u.z mMI of’multiplc peaks in CA 1 cell layer Reid potential. Addition of pe~tob~rhital sodium m Ihe bathing medium reduced the mean number of peaks elicited at aI1 stimulus voitagcs but did not affect the aInpiitLld~ of the primary p~pu~ti~n spike. Error bars represent SEM.

peaks in pentobarbitai-tre~tteri “morphmtzed” comparison to those not previously exposed phine (Figs 4 and 7).

shces in to mor-

~~~rplli~~ produced a significant &et-ation in CA1 cell layer field potentials. The increase in ~~rnplit~l~~ of the cell layer popul~~tion spike and the appearance of multipie peaks confirmed previous results in which ~ntr~~ce~l~lar measures showed excitability changes in CA I wurms foi~~wjng morphine exposure (Deadwyler and Robinson. 1979). In the present study, measures of the ext~~ceiluIar synaptic current recorded from the str~~tum r~~d~at~lrn suggested that the rnorpllit~e~~nd~lc~d excitability changes in C-41 are produced withaut an increased excitatory synaptic input. Several recent reports show similar changes in CAI cell cxcitahility produced by other non-opioid agents (Scb~~~lrt~kroin and Prince, 197X: Wang and Prince, 1979; Anderson, Gjerstad and Langmcn. 197X) which ulso are not accomp~~nied by an increase in excitatory synaptic inputs. The finding that morphine induces convulsive changes in fimbit system neurons also confirms recent reports that intracerebroventricular ad~l~nistr~~t~on of opiates and opioid peptides induces conv~~~soid activity in these structures in rieu (Frenk. McCarty and Liebeskind, 197%; Frenk. tirca and Liebeskind, 1978b: Urea. Frcnk and Lie&kind. 19X0.

Morphine’excitation

toxin to the bathing medium. The present authors have confirmed this latter finding and in addition report here a pronounced depression by pentobarbital of the multiple peak discharges. Recently it has been shown that GABA-type inhibitory synapses are enhanced by pentobarbital in the hippocampal slice (Alger and Nicoll. 1979) and that morphine and endogenous opioid peptides selectively depress the effectiveness of this inhibitory cell type (Zieglgansberger et al., 1979). The above findings confirm earlier reports that morphine and other opiates have convulsant actions when applied irl riw (Frenk rr u1., 1978b; Elazar, Motles. Ely and Simantov. 1978) to the hippocampus and suggest that similar consequences might be exnected from the release of endoaenous ooioid neotides from enkephalinergic terminals in the hippocampus. Thus, the small number of opiate receptors (Atweh and Kuhar. 1977) and the apparent light immunohistochemical staining (Simantov, Kuhar, Uhl and Snyder. 1977: Sar. Stumpf, Miller. Chang and C‘uatrecasas. 1978) of opioid peptides in the hippoI

campus

may

not

indicate

the

powerful

influence

I

of

in this structure. A selective innrrvation of the hippocampal interneuron population by enkephalinergic terminals would provide a mechanism for maintenance and control of the excitability of large numbers of hippocampal pyramidal cells. The results of this and further investigations currently opiate-like

underway

substances

are consistent

with

this notion.

~tc~Xno~~In/~~~~t~~~~~~t.~ An abstract of this work was presented at the Society for Neuroscience Annual Meeting, 1979. Atlanta. GA. This research was supported by NID;\ Grant DA-0204X to S.A.D. We are grateful to Mark West. Stephanie Burgoyne and Terrie McNutt for assistance in preparation of this manuscript.

REFERENCES Alger. B. E. and Nicoll. R. A. (1979). GABA-mediated biphasic inhibitory responses in hippocampus. Nutuw 281: 315 317. Algeri. S.. Brunello. G.. Calderini, G. and Cosolazione. A. (1978). Effect of enkephalins on catecholamine metabolism in r,tt CNS. In: The Endorphins, Adrunces in Biochcvnicul Ps~c,hophur,rluco/o~~ (Costa. E. and Trabucci. M., Eds). pp. 199-210. Raven Press, New York. Andersen. P.. Bliss T. V. P. and Skrede. K. K. (1971). Unit analysis of hippocampal population spikes. E-up/ Bruin Rl~.S. 13: 20X22 I, Andersen. P., Gjerstad. L. and Langmen, I. A. (1978). A cortical epilepsy model irl Ctro. In: Abnormal Neuronul Di\churyrs (Chalazonitis, N. and Boisson, M.. Eds), pp. 79 36. Raven Press. New York. Atweh. S. and Kuhar, M. (1977). Audioradiographic localization of opiate receptor in rat brain. III. The telencephalon. Bruirl Rrs. 134: 393405. Biggio. G.. Casu. M., Corda. M. G.. DiBello, C. and Gessa. G. L. (1978). Stimulation of dopamine synthesis in caudate nucleus by intrastriatal enkephalins and antagonism by naloxone. Science 200: 552-554.

in hippocampal

slices

513

Bradley. P. B.. Gayton, R. J. and Lambert, L. A. (1978). Electrophysiological effects of opiates and opioid peptides. In: Cmrrall~ Acting Peptides (Hughes. J.. Ed.). pp. 215-230. University Park Press. Baltimore. Brase, D. A. (1979). Roles of serotonin and :-aminobutyric acid in opiate effects. In: Nrurochrrnicul Mechunisms of Opiufrs and Endorphins (Loh. H. H. and Ross. D. H.. Eds). pp. 409428. Raven Press. New York. Chang. K.. Cooper. B. R.. Hazum, E. and Cuatrecasas, P. (1979). Multiple opiate receptors: different regional distribution in the brain and differential binding of opiates and opioid peptides. ,vnlec. P1iarniac. 16: 91-104. Corrigall, W. A. and Linseman. M. A. (1979). A specific effect of morphine in the hippocampus in ritro. SW. Nrurosci. Ahstr. 5: 552. Deadwyler. S. A. and Robinson. J. H. (1979). Effects of morphine on hippocampal ce!ls recorded in rirm. Bruits R&Bull. 4: 609-613. Deadwyler. S. A., Dunwiddie, T. and Lunch. G. (1978). Short lasting changes in hippocampal neuronal excitability following repetitive synaptic activation. Bruin Res. 147: 384 389. Deadwyler. S. A., West. J. R., Cotman. C. W. and Lynch. G. S. (1975a). Physiological studies of the reciprocal connections between the hippocampus and entorhinal cortex E.vp/ Neural. 49: 35 57. Deadwyler, S. A., West, J. R.. Cotman. C. W. and Lynch. G. S. (l975b). A neurophysiological analysis of the hippocampal commissural projections to the dentate gyrus of the rat. J. Nwrophy,sio/. 38: 167 184. Della Bella, D.. Casacci. F. and Sassi, A. (1978). Opiate receptors: different ligand afhnity in various brain regions. Adr. Biochrm. Psy. 18: 271-277. Elazar, Z.. Motles, E.. Ely. Y. and Simantov, R. (1978). Acute tolerance to the excitatory effects of enkephalin microiniections into hippocampus. Li/> Sci. 24: 541-548. Frenk. H.: McCarty. B. ‘C. and ‘Liebeskind. J. C. (1978a). Different brain areas mediate the analgesic and epileptic properties of enkephalin. Science 200: 335- 337. Frenk, H., Urea, G. and Lebeskind, J. C. (1978b). Epileptic properties of leucine- and methionine-enkephalin: Comparison with morphine and reversibility by naloxone. Bruin Rrs. 147: 327-337. Fry. J. P., Zieglgansberger, W. and Herz, A. (1979). Specific versus non-specific actions of opioids on hippocampal neurones in the rat brain. Bruin Res. 163: 2955305. Guerrero-Munoz, F., Guerrero-Munoz, M. L. and Way, E. L. (1979a). Effect of B-endorphin on calcium uptake in the brain. Sciewr 206: X9-91. Guerrero-Munoz. F.. Cerrata, M. L., Guerrero-Munoz, M. L. and Way, E. L. (1979b). Effect of morphine on synaptosomal Ca’+ uptake. J. Pharrnac. r.\-p. Thur. 209: 132-136.

Hughes. J., Smith, T. W., Kosterlitz, H. W., Fothorgill, L. A., Morgan, B. A. and Morris, H. R. (1975). Identification of two related pentapeptides from brain with potent opiate agonist activity. Narure. Lond. 258: 577-579. Iwamoto. E. T. and Way. E. L. (1979). Opiate actions and catecholamines. In: .Y~,f~rockurnicrll Mrchonrsms of Opiutrs und Endorphins (Loh. H. H. and Ross. D. H., Eds). pp. 357-40X. Raven Press, New York. Jensen, R. A., Martinez, J. L., Craeger, R., Veliquette. J.. McGaugh. J. L. and Lynch, G. (1979). Site of enkephalin action in the hippocampus. Sot. Nrurosci. Ahsrr. 5: 530. Llorens. C., Martres, M. P.. Baudry. M. and Schwartz, J. C. (1978). Hypersensitivity to noradrenaline in cortex after chronic morphine: Relevance to tolerance and dependence. Nature 274: 603605. Lynch, G., Smith, R. L., Browning, M. D. and Deadwyler, S. A. (1975). Evidence for bidirectional dendritic transport of horseradish peroxidase. In: Physiology and Purholoyy o/ Dendrites. Ad~ancrs in Neurology (Kreutzberg, G., Ed.), Vol. 12. pp. 297-313. Raven Press. New York.

J. H. RCWNWN and S. A. DEADWYLER

514

Martin, W. R., Eades, C. G., Thompson. J. A., Hupplcr, R. E. and Gilbert, P. E. (1976). The effects of morphine and nalorphine-like drugs in the non-dependent and morphine-de~ndent chronic spinal dog. $. Phormac. rup. Tlier. 197: 517~532, Nicoll, R. A., Siggins, G. R., Ling, N., Bloom, F. E. and Guilleman, R. (1977). Neuronal actions of endorphins and enkephalins among brain regions: A comparative microiontophoretic study. Proc. tram. Acad. Sci. U.S.A. 74: 2584-2588. Sar, M.. Stumpf, W. E., Mifier, R. J.. Chanr. K. J. and fuatrecasas.~ P. ( 1978). ~mrnunoh~s:o~he~~a~ locahzation of enkepba~in in rat brain and spinal cord. J. iontp. Neuroi. 1g2: 17-38. Schwartzkroin. P. A. and Pedley, T. (1979). Slow depolartzating potentials in “epileptic” neurons. Brain Rex 20: 2617271. Schwartzkroin, P. A. and Prince, D. A. (1978). Cellular and field potential properties of epileptogenic hjppocampal &es. Or& Rrs. 147: I t 7-130. Schwartzkroin, P. A. and Wester, K. (1975). Long-lasting fdcilitation of a synaptic potential following tetanization in the iu vitro hippocampal slice. hair1 Res. 89: 107-I 19. Segal, M. (1977). Morphine and enkephalin interactions with putative neurotransmitters in rat hippocampus. N~umphUrmilu(~lo~J~

16:

587-592.

Simantov. R., Kuhar, M. J., (Jhl, G. R. and Snyder, S. H. (1977). Gpioid peptide enkephalin: Immunohistochemicat mapping in rat central nervous system. Pro<. rratfr. Ac&. sei., U.S.A. 74: 2167X171. Spencer, H. J.. Gribkoff. V. K., Cotman, C. W. and Lynch, G. S. (1976). GDEE antagonism of ionrophoretic amino acid excitation in the intact hippocampus and in the hippocampal slice preparation. Bruin Res. 105: 471481. Tsang, D., Tan, A. T., Henry, J. L. and Lal, S. (1978). Effect of opioid peptides on L-noradrenaline-stimulated cychc AMP formation in homogenates on rat cerebral cortex and hy~thalamus. Bruin Res. 152: 571&5X. lima, G., Frenk. H. and Liebeskind. 3. C. (i97S). Morphine and enkephahn: Analgesic and epileptic properties. ScirnfV’ 197: 8386. Wong, R. K. S. and Prince, D. A. (1979). Dendritic mechanisms underlying penicillin-induced epileptiform activity. Scietrc~ 204: 122& 1231. Yamamoto, C. (1972).Activation of hippocampal ncurones by mossy fiber stimulation in thin brain sections irz eirro. E.xpi B&&i Res. 14: 423-43s. Ziegigansberger, W., French, E. D.. Siggins. G. R. and Bloom. F. E. (1979). Opioid peptides may excite hippocampal pyramidal neurons by inhibiting adjacent mhibitory interneurons. Science 205: 415417.