- Email: [email protected]
Effect of acute ethanol administration and acute allopregnanolone administration on spontaneous hippocampal pyramidal cell neural activity
Brain Research 967 (2003) 273–280 www.elsevier.com / locate / brainres
Short communication
Effect of acute ethanol administration and acute allopregnanolone administration on spontaneous hippocampal pyramidal cell neural activity Sayaka Tokunaga a , Janelle R. McDaniel a , A. Leslie Morrow b , Douglas B. Matthews a , * a
b
Department of Psychology, The University of Memphis, Memphis, TN 38152, USA Department of Pharmacology and Psychiatry, Bowles Center for Alcohol Studies, University of North Carolina, Chapel Hill, NC 27599, USA Accepted 18 December 2002
Abstract We investigated the effect of acute ethanol administration and acute allopregnanolone administration on spontaneous hippocampal pyramidal cell neural activity. Both agents produced significant reductions in spontaneous firing rate of hippocampal pyramidal neurons at a medium and high doses. Furthermore, blockade of allopregnanolone biosynthesis by preadministration of finasteride, a 5a-reductase blocker, prevented ethanol-induced inhibition on hippocampal pyramidal neural activity. The results further demonstrate similar effects of allopregnanolone and ethanol on hippocampal neurophysiology and that allopregnanolone plays a key role in producing ethanol-induced inhibition of hippocampal neural activity. 2002 Elsevier Science B.V. All rights reserved. Theme: Neural basis of behavior Topic: Drugs of abuse: alcohol, barbiturates, and benzodiazepines Keywords: Allopregnanolone; Finasteride; Hippocampus; Pyramidal neuron; Ethanol
1. Introduction Ethanol’s effect on spatial learning and memory, which depend heavily on hippocampal function, has been extensively investigated [20,21,23]. In rodents, hippocampal lesion greatly degrade the use of spatial information but do not degrade, or can facilitate, the use of non-spatial information [18,25,32,33]. Hippocampal pyramidal neurons (called ‘place cells’) are the primary output neurons of the hippocampus and fire action potentials only in restricted regions of an environment (called ‘place fields’) [28,30]. Consequently, one function of the hippocampus is to create a ‘cognitive map’ [42] that organizes animals’ behaviors. Acute ethanol administration impairs the use of spatial *Corresponding author. Tel.: 11-901-678-5461; fax: 11-901-6782579. E-mail address: [email protected] (D.B. Matthews).
memory, but does not impair, or can facilitate, the use of non-spatial memory [7,13,20,23,46]. At physiologically relevant concentrations, acute ethanol administration demonstrates regionally specific effects on neural activities [9,16,34,35,43–45], including inhibition of hippocampal pyramidal neurons [1,17,24,37,47]. Therefore, it has been suggested that ethanol degrades spatial learning and memory by disrupting hippocampal pyramidal cell neural activities [29,31]. Many of ethanol’s behavioral effects are enhanced by positive GABAA receptor modulators, and are blocked by GABAA receptor antagonist or inverse agonists [2,11,14,15,26,40,41]. Furthermore, allopregnanolone concentrations, an endogenous pregnane steroid that is a potent positive GABAA receptor modulator, is increased by acute ethanol administration [3,27,43]. Preadministration of finasteride, a 5a-reductase inhibitor, reduced ethanol-induced increases in allopregnanolone concentration in cerebral cortex by approximately 48%, and prevented
0006-8993 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0006-8993(02)04266-X
274
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
ethanol-induced reduction in spontaneous firing of MS / DB (medial septal / diagonal band of Broca) neurons [43]. Considering that the MS / DB modulates the hippocampal theta rhythm [38,39], these results suggest that some of the ethanol-induced changes in hippocampal function may be modulated by increased allopregnanolone concentrations in brain. Recently, we have investigated the effect of allopregnanolone on spatial memory and learning in Morris water task. Acute ethanol administration and acute allopregnanolone administration produced strikingly similar pattern of impairments. Consistent with previous findings [19,20], acute ethanol administration degraded performance on spatial memory, but did not degrade performance on nonspatial memory. Interestingly, acute allopregnanolone administration also degraded performance on spatial memory, but did not degrade the performance on non-spatial memory [22]. This similar pattern of selective deficit in spatial memory indicates that allopregnanolone may exert its effects by disrupting hippocampal neural functioning. Because allopregnanolone inhibits hippocampal neurons in vitro [12] and selectively impairs spatial memory, we investigated if allopreananolone modulates ethanol’s electrophysiological effects in vivo. We hypothesize that both agents will inhibit hippocampal pyramidal neural activity. We also investigated the effect of finasteride preadministration on ethanol-induced inhibition on hippocampal pyramidal neurons to further confirm our result.
2. Methods All experiments were conducted in accordance with National Institutes of Health Guideline under Institution Animal Care and Use Committee-approved protocols at the University of Memphis.
2.1. Study 1 Male adult rats (Charles River, Indianapolis, IN) were anesthetized with urethane (approximately 1.5 g / kg, intraperitoneally) and placed in a stereotaxic frame with skull flat orientation. An incision was made in the skin, the skull surface was cleaned, and a burr hole was drilled through the skull over left hippocampus. The center of the hole was roughly 3.5 mm posterior to bregma, 2.5 mm lateral to the midline. Following surgery, the rat was moved to a separate stereotaxic frame located in a Faraday cage. Single-barrel glass micropipettes (A-M Systems, Carlsborg, WA) were pulled (using Model PE-2; Narishige, Tokyo, Japan), and the tip was broken back to |1.0 mm and filled with a 0.9 M NaCl solution saturated with Chicago sky-blue dye. The electrode was lowered into the hippocampus via a hydraulic microdrive (Trent Wells, South Gate, CA). Extracellular action potentials were amplified, filtered (400 Hz and 8 kHz; Fintronics, Orange,
CT), and monitored with a Tektronix oscilloscope and audiomonitor. Neurons were classified as hippocampal complex spike cell using previously established waveform criteria [6]. Briefly, neuron was classified as a complex spike cell when it sometimes fired burst of approximately 2–10 action potentials (spikes) of decreasing amplitude with short spike intervals (#5 ms). Individual action potentials were isolated from background activity with at least 3:1 single-to-noise ratio, and the neuron’s firing rate was stored in 20-s time bins on a computer for later analysis. Following the isolation of a single hippocampal complex spike neuron, a 5-min baseline of spontaneous neural activity was collected prior to an injection, intraperitoneally, of one of four ethanol doses (saline control, 0.5 g / kg, 1.0 g / kg, or 1.5 g / kg 10% ethanol, w / v) or one of four allopregnanolone doses (20% 2-hydroxypropyl-b-cyclodextrin control, 10, 15 or 17.5 mg / kg). Previous studies have shown that 0.5 g / kg ethanol results in a peak blood alcohol level of |50 mg%, whereas doses of 1.0 and 1.5 g / kg ethanol correspond to blood alcohol levels of |85 mg% and |140 mg%, respectively [5,10]. Thereafter an additional 60-min of spontaneous neural activity was recorded. The location of electrode was carefully monitored and the electrode was micromanipulated during the recording to prevent waveform alterations. After completion of the recording, a current was passed through the electrode, thereby depositing dye and marking its location. The brain was then removed from the animal, frozen, and sliced to verify electrode location (Cryostat HM 505E; Microm, Walldorf, Germany). No more than one neuron was recorded per animal.
2.2. Study 2 In order to study if allopregnanolone is necessary to produce inhibition of hippocampal pyramidal neurons, we investigated the effect of finasteride, a 5a-reductese inhibitor, on ethanol-induced inhibition. By blocking biosynthesis of allopregnanolone, we hypothesized, the inhibitory effect of ethanol on hippocampal pyramidal neurons would be reduced. In the ethanol only condition, the procedure was same as study 1, but a single dose, 1.5 g / kg was used. In the finasteride condition, 50 mg / kg finasteride was administered subcutaneously at 4 and 1.5 h before the surgery. Following isolation of a neuron, 1.5 g / kg ethanol was administered and the neuron was recorded for an additional 60 min.
3. Drug preparation Ethanol (95%) was diluted with distilled water to a concentration of 10%. Purified allopregnanolone
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
(Steraloids Inc., Newport, RI) was suspended in 20% (w / v) aqueous 2-hydroxypropyl-b-cyclodextrin and then sonicated for approximately 1 h before each use. Purified finasteride (Steraloids Inc., Newport, RI) was suspended in 20% (w / v) aqueous 2-hydrosypropyl-a-cyclodextrin and then sonicated for approximately 30 min before each use.
4. Results
4.1. Study 1 A total of 20 neurons (n55 for saline group and each dose group) from 20 different male Long–Evans hooded rats (weight range 133–386 g) were recorded in the ethanol condition. A total of 21 neurons (n56 for vehicle group, n55 for each dose group) from 21 different male Long–Evans hooded rats (weight range 174–431 g) were recorded in the allopregnanolone condition. Histological record revealed that neurons recorded were located in the hippocampus. Acute ethanol administration significantly inhibited the spontaneous neural activity of hippocampal pyramidal neurons (Fig. 1, [two-way ANOVA with repeated measures, main effect of ethanol dose; F53.76, df (3, 16), P,0.05]). Post-hoc tests revealed that the amount of neural inhibition was dependent on the ethanol dose. Specifically, hippocampal pyramidal neural activity was significantly reduced by 53.16623.37% in animals administered 1.0 g / kg ethanol (Dunnett’s t-test, P,0.05) and by 59.1168.88% in animals administered 1.5 g / kg ethanol (Dunnett’s t-test, P,0.05) compared to animals in saline
275
control group. There was no significant difference in inhibition between saline control and 0.5 g / kg ethanol group (Dunnett’s t-test, P.0.05). Typical firing rate histograms of single spontaneously active hippocampal pyramidal neurons recorded in each ethanol dose group are shown in Fig. 2. Acute allopregnanolone administration also significantly inhibited the spontaneous neural activity of hippocampal pyramidal neurons (Fig. 3, [two-way ANOVA with repeated measures, main effect of allopregnanolone dose; F57.066, df (3, 17), P,0.01]). Post-hoc tests revealed that the amount of neural inhibition was dependent on the allopregnanolone dose. Specifically, hippocampal pyramidal neural activity was significantly reduced by 63.5168.9% in animals administered 15 mg / kg allopregnanolone (one-way Dunnett’s t-test, P,0.05) and by 49.6769.68% in animals administered 17.5 mg / kg allopregnanolone (one-way Dunnett’s t-test, P,0.05) compared to vehicle control group. There was no significant difference in inhibition between vehicle control and 10 mg / kg allopregnanolone group (one-way Dunnett’s t-test, P. 0.05). Typical firing rate histogram of a single spontaneously active hippocampal pyramidal neuron recorded from an animal in each allopregnanolone dose group are shown in Fig. 4. Table 1 shows the proportion of neurons in each condition that exhibited the inhibition by at least 10, 20, 30, 40 and 50%. This reveals that as the dose of ethanol and allopregnanolone increases, the number of neurons showing higher inhibition percentage also increases. That is, in higher dose groups, more neurons showed greater inhibition.
Fig. 1. Acute ethanol administration inhibits the spontaneous firing rate of hippocampal pyramidal neurons. A 1-h time course of hippocampal pyramidal neuron firing rates for each ethanol dose group (saline control, 0.5, 1.0 and 1.5 g / kg ethanol). n55 neurons per condition and error bars denote S.E.M. Each time bin represents the average percent baseline rate collected in 5 min intervals.
276
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
Fig. 2. Spontaneous firing rate of a single hippocampal pyramidal neuron in an anesthetized rat after: (a) saline administration; (b) 0.5 g / kg ethanol administration; (c) 1.0 g / kg ethanol administration; and (d) 1.5 g / kg ethanol administration. Arrows indicate the time at which i.p. injection was given.
Fig. 3. Acute allopregnanolone administration inhibits the spontaneous firing rate of hippocampal pyramidal neurons. A 1-h time course of hippocampal pyramidal neuron firing rates of each allopregnanolone dose group (20% 20-hydrozypropyl-b-cyclodextrin control, 10, 15 and 17.5 mg / kg allopregnanolone). n56 for control group, and n55 for each allopregnanolone dose group. Error bars denote S.E.M. Each time bin represents the average percent baseline firing rate collected in 5 min intervals.
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
277
Fig. 4. Spontaneous firing rate of a single hippocampal pyramidal neuron in an anesthetized rat after: (a) vehicle administration; (b) 10 mg / kg allpregnanolone administration; (c) 15 mg / kg allopregnanolone administration; and (d) 17.5 mg / kg allopregnanolone administration. Arrows indicate the time at which i.p. injection was given.
4.2. Study 2 A total of five neurons from five different male Long– Evans hooded rats (weight range 188–362 g) were recorded in the ethanol only condition while a total of five neurons from five different male Long–Evans hooded rats (weight range 242–327 g) were recorded in the finasteride condition. Histological record revealed that neurons recorded were located in the hippocampus.
Acute ethanol administration significantly inhibited the spontaneous neural activity of hippocampal pyramidal neurons by 63.58611.41% on average (Fig. 5, [one-way ANOVA with repeated measures, main effect of ethanol; F54.42, df (1, 12), P,0.001]). This result is consistent with the finding from Study 1. Preadministration of finasteride prevented the inhibitory effect of acute ethanol administration (Fig. 5, [two-way ANOVA with repeated measures, main effect of drug condition; F544.78, df (1,
Table 1 Number of neurons in each condition in study 1 showing inhibition of spontaneous neural activity by at least 10, 20, 30, 40 and 50% Percentage inhibited
At least 10% 20% 30% 40% 50% or more Neurons showing overall increase
Ethanol
Allopregnanolone
Saline 5
Low 5
Medium 5
High 5
Vehicle 6
Low 5
Medium 5
High 5
3/5 1/5 0/5 0/5 0/5 2/5
4/5 4/5 2/5 2/5 0/5 1/5
4/5 4/5 4/5 4/5 4/5 1/5
5/5 5/5 4/5 4/5 4/5 0/5
3/6 3/6 2/6 1/6 1/6 3/6
2/5 0/5 0/5 0/5 0/5 3/5
5/5 4/5 4/5 3/5 3/5 0/5
5/5 4/5 4/5 4/5 4/5 0/5
Also the number of neurons showing overall increase in spontaneous neural acitifity is given.
278
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
Fig. 5. Ethanol administration (1.5 g / kg, i.p.) reduced spontaneous neural activity of hippocampal pyramidal neurons. Finasteride (50 mg / kg) pretreatment prevented the inhibition of hippocampal pyramidal neurons produced by ethanol administration.
12), P,0.0001]). Post-hoc test revealed that there was significant difference in inhibition between the ethanol only group and the finasteride group (two-tailed t-test, t52.625, df58, P,0.05). Typical firing rate histogram of a single spontaneously active hippocampal pyramidal neuron recorded from an animal in the ethanol only group and finasteride group are shown in Fig. 6. Importantly, preadministration of finasteride did not alter baseline firing rate (two-tailed t-test, t51.363, df58, P.0.05).
5. Discussion Ethanol-induced inhibitory effect on hippocampal pyramidal cells found in the present study is consistent with the previous findings from several electrophysiological studies [1,37,47]. These electrophysiological data, including the result from the present study, suggest that ethanol inhibits hippocampal pyramidal neural activity either directly or indirectly, especially above a threshold of
Fig. 6. Spontaneous firing rate of a single hippocampal pyramidal neuron in an anesthetized rat after: (a) 1.5 g / kg ethanol administration; and (b) 1.5 g / kg ethanol administration with 50 mg / kg finasteride pretreatment.
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
1.0 g / kg ethanol. Furthermore, the results from electrophysiological studies of single hippocampal neural activity are in general agreement with several behavioral studies demonstrating that ethanol alters hippocampal function [7,8,13,20,23]. These consistent results suggest that the inhibitory effect of ethanol on hippocampal pyramidal neurons might be a cellular mechanism underlying ethanolinduced impairment in spatial learning and memory. The inhibitory effect of acute allopregnanolone administration found in the present study was strikingly similar to that of acute ethanol administration. While ethanol was found not to interact directly with hippocampal GABAA receptor sites [36], allopregnanolone was found to inhibit neural activities in cultured hippocampal neurons [12]. In the present study, these two drugs produced similar inhibitory effects on hippocampal pyramidal neurons. Although the present study is unable to locate the originating site for the inhibitory effects produced by ethanol, considering that ethanol had been found to increase concentration levels of allopregnanolone in the brain [43], it is plausible that ethanol may not directly inhibit hippocampal pyramidal neural activity but instead increase allopregnanolone concentration, resulting in the inhibition on hippocampal pyramidal neurons. It has been found that the dose of 1 g / kg ethanol, which had an inhibitory effect on hippocampal neurons in the present study, does not produce a significant increase in allopregnanolone levels. However, these findings are based on cortical allopregnanolone levels [43]. The investigation on allopregnanolone levels in the hippocampus following ethanol administration is clearly needed. Contrary to the findings of the present study 10 mg / kg allopregnanolone substitutes for ethanol in rats trained to discriminate 1.0 g / kg ethanol [4]. However, it is reasonable to conclude that since substitution tasks likely involve different brain regions, the critical dose needed to produce the effect in substitution tasks is different from the dose needed to produce the inhibitory effect on hippocampal pyramidal neurons. Preadministraiton of finasteride was found to prevent ethanol-induced inhibition on hippocampal pyramidal neural activity. This result provides a strong indication that allopregnanolone plays an important role in producing this effect, and that GABAA receptors are a major site of action for inhibition of pyramidal cell neural activity. Based on this finding, there is a possibility that finasteride preadministration might prevent ethanol-induced impairment in spatial learning and memory. The present study is the only study we know of to demonstrate the dose-dependent inhibitory effect of allopregnanolone on hippocampal pyramidal neurons in vivo. The results from this study clearly demonstrate that ethanol and allopregnanolone reduce hippocampal pyramidal neuron firing rate. Moreover, the present study provides additional evidence that blockade of biosynthesis of allopregnanolone by finasteride prevents ethanol-induced inhibi-
279
tion on hippocampal pyramidal neurons. From these findings, it is reasonable to assume that allopregnanolone is a key mechanism in ethanol’s alterations of hippocampal neurophysiology and this alteration might effect behaviors dependent on hippocampus such as spatial learning and memory.
Acknowledgements This work was supported by an Alcoholic Beverage Medical Research Foundation grant (D.B. Matthews), University of Memphis faculty research grant (D.B. Matthews), and AA10564 (A.L. Morrow).
References [1] Y.I. Alexandorv, Y.V. Grinchenko, S. Laukka, T. Javilehto, V.N. Maz, A.V. Korpusova, Effect of ethanol on hippocampal neurons depends on their behavioral specialization, Acta Physiol. Scand. 149 (1993) 105–115. [2] A.M. Allan, R.A. Harris, Gamma-aminobutyric acid and alcohol actions: neurochemical studies of long sleep and short sleep mice, Life Sci. 39 (1986) 2005–2015. [3] M.L. Barbaccia, D. Affricano, M. Trabucchi, R.H. Purdy, G. Colombo, R. Agabio, G.L. Gessa, Ethanol markedly increases ‘gabaergic’ neurosteroids in alcohol-preferring rats, Eur. J. Pharm. 384 (1999) R1–R2. [4] C.A. Bowen, R.H. Purdy, K.A. Grant, Ethanol-like discriminative stimulus effects of endogenous neuroactive steroids: effect of ethanol training dose and dosing procedure, J. Pharmacol. Exp. Ther. 289 (1999) 405–411. [5] L. Devenport, R.L. Hale, Contributions of hippocampus and neocortex to the expression of ethanol effects, Psychopharmacology 99 (1989) 337–344. [6] S.E. Fox, J.B. Ranck Jr., Electrophysiological characteristics of hippocampal complex-spike cells and theta cells, Exp. Brain Res. 41 (1981) 399–410. [7] W.E. Gibson, Effects of alcohol on radial maze performance in rats, Physiol. Behav. 35 (1984) 1003–1005. [8] B. Givens, Low doses of ethanol impair spatial working memory and reduce hippocampal theta activity, Alcohol Clin. Exp. Res. 19 (1995) 763–767. [9] B.S. Givens, G.R. Breese, Electropysiological evidence that ethanol alters function of medial septal area without affecting lateral septal function, J. Pharmacol. Exp. Ther. 253 (1990) 95–103. [10] B. Givens, K. McMahon, Ethanol suppresses the induction of long-term potentiation in vivo, Brain Res. 688 (1995) 27–33. [11] A.C. Grobin, D.B. Matthews, L.L. Devaud, A.L. Morrow, The role of GABAA receptors in the acute and chronic effects of ethanol, Psychopharmacology 139 (1998) 2–19. [12] N.L. Harrison, M.D. majewska, J.W. Harrington, J.L. Barker, Structure–activity relationships for steroid interaction with the gaminobutyric acidA receptor complex, J. Pharmacol. Exp. Ther. 241 (1987) 346–353. [13] S.E. Hoffmann, D.B. Matthews, Ethanol-induced impairments in spatial working memory are not due to deficits in learning, Alcohol Clin. Exp. Res. 25 (2001) 856–861. [14] S. Liljequist, J. Engel, Effects of GABAergic agonists and antagonists on various ethanol-induced behavioral changes, Psychopharmacology 78 (1982) 71–75. [15] R.G. Lister, M. Linnoila, Alcohol, the chloride ioonphore and
280
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27] [28]
[29] [30]
[31]
S. Tokunaga et al. / Brain Research 967 (2003) 273–280 endogenous ligands for benzodiazepine receptors, Neuropharmacology 30 (1991) 1435–1440. D.M. Lovinger, G. White, F.F. Weight, NMDA receptor-mediated synaptic excitation selectively inhibited by ethanol in hippocampal slice from adult rat, J. Neurosci. 10 (1990) 1372–1379. N. Ludvig, S.E. Fox, J.L. Kubie, B.M. Altura, B.T. Altura, Application of the combined single-cell recording intracerebral microdialysis method to alcohol research in freely behaving animals, Alcohol Clin. Exp. Res. 22 (1998) 41–50. D.B. Matthews, P.J. Best, Fimbria / fornix lesions facilitate the learning of a nonspatial response task, Psychonom. Bull. Rev. 2 (1995) 113–116. D.B. Matthews, P.J. Best, A.M. White, J.L. Vandergriff, P.E. Simson, Ethanol impairs spatial cognitive processing: new behavioral and electrophysiological findings, Curr. Dir. Psychol. Sci. 5 (1996) 111–115. D.B. Matthews, M. Ilgen, A.M. White, P.J. Best, Acute ethanol administration impairs spatial performance while facilitating nonspatial performance in rats, Neurobiol. Learning Memory 72 (1999) 169–179. D.B. Matthews, A.L. Morrow, Effects of acute and chronic ethanol exposure on spatial cognitive processing and hippocampal function in the rat, Hippocampus 10 (2000) 122–130. D.B. Matthews, A.L. Morrow, S. Tokunaga, J.R. McDaniel, Acute ethanol administration and acute allopregnanolone administration impair spatial memory in the Morris water task, Alcohol Clin. Exp. Res. 26 (2002) 1747–1751. D.B. Mattehws, P.E. Simson, P.J. Best, Acute ethanol impairs spatial memory but not stimulus / response memory in the rat, Alcohol Clin. Exp. Res. 19 (1995) 902–909. D.B. Matthews, P.E. Simson, P.J. Best, Ethanol alters spatial processing of hippocampal place cells: a mechanism for impaired navigation when intoxicated, Alcohol Clin. Exp. Res. 20 (1996) 404–407. R.G.M. Morris, P. Garrud, J.N.P. Rawlins, J. O’Keefe, Place navigation impaired in rats with hippocampal lesions, Nature 297 (1982) 681–683. A.L. Morrow, P.D. Suzdak, J.W. Karanian, S.M. Paul, Chronic ethanol administration alters g-aminobutyric acid pentobarbital and ethanol-mediated 36 Cl 2 uptake in cerebral cortical synaptoneurosomes, J. Pharmacol. Exp. Ther. 246 (1988) 158–164. A.L. Morrow, M.J. VanDoren, L.L. Devaud, Effects of progesterone or neuroactive steroid?, Nature 395 (1998) 652–653. R.U. Muller, J.L. Kubie, J.B. Ranck Jr., Spatial firing patterns of hippocampal complex-spike cells in a fixed environment, J. Neurosci. 7 (1987) 1935–1950. L. Nadel, The hippocampus and space revisited, Hippocampus 1 (1991) 221–229. J. O’Keefe, J. Dostrovsky, The hippocampus as a spatial map: preliminary evidence from unit activity in the freely-moving rat, Brain Res. 34 (1971) 171–175. J. O’Keefe, L. Nadel, The Hippocampus as A Cognitive Map, Oxford University Press, Oxford, 1978.
[32] J. O’Keefe, L. Nadel, S. Keightley, D. Kill, Fornix lesions selectively abolish place learning in rat, Exp. Neurol. 48 (1975) 152–166. [33] M.G. Packard, R. Hirsh, N.M. White, Differential effects of fornix and caudate nucleus lesions on two radial maze tasks: evidence for multiple memory systems, J. Neurosci. 9 (1989) 145–147. [34] P.E. Simson, H.E. Criswell, G.R. Breese, Ethanol potentiates gaminobutyric acid-mediated inhibition in the inferior coliculus: evidence for local ethanol / g-aminobutryic acid interactions, J. Pharmacol. Exp. Ther. 259 (1991) 1288–1293. [35] P.E. Simson, H.E. Criswell, G.R. Breese, Inhibition of NMDAevoked electrophysiological activity by ethanol in selected brain regions: Evidence for ethanol-sensitive and ethanol-insensitive NMDA-evoked responses, Brain Res. 607 (1993) 9–16. [36] B.L. Soldo, W.R. Proctor, T.V. Dunwiddie, Ethanol differentially modulate GABAA receptor-mediated chloride currents in hippocampal, cortical, and septal neurons in rat brain slices, Synapse 18 (1994) 94–103. [37] S.C. Steffensen, S.J. Henriksen, Comparison of the effects of ethanol and chlordiazepoxide on electrophysiological activity in the fascia dentate and hippocampus regio superior, Hippocampus 2 (1992) 201–212. [38] M. Stewart, S.E. Fox, Two populations of rhythmically bursting neurons in rat medial septum are revealed by atropine, J. Neurophys. 61 (1989) 982–993. [39] M. Stewart, S.E. Fox, Do septal neurons pace the hippocampal theta rhythm?, Trends Neurosci. 13 (1990) 163–168. [40] P.D. Suzdak, R.D. Schwartz, P. Skolnick, S.M. Paul, Ethanol stimulates g-aminobutyric acid receptor-mediated chloride transport in rat brain synaptoneourosomes, PNAS 83 (1986) 4071–4075. [41] M.K. Ticku, P. Lowrimore, P. Lehoullier, Ethanol enhances GABAinduced 36 Cl 2 influx in primary spinal cord cultured neurons, Brain Res. Bull. 17 (1986) 123–126. [42] E.C. Tolman, Cognitive maps in rats and men, Psychol. Rev. 55 (1948) 189–208. [43] M.J. VanDoren, D.B. Matthews, G.C. Janis, A.C. Grobin, L.L. Devaud, A.L. Morrow, Neuroactive steroid 3a-hydroxy-5a-pregnan20-one modulates electrophysiological and behavioral actions of ethanol, J. Neurosci. 20 (2000) 1982–1989. [44] F.J. Wan, F. Berton, S.G. Madamba, W. Francesconi, G.R. Siggins, Low ethanol consentrations enhance GABAergic inhibitory postsynaptic potentials in hippocampal pyramidal neurons only after block of GABA B receptors, PNAS 93 (1996) 5049–5054. [45] J.L. Weiner, C. Gu, T.V. dunwiddie, Differential ethanol sensitivity of subpopulations of GABAA synapses onto rat hippocampal CA1 pyramidal neurons, J. Neurophys. 77 (1997) 1306–1312. [46] A.M. White, P.E. Simson, P.J. Best, Comparison between the effects of ethanol and diazepam on spatial working memory in the rat, Psychopharmacology 133 (1997) 256–261. [47] A.M. White, P.J. Best, Effects of ethanol on hippocampal place–cell and interneuron activity, Brain Res. 876 (2000) 154–165.
Short communication
Effect of acute ethanol administration and acute allopregnanolone administration on spontaneous hippocampal pyramidal cell neural activity Sayaka Tokunaga a , Janelle R. McDaniel a , A. Leslie Morrow b , Douglas B. Matthews a , * a
b
Department of Psychology, The University of Memphis, Memphis, TN 38152, USA Department of Pharmacology and Psychiatry, Bowles Center for Alcohol Studies, University of North Carolina, Chapel Hill, NC 27599, USA Accepted 18 December 2002
Abstract We investigated the effect of acute ethanol administration and acute allopregnanolone administration on spontaneous hippocampal pyramidal cell neural activity. Both agents produced significant reductions in spontaneous firing rate of hippocampal pyramidal neurons at a medium and high doses. Furthermore, blockade of allopregnanolone biosynthesis by preadministration of finasteride, a 5a-reductase blocker, prevented ethanol-induced inhibition on hippocampal pyramidal neural activity. The results further demonstrate similar effects of allopregnanolone and ethanol on hippocampal neurophysiology and that allopregnanolone plays a key role in producing ethanol-induced inhibition of hippocampal neural activity. 2002 Elsevier Science B.V. All rights reserved. Theme: Neural basis of behavior Topic: Drugs of abuse: alcohol, barbiturates, and benzodiazepines Keywords: Allopregnanolone; Finasteride; Hippocampus; Pyramidal neuron; Ethanol
1. Introduction Ethanol’s effect on spatial learning and memory, which depend heavily on hippocampal function, has been extensively investigated [20,21,23]. In rodents, hippocampal lesion greatly degrade the use of spatial information but do not degrade, or can facilitate, the use of non-spatial information [18,25,32,33]. Hippocampal pyramidal neurons (called ‘place cells’) are the primary output neurons of the hippocampus and fire action potentials only in restricted regions of an environment (called ‘place fields’) [28,30]. Consequently, one function of the hippocampus is to create a ‘cognitive map’ [42] that organizes animals’ behaviors. Acute ethanol administration impairs the use of spatial *Corresponding author. Tel.: 11-901-678-5461; fax: 11-901-6782579. E-mail address: [email protected] (D.B. Matthews).
memory, but does not impair, or can facilitate, the use of non-spatial memory [7,13,20,23,46]. At physiologically relevant concentrations, acute ethanol administration demonstrates regionally specific effects on neural activities [9,16,34,35,43–45], including inhibition of hippocampal pyramidal neurons [1,17,24,37,47]. Therefore, it has been suggested that ethanol degrades spatial learning and memory by disrupting hippocampal pyramidal cell neural activities [29,31]. Many of ethanol’s behavioral effects are enhanced by positive GABAA receptor modulators, and are blocked by GABAA receptor antagonist or inverse agonists [2,11,14,15,26,40,41]. Furthermore, allopregnanolone concentrations, an endogenous pregnane steroid that is a potent positive GABAA receptor modulator, is increased by acute ethanol administration [3,27,43]. Preadministration of finasteride, a 5a-reductase inhibitor, reduced ethanol-induced increases in allopregnanolone concentration in cerebral cortex by approximately 48%, and prevented
0006-8993 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0006-8993(02)04266-X
274
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
ethanol-induced reduction in spontaneous firing of MS / DB (medial septal / diagonal band of Broca) neurons [43]. Considering that the MS / DB modulates the hippocampal theta rhythm [38,39], these results suggest that some of the ethanol-induced changes in hippocampal function may be modulated by increased allopregnanolone concentrations in brain. Recently, we have investigated the effect of allopregnanolone on spatial memory and learning in Morris water task. Acute ethanol administration and acute allopregnanolone administration produced strikingly similar pattern of impairments. Consistent with previous findings [19,20], acute ethanol administration degraded performance on spatial memory, but did not degrade performance on nonspatial memory. Interestingly, acute allopregnanolone administration also degraded performance on spatial memory, but did not degrade the performance on non-spatial memory [22]. This similar pattern of selective deficit in spatial memory indicates that allopregnanolone may exert its effects by disrupting hippocampal neural functioning. Because allopregnanolone inhibits hippocampal neurons in vitro [12] and selectively impairs spatial memory, we investigated if allopreananolone modulates ethanol’s electrophysiological effects in vivo. We hypothesize that both agents will inhibit hippocampal pyramidal neural activity. We also investigated the effect of finasteride preadministration on ethanol-induced inhibition on hippocampal pyramidal neurons to further confirm our result.
2. Methods All experiments were conducted in accordance with National Institutes of Health Guideline under Institution Animal Care and Use Committee-approved protocols at the University of Memphis.
2.1. Study 1 Male adult rats (Charles River, Indianapolis, IN) were anesthetized with urethane (approximately 1.5 g / kg, intraperitoneally) and placed in a stereotaxic frame with skull flat orientation. An incision was made in the skin, the skull surface was cleaned, and a burr hole was drilled through the skull over left hippocampus. The center of the hole was roughly 3.5 mm posterior to bregma, 2.5 mm lateral to the midline. Following surgery, the rat was moved to a separate stereotaxic frame located in a Faraday cage. Single-barrel glass micropipettes (A-M Systems, Carlsborg, WA) were pulled (using Model PE-2; Narishige, Tokyo, Japan), and the tip was broken back to |1.0 mm and filled with a 0.9 M NaCl solution saturated with Chicago sky-blue dye. The electrode was lowered into the hippocampus via a hydraulic microdrive (Trent Wells, South Gate, CA). Extracellular action potentials were amplified, filtered (400 Hz and 8 kHz; Fintronics, Orange,
CT), and monitored with a Tektronix oscilloscope and audiomonitor. Neurons were classified as hippocampal complex spike cell using previously established waveform criteria [6]. Briefly, neuron was classified as a complex spike cell when it sometimes fired burst of approximately 2–10 action potentials (spikes) of decreasing amplitude with short spike intervals (#5 ms). Individual action potentials were isolated from background activity with at least 3:1 single-to-noise ratio, and the neuron’s firing rate was stored in 20-s time bins on a computer for later analysis. Following the isolation of a single hippocampal complex spike neuron, a 5-min baseline of spontaneous neural activity was collected prior to an injection, intraperitoneally, of one of four ethanol doses (saline control, 0.5 g / kg, 1.0 g / kg, or 1.5 g / kg 10% ethanol, w / v) or one of four allopregnanolone doses (20% 2-hydroxypropyl-b-cyclodextrin control, 10, 15 or 17.5 mg / kg). Previous studies have shown that 0.5 g / kg ethanol results in a peak blood alcohol level of |50 mg%, whereas doses of 1.0 and 1.5 g / kg ethanol correspond to blood alcohol levels of |85 mg% and |140 mg%, respectively [5,10]. Thereafter an additional 60-min of spontaneous neural activity was recorded. The location of electrode was carefully monitored and the electrode was micromanipulated during the recording to prevent waveform alterations. After completion of the recording, a current was passed through the electrode, thereby depositing dye and marking its location. The brain was then removed from the animal, frozen, and sliced to verify electrode location (Cryostat HM 505E; Microm, Walldorf, Germany). No more than one neuron was recorded per animal.
2.2. Study 2 In order to study if allopregnanolone is necessary to produce inhibition of hippocampal pyramidal neurons, we investigated the effect of finasteride, a 5a-reductese inhibitor, on ethanol-induced inhibition. By blocking biosynthesis of allopregnanolone, we hypothesized, the inhibitory effect of ethanol on hippocampal pyramidal neurons would be reduced. In the ethanol only condition, the procedure was same as study 1, but a single dose, 1.5 g / kg was used. In the finasteride condition, 50 mg / kg finasteride was administered subcutaneously at 4 and 1.5 h before the surgery. Following isolation of a neuron, 1.5 g / kg ethanol was administered and the neuron was recorded for an additional 60 min.
3. Drug preparation Ethanol (95%) was diluted with distilled water to a concentration of 10%. Purified allopregnanolone
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
(Steraloids Inc., Newport, RI) was suspended in 20% (w / v) aqueous 2-hydroxypropyl-b-cyclodextrin and then sonicated for approximately 1 h before each use. Purified finasteride (Steraloids Inc., Newport, RI) was suspended in 20% (w / v) aqueous 2-hydrosypropyl-a-cyclodextrin and then sonicated for approximately 30 min before each use.
4. Results
4.1. Study 1 A total of 20 neurons (n55 for saline group and each dose group) from 20 different male Long–Evans hooded rats (weight range 133–386 g) were recorded in the ethanol condition. A total of 21 neurons (n56 for vehicle group, n55 for each dose group) from 21 different male Long–Evans hooded rats (weight range 174–431 g) were recorded in the allopregnanolone condition. Histological record revealed that neurons recorded were located in the hippocampus. Acute ethanol administration significantly inhibited the spontaneous neural activity of hippocampal pyramidal neurons (Fig. 1, [two-way ANOVA with repeated measures, main effect of ethanol dose; F53.76, df (3, 16), P,0.05]). Post-hoc tests revealed that the amount of neural inhibition was dependent on the ethanol dose. Specifically, hippocampal pyramidal neural activity was significantly reduced by 53.16623.37% in animals administered 1.0 g / kg ethanol (Dunnett’s t-test, P,0.05) and by 59.1168.88% in animals administered 1.5 g / kg ethanol (Dunnett’s t-test, P,0.05) compared to animals in saline
275
control group. There was no significant difference in inhibition between saline control and 0.5 g / kg ethanol group (Dunnett’s t-test, P.0.05). Typical firing rate histograms of single spontaneously active hippocampal pyramidal neurons recorded in each ethanol dose group are shown in Fig. 2. Acute allopregnanolone administration also significantly inhibited the spontaneous neural activity of hippocampal pyramidal neurons (Fig. 3, [two-way ANOVA with repeated measures, main effect of allopregnanolone dose; F57.066, df (3, 17), P,0.01]). Post-hoc tests revealed that the amount of neural inhibition was dependent on the allopregnanolone dose. Specifically, hippocampal pyramidal neural activity was significantly reduced by 63.5168.9% in animals administered 15 mg / kg allopregnanolone (one-way Dunnett’s t-test, P,0.05) and by 49.6769.68% in animals administered 17.5 mg / kg allopregnanolone (one-way Dunnett’s t-test, P,0.05) compared to vehicle control group. There was no significant difference in inhibition between vehicle control and 10 mg / kg allopregnanolone group (one-way Dunnett’s t-test, P. 0.05). Typical firing rate histogram of a single spontaneously active hippocampal pyramidal neuron recorded from an animal in each allopregnanolone dose group are shown in Fig. 4. Table 1 shows the proportion of neurons in each condition that exhibited the inhibition by at least 10, 20, 30, 40 and 50%. This reveals that as the dose of ethanol and allopregnanolone increases, the number of neurons showing higher inhibition percentage also increases. That is, in higher dose groups, more neurons showed greater inhibition.
Fig. 1. Acute ethanol administration inhibits the spontaneous firing rate of hippocampal pyramidal neurons. A 1-h time course of hippocampal pyramidal neuron firing rates for each ethanol dose group (saline control, 0.5, 1.0 and 1.5 g / kg ethanol). n55 neurons per condition and error bars denote S.E.M. Each time bin represents the average percent baseline rate collected in 5 min intervals.
276
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
Fig. 2. Spontaneous firing rate of a single hippocampal pyramidal neuron in an anesthetized rat after: (a) saline administration; (b) 0.5 g / kg ethanol administration; (c) 1.0 g / kg ethanol administration; and (d) 1.5 g / kg ethanol administration. Arrows indicate the time at which i.p. injection was given.
Fig. 3. Acute allopregnanolone administration inhibits the spontaneous firing rate of hippocampal pyramidal neurons. A 1-h time course of hippocampal pyramidal neuron firing rates of each allopregnanolone dose group (20% 20-hydrozypropyl-b-cyclodextrin control, 10, 15 and 17.5 mg / kg allopregnanolone). n56 for control group, and n55 for each allopregnanolone dose group. Error bars denote S.E.M. Each time bin represents the average percent baseline firing rate collected in 5 min intervals.
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
277
Fig. 4. Spontaneous firing rate of a single hippocampal pyramidal neuron in an anesthetized rat after: (a) vehicle administration; (b) 10 mg / kg allpregnanolone administration; (c) 15 mg / kg allopregnanolone administration; and (d) 17.5 mg / kg allopregnanolone administration. Arrows indicate the time at which i.p. injection was given.
4.2. Study 2 A total of five neurons from five different male Long– Evans hooded rats (weight range 188–362 g) were recorded in the ethanol only condition while a total of five neurons from five different male Long–Evans hooded rats (weight range 242–327 g) were recorded in the finasteride condition. Histological record revealed that neurons recorded were located in the hippocampus.
Acute ethanol administration significantly inhibited the spontaneous neural activity of hippocampal pyramidal neurons by 63.58611.41% on average (Fig. 5, [one-way ANOVA with repeated measures, main effect of ethanol; F54.42, df (1, 12), P,0.001]). This result is consistent with the finding from Study 1. Preadministration of finasteride prevented the inhibitory effect of acute ethanol administration (Fig. 5, [two-way ANOVA with repeated measures, main effect of drug condition; F544.78, df (1,
Table 1 Number of neurons in each condition in study 1 showing inhibition of spontaneous neural activity by at least 10, 20, 30, 40 and 50% Percentage inhibited
At least 10% 20% 30% 40% 50% or more Neurons showing overall increase
Ethanol
Allopregnanolone
Saline 5
Low 5
Medium 5
High 5
Vehicle 6
Low 5
Medium 5
High 5
3/5 1/5 0/5 0/5 0/5 2/5
4/5 4/5 2/5 2/5 0/5 1/5
4/5 4/5 4/5 4/5 4/5 1/5
5/5 5/5 4/5 4/5 4/5 0/5
3/6 3/6 2/6 1/6 1/6 3/6
2/5 0/5 0/5 0/5 0/5 3/5
5/5 4/5 4/5 3/5 3/5 0/5
5/5 4/5 4/5 4/5 4/5 0/5
Also the number of neurons showing overall increase in spontaneous neural acitifity is given.
278
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
Fig. 5. Ethanol administration (1.5 g / kg, i.p.) reduced spontaneous neural activity of hippocampal pyramidal neurons. Finasteride (50 mg / kg) pretreatment prevented the inhibition of hippocampal pyramidal neurons produced by ethanol administration.
12), P,0.0001]). Post-hoc test revealed that there was significant difference in inhibition between the ethanol only group and the finasteride group (two-tailed t-test, t52.625, df58, P,0.05). Typical firing rate histogram of a single spontaneously active hippocampal pyramidal neuron recorded from an animal in the ethanol only group and finasteride group are shown in Fig. 6. Importantly, preadministration of finasteride did not alter baseline firing rate (two-tailed t-test, t51.363, df58, P.0.05).
5. Discussion Ethanol-induced inhibitory effect on hippocampal pyramidal cells found in the present study is consistent with the previous findings from several electrophysiological studies [1,37,47]. These electrophysiological data, including the result from the present study, suggest that ethanol inhibits hippocampal pyramidal neural activity either directly or indirectly, especially above a threshold of
Fig. 6. Spontaneous firing rate of a single hippocampal pyramidal neuron in an anesthetized rat after: (a) 1.5 g / kg ethanol administration; and (b) 1.5 g / kg ethanol administration with 50 mg / kg finasteride pretreatment.
S. Tokunaga et al. / Brain Research 967 (2003) 273–280
1.0 g / kg ethanol. Furthermore, the results from electrophysiological studies of single hippocampal neural activity are in general agreement with several behavioral studies demonstrating that ethanol alters hippocampal function [7,8,13,20,23]. These consistent results suggest that the inhibitory effect of ethanol on hippocampal pyramidal neurons might be a cellular mechanism underlying ethanolinduced impairment in spatial learning and memory. The inhibitory effect of acute allopregnanolone administration found in the present study was strikingly similar to that of acute ethanol administration. While ethanol was found not to interact directly with hippocampal GABAA receptor sites [36], allopregnanolone was found to inhibit neural activities in cultured hippocampal neurons [12]. In the present study, these two drugs produced similar inhibitory effects on hippocampal pyramidal neurons. Although the present study is unable to locate the originating site for the inhibitory effects produced by ethanol, considering that ethanol had been found to increase concentration levels of allopregnanolone in the brain [43], it is plausible that ethanol may not directly inhibit hippocampal pyramidal neural activity but instead increase allopregnanolone concentration, resulting in the inhibition on hippocampal pyramidal neurons. It has been found that the dose of 1 g / kg ethanol, which had an inhibitory effect on hippocampal neurons in the present study, does not produce a significant increase in allopregnanolone levels. However, these findings are based on cortical allopregnanolone levels [43]. The investigation on allopregnanolone levels in the hippocampus following ethanol administration is clearly needed. Contrary to the findings of the present study 10 mg / kg allopregnanolone substitutes for ethanol in rats trained to discriminate 1.0 g / kg ethanol [4]. However, it is reasonable to conclude that since substitution tasks likely involve different brain regions, the critical dose needed to produce the effect in substitution tasks is different from the dose needed to produce the inhibitory effect on hippocampal pyramidal neurons. Preadministraiton of finasteride was found to prevent ethanol-induced inhibition on hippocampal pyramidal neural activity. This result provides a strong indication that allopregnanolone plays an important role in producing this effect, and that GABAA receptors are a major site of action for inhibition of pyramidal cell neural activity. Based on this finding, there is a possibility that finasteride preadministration might prevent ethanol-induced impairment in spatial learning and memory. The present study is the only study we know of to demonstrate the dose-dependent inhibitory effect of allopregnanolone on hippocampal pyramidal neurons in vivo. The results from this study clearly demonstrate that ethanol and allopregnanolone reduce hippocampal pyramidal neuron firing rate. Moreover, the present study provides additional evidence that blockade of biosynthesis of allopregnanolone by finasteride prevents ethanol-induced inhibi-
279
tion on hippocampal pyramidal neurons. From these findings, it is reasonable to assume that allopregnanolone is a key mechanism in ethanol’s alterations of hippocampal neurophysiology and this alteration might effect behaviors dependent on hippocampus such as spatial learning and memory.
Acknowledgements This work was supported by an Alcoholic Beverage Medical Research Foundation grant (D.B. Matthews), University of Memphis faculty research grant (D.B. Matthews), and AA10564 (A.L. Morrow).
References [1] Y.I. Alexandorv, Y.V. Grinchenko, S. Laukka, T. Javilehto, V.N. Maz, A.V. Korpusova, Effect of ethanol on hippocampal neurons depends on their behavioral specialization, Acta Physiol. Scand. 149 (1993) 105–115. [2] A.M. Allan, R.A. Harris, Gamma-aminobutyric acid and alcohol actions: neurochemical studies of long sleep and short sleep mice, Life Sci. 39 (1986) 2005–2015. [3] M.L. Barbaccia, D. Affricano, M. Trabucchi, R.H. Purdy, G. Colombo, R. Agabio, G.L. Gessa, Ethanol markedly increases ‘gabaergic’ neurosteroids in alcohol-preferring rats, Eur. J. Pharm. 384 (1999) R1–R2. [4] C.A. Bowen, R.H. Purdy, K.A. Grant, Ethanol-like discriminative stimulus effects of endogenous neuroactive steroids: effect of ethanol training dose and dosing procedure, J. Pharmacol. Exp. Ther. 289 (1999) 405–411. [5] L. Devenport, R.L. Hale, Contributions of hippocampus and neocortex to the expression of ethanol effects, Psychopharmacology 99 (1989) 337–344. [6] S.E. Fox, J.B. Ranck Jr., Electrophysiological characteristics of hippocampal complex-spike cells and theta cells, Exp. Brain Res. 41 (1981) 399–410. [7] W.E. Gibson, Effects of alcohol on radial maze performance in rats, Physiol. Behav. 35 (1984) 1003–1005. [8] B. Givens, Low doses of ethanol impair spatial working memory and reduce hippocampal theta activity, Alcohol Clin. Exp. Res. 19 (1995) 763–767. [9] B.S. Givens, G.R. Breese, Electropysiological evidence that ethanol alters function of medial septal area without affecting lateral septal function, J. Pharmacol. Exp. Ther. 253 (1990) 95–103. [10] B. Givens, K. McMahon, Ethanol suppresses the induction of long-term potentiation in vivo, Brain Res. 688 (1995) 27–33. [11] A.C. Grobin, D.B. Matthews, L.L. Devaud, A.L. Morrow, The role of GABAA receptors in the acute and chronic effects of ethanol, Psychopharmacology 139 (1998) 2–19. [12] N.L. Harrison, M.D. majewska, J.W. Harrington, J.L. Barker, Structure–activity relationships for steroid interaction with the gaminobutyric acidA receptor complex, J. Pharmacol. Exp. Ther. 241 (1987) 346–353. [13] S.E. Hoffmann, D.B. Matthews, Ethanol-induced impairments in spatial working memory are not due to deficits in learning, Alcohol Clin. Exp. Res. 25 (2001) 856–861. [14] S. Liljequist, J. Engel, Effects of GABAergic agonists and antagonists on various ethanol-induced behavioral changes, Psychopharmacology 78 (1982) 71–75. [15] R.G. Lister, M. Linnoila, Alcohol, the chloride ioonphore and
280
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27] [28]
[29] [30]
[31]
S. Tokunaga et al. / Brain Research 967 (2003) 273–280 endogenous ligands for benzodiazepine receptors, Neuropharmacology 30 (1991) 1435–1440. D.M. Lovinger, G. White, F.F. Weight, NMDA receptor-mediated synaptic excitation selectively inhibited by ethanol in hippocampal slice from adult rat, J. Neurosci. 10 (1990) 1372–1379. N. Ludvig, S.E. Fox, J.L. Kubie, B.M. Altura, B.T. Altura, Application of the combined single-cell recording intracerebral microdialysis method to alcohol research in freely behaving animals, Alcohol Clin. Exp. Res. 22 (1998) 41–50. D.B. Matthews, P.J. Best, Fimbria / fornix lesions facilitate the learning of a nonspatial response task, Psychonom. Bull. Rev. 2 (1995) 113–116. D.B. Matthews, P.J. Best, A.M. White, J.L. Vandergriff, P.E. Simson, Ethanol impairs spatial cognitive processing: new behavioral and electrophysiological findings, Curr. Dir. Psychol. Sci. 5 (1996) 111–115. D.B. Matthews, M. Ilgen, A.M. White, P.J. Best, Acute ethanol administration impairs spatial performance while facilitating nonspatial performance in rats, Neurobiol. Learning Memory 72 (1999) 169–179. D.B. Matthews, A.L. Morrow, Effects of acute and chronic ethanol exposure on spatial cognitive processing and hippocampal function in the rat, Hippocampus 10 (2000) 122–130. D.B. Matthews, A.L. Morrow, S. Tokunaga, J.R. McDaniel, Acute ethanol administration and acute allopregnanolone administration impair spatial memory in the Morris water task, Alcohol Clin. Exp. Res. 26 (2002) 1747–1751. D.B. Mattehws, P.E. Simson, P.J. Best, Acute ethanol impairs spatial memory but not stimulus / response memory in the rat, Alcohol Clin. Exp. Res. 19 (1995) 902–909. D.B. Matthews, P.E. Simson, P.J. Best, Ethanol alters spatial processing of hippocampal place cells: a mechanism for impaired navigation when intoxicated, Alcohol Clin. Exp. Res. 20 (1996) 404–407. R.G.M. Morris, P. Garrud, J.N.P. Rawlins, J. O’Keefe, Place navigation impaired in rats with hippocampal lesions, Nature 297 (1982) 681–683. A.L. Morrow, P.D. Suzdak, J.W. Karanian, S.M. Paul, Chronic ethanol administration alters g-aminobutyric acid pentobarbital and ethanol-mediated 36 Cl 2 uptake in cerebral cortical synaptoneurosomes, J. Pharmacol. Exp. Ther. 246 (1988) 158–164. A.L. Morrow, M.J. VanDoren, L.L. Devaud, Effects of progesterone or neuroactive steroid?, Nature 395 (1998) 652–653. R.U. Muller, J.L. Kubie, J.B. Ranck Jr., Spatial firing patterns of hippocampal complex-spike cells in a fixed environment, J. Neurosci. 7 (1987) 1935–1950. L. Nadel, The hippocampus and space revisited, Hippocampus 1 (1991) 221–229. J. O’Keefe, J. Dostrovsky, The hippocampus as a spatial map: preliminary evidence from unit activity in the freely-moving rat, Brain Res. 34 (1971) 171–175. J. O’Keefe, L. Nadel, The Hippocampus as A Cognitive Map, Oxford University Press, Oxford, 1978.
[32] J. O’Keefe, L. Nadel, S. Keightley, D. Kill, Fornix lesions selectively abolish place learning in rat, Exp. Neurol. 48 (1975) 152–166. [33] M.G. Packard, R. Hirsh, N.M. White, Differential effects of fornix and caudate nucleus lesions on two radial maze tasks: evidence for multiple memory systems, J. Neurosci. 9 (1989) 145–147. [34] P.E. Simson, H.E. Criswell, G.R. Breese, Ethanol potentiates gaminobutyric acid-mediated inhibition in the inferior coliculus: evidence for local ethanol / g-aminobutryic acid interactions, J. Pharmacol. Exp. Ther. 259 (1991) 1288–1293. [35] P.E. Simson, H.E. Criswell, G.R. Breese, Inhibition of NMDAevoked electrophysiological activity by ethanol in selected brain regions: Evidence for ethanol-sensitive and ethanol-insensitive NMDA-evoked responses, Brain Res. 607 (1993) 9–16. [36] B.L. Soldo, W.R. Proctor, T.V. Dunwiddie, Ethanol differentially modulate GABAA receptor-mediated chloride currents in hippocampal, cortical, and septal neurons in rat brain slices, Synapse 18 (1994) 94–103. [37] S.C. Steffensen, S.J. Henriksen, Comparison of the effects of ethanol and chlordiazepoxide on electrophysiological activity in the fascia dentate and hippocampus regio superior, Hippocampus 2 (1992) 201–212. [38] M. Stewart, S.E. Fox, Two populations of rhythmically bursting neurons in rat medial septum are revealed by atropine, J. Neurophys. 61 (1989) 982–993. [39] M. Stewart, S.E. Fox, Do septal neurons pace the hippocampal theta rhythm?, Trends Neurosci. 13 (1990) 163–168. [40] P.D. Suzdak, R.D. Schwartz, P. Skolnick, S.M. Paul, Ethanol stimulates g-aminobutyric acid receptor-mediated chloride transport in rat brain synaptoneourosomes, PNAS 83 (1986) 4071–4075. [41] M.K. Ticku, P. Lowrimore, P. Lehoullier, Ethanol enhances GABAinduced 36 Cl 2 influx in primary spinal cord cultured neurons, Brain Res. Bull. 17 (1986) 123–126. [42] E.C. Tolman, Cognitive maps in rats and men, Psychol. Rev. 55 (1948) 189–208. [43] M.J. VanDoren, D.B. Matthews, G.C. Janis, A.C. Grobin, L.L. Devaud, A.L. Morrow, Neuroactive steroid 3a-hydroxy-5a-pregnan20-one modulates electrophysiological and behavioral actions of ethanol, J. Neurosci. 20 (2000) 1982–1989. [44] F.J. Wan, F. Berton, S.G. Madamba, W. Francesconi, G.R. Siggins, Low ethanol consentrations enhance GABAergic inhibitory postsynaptic potentials in hippocampal pyramidal neurons only after block of GABA B receptors, PNAS 93 (1996) 5049–5054. [45] J.L. Weiner, C. Gu, T.V. dunwiddie, Differential ethanol sensitivity of subpopulations of GABAA synapses onto rat hippocampal CA1 pyramidal neurons, J. Neurophys. 77 (1997) 1306–1312. [46] A.M. White, P.E. Simson, P.J. Best, Comparison between the effects of ethanol and diazepam on spatial working memory in the rat, Psychopharmacology 133 (1997) 256–261. [47] A.M. White, P.J. Best, Effects of ethanol on hippocampal place–cell and interneuron activity, Brain Res. 876 (2000) 154–165.