Basis of the gabamimetic profile of ethanol

This article summarizes the proceedings of a symposium held at the 2005 Research Society on Alcoholism meeting. The initial presentation by Dr. Wallner provided evidence that selected GABA(A) receptors containing the delta subunit display sensitivity to low intoxicating ethanol concentrations and this sensitivity is further increased by a mutation in the cerebellar alpha6 subunit, found in alcohol-hypersensitive rats. Dr. Mameli reported that ethanol affects gamma-aminobutyric acid (GABA) function by affecting neural circuits that influence GABA release. Dr. Parsons presented data from electrophysiological and microdialysis investigations that ethanol is capable of releasing GABA from presynaptic terminals. Dr. Morrow demonstrated that systemic ethanol increases neuroactive steroids in brain, the absence of which alters various functional responses to ethanol. Dr. Criswell presented evidence that the ability of ethanol to increase GABA was apparent in some, but not all, brain regions indicative of regional specificity. Further, Dr. Criswell demonstrated that neurosteroids alone and when synthesized locally by ethanol act postsynaptically to enhance the effect of GABA released by ethanol in a region specific manner. Collectively, this series of reports support the GABAmimetic profile of acutely administered ethanol being dependent on several specific mechanisms distinct from a direct effect on the major synaptic isoforms of GABA(A) receptors.

Differential modulation of GABA- and NMDA-gated currents by ethanol and isoflurane in cultured rat cerebral cortical neurons

Ethanol and the volatile anesthetics share many features including effects on both GABA and NMDA receptors. To determine the degree of similarity between these compounds, we examined the concentration-response curves for ethanol and isoflurane on currents gated by GABA or NMDA. The effects of isoflurane and ethanol on the righting reflex of rats were also observed. The concentration of ethanol causing loss of the righting reflex of rats was 82.3+/-2.9 mM, whereas median concentration of isoflurane exerting that effect was 0.125 mM. Both isoflurane and ethanol inhibited NMDA-gated currents in cultured cerebral cortical neurons at concentrations well below those associated with loss of the righting reflex or anesthesia. However, the effect of isoflurane was greater than that of ethanol and the slope of the concentration-response curve for isoflurane less steep than that for ethanol. Isoflurane enhanced GABA-gated currents at anesthetic concentrations but there was a sharp concentration-response curve with only minimal effects of isoflurane on GABA-gated currents at concentrations associated with loss of the righting reflex. In contrast, ethanol had no effect on GABA-gated currents even at lethal concentrations, i.e. 300 mM or 1.2%. Comparison of the concentration-response curves for the effects of isoflurane on NMDA- and GABA-gated currents has revealed both EC50 and Hill slope for the potentiation of GABA-gated currents were significantly greater than those for inhibition of NMDA-gated currents. These results support the hypothesis that isoflurane has actions on both the GABA and NMDA systems that are not shared by ethanol.

Chronic ethanol consumption alters recovery of spontaneously active medial septal/diagonal band of broca neurons from GABA-microiontophoresis

BACKGROUND

Acute ethanol administration increases GABA-mediated inhibition in a variety of cerebral cortical preparations. Furthermore, chronic ethanol administration blunts ethanol-induced increases in GABA-mediated inhibition and alters GABA A receptor subunit mRNA and peptide expression in the cerebral cortex. The sedative hypnotic effects of ethanol are believed to be modulated by GABA-induced inhibition in medial septum/diagonal band of Broca (MS/DB) neurons, a brain region where acute ethanol administration increases GABA-mediated inhibition of spontaneously active neurons. Chronic ethanol administration produces tolerance to the sedative effects of ethanol. However, it is unknown if chronic ethanol consumption produces alterations in GABA-mediated inhibition in the MS/DB in a manner similar to that found in the cerebral cortex.

METHODS

Animals either consumed ethanol chronically for 14 days via a liquid diet or were pair-fed an equicaloric dextrose-containing control diet. Spontaneously active MS/DB neurons were recorded using multibarrel glass micropipettes while the effect of GABA-microiontophoresis was investigated. The total amount of GABA-mediated inhibition at four ejection currents was analyzed, as was the recovery to spontaneous neural firing rates following GABA inhibition. In a separate group of animals, the medial septum was microdissected, and the relative expression of GABA A receptor alpha1 and alpha4 subunit peptide were analyzed via Western blot analysis.

RESULTS

Chronic ethanol consumption altered recovery of spontaneous neural activity of MS/DB neurons following GABA-microiontophoresis compared to premicroiontophoresis levels. Specifically, the recovery of spontaneous neural activity of MS/DB neurons recorded from animals that chronically consumed ethanol was slower following GABA-microiontophoresis compared to neurons recorded from control animals. This effect was temporary and reversible. Furthermore, the alteration in recovery of spontaneous neural activity was not due to changes in the total amount of inhibition produced by GABA. Finally, there was no significant change in GABA A receptor alpha1 and alpha4 subunit peptide levels in the MS/DB.

CONCLUSIONS

Chronic ethanol consumption alters the frequency of spontaneous MS/DB neural activity following GABA microiontophoresis compared to premicroiontophoresis levels. These data suggest that the kinetics of GABA A receptors in the MS/DB are altered by chronic ethanol consumption independent of changes in the total amount of inhibition or alterations in GABA A receptor alpha1 and alpha4 subunit peptide expression.

Ethanol modulation of gamma-aminobutyric acid (GABA)-mediated inhibition of cerebellar Purkinje neurons: relationship to GABAb receptor input

BACKGROUND

Electrophysiological recording reveals that only a portion of cerebellar Purkinje neurons are sensitive to ethanol enhancement of gamma-aminobutyric acid (GABA) responses. Although activation of beta-adrenergic receptors permits ethanol enhancement of GABA function from some cerebellar Purkinje neurons, other neurons remain insensitive to ethanol. These findings are consistent with the finding that other external neural inputs are required to allow ethanol enhancement of GABA responses from Purkinje neurons. Because of a high expression of GABA(B) receptors on Purkinje cells, we tested whether activation of GABA(B) receptors might modulate the action of ethanol on GABA responsiveness.

METHODS

Extracellular single-unit electrophysiological recording was used to investigate the effects of ethanol on responses to GABA and muscimol (a GABA(A) agonist) from cerebellar Purkinje neurons. Drugs tested were baclophen (a GABA(B) agonist) and CGP35348 (a GABA(B) antagonist).

RESULTS

Ethanol did not enhance responses to GABA and muscimol from all Purkinje neurons. Systemic administration of the GABA(B) agonist, baclophen (3 mg/kg intravenously), permitted ethanol to enhance GABA inhibition from approximately 75% of cerebellar Purkinje neurons not initially enhanced by ethanol. Local iontophoretic application of baclophen to Purkinje neurons also allowed ethanol to enhance GABA and muscimol responsiveness from a portion of neurons in which ethanol initially did not affect their actions. An inhibitory action of ethanol on responses to GABA and muscimol, which was also influenced by baclophen, was observed from some Purkinje neurons. From Purkinje neurons initially sensitive to ethanol enhancement of GABA and muscimol function, administration of CGP35348, a GABA(B) antagonist, diminished the effect of ethanol on the responsiveness of these agonists from the majority (9/15) of neurons.

CONCLUSIONS

The present findings demonstrated that baclophen allows ethanol enhancement of GABA and muscimol responsiveness from some, but not all, cerebellar Purkinje neurons initially not sensitive to ethanol. Likewise, a GABA(B) antagonist can diminish ethanol enhancement of GABA and muscimol responses from some ethanol-sensitive neurons. Thus, these results emphasize that GABA(B) receptors on a portion of Purkinje neurons act as an auxiliary neural input that allows ethanol enhancement of GABA responses. Consequently, receptor structure alone does not account for the action of ethanol on GABA(A) receptor function on this cell type.

Interactive role for neurosteroids in ethanol enhancement of gamma-aminobutyric acid-gated currents from dissociated substantia nigra reticulata neurons

Although previous in vivo electrophysiological studies demonstrated a consistent ethanol enhancement of gamma-aminobutyric acid (GABA) responsiveness from substantia nigra reticulata (SNR) neurons, ethanol applied in vitro to dissociated neurons from the SNR had an inconsistent effect on GABA function. One source for the disparity between these contrasting in vivo and in vitro results could be an endogenous factor (acting on an auxiliary site on GABA(A) receptors) that was not available to the isolated SNR neurons. Because neurosteroids are present in vivo and act on an auxiliary site, it was hypothesized that the presence of a neurosteroid was important for a consistent effect of ethanol on GABA responsiveness from neurons studied in vitro. Alone, the neurosteroid analog alphaxalone produced a significant, concentration-related enhancement of GABA responsiveness from isolated SNR neurons. In contrast to an inconsistent action of 100 mM ethanol on GABA responsiveness in the absence of alphaxalone, the presence of 30 and 100 nM alphaxalone resulted in the majority of isolated neurons responding to this ethanol level. At a concentration of alphaxalone as low as 30 nM, ethanol produced a robust concentration-related increase in GABA-gated currents from this cell type. The neurosteroid 3alpha, 5alpha-tetrahydrodeoxycorticosterone (100 nM) also permitted a reliable concentration-dependent ethanol enhancement of responses to GABA from SNR cells, indicative that the effects of alphaxalone were not unique. This consistent neurosteroid-induced ethanol enhancement of GABA responsiveness from dissociated SNR neurons supports the view that neurosteroids may play a key role in the action of ethanol on postsynaptic GABA(A) receptor function.

Action of ethanol on responses to nicotine from cerebellar Purkinje neurons: relationship to methyllycaconitine (MLA) inhibition of nicotine responses

The effect of ethanol on responses to nicotine from rat cerebellar Purkinje neurons was investigated using extracellular single-unit recording. Systemic administration of ethanol initially enhanced the nicotine-induced inhibition from 50% of the Purkinje neurons. However, irrespective of whether there was an initial enhancement, systemic administration of ethanol antagonized the response to nicotine from the majority of Purkinje neurons. When varying ethanol concentrations were electro-osmotically applied to this neuronal cell type, the responses to nicotine (6/8) were enhanced when a low concentration of ethanol (40 mM) was in the pipette, whereas the majority of nicotine responses (10/11) were antagonized when a higher concentration of ethanol (160 mM) was applied to Purkinje neurons. Thus, the concentration of ethanol presented to the neuron seemed to explain the biphasic consequence of systemically administered ethanol on responses to nicotine. In order to determine whether ethanol affected a specific nACh receptor subtype containing the alpha-7 subunit, it was initially established that the nicotinic antagonists, alpha-bungarotoxin (alpha-BTX) and methyllycaconitine (MLA), which are associated with this subunit, had identical actions on responses to nicotine from Purkinje neurons. When MLA was tested against responses to nicotine from this cell type, MLA antagonized the response to nicotine from 45% (9/20) of the neurons tested. In a direct comparison of the action of ethanol to inhibit responses to nicotine with the action of MLA on the same Purkinje neuron, ethanol inhibited responses to nicotine on all neurons sensitive to MLA. However, ethanol also affected nicotine-induced neural changes from some Purkinje neurons not sensitive to MLA antagonism of nicotine. These data support the supposition that ethanol affects a nACh receptor subtype which has an alpha-7 subunit as well as other nACh receptor subtypes without this specific subunit.

Action of ethanol on responses to nicotine from cerebellar interneurons and medial septal neurons: relationship to methyllycaconitine inhibition of nicotine responses

BACKGROUND

A majority of alcoholics also smoke, suggesting that alcohol and nicotine share a common action on nicotinic cholinergic receptors.

METHODS

Extracellular single-unit recording was used to investigate the effects of ethanol on responses to nicotine from rat cerebellar interneurons and medial septal neurons.

RESULTS

Nicotine produced inhibition from medial septal neurons, but increased neural activity of cerebellar interneurons. When ethanol was applied locally to cerebellar interneurons, the excitatory response to nicotine was enhanced in a dose-related manner. Nicotine-induced inhibition from medial septal neurons was reduced by ethanol from the majority of neurons, but a dose relationship for this inhibition by ethanol was not observed. Ethanol affected responses to nicotine from over 90% of all neurons investigated at these sites. Initially, it was established that the nicotinic antagonists, methyllycaconitine (MLA) and alpha-bungarotoxin, which affect a nicotinic cholinergic (nACh) receptor with an alpha7 subunit, had similar actions on responses to nicotine from individual medial septal cells and cerebellar interneurons. When MLA was tested against responses to nicotine from neurons in the two brain regions, MLA antagonized responses to nicotine from only 27% of the neurons rather than the 90% found for ethanol. This latter observation provided evidence that ethanol was affecting neurons with MLA-insensitive receptors. When the actions of ethanol on responses to nicotine were compared directly with the action of MLA on the same medial septal neurons, both ethanol and MLA caused a greater than 50% antagonism of the response to nicotine, indicative that nACh receptors with the alpha7 subunit were sensitive to ethanol.

CONCLUSIONS

Collectively, these data provide evidence that ethanol affects responses to nicotine not only from nACh receptors on medial septal cells and cerebellar interneurons containing an alpha7 subunit (i.e., MLA-sensitive receptors), but also from nACh receptor subtypes without this specific nACh receptor subunit (i.e., MLA-insensitive receptors).

Action of ethanol and zolpidem on gamma-aminobutyric acid responses from cerebellar Purkinje neurons: relationship to beta-adrenergic receptor input

The observation that cerebellar Purkinje cells contain type-I benzodiazepine-sensitive GABA(A) receptors is consistent with findings in the present work that the majority of Purkinje neurons are sensitive to enhancement of GABA by the type-1 benzodiazepine agonist, zolpidem. Previous work has demonstrated a relation between zolpidem and ethanol enhancement of GABA responses in several brain regions, but had not tested Purkinje neurons. Therefore, given that a majority of Purkinje neurons were found to be sensitive to zolpidem, ethanol would have been expected to enhance GABA responses from this cell type. However, in agreement with earlier electrophysiological studies, ethanol enhanced GABA inhibitory responses from only a small proportion of these cerebellar Purkinje neurons. Rather than enhancement of GABA, local application of ethanol either inhibited or did not affect responses to GABA from a majority of cerebellar-Purkinje neurons. Nonetheless, as previously reported, a portion of the Purkinje neurons initially insensitive to ethanol enhancement of GABA became sensitive to this action of ethanol with co-application of the beta-adrenergic agonist, isoproterenol. Thus, these results collectively implicate a beta-adrenergic input dependency for ethanol enhancement of GABA from some, but not all, cerebellar Purkinje neurons sensitive to zolpidem. Because a beta-adrenergic input did not allow ethanol enhancement of GABA from all Purkinje neurons, future studies should explore the possibility that other auxiliary neural inputs to zolpidem-sensitive cerebellar Purkinje neurons may be required for ethanol enhancement of GABA responsiveness when a beta-adrenergic input does not have this action. Likewise, knowing that the action of zolpidem can predict ethanol enhancement of GABA in other brain regions, the present findings suggest that a future determination be made concerning whether zolpidem-sensitive neurons in these other regions of brain require a beta-adrenergic or an alternative neural input for ethanol enhancement of GABA responses.

Action of zolpidem on responses to GABA in relation to mRNAs for GABA(A) receptor alpha subunits within single cells: evidence for multiple functional GABA(A) isoreceptors on individual neurons

The relationship between zolpidem sensitivity and GABA(A) receptor alpha subunits was studied in individual dissociated neurons from rat brain. Using whole-cell recording, similar EC50 values were demonstrated for the effect of gamma-aminobutyric acid (GABA) on gated-chloride currents from substantia nigra reticulata (SNR) and lateral septal neurons. Subsequently, many neurons from both the SNR or lateral septum were found to exhibit enhanced GABA-gated chloride currents across concentrations of zolpidem ranging from 10 to 300 nM. Some neurons exhibited a greater than 20% increase in responsiveness to GABA at 30 nM of zolpidem without further increase at higher concentrations of zolpidem. Conversely, zolpidem enhancement of GABA from another group of neurons was not observed at 30 nM zolpidem, but between 100 and 300 nM the response to GABA increased greater than 20%. Finally, a third group of neurons reached both of these criteria for zolpidem enhancement of GABA. This latter spectrum of responses to GABA after varying concentrations of zolpidem was consistent with the presence of either two GABA(A) receptors or a single receptor with differing affinities for zolpidem on an individual neuron. Following determination of the sensitivity of neurons from SNR or lateral septum to zolpidem, cytoplasm was extracted from some individual cells to allow identification of cellular mRNAs for the alpha1, alpha2 and alpha3 GABA(A) receptor subunits with RT-PCR. Those neurons that responded to the 30 nM zolpidem concentration invariably expressed the alpha1-GABA(A) receptor subunit. This result is consistent with the GABA(A) alpha1-receptor subunit being an integral part of a functional high-affinity zolpidem type 1-BZD receptor complex on neurons in brain. Those neurons which showed enhancement of GABA from 100 to 300 nM zolpidem contained mRNAs for the alpha2 and/or the alpha3 receptor subunits, a finding consistent with these alpha subunits forming type 2-BZD receptors. Some individual dissociated SNR neurons were sensitive to both low and high concentrations of zolpidem and contained mRNAs for all three alpha-receptor subunits. These latter individual neurons are proposed to have at least two functional GABA(A) receptor subtypes. Thus, the present investigation emphasizes the importance of characterizing the relationship between endogenous GABA(A) receptor function and the presence of specific structural components forming GABA(A) receptor subtypes on neurons.

Differential effects of bilateral dopamine depletion in neonatal and adult rats

Both Lesch-Nyhan syndrome and Parkinson's disease are associated with decreased brain dopamine, yet each disorder is characterized by a different set of motor symptoms. Lesch-Nyhan syndrome is manifested in early childhood, while parkinsonism usually does not appear until adulthood, suggesting that age at the time of dopamine loss is one determinant of the effects of neurotransmitter deficiency. Support for this view is found in studies of animals given dopamine-depleting lesions at different ages and then tested in adulthood. Animals lesioned as neonates show a supersensitivity to dopamine agonists, especially D1-dopamine receptor agonists, and to MK-801, an NMDA receptor antagonist. In addition, neonatally treated animals show a 'priming' effect following repeated exposure to D1-dopamine agonists. Animals depleted of dopamine as adults are more supersensitive to agonists acting on the D2-dopamine receptor, and do not evidence priming to dopamine agonists or an enhanced response to MK-801. These differential pharmacological profiles suggest that the changes in neurotransmitter systems following dopamine depletion are, at least in part, determined by age at the time of the lesion.

Flumazenil blockade of anxiety following ethanol withdrawal in rats

In previous research, the drug flumazenil has been categorized both as a pure benzodiazepine antagonist and as a benzodiazepine partial agonist. The following studies used an elevated plus maze to test whether flumazenil would exert any antianxiety action in rats. While chlordiazepoxide (3.0 mg/kg), ethanol (0.75 g/kg), and the atypical benzodiazepine zolpidem (1.0 mg/kg) all significantly increased time spent on the open arms and percent open arm entries, flumazenil (1-10 mg/kg) alone did not produce any anxiolytic effects on the maze. Withdrawal from chronic ethanol treatment led to a decrease in open arm time and percent open arm entries. Flumazenil (3.0 mg/kg) blocked these changes, suggesting that the effects of flumaxenil are at least partially dependent upon the levels of stress or anxiety in the subjects. An anxiolytic action of flumazenil was not seen following the central administration of the neuropeptide corticotropin-releasing factor (CRF), which reduced open arm time on the elevated plus maze. These results support the hypothesis that the mechanism of action for flumazenil effects on the anxiety observed during ethanol withdrawal involves antagonism of an endogenous benzodiazepine inverse agonist, rather than activity as a partial agonist or blockade of CRF-mediated effects.

Evidence for a selective effect of ethanol on N-methyl-d-aspartate responses: ethanol affects a subtype of the ifenprodil-sensitive N-methyl-d-aspartate receptors

An extracellular electrophysiological approach was used to determine the effect of ethanol on responses to N-methyl-D-aspartate (NMDA) across several brain regions in urethane-anesthetized rats. The results indicated that, in most brain regions, ethanol inhibited the NMDA-induced increases in firing rate for some, but not all, spontaneously active neurons. Ethanol functioned as an NMDA antagonist for some neurons in the medial septum, red nucleus, deep mesencephalic nucleus, substantia nigra reticulata, ventral tegmental area and cerebellum. In the hippocampus, ethanol inhibited NMDA responses from all neurons. However, ethanol was not found to be active against NMDA responses in the lateral septum, suggesting that there is a degree of regional specificity for ethanol inhibition of NMDA responses. It was then established in unanesthetized rats that ethanol also antagonized responses to NMDA in some, but not all, neurons in the medial septum and cortex, indicating that the differential action of ethanol on NMDA responses obtained in the urethane-anesthetized rats was not due to the anesthetic. Based on an earlier study showing that the effects of ifenprodil and ethanol on NMDA responses were correlated, the ability of ethanol to inhibit NMDA responses was compared with changes produced by ifenprodil on the same neurons, where ethanol did or did not affect NMDA responses. In the several brain regions investigated, ethanol inhibited NMDA responses in a subgroup of neurons in which ifenprodil inhibited NMDA-induced increases in firing. For all neurons investigated, if a cell was insensitive to ifenprodil antagonism of NMDA responses then ethanol also was ineffective against the response to NMDA. These results suggest that ethanol acts on an ifenprodil-sensitive NMDA receptor subtype. Given that previous investigations have suggested that the NMDA receptor type 2B subunit is essential for the action of ifenprodil, the positive relationship between the actions of ifenprodil and ethanol on responses to NMDA is consistent with the hypothesis that the combination of specific receptor subunits forming an NMDA receptor on a neuron determines the ability of ethanol to antagonize an NMDA response.

Suppression of alcohol intake after administration of the Chinese herbal medicine, NPI-028, and its derivatives

The Chinese herbal medicine, NPI-028, has been used for centuries in China to counteract alcohol intoxication. The present study used a number of different experimental conditions to determine whether NPI-028 and its derivatives might selectively influence alcohol intake in rodents that naturally exhibit high alcohol intakes. It was determined that intraperitoneal (i.p.) injections of NPI-028 (0.5, 0.75, and 1.0 g/kg) suppressed alcohol intake by up to 30% in both alcohol-preferring P and Fawn-Hooded (FH) rats during a continuous access schedule. These injections did not significantly affect food or water intakes, nor did the highest dose of NPI-028 (1 g/kg) alter blood ethanol levels after an i.p. injection of 2.5 g/kg of ethanol. In P rats, it was found that NPI-028 was orally active with the dose of 1.5 g/kg having a greater effect on ethanol intake than the 1.0 g/kg dose; once again, food and water intakes were not significantly altered. In FH rats maintained on a limited access schedule (1 hr/day), alcohol intake was completely abolished by 1.5 g/kg of NPI-028. Chronic i.p. administration of NPI-028 (0.75 g/kg) for four consecutive days in FH rats maintained on a continuous access schedule did not lead to any diminution of its alcohol-suppressant effects. Thus, NPI-028 has significant effects on alcohol intake without much effect on water and food intake, and tolerance does not readily develop to these effects. The i.p. administration of a partially purified extract (NPI-031) of NPI-028, obtained by countercurrent chromatography, also dose-dependently suppressed ethanol intake in FH rats, but the highest dose 200 mg/kg) also significantly decreased food intake. Finally, the i.p. administration of puerarin (NPI-31G), an isoflavone isolated from NPI-031 by countercurrent chromatography, significantly reduced ethanol intake in FH rats without affecting food or water intake. Therefore, NPI-028 and one of its pure components, NPI-031G, selectively reduced ethanol intake in alcohol-preferring rats.

Nicotine-induced inhibition in medial septum involves activation of presynaptic nicotinic cholinergic receptors on gamma-aminobutyric acid-containing neurons

Neuronal responses to drugs acting on nicotinic cholinergic receptors (nAChRs) were examined in the rat medial septal area by using an in vivo extracellular single-unit recording technique. In the medial septal area, iontophorectically applied nicotine inhibited neuronal activity in 45% of the neurons, but had no effect on the remaining neurons. Dihydro-beta-erythroidine application to neurons in the medial septal area not only blocked nicotine-induced inhibition, but also reduced spontaneous neuronal activity of the neurons. When Mg++ was applied iontophoretically to block presynaptic neurotransmitter release, a significant reduction in spontaneous neural activity also was observed. No further reduction of spontaneous activity by dihydro-beta-erythroidine occurred in the presence of Mg++, suggesting an apparent tonic excitatory input to the majority of neurons in the medial septal area under the control of presynaptic nAChRs. Mg++ abolished the nicotine-induced inhibition in the medial septal area without having an effect on nicotine-induced inhibition in the cerebellum. Thus, these data provide evidence that the inhibitory effects of nicotine in the medial septum are due to an action on presynaptic nAChRs, controlling the release of an inhibitory neurotransmitter. Of the medial septal neurons which showed no response to nicotine, nicotine produced excitation in 21% of the cells after Mg++ application, indicating that nicotine can have a direct action on postsynaptic nAChRs, in addition to its presynaptic action, in the medial septum. Finally, application of the gamma-aminobutyric acid antagonist bicuculline reduced the nicotine-induced inhibition on the majority of medial septal neurons tested, but was without effect on the inhibition produced by nicotine on cerebellar Purkinje neurons. Consequently, it can be concluded that the nicotine-induced inhibition in the medial septum is the result of gamma-aminobutyric acid release due to its action on presynaptic nAChRs present on gamma-aminobutyric acid-containing terminals.

Regional differences in the effects of chronic ethanol administration on [3H]zolpidem binding in rat brain

A strong association has been observed between [3H]zolpidem binding and the presence of gamma-aminobutyric acid (GABAA) receptor mRNA for alpha 1-, beta 2-, and gamma 2-subunits in specific brain regions. This correlates with observed sensitivity of individual neurons to zolpidem and ethanol in these same regions. Previous studies using homogenate binding approaches showed small alterations in [3H]zolpidem binding levels after chronic ethanol exposure. This study was undertaken to ascertain if there is regional specificity of the effects of chronic ethanol administration on [3H]zolpidem binding levels. Chronic ethanol administration induced small, but significant alterations in [3H]zolpidem (5 nM) binding in the inferior colliculus, substantia nigra, and the medial septum. [3H]Zolpidem binding was increased in the inferior colliculus and substantia nigra, and decreased in the medial septum. No significant differences in [3H]zolpidem binding were noted in any other brain area analyzed, including the cortex and cerebellum. These findings show that chronic ethanol administration has small effects on [3H]zolpidem binding, although they occur in a site-specific and bidirectional manner. Moreover, there is no correlation between changes in [3H]zolpidem binding and alterations in GABAA receptor subunit expression.

Effect of zolpidem on gamma-aminobutyric acid (GABA)-induced inhibition predicts the interaction of ethanol with GABA on individual neurons in several rat brain regions

Previous investigations have suggested a relationship between zolpidem binding within specific brain regions and the ability of ethanol or zolpidem to enhance gamma-aminobutyric acid (GABA)-induced inhibition. The purpose of the present study was to extend our electrophysiological analysis to additional brain sites with high levels of zolpidem binding. In the brain regions chosen, red nucleus and globus pallidus, GABA-induced inhibition was shown to be enhanced by either ethanol or zolpidem on some, but not all, neurons. These findings led to the hypothesis that the effect of zolpidem on GABA-induced inhibition would predict the action of ethanol on responses to GABA for that neuron. When zolpidem and ethanol were applied individually to the same neurons in the red nucleus and globus pallidus, those neurons sensitive to zolpidem enhancement of GABA also were sensitive to ethanol. Conversely, if zolpidem did not enhance responses to GABA, ethanol did not enhance responses to GABA at these brain sites. A similar relationship between the abilities of zolpidem and ethanol to enhance GABA-induced inhibition was obtained in 90% of the neurons studied in the medial septum/diagonal band and ventral pallidum. These studies provide further support for the contention that the zolpidem-sensitive GABAA-benzodiazepine isoreceptor also responds to ethanol. Finally, the expression of GABAA subunit mRNAs was analyzed by polymerase chain reaction from micropunches of several brain regions that contain zolpidem binding sites and exhibit sensitivity to ethanol. Polymerase chain reaction analysis proved more sensitive than in situ hybridization in the detection of receptor subunit mRNAs. Several subunits (alpha 1, alpha 2, alpha 3, beta 2, beta 3 and gamma 2) were common to all brain regions in which ethanol and zolpidem enhanced GABA responses. GABAA receptor alpha 4/5, alpha 6, beta 1, gamma 1, gamma 3 and delta subunits were not consistently expressed in association with the presence of zolpidem binding. These data are consistent with the view that one native GABAA receptor to which zolpidem binds, and on which ethanol acts, contains the GABAA receptor subunits alpha 1, beta 2 and gamma 2; however, the present investigation did not preclude the possibility that other subunit combinations can contribute to ethanol and zolpidem enhancement of responses to GABA.

Distribution of [3H]zolpidem binding sites in relation to messenger RNA encoding the alpha 1, beta 2 and gamma 2 subunits of GABAA receptors in rat brain

Localization of the messenger RNAs that encode the alpha 1, beta 2 and gamma 2 subunits of GABAA showed a distinct topographic pattern in rat brain which corresponded with [3H]zolpidem binding in most brain regions. The close topographic correspondence between the specific receptor subunits examined and the distribution of [3H]zolpidem binding sites provides support for the hypothesis that this benzodiazepine type 1 selective ligand binds to a GABAA receptor that consists of alpha 1, beta 2 and gamma 2 subunits in the rat brain. Brain regions with relatively high densities of alpha 1, beta 2 and gamma 2 subunits of GABAA and [3H]zolpidem binding included olfactory bulb, medial septum, ventral pallidum, diagonal band, inferior colliculus, substantia nigra pars reticulata and specific layers of the cortex. Two areas with low [3H]zolpidem binding and a virtual absence of these GABAA receptor subunit messenger RNAs were the lateral septum and the striatum. In contrast to the discrete pattern observed for alpha 1 and beta 2 subunit messenger RNAs, the gamma 2 subunit messenger RNA was distributed more diffusely in brain. Only the hippocampus, layer 2 of the piriform cortex and the cerebellum showed a strong concentration of the gamma 2 subunit messenger RNA. It was determined with a polymerase chain reaction assay that both long and short variants of the gamma 2 subunit messenger RNAs were present within several of the brain sites selected for examination. Sites with high densities of [3H]zolpidem binding sites had a greater relative abundance of the gamma 2 long splice variant, compared to the gamma 2 short variant. There were some regions that expressed high levels of alpha 1, beta 2 and gamma 2S subunit messenger RNAs but low [3H]zolpidem binding, suggesting that gamma 2 splice variant expression may modulate high-affinity [3H]zolpidem binding. To determine relationships between in vitro [3H]zolpidem binding and functional sensitivity in vivo, interactions between zolpidem and GABA were assessed in brain regions that contained high and low densities of [3H]zolpidem binding sites. In the medial septum, a brain region with a high concentration of [3H]zolpidem binding sites, iontophoretic application of zolpidem enhanced the inhibitory effect of GABA responses on 70% of the neurons examined. In the lateral septum, which contains very low densities of [3H]zolpidem binding sites, neurons were not sensitive to zolpidem enhancement of GABA-induced inhibition. These electrophysiological results demonstrate a correspondence between the regional distribution of [3H]zolpidem binding in vitro and functional sensitivity to the drug in vivo.

Effects of ethanol, MK-801, and chlordiazepoxide on locomotor activity in different rat lines: dissociation of locomotor stimulation from ethanol preference

Several lines of research have suggested a link between the reward value of a drug and its ability to stimulate locomotion. One goal of the present study was to determine whether ethanol preferentially stimulates locomotor activity in lines of rat that show a preference for ethanol. A secondary goal was to determine the extent to which the benzodiazepine-like and NMDA antagonistic action of ethanol accounted for its effect on locomotor activity. To meet these goals, the effects of varying doses of ethanol (0.125-1.0 g/kg), MK-801 (0.1-0.3 mg/kg), and chlordiazepoxide (0.3-3 mg/kg) on locomotor activity were studied in several lines of rats that had been habituated to the testing procedure. The effect of low doses of ethanol on motor activity in the Alcohol-Preferring (P) and Fawn-Hooded rats, which show a strong ethanol preference, were similar to those of the alcohol-nonpreferring (NP), Flinders Sensitive Line, and Flinders Resistant Line rats. Only the Flinder Resistant Line rats showed a small, but significant increase in locomotor activity after the administration of ethanol. The highest dose of ethanol (1.0 g/kg) produced locomotor depression in all lines except the P and NP lines, which were not tested at this dose. These findings do not support a link between locomotor stimulation by ethanol and ethanol preference. In contrast, all lines exhibited locomotor stimulation after moderate (0.1-0.3 mg/kg) doses of MK-801, but did not exhibit increases in activity following any dose of chlordiazepoxide. These data indicate that the profiles of activity after MK-801 and chlordiazepoxide were distinct from that of ethanol in the various rat lines.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ethanol, chlordiazepoxide, and MK-801 on performance in the elevated-plus maze and on locomotor activity

The effects of ethanol, chlordiazepoxide, and MK-801 on performance in the elevated-plus maze and on activity measured in a circular activity monitor were compared in Sprague-Dawley rats to determine whether these effects of ethanol could be explained by its action on either GABAA or NMDA receptors. Both ethanol and chlordiazepoxide produced an increase in the time spent in the open arms of the elevated-plus maze and in the ratio of open arm to total arm entries, indicative of an anxiolytic action of these drugs. MK-801 did not alter either the time spent in the open arms or the ratio of open to total arm entries. Chlordiazepoxide and MK-801 produced an increase in total arm entries that suggested that these compounds were increasing locomotor activity. Ethanol also increased total arm entries, but the effect was not statistically reliable. Following habituation to an activity monitor, neither ethanol nor chlordiazepoxide increased activity in this task, whereas MK-801 produced a robust increase in locomotion. Additionally, neither ethanol nor chlordiazepoxide blocked the MK-801-induced locomotor stimulation. The latter finding suggests that the effects of ethanol on GABAA receptors was not blocking an increased activity level produced by its antagonism of NMDA. Additionally, these results indicate that the anxiolytic and locomotor action of ethanol in rats parallel the effects of a benzodiazepine and not those of an NMDA antagonist. Finally, these results suggest that the consequence of ethanol's antagonism of NMDA receptor function is more restricted than that produced by MK-801.

D1 dopamine receptor binding and mRNA levels are not altered after neonatal 6-hydroxydopamine treatment: evidence against dopamine-mediated induction of D1 dopamine receptors during postnatal development

The role of dopaminergic innervation on the postnatal developmental expression of D1 dopamine receptors was investigated. Bilateral destruction of dopamine-containing neurons was achieved by treating rats intracisternally with 6-hydroxydopamine (6-OHDA) on postnatal day 3, and rats were killed on day 21. To ensure effective reduction of D1 receptor activation by residual dopamine, a group of 6-OHDA-lesioned rats was given twice daily injections of the D1 receptor antagonist SCH-23390, from day 4 to 20. D1 dopamine receptor binding was assessed in the caudate-putamen, nucleus accumbens, and olfactory tubercle by quantitative autoradiographic analysis of [3H]SCH-23390 binding. In addition, the relative amount of D1A receptor mRNA was assessed by in situ hybridization of a 35S-labeled riboprobe. In the developing rats, neither the amount of [3H]SCH-23390 binding nor the amount of D1A receptor mRNA was altered by 6-OHDA lesioning followed by chronic treatment with SCH-23390. Thus, bilateral destruction of dopamine-containing neurons and treatment with SCH-23390 in neonatal rats did not interfere with the developmental expression of D1 receptors or alter the levels of mRNA that code for this receptor protein. Treatment of intact rats with SCH-23390 from postnatal day 4 to 20 also did not alter [3H]SCH-23390 binding or levels of D1 receptor mRNA. However, adult rats treated chronically with SCH-23390 exhibited increased [3H]SCH-23390 binding but did not show a significant change in D1 receptor mRNA levels.

Molecular basis for regionally specific action of ethanol on gamma-aminobutyric acidA receptors: generalization to other ligand-gated ion channels

The present investigation provides evidence that there is neuroanatomical specificity for ethanol enhancement of gamma-aminobutyric acid (GABA)-induced inhibition in mammalian brain and that the expression of a specific GABAA isoreceptor is associated with this regional action of ethanol. Ethanol enhanced responses to iontophoretically applied GABA in the medial septum, inferior colliculus, substantia nigra reticulata, ventral pallidum and the diagonal band of Broca. In contrast to these results, responses to GABA applied to cells in the lateral septum, ventral tegmental area and the hippocampus were not affected by ethanol. In those brain regions where ethanol enhanced responses to GABA, a high concentration of zolpidem binding was found, whereas zolpidem binding was much lower or absent in brain regions where ethanol did not enhance GABA. These observations support the hypothesis that ethanol enhances GABA within specific regions of brain by affecting a GABAA receptor with specific structural components. From data obtained with in situ hybridization, there was a strong relationship between the regional distribution of zolpidem binding and the expression of specific mRNAs for the alpha-1, beta-2 and gamma-2 GABAA receptor subunits at sites where ethanol enhanced responses to GABA. The mRNA for the long and short variants of the gamma-2 subunit were found in brain regions both sensitive and insensitive to the action of ethanol on GABA-induced inhibition. These data were not able to address whether the gamma-2 long variant in combination with the alpha-1 and beta-2 subunits is essential for ethanol enhancement of responses to GABA. However, the observation that the long version of the gamma-2 subunit is present in brain areas where ethanol did not affect GABA function suggests that the presence of the long variant of the gamma-2 subunit alone is not sufficient for ethanol's action to enhance responses to GABA. Rather it is concluded that the appropriate combination of GABAA receptor subunits is critical for this action of ethanol. Because the GABAA receptor belongs to a superfamily of ligand-gated ion channels, the action of ethanol was examined on responses to agonists acting on receptors linked to other ion channels. As noted for GABA, local application of ethanol altered responses to NMDA, nicotine and glycine when applied to some, but not all, neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Similar effects of ethanol and flumazenil on acquisition of a shuttle-box avoidance response during withdrawal from chronic ethanol treatment

1. Acquisition of a two-way shuttle-box avoidance response is facilitated by ethanol. This facilitated acquisition of an avoidance response to ethanol was attenuated during withdrawal from chronic-ethanol diet intake (i.e. tolerance developed by ethanol). The deficit in the avoidance task after chronic ethanol treatment could be overcome by increasing the dose of ethanol. 2. Flumazenil, a benzodiazepine antagonist, also facilitated acquisition of the avoidance response in control rats. This response to flumazenil was significantly reduced during withdrawal from chronic-ethanol treatment. This reduced avoidance responding during withdrawal also could be overcome by increasing the dose of flumazenil. 3. The benzodiazepine-inverse agonist, RO 15-4513, produced a deficit in avoidance responding that was antagonized by both ethanol and flumazenil in a dose-related manner. 4. To determine whether flumazenil has the properties of a benzodiazepine agonist, it was established that, unlike the benzodiazepine chlordiazepoxide, flumazenil did not enhance the ethanol-induced deficit in the aerial righting reflex. Additionally, flumazenil blocked the action of chlordiazepoxide in this procedure, consistent with the benzodiazepine antagonist action of flumazenil. 5. Data collected are consistent with the hypothesis that an endogenous substance with the properties of a benzodiazepine-inverse agonist antagonizes the anticonflict actions of acutely administered ethanol during withdrawal from chronic-ethanol exposure.

Evidence for involvement of brain dopamine and other mechanisms in the behavioral action of the N-methyl-D-aspartic acid antagonist MK-801 in control and 6-hydroxydopamine-lesioned rats

Behavioral activation following systemic administration of the N-methyl-D-aspartic acid receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,d) cyclohepten-5,10-imine; dizocilpine] was examined in unlesioned control and in neonatal-6-hydroxydopamine (OHDA) lesioned rats. Neonatal-6-OHDA lesioned animals were found more sensitive than control rats and female rats more sensitive than males to this MK-801-induced behavioral activation. CGS-19755, a competitive NMDA antagonist, also increased activity in neonatally lesioned animals. The increased activity level following MK-801 administration to neonatally lesioned rats was reduced, but not eliminated, by pretreatment with alpha-methyltyrosine, indicating that endogenous catecholamines were partially responsible for this action of MK-801. Furthermore, neither a D1- nor a D2-dopamine antagonist was totally effective alone in reducing MK-801-induced behavioral activation in the neonatally lesioned rats, but MK-801-induced activity was reduced to the level observed after alpha-methyltyrosine when both dopamine antagonists were administered in combination. In contrast to these results, alpha-methyltyrosine virtually eliminated the MK-801-induced activity in adult-lesioned rats. When individual behaviors induced by MK-801 were examined in neonatal-6-OHDA lesioned rats, MK-801 did not produce the same behaviors as L-dihydroxyphenylalanine or a D1- or D2-dopamine agonist. Whereas MK-801 had no major effect on most behaviors induced by specific D1- or D2-dopamine agonists, it blocked some behaviors produced after L-dihydroxyphenylalanine administration, including the self-injurious behavior. Repeated MK-801 treatment resulted in increasingly greater motor activity, but this was not related to increased D1-dopamine receptor sensitization. In support of a regional action of MK-801, MK-801 induced c-fos-like immunoreactivity in the cerebral cortex, but not in the nucleus accumbens or striatum. The action of MK-801 to increase c-fos-like immunoreactivity in cerebral cortex was reduced, but not blocked, by SCH-23390. Additionally, MK-801 reduced, but did not eliminate, D1-dopamine agonist induced c-fos-like immunoreactivity in striatum. These data suggest that MK-801 not only can facilitate dopamine release within specific brain regions, but has behavioral and functional actions distinct from dopamine agonists.

Inhibition of NMDA-evoked electrophysiological activity by ethanol in selected brain regions: evidence for ethanol-sensitive and ethanol-insensitive NMDA-evoked responses

Our laboratory has previously shown that systemically administered ethanol inhibits NMDA-evoked electrophysiological activity in some, but not all, neurons in the medial septum. In the present report, it was found that ethanol, when applied locally via electro-osmosis, potently inhibited NMDA-evoked neuronal activity in a current-dependent manner in the inferior colliculus and hippocampus. In contrast, locally applied ethanol failed to inhibit NMDA-evoked activity in the lateral septum. The inhibition by ethanol of NMDA-evoked activity in the inferior colliculus was specific, in that ethanol failed to inhibit neuronal activity of the inferior colliculus evoked by the excitatory neurotransmitter glutamate. These findings indicate that ethanol can specifically inhibit NMDA-evoked activity in vivo via a local action, and that the ability of ethanol to inhibit NMDA-evoked activity varies regionally in brain. The possibility that these results are explained by the existence of two types of NMDA receptors, one sensitive to ethanol, the other insensitive to ethanol, is discussed.

The neuroanatomical specificity of ethanol action on ligand-gated ion channels: a hypothesis

The studies described will demonstrate that the subunit composition of a GABAA receptor allows ethanol to enhance responses to GABA. Since we have determined that ethanol will influence responses to glycine, nicotine and NMDA in some, but not all, neurons with receptors to these agonists, we hypothesize that specific receptor subtypes of these ligand-gated ion channels will be affected by ethanol.

Augmented sensitivity of D1-dopamine receptors in lateral but not medial striatum after 6-hydroxydopamine-induced lesions in the neonatal rat

Lesioning of neonatal rats with the neurotoxin 6-hydroxydopamine (6-OHDA) reduced striatal dopamine (DA) levels to 3% of control levels and produced marked increases in the behavioral effects of the selective D1-DA receptor agonist SKF-38393 in these animals when tested as adults. However, no differences were observed, either in basal or D1-DA-stimulated striatal cAMP formation or in forskolin-stimulated or GTP-stimulated cAMP production, between control and lesioned animals. C-fos-like immunoreactivity after SKF-38393 was significantly greater in dorsolateral vs. ventromedial aspects of the striatum in lesioned animals. Like the c-fos response, augmented electrophysiological responsiveness to SKF-38393 occurred in lesioned rats in lateral, but not medial, portions of the striatum. No differences were found in nucleus accumbens in sensitivity to SKF-38393 between control and lesioned rats. Although autoradiographic determination of D1-DA receptor binding throughout the striatum and nucleus accumbens revealed no differences between unlesioned and lesioned rats, tyrosine hydroxylase-like immunoreactivity was reduced with a regional distribution inversely related to c-fos-like immunohistochemical expression. These findings demonstrate that regionally enhanced electrophysiological sensitivity of striatal neurons to D1-DA receptor agonists after neonatal 6-OHDA-induced lesions is associated with regional changes in c-fos-like immunoreactivity and tyrosine hydroxylase-like immunohistochemistry, but not with changes in D1-DA receptor autoradiography or D1-DA-stimulated adenylyl cyclase activity. Such regional consequences of 6-OHDA-induced lesions in neonates may contribute to the unique behavioral patterns observed when these rats are challenged with L-dopa or D1-DA agonists as adults.

Pharmacologic evaluation of SCH-39166, A-69024, NO-0756, and SCH-23390 in neonatal-6-OHDA-lesioned rats. Further evidence that self-mutilatory behavior induced by L-dopa is related to D1 dopamine receptors

The purpose of the present investigation was to explore further the hypothesis that the self-injurious behavior induced by L-dihydroxyphenylalanine (L-DOPA) in neonatal-6-hydroxydopamine (OHDA)-lesioned rats is associated with an action on D1 dopamine receptors. This was accomplished by examining the behavioral responses induced by SKF-38393, quinpirole, and L-DOPA after treatment with the D1 antagonist SCH-23390 and three new pharmacologic agents, SCH-39166, NO-0756, and A-69024, reported to be D1 antagonists. All putative D1 antagonists were found to antagonize the action of SKF-38393 without reducing the increased locomotion and behavioral responses induced by quinpirole, consistent with an in vivo action on D1 receptors. The potency hierarchy of the compounds against the action of SKF-38393 on activity, from strongest to weakest, was: SCH-39166 equaled SCH-23390 and these were greater than NO-0756, which was greater than A-69024. All compounds were found to antagonize L-DOPA-induced self-mutilatory behavior (SMB) in neonatal-6-OHDA-lesioned rats in a dose-related manner. The potency hierarchy against this behavior, from strongest to weakest, was: SCH-23390, SCH-39166, NO-0756, and A-69024. The correlation between the ED50 for the ability of these drugs to antagonize SKF-38393-induced activity and their ability to reduce SMB by L-DOPA was greater than 0.99. In conclusion, the present findings provide additional evidence in vivo that NO-0756, SCH-39166, and A-69024 are selective D1 receptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the D1-dopamine agonists SKF-38393 and A-68930 in neonatal 6-hydroxydopamine-lesioned rats: behavioral effects and induction of c-fos-like immunoreactivity

Administration of the selective D1-dopamine receptor agonist 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF-38393) to neonatal 6-hydroxydopamine-lesioned rats results in profound behavioral manifestations and induction of striatal c-fos-like immunoreactivity. The full D1-dopamine agonist I,[R,S]1-aminomethyl-3,4-dihydro-5,6-dihydroxy-3-phenyl-1H-2-benzopyran hydrochloride (A-68930), like SKF-38393, produced a dose-dependent, D1-selective increase in locomotor activity and striatal c-fos-like immunoreactivity. These responses were antagonized by a D1-dopamine antagonist, but not by a D2-dopamine antagonist. A-68930 produced locomotor activation at a lower dose than SKF-38393, but no dose of A-68930 was able to produce the magnitude of locomotor activation seen with SKF-38393. Both A-68930 and SKF-38393 induced similar stereotyped behaviors and possessed similar propensities to induce self-injurious behavior in neonatally lesioned rats; however, A-68930 was significantly more potent than SKF-38393 in inducing these behaviors. When either SKF-38393 or A-68930 were administered repeatedly at 2-week intervals, behavioral sensitization (priming) occurred. However, unlike SKF-38393, a high dose of A-68930 produced seizure activity and markedly desensitized D1-dopamine receptor activation for up to 3 days after administration. These results with A-68930 provide additional evidence that the specific behavioral and biochemical responses observed in neonatally lesioned rats after SKF-38393 administration are due to actions on D1-dopamine receptors, and indicate that A-68930 provides a new tool for investigating D1-dopamine receptor function.

Ethanol potentiates gamma-aminobutyric acid-mediated inhibition in the inferior colliculus: evidence for local ethanol/gamma-aminobutyric acid interactions

The effect of ethanol on gamma-aminobutyric acid (GABA)-mediated inhibition of neurons of the inferior collicular cortex (IC) was investigated. Systemic administration of 0.5 g/kg ethanol, but not 0.25 g/kg ethanol, potentiated the inhibitory effect of GABA on IC neuronal activity. As with systemic administration of ethanol, local application of two concentrations of ethanol potentiated, in a dose-dependent and concentration-dependent manner, GABA-mediated inhibition of IC activity. When utilizing the lower ethanol concentration (270 mM), increasing ethanol ejection currents produced a correspondingly greater amount of augmentation of GABA inhibition without concomitant changes in spontaneous neural activity. At the highest ejection current tested (90 nA), ethanol doubled the inhibitory effects of GABA on IC neuronal activity. Local application of a higher concentration of ethanol (2.7 M) also potentiated GABA-mediated inhibition, with greater ejection currents producing greater potentiation of GABA-inhibition. Compared to the lower ethanol concentration, the higher ethanol concentration required lower ejection currents to produce comparable amounts of potentiation of GABA inhibition. These findings demonstrate that ethanol potentiates GABA inhibition of IC activity via a local action. In contrast to the effects of ethanol on GABA inhibition, locally applied ethanol failed to potentiate the inhibition by glycine of IC activity, indicating that ethanol does not indiscriminately potentiate all types of inhibition of IC neural activity. Additionally, locally applied ethanol failed to potentiate inhibition by GABA in the lateral septum, indicating that ethanol/GABA interactions are site specific. These findings indicate that ethanol specifically potentiates GABA-mediated inhibition of neural activity, and that this action occurs via a local action at specific sites in brain.

Ethanol inhibits NMDA-evoked electrophysiological activity in vivo

Recent studies have demonstrated that ethanol blocks N-methyl-D-aspartate (NMDA) responses in vitro. In the present study, evidence is provided that ethanol, when administered by the systemic route to rats, also inhibits NMDA-evoked electrophysiological activity in vivo at behaviorally relevant doses. Ethanol, at doses in rats ranging from those producing minimal changes in spontaneous behavioral activity (0.75 g/kg) to those producing marked suppression of behavioral activity (2.5 g/kg), produced a dose-dependent inhibition of the ability of NMDA, when iontophoresed onto neurons of the medial septum (MS), to activate MS neurons. However, at all doses of ethanol tested, a proportion of MS neurons responded to ethanol with essentially complete inhibition of NMDA-evoked activity, whereas other MS neurons responded to ethanol with little or no inhibition of NMDA-evoked activity. By way of comparison, MK-801, a non-competitive NMDA antagonist, antagonized NMDA-evoked activity in all MS neurons tested. In contrast to the actions of ethanol, MK-801 increased, rather than decreased, behavioral activity even at doses that completely inhibited NMDA-evoked activity in all MS neurons tested. These latter findings provide evidence that inhibition of NMDA-evoked activity cannot account for all of the behavioral effects of ethanol. In conclusion, while the present results demonstrate for the first time that ethanol can inhibit NMDA-evoked neuronal activity in vivo, they also indicate that additional neural actions must contribute to ethanol's pharmacological profile.

A dopamine deficiency model of Lesch-Nyhan disease--the neonatal-6-OHDA-lesioned rat

Lesch-Nyhan syndrome is characterized by a deficiency of the enzyme hypoxanthine phosphoribosyl transferase (HPRT), compulsive self-mutilatory behavior (SMB), and a loss of central dopaminergic neurons. In order to model the loss of central dopamine-containing neurons in this developmental disorder, neonatal rat pups 3 days of age were given the neurotoxin 6-OHDA intracisternally to reduce brain dopamine. Accompanying the profound loss of dopamine produced by this treatment was an increase in striatal serotonin content. When these neonatally lesioned rats were challenged as adults with systemically administered L-DOPA or with muscimol administration into substantia nigra reticulata (SNR), SMB was observed, a response not observed in unlesioned rats. Thus, the neonatally lesioned rats exhibit increased susceptibility for SMB. Since a D1-dopamine antagonist blocked the SMB response to L-DOPA, it was proposed that D1-dopamine receptors were critical to this behavioral response. Basic investigations concerning D1-dopamine receptor mechanisms in the lesioned rats have been performed and these are reviewed. The data in the neonatally lesioned rats provide convincing evidence that the absence of central dopaminergic neurons is responsible for at least some of the neurological symptoms of the Lesch-Nyhan syndrome, a finding consistent with data collected in mice with an HPRT deficiency.

Long-term D1-dopamine receptor sensitization in neonatal 6-OHDA-lesioned rats is blocked by an NMDA antagonist

Repeated administration of the D1-dopamine agonist SKF-38393 to adult rats having had dopaminergic neurons destroyed early in development results in an increasing enhancement of the behavioral response to SKF-38393 with each dose until a maximum is reached. This increased sensitivity lasts for at least 6 months. In the present study, this long-lasting change in behavioral responsiveness to repeated treatment with SKF-38393, referred to as D1-dopamine receptor priming, was shown to be dose dependent with smaller doses requiring an increased number of administrations to produce a maximal response when compared to higher doses. In addition, priming occurred equally well when treatment intervals ranged from 1 day to 14 days. These latter data reinforced the view that activation of D1-dopamine receptors results in a prolonged change in neural function. In subsequent experiments, D1-dopamine receptor priming was blocked by pretreatment with the NMDA-receptor antagonist MK-801. This antagonism of priming could not be attributed to a blockade of D1-dopamine receptors by MK-801 or to the induction of interfering behaviors. Because an NMDA antagonist interfered with D1-receptor priming as it does with other long-term neural messages, a common requirement for these diverse forms of neuronal plasticity appears to involve activation of the NMDA receptor. This functional link between NMDA receptors and dopaminergic function and its relationship to neuronal plasticity could have relevance to the biochemical mechanisms involved in learning and to symptoms in central disorders during development that worsen over time, particularly those proposed to involve malfunctioning dopaminergic mechanisms.

Pharmacological evaluation of SCH-12679: evidence for an in vivo antagonism of D1-dopamine receptors

The benzazepine compound SCH-12679 has been shown to have clinical efficacy against aggressive behavior in mentally deficient patients. The purpose of the present investigation was to evaluate the potential mechanism of action of SCH-12679. Because of the structural similarity of SCH-12679 to compounds influencing D1-dopamine receptors, even though in vitro studies indicated no direct action on this receptor, investigations focused on the possibility that in vivo SCH-12679 antagonizes the function of this dopamine receptor subtype. After i.p. administration to neonatal-6-hydroxydopamine (6-OHDA)-lesioned rats, SCH-12679 reduced, dose-dependently, the locomotor activity induced by SKF-38393, a D1-dopamine agonist. A dose of SCH-12679 that antagonized the activity induced by SKF-38393 in neonatally lesioned rats also blocked various behaviors observed after administration of this D1-dopamine agonist. SCH-12679 did not alter the activity or behavioral responses induced by quinpirole, a D2-dopamine agonist, when administered to 6-OHDA-lesioned rats. SCH-12679 antagonized the self-mutilation behavior and behavioral responses induced by L-dihydroxyphenylalanine in neonatal-6-OHDA lesioned rats in a manner similar to the prototypic D1-dopamine antagonist SCH-23390 and, like SCH-23390, produced a deficit in avoidance responding in unlesioned rats. SCH-12679 produced a small, transient activation of locomotor activity immediately after administration to neonatal-6-OHDA-lesioned rats that was not observed in unlesioned or adult-6-OHDA-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that lack of brain dopamine during development can increase the susceptibility for aggression and self-injurious behavior by influencing D1-dopamine receptor function

1. Lesch-Nyhan disease has a defined neurological lesion that is accompanied by abnormal motor function, aggression and self-injurious behavior. 2. The dopamine deficiency in Lesch-Nyhan disease has been modelled by destroying dopamine-containing neurons in neonatal rats with 6-hydroxydopamine. 3. Because D1-dopamine antagonists will block self-injurious behavior induced by L-DOPA in neonatal-6-OHDA-lesioned rats, D1-dopamine antagonists are proposed as a potential therapy for aggression and self-injurious behavior in patients with these symptoms. 4. The determination that the drug SCH-12679, which exhibited effectiveness against aggressiveness in mentally retarded patients, is a D1-dopamine antagonist supports the view that new D1-dopamine antagonists being developed will be an effective therapy for some types of aberrant behavior in this population.

A conflict procedure not requiring deprivation: evidence that chronic ethanol treatment induces tolerance to the anticonflict action of ethanol and chlordiazepoxide

A conflict task that does not require food or water deprivation is described. Drugs with known anticonflict action including chlordiazepoxide, methysergide, ethanol, and TRH were active in this test while amphetamine, buspirone, and morphine were not. Because this procedure did not require food or water deprivation, the effect of chronic ethanol administered in a liquid diet on the anticonflict actions of ethanol and chlordiazepoxide could be examined. Seven or 12 days of chronic ethanol treatment resulted in tolerance to the acute anticonflict action of ethanol whereas 3 days of treatment did not. Tolerance was not observed to a dose of ethanol administered 3 hr after a previous ethanol treatment. Following a chronic ethanol liquid diet, cross-tolerance to the anticonflict action of chloridazepoxide was observed. This latter observation is consistent with the view that ethanol influences a neural mechanism associated with benzodiazepine function. The importance of tolerance to the anticonflict action of ethanol is discussed.

Serotonergic innervation of the rat caudate following a neonatal 6-hydroxydopamine lesion: an anatomical, biochemical and pharmacological study

6-Hydroxydopamine (6-OHDA) treatment of neonatal rats resulted in a dose-related loss of striatal dopamine (DA). These reductions corresponded closely with the loss of tyrosine hydroxylase-containing terminals at this brain site. Striatal serotonin (5-HT) concentration increased only after DA was maximally depleted by the highest dose of 6-OHDA. Quantitative immunohistochemistry revealed that the increased 5-HT content after neonatal 6-OHDA lesioning was due to a proliferation of 5-HT nerve terminals. The density of immunoreactive 5-HT-containing terminals appeared to increase more than did the 5-HT content. The present study examined whether 5-HT hyperinnervation was playing a role in behavioral responses induced by D1-DA agonists and antagonists in neonatally lesioned rats, because reports have suggested that these drugs may interact with 5-HT receptors. However, SCH-23390, the D1-DA antagonist (0.3 mg/kg), did not alter behavioral responses to 5-HTP and SKF-38393 (3 mg/kg), a D1-DA agonist did not produce any signs of activating 5-HT receptors in 5,7-DHT-lesioned rats. These data indicate that these compounds affecting D1-DA receptors do not have a significant effect on 5-HT function at doses which have maximal effects on D1-DA receptor function. Pretreatment with the 5-HT antagonist methysergide did not produce a change in apomorphine-induced locomotion and did not antagonize the self-mutilation or the other behaviors produced by L-DOPA or SKF-38393 in neonatally lesioned rats, suggesting that 5-HT hyperinnervation is not responsible for these drug-induced changes in neonatal 6-OHDA-lesioned rats.

Clozapine antagonism of D-1 and D-2 dopamine receptor-mediated behaviors

When tested in rats supersensitive to dopamine agonists, the atypical neuroleptic clozapine displayed pharmacological properties expected of both a D-1 and D-2 receptor antagonist. The locomotor response induced by the D-1 receptor agonist SKF-38393 in neonatal-6-hydroxydopamine (6-OHDA)-lesioned rats was reversed in a dose-related fashion, although a complete blockade of this behavior was not observed indicative for only a partial antagonism of D-1 receptor function. Clozapine also blocked the self mutilation resulting from L-dihydroxyphenylalanine (L-DOPA) administration to neonatal-6-OHDA-lesioned rats, an effect previously linked to D-1 receptor activation. At higher doses, clozapine blocked the locomotor activity elicited by the D-2 agonist LY-171555 in adult-6-OHDA-lesioned rats. Therefore, the action of clozapine on D-1 as well as D-2 receptor-mediated behaviors contributes to its pharmacological effects. The ability of clozapine to stop self-mutilatory behavior in neonatal-6-OHDA-lesioned rats suggests that this drug might be an effective treatment for self-injurious behavior associated with the Lesch-Nyhan syndrome and mental retardation.

Behavioral and neurochemical responses to haloperidol and SCH-23390 in rats treated neonatally or as adults with 6-hydroxydopamine

Behavioral and neurochemical effects of haloperidol (D2-dopamine antagonist) and SCH-23390 (D1-dopamine antagonist) were examined in unlesioned rats and in rats lesioned with 6-hydroxy-dopamine (6-OHDA) as adults or as neonates. In unlesioned rats, chronic haloperidol treatment (15 days) resulted in an increase in D2-dopamine receptor density, as measured with [3H]spiperone, in the nucleus accumbens and in the caudate-putamen. Rats treated as adults with 6-OHDA responded to chronic haloperidol similarly to controls. However, adult rats treated with 6-OHDA as neonates did not exhibit an increase in [3H]spiperone binding in response to chronic haloperidol treatment. Control and adult 6-OHDA-treated rats given haloperidol exhibited a profound akinesia. In contrast, rats that received 6-OHDA as neonates and were tested as adults did not display a significant behavioral response to haloperidol at doses as high as 2 mg/kg. Results similar to those for haloperidol were also found for SCH-23390. Chronic treatment (15 days) with this D1-dopamine antagonist increased [3H]SCH-23390 binding in the nucleus accumbens and caudate-putamen in unlesioned rats as well as in adult 6-OHDA-treated rats. However, after neonatal 6-OHDA treatment, an elevation in [3H]SCH-23390 binding did not occur after chronic SCH-23390 treatment. SCH-23390 produced akinesia similar to that produced by haloperidol in unlesioned and in adult 6-OHDA-treated rats. In contrast, rats lesioned with 6-OHDA as neonates and tested as adults did not exhibit a significant behavioral response to SCH-23390 under our test conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

A simple methodology for opiate self-administration and electrical brain stimulation in the mouse

A laboratory mouse can be partially restrained by placing it inside a cage with its tail protruding through a hole in one wall and taped to a surface outside of the cage. The mouse maintains enough mobility to perform an operant response yet its movements are limited to the extent that electrical or chemical stimulation of the brain or electrical recording from the brain can be accomplished without the use of troublesome swivel connectors. In addition, the lateral tail veins are always available for intravenous infusions. Mice adapt readily to this mild restraint and can be taught to self-administer opiates, either intravenously or intracerebrally, as well as to work for electrical brain stimulation.

A flexible low cost signal averager

A 2 channel signal averager based on the KIM-1 microcomputer module is described which can be easily assembled for under $500. Number of samples per pass can be set to 128 or 256 and sample time is variable from 1 msec to 256 msec. Averaged data can be displayed on an oscilloscope, output slowly for use of a chart recorder, or stored on a low cost audio cassette recorder for later analysis. Simply adding additional memory (for less than $100) allows it to be expanded to 8 channels.

A simple chronic microinjection system for use with chemitrodes

An inexpensive, electrically operated microinjection system is described which is small enough to be mounted on the animal side of a standard Mercury commutator. This eliminates the troublesome swivel joint needed for chronic microinjection in freely moving animals. This system allows simultaneous electrical stimulation or recording and chemical stimulation via a chemitrode using a standard 4-channel Mercury commutator.

Analgesia and hyperractivity following morphine microinjection into mouse brain

Analgesia and a paradoxical hyperreactivity to stimuli of sudden onset have recently been reported following the microinjection of morphine into the periacqueductal gray matter of rats. These effects have not been systematically investigated in other species. In the present study, both analgesia and hyperreactivity were observed as dose dependent effects of morphine microinjection into the periacqueductal gray matter of several strains of mice. Analgesia alone was produced by low doses of morphine while at higher doses analgesia was accompanied by hyperreactivity. Strain differences were noted with B6D2F1 mice being more susceptible to the hyperreactivity following morphine than BALB/c mice.

An injury-induced diffuse slow potential from brain

Three different slow potential (SP) changes resulting from focal brain injury are described. The first is an immediate, high amplitude (in excess of 25 mV)negative shift at the site of injury. The second is a biphasic negative-positive SP wave which spreads throughout the cortex ipsilateral to injury and is similar to spreading depression (SD). The third SP change, called here the injury-induced diffuse slow potential (IDSP)is a prolonged (lasting approx. 2h) negative shift occurring simultaneously in many brain areas, also in those far removed from the injured focus. The SD can be separated from IDSP by the size of focal injury; a 20 mu pucture of the parenchyma will trigger IDSP but not SD. An injury resulting from a larger puncture triggers both, SD and IDSP. IDSPcan not be induced by a re-entry of a previously damaged tissue. The magnitude of IDSP has anatomical specificity in that the largest amplitude occurrs in white as compared to gray of the cortex or of the caudate nucleus. Aso, the magnitude of the hypothalamic IDSP is larger when ipsilateral corpus callosum-commissural regions are injured. Electrical stimulation of the cortex in rats sufficiently strong to result in tonic-clonic convulsions triggers SD and IDSP; these two slow potential changes are similar to those induced by mechanical injury. A transpinnate electrical stimulus strong enough to elicit a grand-mal type of discharge results in a diffuse negative slow potential change similar to IDSP elicited by mechanical damage or direct cortical stimulation.

Spontaneous whole brain slow potential changes during recovery from experimental neurosurgery

Prolonged, nonlocalized brain slow potential changes, frequently associated with cortical spreading depression, occur spontaneously during 5 days following brain implant surgery in rats. These potentials are accompanied by reductions in multiple nerve cell activity and reductions in behavioral motility. The method used in this study provides a tool for evaluating recovery from neurosurgical trauma or other brain injuries, and for testing procedures that facilitate or impede this process.