Selective changes in GABAergic transmission in substantia nigra and superior colliculus caused by ethanol and ethanol withdrawal

Brain-wide functional architecture remodeling by alcohol dependence and abstinence

Alcohol abuse and alcohol dependence are key factors in the development of alcohol use disorder, which is a pervasive societal problem with substantial economic, medical, and psychiatric consequences. Although our understanding of the neurocircuitry that underlies alcohol use has improved, novel brain regions that are involved in alcohol use and novel biomarkers of alcohol use need to be identified. The present study used a single-cell whole-brain imaging approach to 1) assess whether abstinence from alcohol in an animal model of alcohol dependence alters the functional architecture of brain activity and modularity, 2) validate our current knowledge of the neurocircuitry of alcohol abstinence, and 3) discover brain regions that may be involved in alcohol use. Alcohol abstinence resulted in the whole-brain reorganization of functional architecture in mice and a pronounced decrease in modularity that was not observed in nondependent moderate drinkers. Structuring of the alcohol abstinence network revealed three major brain modules: 1) extended amygdala module, 2) midbrain striatal module, and 3) cortico-hippocampo-thalamic module, reminiscent of the three-stage theory. Many hub brain regions that control this network were identified, including several that have been previously overlooked in alcohol research. These results identify brain targets for future research and demonstrate that alcohol use and dependence remodel brain-wide functional architecture to decrease modularity. Further studies are needed to determine whether the changes in coactivation and modularity that are associated with alcohol abstinence are causal features of alcohol dependence or a consequence of excessive drinking and alcohol exposure.

Presynaptic Ethanol Actions: Potential Roles in Ethanol Seeking

Ethanol produces intoxication through actions on numerous molecular and cellular targets. Adaptations involving these and other targets contribute to chronic drug actions that underlie continued and problematic drinking. Among the mechanisms involved in these ethanol actions are alterations in presynaptic mechanisms of synaptic transmission, including presynaptic protein function and excitation-secretion coupling. At synapses in the central nervous system (CNS), excitation-secretion coupling involves ion channel activation followed by vesicle fusion and neurotransmitter release. These mechanisms are altered by presynaptic neurotransmitter receptors and prominently by G protein-coupled receptors (GPCRs). Studies over the last 20-25 years have revealed that acute ethanol exposure alters neurotransmitter secretion, with especially robust effects on synapses that use the neurotransmitter gamma-aminobutyric acid (GABA). Intracellular signaling pathways involving second messengers such as cyclic AMP and calcium are implicated in these acute ethanol actions. Ethanol-induced release of neuropeptides and small molecule neurotransmitters that act on presynaptic GPCRs also contribute to presynaptic potentiation at synapses in the amygdala and hippocampus and inhibition of GABA release in the striatum. Prolonged exposure to ethanol alters neurotransmitter release at many CNS GABAergic and glutamatergic synapses, and changes in GPCR function are implicated in many of these neuroadaptations. These presynaptic neuroadaptations appear to involve compensation for acute drug effects at some synapses, but "allostatic" effects that result in long-term resetting of synaptic efficacy occur at others. Current investigations are determining how presynaptic neuroadaptations contribute to behavioral changes at different stages of alcohol drinking, with increasing focus on circuit adaptations underlying these behaviors. This chapter will discuss the acute and chronic presynaptic effects of ethanol in the CNS, as well as some of the consequences of these effects in amygdala and corticostriatal circuits that are related to excessive seeking/drinking and ethanol abuse.

The Cerebellar GABAAR System as a Potential Target for Treating Alcohol Use Disorder

In the brain, fast inhibitory neurotransmission is mediated primarily by the ionotropic subtype of the gamma-aminobutyric acid (GABA) receptor subtype A (GABA_AR). It is well established that the brain's GABA_AR system mediates many aspects of neurobehavioral responses to alcohol (ethanol; EtOH). Accordingly, in both preclinical studies and some clinical scenarios, pharmacologically targeting the GABA_AR system can alter neurobehavioral responses to acute and chronic EtOH consumption. However, many of the well-established interactions of EtOH and the GABA_AR system have been identified at concentrations of EtOH ([EtOH]) that would only occur during abusive consumption of EtOH (≥40 mM), and there are still inadequate treatment options for prevention of or recovery from alcohol use disorder (AUD, including abuse and dependence). Accordingly, there is a general acknowledgement that more research is needed to identify and characterize: (1) neurobehavioral targets of lower [EtOH] and (2) associated brain structures that would involve such targets in a manner that may influence the development and maintenance of AUDs.Nearly 15 years ago it was discovered that the GABA_AR system of the cerebellum is highly sensitive to EtOH, responding to concentrations as low as 10 mM (as would occur in the blood of a typical adult human after consuming 1-2 standard units of EtOH). This high sensitivity to EtOH, which likely mediates the well-known motor impairing effects of EtOH, combined with recent advances in our understanding of the role of the cerebellum in non-motor, cognitive/emotive/reward processes has renewed interest in this system in the specific context of AUD. In this chapter we will describe recent advances in our understanding of cerebellar processing, actions of EtOH on the cerebellar GABA_AR system, and the potential relationship of such actions to the development of AUD. We will finish with speculation about how cerebellar specific GABA_AR ligands might be effective pharmacological agents for treating aspects of AUD.

Opposite actions of alcohol on tonic GABA(A) receptor currents mediated by nNOS and PKC activity

The molecular mechanisms that mediate genetic variability in response to alcohol are unclear. We found that alcohol had opposite actions (enhancement or suppression) on GABA(A) receptor (GABA(A)R) inhibition in granule cells from the cerebellum of behaviorally sensitive, low alcohol-consuming Sprague-Dawley rats and DBA/2 mice and behaviorally insensitive, high alcohol-consuming C57BL/6 mice, respectively. The effect of alcohol on granule cell GABA(A)R inhibition was determined by a balance between two opposing effects: enhanced presynaptic vesicular release of GABA via alcohol inhibition of nitric oxide synthase (NOS) and a direct suppression of the activity of postsynaptic GABA(A)Rs. The balance of these two processes was determined by differential expression of neuronal NOS (nNOS) and postsynaptic PKC activity, both of which varied across the rodent genotypes. These findings identify opposing molecular processes that differentially control the magnitude and polarity of GABA(A)R responses to alcohol across rodent genotypes.

Primate cerebellar granule cells exhibit a tonic GABAAR conductance that is not affected by alcohol: a possible cellular substrate of the low level of response phenotype

In many rodent brain regions, alcohol increases vesicular release of GABA, resulting in an increase in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) and the magnitude of tonic GABAA receptor (GABAAR) currents. A neglected issue in translating the rodent literature to humans is the possibility that phylogenetic differences alter the actions of alcohol. To address this issue we made voltage-clamp recordings from granule cells (GCs) in cerebellar slices from the non-human primate (NHP), Macaca fascicularis. We found that similar to Sprague Dawley rats (SDRs), NHP GCs exhibit a tonic conductance generated by α6δ subunit containing GABAARs, as evidenced by its blockade by the broad spectrum GABAAR antagonist, GABAzine (10 μM), inhibition by α6 selective antagonist, furosemide (100 μM), and enhancement by THDOC (10-20 nM) and THIP (500 nM). In contrast to SDR GCs, in most NHP GCs (~60%), application of EtOH (25-105 mM) did not increase sIPSC frequency or the tonic GABAAR current. In a minority of cells (~40%), EtOH did increase sIPSC frequency and the tonic current. The relative lack of response to EtOH was associated with reduced expression of neuronal nitric oxide synthase (nNOS), which we recently reported mediates EtOH-induced enhancement of vesicular GABA release in rats. The EtOH-induced increase in tonic GABAAR current was significantly smaller in NHPs than in SDRs, presumably due to less GABA release, because there were no obvious differences in the density of GABAARs or GABA transporters between SDR and NHP GCs. Thus, EtOH does not directly modulate α6δ subunit GABAARs in NHPs. Instead, EtOH enhanced GABAergic transmission is mediated by enhanced GABA release. Further, SDR GC responses to alcohol are only representative of a subpopulation of NHP GCs. This suggests that the impact of EtOH on NHP cerebellar physiology will be reduced compared to SDRs, and will likely have different computational and behavioral consequences.

Tolerance to ethanol sedation and withdrawal hyper-excitability is mediated via neuropeptide Y Y1 and Y5 receptors

AIMS

Neuropeptide Y (NPY) is widely distributed throughout the brain and has been implicated in some of the actions of ethanol. The aim of the present study was to characterize the subtypes of NPY receptors in ethanol induced sedation, tolerance and withdrawal hyper-excitability.

MAIN METHODS

The loss of righting reflex paradigm was used to record the sleep duration in mice.

KEY FINDINGS

The acute administration of ethanol (3-4g per kg, i.p., 20%v/v) resulted in marked sedation. While prolonged ethanol consumption led to the development of tolerance, the mice showed hyper-excitability following ethanol withdrawal. Prior acute intracerebroventricular (i.c.v.) injection of NPY (5-20 ng per mouse) or NPY Y1 and Y5 receptors agonist [Leu(31), Pro(34)]-NPY (0.02-0.2 ng per mouse) potentiated ethanol induced sedation. On the other hand, administration of selective NPY Y1 receptor antagonist BIBP3226 (5 ng per mouse, i.c.v.) inhibited ethanol induced sedation. Chronic concomitant treatment of NPY (20 ng per mouse, i.c.v.) or [Leu(31), Pro(34)]-NPY (0.2 ng per mouse, i.c.v.) to ethanol-fed groups prevented the development of tolerance and attenuated withdrawal hyper-excitability. Moreover, acute treatment of NPY (5 ng per mouse, i.c.v.) or [Leu(31), Pro(34)]-NPY (0.02 ng per mouse, i.c.v.) reversed the peak ethanol withdrawal hyper-excitability.

SIGNIFICANCE

The results underscore a role for NPY Y1 and Y5 receptors in the ethanol induced sedation, tolerance and withdrawal hyper-excitability. We suggest that modulation of NPY Y1 and Y5 receptors may be a strategy to address the ethanol withdrawal conditions.

Role of the superior colliculus and the intercollicular nucleus in the brainstem seizure circuitry of the genetically epilepsy-prone rat

PURPOSE

The neuronal network responsible for the convulsive behavior associated with sound-induced seizures in genetically epilepsy-prone rats (GEPRs) is believed to include the inferior colliculus and other brainstem structures such as the deep layers of the superior colliculus (DLSC), periaqueductal gray, and pontine reticular formation. However, previous studies also suggested that the DLSC and the nearby intercollicular nucleus (ICN) are part of a midbrain anticonvulsant zone capable of suppressing tonic convulsions when activated with bicuculline. Our aim in this study was to investigate the role of the superior colliculus (SC) and the ICN in generalized tonic-clonic seizures (GTCSs).

METHODS

Bilateral lesions of the SC and the ICN as well as bicuculline infusions into the ICN were used to assess the role of this dorsal midbrain region in brainstem seizures induced by sound stimulation in GEPR-9s and GEPR-3s.

RESULTS

Lesions of the SC markedly attenuated audiogenic seizure (AGS) severity by abolishing all behavioral components except the wild running. Lesions of the ICN significantly reduced seizure severity in GEPR-9s, but were somewhat less effective than SC lesions. Bicuculline infusion into the deep layers of the SC and/or the ICN produced audiogenic-like seizures in GEPR-9s.

CONCLUSIONS

These findings support the hypothesis that the SC and ICN are important components of the brainstem seizure network, but suggest they are not necessary for the wild-running component of the seizure. The results further indicate that stimulation of the tectum facilitates GTCSs. Thus these findings suggest that the dorsal midbrain, when stimulated, is proconvulsant rather than anticonvulsant regarding brainstem seizures in GEPRs.

Neurons in the deep layers of superior colliculus are a requisite component of the neuronal network for seizures during ethanol withdrawal

Ethanol withdrawal (ETX) in ethanol-dependent animals and humans often results in seizure susceptibility. The deep layers of superior colliculus (DLSC) are proposed to be involved in the neuronal networks of several types of seizures. In rodents, ETX results in susceptibility to audiogenic seizures (AGS), and the DLSC are implicated as a critical component of the seizure network in a genetic form of AGS. Ethanol inhibits NMDA receptors, and the binding at these receptors is increased during ETX in certain brain regions. Therefore, the effect of focal microinjection into DLSC of a competitive NMDA receptor antagonist, DL-2-amino-7-phosphonoheptanoic acid (AP7) on ETX seizures was examined. AP7 (2 and 5 nmol/side) microinjected bilaterally into DLSC suppressed AGS, supporting a critical role of the DLSC in the AGS network during ETX. DLSC neuronal firing changes in behaving rats were subsequently examined, using chronically implanted microwire electrodes. Acoustically-evoked DLSC firing was significantly suppressed during ethanol intoxication and during ETX. However, DLSC neurons began firing tonically 1-2 s before the onset of the wild running behavior of AGS. Acoustically-evoked DLSC firing was suppressed during post-ictal depression with recovery beginning as the righting reflex returned. These data support a requisite role of the DLSC in AGS during ETX. These neuronal firing changes suggest an important role of DLSC neurons in generation of the wild running phase of AGS during ETX, which may be a general pathophysiological mechanism and a critical event in the initiation of wild running, since a similar pattern was seen previously in a genetic form of AGS.

Modulation of the audiogenic seizure network by noradrenergic and glutamatergic receptors of the deep layers of superior colliculus

Recent studies suggest that the deep layers of superior colliculus (DLSC) play a role in the network for audiogenic seizures (AGS) in genetically epilepsy-prone rats (GEPR-9s). The present study examined the role of glutamatergic and noradrenergic receptors in DLSC in modulation of AGS susceptibility. The study examined effects of a competitive NMDA receptor antagonist [dl-2-amino-7-phosphonoheptanoic acid (AP7)] or an alpha1 noradrenergic agonist (phenylephrine) focally microinjected into DLSC as compared to effects in the inferior colliculus (IC) and pontine reticular formation (PRF), which are major established components of the AGS network. The results demonstrated that blockade of NMDA receptors in DLSC suppressed AGS susceptibility. AP7 microinjection was effective at relatively low doses in IC, but required higher doses in DLSC and PRF. The DLSC was relatively more sensitive to seizure reduction by the alpha1 noradrenergic agonist as compared to the IC and PRF. The anticonvulsant effect of AP7 was longer-lasting than phenylephrine in the DLSC and IC but not in the PRF. These data suggest that neurons in the DLSC are a requisite component for the neuronal network for AGS in GEPR-9s and that NMDA and alpha1 adrenoreceptors in this site may play important roles in the modulation of AGS propagation. The relatively greater sensitivity of DLSC to phenylephrine as compared to IC and PRF indicates that norepinephrine may be more important in the modulation of AGS in DLSC, which contrasts to the role of glutamate modulation. These data support recent neuronal recording data, which indicate that DLSC neurons play a critical role in AGS.

Neurons in the deep layers of superior colliculus play a critical role in the neuronal network for audiogenic seizures: mechanisms for production of wild running behavior

Recent investigations suggest that the deep layers of superior colliculus (DLSC) play a role in the neuronal network for audiogenic seizures (AGS). The present study examined DLSC neuronal firing and convulsive behavior simultaneously in freely-moving genetically epilepsy-prone rats (GEPR-9s) using chronically implanted microwire electrodes. An abrupt onset of acoustically-evoked firing at approximately 80-90 dB was observed in DLSC neurons of GEPR-9s, which was significantly above the normal threshold. DLSC neurons began to exhibit rapid tonic burst firing 1-2 s prior to the onset of the wild running behavior at the beginning of AGS. As the tonic phase of the seizure began, DLSC firing ceased, and only returned towards normal following post-ictal depression. These neuronal mechanisms may be relevant to other seizure models in which the DLSC is implicated. The temporal pattern of neuronal firing during AGS is specific to DLSC and differs markedly from those observed elsewhere in the AGS neuronal network. The temporal firing pattern suggests that the DLSC plays a primary role in the generation of the wild running phase of AGS. Previous studies indicate that the inferior colliculus is dominant during AGS initiation, and the pontine reticular formation is dominant during the tonic extension phase of AGS. Taken together these data suggest that the neurons in the neuronal network undergo a dominance shift as each specific convulsive behavior of AGS is elaborated.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Antisense inhibition of striatal GABAA receptor proteins decreases GABA-stimulated chloride uptake and increases cocaine sensitivity in rats

The functional status of striatal GABAA receptors appears to be inversely related to the magnitude of cocaine-induced behaviors. Exposure of striatum to antisense oligodeoxynucleotides (ASODNs) targeted to the mRNAs for the alpha 2 and the beta 3 subunits of the GABAA receptor should decrease expression of receptor proteins and therefore might be expected to increase cocaine sensitivity. ASODNs, scrambled ODNs or saline were injected into right lateral ventricle of rats and behavioral responses to cocaine were tested 18-20 h after treatment. Animals injected separately with alpha 2 or beta 3 ASODNs exhibited increased behavioral sensitivity to cocaine compared to rats injected with saline or scrambled ODNs including performing more 360 degrees turns to the left than to the right. There was significantly less GABA-stimulated Cl uptake in right striatum compared to left striatum of ASODN-treated rats with no significant difference between sides in control animals. Specific binding to benzodiazepine and convulsant sites on the GABAA receptor was not selectively altered by ASODN treatment. Combined alpha 2 beta 3 ASODN treatment did not affect either cocaine sensitivity or GABAA receptor function. There was no difference between the density of Nissl stained cells in the left and right edges of striatum in control or ASODN-treated rats indicating the absence of significant neurotoxic effects of the ASODN treatment. Injection of fluorescein-conjugated ASODNs indicated that ASODN is present in striatum at times during which behavioral and neurochemical indices of GABA receptor function are decreased. Thus, the functional status of GABAA receptors in striatum may be involved in determining cocaine sensitivity.

Molecular pathogenesis of alcohol withdrawal seizures: the modified lipid-protein interaction mechanism

The phrase alcohol withdrawal seizures (AWS) refers to seizures that result from the withdrawal of alcohol after a period of chronic alcohol administration. A mechanism of AWS is postulated, namely the modified lipid-protein interaction (MLPI) mechanism. This hypothesis is based upon an evaluation of the mechanisms of membrane fluidity, calcium channels, gamma-aminobutyric acid (GABA) and glutamate in the molecular pathogenesis of AWS. The mechanism hypothesizes that acute ethanol treatment alters the neuronal membrane lipids which then perturbs protein events, such as affecting the GABAA receptors, NMDA receptors and voltage-dependent Ca2+ channels synergistically or in combination. Subsequent adaptations in these systems occur after prolonged administration of ethanol. A sudden withdrawal of ethanol then leads to hyperexcitability which results in AWS.

Therapy of alcohol withdrawal syndrome in intensive care unit patients following trauma: results of a prospective, randomized trial

OBJECTIVES

To assess the effect of three different alcohol withdrawal therapy regimens in traumatized chronic alcoholic patients with respect to the duration of mechanical ventilation and the frequency of pneumonia and cardiac disorders during their intensive care unit (ICU) stay.

DESIGN

A prospective, randomized, blinded, controlled clinical trial.

SETTING

A university hospital ICU.

PATIENTS

Multiple-injured alcohol-dependent patients (n=180) transferred to the ICU after admission to the emergency room and operative management. A total of 180 patients were included in the study; however, 21 patients were excluded from the study after assignment.

INTERVENTIONS

Patients who developed actual alcohol withdrawal syndrome were randomized to one of the following treatment regimens: flunitrazepam/clonidine (n=54); chlormethiazole/haloperidol (n=50); or flunitrazepam/haloperidol (n=55). The need for administration of medication was determined, using a validated measure of the severity of alcohol withdrawal (Revised Clinical Institute Withdrawal Assessment for Alcohol Scale).

MEASUREMENTS AND MAIN RESULTS

The duration of mechanical ventilation and major intercurrent complications, such as pneumonia, sepsis, cardiac disorders, bleeding disorders, and death, were documented. Patients did not differ significantly between groups regarding age, Revised Trauma and Injury Severity Score and Acute Physiology and Chronic Health Evaluation II score on admission. In all except four patients in the flunitrazepam/clonidine group, who continued to hallucinate, the Revised Clinical Institute Withdrawal Assessment for Alcohol Scale decreased to <20 after initiation of therapy. ICU stay did not significantly differ between groups (p=.1669). However, mechanical ventilation was significantly prolonged in the chlormethiazole/haloperidol group (p=.0315) due to an increased frequency of pneumonia (p=.0414). Cardiac complications were significantly (p=.0047) increased in the flunitrazepam/clonidine group.

CONCLUSIONS

There was some advantage in the flunitrazepam/clonidine regimen with respect to pneumonia and the necessity for mechanical ventilation. However, four (7%) patients had to be excluded from the study due to ongoing hallucinations during therapy. Also, cardiac complications were increased in this group. Thus, flunitrazepam/haloperidol should be preferred in patients with cardiac or pulmonary risk. Further studies are required to determine which therapy should be considered.

Chronic ethanol exposure enhances [3H]GABA release and does not affect GABAA receptor mediated 36Cl uptake

Chronic ethanol treatment (CET) produces a permanent reduction in hippocampal long-term potentiation (LTP). The CET produced reduction of LTP can be eliminated acutely by pharmacological blockade of gamma-aminobutyric acid (GABA)ergicA synaptic transmission with the specific antagonist, bicuculline methiodide. Since LTP induction is normally modulated by activation of GABAergic synaptic transmission, we hypothesized that CET reduced LTP by enhancing either postsynaptic GABAA channel function or GABA release from presynaptic terminals. In the present study, we examined the long term effects of CET on GABAA channel function by measuring the efficacy of GABA to stimulate and bicuculline to antagonize GABA-stimulated 36Cl- uptake in hippocampal and cortical membrane preparations. CET did not affect basal uptake of chloride or the efficacy of either GABA or bicuculline at the GABAA channel. We next measured the long term effects of CET on basal and stimulated GABA release. When basal and electrically-stimulated [3H]GABA release were measured in superfused hippocampal slices, stimulated release was increased by 30% in CET rats. Basal release was unaffected. Thus it appears that CET may be reducing LTP by enhancing plasticity-related GABA release from presynaptic terminals.

P and NP rats respond differently to the discriminative stimulus effects of nicotine

Rats selectively bred for ethanol preference, that is, alcohol-preferring (P) and -nonpreferring (NP) rats, were trained to discriminate the interoceptive stimuli produced by IP-administered 1,000 mg/kg ethanol (10% v/v) in a two-lever, food-motivated, operant task. Once criterion performance was attained, dose-response experiments indicated an ED50 value for P rats = 354.1 mg/kg, whereas NP rats generated an ED50 value of 495.1 mg/kg, not significantly different from each other. In contrast to these similar sensitivities to ethanol, administration of doses of nicotine (0.4-1.2 mg/kg, SC) indicated that P rats were significantly more sensitive to the ethanol-like effects of nicotine than were NP rats. The results provide additional evidence for a possible hereditary co-occurrence of alcohol and nicotine sensitivity.

FG7142 causes opposite changes in [3H]GABA release from nigrocollicular regions

The activity of GABAergic neurons projecting from the striatum to the substantia nigra (SN) and from the SN to the superior colliculus (SC) may be involved in regulating seizure sensitivity such that striatonigral transmission is decreased and nigrocollicular transmission is increased in proconvulsant states. To test whether these changes occur in FG7142-treated rats, GABA transmission was assessed by measuring [3H]GABA release from superfused slices of the SN and SC and measuring [35S]TBPS binding to GABAA receptors throughout the brain. Nine daily injections of FG7142 (30 mg/kg IP) greatly increased myoclonic seizures in about one half of the animals. These animals exhibited a decrease in stimulated [3H]GABA release from the SN and an increase in both basal and stimulated release from the SC. Animals that were less sensitive to FG7142 treatment also had increased collicular release but not decreased nigral release. [35S]TBPS binding was unchanged by FG7142 treatment. Thus, decreased nigral GABA release may contribute to decreased striatonigral transmission after seizure occurrence whereas increased collicular GABA release may contribute to increased nigrocollicular transmission preceding multiple-seizure occurrence.