Ethanol exposure in early adolescence inhibits intrinsic neuronal plasticity via sigma-1 receptor activation in hippocampal CA1 neurons.

BACKGROUND: We demonstrated previously that rats exposed to chronic intermittent ethanol (CIE) vapors in early adolescence show increased magnitudes of long-term potentiation (LTP) of excitatory transmission when recorded at dendritic synapses in hippocampus. Large amplitude LTP following CIE exposure is mediated by sigma-1 receptors; however, not yet addressed is the role of sigma-1 receptors in modulating the intrinsic properties of neurons to alter their action potential firing during LTP. METHODS: Activity-induced plasticity of spike firing was investigated using rat hippocampal slice recordings to measure changes in both field excitatory postsynaptic potentials (fEPSPs) and population spikes (pop. spikes) concomitantly at dendritic inputs and soma of CA1 pyramidal neurons, respectively. RESULTS: We observed unique modifications in plasticity of action potential firing in hippocampal slices from CIE exposed adolescent rats, where the induction of large amplitude LTP by 100 Hz stimulations was accompanied by reduced CA1 neuronal excitability--reflected as decreased pop. spike efficacy and impaired activity-induced fEPSP-to-spike (E-S) potentiation. In contrast, LTP induction in ethanol-naïve control slices resulted in increased spike efficacy and robust E-S potentiation. E-S potentiation impairments emerged at 24 hours after CIE treatment cessation, but not before the alcohol withdrawal period, and were restored with bath-application of the sigma-1 receptor selective antagonist BD1047, but not the NMDA receptor antagonist d-AP5. Further evidence revealed a significantly shortened somatic fEPSP time course in adolescent CIE-withdrawn hippocampal slices during LTP; however, paired-pulse data show no apparent correspondence between E-S dissociation and altered recurrent feedback inhibition. CONCLUSIONS: Results here suggest that acute withdrawal from adolescent CIE exposure triggers sigma-1 receptors that act to depress the efficacy of excitatory inputs in triggering action potentials during LTP. Such withdrawal-induced depression of E-S plasticity in hippocampus probably entails sigma-1 receptor modulation of 1 or several voltage-gated ion channels controlling the neuronal input-output dynamics.

Emergence of NMDAR-independent long-term potentiation at hippocampal CA1 synapses following early adolescent exposure to chronic intermittent ethanol: role for sigma-receptors.

Adolescent humans who abuse alcohol are more vulnerable than adults to the development of memory impairments. Memory impairments often involve modifications in the ability of hippocampal neurons to establish long-term potentiation (LTP) of excitatory neurotransmission; however, few studies have examined how chronic ethanol exposure during adolescence affects LTP mechanisms in hippocampus. We investigated changes in LTP mechanisms in hippocamal slices from rats exposed to intoxicating concentrations of chronic intermittent ethanol (CIE) vapors in their period of early-adolescent (i.e., prepubescent) or late-adolescent (i.e., postpubescent) development. LTP was evaluated at excitatory CA1 synapses in hippocampal slices at 24 h after the cessation of air (control) or CIE vapor treatments. CA1 synapses in control slices showed steady LTP following induction by high-frequency stimulation, which was fully dependent on NMDAR function. By contrast, slices from early-adolescent CIE exposed animals showed a compound form of LTP consisting of an NMDAR-dependent component and a slow-developing component independent of NMDARs. These components summated to yield LTP of robust magnitude above LTP levels in age-matched control slices. Bath-application of the sigma-receptor antagonist BD1047 and the neuroactive steroid pregnenolone sulfate, but not acute ethanol application, blocked NMDAR-independent LTP, while leaving NMDAR-dependent LTP intact. Analysis of presynaptic function during NMDAR-independent LTP induction demonstrated increased presynaptic function via a sigma-receptor-dependent mechanism in slices from early-adolescent CIE-exposed animals. By contrast, CIE exposure after puberty onset in late-adolescent animals produced decrements in LTP levels. The identification of a role for sigma-receptors and neuroactive steroids in the development of NMDAR-independent LTP suggests an important pathway by which hippocampal synaptic plasticity, and perhaps memory, may be uniquely altered by chronic ethanol exposure during the prepubescent phase of adolescent development.

Steroid pregnenolone sulfate enhances NMDA-receptor-independent long-term potentiation at hippocampal CA1 synapses: role for L-type calcium channels and sigma-receptors.

Severe stress elevates plasma and CNS levels of endogenous neuroactive steroids that can contribute to the influence of stress on memory formation. Among the neuroactive steroids, pregnenolone sulfate (PREGS) reportedly strengthens memories and is readily available as a memory-enhancing supplement. PREGS actions on memory may reflect its ability to produce changes in memory-related neuronal circuits, such as long-term potentiation (LTP) of excitatory transmission in hippocampus. Here, we report a previously undiscovered pathway by which PREGS exposure promotes activity-dependent LTP of field excitatory postsynaptic potentials at CA1 synapses in hippocampal slices. Thus, application of PREGS, but not the phosphated conjugate of the steroid, selectively facilitates the induction of a slow-developing LTP in response to high-frequency (100 Hz) afferent stimulation, which is not induced in the absence of the steroid. The slow-developing LTP is independent of NMDA-receptor function (i.e., dAP5 insensitive) but dependent on functional L-type voltage-gated calcium channels (VGCC) and sigma-receptors. By contrast, PREGS at the highest concentration tested produces a depression in NMDA-receptor-dependent LTP, which is evident when sigma-receptor function is compromised by the presence of a sigma-receptor antagonist. We found that at early times during the induction phase of L-type VGCC-dependent LTP, PREGS via sigma-receptors transiently enhances presynaptic function. As well, during the maintenance phase of L-type VGCC-dependent LTP, PREGS promotes a further increase in presynaptic function downstream of LTP induction, as evidenced by a decrease in paired-pulse facilitation. The identification of complex regulatory actions of PREGS on LTP, involving sigma-receptors, L-type VGCCs, NMDA-receptors, and inhibitory circuits will aid future research endeavors aimed at understanding the precise mechanisms by which this stress-associated steroid may engage multiple LTP-signaling pathways that alter synaptic transmission at memory-related synapses.

Chloral hydrate and ethanol, but not urethane, alter the clearance of exogenous dopamine recorded by chronoamperometry in striatum of unrestrained rats.

Several general anesthetics reduce dopamine (DA) neuronal activity and release. However, anesthetic-induced alterations in DA transporter (DAT) function are unclear. We used high-speed chronoamperometry to examine how urethane, chloral hydrate and ethanol affected clearance of locally-applied DA in the dorsal striatum of unrestrained rats. Maximal DA signal amplitudes (A(max)) and signal decay rate constants (k) were monitored continuously from the awake to anesthetized state. Urethane did not significantly change A(max) or k. Chloral hydrate and ethanol potentiated A(max) by 32% and 37%, respectively, over baseline, indicative of DAT inhibition. These effects were not additive. Only ethanol increased k, an effect blocked by chloral hydrate. These results suggest differential effects of anesthetics on striatal DAT function, with chloral hydrate and ethanol significantly altering DAT activity but urethane producing only minimal effects.

Individual differences in cocaine-induced locomotor sensitization in low and high cocaine locomotor-responding rats are associated with differential inhibition of dopamine clearance in nucleus accumbens.

Behavioral sensitization to cocaine reflects neuroadaptive changes that intensify drug effects. However, repeated cocaine administration does not induce behavioral sensitization in all male Sprague-Dawley rats. Because cocaine inhibits the dopamine (DA) transporter (DAT), we investigated whether altered DAT function contributes to these individual differences. Freely moving rats had electrochemical microelectrode/microcannulae assemblies chronically implanted in the nucleus accumbens so that exogenous DA clearance signals were recorded simultaneous with behavior. The peak DA signal amplitude (A(max)) and efficiency of clearance (k) were used as indices of in vivo DAT function. Low and high cocaine responders (LCRs and HCRs, respectively) were identified based on their locomotor responsiveness to an initial injection of cocaine (10 mg/kg i.p.). Consistent with DAT inhibition, cocaine elevated A(max) and reduced k in HCRs, but not in LCRs. The same dose of cocaine was administered for six additional days and after a 7-day withdrawal. Baseline behavioral and dopamine clearance indices were unaltered by repeated cocaine or after withdrawal. Only LCRs expressed cocaine-induced sensitized locomotor activation, and this was accompanied by cocaine-induced elevations in A(max) and reductions in k. These sensitized responses to cocaine persisted in LCRs after withdrawal. In contrast, neither locomotor nor electrochemical responses were altered by repeated saline administration or a saline challenge after repeated cocaine administration, suggesting that conditioning did not significantly contribute. Our results suggest that increased DAT inhibition by cocaine is associated with locomotor sensitization and that DAT serves as a common substrate for mediating both the initial and sensitized locomotor responsiveness to cocaine.

Kinetic analysis of striatal clearance of exogenous dopamine recorded by chronoamperometry in freely-moving rats.

Previously, we developed technology that coupled high-speed chronoamperometry with microejections of dopamine (DA) to measure DA clearance in the brains of freely-behaving rats. Here, by varying the ejection volumes of DA across a 200-fold difference, the kinetics of striatal clearance were analyzed as a function of time and DA volume from 289 chronoamperometric signals (n=20 rats). Each DA clearance trace was fitted to a first-order exponential decay function to determine the rate constant for DA clearance (k). Additionally, the apparent Michaelis-Menten V(max) and K(m) kinetic constants were determined in freely-moving rats, enabling quantitative comparison of our values with other models of reuptake. The first-order rate constant for DA clearance, which reflects the V(max)/K(m) ratio or clearance efficiency, did not vary significantly when small volumes of DA were ejected resulting in peak DA signal amplitudes (A(max)) of <5 microM. However, following nomifensine-induced DAT inhibition, A(max) was increased and k was attenuated simultaneously with behavioral activation; and A(max) and behavior remained elevated beyond the initial period. Our results indicate that the analysis of kinetic parameters from chronoamperometric DA signals may be useful for investigating drug-induced regulation of DAT kinetics in relation to the behavior of freely-moving rats.

Acute cocaine differentially alters accumbens and striatal dopamine clearance in low and high cocaine locomotor responders: behavioral and electrochemical recordings in freely moving rats.

Behavioral responses of rodents to cocaine are characterized by marked individual variability. Here, outbred male Sprague-Dawley rats were profiled based on concomitant recording of behavioral and electrochemical responses. Rats were categorized as either low or high cocaine responders (LCRs or HCRs, respectively) based on their differential locomotor responsiveness to an acute, low-dose injection of cocaine (10 mg/kg i.p.). LCRs and HCRs also differed in other cocaine-induced behaviors. The role of the dopamine transporter (DAT) in mediating the behavioral differences in cocaine responsiveness in LCRs and HCRs was investigated by high-speed chronoamperometric recording of exogenous dopamine (DA) clearance signals in nucleus accumbens (NAc) and dorsal striatum (dSTR). Higher volumes of DA were required in NAc of HCRs, than of LCRs, to produce equivalent peak DA signal amplitude (A(max)) responses. In HCRs, systemic cocaine administration evoked an immediate and prolonged 2-fold augmentation in A(max) in both brain regions, coincident with locomotor activation. The cocaine-induced decrease in the efficiency of DA clearance (k) in NAc of HCRs was more immediate and prolonged than in dSTR, where the transient decrease coincided with maximal stereotypic behavior. In contrast, in LCRs, A(max) was not altered by cocaine, and decay rate constant (k) was transiently attenuated only in dSTR. Correlation analyses of individual responses revealed that cocaine-induced changes in DA clearance signal parameters accounted for 20 to 40% of the variation in behavioral responsiveness to cocaine. Overall, our findings emphasize the importance of characterizing individual responses to understand more fully the range of functional consequences resulting from DAT inhibition.