Potential pleiotropic effects of Mpdz on vulnerability to seizures.

We previously mapped quantitative trait loci (QTL) responsible for approximately 26% of the genetic variance in acute alcohol and barbiturate (i.e., pentobarbital) withdrawal convulsion liability to a < 1 cM (1.8 Mb) interval of mouse chromosome 4. To date, Mpdz, which encodes the multiple PSD95/DLG/ZO-1 (PDZ) domain protein (MPDZ), is the only gene within the interval shown to have allelic variants that differ in coding sequence and/or expression, making it a strong candidate gene for the QTL. Previous work indicates that Mpdz haplotypes in standard mouse strains encode distinct protein variants (MPDZ1-3), and that MPDZ status is genetically correlated with severity of withdrawal from alcohol and pentobarbital. Here, we report that MPDZ status cosegregates with withdrawal convulsion severity in lines of mice selectively bred for phenotypic differences in severity of acute withdrawal from alcohol [i.e., High Alcohol Withdrawal (HAW) and Low Alcohol Withdrawal (LAW) lines] or pentobarbital [High Pentobarbital Withdrawal (HPW) and Low Pentobarbital Withdrawal (LPW) lines]. These analyses confirm that MPDZ status is associated with severity of alcohol and pentobarbital withdrawal convulsions. Using a panel of standard inbred strains of mice, we assessed the association between MPDZ status with seizures induced by nine chemiconvulsants. Our results show that MPDZ status is genetically correlated with seizure sensitivity to pentylenetetrazol, kainate and other chemiconvulsants. Our results provide evidence that Mpdz may have pleiotropic effects on multiple seizure phenotypes, including seizures associated with withdrawal from two classes of central nervous system (CNS) depressants and sensitivity to specific chemiconvulsants that affect glutaminergic and GABAergic neurotransmission.

Behavior-related modulation of substantia nigra pars reticulata neurons in rats performing a conditioned reinforcement task.

Motor-control models of basal ganglia function have emphasized disinhibition through reduction of tonic, inhibitory output. Although these models have shed important light on basal ganglia operations, evidence emerging from electrophysiological studies of behaving primates suggests that disinhibition alone may not adequately explain the role of the basal ganglia in movement. To assess this role in the rat, the most frequently used subject in studies of basal ganglia function, we recorded neuronal activity in the primary output nucleus, the substantia nigra pars reticulata, during an operant task. After rats were trained to nosepoke into an illuminated hole for access to a 10% sucrose solution delivered through a spout, single- and multiple-unit activity was recorded during 60-120 nosepoke trials. Compared to the period 60 s before the start of the first trial in the task, 110 of 225 reticulata units increased firing >200% while 17 of 225 decreased to 40% of baseline. Of these 225 units, >60% responded coincident with specific task events such as nosepokes and spout licking. Most nosepoke-responsive units showed either excitation (>50%) or a combination of excitation and inhibition (>25%) rather than inhibition alone (>20%). Increases in firing were also common during approach and licking at the spout, with inhibitions alone comprising 30% of responses. In some units, there was evidence of reward-related responding, with changes occurring in anticipation of reward delivery or during the delivery of sucrose, but not the persistent licking that continued for several seconds after its offset. While 70% of units responded during both nosepokes and spout licking, changes in firing were typically unique depending on the motor behavior required (i.e. nosepoking vs. licking). Our results, which indicate a prominent role for increases in nigra reticulata activity during movement, add to growing evidence that although inhibitions may allow desired motor responses to emerge, excitations may help shape behavioral output by suppressing competing motor programs.

Long T methamphetamine schedules produce circadian ensuing drug activity in rats.

Eight female Sprague-Dawley rats were housed in isolated continuous 24-h/day environments under conditions of constant dim light and a rate-limited feeding schedule. Following 2 months of free-running activity, all animals were administered methamphetamine (MA) i.p. (2 mg/kg) once every 31 h for 24 injection cycles. Average wave forms of wheel-running activity showed that animals did not anticipate the 31-h schedule of MA injections, but rather displayed circadian ensuing drug activity (CEDA) between 24 and 28 h following the injections. Post-injection meals failed to meet reliably the threshold necessary to achieve food-engendered anticipatory or ensuing activity. Cosinor analysis showed that the intensity of CEDA was strongly influenced by the relative phase of the free-running rhythm. CEDA was moderately influenced by the size of the post-injection bout of activity. Because injection times rotated daily throughout local time without repeating a time of day, CEDA resulting from a long T schedule of MA administration appeared to be based on one-trial resetting of a circadian-related mechanism by a major drug of abuse.

Effects of ethanol on rat somatosensory cortical neurons.

In this study, we characterized the local effects of ethanol (EtOH) on postsynaptic potentials (PSPs) and membrane properties of layer II-III (L2-3) and layer V (L5) somatosensory cortical neurons. Intracellular recordings were done using the in vitro slice preparation of rat somatosensory cortex. Our results show that EtOH exerts local effects on cortical cell membrane at physiologically relevant concentrations. A predominant effect of EtOH was to reduce excitability of L2-3 and L5 neurons by increasing the rheobase, decreasing input resistance and repetitive firing, reducing PSPs amplitude and the probability of evoking action potentials. Early (6 ms) and late (18 ms) PSP components were affected differentially by EtOH, the late components being more suppressed. Overall, EtOH-mediated suppression of PSPs was stronger in L5 neurons. Cortical neurons were divided into three subtypes: regular spiking adapting (RS-A), regular spiking non-adapting (RS-NA) and bursting (D-IB) neurons. PSPs evoked in RS-A neurons were more sensitive to EtOH suppressant effects. EtOH effects on input resistance were distributed differentially among the three groups of neurons. These results support the notion that EtOH disrupts higher processing of somatosensory information via a differential alteration of cortical neuron's membrane properties and synaptic transmission.

Circadian activity precedes daily methamphetamine injections in the rat.

Scheduled daily injections of methamphetamine (MA) produced locomotor activity that preceded and followed the usual time of injection in rats housed under conditions of constant, moderately dim light and temporally distributed feeding. A circadian basis for pre-injection time activity was supported by its anticipatory timing in the apparent absence of reliable preceding external cues and by its persistence on a test day on which the rats remained undisturbed. Post-injection time locomotor activity also persisted on the test day, occurring from 24 to 29 h after the final MA injection. These results indicate that MA injections engage circadian processes underlying locomotor activity, and they raise the possibility that intake of drugs of abuse by humans may facilitate drug taking or relapse at times of day related to previous drug use.

Behavioral associations of neuronal activity in the ventral tegmental area of the rat.

The ventral tegmental area (VTA) is a central element in a system that mediates the reinforcing properties of natural stimuli (such as food), brain stimulation, and drugs of abuse. Although considerable effort has been applied to understanding how drugs of abuse influence this system, relatively little work has examined its function during conditioned reinforcement tasks in awake, behaving animals. In the present studies, bundles of four to eight microwire electrodes were chronically implanted in the VTA or prefrontal cortex (PFC) of male Wistar rats. Following recovery from surgery, simultaneous recordings from multiple single neurons and unit clusters were obtained in rats pressing a lever for a sucrose solution on a fixed-ratio schedule of reinforcement. Consistent with the hypothesis that these neurons encode information related to motivation, most of the neurons in both VTA and PFC showed significant modulation of firing rate associated with one or more events occurring within the response/reinforcement cycle. These events included lever presses, onset and end of a tone signaling sucrose delivery, and onset and end of sucrose consumption. Significant decreases in firing rate were observed, coincident with onset of the tone and sucrose delivery, or with consumption. These decreases were sustained through the end of sucrose consumption. A number of neurons also discharged bursts of activity associated with individual lever presses. These findings provide a clear demonstration that VTA neuronal activity is modulated during motivated behavior. Similar information about events within the ongoing response/reinforcement cycle appears to be distributed through many neurons within the VTA, and may be mirrored in target structures such as PFC.

Sensitivity to N-methyl-D-aspartic acid-induced convulsions is genetically associated with resistance to ethanol withdrawal seizures.

Mice genetically selected to be resistant (withdrawal-seizure resistant, WSR) or prone (withdrawal-seizure prone, WSP) to handling-induced convulsions during ethanol withdrawal were tested for sensitivity to convulsions induced by timed intravenous (i.v.) infusion of N-methyl-D-aspartic acid (NMDA). WSR mice displayed convulsions at infused doses of NMDA that averaged 20% lower than WSP mice. This result was present in both genetically independent replicates of the WSR and WSP mice and provides strong evidence for an involvement of the NMDA system in the difference in withdrawal seizures present in these lines.

Neural sensitivity to pentylenetetrazol convulsions in inbred and selectively bred mice.

In these experiments, sensitivity to the convulsant drug pentylenetetrazol (PTZ) was measured in 10 inbred mouse strains, and in 4 mouse lines selectively bred for severe (WSP1, WSP2) or minimal (WSR1, WSR2) ethanol withdrawal convulsions. Using a timed infusion procedure, sensitivity to convulsions was assessed by measures of latency to convulsion, effective dose (ED) infused at time of convulsion, and brain concentration (BC) of PTZ at time of convulsion. In addition, ED and latency to convulsion were measured in WSP and WSR mice at 5 different concentrations of PTZ. Higher concentrations, which increased rate of drug infusion, reduced latency but had little effect on ED. WSP1 mice were slightly more sensitive to PTZ than WSR1 mice, but WSP2 mice were equally or less sensitive than WSR2 mice. Among the inbred strains, latency, ED and brain PTZ concentration were found to be highly correlated, suggesting that pharmacokinetic factors do not significantly influence access of PTZ to sites of action in the central nervous system. The C57BL/6J strain was least sensitive by all measures, while DBA/2J mice were highly sensitive. The BALB/cJ strain was the most sensitive strain as assessed by ED and latency, but BC indicated relatively average sensitivity. Apparently, pharmacokinetic factors in this strain result in a relatively rapid accumulation of drug in brain, making it appear to be more sensitive. Thus, although ED provides a reliable estimate of neural sensitivity in general, genetic factors exist which, in some strains, modify access of PTZ and possibly other drugs to brain, potentially affecting determination of sensitivity in the absence of a measure of brain drug concentration.

Genetic correlations among inbred strain sensitivities to convulsions induced by 9 convulsant drugs.

Inbred mouse strains differed significantly in sensitivity to convulsions induced by 9 convulsant drugs administered using a timed infusion procedure. Some strains (e.g. BALB/cJ, A/J) were generally seizure-susceptible, while some were generally seizure resistant (e.g. C57BL/6J, SWR/J). However, the overall pattern of strain sensitivities was complex, and depended upon drug and convulsant sign. Five of the drugs (bicuculline, DMCM, picrotoxin, TBPS and pentylenetetrazol (PTZ] produce convulsions, at least in part, through their interactions with the GABA receptor, while the other 4 (strychnine, CHEB, 4-aminopyridine and kainic acid) act through independent mechanisms. We predicted that responses to drugs with similar mechanisms of action would be genetically correlated. However, strains sensitive to picrotoxin-induced convulsions were not necessarily sensitive to convulsions elicited by PTZ or TBPS. Furthermore, different convulsant signs produced by a single drug were not always strongly correlated. Instead, genetic correlations were found among inbred strains for sensitivity to similar convulsant signs produced by different drugs. This suggests that genetic variation in sensitivity to these convulsant drugs arises primarily from variation in systems important for the expression of the convulsion.