Phospho-regulation of synaptic and extrasynaptic N-methyl-d-aspartate receptors in adult hippocampal slices.

Recent evidence demonstrates that N-methyl-d-aspartate receptor (NMDAR) trafficking contributes to synaptic plasticity in the hippocampus. Phosphorylation of tyrosine residues, especially NR2B tyrosine 1472, appears to be a mechanism by which NMDAR endocytosis is prevented, suggesting that the tyrosine phosphorylation and surface expression of NMDARs are positively correlated. Previous work from our laboratory and others has confirmed that modulation of tyrosine phosphatase and kinase activity alters the surface expression of NMDARs. However, the changes in NMDAR surface expression described in those studies were in terms of total surface membrane versus intracellular receptors. Within the plasma membrane of glutamatergic synapses, distinct populations of NMDARs exist. Namely, receptors at the surface can be differentiated into synaptic and extrasynaptic pools based on their association with the post-synaptic density (PSD) and availability to glutamate. In the present study, we utilized a subcellular fractionation approach coupled with detergent extraction to prepare synaptic and extrasynaptic NMDARs from adult rat hippocampal slices. Using this method, we examined how tyrosine phosphatase and Src-family tyrosine kinase (SFK) inhibitors modulate the phosphorylation and localization of these different pools of NMDARs. We found that both synaptic and extrasynaptic NMDARs were modulated by tyrosine phosphatase and SFK inhibitors; however subunit- and residue-specific effects were observed. Specifically, phosphorylation of NR2B tyrosine 1472 was associated with enrichment of synaptic NMDARs, whereas phosphorylation of NR2B tyrosine 1336 was associated with enrichment of extrasynaptic NMDARs. Using electrophysiological methods, we also reveal that the biochemical modifications produced by these inhibitors were associated with corresponding changes in NMDAR function.

Synaptic GABAergic and glutamatergic mechanisms underlying alcohol sensitivity in mouse hippocampal neurons.

This study was designed to examine the neuronal mechanisms of ethanol sensitivity by utilizing inbred short sleep (ISS) and inbred long sleep (ILS) mouse strains that display large differences in sensitivity to the behavioural effects of ethanol. Comparisons of whole-cell electrophysiological recordings from CA1 pyramidal neurons in hippocampal slices of ISS and ILS mice indicate that ethanol enhances GABAA receptor-mediated inhibitory postsynaptic currents (GABAA IPSCs) and reduces NMDA receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) in a concentration- and strain-dependent manner. In ILS neurons, these receptor systems are significantly more sensitive to ethanol than those in ISS neurons. To further examine the underlying mechanisms of differential ethanol sensitivities in these mice, GABAB activity and presynaptic and postsynaptic actions of ethanol were investigated. Inhibition of GABAB receptor function enhances ethanol-mediated potentiation of distal GABAA IPSCs in ILS but not ISS mice, and this blockade of GABAB receptor function has no effect on the action of ethanol on NMDA EPSCs in either mouse strain. Thus, subregional differences in GABAB activity may contribute to the differential ethanol sensitivity of ISS and ILS mice. Moreover, analysis of the effects of ethanol on paired-pulse stimulation, spontaneous IPSC events, and brief local GABA or glutamate application suggest that postsynaptic rather than presynaptic mechanisms underlie the differential ethanol sensitivity of these mice. Furthermore, these results provide essential information to focus better on appropriate target sites for more effective drug development for the treatment of alcohol abuse.

Intranigral transplantation of solid tissue ventral mesencephalon or striatal grafts induces behavioral recovery in 6-OHDA-lesioned rats.

Parkinson's disease (PD) is characterized by a degeneration of the dopamine (DA) pathway from the substantia nigra (SN) to the basal forebrain. Prior studies in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats have primarily concentrated on the implantation of fetal ventral mesencephalon (VM) into the striatum in attempts to restore DA function in the target. We implanted solid blocks of fetal VM or fetal striatal tissue into the SN to investigate whether intra-nigral grafts would restore motor function in unilaterally 6-OHDA-lesioned rats. Intra-nigral fetal striatal and VM grafts elicited a significant and long-lasting reduction in apomorphine-induced rotational behavior. Lesioned animals with ectopic grafts or sham surgery as well as animals that received intra-nigral grafts of fetal cerebellar cortex showed no recovery of motor symmetry. Subsequent immunohistochemical studies demonstrated that VM grafts, but not cerebellar grafted tissue expressed tyrosine hydroxylase (TH)-positive cell bodies and were associated with the innervation by TH-positive fibers into the lesioned SN as well as adjacent brain areas. Striatal grafts were also associated with the expression of TH-positive cell bodies and fibers extending into the lesioned SN and an induction of TH-immunolabeling in endogenous SN cell bodies. This finding suggests that trophic influences of transplanted fetal striatal tissue can stimulate the re-expression of dopaminergic phenotype in SN neurons following a 6-OHDA lesion. Our data support the hypothesis that a dopaminergic re-innervation of the SN and surrounding tissue by a single solid tissue graft is sufficient to improve motor asymmetry in unilateral 6-OHDA-lesioned rats.

Potentiation of ethanol effects in cerebellum by activation of endogenous noradrenergic inputs.

We previously found that beta adrenergic agonists such as norepinephrine and isoproterenol potentiate the depressant actions of ethanol (EtOH) on cerebellar Purkinje neurons. Furthermore, antagonism of the beta adrenergic effects of endogenously released catecholamines with timolol reduced EtOH-induced depressions of neuronal activity in that brain area. In the present study, we further investigated the hypothesis that activity of the endogenous noradrenergic innervation to the cerebellar cortex can potentiate this EtOH action. We investigated the interaction of synaptically released catecholamines on EtOH-induced depressions of cerebellar Purkinje neurons in three different experiments: (1) endogenous catecholamine release was facilitated by applying the catecholamine uptake inhibitor desmethylimipramine, (2) activity of the noradrenergic innervation of the cerebellar cortex from locus ceruleus was increased by causing acute withdrawal from 7 days of chronic morphine treatment with the opiate antagonist naloxone, and (3) the noradrenergic innervation of the cerebellum was activated directly by electrical stimulation of the locus ceruleus. We found that all three conditions potentiated EtOH-induced depressions in the cerebellum and that this potentiation of ethanol effects could be antagonized by the systemic administration of the beta adrenergic antagonist propranolol. Furthermore, morphine withdrawal also caused potentiation of the depressant effects of phencyclidine, which are known to be regulated by the endogenous catecholamine innervation in this brain area. Taken together with our previous data demonstrating a beta adrenergic facilitation of EtOH actions in this brain area, the present results suggest that the activity of endogenous noradrenergic synapses can regulate the depressant effects of EtOH on cerebellar Purkinje neurons.

Functional interactions between spinal cord grafts suggest asymmetries dictated by graft maturity.

Fetal spinal cord tissue grafts have been advocated as a possible repair strategy for spinal cord injury. In the present study, we used intraocular spinal cord grafts to model the interactions which may occur between fetal and adult spinal cord after making such a graft and to study to which extent functional connections can be expected to occur between the host and graft tissue. We first grafted fetal spinal cord to the anterior chamber of the eye where it was allowed to mature. A second piece of fetal spinal cord was then sequentially grafted in contact with the first graft. Electrophysiological recordings made from the older graft while electrically stimulating the younger graft provided evidence for an excitatory innervation from the younger spinal cord graft to the mature spinal cord which appeared to be glutamatergic. However, we only rarely found excitatory inputs from the first, mature spinal cord graft to the younger graft. Fiber connections between the two spinal cord grafts were verified by retrograde tracing and neurofilament immunohistochemistry. In no case was a trophic influence on graft volume observed between spinal cord grafts regardless of whether the transplantations were performed sequentially or at the same time. Even the introduction of a second graft to immature spinal cord tissue was ineffective. In contrast, we found a marked trophic, neuron-rescuing effect of spinal cord grafts upon cografts of fetal dorsal root ganglia. This latter observation is consistent with the hypothesis that spinal cord tissue can exert a trophic effect on developing sensory ganglia and demonstrates that many sensory neurons can survive in the presence of a central target and in the absence of the appropriate peripheral target. These intraocular experiments predict that fetal spinal cord grafted to the injured adult spinal cord may develop effective excitatory inputs with the host, while host-to-graft inputs may develop to a considerably smaller extent. Our results also suggest that the adult spinal cord does not exert marked trophic effects on growth of fetal spinal cord, while it does exert a trophic influence on central projections of dorsal root ganglia.

Beta adrenergic sensitization of gamma-aminobutyric acid receptors to ethanol involves a cyclic AMP/protein kinase A second-messenger mechanism.

Previous studies have found that ethanol (EtOH) will consistently potentiate gamma-aminobutyric acid (GABA) receptor function in the cerebellum during beta adrenergic receptor activation. One consequence of beta adrenergic receptor stimulation is to increase cAMP levels, which, in turn, activate protein kinase A (PKA)-mediated phosphorylation of intracellular protein sites. In the present study, we investigated three cAMP analogues, two activators and one inhibitor of PKA to determine whether this cAMP-mediated second-messenger system may be one mechanism involved in the previously observed beta adrenergic interaction of EtOH with the GABA(A) receptor. Furthermore, because the phosphorylation state of the GABA(A) receptor may be an important determinant of function, we investigated the effect of the block of phosphatase activity on EtOH/GABA receptor interactions. We found that similar to the beta adrenergic agonist isoproterenol, local applications of the membrane-permeable cAMP analogues 8-bromo-cAMP and Sp-cAMP could modulate responses to iontophoretically applied GABA and that these modulated GABA responses were sensitized to the potentiative effects of EtOH. EtOH did not facilitate unmodulated GABA effects or GABA responses that were maximally modulated by 8-bromo-cAMP, suggesting that the cAMP mechanism mediates the observed EtOH interaction with GABA mechanisms. Furthermore, the PKA inhibitor Rp-cAMP reversed the EtOH-induced potentiation of the isoproterenol-modulated GABA responses. Finally, microcystin-LR and okadaic acid, which are type I and IIa phosphatase inhibitors, could also modulate and sensitize GABA responses to EtOH. These data suggest that beta adrenergic sensitization of GABA(A) receptors to EtOH involves the intracellular cAMP/PKA second-messenger cascade.

Differential development and characterization of rapid acute neuronal tolerance to the depressant effects of ethanol on cerebellar Purkinje neurons of low-alcohol-sensitive and high-alcohol-sensitive rats.

Rapid acute neuronal tolerance (RANT) to the depressant effects of ethanol (EtOH) is a desensitization of EtOH-induced depression of neuronal firing that develops over the first 5 to 7 min of EtOH exposure. This phenomenon has been hypothesized to play a role in acute behavioral insensitivity to EtOH and is expressed by cerebellar Purkinje neurons in animals selectively bred for insensitivity to EtOH-induced ataxia, such as low-alcohol-sensitive (LAS) rats and short-sleep mice. Purkinje neurons of animals bred for high sensitivity to EtOH-induced behavioral ataxia, such as high-alcohol-sensitive (HAS) rats and long-sleep mice, only infrequently express such acute tolerance to EtOH-induced depression of neuronal activity. However, because higher EtOH doses are required to depress Purkinje neuron activity in LAS rats than in HAS rats, it was not known whether the higher EtOH doses that depress LAS neurons would also induce RANT to EtOH in HAS rats, which were generally not exposed to such high EtOH doses in previous studies. Furthermore, the conditions for development and maintenance of RANT to EtOH had not been characterized. We found that RANT to EtOH-induced depression of cerebellar neurons principally developed within 5 min of EtOH application and recovered within 20 min of the last EtOH exposure and that neurons in HAS rats did not develop acute tolerance to the higher EtOH doses that were effective in LAS rats. We conclude that this rapid tolerance contributes to the acute EtOH sensitivity difference between LAS and HAS rats.

Ethanol depression of cerebellar Purkinje neuron firing involves nicotinic acetylcholine receptors.

Local application of ethanol (EtOH) has been reported to inhibit Purkinje neuron firing. EtOH-induced depressions can be antagonized by bicuculline, suggesting involvement of GABAA receptors. Since there is evidence from other studies indicating that nicotine may interact with EtOH responses, in this study we investigated whether nicotinic acetylcholine receptors (nAChR's) might be also involved in EtOH-induced depressions of these neurons in urethane-anesthetized Sprague-Dawley rats. Using local application (micropressure ejection) of drugs onto cerebellar Purkinje neurons while recording extracellular firing rates, we found that depressant responses to EtOH could be potentiated by subdepressant doses of nicotine. Furthermore, EtOH-induced depressions of firing could be antagonized by mecamylamine, a nicotinic acetylcholine receptor (nAChR) antagonist. Results from the present study indicate that EtOH-induced depressions may involve nAChRs in the cerebellum.

8-Bromo-cAMP mimics beta-adrenergic sensitization of GABA responses to ethanol in cerebellar Purkinje neurons in vivo.

Previous studies in our laboratory indicated that electrophysiological responses of cerebellar Purkinje neurons to GABA were not routinely potentiated by ethanol (EtOH), and the potentiation was not large when it occurred. In the presence of beta-adrenergic agonists, such as isoproterenol, however, GABA inhibitions became sensitive to potentiation by EtOH in nearly every Purkinje neuron tested. beta-adrenergic receptor activation alone also modulates (potentiates) GABA responses on Purkinje neurons, and this has been reported to be mediated by a cAMP second messenger system. Herein, we report that the membrane-permeable cAMP analog, 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP), but not the membrane-impermeable cAMP, can also modulate GABA responses and that EtOH potentiates this facilitatory action of 8-Br-cAMP. These effects are not likely caused by adenosine receptor mechanisms, because this 8-bromoadenosine mediated modulation and sensitization was observed in the presence of systemic theophylline. These data suggest that the beta-adrenergic modulation and sensitization to EtOH of cerebellar Purkinje neuron GABA responses occur via a cAMP second messenger mechanism.

Ethanol-induced depressions of cerebellar Purkinje neurons are potentiated by beta-adrenergic mechanisms in rat brain.

Electrophysiological studies indicate that EtOH decreases the firing rate of cerebellar Purkinje neurons in vivo and in vitro through a GABAA mechanism. These neurons receive a prominent noradrenergic input from the locus coeruleus. Stimulation of the locus coeruleus or local application of beta-adrenergic agonists potentiates Purkinje neuron responses to GABA and sensitizes GABA responses to the potentiative effects of EtOH. In the present study, we found that the modulatory influences of the beta-adrenergic agonist isoproterenol potentiated EtOH-induced depressions of Purkinje neuron firing. This isoproterenol interaction with EtOH was antagonized by the beta-adrenergic antagonist timolol. We found evidence that endogenous catecholamines can cause this effect as well. Timolol antagonized EtOH-induced depressions on 20% of the neurons studied. This was the same frequency as that previously found for EtOH-induced potentiations of GABA depressions in this brain area. These data suggest that the Purkinje neurons showing this interaction receive spontaneously active catecholamine inputs that sensitize the GABA effects to the potentiative effects of ethanol. Consistent with this hypothesis, we also found that timolol antagonized this GABA/EtOH interaction. Taken together, these results are consistent with the hypothesis that EtOH-induced depressions of Purkinje neurons involved endogenous GABA actions that may be regulated by beta-adrenergic mechanisms.

Spinal cord-skeletal muscle cografts: trophic and functional interactions.

Skeletal muscle from embryonic day 20 (E20) was combined with E15 rat spinal cord in the anterior chamber of the eye of adult albino rats. The two grafts were either transplanted concomitantly or sequentially, in which case muscle tissue was added 4 months after the spinal cord. Control groups received a single graft of either spinal cord or skeletal muscle. Survival and intraocular growth were observed through the cornea. After maturation in oculo, the double grafts were examined immunohistologically utilizing antisera to neurofilament (NF) and acetylcholinesterase (AChE). The grafts were also evaluated using electrical stimulation to determine functional connectivity. The spinal cord and skeletal muscle grafts were found to exert reciprocal trophic effects on each other, evidenced as a larger muscle mass in skeletal muscle grafts allowed to develop in the presence of spinal cord tissue, and a larger volume of spinal cord grafts allowed to develop together with a skeletal muscle graft, respectively. Immunohistochemistry revealed NF-positive nerve fibers leaving the spinal cord graft and entering the muscle tissue. AChE-positive endplates developed in the muscle grafts. Electrical stimulation of the spinal cord part of double-graft combinations generally elicited contractile responses in specific areas of the muscle cograft. These results demonstrate both structural and functional connections between grafts of spinal cord and skeletal muscle tissue in vivo. The fact that such connections were also established between a mature (adult) spinal cord graft and fetal skeletal muscle tissue suggests that some alpha-motoneurons are able to survive for many months in the intraocular grafts without an appropriate target, and that they are able to subsequently innervate skeletal muscle targets.

Differential effects of ethanol on the firing rates of Golgi-like neurons and Purkinje neurons in cerebellar slices in vitro.

Previous studies have demonstrated that ethanol (EtOH) inhibits the firing rate of Purkinje neurons both in vitro and in vivo. However, little is known about the response of cerebellar interneurons to EtOH. In this report, we describe the effects of locally applied EtOH on the firing of one type of cerebellar interneuron, tentatively identified as Golgi neurons, and on Purkinje cells in brain slices in vitro. The Golgi neurons were excited by EtOH, whereas EtOH depressed the firing rate of Purkinje neurons. To the best of our knowledge, this is the first report of responses of cerebellar Golgi neurons to local applications of EtOH.

Electrophysiological interactions of ethanol with GABAergic mechanisms in the rat cerebellum in vivo.

Biochemical studies indicate that ethanol (EtOH) will facilitate the activation of the GABAA/Cl- channel, and behavioral studies demonstrate that EtOH-induced sedative and incoordinating effects can be potentiated by GABA mimetics and blocked by GABA antagonists. It has been difficult, however, to demonstrate an EtOH-induced potentiation of the depressant electrophysiological effects of locally applied GABA in mammalian brain in vivo. Similarly, in this study, local EtOH applications only infrequently caused potentiations of the depressant effects of microiontophoretically applied GABA on cerebellar Purkinje neurons, and this interaction was modest when present. The predominant interaction of locally applied EtOH was an antagonism of GABA-induced depressions of neuronal activity. However, the GABAA receptor antagonist bicuculline reversibly and apparently competitively blocked the depressant effects of locally applied EtOH on single cerebellar Purkinje neurons. Our data suggest that EtOH potentiation of GABA responses alone is insufficient to account for EtOH-induced depressions of cerebellar Purkinje neurons. However, these data clearly imply that activation of a GABAA receptor is required for the expression of EtOH-induced depressions of neuronal activity in this brain area. It is less clear how lower, nondepressant doses of EtOH interact with GABA mechanisms. We hypothesize that either the GABAA receptor mechanism must be sensitized to the potentiative effects of EtOH through the influences of neuromodulatory and/or hormonal regulation, or that EtOH interacts directly with these regulatory processes.

Sensitization of gamma-aminobutyric acid-induced depressions of cerebellar Purkinje neurons to the potentiative effects of ethanol by beta adrenergic mechanisms in rat brain.

We previously reported that both systemic administration and brief local application of ethanol potentiated gamma-aminobutyric acid (GABA)-induced depressions of cerebellar Purkinje neurons when the GABA responses were concomitantly facilitated (modulated) by catecholaminergic agonists. In the present study, we further investigated the effects of prolonged local applications of ethanol, which more closely mimic the systemic application of ethanol, and we characterized the pharmacological specificity of the catecholaminergic interaction with these ethanol effects. As has been previously observed, iontophoretic applications of isoproterenol (ISO), a beta adrenergic agonist, facilitated GABA-induced depressions of cerebellar Purkinje neurons. The prolonged local application of ethanol produced a long-lasting potentiation of the ISO-modulated GABA responses that was similar in duration to that caused by systemic ethanol administration. The ethanol-induced augmentation of the ISO-modulated GABA responses was diminished both by terminating the beta adrenergic agonist application as well as by administering the beta adrenergic antagonist timolol. The alpha adrenergic agonist phenylephrine, on the other hand, either attenuated or had no effects on the GABA-induced depressions of cerebellar Purkinje neurons, and a subsequent application of ethanol did not potentiate GABA responses in the presence of phenylephrine. We conclude that prolonged local application of ethanol mimics the interaction of systemic ethanol with GABA-induced depressions of cerebellar Purkinje neurons. Furthermore, the catecholaminergic sensitization of GABA responses to these potentiative effects of ethanol is mediated by a beta adrenergic mechanism.

Ethanol potentiation of GABA-induced electrophysiological responses in cerebellum: requirement for catecholamine modulation.

In this study, we confirmed that microiontophoretically applied norepinephrine (NE) and isoproterenol potentiate the depressant effects of locally-applied gamma-aminobutyric acid (GABA) on cerebellar Purkinje neurons of anesthetized rats. Although ethanol (EtOH) does not reliably or efficaciously potentiate GABA-induced depressions of neuronal activity, we found that systemic or locally-applied EtOH does markedly potentiate GABA-induced inhibitions of Purkinje neuron firing rate if that response is concomitantly modulated by NE or isoproterenol. This study suggests that the EtOH sensitivity of the GABA mechanism of electrophysiological responses in the cerebellar cortex is regulated by the neuromodulatory effect of beta-adrenergic receptor activation.

Nicotine interferes with GABA-mediated inhibitory processes in mouse hippocampus.

Previous studies indicated that the excitatory effects of nicotine may be mediated via interference with GABAergic transmission. Here, several variants of the paired-pulse paradigm were employed to ascertain whether nicotine interferes with endogenous inhibitory circuits in the hippocampus. Nicotine attenuated the inhibition evoked by antidromic (alvear) stimulation in the CA1 region in a concentration-dependent manner (EC50 = 60-75 microM). This same phenomenon was also observed for the GABAA receptor antagonist bicuculline (0.1 microM). Orthodromic-orthodromic paired-pulse paradigms were found to be unsuitable for investigating the effects of epileptogenic agents such as nicotine and bicuculline on endogenous inhibition.

5 alpha-Pregnan-3 alpha-ol-20-one blocks nicotine-induced seizures and enhances paired-pulse inhibition.

5 alpha-Pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) blocked seizures induced by nicotine (4 mg/kg, i.p.) in C3H male mice with an ID50 of 2.37 +/- 0.66 mg/kg (average +/- 95% confidence limit). This steroid (1 microM) also increased paired-pulse inhibition in the hippocampus approximately 40% after 50 min exposure; nicotine (200 microM) partially reversed this effect. Since nicotine and 3 alpha-OH-DHP may have opposite effects on endogenous inhibitory systems, it is proposed that nicotine-induced seizures may involve a disinhibitory mechanism and that 3 alpha-OH-DHP protects against seizures by preventing disinhibition.

Nicotine effects in mouse hippocampus are blocked by mecamylamine, but not other nicotinic antagonists.

Previous data indicated that bath-application of nicotine to mouse hippocampal slices resulted in a concentration-dependent increase in the amplitude of the orthodromic population spike and the appearance of multiple population spikes in the CA1 pyramidal cell layer. d-Tubocurarine (4-100 microM), alpha-bungarotoxin (10-160 microM), and atropine (40-200 microM) had similar effects, although for alpha-bungarotoxin these excitatory effects were transient. Mecamylamine (1.6-3.2 mM) inhibited the population spike, while hexamethonium (3.2 mM) had no effect. These cholinergic antagonists were tested for their ability to block excitatory effects of nicotine (800 microM) at antagonist concentrations which were at or near threshold for intrinsic effects. Of the 5 antagonists tested, only mecamylamine (400 microM) effectively inhibited the nicotine-induced increase of the population spike amplitude and the appearance of multiple population spikes. These results suggest that nicotine exerts electrophysiological effects via a subclass of nicotinic cholinergic receptors that is neither neuromuscular nor ganglionic in the classical sense; these brain nicotinic receptors are sensitive to mecamylamine, but not to hexamethonium, alpha-bungarotoxin, or D-tubocurarine.

Differential response to flurazepam in long-sleep and short-sleep mice.

In addition to differing in ethanol sensitivity, long-sleep (LS) and short-sleep (SS) mice also differ in response to GABAergic agents. In the present study the sensitivity of LS and SS mice to the anesthetic, hypothermic and anticonvulsant effects of benzodiazepine, flurazepam, was determined. Flurazepam (75-300 mg/kg) induced a dose-dependent loss of righting response in both lines. The LS line displayed a two-fold greater sensitivity to the anesthetic effects of flurazepam. A dose-dependent decrease in body temperature was also observed following administration of flurazepam (25-150 mg/kg), but the two lines did not differ on this measure. Determination of the anticonvulsant effects of flurazepam (1-6 mg/kg) against seizures induced by 3-mercaptopropionic acid revealed that the SS line was more sensitive to the anticonvulsant effects of this benzodiazepine. These studies demonstrate that LS and SS mice differ in response to flurazepam, but the nature of the difference depends on the type of response measured and the dose of flurazepam employed.

Genetic differences in plasma corticosterone levels in response to nicotine injection.

Changes in plasma corticosterone (CCS) levels following intraperitoneal injections of nicotine were measured in four inbred mouse strains: DBA/2Ibg, C57BL/6Ibg, C3H/2Ibg, and A/J. In all four strains, nicotine produced a dose-dependent (0.5-2.0 mg/kg nicotine) increase in plasma CCS levels which peaked 10-30 min after injection. Saline increased plasma CCS levels in C57BL, A, and C3H, but not in DBA mice. After correcting for plasma CCS levels produced by saline injection, the nicotine-induced rise in plasma CCS was significantly lower for the C57BL strain than for the other three strains tested. These mouse strains also varied in their responses to saline injection with the rank order: C57BL greater than A = C3H greater than DBA. However, the two most divergent strains (C57BL and DBA) did not differ in the effects of a cold water stress. The response to nicotine was completely inhibited by mecamylamine in two strains tested (C3H and C57BL) whereas the response to saline injection was unaffected, suggesting that only the response to nicotine was mediated by nicotinic receptors. It is clear that elevations in plasma CCS induced either by saline injection or by nicotine are influenced by genetic factors.