Lawrence Isaac

A novel interaction between dynorphin(1–13) and an N-methyl-d-aspartate site

Dynorphin injected intrathecally in the rat results in a neurotoxicity behaviorally expressed as an irreversible loss of the thermally evoked tail-flick reflex. The excitatory amino acid antagonists DL-2-amino-5-phosphonovalerate (APV) and gamma-D-glutamylglycine (DGG) blocked the loss of the tail-flick reflex. The order of potency (APV greater than DGG) suggests that the N-methyl-D-aspartate (NMDA) subclass of excitatory amino acid receptors participate in the neurotoxicity. Additionally, intrathecal injection of APV results in a reversible loss of the tail-flick reflex, whereas with DGG doses which block the tail-flick reflex also result in hindlimb paralysis. These data suggest that neurotransmission in the tail-flick reflex pathway is, in part, mediated by NMDA receptors. From these and previous findings it was concluded that dynorphin neurotoxicity results from enhanced, excitotoxic, transmission across these synapses utilizing NMDA receptors.

Intrathecal dynorphin (1–13) results in an irreversible loss of the tail-flick reflex in rats

Intrathecal injection of dynorphin produced a loss of the tail-flick reflex that lasted throughout the 14-day experimental period whereas, the inclined plane test of motor function and tail-shock vocalization recovered within an hour. An important aspect of the loss of the tail-flick reflex was that it was an all-or-none event. At any dose of tail-flick latency either remained unchanged when compared with pre-injection latencies or the latency was elevated to the cut off time of 14 s. The ED50s +/- S.E.M. for tail-flick, inclined plane and tail-shock vocalization were 65.4 +/- 5.0, 67.7 +/- 5.0 and 68.0 +/- 3.9 nmol respectively. Results from the hot-plate test revealed no statistical difference between saline and dynorphin injected animals one day following the injection. Animals injected with morphine sulphate s.c. lost the tail-flick reflex but completely recovered by 24 h. Histology of the spinal cord of animals treated with dynorphin 24 h prior to sacrifice revealed dead neurons primarily in the ventral horn with little or no damage in the dorsal horn. These data demonstrate that dynorphin(1-13) injected intrathecally results in a rather specific neurotoxic action in the spinal cord.

MK-801 blocks dynorphin A (1−13)-induced loss of the tail-flick reflex in the rat

Dynorphin A (1-13) administered intrathecally to rats results in a dose-dependent loss of the tail-flick reflex. This effect is mediated, at least in part, by N-methyl-D-aspartate receptors. We examined the influence of pretreatment or post-treatment with MK-801 on this behavioral response. MK-801 administered i.p. 30 min prior to dynorphin provided dose-dependent protection against loss of the tail-flick reflex with an ED50 of 0.06 mg/kg. MK-801 administered after dynorphin had a dose- and time-dependent protective action. The dose of 0.06 mg/kg protected 63% of the animals from loss of the tail-flick reflex when injected 15 min after dynorphin. In contrast, 3 mg/kg did not protect animals when injected 15 min after dynorphin, but did protect 50% of the animals when injected 30 min post-dynorphin. Although we cannot exclude other effects mediated by MK-801, these data support our previous findings that dynorphin-induced loss of the tail-flick reflex involves the N-methyl-D-aspartate-receptor complex and support the contention that the process(es) initiated by dynorphin injection proceed rapidly (minutes rather than hours).

Similar effects of treatment with desipramine and electroconvulsive shock on 5-tiydroxytryptamine1A receptors in rat brain

The effect of chronic and acute treatment with desipramine (DMI) and electroconvulsive shock (ECS) on 5-hydroxytryptamine1A (5-HT1A) receptors was determined in the cortex and the hippocampus brain regions of rats. We observed that chronic treatment with both DMI and ECS significantly decreased 5-HT1A receptors, as determined by [3H]8-hydroxy-2-(di-n-propylamino)tetralin [( 3H]8-OH-DPAT) binding, in the cortex but not in the hippocampus. Acute treatment with DMI or ECS did not significantly alter the 5-HT1A receptors in the cortex. Neither chronic nor acute treatment influenced KD of [3H]8-OH-DPAT binding in the cortex or hippocampus. These results thus suggest that in contrast to the effects on 5-HT2 receptors, tricyclics as well as ECS produce similar effects on 5-HT1A receptors, suggesting that this site may represent a common site of action for antidepressant treatment.

Prenatal exposure to diazepam results in enduring reductions in brain receptors and deep slow wave sleep

After prenatal exposure to diazepam (Valium), mature rats at 4 months of age displayed slow wave sleep (SWS) electroencephalographic patterns indicating impaired synchronization and SWS mechanisms. These animals spent a much greater portion of their SWS in the lighter SWS I, as compared to the control group which showed a predominance of the deeper SWS II. At one year of age, the diazepam-exposed rats had much fewer diazepam-specific binding sites in the thalamus than the vehicle-exposed controls. These results provide first evidence for a physiological role for benzodiazepine receptors by showing that prenatal exposure to diazepam has an enduring and detrimental effect on their ontogenesis and sleep mechanisms.

Localization of dynorphin-induced neurotoxicity in rat spinal cord

Intrathecally injected dynorphin A (1-13) in rats results in a reversible hindlimb paralysis and an irreversible loss of the tail-flick reflex. Histologic examination of the spinal cords of dynorphin treated rats demonstrated dead and/or dying neurons predominately localized in the central area which approximates Rexed lamina VII and X. In this area a maximum effect of the dynorphin-induced neurotoxicity is evident. Thus, the dynorphin-induced neuron death is suggestive of an anatomical selectivity.

Electroconvulsive shock increases endogenous monoamine oxidase inhibitor activity in brain and cerebrospinal fluid

Chronic daily administration of electroconvulsive shock (ECS) to cats resulted in a progressive elevation of seizure threshold which was accompanied by a sustained elevation in the activity of an endogenous monoamine oxidase inhibitor (EMAOI) present in cerebrospinal fluid (CSF). The increase in EMAOI activity in CSF following chronic ECS was observed maximally at 24-48 h. In rats, a single application of ECS resulted in a rapid but short-lasting increase in EMAOI activity present in the crude membrane fraction from brain. These findings demonstrate that both acute and chronic ECS modify the activity of an EMAOI in brain and CSF which may contribute to both the antidepressant and anticonvulsant effects of ECS treatment.

Effect of electroconvulsive shock on 5-HT2 and α1-adrenoceptors and phosphoinositide signalling system in rat brain

We studied the effect of repeated administration of electroconvulsive shock (ECS) on alpha 1-adrenoceptor subtype (alpha 1A and alpha 1B) and 5-HT2 (serotonin-2) receptors and receptor-mediated phosphoinositide (PI) hydrolysis in rat cerebral cortex. We observed that repeated administration with ECS significantly increased the density of 5-HT2 receptors, as labeled by [3H]ketanserin, as well as 5-HT-stimulated [3H]inositol-1-phosphate ([3H]IP1) in rat cerebral cortex. We also observed that repeated ECS administration caused a significant increase in the number of alpha 1-adrenoceptors and the alpha 1B-adrenoceptor subtype as measured by (+/-)-beta-([125I]iodo-4-hydroxyphenyl)-ethyl-aminomethyl-tetralone binding. However, it had no significant effects on norepinephrine (NE)-stimulated [3H]IP1 formation or alpha 1A-adrenoceptor subtype. These results thus suggest that up-regulation of 5-HT2 receptors after administration with ECS is associated with increased 5-HT-stimulated [3H]IP1 formation. The lack of effects on NE-stimulated PI turnover in ECS treated rats may be due to its lack of effect on the alpha 1A-adrenoceptor subtype.

Alteration of electroconvulsive threshold by cerebrospinal fluid from cats tolerant to electroconvulsive shock

Daily electroconvulsive shocks for 22 days resulted in a progressive elevation of electroconvulsive threshold. When these shocks were discontinued, the thresholds returned to untreated levels in approximately 20 days. Cerebrospinal fluid collected from cats with elevated thresholds and repeatedly transferred to the ventricular space of untreated animals elevated the electroconvulsive threshold of the recipient. This finding demonstrates that a substance present in the cerebrospinal fluid of cats with elevated electroconvulsive threshold can alter the threshold of untreated animals.

A strychnine-sensitive site is involved in dynorphin-induced paralysis and loss of the tail-flick reflex

Dynorphin A(1-13) administered intrathecally to rats induces a reversible hindlimb paralysis and permanent loss of the tail-flick reflex in a dose-dependent and all-or-none manner. The loss of the tail-flick reflex has been determined to result from neurotoxicity linked to the N-methyl-D-aspartate (NMDA) receptor. Recently, it has been reported that NMDA antagonists attenuate irreversible paralysis induced by dynorphin A(1-17) and dynorphin A(2-17). In the present studies, we examined whether repeated injections of dynorphin A(1-13) acetate salt could change the characteristics of the reversible paralysis. Injections repeated every 48 h resulted in hindlimb paralysis upon each injection which was not different in terms of magnitude or duration (P greater than 0.60). Injections repeated at 2 h intervals resulted in desensitization of the paralytic effects (P less than 0.05). We also examined if strychnine sulfate, a glycine antagonist would alter the paralytic response to dynorphin. Strychnine protected rats from paralysis (P less than 0.01) and loss of the tail-flick reflex with an ED50 of 7 nmol. We conclude that the reversible paralysis induced by dynorphin A(1-13) is repeatable which suggests that the paralysis results from nontoxic or subtoxic actions of dynorphin. Desensitization to the paralytic effects occurs with closely spaced injections by some unknown mechanism. In addition, we conclude that the spinal glycinergic inhibitory system may participate in the induction of the paralysis because strychnine antagonizes dynorphin-induced paralysis.(ABSTRACT TRUNCATED AT 250 WORDS)