John G. Sinclair

Ethanol blocks tetanic and calcium-induced long-term potentiation in the hippocampal slice

Long-term potentiation (LTP) of the CA1 population spike in the hippocampal slice was produced by a tetanic stimulus in the stratum radiatum or by doubling the calcium concentration in the medium to 4 mM for 10 min. Ethanol (0.1 M) antagonized both tetanic- and ethanol-induced LTP. Since the hippocampus is known to be involved with memory, and ethanol interferes with memory and the production of LTP, the results are consistent with the notion that LTP may be related to memory processes.

Noxious stimuli produce prolonged changes in the CA1 region of the rat hippocampus

&NA; Limbic structures including the hippocampus are thought to be involved in pain though not much is known of their neuronal responses to noxious stimuli. In this report we show that a prolonged and substantial depression of the dorsal hippocampal CA1 pyramidal cell population spike is produced by a brief but intense noxious stimulus applied to the tail of lightly anaesthetized rats. This depression is temperature‐dependent and habituates to subsequent noxious stimuli applied more than 1 h later. Further, the depression is absent when noxious heat is applied in the presence of hippocampal theta rhythm.

Responses in the CA1 region of the rat hippocampus to a noxious stimulus

We report here some physiological and pharmacological characteristics of noxious stimuli-induced changes in the hippocampal CA1 pyramidal cell synaptic excitability to field CA3 stimulation. A noxious heat stimulus applied to the left hind paw (LHP) produced a persistent depression of the CA1 population spike (PS) which habituated to a repetition of the stimulus. Interestingly, exposure of the tail to a noxious stimulus following habituation of the LHP produced a depression of the CA1 PS. This finding suggested that persistent depression and habituation are topographically represented. In separate experiments we determined that while the persistent depression of the CA1 population spike was accompanied by, in most cases, a prolonged increase in the amplitude of the CA1 antidromic field potential, there was a concurrent persistent depression and habituation of the CA1 PS and the corresponding apical dendritic field excitatory postsynaptic potential (dfEPSP). This suggested that noxious stimulus-induced CA1 synaptic depression is mediated at the apical dendritic region, perhaps postsynaptically at the dendrites and/or presynaptically on CA3 afferent terminals. Furthermore, atropine sulfate (40 mg/kg ip), which prevented the depression of the CA1 PS, also blocked the depression of dfEPSP when iontophoresed at the apical dendritic recording site. In addition atropine antagonized the depression of the dfEPSP produced by iontophoretic acetylcholine (Ach) but not gamma-aminobutyric acid. However, iontophoretic atropine at the cell body recording site did not prevent the depression of the CA1 PS. These results are consistent with the notion that Ach release in the apical dendrites of CA1 pyramidal cells following a noxious stimulus depresses CA1 synaptic excitability.

Spinal vs. supraspinal actions of morphine on the rat tail-flick reflex

&NA; A controversy exists as to whether morphine attenuates spinal cord nociceptive transmission through a supraspinal site of action. The approach of examining the effects of morphine on spinal cord nociceptive transmission in the presence and absence of spinal cord conduction has led to conflicting conclusions. We have compared the effects of morphine on the rat tail‐flick reflex (TFR) in lightly anaesthetized animals in the presence and absence of a spinal cord cold‐block. Morphine, administered systemically, was found to be more potent in increasing the latency of the reflex when the spinal cord conduction was present. However, when low doses of morphine were injected intrathecally, morphine was more potent when spinal cord conduction was blocked. These data indicate that systemically administered morphine, at low doses, has a supraspinal site of action in prolonging the onset of the TFR. Conflicting results on this issue do not appear to be due to plasticity changes following spinal cord section or lesions, psychological stress in conscious animals or the presence of tonic bulbospinal inhibition.

Tonic inhibitory influence of a supraspinal monoaminergic system on presynaptic inhibition of an extensor monosynaptic reflex.

Presynaptic inhibition of the extensor (quadriceps, QUAD) monosynaptic reflex (MSR) in unanaesthetized decerebrate cats was antagonized by imipramine hydrochloride (2-5 mg/kg), 5-hydroxytryptophan (75 mg/kg) and a specific 5-hydroxytryptamine (5-HT) neuronal uptake blocker, fluoxetine hydrochloride (Lilly 110140, 0.25-6 mg/kg). These effects of imipramine and fluoxetine were partially reversed by the 5-HT antagonist, cyproheptadine hydrochloride (5 mg/kg), and completely reversed by the application of a thoracic cold block which prevents supraspinal inputs to the caudal spinal cord. Imipramine, however, failed to antagonize this inhibition in animals pretreated with either DL-p-chlorophenylalanine (p-CPA, 300 mg/kg i.p. for 2 consecutive days) or DL-a-methyl-p-tyrosine methyl ester hydrochloride (a-MPt, 125 mg/kg i.p. 16 and 4 h prior to the experiment). Cyproheptadine (2.5--5 mg/kg); phenoxybenzamine hydrochloride (2.5-5 mg/kg) and a cold block enhanced the inhibition of this extensor MSR but a cold block failed to alter the inhibition in animals pretreated with p-CPA or a-MPT. Presynaptic inhibition of the flexor (posterior biceps-semitendinosus, PBST) MSR was however not blocked by imipramine, fluoxetine or a cold block nor enhanced by cyproheptadine or phenoxybenzamine. The effects of the drugs tested and a cold block on the excitability of the QUAD group Ia afferents were reciprocal to those on the MSR during presynaptic inhibition. The results of this study indicate that descending tonically active systems (1) involving 5-HT and noradrenaline, antagonize presynaptic inhibition of the QUAD but not the PBST-MSR, (2) decrease the excitability of the QUAD Ia afferents and (3) increase the excitability of QUAD motoneurones.

Ethanol-GABA interactions at the rat Purkinje cell

Single unit cerebellar Purkinje cell activity was recorded extracellularly in urethane-anaesthetised rats. An inhibition of these cells, believed to be GABA-mediated, which is produced by local surface stimulation of the cerebellar cortex was antagonized by the local, micropressure application of ethanol. The inhibition of cerebellar Purkinje cells produced by local micropressure application of GABA was similarly antagonized by a slow i.v. infusion of ethanol (1.5 g/kg over 10 min). When both ethanol and GABA were applied to the Purkinje cells by local micropressure the results were difficult to interpret due to an ethanol-induced decrease in the baseline firing rate. There was a decrease in the absolute GABA-mediated inhibition but no change in the relative inhibition. In general, it appears that ethanol produces an antagonism of GABA-mediated inhibition of cerebellar Purkinje cells.

Spinal vs supraspinal actions of morphine on cat spinal cord multireceptive neurons

To examine whether morphine elicits a supraspinal mediated spinal inhibition of nociceptive transmission, several investigators have compared the effects of morphine on nociceptive transmission in animals with the spinal cord intact vs transected or cold-blocked. The results have been conflicting, possibly due to different methods of analysis. For example, some investigators have found i.v. administered morphine produces a greater percentage decrease in nociceptive transmission when the spinal cord is intact compared to the transected state. Therefore, they concluded that morphine elicits a supraspinal-mediated inhibition. Conversely, others have reported that the increase in noxious stimulus-evoked responses of dorsal horn neurons upon cold blocking the spinal cord was reduced by i.v. morphine. They therefore concluded that morphine decreases descending inhibition. We tested the effects of i.v. morphine on spinal cord multireceptive neurons in the presence and absence of descending inhibition. Using the above methods of analysis, our results were found to be consistent with their findings which indicate that the method of analysis used is critical to the interpretation reached. To determine how these calculations would be affected by a depressant effect on the spinal cord neurons only, we performed similar experiments iontophoresing gamma-aminobutyric acid (GABA) onto these dorsal horn neurons. The similarity between the morphine and GABA data suggests that the effects of systemically administered morphine on multireceptive dorsal horn neurons can be adequately explained by a spinal cord site of action.

Serotonin involvement in the blockade of bulbospinal inhibition of the spinal monosynaptic reflex

Bulbospinal inhibition of the extensor quadriceps monosynaptic reflex (MSR) was antagonized by the serotonin precursor, 5-hydroxytryptophan (5-HTP, 75 mg/kg), in unanesthetized, mid-collicular, decerebrate cats. Fluoxetine HCl (Lilly 110140, 0.25 - 6 mg/kg), a specific serotonin neuronal uptake blocker, also blocked this inhibition as well as bulbospinal inhibition of the flexor posterior biceps-semi-tendinosus MSR. The serotonin antagonist, cyproheptadine HCl (5 mg/kg), partially reversed the above blocking actions of 5-HTP and fluoxetine and enhanced bulbospinal inhibition when administered alone in doses of 2.5-5 mg/kg. Imipramine HCl (0.125 - 4 mg/kg) was more potent in antagonizing bulbospinal inhibition of the dorsal root-ventral root MSR when administered intra-arterially to the spinal cord than when injected intra-arterially to the brain stem or intravenously, indicating that the spinal cord is the site of imipramines action. These results support our earlier proposal that a 5-HT system antagonizes bulbospinal inhibition of the MSR. They also indicate that the 5-HT system is tonically active and exerts its blocking action in the spinal cord.

Evidence that noradrenaline reduces tonic descending inhibition of cat spinal cord nociceptor-driven neurones

Abstract To determine whether noradrenaline (NA) is involved in the powerful tonic descending inhibition which exists on dorsal horn nociceptor‐driven neurones, their response to noxious radiant heat was tested with drugs that decrease (reserpine) or enhance (nisoxetine, desipramine) NA synaptic transmission. In animals that were depleted of NA by reserpine (1.0 mg/kg i.p.) the degree of tonic inhibition (as determined by comparing the response in the normal vs. cold block states of the cord) was greater when compared to controls. Conversely, the extent of tonic descending inhibition on these neurones was decreased after the intravenous administration of NA uptake blockers, nisoxetine HC1 and desipramine HC1 (6 mg/kg). Thus we conclude that NA is not involved in mediating, but rather appears to reduce this tonic descending inhibition.

Morphine, but not atropine, blocks nociceptor-driven activity in rat dorsal hippocampal neurones.

Neurones in the CA1 region of the dorsal hippocampus of urethane-anaesthetized rats were recorded extracellularly with microelectrodes, and tested for their response to non-noxious and noxious peripheral stimuli. None of the cells responded to non-noxious stimuli but 91 of 216 cells were excited by noxious stimuli. Morphine (5.0 mg/kg i.v.) increased the background firing rate in 7 of 10 cells tested and blocked the response to noxious heating of the tail in all cases. Naloxone (0.5 mg/kg i.v.) reversed these effects of morphine. Atropine (0.1 plus 0.2 mg/kg i.v.), in another group of animals, produced only transient changes of an inconsistent nature in 7 cells tested.