Diazepam modification of evoked and spontaneous lateral geniculate activity

Benzodiazepines and the mammalian retina. II. Actions on retinal ganglion cells

The effects of intravenously and iontophoretically applied benzodiazepines, midazolam and flurazepam and their receptor antagonist, flumazepil (RO 15 1788), on visually evoked and spontaneous activities of the mammalian retinal ganglion cells have been studied. Intravenously applied midazolam and flurazepam suppressed both light-evoked and spontaneous firing of rat optic tract fibres. They reduced both the sensitivity to light and the temporal resolution of the fibres. Flumazepil (RO-15-1788), on the other hand, enhanced both light-evoked and spontaneous firing of the optic tract fibres. It increased their light sensitivity but did not affect their temporal resolution. In the cat, iontophoretically applied benzodiazepines suppressed and flumazepil increased the receptive field centre and surround response as well as the spontaneous firing of ON-type retinal ganglion cells which mainly receive GABAergic inputs. However, these drugs did not affect the activities of the OFF-type cells which mainly receive glycinergic inputs. These results suggest not only that the action of benzodiazepines on the retinal ganglion cells is mediated by benzodiazepine receptors that are linked with GABA receptors, but also that the retinal benzodiazepine receptors receive an endogenous benzodiazepine receptor ligand.

Visual evoked potentials in rats selected for high or low self-stimulation

Visual evoked potentials (VEPs) were analyzed in order to distinguish between rats from genetically high (HI) and low (LO) self-stimulation lines (LC2-HI and LC2-LO). Secondary VEP components - slow secondary negative wave (SNW) and sensory afterdischarge (SAD) - which are considered to be most sensitive indices of normal and pharmacologically-induced behavioral changes, were used for the comparison. Small, albeit statistically significant enhancement of SNW and SAD was obtained in LO rats. Unlike LO animals, HI rats gained in SNW amplitude and SAD area during repeated photic stimulation. The difference being highly significant. D,L-Amphetamine (1 mg/kg, i.p.) suppressed SAD and reduced the SNW amplitude in both HI and LO animals, although the predrug difference in their values remained practically unaltered. Apomorphine (0.25, 2.75, 5.25 mg/kg i.p.) had no measurable effect on VEP parameters even though it caused a regular picture of dose-related enhancement of locomotion and stereotypy. The effect of amphetamine can, therefore, be attributed to the activation of the norepinephrinergic system. Correspondingly, VEP variance in the two lines of rats is interpreted as related to the peculiarities of norepinephrine modulation of neocortical activity.

Pharmacologic analysis of sodium valproate-induced suppression of secondary components of visual evoked potentials in albino rats

Sodium Valproate (VPA) administered to rats in a dose of 10 or 200 mg/kg IP suppressed the slow negative wave (SNW) and photically-induced afterdischarge (SAD) of VEP (when they were present) within 15-30 min. The recovery of VEP amplitude began at 3 hr. This effect was antagonized by subconvulsive doses of convulsant benzodiazepine RO 5-3663 (2 mg/kg) and metrazol (15 mg/kg) but not by picrotoxin (2 mg/kg) and naloxone (10 mg/kg). The SNW suppression may be attributed to a disinhibitory action of a system located presynaptically on recurrent collaterals of the output neurons, or nerve terminals of inhibitory interneurons or both. Alternative conjecture suggests that VPA depolarizes the dendritic tree masking thereby somatic inhibition produced by recurrent circuits.

Effects of dipropylacetic acid on late components of the photically evoked potential and afterdischarge in rat

The effects of dipropylacetic acid (DPA) on primary, secondary and afterdischarge waves of the photically evoked potential were examined. It was observed that DPA selectively attenuated late wave components and afterdischarge. A thalamic locus of action for these effects is postulated.

Comparison of effects of bicuculline, strychnine, and picrotoxin with those of pentylenetetrazol on photically evoked afterdischarges

The effects of bicuculline, strychnine, and picrotoxin on photically evoked afterdischarges (PhADs) in rat visual cortex were compared with those of pentylenetetrazol (a known PhAD potentiator) and a diazepam (a known PhAD suppressor) challenge. Bicuculline was found to augment PhADs in a manner similar to that found with pentylenetetrazol, with the exception that potentiation only occurred at convulsive levels whereas pentylenetetrazol augmentation occurred at both subconvulsive and convulsive levels. Diazepam suppressed bicuculline-potentiated PhADs. Picrotoxin was found to have some limited augmenting effect on PhAD activity but in a manner unlike that observed with either bicuculline or pentylenetetrazol. Strychnine had no systematic augmenting effect on PhADs. These results were discussed in terms of the possible role of GABA in thalamic systems responsible for PhAD production.

Metrazol potentiated after-discharges: dose-response relationships and effects of selective lesions

In Experiment 1 the dose-response effects of pentylentetrazol (Metrazol) on photically evoked after-discharge (PhAD) parameters were examined. Metrazol potentiated PhAD activity by affecting all measured parameters - PhAD frequency, amplitude, burst duration and spindle composition - in particular PhAD burt duration and spindle composition. A mimimum effective dose of 10 mg/Kg of Metrazol was required for some statistically reliable potentiation. Metrazol dosage levels of 20 and 25 mg/Kg induced lengthy bouts of EEG spindling and spiking as well as near maximized PhAD component augmentation. In Experiment 2 stereotaxically oriented knife-cuts isolated the thalamus from cortical and/or midbrain and brainstem input. Such lesions did not block the capacity of Metrazol to potentiate PhADs, although the lesions altered evoked activity. These findings are discussed in terms of the current thought of Metrazol action and thalamic mechanisms in the control of after-discharge activity.

Principal cells in lateral geniculate: effects of metrazol on capacity to after-discharge

The effect of metrazol on the capacity of dorsal lateral geniculate nucleus (dLGN) principal (P) cells to repetitively burst (after-discharge) following visual system stimulation was examined in P cells which differed in terms of response patterns to visual stimulation (latency of initial spike, extent of repetitive bursting, and on- or off-type responding cells). Twenty-seven P cells were examined in as many subjects. Metrazol augmented repetitive bursting irrespective of the type of P cell as long as repetitive bursting was present in the pre-metrazol period. P cells that displayed only a single initial burst to photic stimulation did not exhibit after-discharge bursting during the metrazol challenge. In all but one cell metrazol enhanced baseline firing rate. These results are discussed in terms of the putative nature of inhibition in the rat dLGN.