Biphasic effects of glycine on synaptic responses of the frog optic tectum in vitro

Distribution of GABA, glycine, and glutamate in neurons of the medulla oblongata and their projections to the midbrain tectum in plethodontid salamanders

In the medulla oblongata of plethodontid salamanders, GABA-, glycine-, and glutamate-like immunoreactivity (ir) of neurons was studied. Combined tracing and immunohistochemical experiments were performed to analyze the transmitter content of medullary nuclei with reciprocal connections with the tectum mesencephali. The distribution of transmitters differed significantly between rostral and caudal medulla; dual or triple localization of transmitters was present in somata throughout the rostrocaudal extent of the medulla. Regarding the rostral medulla, the largest number of GABA- and gly-ir neurons was found in the medial zone. Neurons of the nucleus reticularis medius (NRM) retrogradely labeled by tracer application into the tectum revealed predominantly gly-ir, often colocalized with glu-ir. The NRM appears to be homologous to the mammalian gigantocellular reticular nucleus, and its glycinergic projection is most likely part of a negative feedback loop between medulla and tectum. Neurons of the dorsal and vestibular nucleus projecting to the tectum were glu-ir and often revealed additional GABA- and/or gly-ir in the vestibular nucleus. Regarding the caudal medulla, the highest density of GABA- and gly-ir cells was found in the lateral zone. Differences in the neurochemistry of the rostral versus caudal medulla appear to result from the transmitter content of projection nuclei in the rostral medulla and support the idea that the rostral medulla is involved in tecto-reticular interaction. Our results likewise underline the role of the NRM in visual object selection and orientation as suggested by behavioral studies and recordings from tectal neurons.

Glycine induces a novel form of long-term potentiation in the superficial layers of the superior colliculus

1. The mammalian superior colliculus (SC) is a midbrain nucleus containing space maps of different sensory modalities which show various forms of age- and activity-dependent plasticity in vivo and in vitro. In the present study, we aimed to characterize the role of glycine (Gly) receptors in the SC, and we observed that application of glycine (Gly; 500 microM and 3 mM) for 7 min to SC slices of adult guinea-pigs caused a novel form of long-term potentiation (termed LTPgly) of evoked excitatory postsynaptic potentials recorded in the superficial layers. 2. The strength of potentiation was found to be concentration-dependent and partially independent from synaptic stimulation. 3. LTPgly did not involve NMDA receptor activation as proven by the lack of inhibition by 100 microM D,L-2-amino-5-phosphonovaleric acid (APV) and 10 microM MK-801. 4. LTPgly could only be masked but not prevented by strychnine (100 microM) and remained undisturbed in the presence of picrotoxin (100 microM). 5. Inhibition of carbonic anhydrase by acetazolamide (20 microM) had no effect on LTPgly suggesting that the excitatory action of Gly is not due to a differential breakdown of the Cl-/HCO3 gradients. 6. As indicated by the inhibition of LTPgly of the fEPSP slope by the L-type calcium channel blocker nifedipine (20 microM), voltage-dependent calcium channels are the source for Ca2+ elevation as the intracellular trigger. 7. Our data provide the first evidence for a role of Gly in SC synaptic transmission. They illustrate a so far unknown action of Gly which can lead to long-lasting changes of synaptic efficacy and which is not mediated via NMDA-related or strychnine-sensitive binding sites.

Immunocytochemical evidence for the involvement of glycine in sensory centers of the rat brain

Glycine-like immunoreactivity was localized to a number of sites in the rat brain which are involved in processing sensory information. In the auditory and vestibular systems, glycine immunoreactivity was seen in dorsal and ventral cochlear nuclei, superior olive, trapezoid body, medial and lateral vestibular nuclei, and inferior colliculus. Staining in the visual system was seen in retina, dorsal lateral geniculate nucleus, and superior colliculus. The olfactory system exhibited staining in the olfactory bulb and accessory olfactory formation. Somatosensory centers with glycine immunoreactivity included the dorsal column nuclei, spinal trigeminal nucleus, principal sensory nucleus of V, reticular formation, and periaqueductal gray. Glycine-immunoreactive neurons were also seen in cerebellar cortex, deep cerebellar nuclei, hippocampus, cerebral cortex, and striatum. The distribution of staining indicates that glycine plays a major role in sensory centers with actions at both strychnine-sensitive and strychnine-insensitive receptors.

The distribution and function of gamma-aminobutyric acid (GABA) in the superior colliculus

Laminer analysis of the distribution of GABA and GAD in the superior colliculus has shown that the distribution pattern of GABA within the SC is similar in rabbit, cat, and guinea pig. The highest levels of GABA were found in the superficial gray layer (SGL), averaging 37-40 mmol/kg dry weight. The GABA concentrations in the deep layers were each only half that of the levels in the SGL. The concentrations of both GABA and GAD in the upper half of SGL are the same as those in the substantia nigra and medial forebrain bundle which have the highest amounts of GABA in the CNS. Denervation studies of the fibers projecting to SGL suggest that the GABA concentrated in the SGL is intrinsic to the layer. The results obtained from immunohistochemical and electron microscopic studies on the localization of GABA neurons corresponds well with the regional distribution pattern of GABA and GAD reported here. However, pharmacological and electrophysiological studies do not necessarily accord well with the GABA distribution studies because they indicate that there are many GABA sensitive neurons in both the SGL and DGL. To investigate the role of GABA in the SGL, the effect of GABA and its agonists and antagonists on neurotransmission in SGL has been studied in SC slices in a perfusion system. Bath applied GABA (100 microM to 1 mM) enhanced the amplitude of postsynaptic field potentials (PSP) in SGL in a dose-dependent fashion and at concentrations above 1 mM it depressed the PSP in a dose-dependent fashion. A similar response pattern was obtained with muscimol (0.1-10 microM excitation; greater than 10 microM inhibition). However (-)-baclofen only inhibited the PSP. Bicuculline (1 microM) shifted the dose-response inhibitory curve of GABA to the right, while the excitatory effect was enhanced. These results indicate that GABA has an excitatory and inhibitory action on neurotransmission in the SGL. The nigro-tectal GABAergic fibers terminate in the intermediate and deep layers of SC. Inhibition of GABAergic activity in the SC causes irrepressible saccades made toward the center of the movement field while GABA activation delays and slows saccadic eye movements. Thus, GABA in the SC plays an important role in the control of eye movements. The same GABAergic projection is also related to the propagation of generalized seizures. There exist collicular neurons which suppress the propagation of seizures.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of GABA on the frog optic tectum is sensitive to ammonium and to penicillin

Excitatory postsynaptic potentials (termed U1 and U2) were extracellularly recorded from the frog optic tectum in vitro following electrical stimulation of the contralateral optic nerve. Gamma-Aminobutyric acid (GABA) and glycine elicited a large and sustained enhancement of these synaptic waves. In the presence of the Cl- transport inhibitor ammonium (NH+4) the effects of GABA or glycine were progressively reduced to about 50% of their initial action without changes in the control synaptic waves. In 50% Cl- media the depression of GABA and glycine responses by NH+4 was more intense. Other Cl- transport inhibitors such as bumetanide, piretanide and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonate (SITS) were inactive against responses to GABA or glycine. Penicillin, a Cl- channel blocker, antagonized the action of GABA and glycine, while increasing the amplitude of the U2 waveform. The present results provide pharmacological evidence in support of the Cl- dependence of the unusual action of GABA or glycine in facilitating excitatory synaptic transmission in the optic tectum.

The effect of GABA on neurotransmission in frog tectal slices

The regional distribution of gamma-aminobutyric acid (GABA) was determined in the central nervous system of the frog. The highest amount of GABA was found in the tectum (35.7 +/- 2.3 mmol/kg protein). A thin tectal slice (400-600 microns thick) preparation was developed in which the postsynaptic potential (PSP) could be elicited. The addition of GABA to the perfusion medium depressed the PSP in a dose-dependent manner in the concentration range 1-12.5 mM at 28 degrees C. Nipecotic acid, a GABA uptake inhibitor, at a concentration of 0.5 mM shifted the dose-response curve for GABA to the left. These results from the frog, a cold-blooded animal, were in good agreement with those for hippocampal slices from the guinea-pig.

Excitatory and inhibitory action of GABA on synaptic transmission in slices of guinea pig superior colliculus

The laminar distribution of gamma-aminobutyric acid (GABA) in the superior colliculus of the guinea pig was analyzed. The superficial gray layer, especially the upper half, was found to contain the highest amount, 37.4 +/- 1.1 mmol/kg dry. To investigate the role of GABA in the superficial gray layer, the effect of GABA on neurotransmission in the superficial gray layer was studied in superior colliculus slices in a perfusion system. Bath-applied GABA, 100 microM-1 mM, enhanced the amplitude of the postsynaptic field potential (PSP) in the superficial gray layer dose dependently and at concentrations above 1 mM it depressed the PSP dose dependently. A similar response pattern was obtained with muscimol (0.1-10 microM, enhancement; greater than 10 microM, depression). However, (-)-baclofen only inhibited the PSP dose dependently (0.1-1 microM). The excitatory effect of GABA was enhanced at concentrations of nipecotic acid less than 0.5 mM. Bicuculline 1 microM shifted the dose-response curve of GABA to the right and the excitatory effect was also enhanced. These results indicate that GABA has a dual action on neurotransmission in the superficial gray layer: an excitatory effect, possibly mediated by GABAA receptors and an inhibitory effect mediated by both GABAA and GABAB receptors.

Complex effects of baclofen on synaptic transmission of the frog optic tectum in vitro

In the frog optic tectum in vitro, low concentrations of the GABAB agonist (-)-baclofen reversibly depressed field potentials evoked by optic nerve stimulation. This effect, which was dose-dependent and stereo-specific, differed from the potent facilitatory action of bath-applied GABA. A transient enhancement in field potential amplitude often preceded the more consistently observed depression. Both phases of the baclofen response were insensitive to the GABAA antagonist picrotoxin. Since the effects of baclofen and GABA were different, it is suggested that GABAB receptors were unlikely to mediate the action of GABA on synaptic transmission in the optic tectum.