Optic tract stimulation evokes GABAA but not GABAB IPSPs in the rat ventral lateral geniculate nucleus

Feedforward excitation and inhibition evoke dual modes of firing in the cat's visual thalamus during naturalistic viewing

Thalamic relay cells transmit information from retina to cortex by firing either rapid bursts or tonic trains of spikes. Bursts occur when the membrane voltage is low, as during sleep, because they depend on channels that cannot respond to excitatory input unless they are primed by strong hyperpolarization. Cells fire tonically when depolarized, as during waking. Thus, mode of firing is usually associated with behavioral state. Growing evidence, however, suggests that sensory processing involves both burst and tonic spikes. To ask if visually evoked synaptic responses induce each type of firing, we recorded intracellular responses to natural movies from relay cells and developed methods to map the receptive fields of the excitation and inhibition that the images evoked. In addition to tonic spikes, the movies routinely elicited lasting inhibition from the center of the receptive field that permitted bursts to fire. Therefore, naturally evoked patterns of synaptic input engage dual modes of firing.

Nature of inhibitory postsynaptic activity in developing relay cells of the lateral geniculate nucleus

Using intracellular recordings in an isolated (in vitro) brain stem preparation, we examined the inhibitory postsynaptic responses of developing neurons in the dorsal lateral geniculate nucleus (LGN) of the rat. As early as postnatal day (P) 1-2, 31% of all excitatory postsynaptic (EPSP) activity evoked by electrical stimulation of the optic tract was followed by inhibitory postsynaptic potentials (IPSPs). By P5, 98% of all retinally evoked EPSPs were followed by IPSP activity. During the first postnatal week, IPSPs were mediated largely by GABA(A) receptors. Additional GABA(B)-mediated IPSPs emerged at P3-4 but were not prevalent until after the first postnatal week. Experiments involving the separate stimulation of each optic nerve indicated that developing LGN cells were binocularly innervated. At P11-14, it was common to evoke EPSP/IPSP pairs by stimulating either the contralateral or ipsilateral optic nerve. During the third postnatal week, binocular excitatory responses were encountered far less frequently. However, a number of cells still maintained a binocular inhibitory response. These results provide insight about the ontogeny and nature of postsynaptic inhibitory activity in the LGN during the period of retinogeniculate axon segregation.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.

An oriented framework of neuronal processes in the ventral lateral geniculate nucleus of the rat demonstrated by NADPH diaphorase histochemistry and GABA immunocytochemistry

This study investigated the morphology and quantitative distribution of neurons containing NADPH diaphorase activity in the ventral lateral geniculate nucleus of the rat. The pattern of diaphorase staining revealed a strongly reactive lateral subdivision and a weakly staining medial subdivision. A characteristic feature of the diaphorase staining in the lateral part was its "stripe-like" appearance. These "diaphorase stripes" resulted from regions of strong somatic and neuropil diaphorase activity lying between unstained fibre bundles coursing dorsoventrally through the nucleus. Two distinct populations of diaphorase reactive cell types were present--class A and class B neurons. The ratio of class A to class B diaphorase neurons was approximately 14:1 (A:B). Diaphorase reactive neurons made up 73% of the total neuron population in the lateral subdivision, and 31% in the medial subdivision. A third population of cells was found exclusively in the optic tract--class C neurons. Quantitative analyses in the coronal and sagittal planes indicated that the principal processes of both class A and class B neurons were oriented preferentially--either parallel with, or perpendicular to the outlying optic tract. Diaphorase enzyme histochemistry in combination with GABA immunocytochemistry demonstrated the co-localization of GABA immunoreactivity in the majority of class B neurons, whereas class A and class C neurons were GABA immunonegative. Furthermore a large population of GABA-immunoreactive neurons was present that were not stained for diaphorase activity. From this and previous studies, it can be concluded that a high proportion of the diaphorase reaction class A neurons are geniculotectal projection cells, while diaphorase reaction class B neurons represent a numerically small subpopulation of "local-circuit" inhibitory neurons. Since diaphorase activity co-localizes with nitric oxide synthase, the results indicate the likely involvement of nitric oxide in the neuronal operations of both subpopulations of geniculotectal projection neurons and "local-circuit" GABAergic neurons in the rat's ventral lateral geniculate nucleus.

Intracellular electrophysiological study of suprachiasmatic nucleus neurons in rodents: inhibitory synaptic mechanisms

1. The mechanisms responsible for evoked and spontaneous fast inhibitory postsynaptic potentials (IPSPs) in the hypothalamic suprachiasmatic nucleus (SCN) were studied with intracellular recording. SCN neurons, primarily ones identified as receiving excitatory optic nerve input, were recorded in rat and guinea-pig brain slice preparations maintained in vitro. 2. In normal medium, electrical stimulation of a site dorsocaudal to the SCN evoked IPSPs from thirty-three of thirty-six neurons. The evoked IPSPs rose to the peak quickly (8.7 +/- 0.9 ms, mean +/- S.E.M.; n = 15 neurons) and decayed gradually with a time constant of 25 +/- 3 ms. Spontaneous IPSPs were present in each of the thirty-six neurons. These IPSPs had a rise-to-peak time of 7.2 +/- 1.0 ms (n = 6 neurons) and a decay time constant of 14 +/- 5 ms. 3. When recorded with potassium acetate-filled electrodes, the evoked and spontaneous IPSPs were hyperpolarizing at resting membrane potential (less negative than -70 mV) and had a reversal potential of around -75 mV. On the other hand, when recorded with potassium chloride-filled electrodes, the IPSPs were depolarizing at membrane potentials more negative than -50 mV and had an estimated reversal potential less negative than spike threshold. 4. Bath application of bicuculline (10-50 microM), a gamma-aminobutyric acidA (GABAA) receptor antagonist, resulted in a complete blockade of both the evoked (n = 16) and spontaneous (n = 13) IPSPs. The bicuculline effects were reversible, and were not associated with any significant and consistent change in baseline membrane potential or input resistance. The neurons impaled in bicuculline-containing medium (n = 11) exhibited neither spontaneous IPSPs nor evoked IPSPs. 5. In some neurons bicuculline-resistant hyperpolarizing potentials, which were similar to the fast IPSPs in time course, occurred spontaneously or were evoked by electrical stimulation of the optic nerve or the dorsocaudal site. A fast prepotential always preceded the hyperpolarizing potential, and hyperpolarizing currents blocked these events, indicating that they were not synaptic in origin. No slow IPSPs were detected. 6. The results suggest that fast IPSPs from non-retinal afferents exist in virtually all SCN neurons receiving excitatory retinal input, and that GABAA receptors associated with Cl- channels mediate the fast IPSPs.

Adenosine promotes burst activity in guinea-pig geniculocortical neurones through two different ionic mechanisms

1. The mechanisms of action of adenosine were examined in relay neurones of the dorsal lateral geniculate nucleus (LGND) using in vitro intracellular recording techniques in guinea-pig thalamic slices. 2. Adenosine hyperpolarized LGND relay neurones due to an increase in membrane potassium conductance. The K+ currents generated by near maximal stimulation of adenosine and GABAB receptors were non-additive. 3. Blockage of membrane K+ conductances by barium unmasked a second response to adenosine; an outward shift of the current versus voltage relationship negative to -65 mV associated with an increase in membrane input resistance. The beta-adrenoceptor agonist isoprenaline elicited an inward current in the same voltage range, which was inhibited and replaced by an outward current during activation of adenosine receptors. The effects of adenosine were due to a decrease in amplitude and rate of rise of the hyperpolarization-activated cation current, Ih. Maximal reduction by 66% of Ih amplitude occurred near the range of half-activation. 4. Both responses to adenosine were mimicked by the selective A1 receptor agonists N6-cyclopentyladenosine or N6-cyclohexyladenosine, and reversibly blocked by the selective A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). 5. The decrease in Ih by adenosine may be mediated by an inhibition of adenylyl cyclase activity and hence a decrease in the intracellular level of cyclic AMP, since local application of the adenylyl cyclase inhibitor 2',3'-dideoxyadenosine imitated the decrease in Ih. Local application of the adenylyl cyclase stimulant forskolin or 8-bromo-cyclic AMP resulted in an enhancement in Ih, and forskolin inhibited the action on Ih evoked by N6-cyclopentyladenosine. 6. The adenosine-induced effects interacted with the intrinsic electrophysiological properties of LGND neurones in that (i) the hyperpolarization due to an increase in K+ conductance inhibited single spike firing and promoted calcium-mediated burst discharges, and (ii) the decrease in Ih inhibited the dampening effect on Ca(2+)-mediated rebound activity of beta-adrenergic receptor stimulation. 7. It is suggested that during increased levels of extracellular adenosine the response of LGND relay neurones to activating brainstem influences will be depressed, and a pattern of Ca(2+)-mediated burst firing will be favoured.

Paroxysmal inhibitory potentials mediated by GABAB receptors in partially disinhibited rat hippocampal slice cultures

1. Intracellular recording techniques were used to study synaptic potentials in CA3 pyramidal cells elicited with mossy fibre stimulation in partially disinhibited hippocampal slice cultures. Two experimental protocols were used: (1) high concentrations (20-40 microM) of the A-type gamma-aminobutyric acid (GABAA) receptor antagonist bicuculline plus low concentrations (2-4 microM) of the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), or (2) low concentrations (1-2.5 microM) of bicuculline alone. 2. Under the first condition, stimulation of mossy fibre afferents evoked epileptic bursts alternating with a response consisting of an excitatory postsynaptic potential (EPSP) followed by an unusually large and long-lasting hyperpolarizing potential with a maximal amplitude in the range of -30 mV from the resting membrane potential. 3. This paroxysmal inhibitory potential (PIP) had a reversal potential near that of potassium. The amplitude of the PIP was not dependent on action potentials superimposed on the preceding EPSP, and was present in cells recorded with microelectrodes containing the Ca2+ chelator EGTA. These data suggest that the PIP is not a Ca(2+)-activated K+ potential. 4. The PIP was prolonged by the GABA-uptake blocker nipecotic acid, was reduced by hyperpolarizing interneurons with the opioid agonist FK 33-824, and was abolished by the GABAB-receptor antagonist CGP 35 348. These data indicate that the PIP is mediated by the activation of GABAB receptors following GABA release from interneurons. 5. The NMDA-receptor antagonist D-2-amino-5-phosphonovalerate (D-APV) strongly reduced the amplitude of the PIP, but had no effect on the GABAB receptor-mediated inhibitory postsynaptic potential (IPSP) under control conditions. 6. Under the first condition, regular stimulation elicited a cyclical pattern of evoked responses. There was either an alternation between an epileptic burst and a PIP or, at shorter interstimulus intervals, a sequence of gradually increasing PIPs followed by an epileptic burst, which then reset the cycle. 7. Under the second condition, in low concentrations of bicuculline alone, the early GABAA-mediated IPSP was little affected, but the late GABAB-mediated IPSP was greatly enhanced. These enhanced late IPSPs were comparable in amplitude and duration to the PIPs seen under the first conditions, could exhibit cyclical behaviour, and were reduced by D-APV. 8. Application of CGP 35 348 abolished the late IPSP under control conditions, but had no effect on hippocampal excitability. In contrast, CGP 35 348 blocked the PIP elicited in low bicuculline, and consequently led to intense epileptic discharge.(ABSTRACT TRUNCATED AT 400 WORDS)

Low-frequency oscillatory activities intrinsic to rat and cat thalamocortical cells

1. Low-frequency membrane potential oscillations recorded intracellularly from thalamocortical (TC) cells of the rat and cat dorsal lateral geniculate nucleus (dLGN) and of the rat ventrobasal nucleus (VB) maintained in vitro were investigated. On the basis of their electrophysiological and pharmacological properties, four types of activity were distinguished and named: the pacemaker oscillations, the spindle-like oscillations, the 'very slow' oscillations and the 'N-methyl-D-aspartate' (NMDA) oscillations. 2. The pacemaker oscillations (95 out of 173 cells) consisted of rhythmic, large-amplitude (10-30 mV) depolarizations which occurred at a frequency of 1.8 +/- 0.3 Hz (range, 0.5-2.9 Hz) and could often give rise to single or a burst of action potentials. Pacemaker oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV, but in a given cell the upper and lower limits of this voltage range were separated by only 13.1 +/- 0.5 mV. Above -45 mV tonic firing consisting of single action potentials was seen in the cells showing this or the other types of low-frequency oscillations. 3. The spindle-like oscillations were observed in thirty-nine (out of 173) TC cells and consisted of rhythmic (2.1 +/- 0.3 Hz), large-amplitude depolarizations (and often associated burst firing) similar to the pacemaker oscillations but occurring in discrete periods every 5-25 s and lasting for 1.5-28 s. The spindle-like oscillations were observed when the membrane potential was moved negative to -55 and positive to -80 mV and in two cells they were transformed into continuous pacemaker oscillations by depolarization of the membrane potential to -60 mV. 4. Pacemaker and spindle-like oscillations were unaffected by tetrodotoxin (TTX) or by selective blockade of NMDA, non-NMDA, GABAA, GABAB, nicotinic, muscarinic, alpha- and beta-noradrenergic receptors. 5. The 'very slow' oscillations consisted of a TTX-insensitive, slow hyperpolarization-depolarization sequence (5-15 mV in amplitude) which lasted up to 90 s and was observed in nine dLGN cells and in two VB cells. The pacemaker and the spindle-like oscillations were recorded in one cell each which also showed the 'very slow' oscillations. 6. The 'NMDA' oscillations were observed only in a 'Mg(2+)-free' medium (0 mM-Mg2+, 2-4 mM-Ca2+; 64 out of 72 cells) and consisted of large-amplitude (10-25 mV) depolarizations that did not occur at regular intervals and were intermixed with smaller depolarizations present on the baseline and on the falling phase of the larger ones.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of GABA and benzodiazepines on neurones in the suprachiasmatic nucleus (SCN) of Syrian hamsters

Administration of benzodiazepines at appropriate times in the circadian cycle induce phase-shifts in circadian locomotor activity. The possibility that benzodiazepine-induced shifts are mediated at the level of the suprachiasmatic nuclei (SCN), identified as the circadian pacemaker in mammals, was examined electrophysiologically. Extracellular recordings were made from Syrian hamster (Mesocricetus auratus) hypothalamic SCN neurones in vitro to assess (1) the effects of gamma-aminobutyric acid (GABA) on SCN neuronal activity and (2) the effects of benzodiazepines (chlordiazepoxide and flurazepam) on GABA-evoked responses. Of 93 SCN cells tested, 86 were suppressed by iontophoresed GABA (20 mM) in a current(dose)-dependent manner, while 6 were unaffected; suppression was found during both the projected light and dark phases of the circadian cycle. Application of bicuculline methiodide alone elevated mean discharge activity, while GABA-evoked suppressions were blocked by bicuculline (n = 9/11 cells). Iontophoresis of chlordiazepoxide or flurazepam (20 mM; 1-10 nA) alone produced a current(dose)-dependent prolonged suppression of cell firing which was antagonised by bicuculline. These results indicate that benzodiazepine/GABA-evoked responses are at least partially mediated by GABAA receptors within the SCN and suggest that SCN may be a possible locus for the action of benzodiazepines in their induction of phase-shifts in circadian function.

Postsynaptic potentials and morphology of tectal cells responding to electrical stimulation of the bullfrog nucleus isthmi

Postsynaptic responses of tectal cells in the bullfrog (Rana catesbeiana) were intracellularly recorded following electrical stimulation of the optic tract and the nucleus isthmi, and fluorescent dye, Lucifer yellow, was injected into some of the impaled cells to show their morphologies. Two main response types were found: The first type was an EPSP followed by an IPSP, and the second type was single IPSP. The first type predominates in cells responding to the optic tract stimulation and the second type prevails in cells responding to the isthmic stimulation. Fifteen cells stained with Lucifer yellow were localized in layer 6 (11 cells), layer 7 (1 cell), and layer 8 (3 cells). They were mainly identified as pear-shaped cells, large ganglionic cells, and stellate cells. Three injections demonstrated "dye-coupling," which labeled up to six cells following one injection. Comparisons of postsynaptic potentials with cellular morphologies suggested that the nucleus isthmi could directly excite large ganglionic neurons in layer 6. Synaptic mechanisms for strong isthmic inhibition on the tectal neurons remain unknown.

On the properties and origin of the GABAB inhibitory postsynaptic potential recorded in morphologically identified projection cells of the cat dorsal lateral geniculate nucleus

Intracellular recordings were performed from projection cells of the cat dorsal lateral geniculate nucleus in vitro to investigate the properties and origin of optic tract evoked inhibitory postsynaptic potentials mediated by GABAB receptors and their relationship to the physiologically different cell classes present in this nucleus. In all three main laminae of the dorsal lateral geniculate nucleus, stimulation of the optic tract evoked an excitatory postsynaptic potential followed by two inhibitory postsynaptic potentials. The first is a GABAA receptor mediated inhibitory postsynaptic potential since it was blocked by bicuculline, reversed in polarity following intracellular Cl- injection and had a reversal potential similar to the bicuculline sensitive hyperpolarizing effect of GABA. The second is a GABAB receptor mediated inhibitory postsynaptic potential. Its amplitude was not linearly related to membrane potential (maximal amplitude at -60 mV), it decreased when using frequencies of stimulation higher than 0.05 Hz and it was reversibly increased by addition of bicuculline to the perfusion medium. The reversal potential of GABAB inhibitory postsynaptic potentials was dependent on the extracellular K+ concentration but did not change in the presence of bicuculline or when recording with Cl- filled microelectrodes. While GABAA inhibitory postsynaptic potentials always abolished repetitive firing of projection cells, GABAB inhibitory postsynaptic potentials were able to block weak firing but unable to decrease strong activation of projection cells evoked by direct current injection. Optic tract evoked GABAB (as well as GABAA) inhibitory postsynaptic potentials could be recorded in slices which did not include the perigeniculate nucleus, thus indicating that they are generated by the interneurons of the dorsal lateral geniculate nucleus. Using intracellular injection of horseradish peroxidase, we have found that the GABAB inhibitory postsynaptic potentials are present in projection cells showing many different types of neuronal morphologies. In conclusion, GABA released from interneurons in the dorsal lateral geniculate nucleus is capable of evoking an early, short-lasting GABAA and a late, long-lasting GABAB inhibitory postsynaptic potential in projection cells with diverse morphology, indicating that the late inhibition in the dorsal lateral geniculate nucleus can no longer be associated exclusively with the recurrent inhibitory pathway through the perigeniculate nucleus.