Homosynaptic and heterosynaptic changes in driving of dentate gyrus interneurons after brief tetanic stimulation in vivo

GABAergic Interneurons with Nonlinear Dendrites: From Neuronal Computations to Memory Engrams

GABAergic interneurons (INs) are a highly diverse class of neurons in the mammalian brain with a critical role in orchestrating multiple cognitive functions and maintaining the balance of excitation/inhibition across neuronal circuitries. In this perspective, we discuss recent findings regarding the ability of some IN subtypes to integrate incoming inputs in nonlinear ways within their dendritic branches. These recently discovered features may endow the specific INs with advanced computing capabilities, whose breadth and functional contributions remain an open question. Along these lines, we discuss theoretical and experimental evidence regarding the potential role of nonlinear IN dendrites in advancing single neuron computations and contributing to memory formation.

Phrenic motoneuron structural plasticity across models of diaphragm muscle paralysis

Structural plasticity in motoneurons may be influenced by activation history and motoneuron-muscle fiber interactions. The goal of this study was to examine the morphological adaptations of phrenic motoneurons following imposed motoneuron inactivity while controlling for diaphragm muscle inactivity. Well-characterized rat models were used including unilateral C2 spinal hemisection (SH; ipsilateral phrenic motoneurons and diaphragm muscle are inactive) and tetrodotoxin phrenic nerve blockade (TTX; ipsilateral diaphragm muscle is paralyzed while phrenic motoneuron activity is preserved). We hypothesized that inactivity of phrenic motoneurons would result in a decrease in motoneuron size, consistent with a homeostatic increase in excitability. Phrenic motoneurons were retrogradely labeled by ipsilateral diaphragm muscle injection of fluorescent dextrans or cholera toxin subunit B. Following 2 weeks of diaphragm muscle paralysis, morphological parameters of labeled ipsilateral phrenic motoneurons were assessed quantitatively using fluorescence confocal microscopy. Compared to controls, phrenic motoneuron somal volumes and surface areas decreased with SH, but increased with TTX. Total phrenic motoneuron surface area was unchanged by SH, but increased with TTX. Dendritic surface area was estimated from primary dendrite diameter using a power equation obtained from three-dimensional reconstructed phrenic motoneurons. Estimated dendritic surface area was not significantly different between control and SH, but increased with TTX. Similarly, TTX significantly increased total phrenic motoneuron surface area. These results suggest that ipsilateral phrenic motoneuron morphological adaptations are consistent with a normalization of motoneuron excitability following prolonged alterations in motoneuron activity. Phrenic motoneuron structural plasticity is likely more dependent on motoneuron activity (or descending input) than muscle fiber activity.

Cooperativity between hippocampal–prefrontal short-term plasticity through associative long-term potentiation

The hippocampal-medial prefrontal cortex (mPFC) pathway provides highly convergent input to the mPFC in rats and shows two types of short-term plasticity in terms of paired-pulse facilitation (PPF) of the field potential under urethane anesthesia. We now report that stimulating either the dorsal or ventral subregions of the posterior hippocampus elicited PPF (by about 335 and 120%, respectively) of field potentials recorded in the mPFC at 100 ms interpulse interval. This PPF-like interaction occurred when projections were stimulated in the ventral-dorsal order (by about 200% of the single-pulsed response), but not vice versa. When weak long-term potentiation (LTP) of the dorsal projection was evoked simultaneously with strong LTP of the ventral projection, an associative effect was revealed (about +55%), although the magnitudes of LTP in each projection were not correlated. Even when the impermutable PPF-like facilitation was further enhanced (by about +120%), the enhancement was not correlated with either form of LTP, but exhibited the interaction of changes in the dorsal PPF, rather than in the heterotopic priming effect through the ventral projection. Moreover, this change was correlated with the associated LTP ratio of dorsal to ventral projection LTP (i.e., LTP associativity). Larger increases in LTP associativity correlated with greater impermutable PPF-like facilitation; in addition, there was hardly attenuation of the response to the dorsal projection by subsequent electrolytic lesions of the ventral subregion. These results indicate that the mPFC functionally integrates discrete sources of hippocampal information via cooperativity between short- and long-term plasticity.

Long-term potentiation/depotentiation are accompanied by complex changes in spontaneous unit activity in the hippocampus

Typically, long-term potentiation (LTP) has been assessed as long-lasting changes in field potentials or intracellularly recorded postsynaptic potentials evoked by activation of a set of afferents. In the present experiment, we determined changes in spontaneous unit activity in the dentate gyrus (DG) following high-frequency (HFS) or low-frequency stimulation (LFS) of the medial perforant pathway. Experiments were performed in anesthetized rats. Field potentials and unit recordings were obtained alternatively from the same recording electrode. Of 39 single units isolated (from 25 independent sessions), the spontaneous discharges of 13 units (33%) increased, while 7 units (18%) decreased their discharges following HFS that induced significant LTP of the field potentials. Such opposing modulations of unit discharges following HFS were observed on simultaneously recorded units. LFS applied following HFS also induced bi-directional effects on unit discharges. Of 20 single units isolated from a subset of recordings (12 experiments) to which LFS was applied, 6 units increased and 4 units decreased their discharges. LFS produced a long-lasting (>20 min) depotentiation, to the baseline level, on field potentials in four recording cases. The autocorrelation functions indicated that the isolated unit discharges were comparable to those of the putative DG granule cells and interneurons, shown in previous studies. The results suggest that changes in synaptic efficacy following HFS or LFS produce rather dynamic changes in cell activity in the DG.

GABAergic modulation of neocortical long-term potentiation in the freely moving rat

Although the neocortex has generally been considered resistant to the induction of long-term potentiation (LTP), we have recently shown that LTP can be reliably induced in the freely moving rat provided that the stimulation sessions are spaced and repeated. Here, we report that the induction of LTP in this preparation can be modulated by both GABAergic agonism and antagonism. The delivery of stimulation trains in the presence of the GABA(A) agonist diazepam blocked the induction of neocortical LTP, while the GABA(A) antagonist picrotoxin slowed the development of potentiation. When animals that had previously received high-frequency stimulation combined with diazepam were repotentiated, they showed greater resistance to LTP induction than animals that had received diazepam alone. These data suggest that the inhibitory circuits themselves may have potentiated. The demonstration that diazepam blocks neocortical LTP provides further support for the notion that LTP plays a role in memory formation.

Physiological evidence for hippocampal disinhibition resulting from activation of the median raphe

The mechanism underlying median raphe (MR)-induced facilitation of hippocampal synaptic transmission was investigated by recording stimulus-evoked field potentials and unitary responses in urethane-anesthetized rats. Stimulation of the MR 40 ms prior to perforant path (PP) activation significantly increased the magnitude of PP-evoked granule cell population spikes (median increase = 78%) without affecting population EPSP slope. Injection of homocysteic acid into the vicinity of the MR also facilitated PP-evoked granule cell population spikes, in a dose-dependent manner. Nineteen dentate hilar units were characterized as putative interneurons on the basis of their waveform characteristics and their response to PP stimulation. Electrical activation of the MR inhibited spontaneous or PP-evoked activity in the majority (75%) of these cells; the remaining cells were unaffected. MR stimulation also inhibited spontaneous activity in a large proportion (60%) of putative interneurons in CA1. The present results provide evidence that neurons within the raphe modulate hippocampal throughput by altering discharge of non-principal cells. These data, thus, support the idea that disinhibition is a common mechanism by which extrahippocampal structures modulate information flow through the hippocampus.