Morphology of local axon collaterals of electrophysiologically characterised neurons in the rat medial septal/ diagonal band complex.

Neurons in the medial septal/diagonal band complex (MS/DB) in vivo exhibit rhythmic burst-firing activity that is phase-locked with the hippocampal theta rhythm. The aim was to assess the morphology of local axon collaterals of electrophysiologically identified MS/DB neurons using intracellular recording and biocytin injection in vitro. Cells were classified according to previous criteria into slow-firing, fast-spiking, regular-spiking, and burst-firing neurons; previous work has suggested that the slow-firing neurons are cholinergic and that the other types are GABAergic. A novel finding was the existence of two types of burst-firing neuron. Type I burst-firing neurons had significantly longer duration after hyperpolarisation potentials when held at -60 mV, and at -75 mV, type I neurons exhibited a low-threshold spike with more rapid activation and inactivation kinetics than those of type II neurons. We have, also for the first time, described the main features of the local axon collaterals of the five neuron types. All filled neurons possessed a main axon that gave forth 1-12 local primary axon collaterals. All electrophysiological types, except for the type I burst-firing neuron, had a main axon that coursed toward the fornix. Myelination of the main axon was a prominent feature of all but the slow-firing neurons. Branching of the primary axon collaterals of the fast-spiking and type I burst-firing neurons was more extensive than that of the other cell types, with those of the slow-firing neurons exhibiting the least branching. All cell types possessed axon collaterals of the en passant type, and some in addition had twiglike or basketlike axon terminals. All cell types made synapses on distal dendrites; a proportion of the fast-spiking and burst-firing cells in addition had basketlike terminals that made synaptic contacts on proximal dendrites and on somata. Two morphological types of somata were postsynaptic to the basket cells: large (20-30-microm) oval cells with dark cytoplasm, and large oval cells with paler cytoplasm, often with an apical dendrite. The presence of lamellar bodies in the large dark neurons suggests that they may be cholinergic neurons, because previous work has localised these structures in some neurons that stain for choline acetyltransferase. Our work suggests therefore that there may be GABAergic neurons in the MS/DB that form basket synaptic contacts on at least two types of target cell, possibly cholinergic and GABAergic neurons, which means that the basket cells could play a key role in the generation of rhythmic activity in the MS/DB.

Synaptic plasticity and memory: an evaluation of the hypothesis.

Changing the strength of connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. In its most general form, the synaptic plasticity and memory hypothesis states that "activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the information storage underlying the type of memory mediated by the brain area in which that plasticity is observed." We outline a set of criteria by which this hypothesis can be judged and describe a range of experimental strategies used to investigate it. We review both classical and newly discovered properties of synaptic plasticity and stress the importance of the neural architecture and synaptic learning rules of the network in which it is embedded. The greater part of the article focuses on types of memory mediated by the hippocampus, amygdala, and cortex. We conclude that a wealth of data supports the notion that synaptic plasticity is necessary for learning and memory, but that little data currently supports the notion of sufficiency.

Effects of afferent firing frequency on the amplitude of the monosynaptic EPSP elicited by trigeminal spindle afferents on trigeminal motoneurones.

We have examined the effect of changes in afferent firing frequency on the monosynaptic EPSP elicited by trigeminal muscle spindle afferents in elevator motoneurones, using both paired pulse and repetitive activation of the masseter nerve. Both modes of stimulation resulted in significant facilitation of EPSP amplitude over intervals of 5-19 ms, but no significant change in EPSP amplitude at either longer or shorter intervals. The facilitation obtained stands in contrast to the predominant depression of EPSP amplitude reported in hindlimb motoneurones following stimulation at decreasing intervals.

Differences in monosynaptic EPSPs elicited by masseter and temporalis spindle afferents in anaesthetised rats.

We have used the intracellular variant of the spike triggered averaging method to examine the monosynaptic connexions of masseter and temporalis spindle afferents on jaw-elevator motoneurones. Temporalis spindle afferents elicited larger averaged EPSPs in motoneurones than masseter spindle afferents, in part because transmission at synapses of temporalis afferents was associated with lower incidences of failures. We conclude that EPSP amplitude in this motor system in governed, at least in part, by the presynaptic neurone rather than the identity of the postsynaptic neurone.

Monosynaptic EPSPs elicited by single interneurones and spindle afferents in trigeminal motoneurones of anaesthetized rats.

1. Our aim has been to quantify the monosynaptic connections of trigeminal interneurones and spindle afferents onto jaw-elevator motoneurones as a step towards identifying common features in organization of monosynaptic inputs onto motoneurones. We have used the intracellular variant of the spike-triggered averaging method to examine the connections of single identified trigeminal interneurones and jaw-elevator muscle spindle afferents onto single jaw-elevator motoneurones. The interneurones examined lay in the region immediately caudal to the trigeminal motor nucleus. The experiments were performed on rats anaesthetized with pentobarbitone, paralysed and artificially ventilated. 2. Ten EPSPs and eight IPSPs were obtained from examining the connections of seventeen interneurones to thirty-six motoneurones, suggesting a functional connectivity of 50% for individual interneurones onto elevator motoneurones. Fourteen EPSPs were obtained from examining the connections of thirteen spindle afferents onto twenty-seven motoneurones, giving a functional connectivity of 52% for individual spindle afferents onto elevator motoneurones. The amplitudes of the EPSPs elicited by interneurones ranged from 7-48 microV (mean = 17, S.D. = 12.5, n = 10) and from 7 to 289 microV (mean = 64, S.D. = 76.0, n = 14) for the spindle-mediated EPSPs; the difference in the two means was not significant (P = 0.07). 3. However, the amplitude of averaged responses obtained by signal averaging methods are dependent on the assumption that the postsynaptic response occurs following every impulse in the presynaptic neurone. We therefore estimated the percentage of sweeps which contained EPSPs triggered by the presynaptic neurone under study. In essence the method used consisted of visual inspection of the individual sweeps comprising an average in order to assess the occurrence of EPSPs within six separate time windows, each of duration +/- 0.3 ms. Five windows were placed at randomly selected times on average and were used to provide an estimate of the frequency of occurrence of randomly triggered EPSPs. The sixth window was centred on the start of the averaged EPSP and the frequency of occurrence of randomly triggered EPSPs was subtracted from the frequency of occurrence of EPSPs in this window to produce an estimate of the incidence of EPSPs triggered by the presynaptic neurone under study. 4. Values of the incidence of occurrence of EPSPs triggered by the presynaptic neurones ranged from 4.3 to 92% for the fifteen averaged EPSPs which could be analysed in this manner (two elicited by interneurones and thirteen by spindle afferents).(ABSTRACT TRUNCATED AT 400 WORDS)