SYNAPTIC PLASTICITY IN THE RED NUCLEUS

Nakaakira Tsukahara,     Department of Biophysical Engineering, Faculty of Engineering Science, Osaka University, 1–1, Machikaneyama, Toyonaka, Osaka and National Institute for Physiological Sciences, Myodaiji, Okazaki, 444 Japan

Publisher Summary

The morphological discovery that synaptic reorganization takes place after the partial deafferentation of septal nucleus in adult rats indicates that the neuronal connection in the mammalian central nervous system is not as rigid as has long been considered. Red nucleus (RN) represents a suitable preparation to determine whether new, functionally effective synaptic connections are formed. Cortico-rubral fibers terminate on the distal dendrites and fibers from the contralateral nucleus interpositus (IP) of the cerebellum make synaptic contact on the soma. The cortico-rubral dendritic EPSPs are characterized with slow-rising time course, whereas the somatic IP-excitatory postsynaptic potentials (EPSPs) are characterized with fast-rising time course. EPSPs from medial lemniscus have rise time between those of IP-rubral and cortico-rubral EPSPs. Sprouting and synaptic reorganization are not limited to the cases of removal of the direct synaptic inputs of RN neurons. A major goal of the study of neuronal plasticity is to provide a neuronal basis for behavioral plasticity, such as learning and memory. These studies range from the examination of simple behavioral phenomena to classical conditioning phenomena.

Neuronal substrates of learning and memory have been a subject of great interest in neurophysiology for several decades. The morphological discovery that synaptic reorganization takes place after partial deafferentation of septal nucleus in adult rats (Raisman, 1969) indicates that the neuronal connection in the mammalian central nervous system is not as rigid as has long been considered. There has been a great progress in our understanding of the neuronal plasticity in the central nervous system.

Red nucleus (RN) represents a suitable preparation to determine whether new, functionally effective synaptic connections are formed. Detailed information is now available about the synaptic organization of RN neurons. Both physiological and electron microscopic studies revealed a clear segregation of synaptic sites among several inputs on the soma-dendritic membrane of RN cells. Cortico-rubral fibers terminate on the distal dendrites and fibers from the contralateral nucleus interpositus (IP) of the cerebellum make synaptic contact on the soma. Recent investigation suggests that afferent fibers from the medial lemniscus make synaptic contacts on dendrites in an intermediate position. Thus the cortico-rubral dendritic EPSPs are characterized with slow-rising time course, whereas the somatic IP-EPSPs are characterized with fast-rising time course. EPSPs from medial lemniscus have rise time between those of IP- and cortico-rubral EPSPs.

LESION-INDUCED SPROUTING AND FORMATION OF FUNCTIONAL SYNAPSES IN ADULT FELINE RED NUCLEUS

After chronic lesions of the IP in adult cats, a new fast-rising component appears superimposed on the slow-rising cortico-rubral EPSPs. A slight change in cable properties of dendrites (electrotonic length) of RN neurons after IP lesions accounts for only a minor portion (less than 5%) of the observed change in time to peak of the cortico-rubral EPSPs. Thus it was concluded that new and active synapses are formed on the proximal portion of soma-dendritic membrane of RN cells (Tsukahara et al., 1974; 1975a). This conclusion was corroborated by the electron microscopic studies of Nakamura et al. (Nakamura et al., 1974) and Hanaway and Smith (1978) (see aslo Nakamura et al., 1978).

Fig. 1 Lesion-induced sprouting in adult feline red nucleus
A: Synaptic organization of normal red nucleus (RN). Monosynaptic excitatory input from ipsilateral sensorimotor cortex (SM) through the cerebral peduncle (CP) impinges on the distal dendrites and that from the contralateral nucleus interpositus (IP) of the cerebellum on the soma. Stimulation of CP produes a slow-rising EPSP in a RN cell and stimulation of IP produces a fast-rising EPSP as shown in the inset. B: After IP-lesion a fast-rising component appears superimposed on the slow-rising CP-EPSPs as shown in the inset. C: Frequency distribution of the time-to-peak of the CP-EPSPs of normal cats as measured as the inset of A. D: Same as C but CP-EPSPs of cats with chronic IP-lesions measured as the inset of B.

Analysis of the unitary cortico-rubral EPSPs before and after IP lesions further supports this by adding several details of the unitary EPSPs; two groups of the unitary EPSPs, one with shorter time-to-peak and larger amplitude than in normal cats and the other with time-to-peak and amplitude of normal range exist. The former is more sensitive to membrane potential displacement than the latter. The relation of the time-to-peak and the amplitude of the cortico-rubral unitary EPSPs before and after chronic IP lesion can be fitted to the theoretical relation derived from Rall’s compartment model (Rall, 1964; Murakami et al., 1977a; Sato and Tsukahara, 1976).

The time course of facilitation at the newly-formed corticorubral synapses, as investigated by a pair of stimuli, shows no significant difference from those at the normal corticorubral synapses (Fig. 2). Post-tetanic potentiation was also found in both normal and newly-formed synapses (Murakami et al., 1977b).

Fig. 2 Facilitation of corticorubral EPSPs
A: Corticorubral EPSPs produced by a single CP stimulation (upper trace) and those produced by a pair of stimuli of the same intensity (lower trace) in an operated cat. B: Same as A but in a normal cat. Responses exemplified in B were averaged by a computer (30 traces) and displayed in C. Arrows indicate the onset of stimuli. D,E: Time course of facilitation of the EPSPs. Ordinate, the degree of facilitation expressed as shown in the inset diagram on a logarithmic scale. Abscissa, interval between two CP stimuli. Each point is the average of 14 EPSPs in operated cats (D) and 12 EPSPs in normal cats (E). The plotted points (open circles) could be fitted by a straight line (dotted lines) were replotted on the same graphs (filled circles). These values could be fitted by straight lines with time constants of 6 and 3 msec for D and E, respectively. (modified from ref.6).

The EPSPs of the medial lemniscus input was usually subthreshold for spike initiation. After chronic IP lesions, the EPSPs induced by stimulation of the medial lemniscus had a faster rise time and produced spike potentials more frequently, suggesting that lemniscal fibers also sprout after IP lesion (Tsukahara et al., unpublished). This observation may account for the slow restoration of multiunit activities of the RN after chronic lesions of the IP and the cerebral cortex (Bromberg and Gilman, 1978).

LESION-INDUCED SPROUTING IN KITTEN RED NUCLEUS

It is generally agreed that the degree and extent of sprouting is more remarkable after denervation at the neonatal stage than at the adult stage (Tsukahara, 1981 for the review). Synaptic organization of the kitten RN neurons is essentially the same as that of the adult cat. They receive two major excitatory inputs, one from the contralateral IP on the soma and the other from the ipsilateral cerebrum on the distal dendrites.

After lesion of the contralateral IP by hemicerebellectomy in early developmental stage within several weeks after birth, new functional connections appeared from the ipsilateral IP. Stimulation of the ipsilateral IP produced monosynaptic EPSPs in some RN cells which have not been found in both after adult lesions and in normal kittens.

Cerebral lesion destroying the ipsilateral corticorubral fibers was found to induce sprouting from three sources; 1) most importantly from the contralateral cerebral cortex via the contralateral cerebral peduncle (Nah & Leong, 1976a, b), 2) contralateral IP, and 3) ipsilateral IP. As shown in Fig. 3A, stimulation of the contralateral CP produces a slow-rising EPSPs in a kitten in which ipsilateral cerebral snesorimotor cortex was destroyed previously at 27th day postnatally. The latency of the CP-EPSPs induced from the contralateral CP was 1.8 msec on the average with time-to-peak of 3,2 msec on the average. Similar slow-rising EPSPs were also produced by stimulating the sensorimotor cortex. Judging from their latency, they were mediated by the slow conducting corticofugal fibers as in the normal ipsilateral corticorubral EPSPs. The monosynaptic nature of the CP-EPSP was tested by the double shock experiment in which the second CP-EPSP was abolished abruptly with stimulus intervals of about 0.5 msec. The area producing the slow-rising EPSPs in RN cells is somatotopically organized. The RN cells innervating the upper spinal segment (C-cell) receive EPSPs from the lateral part of the sensorimotor cortex. On the other hand, RN cells innervating the lower spinal segment (L-cell) receive EPSPs predominantly from the medial part of the sensorimotor cortex. Therefore, the newly-appeared corticorubral projection from the contralateral cerebrum has a “topographical specificity”. The fast conducting pyramidal neurons do not project onto the RN neurons as in normal adult cats and slow conducting pyramidal and corticorubral...