Development of responses of globus pallidus and entopeduncular nucleus neurons to stimulation of the caudate nucleus and precruciate cortex

Extracellular recordings were made from neurons in the globus pallidus and entopeduncular nucleus (GP-ENTO) of anesthetized kittens of 2 to 177 days of age and from four adult cats. Stimulation of the striatum and the precruciate cortex produced responses in GP-ENTO neurons of the youngest kittens tested (2 days of age). In kittens of 1 to 10 days, about 70% of the GP-ENTO neurons responded to either caudate or cortical stimulation with a purely excitatory response (i.e., an evoked action potential). With increasing age the frequency of occurrence of this type of response decreased and the occurrence of inhibitory responses or of sequences of excitation followed by inhibition increased. In addition to these changes in the form of the evoked responses, other response parameters exhibited age-dependent alterations. Latency to response decreased with age and the ability of GP-ENTO neurons to follow repetitive stimuli increased as the kittens became older. These findings suggest that although GP-ENTO neurons are functional as early as 2 days postnatally in the kitten, subsequent maturation of the responsiveness of these neurons continues for several postnatal months.     

A neurophysiological analysis of the development of cortico-caudate connections in the cat.

Extracellular single-unit recordings were made from the caudate nuclei of a series of kittens ranging in age from 1 to 50 days. Each of these neurons was tested for ability to respond to stimulation of the ipsilateral precruciate cortex. The results indicate that the synaptic effect of cortical efferents on caudate neurons in neonates is qualitatively similar to that observed in the adult striatum. During the course of development the proportion of responsive units increased, response latency decreased, and the ability to follow repetitive stimuli improved.     

Amphetamine alters evoked responses of nigral neurons in kittens and adult cats

The effects of i.p. amphetamine administration (5 mg/kg) on the evoked unitary responses of substantia nigra (SN) neurons to electrical stimulation of their afferents were tested in 4 kittens (3–27 days of age) and 4 adult cats. In adults, amphetamine had two major effects: (1) it blocked temporarily (15–30 min) all neuronal responses to caudate (Cd) and cortical (Cx) stimulation; neuronal responsiveness recovered by 75 min post-drug; and (2) after 15 min postdrug, Cd and Cx stimulation evoked initial excitatory responses that were almost never found predrug. The latencies of Cd-evoked excitations indicated the existence of a mono- or oligosynaptic excitatory strionigral pathway while latencies of Cx-evoked excitations suggested that corticonigral excitatory influences were mediated multisynaptically. In kittens, amphetamine also produced an initial blockade of Cd- and Cx-evoked responses. However, the sign of initial responses to Cd stimulation was not altered since excitations were found both before and after drug treatment. These results indicated that amphetamine reveals excitatory evoked responses of SN neurons to striatal and striatally-mediated inputs that are masked during the course of normal postnatal development. Drug-related alterations of afferent inputs to SN neurons may underlie amphetamine-induced shifts in spontaneous neuronal activity which have been reported frequently.     

Changes in Fos-like immunoreactivity evoked by maturation of the sneeze reflex triggered by nasal air puff stimulation in kittens

The sneeze reflex is a valuable tool for exploring the maturation of the respiratory control in the newborn as it alters both inspiratory and expiratory activities. Air puff stimulation of the superior nasal meatus innervated by ethmoidal afferents consistently evokes sneeze in adult cats. Such stimulation evokes only a reinforcement of expiratory activities in newborn kittens. This study demonstrates that the pattern of Fos-like immunoreactivity evoked by nasal stimulation changes during functional maturation of sneeze. Nasal stimulation evoked immunoreactivity (i) in the trigeminal sensory complex, at the levels where nasal afferents project, (ii) in the reticular formation, (iii) in the solitary complex and (iv) in the parabrachial area of mature kittens. The evoked immunoreactivity was the same in newborn kittens as in mature kittens in the projection areas of the nasal primary afferents. Fos response was less than half that in mature kittens in the reticular formation and absent in the solitary complex or the parabrachial area. Sneeze can be elicited from the time when evoked immunoreactivity in the solitary complex and the parabrachial area is above control levels. These data provide evidence that the maturation of sneeze is dependent on the development of central relays allowing peripheral inputs to be integrated by neurons engaged in respiratory control.     

Reorganization of the primary afferent termination in the rat spinal dorsal horn during post-natal development

To study the reorganization of the primary afferent input in the spinal dorsal horn during post-natal development, synaptic responses evoked by large Abeta and fine Adelta afferents were recorded from substantia gelatinosa (SG) neurons in slices obtained from immature (post-natal days 21-23) and mature rats (post-natal days 56-60). Threshold stimulus intensities and conduction velocities (CVs) of Abeta and Adelta afferents were determined by intracellular recordings of the antidromic action potentials from dorsal root ganglion (DRG) neurons isolated from immature and mature rats. In immature rats, excitatory postsynaptic currents (EPSCs) were elicited by stimulation sufficient to activate Abeta afferents in the majority of SG neurons (64.9%, 24 of 37 neurons), while most EPSCs observed in mature rats were elicited by stimulation of Adelta afferents (62.5%, 25 of 40 neurons). These observations suggest that the primary afferents innervating SG neurons were reorganized following maturation; Abeta afferents were the predominant inputs to the SG neurons in the immature state, thereafter Adelta afferents were substituted for the Abeta afferents to convey sensory information to the SG neurons. This relatively slow reorganization of the sensory circuitry may correlate with slow maturation of the SG neurons and with a delay in the functional connections of C afferents to the SG neurons.     

Intracellular responses of caudate neurons to temporally and spatially combined stimuli

Intracellular recordings of caudate neuronal responses evoked by temporally combined stimulations of cortex, thalamus, and substantia nigra were made in the cat. Excitatory postsynaptic potentials (EPSPs) which temporally coincided were additive. EPSPs which coincided with an inhibitory postsynaptic potential (IPSP) previously evoked from the same stimulus site were enhanced. The cortical stimulus was prepotent in the sense that EPSPs evoked from thalamic or nigral stimulation were inhibited by the cortical IPSP. The cortical EPSP was enhanced if it was evoked during a nigral or thalamic IPSP. These results are discussed in the context of recent reports concerning the fine structure of synaptic contacts of input fibers to the caudate nucleus.     

Postnatal development of thalamic synaptic events underlying evoked recruiting responses and electrocortical activation

Postsynaptic potentials were intracellularly recorded from thalamic neurons in kittens of different ages during stimulation of medial thalamic (MTh) regions at low and high frequencies. In neonatal and young kittens (< 10 days old) low-frequency (3.3/sec) MTh stimulation elicited IPSPs in thalamic neurons with a mean duration of 50 msec. EPSPs at this age were weak and variable. In older kittens (2–3 weeks old) MTh-evoked IPSPs exhibited a mean duration of 125 msec and EPSPs were more prominent in EPSP-IPSP sequences. High-frequency (80/sec) MTh stimulation in young kittens produced sustained summation of IPSPs and suppression of cell discharge. IPSPs were also elicited by high-frequency MTh stimulation in 2–3-week-old kittens but these IPSPs were rapidly terminated by powerful and sustained EPSPs. The relationship of these observations to previous studies of evoked synchronizing and desynchronizing processes in thalamic neuronal organizations in adult animals indicates that: (1) the development of evoked thalamic neuronal synchronization is associated with the functional maturation of interneuronal pathways involved in the production of EPSP-IPSP sequences in thalamic neurons; and (2) the development of electrocortical activation subsequent to high-frequency MTh stimulation occurs pari passu with an increasing capacity of excitatory synaptic inputs to elicit powerful and sustained EPSPs in thalamic neurons. Thalamic synaptic mechanisms underlying evoked synchronization and desynchronization of electrocortical activity attain functional maturation by the end of the third postnatal week in the kitten.     

Deafferentation Weakens Excitatory Synapses in the Developing Central Auditory System

Decreased excitatory synaptic activity during development often leads to pre- and postsynaptic atrophy, as assessed anatomically. The present study considers the effect of decreased excitatory transmission on the maturation of synaptic strength. Towards this end, cochlear nucleus neurons, which project to the ipsilateral lateral superior olive (LSO), were denervated in gerbils at postnatal day 7, before the onset of hearing. This manipulation was intended to disrupt spontaneous glutamatergic transmission in the LSO while sparing the glycinergic afferents from the medial nucleus of the trapezoid body (MNTB). Afferent-evoked synaptic activity was assessed 1–6 days after ablation in a brain slice preparation using whole-cell current- and voltage-clamp recordings. In control animals, ipsilaterally evoked excitatory postsynaptic potentials (EPSPs) were present in 91% of neurons tested, but were observed in only 60% of neurons following cochlea removal. The maximum EPSP amplitude was significantly smaller in manipulated neurons compared with controls, and this was accompanied by a higher incidence of ipsilaterally evoked inhibitory postsynaptic potentials (IPSPs). To study the efficacy of excitatory synapses in greater detail, voltage-clamp recordings were made in the presence of strychnine and AP-5 [d(O)-2-amino-5-phosphonopentanoic acid]. The minimum excitatory postsynaptic current (EPSC) amplitude, presumed to reflect the efficacy of a single glutamatergic afferent, was ～40% smaller in manipulated neurons. In contrast, MNTB-evoked IPSPs were similar in neurons from control and ablated animals. However, manipulated neurons often exhibited a rebound depolarization after a hyperpolarizing current pulse or an afferent-evoked IPSP. In 70% of manipulated neurons, synaptically evoked rebound depolarizations were reduced, but not eliminated, by glutamate receptor antagonists. The glycine receptor antagonist strychnine did eliminate the IPSP-associated depolarization in these neurons. Collectively, these results suggest that functional denervation of excitatory afferents decreases their synaptic efficacy as result of both cell loss as well as decreased strength of individual surviving synapses.     

A GABAergic, strongly inhibitory projection to a thalamic nucleus in the zebra finch song system.

The anterior forebrain pathway (AFP) of the oscine song system is essential for song learning but not song production. Most cells recorded in this serially connected pathway show increased firing in response to song playback, suggesting largely excitatory connections among AFP nuclei. However, the neurons forming a key projection in this pathway, from area X to the medial nucleus of the dorsolateral thalamus (DLM), express glutamic acid decarboxylase in their somata and terminals, suggesting an inhibitory connection. To investigate the firing properties of DLM neurons and the functional influence of area X afferents in DLM, we made whole-cell recordings from DLM neurons in brain slices from adult male zebra finches. Most cells had intrinsic properties closely resembling those of mammalian thalamocortical cells, including a low-threshold Ca(2+) spike and time-dependent, hyperpolarization-activated inward rectification. Activation of afferents from area X evoked a strong, all-or-none IPSP whose amplitude and latency were unchanged by application of glutamate antagonists, consistent with a monosynaptic contact. The IPSP had a reversal potential near -70 mV and was blocked by the GABA(A) receptor antagonist bicuculline methiodide. Post-inhibitory rebound firing occurred in DLM neurons with a delay near 50 msec. Strong inhibition can combine with the intrinsic properties of DLM neurons to allow signaling on disinhibition. Our data are consistent with the hypothesis that the AFP corresponds to the mammalian corticobasal ganglia-thalamocortical loop. The similar functional properties of avian and mammalian thalamic neurons suggest conserved forebrain mechanisms of sensorimotor information processing across vertebrate taxa.     

Convergence of projections from the rat hippocampal formation, medial geniculate and basal forebrain onto single amygdaloid neurons: an in vivo extra- and intracellular electrophysiological study.

We recorded extra- and intracellular responses from rat amygdaloid neurons in vivo after electrical stimulation of the hippocampal formation (dentate gyrus, hippocampal fields CA3 and CA4, entorhinal cortex, subicular complex); medial geniculate; and basal forebrain (diagonal band, ventral pallidum, olfactory tubercle, nucleus accumbens, bed nucleus of stria terminalis, lateral preoptic area, substantia innominata). Stimulation of hippocampal formation structures evoked IPSPs or EPSP-IPSP sequences in which the IPSP had a lower threshold than the EPSP. Recordings from candidate inhibitory neurons in the amygdala indicated that excitatory afferents from the hippocampal formation contact both amygdaloid inhibitory and principal neurons (feedforward inhibition), and that the inhibitory neurons have a lower threshold of activation. Medial geniculate stimulation also evoked EPSP-IPSP sequences. In marked contrast to these results, stimulation of basal forebrain structures evoked short latency IPSPs in amygdaloid neurons. This provides the first physiological evidence for direct inhibition of the amygdala by the basal forebrain. Basal forebrain stimulation also evoked EPSP-IPSP sequences in amygdaloid neurons. Individual amygdaloid neurons could show responses to stimulation of the hippocampal formation, basal forebrain, and medial geniculate, indicating that synaptic input from these areas converges onto single amygdaloid cells. The findings provide further information about the synaptic organization of afferents to the amygdala, and indicate that single amygdaloid neurons play a role in the synaptic integration of input from these diverse sources.     

Postnatal development of sensory influences on neurons in the ventromedial hypothalamic nucleus of the rat

Extracellular unitary records were obtained from neurons in the ventromedial hypothalamic nucleus (VMH) of very young (1-25 days of postnatal age) and adult rats. Spontaneous unitary activity and evoked responses to both external (somatic, gustatory, and olfactory) and internal sensory (systemic administration of hypertonic saline and glucose solutions) stimulation were determined in order to assess the functional development of VMH neurons and their afferents. The basic electrophysiological characteristics of VMH neurons were established prenatally. From the date of birth, many VMH neurons had: spontaneous action potential generation; evoked responses to external or internal sensory stimulation; and convergent sensory inputs. In contrast, the major developmental change in the neurophysiological properties of VMH neurons was the diminution with increasing age of the convergence of external and internal sensory influences. This developmental 'fine-tuning' of a complex functional feature of VMH neurons is important because the maturation of convergence coincides with a 'critical period' of VMH ontogenesis demonstrated in behavioral and experimental brain damage reports.     

Caudate neurons respond to excitatory and inhibitory amino acids in early postnatal periods in the cat

This study was designed to determine whether caudate neurons would respond to microphoretic application of glutamate and gamma-aminobutyric acid (GABA) in early postnatal periods in the cat. Extracellular recordings were performed in 175 neurons in developing kittens and 114 neurons in adult cats. At the earliest ages tested (1-10 days), caudate cells were excited by microphoretic application of glutamate and were inhibited by application of GABA. The results also indicated that caudate units have lower response thresholds to application of glutamate and GABA in early postnatal periods than in later periods. Since previous findings indicated that synaptically mediated inhibitory potentials develop during later postnatal periods in the cat, the present findings suggest that receptors for GABA may be capable of functioning before presynaptic endings make operational contacts.     

Development of the evoked potentials in the thalamus and cerebral cortex after stimulation of the stellate ganglion afferents in kittens.

The primary evoked potentials (EP) were studied in newborn, 10-, 20-day-old, 1-month-old kittens in the thalamus and cerebral cortex after stimulation of the stellate ganglion (SG) afferents. The nerves were irritated with single square-wave pulses with an amplitude of 1.2-1.3 threshold for EP appearance. In all kittens, EP were registered in the same zones as in adult animals. In the thalamus, this zone was represented by the centromedial part of the ventralis posterior lateralis (n. VPL), contralaterally to the stimulation place. In the cerebral cortex, the EP were observed in the cerebral cortex, specifically in the somatosensory zones S1 and S2 also contralaterally to the stimulation place. In postnatal ontogenesis, the latency of evoked potentials decreases, amplitude increases and the form of these potentials transforms from negative in newborn kittens to positive-negative wave in 1-month-old animals. Maturation of the SG afferent pathways exists in parallel with the somatosensory ones and does not finish at 1 month of life.     

Postnatal development of identified medium-sized caudate spiny neurons in the cat

The morphology of intracellularly recorded neurons in the cat caudate nucleus (Cd) was studied during postnatal development. After intracellular recording of evoked responses in these neurons, horseradish peroxidase (HRP) was injected iontophoretically through the recording micropipette. Fifty-eight Cd neurons in cats ranging from 6 days of age through adulthood were identified morphologically. All of the recovered Cd cells were medium-sized spiny neurons. The basic somatodendritic morphology of these neurons was evident in the youngest kittens. The most striking morphological change was the postnatal formation of an extensive local axonal collateral plexus. The development of these local axonal collaterals was also quantified with computer assistance in medium-sized Cd spiny neurons selected from silver-impregnated material. This analysis showed that the major development of the branches of this local plexus occurred between birth and 3-4 months of postnatal age. Data from both the HRP-filled and silver-stained axons indicated that the postnatal growth of the local axonal collaterals of the medium spiny cells was associated with the elaboration and increasing prevalence of evoked inhibitory postsynaptic potentials in Cd neurons.     

Pallidal and entopeduncular intracellular responses to striatal, cortical, thalamic, and sensory inputs

Intracellular recordings were made from pallidal and entopeduncular neurons in cats. Responses were evoked by direct brain stimulation and auditory and somatosensory stimuli. Brain sites stimulated were caudate nucleus, the precruciate area of the cerebral cortex, and the central median-parafascicular region of the thalamus. The predominant synaptic response pattern for all types of stimulation was an EPSP-IPSP sequence. Thirty percent of the responses were IPSPs only. Relatively few “pure” EPSPs were recorded. These patterns of synaptic responses were compared with those evoked by comparable stimuli to caudate neurons. In particular, the relatively high percentage of “pure” IPSPs in pallidal and entopeduncular cells contrasted with the rate occurrence of “pure” IPSPs in caudate neurons. This difference in incidence of response types may be attributed to anatomical differences in the fine structure of these nuclei.     

Effect of electrocutaneous stimulation on responses of neurons in the parvocellular portion of the medial geniculate body induced by acoustic stimulation

The effect of contralateral forepaw electrical stimulation on responses evoked by a click in neurons of the parvocellular part of the medial geniculate body was studied in cats anesthetized by thyopental sodium and immobilized with myorelaxine. Neurons in which responses to a click were inhibited by conditioning forepaw stimulation were located in areas of the parvocellular part of the medial geniculate body adjoining the ventro-posterior nucleus and the magnocellular part of the medial geniculate body. Forepaw stimulation did not affect the neurons located in lateral area of the parvocellular part of the medial geniculate body. In most neurons intracellular recording revealed EPSP-spike-IPSP or EPSP-IPSP sequence in response to a click while forepaw stimulation evoked exclusively IPSPs. Inhibition of neuronal responses to a click produced by forepaw stimulation may develop both during IPSP and without it. The mechanisms of inhibitory effect of forepaw stimulation on responses evoked by a click in neurons of the parvocellular part of the medial geniculate body are discussed.     

Delayed development of song control nuclei in the zebra finch is related to behavioral development

The postnatal development of two visual areas (nucleus rotundus and ectostriatum) and two song control areas (hyperstriatum ventrale pars caudale, HVc, and nucleus robustus archistriatalis, RA) of the zebra finch brain was followed from birth to adulthood. The following parameters were investigated: (1) neuron size, (2) volume of the brain nuclei, and (3) myelination of axons. The nucleus rotundus, the diencephalic station of the tectofugal pathway, exhibits the fastest development: rotundal neurons reach their maximum size at 20 days of age; the volume of this structure reaches adult size at the same time. The process of myelination begins between day 5 and day 10 and is completed at 40 days of age. A similar temporal sequence of development is seen in the ectostriatum, except myelination starts some days later. Thus the development of these visual areas is completed at 40 days. In contrast, the development of the song control nuclei is delayed. Neurons in RA and HVc grow steadily up to 40 days of age, attaining a size larger than that observed in adults. Whereas the volume of HVc increases until day 40 and remains stable thereafter, RA volume increases until day 70 and evidences a decrease thereafter. It is not until postnatal day 20 (RA) and day 40 (HVc) that the myelination process starts in the song control areas. Adult myelin density is achieved by 70 days in RA and by 100 days in HVc. It can be demonstrated that the development of the visual system parallels the development of visual performance of the birds. Delayed growth of song control nuclei coincides with development of song.     

Loss and spontaneous recovery of forelimb evoked potentials in both the adult rat cuneate nucleus and somatosensory cortex following contusive cervical spinal cord injury

Varying degrees of neurologic function spontaneously recovers in humans and animals during the days and months after spinal cord injury (SCI). For example, abolished upper limb somatosensory potentials (SSEPs) and cutaneous sensations can recover in persons post-contusive cervical SCI. To maximize recovery and the development/evaluation of repair strategies, a better understanding of the anatomical locations and physiological processes underlying spontaneous recovery after SCI is needed. As an initial step, the present study examined whether recovery of upper limb SSEPs after contusive cervical SCI was due to the integrity of some spared dorsal column primary afferents that terminate within the cuneate nucleus and not one of several alternate routes. C5-6 contusions were performed on male adult rats. Electrophysiological techniques were used in the same rat to determine forelimb evoked neuronal responses in both cortex (SSEPs) and the cuneate nucleus (terminal extracellular recordings). SSEPs were not evoked 2 days post-SCI but were found at 7 days and beyond, with an observed change in latencies between 7 and 14 days (suggestive of spared axon remyelination). Forelimb evoked activity in the cuneate nucleus at 15 but not 3 days post-injury occurred despite dorsal column damage throughout the cervical injury (as seen histologically). Neuroanatomical tracing (using 1% unconjugated cholera toxin B subunit) confirmed that upper limb primary afferent terminals remained within the cuneate nuclei. Taken together, these results indicate that neural transmission between dorsal column primary afferents and cuneate nuclei neurons is likely involved in the recovery of upper limb SSEPs after contusive cervical SCI.     

Synaptic potentials of hypoglossal motoneurons and a common inhibitory interneuron in the trigemino-hypoglossal reflex

The responses of the hypoglossal motoneurons to stimulation of the inferior alveolar nerve (IAN) and the masseter nerve (MN) have been studied in cats anesthetized with sodium pentobarbital or decerebrated. On the basis of intracellulary recorded responses, hypoglossal motoneurons could be divided into 3 types. (1) DD type, in which a depolarizing-hyperpolarizing (EPSP-IPSP) potential was evoked by stimulation of IAN and MN. (2) DH type, in which a depolarizing-hyperpolarizing (EPSP-IPSP) potential was evoked by IAN stimulation and a hyperpolarizing potential (IPSP) was evoked by MN stimulation. (3) HH type, in which a hyperpolarizing potential (IPSP) was evoked by both stimuli.In the vicinity of the hypoglossal nucleus, intracellular and extracellular recordings were made from a group of neurons which could not be antidromically activated by stimulation of the hypoglossal nerve. These neurons fired in regular bursts of 3–18 spikes, with frequencies of up to 900/sec following stimulation of the ipsi-IAN, the contra-IAN, the ipsi-MN and the ipsilateral hypoglossal nerve. With a single shock to the ipsi-IAN, the mean latency of the peak of the initial spikes was 0.56 msec shorter than that of the onset of IPSPs of hypoglossal motoneurons. There was a correlation between the number of spikes of these neurons and the amplitude of IPSPs of the hypoglossal motoneuron evoked by varied stimulus intensities applied to the ipsi-IAN and the ipsi-MN. Electrophoretic injection of dye (fast green FCF) through the recording microelectrode revealed that all these neurons were located in the region ventral to the hypoglossal nucleus around the radix of the hypoglossal nerve. These results suggest that these perihypoglossal neurons are inhibitory interneurons which synapse on the hypoglossal motoneurons in the trigemino-hypoglossal reflex.     

Responses of globus pallidus neurons to cortical stimulation: intracellular study in the rat.

The responses of globus pallidus (GP) neurons to stimulation of the sensorimotor cortex, the neostriatum, and the subthalamic nucleus were intracellularly recorded in anesthetized rats. Stimulation of the cortex evoked a sequence of postsynaptic responses including an initial short EPSP, a short IPSP, and a late EPSP with multiple spikes in most of the repetitively firing GP neurons. The response pattern was very similar to those evoked by striatal stimulation, except that the latencies were longer. An acute knife cut placed immediately caudal to the substantia nigra caused no significant change in the responses to cortical and striatal stimulation. Stimulation of the subthalamic nucleus evoked a short latency EPSP overlapped with an IPSP. The polarity of all the IPSPs was reversed by a Cl- injection. A systemic injection of picrotoxin abolished all the IPSPs and unmasked large depolarizations with multiple spikes. An ibotenic acid lesion of the subthalamic nucleus eliminated both the initial short latency and late EPSPs to cortical and striatal stimulation and disclosed a prominent IPSP. Stimulation of the lesioned subthalamic nucleus also evoked large, short latency IPSPs without noticeable EPSPs. These results indicate that (i) the IPSPs evoked by cortical, striatal, and subthalamic stimulation were mediated by a GABAA receptor, (ii) both the initial and late EPSPs to cortical and striatal stimulation involved activation of the subthalamic nucleus but not brainstem nuclei, and (iii) cortically derived signals mediated through the neostriatum (i.e. long latency IPSPs) and the subthalamic nucleus (i.e. short latency EPSPs) converged on most GP neurons.