E. Hazel Murphy

Altered neuronal distribution of parvalbumin in anterior cingulate cortex of rabbits exposed in utero to cocaine

Previous studies of rabbits exposed in utero to cocaine have revealed an increase in the number of neurons which are GABA immunoreactive in the anterior cingulate cortex (ACC), suggesting a cocaine-elicited modification in the balance of excitatory and inhibitory interactions. Of the major calcium binding proteins expressed by different subgroups of GABAergic neurons, parvalbumin has been observed in conditions involving excess excitation, and may serve to protect neurons from excitotoxicity. In the present study, we used immunocy-tochemistry to compare the effects of prenatal cocaine exposure on the postnatal development of parvalbumin immunoreactivity in interneurons of the visual cortex (VC) and ACC. We determined the number and laminar distribution of parvalbumin immunoreactive neurons, and we also assessed the distribution of parvalbumin immunoreactivity within primary, secondary and tertiary dendrites of neurons in these two cortical areas. In both ACC and VC, parvalbumin immunoreactive neurons were first observed around postnatal day 10 (P10) and their number increased rapidly from P10 to P20. At all ages studied (P10 to P60) there was no difference between cocaine-exposed and saline control animals in the number or laminar distribution of parvalbumin immunoreactive neurons in either cortical area. However, the distribution of parvalbumin immunoreactivity within dendrites revealed a significant difference between cocaine-exposed and saline control animals in ACC but not in VC. In ACC, at all ages studied, there was an increase in the number of primary, secondary and tertiary dendrites which were parvalbumin immunoreactive in cocaine-exposed animals compared with saline controls. This difference was most striking in secondary dendrites, and in laminae V and VI. The effect was observed at doses of 4 and 3 mg/kg per injection but not at 2 mg/kg per injection. In contrast to ACC, in VC there was no difference in the number of immunoreactive dendrites in cocaine-exposed animals compared with saline controls. These observations are consistent with the hypothesis that the ACC of rabbits exposed in utero to cocaine is characterized by altered excitatory/inhibitory interactions. ACC receives a dense dopaminergic input, but VC receives minimal dopaminergic innervation. Mechanisms by which the action of cocaine on the developing dopaminergic system may modify the balance of excitation and inhibition in ACC are discussed.

The effects of epileptic cortical activity on the development of callosal projections

The effect of epileptic neural activity on the postnatal development of the corpus callosum was studied. Epileptiform activity was induced in the visual cortex of postnatal rabbits by continuous infusion of penicillin. Callosal projections of the occipital cortex were studied in rabbits aged at least 4 weeks. In these penicillin-exposed rabbits, the visual callosal projections extended through most of area 17 in a projection pattern characteristic of neonatal rabbits, rather than being restricted to a narrow callosal zone at the lateral border of area 17, as they usually are by this age. The results indicate that epileptic cortical activity stabilizes immature callosal projections which are normally eliminated during development. The maintenance of such immature and non-specific projections in the mature CNS may interfere with normal cortical functions and could underlie the cognitive deficits which have been observed following childhood epilepsy.

The critical period for alteration in cortical binocularity resulting from divergent and convergent strabismus

Abstract Kittens were reared with unilateral convergent (esotropia) or divergent (exotropia) strabismus which was surgically induced at different ages during the first 3 months. Visual field testing at ages 3–4 1 2 months showed no loss of the nasal field in esotropic kittens and a slight loss of the nasal field in exotropic kittens. Recording from single units in area 17 showed that early surgically induced strabismus results in a loss of binocularity of cortical cells and an increase in the size of the receptive fields of the cells. However, the critical periods for the two effects differ, with the critical period for the loss of cortical binocularity ending before the critical period for the abnormally large receptive fields. A special group of complex cells, the complex-spot cells, was relatively resistant to the loss of binocularity caused by strabismus. In both esotropic and exotropic animals the normal eye dominated more units than the operated eye. Although the major effects of esotropia and exotropia in cortical physiology were similar, some differences were found between the two groups. In the esotropic cats, the few binocularly driven units had receptive fields significantly larger than the monocularly driven units. In contrast, in the exotropic cats there was no significant differences in receptive field size of monocularly and binocularly driven units. In esotropic animals complex cells driven by the strabismic eye had larger receptive fields than complex cells driven by the unoperated eye. This difference did not occur in exotropic animals. In addition, the overall percentage of binocularly driven units was higher in exotropic (25%) than in esotropic (13%) animals. These results suggest that functional abnormalities in the visual cortex are slightly more severe in esotropic animals than in exotropic ones.

Normal development of tyrosine hydroxylase and serotonin immunoreactive fibers innervating anterior cingulate cortex and visual cortex in rabbits exposed prenatally to cocaine

Anterior cingulate cortex develops abnormally in rabbits exposed to cocaine in utero but visual cortex is normal. The interactions of cocaine with the dopamine, norepinephrine and serotonin systems makes each a potential candidate for influencing these developmental effects. Here, we report no differences in the distribution and density of serotonin and tyrosine hydroxylase-immunoreactive fibers in the anterior cingulate cortex in postnatal rabbits exposed prenatally to cocaine or saline. Because the pattern and extent of cortical innervation by these systems appears normal, the data suggest that the effects of prenatal exposure to cocaine may involve changes in monoamine signaling distinct from morphological reorganization of the cortical afferents.

Altered Expression of Microtubule-Associated Proteins in Cat Trochlear Motoneurons after Peripheral and Central Lesions of the Trochlear Nerve

Neurons lesioned in the peripheral nervous system (PNS) generally regenerate and survive, while neurons lesioned in the central nervous system (CNS) do not regenerate and often die. Investigators have traditionally compared the neuronal responses to PNS and CNS lesions in two separate populations of neurons. In this study, we compared the effects of PNS and CNS lesions on the expression of cytoskeletal proteins in a single neuronal population, the trochlear motoneurons of the cat. The trochlear nerve was lesioned either unilaterally in the PNS or bilaterally in the CNS (within the anterior medullary velum), and animals were allowed to survive 1, 2, or 4 weeks. Brain sections were reacted immunocytochemically using antibodies against microtubule -associated protein-2 (MAP-2) and a phosphorylated isoform of MAP1B, termed MAP1B-P. MAP-2 immunoreactivity (IR) was significantly decreased in the CNS-lesioned trochlear nucleus, compared to the lesioned and the unlesioned trochlear nucleus of PNS-lesioned animals. MAP1B-P IR was significantly increased in PNS- and CNS- lesioned trochlear axons, compared to axons in the unlesioned trochlear nerve of PNS-lesioned animals, and appeared in a small percentage of PNS- and CNS-lesioned cell bodies. These results support the growing body of evidence that MPA-2 can serve as a marker for cells that will eventually die following neuronal insult. The increased immunostaining of MAP1B-P in lesioned axons and its appearance in lesioned cell bodies are characteristic of the immature CNS and may reflect an initial recapitulation of early development, when the levels of this protein are high.

Visual callosal projections in the adult ferret

The laminar and tangential organization of visual callosal projections of areas 17 and 18 were investigated in the adult ferret, using histochemical methods to visualize axonally transported horseradish peroxidase (HRP). Normal adult ferrets were given injections of HRP throughout one visual cortex or had gelfoam soaked in HRP applied to the transected corpus callosum. The ferret callosal cell distribution has a greater tangential extent in area 18 than in area 17. In addition, the radial organization of callosal cells in areas 17 and 18 differs: three times as many infragranular cells are present in area 18 than in area 17, although the number of supragranular cells is similar for both areas 17 and 18. Since the projections of alpha retinal ganglion cells are reported to be exclusively contralateral in the ferret (Vitek et al., 1985), callosal projections may make a major contribution to the binocularity of neurons in area 18.

The effects of neonatal monocular enucleation on the organization of ipsilateral and contralateral retinothalamic projections in the rabbit

Autoradiographic methods were used to compare the ipsilateral and contralateral retinothalamic projections in pigmented Dutch-Belted rabbits that had neonatal monocular enucleation with the projections found in normally reared rabbits. In the normal adult rabbit, there is dense label throughout the dorsal lateral geniculate nucleus (LGd) except for a decreased label density in the region corresponding to the ipsilateral input. Following neonatal monocular enucleation, the contralateral projection fills in the part of the LGd corresponding to the ipsilateral input. Our data indicate that following monocular enucleation, two processes occur: an arrest of the segregation process and an expansion of the contralateral projection into the space normally containing the terminals of the ipsilateral projection. In addition, this filling in of the terminal space occurs relatively rapidly and is completed by day 14. No changes, however, were observed in the ipsilateral projection to the LGd. Unlike the LGd, the ventral lateral geniculate nucleus and the intergeniculate leaflet showed increases in the size of the ipsilateral projection region, and no changes in the contralateral projection. The present findings suggest that there may be different mechanisms governing whether alterations in the distribution of retinothalamic projections will occur in either the ipsilateral or contralateral nucleus.

Axotomy-induced changes in cytochrome oxidase activity in the cat trochlear nucleus

Following a unilateral section of the trochlear nerve, the effects of axotomy on cytochrome oxidase levels in the trochlear nucleus were studied. Cytochrome oxidase levels in the axotomized nucleus were significantly lower than in the control nucleus. The maximal decrease was observed at 2 weeks. Following partial restoration during weeks 3 and 4, cytochrome oxidase levels stabilized at levels only slightly below normal. Since a significant number of trochlear motoneurons die following axotomy, the restoration of cytochrome oxidase levels close to normal suggests that the surviving neurons may compensate for an increased load with a permanent increase in oxidative metabolism.

Survival of the ganglion cell population in the rabbit retina following neonatal visual cortex ablation.

Unilateral visual cortex ablations in neonatal rabbits produced no detectable loss of ganglion cells in the contralateral retina following a survival period of 3, 4 or 8 months. Analysis of neuron size distributions and neuron densities from whole-mounted retinas indicate that transneuronal retrograde degeneration does not occur in the rabbit following neonatal visual cortex removal. The results support the hypothesis that axon collaterals play a role in retinal ganglion cell survival in neonatally operated animals. The paradoxical relationship between functional sparing and ganglion cell survival is discussed.

Calcitonin Gene-Related Peptide Increases Following Axotomy of Trochlear Motoneurons

Neuropeptides have long been considered to act as neurotransmitters or neuromodulators, but they may also contribute to a variety of regulatory and trophic neuronal functions. In the present study, we determined the effects of axotomy on the levels of the neuropeptide calcitonin gene-related peptide (CGRP) in trochlear motoneurons (TMNs) of adult cats. The number of neurons with detectable CGRP immunoreactivity, and the intensity of their CGRP immunoreactivity, increased dramatically 1 week postaxotomy, gradually returned toward normal levels, but remained significantly higher than normal 12 weeks postaxotomy--a time when axonal regeneration and functional reinnervation of the superior oblique muscle should be complete. Our observation that CGRP levels in TMNs increase after axotomy suggests a role for this peptide in the response of motoneurons to injury and in regeneration. In addition, since many axotomized TMNs die, we suggest that the maintenance of high CGRP levels even after regeneration is complete may reflect an increased load on those TMNs that survive.