Age as a factor in 6-hydroxydopamine-induced plasticity in the hypothalamus

Catecholamine reinnervation of supraoptic nucleus after deafferenting mechanical lesions and superior cervical ganglionectomy: histofluorescence and functional assessments in young adult and aged rats

Catecholaminergic innervation of the hypothalamic vasopressin (VP)-secreting supraoptic nucleus (SON) was examined at selected intervals after deafferenting neurosurgical lesions, with respect to potential contribution of peripheral vascular sympathetic fibers. Young adult (3 months) and aged (20 months) male F344 rats were subjected to mechanical knife-cut lesion just caudal and medial to the SON, superior cervical ganglionectomy (SCGx), or both surgeries. SON and lesion sites were assessed at 4, 14, 30 or 45 days after surgery, by CA histofluorescence. Functional evaluation in rats subjected to chronic lesions consisted of monitoring water balance (water consumption and urine volumes, and urine osmolality and VP content) in individual rats for presurgical and postsurgical intervals. Histofluorescence evaluation showed that SCGx did not affect the overall SON fluorescence pattern, although a minor sympathetic contribution to that pattern was discerned by comparing SON in rats subjected to lesion alone vs SCGx + lesion. Morphological reinnervation of SON was accomplished at 30 days in young rats, and 45 days in aged rats, after both lesion and SCGx. In young rats, histofluorescence density 30 days after deafferentation was denser than the innervation pattern seen in intact (sham-lesioned) animals, while reinnervation at 45 days postsurgically in aged rats only approximated the presurgical pattern. Vasopressin excretion and corresponding water conservation measures were compromised by SON deafferentation at both ages; excreted VP levels and water balance did not rebound to presurgical values at chronic postsurgical intervals in either young or aged rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Reactive synaptogenesis in hippocampal area CA1 of aged and young adult rats

Selective lesions that result in a partial loss of neuronal input appear to signal residual, undamaged inputs to sprout and replace synaptic connections that have been lost. Previous investigations have compared this process of reactive synaptogenesis in young and old animals in the hippocampal dentate gyrus and have demonstrated that the aged brain has a diminished capacity for reinnervation following massive denervation of a target area. This investigation has focused on the lesion-induced plasticity of an adjacent area of the hippocampal formation, area CA1 of regio superior, in young adult and aged rats. Young adult aged Fischer 344 rates were subjected to a unilateral, intraventricular injection of kainic acid that selectively destroyed the CA3-CA4 hippocampal pyramidal neurons. Following a 2-day interoperative interval, the rats sustained an ipsilateral transection of the fimbria-fornix. Animals were killed at 4, 10, 30, and 60 days following the second transection and processed for electron microscopic analysis. Photographic montages were constructed of area CA1 extending from the alveus to the hippocampal fissure. The density of synapses, both intact and degenerating, was determined and analyzed as a function of age, days postlesion, and zone of analysis. Synaptic density decreased 30-40% contralaterally and 60-70% ipsilaterally in both aged and young adult rats. While both age groups restored synaptic density to preoperative levels, aged subjects required significantly more time. Aged rats appeared to be retarded in the initial phases of synaptic replacement. The delay in the aged animals' reactive response was not due to any differences in degeneration clearance between the age groups.

Effects of triiodothyronine and propylthiouracil on regeneration of catecholaminergic nerve terminals in the paraventricular hypothalamic nucleus of the adult rat

Effects of triiodothyronine and the antithyroid drug, propylthiouracil, on regeneration of catecholaminergic nerve terminals in the paraventricular hypothalamic nucleus of adult rats were studied. Lesions were produced by 6-hydroxydopamine neurotoxin and then the animals were treated with triiodothyronine or propylthiouracil inducing hyperthyroidism or hypothyroidism, respectively, as determined by radioimmunoassay. Although catecholaminergic varicosities increased with time in the paraventricular hypothalamic nucleus of rats after lesion, fluorescent microscopic quantitation showed no statistical difference in their number between rats treated with triiodothyronine and the vehicle for as long as 56 days. Furthermore, electron microscopic quantitation at 56 days postlesion showed no significant difference between the triiodothyronine-treated and control rats in terms of the density, proportion, size, types of synapses, and synaptic frequency of catecholaminergic nerve terminals. There were growth cones in the paraventricular hypothalamic nucleus, suggesting growth activity after lesion. However, we found that exogenous administration of large doses of triiodothyronine at 25 micrograms/kg had little effect on the enhancement of regeneration of central catecholaminergic terminals after their destruction by 6-hydroxydopamine.

Effects of intracerebroventricular 6-hydroxydopamine on catecholaminergic fibers in the rat hypothalamus

Regeneration in the central nervous system has been claimed to be very limited and abortive, although functional regeneration of some of its pathways after destruction has been observed. The exact mechanisms by which axons regenerate fully or fail to have functional regeneration remain to be studied further. We explored whether or not there is a regional difference in regeneration of central catecholaminergic (CA) neurons in the hypothalamus of young adult rats after 6-hydroxydopamine (6-OH-DA) treatment. Four days after treatment, the numbers of CA terminals and axons were significantly reduced in the paraventricular hypothalamic nucleus, periventricular hypothalamic nucleus, supraoptic commissure (SOC), and dorsomedial hypothalamic nucleus as assessed by a morphometric quantitation on fluorescence microscopy micrographs; CA axons were gradually increased in numbers after the treatment. The number of CA varicosities in the supraoptic commissure was restored to 96% of control 180 days after the 6-OH-DA lesion, whereas the actual numbers of CA varicosities in the paraventricular, periventricular, and dorsomedial hypothalamic nuclei were attained at 79, 79, and 68% of control values, respectively. Our results indicate that CA fibers in the supraoptic commissure possess more regenerative potential than the three other hypothalamic regions studied, suggesting a regional difference in CA nerve sprouting during neuroplasticity within the hypothalamus. The favorable regeneration of CA axons in the supraoptic commissure implies to us that some trophic features along that pathway, particularly near the third ventricle, may have been stimulated after chemical lesion using 6-OH-DA, and gradually released in the distal field of the supraoptic commissure to attract CA stumps to sprout. These factors may thus induce both regenerative sprouting and collateral sprouting resulting in vigorous regrowth of CA fibers in the supraoptic commissure.

Plasticity of catecholaminergic neurons in aged rat brain: reinnervation and functional recovery after axotomy

Regenerative growth at the lesion site, reinnervation of a target nucleus and functional manifestations of recovery were studied in aged (20 and 30 months old) rats subjected to long-term transection of catecholaminergic (CA) fibers which contact and influence neurons of the supraoptic nucleus (SON). Small bilateral knife cuts were placed stereotaxically just caudal and medial to the SON. CA histofluorescence, induced by formaldehyde-glutaraldehyde (FAGLU) or aluminum-formaldehyde (ALFA) methods, was examined in hypothalamus at 2, 14, 21 and 60 days postsurgically. Water consumption, and urine volume and osmolality, were monitored presurgically, and through survival times. Subtotal CA denervation in the SON, and typical axonal transmitter "pile-up" at the lesion site, were evident two days after surgery. Among these degenerative profiles, which persisted for up to three weeks, fine-sized new fibers were apparent at the lesion, beginning between 2 and 14 days, and persisting throughout the period studied. At 21 days, and progressively thereafter, SON neurons were rimmed with fluorescent varicosites. Water consumption initially was depressed, but returned to presurgical mean levels by nine days. Urine volume returned to normal by 32 days. Urine osmolality showed a recovery by approximately three weeks. These functional parameters rebounded to levels higher than presurgical means among 20 month old, but not 30 month old, rats beyond 6 weeks survival, concurrent with a morphological hyperinnervation. The results reaffirm morphological regeneration, and support reinnervation and functional recovery, which extend considerably into the aging process.

Synaptic remodelling and astrocytic hypertrophy in rat cerebral cortex from early to late adulthood

Ultrastructural observation of the molecular layer of the parietal cortex of rats, aged 3, 6, 10 and 17 months, revealed various atypical synaptic profiles besides typical synapses. The atypical synapses were frequently in the vicinity of hypertrophied astrocytic profiles, and were sometimes completely surrounded by astrocytic processes. The presynaptic terminal contained either no vesicles or a few small distorted vesicles. Vacant postsynaptic terminals were occasionally seen. The total surface area of astrocytic profiles and the numbers of atypical synapses increased significantly between 3 and 10 months. The astrocytic acquisition of degenerating terminals was repeatedly observed over this period. Since there was no decrease in total synaptic number at this age, the astrocytic phenomenon may represent a stage in a continuous cycle of synaptic loss and replacement in the normal brain. By 17 months, when total synapse numbers decrease, synaptic replacement may be less than optimal.