Up-regulation of site-specific remodeling without accumulation of microcracking and loss of osteocytes.

Functional adaptation of bone normally protects the skeleton from fracture during daily activity. Accumulation of microcracking and loss of osteocytes have been implicated in the regulation and initiation of targeted (reparative) remodeling of bone and, in certain situations, the development of fatigue or stress fracture. We performed a histologic study of the dorsal cortex of the mid-diaphysis of the third metacarpal (Mc-III) bone of Thoroughbred racehorses after bones were bulk-stained in basic fuchsin and transverse calcified sections were prepared. The Thoroughbred racehorse is an extreme athlete whose Mc-III bone experiences particularly high cyclic strains during training and racing. A group of non-athletic horses was also included in the experiment. The following variables were quantified: activation frequency (Ac.f); bone formation rate (BFR); resorption space density (Rs.N/T.Ar); microcrack density (Cr.Dn); microcrack mean length (Cr.Le); microcrack surface density (Cr.S.Dn); osteocyte density (Ot.N/T.Ar; Ot.N/B.Ar); and bone volume fraction (B.Ar/T.Ar). Ac.f and BFR were estimated using a mathematical algorithm. Using confocal microscopy, bones were examined for fine microcracks, diffuse matrix injury, and disruption of the osteocyte syncytium. Low values for Cr.Dn (#/mm2) were found in both groups (0.022+/-0.008 and 0.013+/-0.006 for racing Thoroughbreds and non-athletic horses, respectively). There was no significant relationship between Cr.Dn and Ot.N/T.Ar; Ot.N/B.Ar, B.Ar/T.Ar, and Ot.N/T.Ar; Ot.N/B.Ar, and remodeling (Ac.f, Rs.N/T.Ar) and Ot.N/T.Ar; Ot.N/B.Ar. Intense remodeling of the Mc-III dorsal cortex was found in the racing Thoroughbreds (Ac.f 12.8+/-7.4 #/mm2/year; BFR 31.5+/-15.6%; Rs.N/T.Ar 0.19+/-0.09 #/mm2) and was significantly increased compared with non-athletic horses. Overall, remodeling was weakly correlated with Cr.Dn (r2=0.15, P<0.05). Subtle matrix injury, not detectable by bright-field microscopy, was particularly evident adjacent to resorption spaces in Thoroughbred bone. In non-athletic horses, disruption of the dendritic cell processes of osteocytes associated with cement lines and interstitial fragments was more evident. Taken together, these findings suggest that site-specific (targeted) induction of remodeling during functional adaptation of bone in a high-strain skeletal site is not dependent on accumulation of microcracking or loss of osteocytes. We hypothesize that athleticism can directly influence bone turnover in this extreme athlete through pathways that do not involve classical linear microcracks.

Properties of a projection pathway from the medial preoptic nucleus to the midbrain periaqueductal gray of the rat and its role in the regulation of cardiovascular function.

In this study we examined (1) the effect of stimulation of the MPO on the firing activity of neurons in the PAG, (2) the role of glutamic acid in this interaction and, (3) whether reversible blockade of neuronal activity in the PAG by lidocaine can alter the effect of stimulation of the MPO on arterial blood pressure. Single pulse stimulation of the MPO produced a biphasic response in 2/32 cells and inhibited 3/32 cells. Train electrical stimulation excited 21/54 cells and inhibited 12/54 cells. The latencies to the onset of the excitatory and the inhibitory effects were not different, but the duration of the excitatory effect was slightly longer than that of the inhibitory effect. Chemical stimulation of the MPO excited 17/97 cells and inhibited 16/97 cells. The latency to onset of the excitatory response to stimulation of the MPO was longer but the duration was shorter than that of the inhibitory response. In 83% of the animals (29/35), stimulation of the MPO produced a decrease in mean arterial pressure (MAP). The duration of the response was 196.9 +/- 20.9 s and the average decrease in the MAP was 18.2 +/- 1.4 mmHg. Application of KYN blocked the excitatory response to stimulation of the MPO in 8/16 cells and the inhibitory response of 3/10 cells. Injection of lidocaine into the PAG by itself had no effect on the arterial blood pressure. However, in all animals (n = 10) lidocaine totally or significantly reduced the magnitude of the blood pressure change produced by stimulation of the MPO in a reversible manner. These studies electrophysiologically confirm a pathway between the MPO and the PAG that is, in part, under glutamatergic control. In addition, our results demonstrate that stimulation of the MPO produces a distinctive depressor effect that is mediated through the PAG.

The role of the basolateral nucleus of the amygdala in the pathway between the amygdala and the midbrain periaqueductal gray in the rat.

We electrophysiologically examined the connection between the basolateral nucleus of the amygdala (BLA) and the periaqueductal gray (PAG) and examined the role of the central nucleus of the amygdala (CNA) in this pathway. Train electrical stimulation of the BLA excited 21% (7/33) and inhibited 27% (9/33) of the cells recorded in the PAG. Chemical stimulation of the BLA excited 23% (13/56) and inhibited 16% (9/56) of the cells recorded in the PAG. Injection of lidocaine into the CNA by itself had no effect on PAG cells (n = 9) or on blood pressure but blocked the effect of BLA stimulation on PAG neurons in 78% of the cells recorded. It was concluded that: (1) PAG cells respond to BLA stimulation; (2) the majority of these cells are located in the dorsolateral and lateral columns of the PAG; and (3) the CNA modulates a majority of the activities of the BLA in the PAG.

An electrophysiological characterization of the projection from the central nucleus of the amygdala to the periaqueductal gray of the rat: the role of opioid receptors.

The midbrain periaqueductal gray (PAG) and the central nucleus of the amygdala (CNA) are both known to be involved in fear and anxiety, analgesia, vocalization, cardiovascular and respiratory changes, and freezing. Anatomical studies have shown that a connection between these two regions exists but little is known about the physiology or the neurochemical constituents of this pathway. The goals of this study were to characterize the projection from the CNA to the PAG using electrophysiological techniques and to determine whether mu- and/or delta-opioid receptors, which play a large role in a majority of the functions of the PAG, are involved in this pathway. Of the 38 PAG cells tested with single shock stimulation of the CNA, 44% responded; of those, 46% were excited and 54% were inhibited. The latency to onset of response for the inhibitory cells (12.71 +/- 6.61 ms) was shorter than that of the excitatory cells (22.33 +/- 4.04 ms). Forty-six percent of the 129 PAG cells tested with train electrical stimulation of the CNA responded; 44% were excited and 56% were inhibited. Chemical stimulation of the CNA (10 mM D,L-homocysteic acid) produced similar results; 48% (62/128) of PAG cells responded; 45% of cells were excited and 55% were inhibited. The baseline firing rate of the inhibitory cells was significantly higher compared to the excitatory cells. Chemical stimulation of the CNA produced an increase in blood pressure in 12 animals, a decrease in two animals, and had no effect on the blood pressure of 68 animals. The blood pressure changes ranged between 8.5 and 26.3 mmHg with a mean of 16.2 +/- 2.2 mmHg. The effect of naloxone (given either on site in the PAG or systemically) on the response to CNA stimulation was tested in 21 cells. Twenty-five percent of the excitatory cells (2/8) and 77% (10/13) of the inhibitory cells were blocked by naloxone with the majority of the blocked cells located in the ventrolateral PAG. It is concluded that: (1) Approximately 50% of cells in the lateral and ventrolateral columns of the PAG respond to CNA stimulation; (2) the inhibitory response is mediated by a faster conducting or a more direct pathway than the pathway that mediates the excitatory response; (3) neurons that are inhibited by CNA stimulation have a significantly higher baseline firing rate than neurons that are excited, suggesting that they may be tonically active interneurons; and (4) at least one link in the CNA-PAG pathway utilizes mu- or delta-opioid receptors.