The effects of age, season and fertility status on plasma and intratesticular insulin-like growth factor I concentration in stallions.

The purposes of this study were to establish the basal plasma and testicular insulin-like growth factor-I (IGF-I) values for stallions ranging in age from 6 months to 23 years and to determine if IGF-I could be used as a marker for declining fertility. Blood and testes were obtained from 28 light horse stallions and colts. Of the 28 stallions, 22 were considered fertile and were categorized by age (<2 y, 5 to 10 y, 11 to 15 y, and 16 to 23 y); 12 age-matched stallions were grouped as to fertility status (fertile, subfertile, infertile); and all 28 stallions were grouped as to season of castration (breeding season vs. non-breeding season). In colts less than 2 years of age, IGF-I concentrations in plasma and testicular extracts were higher (P < 0.01) than in the other age groups and were higher in the breeding season than in the non-breeding season (P < 0.01). No significant differences in plasma or testicular extract concentrations of IGF-I were found among fertility groups. The results of this study demonstrate that plasma and testicular IGF-I levels are high in stallions younger than 2 years of age and then decline and plateau in stallions older than 5 years of age, suggesting that IGF-I may be involved in testicular development. The results allude to a possible seasonal effect on IGF-I production. However, it is difficult to separate true seasonality and the effect of age as only those stallions less than 2 years old exhibited variation between seasons. The IGF-I does not appear to have a direct relationship with declined fertility in the stallions tested, suggesting that IGF-I may not be a reliable biomarker for the diagnosis of subfertility and infertility.

Inhibition of the expression of conditioned cardiac responses in the developing rat.

It is generally recognized that during development the capacity to express learning in terms of changes in somatomotor activity is evident earlier than the capacity for learned changes in autonomic responding (e.g., heart rate). In this series of experiments, findings indicate that changes in heart rate to a visual conditioned stimulus (CS) paired with a footshock unconditioned stimulus (US) can be observed as early in development in the rat as freezing responses. However, cardiac responses are inhibited from being expressed by preweanlings (but not adults) during CS-US pairings, the time when heart rate responses are often measured. This inhibition appears to arise from US exposure, and dissipates completely within 2 hr of training. These findings are discussed with respect to developmental changes in US-evoked autonomic arousal and response system dissociations.

Emergence of conditioned cardiac responses to an olfactory CS paired with an acoustic startle UCS during development: form and autonomic origins.

Pairing an olfactory conditioned stimulus (CS) with an acoustic startle unconditioned stimulus (UCS) produced conditioned bradycardia in rats ranging from 12 days of age to adulthood but failed to produce conditioning in 10-day-olds (Experiment 1). The second experiment compared the effects of two different UCSs, acoustic startle and electric shock. The startle UCS produced conditioned bradycardia accompanied by immobility. In contrast, the shock UCS produced conditioned tachycardia accompanied by behavioral activation. Pharmacological analysis revealed that activation of the parasympathetic nervous system (PNS) produced the conditioned bradycardia (Experiment 3) and that activation of the sympathetic nervous system (SNS) produced the conditioned tachycardia (Hunt, Hess, & Campbell, 1994). Further comparisons revealed that the startle UCS established conditioning several days earlier in development than the shock UCS. The ability of the PNS to respond to phasic stimulation earlier in development than the SNS was discussed as a possible mechanism for the early development of conditioned bradycardia.