To examine a possible role for gonadal steroid hormones in the enhanced accumulation of hypothalamic oxytocin (OT) messenger RNA (mRNA) and peptide in late pregnancy, we used an established model (22) in which sequential administration of estradiol (E2) and progesterone (P) SILASTIC capsules to ovariectomized rats is followed by removal of P. Long term and sustained E2 combined with abrupt P withdrawal mimics the gonadal steroid hormone pattern of late gestation in the rat (22). Using this paradigm, we demonstrate that OT mRNA is increased in the rat hypothalamus after long term P treatment, but only in the presence of E2 and only when P capsules are removed 48 h before killing. Furthermore, we show that P replacement in primiparous rats during late pregnancy blunts the increase in OT mRNA normally observed at the end of gestation. Our results support a role for E2 priming and P withdrawal in the enhanced accumulation of OT mRNA in the hypothalamus of the female rat.

Monkeys can gradually change the amplitude of the biceps spinal stretch reflex (SSR) without change in initial muscle length or biceps background electromyographic activity (EMG) (17). We investigated the concurrent behavior of synergist (brachialis and brachioradialis) and antagonist (triceps) muscles. Synergist background EMG remained stable while marked change occurred in biceps SSR amplitude. Triceps background EMG was minimal under all conditions. Thus biceps SSR amplitude change was not due to change in the background activity of closely related muscles. When biceps SSR amplitude changed, synergist SSR amplitude changed similarly but to a lesser extent. Brachialis change averaged 72% of biceps change, while brachioradialis change averaged 33%. By indicating that SSR amplitude change is relatively specific to the agonist muscle, this finding eliminates a number of nonspecific mechanisms as possible origins of SSR amplitude change. Thus it supports the potential value of the SSR as a system for studying the neuronal and synaptic bases of memory in the primate central nervous system (CNS).

Monkeys can gradually change the amplitude of the wholly segmental, largely monosynaptic, spinal stretch reflex (SSR) when confronted by a task requiring such change (15-19). Change develops over months and may reverse and redevelop at similarly slow rates. We investigated the persistence of SSR amplitude change over nonperformance periods of up to 38 days. Eight animals with chronic EMG electrodes learned to maintain elbow angle and a given level of biceps background EMG against constant extension torque. At random times, a brief additional extension torque pulse elicited the biceps SSR. In the control mode, reward always followed. Under the SSR increase or SSR decrease mode, reward occurred only if the absolute value of biceps EMG in the SSR interval was above or below a set value. Animals completed 3,000-6,000 trials/day over data-collection periods of 2-17 mo. Animals worked first under the control mode for up to 60 days and then under the SSR increase or SSR decrease mode for up to 274 days. Mode was switched once or twice more (SSR increase to SSR decrease or vice versa) over subsequent months. Animals responded to each SSR increase or SSR decrease mode exposure with gradual mode-appropriate change in SSR amplitude. Mode exposures were interrupted by gaps in performance of 10-38 days. Gaps produced transient 10- to 15% decreases in SSR amplitude under the control mode. This nonspecific decrease disappeared over the first week of postgap performance. Under the control mode, gaps had no other effects on SSR amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)

We previously reported that gonadal steroids modify the expression of arginine vasopressin (AVP) in the hypothalamus of rats administered 2% sodium chloride solution for 5 days. Gonadectomy prevented, and testosterone (T) replacement restored, enhanced AVP mRNA levels in the supraoptic nucleus (SON) of male rats receiving this hyperosmotic challenge. The present study investigated the effects of the androgenic and estrogenic metabolites of T on hypothalamic AVP mRNA levels in response to chronic hyperosmolality. Gonadectomized male rats receiving 2% NaCl for 5 days and treated with T, dihydrotestosterone (DHT), or DHT + estradiol (E₂), but not E₂ alone or empty implants, had increased AVP mRNA levels compared to gonadectomized animals receiving tap water. Our results support a role for T and DHT-mediated effects upon the enhanced accumulation of AVP mRNA in the SON of male rats receiving a chronic hyperosmotic challenge.

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