Inhibitory action of dopamine involves a subthreshold Cs(+)-sensitive conductance in neostriatal neurons.

Intracellular recordings in in vitro slice preparations of rat brain were used to compare the actions of dopamine and dopamine receptor agonists on the subthreshold membrane properties of neostriatal neurons. A reproducible response for dopaminergic agonists was evoked after firing produced by current ramp injections that induced a subthreshold voltage displacement. Dopamine (10-100 microM) decreased both firing rate and membrane slope input resistance in virtually all cells tested. Input resistance change appeared as an increase in inward rectification. Approximate reversal potential was around -87 mV. The D1 receptor agonists SKF 38393 and Cl-APB (1-10 microM) mimicked both dopamine effects with a reversal potential around -89 mV. The effects were blocked by the presence of 5-10 mM caesium (Cs+) but not by 1 microM tetrodotoxin, suggesting that main D1 effects on input resistance are due to subthreshold Cs(+)-sensitive conductances. cAMP analogues mimicked the actions of D1 receptor agonists. The D2 agonist, quinpirole (1-10 microM), did not produce any input resistance change, nonetheless, it still produced a decrease in firing rate. This suggests that the main D2 effect on firing is due to actions on suprathreshold ion conductances. All effects were blocked by D1 and D2 antagonists, respectively. D1 or D2 effects were found in the majority of cells tested.

Subthreshold rectification in neostriatal spiny projection neurons.

Intracellular recordings from slice preparations were used to assess the subthreshold electrophysiological behavior of rat neostriatal projection neurons. Both current steps and ramp currents were used to estimate the current-voltage relationship (I-V plot). Inward rectification in the subthreshold range was a characteristic of most neurons. The amount of rectification varied greatly, and it was complex: membrane voltage trajectories in response to ramps were made up by almost piece-wise changes in the rate of voltage rise, suggesting that multiple conductances contribute to the subthreshold range. Inward current blockers such as tetrodotoxin (TTX) or Cd2+ decreased inward rectification, whereas outward current blockers such as tetraethylammonium (TEA) or 4-aminopyridine (4-AP) increased inward rectification. However, most inward rectification was due to TEA- and Cs(+)-sensitive conductances and not to TTX- or Cd(2+)-sensitive conductances. Cs(+)-sensitive conductances predominated at more negative membrane potentials, whereas 4-AP-sensitive conductances predominated at just +/- 10 mV below the firing threshold. In spite of a very slow activation, there was evidence for transient outward currents modulating the response, i.e., 4-AP-sensitivity, and voltage-sensitivity for firing frequency and threshold. TEA-sensitive conductances also contributed toward fixing the firing threshold. These results imply the contribution of various ion conductances on the shaping of the characteristic physiological firing recorded in vivo. Modulation of these responses by transmitters or peptides may help to understand neural processing in the neostriatum.

Reduction of motor impairment by adrenal medulla transplants in aged rats.

Recently we have reported that drugs that enhance dopaminergic transmission, such as L-dopa and D-amphetamine, substantially improve the age-related deterioration of extrapyramidal motor functions, as assessed by the narrow uphill beam test. Here we report the effect of fetal adrenal medullary transplants upon the motor performance of aged rats in such a test. Rats were grafted with 300,000 cultured adrenal medullary cells, placed into the head of either the left caudate nucleus, or into both caudates. A third group was grafted with one freshly dissected adrenal medulla placed as a block of tissue into the lateral ventricle, whereas the control group sustained sham grafting. Evaluation of the motor performance of cultured and sham-grafted rats showed no improvement along the testing phase. Only adrenal block-grafted rats exhibited a significant recovery of motor coordination. Histologically, cultured cell grafts had a deteriorated appearance with poor survival rates, while block grafts exhibited chromaffin cells with round and neuron-like shapes. The results suggest that cultured adrenal grafts may not induce motor improvement due to their extremely low survival and poor integration, at least in the aged host brain, while fresh adrenal transplants may improve motor coordination for as long as 84 days.

Dibutyryl cAMP stimulates analgesia in rats bearing a ventricular adrenal medulla transplant.

In the present study, a significant increase in pain threshold (current to elicit vocalization to tail shock) was found 15 and 60 min after injection of dibutyryl cyclic AMP (db cAMP) (30 micrograms) into the lateral ventricle in rats bearing a transplant of fetal adrenal medulla (AM). By contrast, no effect on pain threshold was observed in rats bearing an AM transplant but receiving no db cAMP, or in rats receiving db cAMP but not bearing an AM transplant. In primary cultures of rat fetal chromaffin cells, db cAMP increased the number of neuron-like cells that showed both vasoactive intestinal polypeptide (VIP)- and tyrosine hydroxylase (TH)-like immunoreactivity. These findings indicate that db cAMP exerts a pharmacological modulation of the functional activity (i.e. elevation in pain thresholds) of fetal adrenal AM transplants, and induces phenotypic changes in cultured chromaffin cells with expression of a peptide that elevates pain threshold.

Vasoactive intestinal polypeptide induces REM recovery in insomniac forebrain lesioned cats.

Basal forebrain (BF) lesions in cats produces insomnia by reducing both slow wave sleep (SWS) and rapid-eye-movement (REM) sleep time. Recently it has been shown that vasoactive intestinal polypeptide (VIP) may be a specific REM inductor in the parachlorophenylalanine (PCPA) insomniac model. The purpose of this study was to test the hypnogenic properties of VIP in a nonpharmacological model of insomnia. Cats were rendered insomniac by delivering a DC current through stainless steel tripolar electrodes implanted in the basal forebrain area (BFA). Sleep-waking cycle recordings were done prior to lesions and on days 7, 9, 10, 11, 14, and 21 days after BF lesion. On day 10 after the lesion, 200 ng of VIP was injected into the 4th ventricle. Results showed that on postlesion days 7 and 9, SWS and REM sleep total times decreased, while waking time increased significantly. VIP restored REM sleep total time and frequency for almost 48 h, and SWS sleep total time for 24 h. On days 14 and 21 postlesion, insomnia was reestablished. Results are discussed in terms of the possible anatomical and neurochemical substrates whereby VIP can induce the recovery of sleep-waking control values.

In vitro effects of androgens upon the spontaneous rat uterine contractility.

The effectiveness of ring A reduced (5 alpha and 5 beta) testosterone (T) derivatives upon the rat uterus spontaneous contractility was tested in vitro. Compounds with the 3 alpha-hydroxy-5 alpha reduced configuration, such as androsterone and androstanediol, and one 5 beta reduced (5 beta-dihydrotestosterone) elicited a remarkable inhibitory effect upon the myometrial activity. Although steroids with 5 beta reduction were less potent than 3 alpha-hydroxy-5 alpha reduced compounds for depressing the myometrial activity, they were somewhat more potent than T, DHEA, androstenedione and the remaining 5 alpha reduced compounds tested. Therefore, T could act as a "prehormone", accounting for the maintenance of the physiological myometrial activity through its 5-reduced derivatives.