The stria terminalis sends fibres to a well circumscribed zone in the dorsal part of the preoptic area. This ‘strial part of the preoptic area’ contains cells with beaded, spine-poor dendrites arranged horizontally so as to intersect at right angles with the strial axons; it also receives aminergic fibres from the reportedly noradrenergic plexus in the bed nucleus of the stria terminalis. The strial part of the preoptic area is separated from the ventral part of the strial bed nucleus by a compact mass of cells (the ‘round nucleus’) which, unlike the other two areas, does not receive either aminergic fibres or synapses from the strial axons.
The incidences of various types of synapses have been counted in the neuropil of the strial part of the preoptic area and in the region lying between the ventromedial and arcuate hypothalamic nuclei. Synapses on dendritic shafts outnumber those on dendritic spines in the ventromedial nucleus, and to an even greater degree in the preoptic area. In both regions fibres of amygdaloid origin passing through the stria terminalis establish synapses (identified by the electron dense reaction of orthograde degeneration two days after axotomy) on both dendritic shafts and (especially in the ventromedial nucleus) on spines.
There is no sexual dimorphism in the incidences of the shaft synapses or any of the synapses of amygdaloid origin in the preoptic area, and none in any category of synapse in the ventromedial nucleus. However, in the normal females the number of non-amygdaloid synapses on dendritic spines in the preoptic area is higher than in the male. The suggestion that this difference could be related to the ability of the female to maintain a cyclic pattern of gonadotrophin release and/or behavioural oestrus is supported by published work implicating the preoptic area in the control of ovulation and mating behaviour.
Castration of the male within 12 h of birth (but not at 7 days of age) causes an increase to the female level in the number of spine synapses and permits the development of a cyclic pattern of gonadotrophin release and the ability to show a progesterone facilitated increase in receptivity. Conversely, females treated on day 4 (but not on day 16) with 1.25 mg of testosterone propionate have a low number of spine synapses (in the male range); this treatment also abolishes the cyclic pattern of gonadotrophin release and the progesterone facilitated increase in receptivity.